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Contributions to the geology and petrology of the Trans-Mexican Volcanic belt Nixon, Graham Tom 1986

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CONTRIBUTIONS TO THE GEOLOGY AND PETROLOGY OF THE TRANS-MEXICAN VOLCANIC BELT by GRAHAM T. NIXON B.Sc. (Hons.) Leeds University, 1971 M.Sc. Memorial University, 1975 A THESIS SUBMITTED IN THE REQUIREMENTS DOCTOR OF PARTIAL FULFILLMENT OF FOR THE DEGREE OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES Department of Geological Sciences We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1986 (c\ Graham Tom Nixon In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date O c V , V U k DE-6(3/81) ABSTRACT The composition and s p a t i a l d i s t r i b u t i o n of Quaternary volcanism i n the Trans-Mexican Volcanic Belt (TMVB) exhibit some remarkable co r r e l a t i o n s with the se i s m i c i t y , age, and structure of ocean lithosphere being consumed at the Middle America Trench. In the west, the TMVB i s related to aseismic subduction of the Rivera plate (2 cm/yr) and i n the east to a moderately dipping (20-30°) rapidly subducting Cocos plate (6-9 cm/yr). These con-t r a s t i n g arc segments are bounded by the Colima Graben, a zone of high-angle f a u l t i n g and contemporaneous a l k a l i n e / c a l c - a l k a l i n e volcanism, s i t -uated above a s i n i s t r a l transform f a u l t (4 cm/yr) developed i n the downgo-ing slab at the Cocos/Rivera juncture. Geologic mapping and K-Ar dating of the lavas of I z t a c c i h u a t l , a major c a l c - a l k a l i n e volcano i n the TMVB, have established two main phases of eruptive a c t i v i t y that began p r i o r to 0.9 Ma. The substructure of Iz t a c c i h u a t l (>0.6 Ma) i s composed p r i n c i p a l l y of two-pyroxene andesites 3 and dacites (300 km ) erupted from Llano Grande and Ancestral Pies v o l -canoes. The second stage of cone construction (<0.6 Ma) involved horn-3 blende dacites and andesites (150 km ) extruded from NNW-SSE oriented vents to form the modern summit region. The e a r l i e s t g l a c i a l deposits date p r i o r to 0.27 Ma but moraine complexes on the flanks are Wisconsin to Neoglacial. The phenocryst mineralogy and chemistry of the Younger Andesites and Dacites ind i c a t e that magma mixing has played an important role i n the evolution of I z t a c c f h u a t l magma chambers. Mixed lavas characterized by dis e q u i l i b r i u m phenocryst assemblages involving magnesian o l i v i n e and quartz are derived by mechanical mixing of hornblende dacite magma re s i d i n g i i i i n the crust and o l i v i n e - p h y r i c b a s a l t i c magma ascending from depth. Mixing and homogenization are effected by l i q u i d blending and dynamic f r a c -t i o n a l c r y s t a l l i z a t i o n i n turbulently convecting hybrid l i q u i d s . The whole-rock geochemistry of mixed lavas and hornblende dacites i s used to derive the composition of each batch of b a s a l t i c magma p e r i o d i c a l l y replenishing c r u s t a l magma chambers. B a s a l t i c end-members exhibit s i g n i f -icant v a r i a t i o n s i n large-ion l i t h o p h i l e elements and Sr i s o t o p i c composi-t i o n which are at t r i b u t e d to heterogeneities i n mantle source regions. The primitive compositions of these magmas are compatible with an o r i g i n i n -volving p a r t i a l melting of f e r t i l e p e r i d o t i t e under hydrous high-pressure conditions. iv TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES i x LIST OF PLATES x i ACKNOWLEDGEMENTS x i i CHAPTER 1. THE RELATIONSHIP BETWEEN QUATERNARY VOLCANISM • IN CENTRAL MEXICO AND THE SEISMICITY AND STRUCTURE OF SUBDUCTED OCEAN LITHOSPHERE 1.1. Introduction 1 1.2. The Trans-Mexican Volcanic Belt 2 1.3. Trench and Continental Margin 8 1.4. Seismicity 9 1.5. Evolution of Ocean Lithosphere and i t s Bearing on the Modern Arc 17 1.6. Segmentation a t Subduction Zones: Discussion 28 CHAPTER 2. GEOLOGY OF IZTACCIHUATL VOLCANO AND THE NORTHERN SIERRA NEVADA, CENTRAL MEXICO 2.1. Introduction 32 2.2. A n a l y t i c a l Techniques 35 2.3. Previous Work and Regional Geology 36 2.4. Nomenclature and Chemical C l a s s i f i c a t i o n of the Lavas ... 39 2.5. I z t a c c i h u a t l Volcano 5 9 2.5.1. K-Ar Geochronometry and Eruptive History 61 2.5.2. Older Volcanic Series: Morphology and Structure 66 2.5.2.1. Llano Grande Volcano 67 2.5.2.2. Ancestral Pies 68 2.5.3. Mineralogy and Petrography of the Older Volcanic Series 69 2.5.4. Older Flank A c t i v i t y 80 2.5.4.1. La Trampa Flows 80 2.5.4.2. Tlacupaso Rhyodacite 81 2.5.5. Younger Volcanic Series: Morphology and Structure .... 82 2.5.5.1. Pies 82 2.5.5.2. Summit Series: Cabeza, Pecho, R o d i l l a s 83 2.5.6. Mineralogy and Petrography of the Younger Volcanic Series 85 2.5.7. Pies Dacite Plug 89 2.5.8. Younger Flank A c t i v i t y 8 9 2.5.8.1. Teyotl Dacite 8 9 2.5.8.2. La J o y a Lava F i e l d 90 2.5.9. P y r o c l a s t i c Breccias 94 2.5.10. Xenoliths 95 2.5.11. Hydrothermal A l t e r a t i o n 96 2.6. Volcanic Rocks of the S i e r r a Nevada 97 V TABLE OF CONTENTS (Cont'd) Page 2.6.1. Rio F r i o Pumice Deposit 97 2.6.2. Papayo Dacite 97 2.6.3. I z t a l t e t l a c Cone 100 2.6.4. Buenavista Dacite 101 2.7. Volcanic Rocks of the Valley of Mexico: Chichinautzin Group 102 2.8. Tephra Deposits 103 2.9. E p i c l a s t i c Volcanic Breccias, Loess, and A l l u v i a l Deposits 107 2.10. G l a c i a t i o n and G l a c i a l Deposits 108 2.10.1. Older T i l l s and T i l l - l i k e Sediments I l l 2.10.2. Nexcoalango T i l l 113 2.10.3. Hueyatlaco T i l l 113 2.10.4. Milpulco T i l l 115 2.10.5. Ayoloco T i l l 116 2.11. Summary 116 CHAPTER 3. PETROLOGY OF THE YOUNGER ANDESITES AND DACITES OF IZTACCIHUATL VOLCANO, MEXICO: 1. DISEQUILIBRIUM PHENOCRYST ASSEMBLAGES AS INDICATORS OF MAGMA CHAMBER PROCESSES 3.1. Introduction 124 3.2. General Geology and Sampling 125 3.3. Lava Petrography 131 3.3.1. Hornblende Dacites 136 3.3.2. Cpx-Opx Dacites 137 3.3.3. Mixed Lavas 137 3.3.4. B a s a l t i c Rocks 139 3.4. Mineralogy and Phase Chemistry 139 3.4.1. O l i v i n e 142 3.4.2. Pyroxenes 143 3.4.3. Amphibole 152 3.4.4. Plagioclase 155 3.4.5. Oxides 162 3.4.6. B i o t i t e and Quartz 166 3.4.7. Accessory Minerals 168 3.5. O l i v i n e - L i q u i d E q u i l i b r i a 169 3.6. O l i v i n e Morphology-Composition Relations 173 3.7. Pyroxene Compositional Variations 178 3.7.1. Clinopyroxene 182 3.7.2. Orthopyroxene 185 3.7.3. Coexisting Pyroxenes 187 3.7.4. T i - A l Relations 191 3.8. Amphibole-Liquid E q u i l i b r i a 198 3.9. Plagioclase Compositional Variations 202 3.9.1 Or i g i n of Plagioclase Textures and Compositions 205 3.10. Geothermometry and Oxygen Barometry 212 3.11. Or i g i n of Disequilibrium Phenocryst Assemblages 218 3.12. Phenocryst Residence Times 223 3.12. Mixing Dynamics 225 v i TABLE OF CONTENTS (Cont'd) Page 3.14. Concurrent Magma Mixing and F r a c t i o n a l C r y s t a l l i z a t i o n : Implications f o r Magma Chamber Processes 230 CHAPTER 4. PETROLOGY OF THE YOUNGER ANDESITES AND DACITES OF IZTACCIHUATL VOLCANO,'MEXICO: 2. CHEMICAL STRATIGRAPHY, MAGMA MIXING, AND THE COMPOSITION OF BASALTIC MAGMA INFLUX 4.1. Introduction 237 4.2. Geologic Setting and Eruptive Behaviour 238 4.3. Petrography 239 4.4. Sampling Procedures and Stratigraphic C o r r e l a t i o n 242 4.5. A n a l y t i c a l Techniques 245 4.6. Whole-Rock Composition and C l a s s i f i c a t i o n 247 4.7. Chemical Stratigraphy and Evidence f o r Magma Chamber Recharge 266 4.7.1 Cabeza 270 4.7.2. Pecho 270 4.7.3. Ro d i l l a s 273 4.7.4. Pies 278 4.8. S p a t i a l and Temporal Evolution 280 4.9. Strontium Isotopic Composition 282 4.10. Magma Mixing: Characterization of Binary Mixtures and End-Member Compositions 287 4.10.1. Mixing Systematics 288 4.10.2. Fine Structure of Mixed Lavas 292 4.10.3. Inhomogeneous Mixing 296 4.10.4. Mixing Proportions 297 4.10.5. Calculated Composition of B a s a l t i c End-Members 302 4.11. Summary and Discussion 315 CHAPTER 5. ORIGIN OF PRIMITIVE OROGENIC MAGMAS IN THE TRANS-MEXICAN VOLCANIC BELT AND THE NATURE OF MANTLE SOURCE REGIONS 5.1. Introduction 322 5.2. Experimental Evidence f o r Mantle-Derived Melts 324 5.3. Behaviour of Major Elements during P a r t i a l Melting 328 5.3.1. Conditions and Extents of Melting 331 5.3.2. MgO vs. FeO Diagram 335 5.4. Incompatible Elements 343 5.4.1. Normalized Abundance Patterns 345 5.5. Causes of Chemical Heterogeneity i n Prim i t i v e Magmas .... 356 5.6. Summary and Discussion 360 CHAPTER 6. GENERAL CONCLUSIONS 362 REFERENCES CITED 368 APPENDIX A: Instrumental P r e c i s i o n and Accuracy of XRF Analyses 391 APPENDIX B: Microprobe Data 392 APPENDIX C: LINMIXER (Version 2): A Computer Program f o r Calculating End-Member Compositions Involved i n Magma Mixing 404 v i i LIST OF TABLES Page Table 2.1: Chemical Analyses and Cation Normative Compositions of Lavas of I z t a c c i h u a t l Volcano and the Northern S i e r r a Nevada 40 Table 2.2: Vent Type, Exposed Surface Area, and Estimated Volume of Eruptive Products f o r I z t a c c i h u a t l and the Northern S i e r r a Nevada 60 Table 2.3: K-Ar Geochronometry of I z t a c c i h u a t l Lavas 62 Table 2.4: Modal Analyses of I z t a c c i h u a t l Lavas 76 Table 3.1: Modal Analyses of Younger Andesites and Dacites 135 Table 3.2: Representative Microprobe Point Analyses of O l i v i n e s .... 140 Table 3.3: Representative Microprobe Point Analyses of Clinopyroxenes 146 Table 3.4: Representative Microprobe Point Analyses of Orthopyroxenes 147 Table 3.5: Representative Microprobe Point Analyses of Coexisting Orthopyroxenes and Clinopyroxenes 150 Table 3.6: Representative Microprobe Point Analyses of Amphiboles 154 Table 3.7: Representative Microprobe Point Analyses of Plagioclase Feldspars 158 Table 3.8: Microprobe Point Analyses of Oxides 163 Table 3.9: Microprobe Point Analyses of B i o t i t e s 167 Table 3.10: Plagioclase Phenocryst Textures and Compositional Variations 210 Table 4.1: Chemical Analyses and Cation Norms of the Younger Andesites and Dacites of I z t a c c i h u a t l Volcano 248 Table 4.2: Chemical Analyses and Cation Norms of Primitive and Evolved B a s a l t i c Lavas of La Joya ( I z t a c c i h u a t l ) and the Valley of Mexico 255 Table 4.3: Strontium Isotopic Composition of Volcanic Rocks of I z t a c c i h u a t l and the Valley of Mexico 283 Table 4.4: Binary Mixing Models of the Compositions of B a s a l t i c Magma Batches at I z t a c c i h u a t l Volcano 304 v i i i LIST OF TABLES (Cont'd) Page Table 4.5: Calculated Composition and Cation Norms of End-Member B a s a l t i c Components i n Mixed Lavas 310 Table 5.1: Comparison of Mexican B a s a l t i c Compositions with P o t e n t i a l Primary Magmas and Experimental Liquids Produced by Anhydrous Melting of P e r i d o t i t e 332 Table B . l : Microprobe Point Analyses of Olivines 392 Table B.2: Microprobe Point Analyses of Pyroxenes 394 Table B.3: Microprobe Point Analyses of Amphiboles 398 Table B.4: Microprobe Point Analyses of Plagioclase Feldspars 399 Table B.5: Microprobe Point Analyses of B i o t i t e s 402 Table B.6: C o r r e l a t i o n of Sample Numbers Used i n the F i e l d and i n the Text 403 ix LIST OF FIGURES Page Figure 1.1: Generalized Tectonic Map of Mexico 4 Figure 1.2: Plot of Shallow Earthquakes for 1963-1974 11 Figure 1.3: Plot of Intermediate Earthquakes f o r 1963-1974 14 Figure 1.4: Age and Structure of Ocean Lithosphere being Subducted at the Middle America Trench 20 Figure 1.5: Vector Diagram f o r Relative Motions between the Rivera, Cocos, P a c i f i c , and NOAM Plates 23 Figure 2.1: AFM Diagram f o r Volcanic Rocks of I z t a c c i h u a t l and the Northern S i e r r a Nevada 56 Figure 2.2: Histogram of S i l i c a D i s t r i b u t i o n of Analyzed Samples .... 58 Figure 2.3: Diagrammatic Cross-Sections showing Stages i n the Evolution of I z t a c c i h u a t l Volcano 64 Figure 2.4: Grain-Size D i s t r i b u t i o n of Phenocrysts i n I z t a c c i h u a t l Lavas 71 Figure 2.5: Mineralogy and Petrographic C h a r a c t e r i s t i c s of I z t a c c i h u a t l Lavas 73 Figure 2.6: S t r a t i g r a p h i c Sections and Correlation of Recent Sediments and Tephra Deposits 105 Figure 2.7: G l a c i a l Chronology of I z t a c c i h u a t l 110 Figure 3.1: Generalized Geologic Map and Cross-Section of I z t a c c i h u a t l and the Northern S i e r r a Nevada 127 Figure 3.2: Geologic Map of the Summit Region of I z t a c c i h u a t l Showing Sample Locations 130 Figure 3.3: Phenocryst Mineralogy and Petrographic C h a r a c t e r i s t i c s of the Younger Andesites and Dacites 133 Figure 3.4: Plot of O l i v i n e Phenocryst Compositions vs. Host Rock Mg-number 171 Figure 3.5: D i s t r i b u t i o n of O l i v i n e Morphologies and Composition .... 176 Figure 3.6: Microprobe Point Analyses of Pyroxenes Plotted i n the Ca-Mg-Fe Qu a d r i l a t e r a l 181 Figure 3.7: Microprobe Point Analyses of Pyroxenes i n Various Lava Types Plotted i n the Ca-Mg-Fe Qua d r i l a t e r a l 184 X LIST OF FIGURES (Cont'd) Page Figure 3.8: Microprobe Point Analyses of Coexisting Pyroxenes Plotted i n the Ca-Ma-Fe Qu a d r i l a t e r a l 189 Figure 3.9: T i vs. ZA1 Plot f o r Pyroxenes 193 Figure 3.10: T i / z A l vs. Mg-number Plot f o r Pyroxenes 195 Figure 3.11: Al(IV) vs. (Na+K) Site Occupancy Plot f o r Amphiboles ... 200 Figure 3.12: Microprobe Point Analyses of Plagioclase Feldspars Plotted i n the Or-Ab-An diagram 204 Figure 3.13: Diagram of the Plagioclase Binary System 207 Figure 3.14: T-f Plot 215 °2 Figure 3.15: Inferred O r i g i n and Evolution of Phenocryst Assemblages . 221 Figure 4.1: Generalized Geologic Map of the Summit Region of I z t a c c i h u a t l Showing Sample L o c a l i t i e s 243 Figure 4.2: Harker V a r i a t i o n Diagrams 258 Figure 4.3: AFM Diagram 261 Figure 4.4: K^O vs. S i 0 2 Plot 263 Figure 4.5: Chemical Stratigraphy Plots 268 Figure 4.6: Plot of 8 ? S r / 8 6 S r vs. 8 ? R b / 8 6 S r 285 Figure 4.7: V a r i a t i o n Diagrams showing the E f f e c t s of Temporal Variations i n End-Member Composition on Binary Mixing Trends 290 Figure 4.8: MgO and Ni vs. S i 0 2 Plots 294 Figure 4.9: Mean Proportions of B a s a l t i c End-Members i n Mixed Lavas . 301 Figure 4.10: V a r i a t i o n Diagrams Showing the Calculated Mean Composition of B a s a l t i c End-Members i n Relation to Mixed Lavas, Hb Dacite, and Analyzed B a s a l t i c rocks 312 Figure 5.1: Summary of Melting Relations f o r IZ-839 326 Figure 5.2: MgO vs. FeO Plot (cation mole %) f o r Primitive B a s a l t i c Magmas and Melt F i e l d s f o r Selected P e r i d o t i t e s 337 Figure 5.3: Normalized Abundance Patterns of Incompatible Elements for Selected Oceanic, Continental, and Mexican B a s a l t i c Compositions and Subducted Sediment 347 LIST OF PLATES Page Geologic Map of I z t a c c i h u a t l Volcano and the _ p ^ * «' Northern S i e r r a Nevada complete with Geologic *w * Cross-Sections (Rear Pocket) Plate 2.1: Airphoto mosaic of the Summit Region of I z t a c c i h u a t l 34 ACKNOWLEDGEMENTS Fieldwork at I z t a c c i h u a t l was i n i t i a t e d with the aid of successive Penrose grants awarded by the Geological Society of America and the Nixon Personal Chequing Account, and subsequently funded by NSERC operating grants A-8841 to R. L. Armstrong and A-8302 to H. R. Wynne-Edwards. Logis-t i c s i n the f i e l d were greatly eased by the pro v i s i o n of a f i e l d vehicle and v i s i t i n g researcher stipend granted by the I n s t i t u t o de Ge o f i s i c a , Universidad Nacional Autonoma de Mexico (UNAM), through the o f f i c e s of Drs. Luis Del C a s t i l l o G., Head of the Department of Exploration, and J u l i a n Adem, Director of the I n s t i t u t e . A f i e l d vehicle was also kindly provided by the I n s t i t u t o de Geologia through Drs. Jose Guerrero, Head of the De-partment of Geology, and Diego A. Cordoba, Director of the I n s t i t u t e . Dr. and Mrs. Lloyd Staples, Eugene, Oregon, were instrumental i n easing problems of accomodation i n Mexico. Their daughter and son-in-law, Ann Staples and Renan Perez de Priego, and family, welcomed me into t h e i r home on numerous v i s i t s . I thank them as friends and hope someday to repay t h e i r h o s p i t a l i t y . I also enjoyed the cameraderie of 'Los Muchachos del Altzomoni' and thank Ings. Leopoldo Pefia and Jose Fernandez Arauna of Telesistema Mexicano for making my stay possible. A n a l y t i c a l help and guidance was provided by Rob Berman (trace element and v o l a t i l e a n a l y s i s ) , John Knight (microprobe), Dick Armstrong and Dave Whitford (trace element and Sr i s o t o p i c a n a l y s i s ) , K r i s t a Scott (K anal-y s i s ) , and Joe Harakal (Ar isotope analysis and data reduction). Dan Au, Department of Geophysics, U.B.C., kindly provided computer programs to manipulate the earthquake data base. Ed Montgomery kindly printed the x i i i plate and Gord Hodge drafted many of the diagrams (the better ones) and lent h i s 'perspective' to the others and the geology map. Early versions of part or a l l of the manuscript were c a r e f u l l y reviewed and c r i t i c i z e d by Drs. R. L. Armstrong, R. L. Chase, K. C. McTaggart, and W. M. Mathews, Univ e r s i t y of B r i t i s h Columbia, R. P. P h i l l i p s and G. R. G a s t i l , University of C a l i f o r n i a , San Diego, M. J . Carr, Rutgers Univer-s i t y , and Z. de Cserna, UNAM. I greatly appreciate the help and support of Dick Armstrong who took i t upon himself to harbour a ' f u g i t i v e ' , though no-one quite anticipated the length of his 'sentence'. F i n a l l y , thank you Barbara for putting up with t h i s i n s a n i t y , and most of a l l f o r 'being there'. 1 CHAPTER 1 THE RELATIONSHIP BETWEEN QUATERNARY VOLCANISM IN CENTRAL MEXICO AND THE  SEISMICITY AND STRUCTURE OF SUBDUCTED OCEAN LITHOSPHERE 1.1 INTRODUCTION Studies of convergent plate margins during the past decade have as-sembled a wealth of geological and geophysical evidence that permits a tectonic subdivision of subduction zones (Stoiber and Carr, 1973; Carr et a l . , 1974; Stauder, 1973, 1975; Barazangi and Isacks, 1976). According to models developed by Stoiber and Carr (1973), the descending lithosphere i s broken by tear f a u l t s , propagated at the trench, that divided the( slab i n t o d i s c r e t e segments, t y p i c a l l y less than 300 km across. Each segment of oceanic lithosphere descends into the mantle with a d i f f e r e n t s t r i k e and dip, producing o f f s e t s i n features such as the i n c l i n e d seismic zone, trench axis and alignment of cones along the "volcanic f r o n t . " The boun-daries between segments are recognized by transverse features such as mapped f a u l t zones (commonly with s t r i k e - s l i p displacement), elongate c l u s t e r s of cinder cones or l o c i of large volcanic eruptions, and concen-tr a t i o n s of shallow earthquakes. Where a l l these c r i t e r i a are used i n combination, the weight of evidence generally favours segmentation, although the actual number of segments i n any p a r t i c u l a r arc may be d i s -puted. As the model i s extended i n t o areas of more complex plate i n t e r a c -t i o n - f o r example, near the t r i p l e junction of the Cocos, Caribbean, and North American (NOAM) plates (Carr, 1976) - or i f i t i s applied to arcs where only a l i m i t e d number of the c r i t e r i a that purport to d i s t i n g u i s h segments are present, as i n the Cascade volcanic chain (Hughes et a l . , 2 1980) - the rel a t i o n s h i p s between the subducted slab and tectonic f a b r i c of the overriding plate become more questionable. Regional lineaments i n the TMVB formed by Holocene cinder cones and s i t e s of h i s t o r i c eruptions served as the basis for a segmentation model for the Mexican arc (Stoiber and Carr, 1973; Carr et a l . , 1974). The arc was subdivided i n t o s i x segments, about 150 to 200 km i n width, whose boundaries were assumed to l i e p a r a l l e l to the northeasterly d i r e c t i o n of underthrusting at the trench. Study of earthquake f o c a l mechanisms of the Central American arc by Dean and Drake (1978) did not substantiate the proposed segmented nature of the Mexican continental margin. I w i l l review the general structure and compositional v a r i a b i l i t y of volcanism i n ce n t r a l Mexico and r e l a t e these features to the sei s m i c i t y and structure of young ocean lithosphere presently being consumed at the Middle America Trench. It i s shown that the tectonic evolution and age of the ocean lithosphere may play an important role i n determining both the nature of segmentation and the seismic signature of the subducted slab. 1.2 THE TRANS-MEXICAN VOLCANIC BELT The locus of and e s i t i c volcanism i n cen t r a l Mexico extends i n a west-east d i r e c t i o n f or more than 1,000 km from the P a c i f i c Coast to the margins of the High Mexican Plateau overlooking the Gulf of Mexico. Inspection of the Tectonic Map of Mexico (Figure 1.1; de Cserna, 1961) reveals the complex nature and the extreme d i v e r s i t y of "basement" terranes underlying the volcanic belt as i t transects the s t r u c t u r a l grain of the Mexican continent (de Cserna, 1965, 1976; Demant and Robin, 1975). In the west, the TMVB i s underlain by the ignimbrite province of the S i e r r a Madre Oc-Figure 1.1: Generalized tectonic map of Mexico modified from de Cserna (1961). Trans-Mexican Volcanic Belt: v e r t i c a l r u l i n g = western arc; h o r i z o n t a l r u l i n g = c a l c - a l k a l i n e / a l k a l i n e province of the Colima Graben; V-pattern = c e n t r a l and eastern arc. F i l l e d t r i a n g l e s denote major c a l c - a l k a l i n e cones of the " v o l -canic front"; open t r i a n g l e s represent selected smaller cones; f i l l e d c i r c l e s indicate caldera complexes. 1 = San Juan; 2 = Sanganguey; 3 = Ceboruco; 4 = Tequila; 5 = S i e r r a La Prima-vera; 6 = Nevado de Colima; 7 = Volcan Colima; 8 = P a r i c u t i n ; 9 = Nevado de Toluca; 10 = Popcatepetl; 11 = I z t a c c l h u a t l ; 12 = La Malinche; 13 = Los Humeros; 14 = Pico de Orizaba; 15 = San Andres Tuxtla; 16 = E l Chichon; 17 = Tacana. 5 c i d e n t a l which extends northward along the western C o r d i l l e r a of Mexico to the United States border. Where the two provinces i n t e r s e c t , gently dipp-ing volcanic formations of the S i e r r a Madre Occidental are cut by l o n g i t u -d i n a l graben structures associated with Quaternary volcanic a c t i v i t y within the TMVB. High-angle f a u l t s and tensional fractures extend from Volcan Sanganguey, near Tepic, to the Chapala region, 50 km south of Guadalajara (Demant et a l . , 1976; Demant, 1978). S i l i c i c p y r o c l a s t i c rocks and i n t e r -calated b a s a l t i c flows of the Si e r r a Madre Occidental just north of Guada-l a j a r a have yielded K-Ar ages of 4.5 to 9.5 Ma (Watkins et a l . , 1971). Ignimbrites of th i s older province extend along the northern edge of the TMVB as f a r as Pachuca, about 100 km northeast of Mexico C i t y (Demant, 1978). East of the Valley of Mexico, the axis of Quaternary volcanism transgresses folded marine sedimentary s t r a t a of the S i e r r a Madre O r i e n t a l and Pliocene-Miocene plateau lavas belonging to an eastern a l k a l i n e pro-vince (Demant and Robin, 1975; Robin and Tournon, 1978). The peculiar geometry of the Mexican arc i n comparison to other circum-Pacific andesite provinces has generated a remarkable array of concepts concerning i t s o r i g i n . Previous studies have related volcanism to (1) a continental prolongation of the C l a r i o n Fracture Zone (Mooser and Maldonado-Koerdell, 1961); (2) an extension of the San Andreas f a u l t system from the Gulf of C a l i f o r n i a ( G a s t i l and Jensky, 1973); (3) an ancient geostructure subjected to l e f t - l a t e r a l transcurrent displacement and l a t e r reactivated i n middle T e r t i a r y time (Mooser, 1972); and (4) a phenomenon related to subduction at the Middle America Trench (Gunn and Mooser, 1970; Mooser, 1972; Demant and Robin, 1975; Robin and Nicolas, 1978; Menard, 1978). These hypotheses and many more have been summarized by Demant (1978). 6 A source of confusion a r i s i n g from the e a r l i e r work concerns the age connotation of the popular term "Trans-Mexican Volcanic B e l t " which varies from author to author even though the same geographic e n t i t y i s t a c i t l y implied. For example, Mooser (1972), conducting investigations i n the Valley of Mexico, considered that volcanism i n the TMVB began about 30 Ma ago, but that volcanic a c t i v i t y was not widespread u n t i l mid-Miocene time. An Oligocene to Holocene age was accepted i n l a t e r studies of the same region (Negendank, 1972, 1973; Bloomfield, 1975; Bloomfield and Valastro, 1977; Richter and Negendank, 1976). At the eastern extremity of the volcanic b e l t , Robin (1976) recognized a " p r i m i t i v e " TMVB, comprising Miocene andesites, and a l a t e r phase of "Neovolcanic" a c t i v i t y , commencing approximately 2.5 Ma (Robin and Nicolas, 1978; Cantagrel and Robin, 1978). In f a c t , Cantagrel and Robin (1978) state that the east-west trend of contemporary c a l c - a l k a l i n e volcanism has changed l i t t l e since the mid-Miocene. Although t h i s claim may indeed be correct, i t c e r t a i n l y requires substantiating by further K-Ar geochronometry i n cent r a l and western Mexico. New K-Ar dates obtained on and e s i t i c lavas associated with the e a r l i e s t stages of cone-building at I z t a c c i h u a t l , near Mexico C i t y , and at Volcan Tequila, i n the western part of the arc, y i e l d ages of approximately 1 Ma (Nixon et a l . , i n press). This study, therefore, i s concerned s p e c i -f i c a l l y with volcanic rocks of the TMVB younger than =1 Ma. Chemical and petrographic data show that the TMVB may be divided i n t o two d i s t i n c t c a l c - a l k a l i n e provinces (Figure 1.1): (1) a western arc, averaging 60 km i n width and extending from the P a c i f i c coast to the Colima Graben; and (2) a ce n t r a l and eastern arc, stretching from the Colima volcanoes through the a r e a l l y extensive cinder cones and lava flows of Michoacan to the l o c a l l y more r e s t r i c t e d volcanism associated with major 7 volcanic lineaments oriented north-south i n the S i e r r a Nevada (Iztaccihuatl-Popocatepetl) and Orizaba-Cofre de Perote regions. Major cones of the western TMVB are b u i l t predominantly of two-pyroxene andesite, stand less than 3,000 m above sea l e v e l , and comprise 3 volumes less than 70 km (Demant et a l . , 1976; Thorpe and Francis, 1975; Nelson, 1976; Luhr and Nelson, 1980). From the Colima volcanoes eastward, the major volcanic e d i f i c e s possess summit elevations i n the range 4,000 to 3 6,000 m, are more voluminous (generally >200 km ), and are constructed with a high proportion of amphibole-bearing andesite and dacite (Bloomfield and Valastro, 1977; Demant et a l . , 1975; Nixon, 1979). This portion of the arc averages 100 to 200 km i n width and l i e s behind an arc-trench gap of more than 300 km at i t s eastern extremity. The boundary between these subprovinces i s occupied by the Colima Graben, a region of high-angle f a u l t i n g oriented north-south and i n t e r s e c t i n g the northwest-southeast s t r u c t u r a l trends of the Guadlajara area. The southern end of t h i s feature i s dominated by Volcan Colima, the most active volcano i n Mexico, and the e x t i n c t ( ? ) Nevado de Colima. On the northern and western margins of the Colima volcanoes, Holocene s c o r i a cones of strongly undersaturated analcime-bearing basanites and minettes (Luhr and Carmichael, 1979) nestle incongruously among the many c a l c - a l k a l i n e cinder cones i n the region. The comenditic dome complex of S i e r r a La Primavera l i e s further north, j u s t west of the c i t y of Guadalajara (Mahood, 1977, 1978). The Colima Graben, therefore, represents a region where a l k a l i n e and c a l c - a l k a l i n e volcanism have developed contemporaneously at the "volcanic f r o n t " of a convergent plate margin. Quaternary v o l c a n i c a c t i v i t y between the eastern terminus of the TMVB and the Guatemalan volcanic chain i s r e s t r i c t e d to two i s o l a t e d regions 8 (Figure 1.1). H i s t o r i c a l l y active volcanism at San Martin Tuxtla has produced a l k a l i n e lavas of sodic a f f i n i t y , i ncluding p i c r i t i c basalts, basanitoids, and hawaiites ( P i c h l e r and Weyl, 1976; Thorpe, 1977), rocks quite d i s t i n c t from the potassic suites of the Colima Graben. Farther south, i n the Chiapanecan arc (Damon and Montesinos, 1978), c a l c - a l k a l i n e volcanism of Quaternary age i s present but r e s t r i c t e d to E l Chichon (Figure 1.1) and a small centre further south; volcanic a c t i v i t y does not become extensive u n t i l the Mexico-Guatemala border. 1.3 TRENCH AND CONTINENTAL MARGIN Several d e t a i l e d studies have been made along the northern part of the Middle America Trench (Fisher, 1961; Shor and Fisher, 1961; Ross and Shor, 1965; Ross, 1971; Karig et a l . , 1978). The continental margin can be subdivided into two morphologic provinces that are separated by a sharp i n f l e c t i o n i n the trench axis where the Tehuantepec Ridge i n t e r s e c t s the continental margin. Northwest of t h i s junction, the continental shelf i s quite narrow, and the trench i s U-shaped i n cross section, reaching depths of about 5 km below sea l e v e l . The trench extends northward as f a r as Isla s Tres Marias, where i t ends abruptly against a southeast-trending f a u l t scarp that l i k e l y separates oceanic crust i n the south from a thinned continental crust to the north (Shor and Fisher, 1961). Southeast of i t s junction with the Tehuantepec Ridge, the trench i s characterized by a broader continental shelf, a V-shaped p r o f i l e , and water depths i n excess of 6 km. Factors that influence these changes include the contrasting age of ocean crust across the Tehuantepec Ridge (Truchan and Larson, 1973), the change i n dip of the subducted plate across t h i s lineament (discussed 9 below), and the absolute motions of the Caribbean and NOAM plates r e l a t i v e to Cocos convergence. The truncated nature of the Mexican continental margin indicates that a s l i v e r of continental lithosphere has been removed (de Cserna, 1961, 1965, 1976), but the timing, mechanism, d i r e c t i o n of transport, and nature of t h i s missing fragment have not yet been resolved (Malfait and Dinkelman, 1972; Kesler, 1973; Karig et a l . , 1978). From the l a t e Miocene to Holo-cene age of the trench f i l l and from the morphology of the trench slope, Karig et a l . (1978) concluded that accretion at the trench probably began i n the Miocene and postdated t r a n s l a t i o n of marginal terranes. The con-temporaneity of volcanism within the TMVB suggests that, c e r t a i n l y by Quaternary time, subduction played a dominant role along the Mexican coast. 1.4 SEISMICITY Seismic a c t i v i t y along the Middle America arc i s intense (Kelleher et a l . , 1973) and exhibits many of the c h a r a c t e r i s t i c s associated with sub-duction processes. Previous investigations have studied the geometry of the plate boundaries and the sense of motion at earthquake hypocenters, and they have examined r e l a t i o n s h i p s between these features and continental margin volcanism (Sykes, 1967; Molnar and Sykes, 1969; Isacks and Molnar, 1969; Stoiber and Carr, 1973; Carr, 1976; Dean and Drake, 1978). The same data employed by these e a r l i e r workers were obtained f o r t h i s study; the Earthquake Data F i l e of the United States Geological Survey was acces-sed f o r the period 1963 to 1974. The locations of epicenters of magnitude (mb) >4 are plotted i n Figures 1.2 and 1.3. Earthquakes with hypocenters 0 to 33 km i n depth are concentrated Figure 1.2: Plot of shallow (0 to 33 km) earthquakes of magni-tude (mb)>4 that occurred during the period 1963 to 1974. Arrows at continental margin indicate s l i p vectors of focal-mechanism solutions f o r shallow (0 to 76 km) underthrusting events taken from Dean and Drake (1978) and Molnar and Sykes (1969). Arrows on oceanic crust approximate Cocos-NOAM and Rivera-NOAM motion. Magnetic l i n e a t i o n s are given i n Ma. The trench contour i s 2,200 fathoms (4 km). C = Cocos plate; R = Rivera plate, P = P a c i f i c plate; EPR = East P a c i f i c Rise; TFZ = Tamayo Fracture Zone; RFZ = Rivera Fracture Zone; OFZ = Orozco Fracture Zone; TR = Tehuantepec Ridge: IS = Isthmus f a u l t ; M = Motagua f a u l t system; CP = Cuilco-Chixoy-Polochic f a u l t system; CT = Cayman Trough. A l l other symbols are the same as Figure 1.1, except the smaller cones which are represented by f i l l e d t r i a n g l e s . 12 along the East P a c i f i c Rise and associated transform f a u l t s , and along the inner trench slope. Strong seism i c i t y related to r i g h t - l a t e r a l d i s p l a c e -ment along the Rivera fracture zone (Molnar, 1973) extends beyond the i n t e r r i d g e segment, eastward, to i n t e r s e c t the trench at approximately 104° 30' W, suggesting the presence of a plate boundary. Seismic a c t i v i t y between t h i s point and longitude 101° W i s as intense as that associated with subduction of the Rivera p l a t e . Focal-mechanism solutions f o r shallow-focus earthquakes (<76 km) at the continental margin ind i c a t e a northeasterly d i r e c t i o n of underthrusting of oceanic lithosphere (Molnar and Sykes, 1969; Dean and Drake, 1978). The azimuths of s l i p vectors for the Mexican arc vary from N41° E to N34° E, using the Cocos-NOAM pole p o s i t i o n of Minster et a l . (1974). An i n t e -r e s t i n g r e l a t i o n s h i p noted by Dean and Drake concerns the attitudes of f a u l t planes along the arc. Northwest of the Gulf of Tehuantepec, the plunge of s l i p vectors averages 15°, while that for vectors to the south i s approximately 21°. This d i s c o n t i n u i t y occurs across the landward extension of the Tehuantepec Ridge and coincides with d i s t i n c t morphological changes noted i n the trench. The d i s t r i b u t i o n of earthquake f o c i at intermediate depths (>100 km) i s shown i n Figure 1.3. Contours representing the depth to the top of the Benioff Zone are drawn at 50 km i n t e r v a l s , and the termination of the i n c l i n e d seismic zone i s i n d i c a t e d . The l o c a t i o n of the contours was co n t r o l l e d by three cross-sections (not shown) oriented perpendicular to the axis of the trench and constructed through Pico de Orizaba i n the east, Nevado de Toluca, and the region between P a r i c u t f n and Volcan Colima i n the west (Figure 1.1). A l l projections were corrected for earth curvature, and events f a r t h e r than approximately 120 km from the l i n e of section were Figure 1.3: Map of intermediate earthquakes of magnitude (mb)>4 fo r the period 1963 to 1974. Diamonds = 100- to 150-km f o c a l depths; squares = 150- to 200-km f o c a l depths; and open c i r c l e s = f o c i >200 km i n depth. Contours of 50 km and 100 km mark the depth to the top of the i n c l i n e d seismic zone, and the dot-dash l i n e delineates the geographic l i m i t of seismici t y within the slab. The trench axis i s black below 2,200 fathoms (4 km). A l l other symbols are the same as Figures 1.1 and 1.2. 1 5 excluded. A d d i t i o n a l control was provided by s i x c l o s e l y spaced seismic sections of the Mexican arc produced by Molnar and Sykes (1969), using relocated hypocenters from the same data base. A number of features i n Figure 1.3 are notable: 1. The i n c l i n e d seismic zone extends to a depth of less than 150 km and i s extremely short (<250 km) i n comparison to arc systems of the eas-tern P a c i f i c and South America that involve older oceanic crust (Isacks et a l . , 1968). 2. The majority of the volcanoes making up the TMVB are located more than 50 km beyond the terminus of the i n c l i n e d seismic zone. 3. Subduction of the Rivera plate i s not associated with Benioff Zone a c t i v i t y . 4. A s i g n i f i c a n t gap exists i n the d i s t r i b u t i o n of intermediate-depth earthquakes between longitudes 99° 30' W and 96° W. The dip of the Benioff Zone decreases eastward along the arc, from about 30° i n the v i c i n i t y of Volcan Colima (situated 100 km above the seismic plane) to perhaps as l i t t l e as 20° beneath Toluca and near San AndreTuxtla. At E l Chichon, the Benioff Zone subtends an angle of about 30°, and t h i s angle increases southeastward to about 40°. below Central American volcanoes (Stoiber and Carr, 1973; Carr, 1976). This d i s c o n t i n u i t y i n the dip of the seismic zone across the Tehuantepec Ridge i s i n the same sense as that observed for the plunge of s l i p vectors i n underthrust solutions (Dean and Drake, 1978) and suggests that a rupture exi s t s i n the downgoing plate at t h i s l o c a l i t y . A second major d i s c o n t i n u i t y i s apparent i n the western part of the arc. Here, contours of depth to Benioff Zone terminate abruptly at the Colima Graben, which marks a 100 km o f f s e t i n the alignment of major 16 volcanoes. This o f f s e t o v e r l i e s the boundary between Rivera and Cocos subduction (discussed below). The "thickness" of the seismic zone beneath the Mexican arc i s about 50 km, but this measurement i s considered to be more i n d i c a t i v e of the uncertainties i n hypocentral l o c a t i o n than a measure of true thickness of the Benioff Zone (note the scatter of epicenters toward the 50-km contour at longitude 101° W i n Figure 1.3). The more precise (relocated) hyp-ocenters of Carr (1976) imply a thickness of about 15 km f o r the Benioff Zone beneath Guatemala, and focal-mechanism solutions for these l a t t e r events generally reveal down-dip extension i n the subducted slab (Isacks and Molnar, 1969; Dean and Drake, 1978). Unfortunately, no such data e x i s t f or the Mexican seismic zone. The most curious features of Figure 1.3 r e l a t e to the d i s t r i b u t i o n and frequency of the intermediate-depth earthquakes along the arc. F i r s t , s e i s m i c i t y i s absent between approximately 99° 30' W and 96° W, except for f i v e earthquakes centered at 97° 30' W which scatter between 100 and 140 km i n depth. These l a t t e r events f a l l w e l l below the main body of seismic a c t i v i t y and may be related to s t r a i n s developed near the lower boundary of the downgoing slab. The top of the Benioff Zone i n t h i s region i s situated at about 50 km, and shallow s e i s m i c i t y at the inner w a l l of the trench i s the highest i n the arc. Secondly, the most intense s e i s m i c i t y at i n t e r -mediate depths i s located just east of the Rivera Fracture Zone and i s associated with subduction of extremely young oceanic lithosphere. This s i t u a t i o n appears paradoxical i n view of the t h e o r e t i c a l l y predicted r e -l a t i o n s h i p s between the age of oceanic lithosphere and i t s seismic signat-ure during subduction (Griggs, 1972; McKenzie, 1969). However, the h i s -tory of the ocean f l o o r i n t h i s region provides a r a t i o n a l explanation f o r 17 the d i s t r i b u t i o n of seismic i t y along the arc. 1.5 EVOLUTION OF OCEAN LITHOSPHERE AND ITS BEARING ON THE MODERN ARC The structure and i n t e r p r e t a t i o n of the ocean crust off the Mexican coast i s somewhat co n t r o v e r s i a l (Atwater, 1970; Larson, 1972; Truchan and Larson, 1973; Molnar, 1973; Lynn and Lewis, 1976; Menard, 1978). However, there does appear to be general agreement that the Cocos plate represents a remnant of the larger F a r a l l o n plate (Atwater, 1970) which was undergoing subduction p r i o r to 55 Ma. After that time, Menard (1978) en-visaged that fragmentation of the F a r a l l o n plate produced two small, t r i a n -gular plates subjected to a regime of pivoting subduction. During the pivo t i n g process, the migrating t r i p l e junction of the southern (Guadelupe) plate formed the pole of r o t a t i o n f o r ridge segments and transforms to the south. The Cocos plate began to form about 12 to 17 Ma as a r e s u l t of subduction of the northern portion of the Guadelupe pl a t e . Pivoting con-tinued, but over the l a s t several m i l l i o n years, the Cocos-Pacific pole of r e l a t i v e motion migrated from the t r i p l e point to i t s present l o c a t i o n at l a t i t u d e 41° N longitude 108° W (Minster et a l . , 1974). In an e a r l i e r model, Lynn and Lewis (1976) concluded that the curved trace of major transforms and marked fanning of magnetic anomalies over the past 10 Ma (Figure 1.4) could be accomodated by a clockwise r o t a t i o n of the ridge coupled with an encroaching Cocos-Pacific pole. Discrimination between these alternate hypotheses requires a more accurate knowledge of the trends of transforms and magnetic anomalies west of the East P a c i f i c Rise and precise estimates of the l o n g i t u d i n a l v a r i a t i o n of spreading rates. Both propositions embody the same basic arrangement of magnetic l i n e a t i o n s and transforms east of the active ridge and recognize that ocean-floor evolu-18 t i o n has been complicated by ridge jumps between the Rivera and Siqueiros Fracture Zones. A t e c t o n i c reconstruction of the age of ocean lithosphere currently involved i n subduction at the Middle America Trench i s shown i n Figure 1.4. The trends of ridge crests and magnetic l i n e a t i o n s are based on data pre-sented by Larson (11972) for the Rivera plate and on a generalized diagram of the northern part of the Cocos plate taken from Lynn and Lewis (1976, Figure 3B). F o s s i l transforms i n the eastern part of the Cocos plate trend northeasterly and represent the extensions of s e i s m i c a l l y active east-west transforms located between ridge segments I and IV of the East P a c i f i c Rise. For example, the Tehuantepec Ridge forms the eastern prolongation of an unnamed fracture zone at 10.5° N and separates older crust of the Guate-mala Basin from young crust to the north. An extension of the Siqueiros Fracture Zone may form the southern boundary of the Guatemala Basin. In areas affected by ridge jumps, the apparent age of subducted lithosphere i s deceptive. Ridge segments I and III both contain f o s s i l ridge crests embedded i n the P a c i f i c plate about 600 km west of the pre-sently active spreading centers. The Clipperton Ridge jumped eastward about 8 Ma and resumed spreading i n crust already 4.5 Ma old (Anderson and Davis, 1973). P r i o r to t h i s event, the age o f f s e t across the fracture zone between ridge segments II and I I I was approximately 8 Ma. According to Lynn and Lewis (1976, Figure 3B), the age o f f s e t across the Tehuantepec Ridge between the same ridge segments i s about 10 Ma. The l a t t e r value apparently r e l i e s on the accuracy of magnetic l i n e a t i o n s i n the Guatemala Basin, because the p o s i t i o n and trend of the 12 Ma age contour agree c l o s e l y with Anomaly 5A (about 12 Ma according to the revised magnetic-polarity time scale of LaBrecque et a l . , 1977), determined by Karig et a l . (1978) to l i e Figure 1.4: Si m p l i f i e d reconstruction of the age and structure of ocean lithosphere presently involved i n subduction at the Middle America Trench. Trend and age of magnetic li n e a t i o n s i n the ocean crust are taken from Larson (1972), Lynn and Lewis (1976), and Karig and others (1978). Transforms and magnetic l i n e a t i o n s beneath the Mexican continent were projected to allow for v a r i a t i o n s i n Benioff Zone dips along the arc. Ridge seg-ments l a b e l l e d I through IV are referred to i n the text. Trench contours are given i n fathoms. The Rivera plate i s shaded l i g h t grey. TP = Cocos-Rivera-NOAM t r i p l e junction; PB = transform boundary between the Rivera and Cocos plates predicted from Figure 1.5; PR = trend of a proto-Rivera fracture zone i f one was indeed present i n the subducted plate; M = Mathematician Ridge; CI = Clipperton Ridge; other symbols are those of F i g -ures 1.1 and 1.2. 20 O if) o c\j , -O 21 on the seaward side of the trench at 100° 30' W. The Mathematician Ridge ceased to be an active spreading center about 4 Ma and records an age o f f s e t of 8 Ma between ridge segments I and II immediately p r i o r to the ridge jump. Assuming these age o f f s e t s can be extrapolated back i n time, and assuming that spreading remained approximately constant between about 12 and 30 Ma, magnetic l i n e a t i o n s i n the subducted slab can be recon-structed. Within t h i s evolutionary framework, f o s s i l transforms and magnetic anomalies were projected onto the Mexican continent, allowing for l a t e r a l v a r i a t i o n s i n the angle of subduction i n f e r r e d from the i n c l i n a t i o n of the Benioff Zone. If the p i v o t i n g subduction model of Menard (1978) were implemented, the projected transforms would curve s l i g h t l y convex southward, and magnetic l i n e a t i o n s would remain approximately perpendicular to them, but t h i s would not s i g n i f i c a n t l y change the general topology. In the region where the Cocos and Rivera plates i n t e r a c t , the factors a f f e c t i n g t h i s tectonic reconstruction are more complex. The r e l a t i v e motion of the Rivera with respect to the P a c i f i c plate (Figure 1.5A) i s taken to be 6 cm/yr from spreading rates derived for the past 4 Ma by Larson (1972); NOAM-Pacific motion of 5.6 cm/yr i s given by Minster et a l . (1974). Both vector orientations, however, represent averages of s l i p d i r e c t i o n s given by Molnar (1973) f o r focal-mechanism solutions derived for Pacific-NOAM motion near the Tamayo Fracture Zone and for R i v e r a - P a c i f i c motion along the Rivera Fracture Zone. It was these solutions that Molnar used to substantiate Atwater's (1970) suggestion that the Rivera c o n s t i -tuted an independent plate. This produces a resultant vector f o r Rivera-NOAM convergence of approximately 2 cm/yr, trending northeasterly. When th i s vector i s combined with a vector for Cocos-NOAM motion (Figure 1.5B) Figure 1.5: Vector diagrams f o r deducing r e l a t i v e motions between the Rivera, Cocos, P a c i f i c , and NOAM plates. A: Rivera-Pacific-NOAM r e l a t i v e motions, using data from Larson (1972), Molnar (1973), and Minster and others (1974). B: r e l a t i v e motion between Cocos-NOAM (Minster and others, 1974) and Rivera-NOAM (from Figure 1.5A) at the trench-trench-transform t r i p l e point (TP) shown i n Figure 1.4. E54 = focal-mechanism s o l u t i o n for s t r i k e - s l i p event 54 of Dean and Drake (1978). 24 calculated from the pole of Minster et a l . (1974) for a point located at the trench-trench-transform t r i p l e junction (TP i n Figures 1.4 and 1.5), i t i s evident that the boundary between the Cocos and Rivera plates i s es-s e n t i a l l y one of l e f t - l a t e r a l s t r i k e s l i p at at rate of approximately 4 cm/yr. Uncertainties i n Molnar's s l i p vectors could perhaps accommodate the proposition that the Rivera plate has recently been accreted to NOAM (Larson, 1972; Menard, 1978). In t h i s case, more complex arguments are required to explain the s e i s m i c i t y and contemporaneity of volcanism along the arc. Even i f such a recent accretion had taken place, i t would not s i g n i f i c a n t l y change the d i r e c t i o n and sense of r e l a t i v e motion between the Cocos and any subducted portion that remained of the Rivera plate; only the magnitude of t h i s motion would be affected. The Cocos-Rivera vector may be compared with a focal-mechanism s o l u -t i o n f o r a s t r i k e - s l i p event that occurred i n the v i c i n i t y of the t r i p l e junction at a depth of 45 km w i t h i n the downgoing slab (Figure 1.5B; Dean and Drake, 1978, event 54). The azimuthal difference between the selected fault-plane and the predicted Cocos-Rivera transform probably l i e s within the error of t h i s poor q u a l i t y (C) event; the sense of motion i s as ex-pected. Two other features constrain the l o c a t i o n of t h i s t r i p l e junction: a zone of shallow seism i c i t y extending into the trench from the Rivera Frac-ture Zone (Figure 1.2), and an observation made by Fisher (1961) concerning the i n t e r s e c t i o n of the trench axis at t h i s point with a submarine mountain range, interpreted here as representing the trace of a transform i n the ocean crust. The l o c a t i o n of a transform-ridge-transform t r i p l e junction to the southwest i s less constrained and was placed at the southern l i m i t of the zone of shallow s e i s m i c i t y i n t h i s region (Figure 1.2). Emanating from the t r i p l e point (TP) i n Figure 1.4 are the presently active Cocos-Rivera boundary (predicted from Figure 1.5) and the expected trace of an ancient or "proto-Rivera" transform i n the ocean f l o o r . The o r i e n t a t i o n of these lineaments i s quite s i m i l a r , and i f i n fac t the Rivera Fracture Zone did have an eastern extension p r i o r to subduction, l i k e other f r a c t u r e zones to the south, i t could accommodate a portion, i f not a l l , of current Cocos-Rivera motion. Using the above assumptions, four d i s t i n c t provinces can be recognized within the subducted lithosphere, each bounded by f a u l t s trending s u b p a r a l l e l to the d i r e c t i o n of plate convergence. When the age of the ocean lithosphere and the d i s t r i b u t i o n of intermediate-depth earthquakes within the downgoing slab are examined, some systematic r e l a t i o n s h i p s are revealed (Figures 1.3 and 1.4). The most intense s e i s m i c i t y at these depths occurs i n ridge segments I and III of the Cocos plate, where the subducted lithosphere i s older than approximately 20 Ma. For the same slab length measured perpendicular to the trench, the frequency of events i n segment I increases markedly toward the east and corresponds to the d i r e c -t i o n of increasing age of ocean f l o o r . The eastern l i m i t of t h i s seismic a c t i v i t y ends abruptly at a f o s s i l transform which marks the boundary between c r u s t a l provinces of d i f f e r i n g age i n the subducted slab. This c o r r e l a t i o n i s quite remarkable i n view of the completely independent manner i n which each of these diagrams was constructed. Seismic a c t i v i t y does not reappear u n t i l the eastern part of ridge segment I I , where i t seems to be con t r o l l e d by an age contour of approximately 20 Ma. The gap i n sei s m i c i t y noted e a r l i e r , therefore, corresponds to a wedge of extremely young oceanic lithosphere and implies that a thin, hot oceanic slab younger than about 20 Ma can be subducted aseismically at these rates of converg-26 ence (6 to 8.5 cm/yr). Hence, aseismic subduction of the young Rivera plate at a much slower convergence rate of 2 cm/yr (and presumably a lower s t r a i n rate) i s reasonable. If t h i s i n t e r p r e t a t i o n i s correct, then c l e a r l y the i n c l i n e d seismic zone does not necessarily place constraints on the extent or presence of a downgoing slab. The observed length of the seismic zone beneath the Mexican arc suggests that the time constant f o r thermal r e l a x a t i o n i n the descending slab i s about 4 Ma, as subduction at the continental margin has probably been continuous since the Miocene (Karig et a l . , 1978). Although s e i s m i c i t y along the arc may be related to the age and structure of the subducted slab, there i s no reason why c a l c - a l k a l i n e volcanism so f a r removed from the trench should be rela t e d to the same tectonic framework, unless i t , too, i s clo s e l y associated with the subduction process. In f a c t , some remarkable c o r r e l a t i o n s e x i s t between the structure and the nature of the TMVB and the downgoing lithosphere. At the Cocos-Rivera juncture, the TMVB i s characterized by: (1) a pronounced o f f s e t or transverse d i s c o n t i n u i t y i n arc volcanism; (2) a r e s t r i c t e d zone of concurrent a l k a l i n e and c a l c a l k a l i n e volcanic a c t i v i t y ; and (3) a region of east-west extension effected by high-angle f a u l t i n g along the Colima Graben. A l l of these features may be related to a hinge-fault mechanism operating within the downgoing slab and transmitting stresses to the base of the continental lithosphere. The geometry of the arc-trench gap suggests that the subducted portion of the Rivera plate i s i n c l i n e d less steeply than the 30° dip i n f e r r e d f o r the Cocos plate just east of Colima. The lack of a zone of intermediate-depth earthquakes along t h i s boundary promotes the l i k e l i h o o d of a hinge f a u l t which could be caused by 27 differences i n buoyancy between young and old oceanic lithosphere (Molnar and Atwater, 1978; Menard, 1978). Since the volcanic products of the TMVB are very young and the t r i p l e junction i s stable i n the sense of McKenzie and Parker (1967), movement of the Cocos-Rivera boundary with time i s unimportant. The sea f l o o r east of ridge segments I and II exhibits a pronounced difference i n intermediate-depth seismi c i t y across the prolongation of the Orozco Facture Zone, yet there i s no evidence to suggest a hinge f a u l t at t h i s l o c a t i o n . Segments I and I I , therefore, are considered as a continuous s t r u c t u r a l e n t i t y that i s being subducted beneath amphibole-bearing andesites of the ce n t r a l and eastern arc. Available chemical data ind i c a t e that a n d e s i t i c rocks i n the c e n t r a l (Toluca-Iztaccxhuatl) region are t y p i c a l l y more magnesian (higher Mg/(Mg + Fe ) than rocks of s i m i l a r s i l i c a content i n the western arc, and they are quite d i f f e r e n t from many Cascade andesites (Whitford and Bloomfield, 1976; Nixon, 1980). A l k a l i n e volcanic a c t i v i t y at San Andres Tuxtla i s enigmatic, with no obvious r e l a t i o n s h i p to subduction of the Tehuantepec Ridge and i t s l i k e l y prolongation i n the slab as a tear f a u l t . However, compositionally s i m i l a r volcanism i s found far t h e r north along the Gulf Coast (Robin and Tournon, 1978). Thorpe (1977) suggested that t h i s a l k a l i n e volcanism was re l a t e d to f r a c t u r i n g around the margins of the Gulf of Mexico, and a possible r e -l a t i o n s h i p does ex i s t between the Tuxtla region and the Isthmus f a u l t (Figure 1.3). This f a u l t zone i s s e i s m i c a l l y active and may be associated with extensional tectonism bordering the Cocos-NOAM-Caribbean t r i p l e junc-t i o n . Muehlberger and R i t c h i e (197 5) selected the Polochic-Chixoy-Cuilco f a u l t system as the present NOAM-Caribbean plate boundary, noted i t s b i -28 f u r c a t i o n into a region of complex high-angle f a u l t i n g extending into the Isthmus of Tehuantepec, and chose a f a u l t trending NE-SW at the Guatemala-Mexico f r o n t i e r as the continuation of this boundary toward the trench. The Guatemala earthquake of February 4, 1976, however, rather dramatically demonstrated that the Motagua f a u l t system i s also active and that the juncture between Caribbean and NOAM plates l i k e l y encompasses a l l of these f a u l t zones. In the past, t h i s plate boundary has been represented by a series of en echelon, curving fault-zones extending from the northern terminus of the Guatemalan volcanoes to Honduras (Muehlberger and R i t c h i e , 1975; Plafker, 1976; Ma l f a i t and Dinkelman, 1972). These e a r l i e r workers have suggested that the western part of the Caribbean plate i s being pinned by Cocos subduction and i s undergoing extension as the main mass of the Caribbean plate moves eastward. It i s apparent i n Figure 1.4 that t h i s region, with i t s l o c a l i z e d volcanic a c t i v i t y , coincides with the subduction of a segment of older oceanic lithosphere. The d i f f u s e t r i p l e junction i s bounded i n the north by the Isthmus f a u l t and i n the south by the NE-SW -trending lineament pointed out by Muehlberger and Rit c h i e (1975). The seaward extension of t h i s l a t t e r structure i s marked by a l i n e of earthquakes extending into the trench (Kelleher et a l . , 1973) and may coincide with a f o s s i l transform at the southern margin of the Guatemala Basin (Figures 1.3 and 1.4). 1.6 SEGMENTATION AT SUBDUCTION ZONES: DISCUSSION A detailed analysis of the Mexican arc suggests that l a t e Quaternary volcanism of the Trans-Mexican Volcanic Belt i s rela t e d to subduction at the Middle America Trench and confirms the segmented nature of t h i s continental margin. The subducted slab i s broken i n t o three separate 29 segments bounded by hinge f a u l t s which are related to s t r u c t u r a l lineaments formed i n the ocean f l o o r . L a t e r a l v a r i a t i o n s within the TMVB r e l a t e to these segments as follows: (1) a western arc associated with aseismic subduction of the Rivera plate; (2) a ce n t r a l and eastern arc re l a t e d to subduction of a shallowly dipping segment of the Cocos plate, extending from the Rivera Fracture Zone to the Tehuantepec Ridge; and (3) a t r a n s i t i o n zone, the Colima Graben, where a l k a l i n e volcanism o v e r l i e s a hinge f a u l t at the Cocos-Rivera boundary. Weakly-developed c a l c - a l k a l i n e volcanism extending from the Isthmus of Tehuantepec to the Guatemala-Mexico border i s rela t e d to the subduction of an older segment of ocean l i t h s p h e r e . This region represents a d i f f u s e t r i p l e junction between the NOAM, Caribbean, and Cocos plates. A l k a l i n e volcanism at San Andres Tuxtla may be related to extensional tectonism i n the v i c i n i t y of the Isthmus f a u l t , which marks the northern l i m i t of t h i s t r i p l e junction. The tectonic elements of the Mexican arc described above bear l i t t l e r e l a t i o n s h i p to those proposed previously (Stoiber and Carr, 1973; Carr et a l . , 1974). I m p l i c i t to e a r l i e r segmentation models of convergent plate margins i s the concept that tear f a u l t s propogated at the trench divide the downgoing slab i n t o segments t y p i c a l l y 100 to 300 km i n width. Each seg-ment i s capable of moving independently i n response to subduction; con-sequently, deep f a u l t s may develop i n the overlying lithosphere p a r a l l e l to subducted segment boundaries. In the TMVB, elongate c l u s t e r s of cinder cones oriented northeast-southwest were regarded as the s u r f i c i a l expression of segment boundaries i n the subducted slab because of t h e i r coincidence with the d i r e c t i o n of plate convergence. In f a c t , Holocene lineaments of s i m i l a r magnitude but 30 d i f f e r e n t o r i e n t a t i o n are found throughout the TMVB. For example, the prevalent s t r u c t u r a l d i r e c t i o n i n the western arc i s northwest-southeast, whereas vents i n the Valley of Mexico are commonly aligned east-west, and i n Michoacan, cinder cones are randomly oriented (Demant, 1978; Nixon et a l . , i n press). When the north-south lineaments of l a t e Pleistocene v o l -canoes i n the Colima Graben and eastern arc are included, the structure along the arc i s seen to be quite complex, and, i n part, r e f l e c t s the l o c a l i z e d trends of "basement" fracture zones. If the northeasterly a l i g n -ment of cinder cones i s connected with deepseated fault-zones formed i n the manner suggested by Stoiber and Carr (1973), then at least a few of these might be expected to be seismically active and extend beyond the "volcanic f r o n t " towards the trench. Other aspects of the segmentation concept, as o r i g i n a l l y proposed by Stoiber and Carr (1973), do appear to have a p p l i -cation i n Mexico. For example, the subduction of an active transform at the Cocos-Rivera junction i s r e f l e c t e d i n the structure of the continental lithosphere and composition of Quaternary volcanism i n this region, even though the slab i s about 100 km deep. Several conclusions of th i s study pertain to segmentation models i n general: 1. Struct u a l boundaries such as ancient transforms, aseismic ridges, and possibly f o s s i l ridge crests i n the descending slab are p o t e n t i a l zones of weakness and may determine segment boundaries i n the subducted lithosphere, at least to a f i r s t order. Such features i n the Nazca and P a c i f i c plates currently trend s u b p a r a l l e l to convergence d i r e c t i o n s and may control segmentation i n South American and western P a c i f i c arcs. 2. Deeper seismic i t y i s dependent on the age of subducted lithosphere, both along the arc and perpendicular to i t . L a t e r a l v a r i a t i o n s i n Benioff-Zone a c t i v i t y may be most apparent i n regions where young ocean lithosphere i s being consumed. 3. The complete record of volcanism i n the Trans-Mexican Volcanic Belt over the past m i l l i o n years can be related to the present p l a t e - t e c t o n i c configuration. Unlike many other P a c i f i c arcs, Quaternary volcanoes i n Mexico o v e r l i e the aseismic extension of a f a i r l y young subducted slab. Compositional v a r i a t i o n of volcanic products along the arc i s extensive, although a l l of these rocks may be related to the subduction process (sensu l a t o ) . F i n a l l y , i t should be recognized that mature convergent margins have a complex tectonic h i s t o r y and that regional alignments of volcanoes and fault-zones may r e f l e c t t h i s s t r u c t u r a l heritage rather than t e c t o n i c elements of the downgoing plate. 32 CHAPTER 2 THE GEOLOGY OF IZTACCIHUATL VOLCANO AND THE NORTHERN SIERRA NEVADA, CENTRAL MEXICO 2.1 INTRODUCTION I z t a c c i h u a t l i s a major Quaternary volcano within the Trans-Mexican Volcanic Belt, a c a l c - a l k a l i n e province that traverses the continent from the P a c i f i c to the Gulf of Mexico. A summary of the geology and structure of the Trans-Mexican Volcanic Belt i s given by Demant (1978, 1981) and Verma (1985), and i t s r e l a t i o n s h i p to subduction of the northern Cocos and Rivera plates was reviewed i n Chapter 1. I z t a c c i h u a t l i s situated approximately 60 km southeast of Mexico C i t y at l a t i t u d e 19° 10.7' N longitude 98° 38.5' W. It occupies the ce n t r a l part of the S i e r r a Nevada, a mountain range over 40 km i n length that forms part of the eastern margin of the Valley of Mexico. The southern extremity of the S i e r r a Nevada i s formed by the nearly symmetrical cone of Volcan Popocatepetl (5452 m) located j u s t 15 km south of I z t a c c i h u a t l . The nor-thern l i m i t of the S i e r r a Nevada occurs at a low mountain pass occupied by Rio F r i o and the main highway connecting Mexico C i t y with Puebla. The northward prolongation of the mountain range i s known as the S i e r r a Rio F r i o which continues at reduced elevation towards the c i t y of Pachuca. The unique form of I z t a c c i h u a t l has been constructed by lava and sculpted by g l a c i a l i c e into a p r o f i l e that resembles a r e c l i n i n g maiden, es p e c i a l l y perceptible when snow-covered and accounting f o r the Aztec name Plate 2.1: A i r photomosaic of the summit region of I z t a c c i h u a t l volcano showing the main peaks of Cabeza (5146 m), Pecho (5286 m), Rodillas (5100 m), and Pies (4703 m). Also shown are the p a r a s i t i c lava f i e l d s of Teyotl and La Joya on the northern and southern flanks r e s p e c t i v e l y , and the plug domes of E l S o l i t a r i o and Los Yautepemes on the northwestern slopes. 35 W^oman i n White'. (The Spanish designation ^La Mujer Gorda' i s f a r less appealing!). The anatomy, from head to toe, comprises four main peak regions: Cabeza (5146 m), the ice-capped summit of Pecho (5286 m), R o d i l -las (5100 m), and the lower peak of Pies (4703 m) l i s t e d from north to south respectively (Plate 1.1). Volcanic a c t i v i t y at I z t a c c i h u a t l ceased before the l a s t major g l a c i a t i o n whereas her neighbour, Volcan Popocatepetl, continued to erupt into h i s t o r i c time with most recent a c t i -v i t y occurring i n the years 1920-1927 (Mooser and others, 1958). The geologic map accompanying t h i s report covers an area of ap-proximately 900 km2 that extends from Paso de Cortes, the divide between the lavas of I z t a c c i h u a t l and those of Popocatepetl, to Rio F r i o . The topographic base was prepared from map sheets E14B31 (Chalco) and E14B41 (Amecameca), at a scale of 1:50,000 and preliminary maps E14B32 and E14B42 at the same scale obtained from DETENAL, Mexico D.F.. Roads on the eastern flanks of the S i e r r a Nevada were transferred to the base from a e r i a l photo-graphs. 2.2 ANALYTICAL TECHNIQUES Chemical analyses of volcanic rocks i n the map area were performed by X-ray fluorescence techniques using a P h i l i p s PW1410 spectrometer. Major elements were determined on fused glass discs according to the procedures of Norrish and Hutton (196 9) and c a l i b r a t e d against i n t e r n a t i o n a l standard-s(Abbey, 1977) using methods established by Harvey and others (1973). Na 20 was analyzed using pressed powder p e l l e t s and l e f t uncorrected f o r matrix e f f e c t s . H 20 and H 20 + were measured gra v i m e t r i c a l l y by successively heating a one gram sample i n a furnace at 120°C and 1000°C, r e s p e c t i v e l y . Trace element analyses were performed on powder briquettes and corrected 36 for instrument d r i f t , background, s p e c t r a l interference, and matrix e f -f e c t s . Estimates of accuracy and p r e c i s i o n are given i n Appendix A and further d e t a i l s of a n a l y t i c a l methods and operating conditions are given by Armstrong and Nixon (1980). K-Ar age determinations were obtained on whole-rock material and plagioclase separates. Potassium analyses were made i n duplicate by atomic absorption on a Techtron AA4 spectrophotometer. Argon analyses were c a r r -ied out on a AEI MS10 mass spectrometer, operated s t a t i c a l l y , following standard isotope d i l u t i o n techniques using a high p u r i t y A r 3 8 spike. Further d e t a i l s of a n a l y t i c a l procedures and equipment are provided by White and others (1967). P r i o r to fusion, a l l whole-rock samples were baked at 130°C f o r approximately 15 hours to reduce atmospheric argon contamination. Plagioclase separates were etched i n 5% HF f o r 20 minutes to remove adhering volcanic groundmass, rinsed i n d i s t i l l e d water, and dried p r i o r to argon measurement. Constants used i n the age equations are those recommended by Steiger and Jager (1977). 2.3 PREVIOUS WORK AND REGIONAL GEOLOGY Among the early attempts to e s t a b l i s h a formal chronology f o r volcanic rocks of the Valley of Mexico was that of Mooser (1957). He subdivided the S i e r r a Nevada into two main andesite s e r i e s : " l a s e r i e andesitica de l a S i e r r a Nevada', superposed by " l a s e r i e a n d e s i t i c a del I z t a c c i h u a t l ' , both of which he believed were l a r g e l y Pliocene i n age. Quaternary to Recent volcanic rocks, represented by numerous s c o r i a and block cones that modif-ied former drainage patterns i n the Valley of Mexico, he assigned to the Chichinautzin Group or " l a s e r i e b a s a l t i c a Chichinautzin'. In a l a t e r p u b l i c a t i o n , Mooser and others (1974) revised these ages, recognizing a 37 ^Lower S i e r r a Group' or ^Basic Complex of the S i e r r a Nevada' which formed during the Miocene, and an ^Upper S i e r r a Group', to which they assigned a Miocene-Pliocene age (14-4Ma), which i s surmounted by the P l i o - P l e i s t o c e n e to Recent volcanic cones of I z t a c c i h u a t l and Popocatepetl. Other i n -vestigators, f o r example Schlaepfer (1968) and Negendank (1972), have subscribed to t h i s general chronology. The Miocene-Pliocene ages of older andesites i n the S i e r r a Nevada were supported by K-Ar geochronometry on samples c o l l e c t e d from I z t a c c i h u a t l (reported i n Steele, 1971). However, new K-Ar determinations performed as part of t h i s study i n d i c a t e that these e a r l i e r dates are spurious and provide evidence f o r Pleistocene to Recent evolution of the S i e r r a Nevada. The basement upon which volcanoes of the S i e r r a Nevada are b u i l t i s not exposed within the Valley of Mexico which i s underlain by Chichinautzin Group volcanic rocks and interbedded sedimentary d e t r i t u s . According to Mooser and others (1974) outcrops of Oligocene-Miocene a n d e s i t i c rocks, the Xochitepec Group of Mooser (1957), are found i n the S i e r r a de Las Cruces which forms part of the western margin of the Valley of Mexico. They also occur further south near Cuernavaca where they are correlated with l i t h o l o -g i c a l l y s i m i l a r rocks of the Tepoztlan and Zempoala Formations described by Fr i e s (1960). Marine limestones, limestone breccias, and pelagic sedimentary rocks of Cretaceous age are also exposed i n the Cuernavaca region, and occur i n the Puebla Basin to the east, and form i n l i e r s near Tula 50 km northwest of Mexico C i t y . These rocks may be the source of rare metapelite xenoliths recovered from the lavas of I z t a c c i h u a t l and La Malinche, situated 50 km east of I z t a c c i h u a t l , as w e l l as fragments of epidotized sandstone found w i t h i n Recent ejecta of Popocatepetl. A borehole east of Mexico C i t y , Pozo 38 Texcoco, i n t e r s e c t s Late Quaternary lake f i l l and T e r t i a r y volcanic rocks before penetrating Cretaceous sediment at 2060 m (Mooser and others, 1974). In cross-sections accompanying her geologic map, Schlaepfer (1968) likewise in t e r p r e t s the basement of the S i e r r a Nevada as comprising rocks of Creta-ceous and T e r t i a r y age. Several investigations have been conducted on the g l a c i a l sequences, tephra deposits, and palaeomagnetic stratigraphy of the S i e r r a Nevada. Steele (1971) conducted a flow by flow i n v e s t i g a t i o n of I z t a c c i h u a t l lavas occurring above t r e e - l i n e (4000 m). None of the flows he examined exhib-i t e d reversed magnetic p o l a r i t y which led him to conclude that the summit region of I z t a c c i h u a t l was constructed during the Bruhnes normal p o l a r i t y i n t e r v a l . He suggested that Pies lavas, because of t h e i r severely eroded nature, may belong to an e a r l i e r period of normal p o l a r i t y . Recently, Steele (1985) revised his i n t e r p r e t a t i o n i n accordance with the new K-Ar dates contained i n t h i s report. White (1956, 1962) established a Late Pleistocene to Recent glaciochro-nology f o r the west side of I z t a c c i h u a t l which has been extended i n t h i s study to the northern and eastern flanks of the volcano and correlated with ^C-dated moraines on La Malinche (Heine, 1973). Aprons of e p i c l a s t i c d e t r i t u s and "toba' sediments (a term used to describe yellow-brown l o e s s -l i k e deposits commonly e x h i b i t i n g coarse interlayered c l a s t i c d e t r i t u s ) that flank the S i e r r a Nevada have been examined by Heine and Schonhals (1973), and Recent tephra from Popocatepetl have been dated by radiocarbon techniques (Heine and Heide-Weise, 1973). The chemical composition of volcanic rocks i n the S i e r r a Nevada and Valley of Mexico has been examined p r i n c i p a l l y by Schlaepfer (1968), Gunn and Mooser (1971), and Negendank (1972, 1973). Schlaepfer (1968) included 39 several wet chemical analyses of lavas from I z t a c c i h u a t l and Popocatepetl, and young dacite flows near Rio F r i o (known as Papayo dacite i n th i s r e -po r t ) . She also provided a compilation of analyses made at the turn of the century, additions to which may be found i n Mooser and others (1958). In a study of the geochemistry of volcanic rocks i n ce n t r a l Mexico, Gunn and Mooser (1971) provided a large number'of X-ray fluorescence analyses f o r both major and trace elements. They included samples c o l l e c t e d from the S i e r r a Nevada and Valley of Mexico (Chichinautzin Group) i n addition to and e s i t i c rocks of the Si e r r a Rio F r i o and pumice taken from a p y r o c l a s t i c flow deposit at Rio F r i o . Supplementary data on Quaternary cones and flows of the Chichinautzin Group have been reported by Negendank (1972) working i n the Valley of Mexico, and by Bloomfield (1973, 1975) from the neighbouring Lerma Basin located southeast of the c i t y of Toluca. 2.4 NOMENCLATURE AND CHEMICAL CLASSIFICATION OF THE LAVAS Major and trace element analyses of volcanic rocks are l i s t e d i n Table 2.1 along with cation normative compositions calculated using a constant Fe a0 3/FeO r a t i o of 0.15 (Brooks, 1976). The most mafic lavas are o l i v i n e -hypersthene normative but the majority of rock compositions are Qz-normative. The various groups of lavas are plotted i n AFM diagrams i n Figure 2.1 where they exhibit an o v e r a l l c a l c - a l k a l i n e trend of l i t t l e to no iron-enrichment with increasing a l k a l i e s (and s i l i c a ) . M i n e r a l o g i c a l l y , t h i s trend i s equivalent to the hypersthenic rock series of Kuno (1950) since lavas lack pigeonite and carry orthopyroxene i n the groundmass. The presence of o l i v i n e i s accompanied by a reaction r e l a t i o n s h i p i n a l l but the most quickly c h i l l e d specimens. Phenocrysts of plagioclase, ortho-pyroxene, augite, hornblende, + o l i v i n e , ± b i o t i t e , ± quartz are t y p i c a l of Table 2.1: Chemical Analyses and Cation Normative Compositions  of Lavas of I z t a c c i h u a t l Volcano and the Northern S i e r r a Nevada I z t a c c i h u a t l : Older Andesites and Dacites Llano Grande Volcano Sample LG-5 LG-8 LG-4 LG-10 LG-6 Latitude 19 12 07 19 10 47 19 13 30 19 10 14 19 13 18 Longitude 98 41 40 98 32 45 98 41 54 98 44 44 98 41 45 sio 2 59.58 61.82 62.00 62.11 62.85 T i 0 2 0.91 0.85 0.63 0.71 0.80 A1 20, 16.56 16.96 15.68 16.24 16.38 ZFe 20, 6.41 5.50 5.02 5.56 5.22 MnO 0.10 0.09 0.08 0.10 0.08 MgO 5.36 3.46 4.65 4.36 3.60 CaO 4.76 4.89 5.21 5.21 5.02 Na 20 3.51 4.31 4.18 3.65 4.61 K 20 1.54 1.71 1.64 1.81 1.68 P*°+ 0.21 0.23 0.16 0.19 0.24 H 20 0.34 0.25 0.58 0.44 0.09 H 20 0.66 0.48 0.38 0.38 0.04 Sum 99.94 100.55 100.21 100.76 100.61 EFeO 5.83 4.96 4.55 5.00 4.67 Mg # 66.08 59.44 68.34 64.63 61.64 Cr 134 84 176 193 101 V 116 102 96 111 97 Ni 108 38 125 74 57 Rb 44 50 50 60 50 Sr 391 446 421 375 505 Ba 501 497 399 425 482 Zr 175 183 134 163 147 Nb 8 11 7 8 6 Y 27 22 16 25 22 Q 13.28 13.32 12.97 14.75 13.01 Or 9.21 10.12 9.72 10.72 9.85 Ab 31.87 38.97 37.72 33.11 41.33 An 22.51 21.80 19.20 22.51 18.76 Di 0.0 0.79 4.55 1.73 3.52 En 14.97 9.23 10.88 11.34 8.33 Fs 2.84 1.65 1.53 2.15 1.22 Mt 2.52 2.45 2.21 2.31 2.39 11 1.28 1.19 0.88 0.99 1.11 Ap 0.44 0.48 0.34 0.40 0.50 C 1.07 0.0 0.0 0.0 0.0 4 1 Table 2.1 (Cont'd): Llano Grande Volcano Sample LG-1 LG-2 LG-3 LG-9 LG-7 Latitude 19 14 40 19 14 38 19 15 18 19 12 35 19 16 : Longitude 98 39 45 98 40 03 98 39 58 98 44 05 98 38 S i 0 2 62.86 65.11 65.27 65.40 65.50 T i 0 2 0.72 0.65 0.59 0.61 0.62 A1 20, 16.34 15.72 16.76 16.32 16.32 ZFe 20, 5.30 4.40 3.84 4.09 4.12 MnO 0.10 0.07 0.06 0.07 0.06 MgO 3.65 2.90 1.83 2.23 2.36 CaO 4.89 4.43 3.74 4.42 4.27 Na 20 3.89 4.31 4.28 4.67 4.50 K 20 1.98 1.99 1.86 1.70 1.90 p2° + 0.21 0.19 0.18 0.21 0.20 H 20 0.10 0.16 0.83 0.00 0.28 H 20 0.40 0.26 0.73 0.12 0.26 Sum 100.44 100.19 99.97 99.84 100.39 ZFeO 4.77 3.97 3.51 3.69 3.71 Mg # 61.61 60.56 52.61 55.95 57.17 Cr 86 81 40 43 52 V 107 89 74 85 84 Ni 47 30 19 19 27 Rb 66 58 57 50 58 Sr 414 398 439 515 479 Ba 609 588 507 485 641 Zr 174 180 146 146 158 Nb 9 12 9 7 8 Y 30 20 18 19 19 Q 15.39 17.94 21.58 18.30 18.39 Or 11.73 11.79 11.20 10.07 11.24 Ab 35.18 38.90 39.15 41.98 40.64 An 21.26 17.69 17.71 18.64 18.67 Di 1.36 2.49 0.0 1.58 0.99 En 9.53 6.91 5.15 5.45 6.07 Fs 1.79 0.73 0.31 0.49 0.50 Mt 2.32 2.25 2.20 2.20 2.21 11 1.01 0.91 0.84 0.85 0.87 Ap 0.44 0.40 0.38 0.44 0.42 C 0.0 0.0 1.49 0.0 0.0 42 Table 2.1 (Cont'd): — Ancestral Pies Sample PA-6 PA-3 PA-7 PA-8 Latitude 19 07 17 19 08 42 19 07 40 19 08 : Longitude 98 39 30 98 38 33 98 39 53 98 41 : sio 2 60.45 61.07 62.38 63.65 T i 0 2 0.76 0.75 0.75 0.99 A1 20, 16.00 16.18 16.43 16.99 ZFe 20 3 5.74 5.48 5.16 5.41 MnO 0.10 0.09 0.09 0.08 MgO 5.13 4.06 3.49 2.20 CaO 5.52 5.23 5.01 4.60 Na 20 3.57 3.61 4.18 4.01 K 20 1.66 2.03 2.09 2.16 P 20 0.21 0.21 0.28 0.24 H 20 0.32 0.96 0.20 0.26 H 20 0.72 0.55 0.22 0.34 Sum 100.18 100.22 100.28 100.93 ZFeO 5.21 4.99 4.65 4.85 Mg # 67.56 63.32 61.18 48.65 Cr 207 151 93 22 V 115 109 102 103 Ni 162 91 47 13 Rb 53 58 60 72 Sr 413 394 536 372 Ba 481 423 684 665 Zr 162 162 175 212 Nb 10 7 7 12 Y 26 29 25 30 Q 12.92 14.16 13.61 17.48 Or 9.89 12.18 12.36 12.81 Ab 32.39 32.99 37.68 36.49 An 22.90 22.25 19.86 21.34 Di 2.67 2.16 2.55 0.0 En 13.14 10.47 8.55 6.10 Fs 2.22 1.92 1.40 1.07 Mt 2.36 2.36 2.35 2.61 11 1.07 1.06 1.05 1.38 Ap 0.44 0.45 0.59 0.50 C 0.0 0.0 0.0 0.22 43 Table 2.1 (Cont'd): — Ancestral Pies Sample PA-1 PA-2 PA-4 PA-5 Latitude 19 09 05 19 09 03 19 08 45 19 06 . Longitude 98 38 35 98 38 30 98 39 40 98 40 : S i 0 2 63.90 64.95 65.12 65.19 T i 0 2 0.73 0.64 0.67 0.65 A1 20 3 17.23 17.08 16.62 16.89 2Fe 20 3 3.82 4.19 4.41 4.44 MnO 0.06 0.05 0.08 0.08 MgO 2.38 1.28 2.15 2.14 CaO 3.87 3.38 4.10 4.13 Na 20 4.16 3.78 3.93 4.22 K 20 1.98 2.16 2.10 2.06 0.23 0.19 0.22 0.21 H 20 0.79 0.97 0.24 0.34 H 20 0.98 0.88 0.42 0.18 Sum 100.13 99.55 100.06 100.53 ZFeO 3.49 3.86 3.99 3.99 Mg # 59.21 41.58 53.18 52.90 Cr 80 38 42 32 V 101 73 76 66 Ni 38 23 18 18 Rb 58 70 67 63 Sr 444 398 426 428 Ba 560 579 588 575 Zr 174 189 188 186 Nb 9 9 10 10 Y 16 20 23 19 Q 19.69 24.74 21.00 19.22 Or 11.90 13.19 12.56 12.21 Ab 38.04 34.91 35.74 38.22 An 18.00 16.05 19.13 19.18 Di 0.0 0.0 0.0 0.0 En 6.68 3.65 6.01 5.93 Fs 0.0 0.64 0.83 0.91 Mt 2.02 2.27 2.28 2.25 11 1.03 0.92 0.94 0.91. Ap 0.49 0.41 0.47 0.44 C 1.94 3.23 1.05 0.74 Table 2.1 (Cont'd): Older Flank A c t i v i t y La Trampa Tlacupaso Sample LG-12 LG-13 LG-11 Latitude 19 16 17 19 15 50 19 15 t Longitude 98 41 40 98 41 35 98 37 i S i 0 2 65.53 71.38 68.16 T i 0 2 0.59 0.49 0.34 A1 20 3 16.73 14.05 15.55 ZFe 20 3 4.03 3.25 2.50 MnO 0.07 0.02 0.06 MgO 2.26 0.43 1.11 CaO 4.22 2.83 2.97 Na 20 4.31 3.10 4.72 K 20 1.73 1.73 2.23 0.21 0.18 0.12 H 20 0.38 0.85 1.22 H 20 0.25 1.43 0.58 Sum 100.31 99.74 99.56 ZFeO 3.64 3.00 2.30 Mg # 56.65 23.56 50.85 Cr 42 38 31 V 75 69 48 Ni 25 17 13 Rb 48 57 68 Sr 557 379 427 Ba 565 450 587 Zr 137 127 124 Nb 6 5 6 Y 18 15 17 Q 20.15 39.48 23.32 Or 10.27 10.73 13.47 Ab 39.02 29.16 43.14 An 19.66 13.51 14.27 Di 0.0 0.0 0.0 En 6.27 1.25 3.13 Fs 0.55 0.0 0.0 Mt 2.19 1.82 0.90 11 0.83 0.72 0.48 Ap 0.44 0.40 0.26 C 0.63 2.73 0.33 Table 2.1 (Cont'd): I z t a c c i h u a t l : Younger Andesites and Dacites Pies Sample P-15 P-16 P-14 P - l l P-12 Latitude 19 09 40 19 09 10 19 08 15 19 09 31 19 09 ( Longitude 98 36 32 98 35 46 98 38 54 98 37 50 98 38 . S i 0 2 58.82 63.19 63.27 63.28 64.34 T i 0 2 0.80 0.81 0.79 0.74 0.70 A1 20, 16.15 16.95 16.95 15.99 15.61 ZFe 20 3 5.99 5.08 5.21 4.92 4.36 MnO 0.10 0.09 0.09 0.08 0.07 MgO 5.50 2.68 2.78 3.17 2.90 CaO 6.00 4.76 4.58 4.41 4.05 Na 20 4.02 4.04 3.72 4.23 3.86 K 20 1.43 2.03 2.14 2.03 2.17 P 20 0.16 0.17 0.26 0.15 0.17 H 20 0.00 0.07 0.11 0.26 1.46 H 20 0.16 0.14 0.31 0.19 0.59 Sum 99.13 100.01 100.21 99.45 100.28 ZFeO 5.45 4.58 4.70 4.47 3.99 Mg # 68.15 55.14 55.42 60.02 60.78 Cr 188 42 55 74 83 V 120 98 107 107 98 Ni 104 17 23 40 30 Rb 42 62 64 60 72 Sr 393 440 486 394 380 Ba 366 529 582 523 512 Zr 145 178 189 172 166 Nb 9 8 9 11 9 Y 22 27 27 22 20 Q 8.97 16.40 18.08 15.98 19.56 Or 8.50 12.06 12.75 12.13 13.10 Ab 35.98 36.50 33.76 38.19 35.51 An 22.09 22.25 21.21 18.97 19.22 Di 5.44 0.31 0.0 1.73 0.14 En 12.95 7.31 7.74 8.10 8.12 Fs 2.22 1.25 1.53 1.20 0.67 Mt 2.39 2.42 2.40 2.35 2.32 11 1.12 1.14 1.11 1.04 1.00 Ap 0.34 0.36 0.55 0.32 0.36 C 0.0 0.0 0.88 0.0 0.0 46 Table 2.1 (Cont'd): Pies R o d i l l a s — Sample P-10 IZ-134 R-9 R-12 R-6 Latitude 19 09 39 19 06 20 19 09 27 19 09 00 19 09 -Longitude 98 37 52 98 35 58 98 38 56 98 40 01 98 38 : S i 0 2 65.69 63.90 57.86 59.89 62.09 T i 0 2 0.67 0.64 0.93 0.82 0.73 A1 20, 16.15 16.05 15.41 15.53 15.89 ZFe 20 3 4.37 4.22 6.56 5.84 5.40 MnO 0.07 0.08 0.11 0.10 0.09 MgO 2.52 2.58 6.55 5.54 4.46 CaO 4.28 4.28 6.44 6.02 5.46 Na 20 4.47 3.84 4.09 4.13 4.34 K 20 1.95 2.25 1.66 1.77 1.92 0.16 0.16 0.30 0.29 0.23 H 20 0.15 0.75 0.11 0.00 0.04 H 20 0.10 0.29 0.18 0.14 0.21 Sum 100.58 99.07 100.20 100.07 100.86 ZFeO 3.92 3.87 5.91 5.26 4.83 Mg # 57.33 58.75 69.94 68.85 65.80 Cr 47 54 224 193 150 V 88 82 125 115 111 Ni 20 22 158 128 61 Rb 55 69 44 48 52 Sr 445 395 645 657 464 Ba 528 585 542 557 533 Zr 168 162 144 145 148 Nb 10 12 10 10 6 Y 18 22 24 23 21 Q 18.15 19.24 5.34 8.78 11.13 Or 11.49 13.62 9.74 10.41 11.22 Ab 40.27 34.99 36.55 36.93 38.90 An 18.08 20.58 18.62 18.51 17.84 Di 1.65 0.09 8.93 7.46 5.98 En 6.19 7.26 14.12 11.98 9.60 Fs 0.64 0.71 2.28 1.80 1.53 Mt 2.26 2.25 2.51 2.40 2.31 11 0.93 0.91 1.29 1.14 1.01 Ap 0.33 0.34 0.62 0.60 0.48 C 0.0 0.0 0.0 0.0 0.0 Table 2.1 (Cont'd): R o d i l l a s — Sample R-16 IZ-112 Latitude 19 10 02 19 09 21 Longitude 98 37 42 98 38 43 S i 0 2 63.21 64.96 TiO s 0.74 0.64 A1 20 3 15.31 15.79 IFe 20, 5.16 4.19 MnO 0.09 0.08 MgO 3.95 2.56 CaO 5.10 4.23 Na 20 4.25 3.81 K 20 2.04 2.04 P 20 0.28 0.20 H20_ 0.17 1.42 H 20 0.13 0.70 Sum 100.43 100.62 ZFeO 4.64 3.83 Mg # 64.07 58.74 Cr 105 57 V 90 92 Ni 53 20 Rb 59 58 Sr 586 392 Ba 734 510 Zr 150 159 Nb 10 12 Y 20 19 Q 14.01 20.97 Or 12.02 12.30 Ab 38.24 35.13 An 16.55 20.08 Di 5.49 0.0 En 8.49 7.21 Fs 1.25 0.66 Mt 2.33 2.25 II 1.03 0.91 Ap 0.58 0.43 C 0.0 0.07 Pecho R-3 PE-12 PE-11 19 09 36 19 10 58 19 13 38 98 38 29 98 42 23 98 36 57 65.25 61.70 62.23 0.67 0.73 0.88 15.83 16.46 15.72 4.50 5.24 5.64 0.07 0.09 0.09 2.62 4.21 3.94 4.35 4.87' 4.89 4.43 3.83 4.25 2.10 1.69 2.11 0.20 0.17 0.25 0.02 0.33 0.02 0.16 0.64 0.13 100.20 99.96 100.15 4.05 4.76 5.07 57.56 65.18 61.94 58 112 106 88 92 103 31 60 42 63 36 59 423 377 429 495 491 474 158 168 170 6 7 12 21 22 24 17.55 14.82 12.85 12.42 10.09 12.46 39.90 34.73 38.21 17.08 22.97 17.55 2.57 0.25 4.06 6.10 11.63 9.13 0.76 1.79 1.50 2.27 2.33 2.48 0.93 1.03 1.23 0.42 0.36 0.52 0.0 0.0 0.0 Table 2.1 (Cont'd): Pecho Cabeza Plug Dome (Yautepemes) Sample PE-10 PE-3 PE-13 C-2 IZ-126 Latitude 19 10 30 19 10 18 19 10 11 19 11 09 19 11 : Longitude 98 37 38 98 38 55 98 42 10 98 38 56 98 39 . S i 0 2 64.38 64.60 64.63 62.83 62.41 T i 0 2 0.67 0.75 0.67 0.75 0.75 A1 20 3 15.91 15.91 15.89 16.16 15.80 ZFe 20 3 4.53 4.81 4.49 5.22 5.13 MnO 0.08 0.08 0.07 0.08 0.09 MgO 2.95 3.47 2.77 3.51 3.74 CaO 4.44 4.71 4.31 4.90 5.13 Na 20 4.35 4.21 4.37 4.31 4.32 K 20 2.18 2.08 2.00 1.90 1.96 p2 o + 0.18 0.19 0.17 0.20 0.25 H 20 0.33 0.00 0.14 0.00 0.36 H 20 0.24 0.13 0.21 0.18 0.43 Sum 100.24 100.94 99.72 99.94 100.37 ZFeO 4.09 4.29 4.07 4.70 4.63 Mg # 60.27 62.70 58.97 61.04 62.94 Cr 60 78 56 65 80 V 87 90 85 102 104 Ni 26 29 20 34 49 Rb 62 63 55 57 54 Sr 420 421 419 430 597 Ba 501 493 523 454 592 Zr 156 151 167 141 152 Nb 15 10 9 8 7 Y 21 22 21 23 22 Q 16.24 16.04 17.51 14.17 13.25 Or 12.92 12.19 11.90 11.24 11.61 Ab 39.27 37.86 39.41 38.72 39.03 An 17.45 18.04 18.03 19.17 17.90 Di 2.77 3.13 1.92 3.09 4.78 En 6.93 8.09 6.85 8.38 8.27 Fs 0.83 0.89 0.82 1.43 1.24 Mt 2.26 2.34 2.27 2.35 2.34 11 0.94 1.04 0.94 1.05 1.05 Ap 0.38 0.39 0.36 0.42 0.52 C 0.0 0.0 0.0 0.0 0.0 49 Table 2.1 (Cont'd): I z t a c c i h u a t l : Younger Flank A c t i v i t y La Joya Sample IZ-839 IZ-10 IZ-86 IZ-143 IZ-177 Latitude 19 07 25 19 08 19 19 07 28 19 06 41 19 07 : Longitude 98 39 30 98 38 38 98 38 59 98 39 06 98 42 1 S i 0 2 52.53 54.88 59.47 62.19 58.24 T i 0 2 0.77 0.87 0.80 0.82 1.09 14.98 16.41 16.64 16.51 17.63 ZFe 20 3 7.85 7.56 5.93 5.38 6.43 MnO 0.13 0.12 0.09 0.09 0.10 MgO 9.92 7.97 5.11 3.92 3.71 CaO 8.89 7.59 5.80 4.60 6.20 Na 20 3.33 3.66 4.56 4.48 4.46 K 20 1.66 0.99 1.17 1.70 1.51 P 20 0.44 0.20 0.19 0.22 0.30 H 20 0.00 0.00 0.21 0.15 0.46 H 20 0.16 0.33 0.23 0.34 0.46 Sum 100.66 100.58 100.20 100.40 100.59 ZFeO 7.03 6.78 5.35 4.84 5.80 Mg # 74.65 71.07 66.75 62.93 57.34 Cr 438 372 199 101 25 V 185 155 114 95 134 Ni 254 179 115 70 24 Rb 38 24 31 48 38 Sr 895 401 441 490 550 Ba 798 315 349 582 417 Zr 137 128 145 183 186 Nb 2 8 14 9 11 Y 27 23 20 23 27 Q 0.0 1.86 8.18 12.83 7.75 Or 9.62 5.78 6.88 10.02 8.93 Ab 29.53 32.68 40.81 40.30 40.36 An 20.53 25.04 21.33 19.79 23.54 Di 15.94 8.72 4.85 1.24 4.24 En 6.84 18.14 11.97 10.26 8.51 Fs 1.32 3.73 2.10 1.56 1.81 Fo 10.01 0.0 0.0 0.0 0.0 Fa 1.93 0.0 0.0 0.0 0.0 Mt 2.32 2.44 2.38 2.41 2.70 11 1.05 1.20 1.11 1.14 1.52 Ap 0.90 0.41 0.40 0.46 0.63 C 0.0 0.0 0.0 0.0 0.0 50 Table 2.1 (Cont'd): Teyotl Sample IZ-1057 IZ-244 IZ-121 IZ-243 Latitude 19 13 08 19 11 51 19 11 44 19 11 Longitude 98 37 50 98 38 20 98 38 25 98 38 : S i 0 2 63.26 63.38 64.49 64.77 T i 0 2 0.70 0.70 0.70 0.71 A1 20, 16.72 16.50 16.54 16.66 ZFe 2 0 3 4.69 4.36 4.37 4.53 MnO 0.08 0.08 0.08 0.08 MgO 2.84 2.34 2.33 2.46 CaO 5.02 4.42 4.67 4.64 Na 20 4.32 4.19 4.30 4.05 K 20 1.58 1.70 1.69 1.74 p2° + 0.15 0.15 0.17 0.21 H20_ 0.00 0.91 0.24 0.03 H 20 0.17 0.59 0.09 0.17 Sum 99.53 99.32 99.67 100.05 ZFeO 4.25 4.01 3.96 4.08 Mg # 58.52 55.56 55.40 55.85 Cr 56 44 34 28 V 95 91 83 108 Ni 25 15 13 14 Rb 42 51 48 49 Sr 538 496 526 563 Ba 449 473 487 522 Zr 146 163 158 150 Nb 7 6 6 7 Y 22 19 21 21 Q 16.36 18.77 18.69 19.64 Or 9.40 10.30 10.08 10.34 Ab 38.89 38.31 38.84 36.61 An 21.82 21.49 21.11 21.79 Di 1.83 0.0 0.93 0.0 En 7.10 6.63 6.07 6.83 Fs 1.00 0.71 0.62 0.84 Mt 2.30 2.32 2.31 2.32 11 0.98 1.00 0.98 1.00 Ap 0.32 0.32 0.36 0.44 C 0.0 0.16 0.0 0.19 5 1 Table 2.1 (Cont'd): - m y o i i Sample IZ-122 IZ-119 IZ-25' Latitude 19 11 47 19 11 43 19 11 -Longitude 98 38 20 98 38 13 98 37 : S i 0 2 64.82 64.73 64.96 T i 0 2 0.71 0.69 0.69 A1 20, 16.81 16.56 16.50 ZFe 20 3 4.49 4.35 4.33 MnO 0.08 0.08 0.08 MgO 2.38 2.28 2.29 CaO 4.62 4.66 4.56 Na 20 4.44 4.34 4.33 K 20 1.70 1.63 1.71 p2° + 0.18 0.18 0.19 H20_ 0.13 0.13 0.03 H 20 0.23 0.09 0.12 Sum 100.59 99.71 99.79 ZFeO 4.03 3.93 3.91 Mg # 55.26 54.98 55.20 Cr 41 33 33 V 87 88 87 Ni 14 12 13 Rb 48 44 47 Sr 547 595 571 Ba 492 498 558 Zr 152 147 155 Nb 8 11 8 Y 21 22 20 Q 17.75 18.96 19.08 Or 10.03 9.71 10.17 Ab 40.07 39.16 39.05 An 20.78 21.12 20.72 Di 0.76 0.80 0.65 En 6.22 5.98 6.07 Fs 0.72 0.63 0.60 Mt 2.31 2.29 2.29 11 0.99 0.97 0.97 Ap 0.38 0.38 0.40 C 0.0 0.0 0.0 52 Table 2.1 (Cont'd): S i e r r a Nevada: Sample IZ-1041 IZ-1042 Latitude 19 18 24 19 18 Longitude 98 38 19 98 38 : S i 0 2 64.84 65.90 T i 0 2 0.51 0.50 A 12 ° a 17.12 16.21 ZFe 20 3 4.21 4.10 MnO 0.08 0.07 MgO 2.69 2.28 CaO 4.37 4.23 Na 20 4.56 4.32 K 20 1.41 1.69 p2 o + 0.17 0.17 H20_ 0.41 0.39 H 20 0.24 0.59 Sum 100.61 100.45 EFeO 3.79 3.71 Mg # 59.82 56.44 Cr 92 100 V 69 76 Ni 42 47 Rb 22 46 Sr 459 435 Ba 500 469 Zr 121 118 Nb 10 5 Y 17 24 Q 17.82 20.26 Or 8.31 10.06 Ab 41.13 39.26 An 20.54 19.91 Di 0.0 0.09 En 7.41 6.30 Fs 1.08 0.96 Mt 2.09 2.09 11 0.71 0.70 Ap 0.35 0.36 C 0.55 0.0 Papayo IZ-1043 IZ-737 IZ-1044 19 17 50 19 16 51 19 18 02 98 39 20 98 42 38 98 41 42 62.79 62.51 65.76 0.70 0.72 0.67 16.09 16.32 15.97 4.63 4.78 4.40 0.08 0.08 0.08 3.00 3.03 2.83 4.68 4.80 4.51 3.98 4.04 4.20 2.00 1.97 2.03 0.21 0.18 0.19 0.73 0.79 0.00 0.56 0.62 0.09 99.45 99.84 100.73 4.25 4.37 3.93 60.15 59.63 59.98 70 67 64 94 92 88 31 30 27 55 52 58 434 443 423 600 555 557 189 186 174 8 8 7 20 23 19 17.02 16.04 18.59 12.07 11.84 11.94 36.29 36.86 37.82 20.67 20.98 18.50 1.32 1.65 2.03 7.88 7.79 6.85 1.01 1.11 0.67 2.31 2.33 2.26 1.00 1.02 0.93 0.45 0.38 0.40 0.0 0.0 0.0 Table 2.1 (Cont'd): — I z t a l t e t l a c — Sample IZ-1046B IZ-1046C Latitude 19 17 30 19 17 30 Longitude 98 43 45 98 43 45 S i 0 2 55.87 56.68 T i 0 2 1.03 1.04 A1 20 3 16.75 17.40 ZFe 20 3 7.30 7.24 MnO 0.12 0.13 MgO 5.80 5.36 CaO 7.40 6.68 Na 20 3.75 3.58 K 20 1.10 1.25 P 20 0.20 0.18 H20_ 0.04 0.92 H 20 0.46 0.43 Sum 99.82 100.89 ZFeO 6.61 6.54 Mg # 64.93 63.30 Cr 180 135 V 149 119 Ni 75 65 Rb 36 28 Sr 469 451 Ba 289 343 Zr 139 148 Nb 5 7 Y 26 30 Q 5.68 7.57 Or 6.54 7.42 Ab 33.80 32.61 An 25.80 27.72 Di 7.85 3.54 En 12.91 13.45 Fs 2.93 3.20 Mt 2.64 2.65 II 1.44 1.46 Ap 0.42 0.38 C 0.0 0.0 Buenavista Rio F r i o IZ-132 RF-3 RF-4 19 05 37 19 20 57 19 20 ! 98 36 20 98 40 12 98 40 : 62.43 75.74 72.45 0.79 0.07 0.06 16.45 13.37 14.91 5.32 0.99 0.99 0.09 0.07 0.08 3.27 0.12 0.12 5.17 0.58 0.55 4.56 4.16 3.37 1.62 4.01 4.20 0.21 0.05 0.03 0.00 0.79 2.37 0.17 0.23 0.44 100.08 100.18 99.57 4.79 0.90 0.92 58.88 22.02 22.02 66 7 3 104 21 20 44 3 3 43 201 194 451 26 33 378 115 89 141 72 72 11 14 15 23 35 36 13.37 32.78 34.37 9.57 24.04 25.87 40.97 37.98 31.41 19.62 2.59 2.64 3.73 0.0 0.0 7.45 0.34 0.35 1.37 0.0 0.0 2.38 0.02 0.07 1.10 0.10 0.09 0.44 0.11 0.07 0.0 1.37 4.46 Table 2.1 (Cont'd): 54 Valley of Mexico: Chichinautzin Group Sample IZ-263 IZ-264 IZ-270 Latitude 19 07 45 19 07 45 19 07 Longitude 98 46 30 98 46 30 98 43 : S i 0 2 56.78 65.66 59.35 T i 0 2 1.18 0.44 1.04 A 12 ° 3 17.65 16.59 17.78 ZFe 20 3 7.17 3.42 6.22 MnO 0.11 0.07 0.10 MgO 4.51 1.03 3.70 CaO 7.22 3.66 6.45 Na 20 4.34 4.84 4.30 K 20 1.22 2.14 1.42 P 20 0.25 0.21 0.29 H 20 0.20 0.90 0.00 H 20 0.23 0.62 0.10 Sum 100.86 99.58 100.75 ZFeO 6.42 3.14 5.56 Mg # 59.44 41.24 58.09 Cr 59 8 33 V 144 47 130 Ni 22 3 22 Rb 30 61 35 Sr 501 473 564 Ba 267 467 365 Zr 182 249 190 Nb 13 7 9 Y 29 22 26 Q 5.15 19.37 9.40 Or 7.16 12.89 8.33 Ab 39.07 44.12 38.64 An 24.75 17.12 24.70 Di 7.38 0.0 4.16 En 9.38 2.90 8.41 Fs 2.17 0.27 1.67 Mt 2.78 2.03 2.64 11 1.63 0.63 1.44 Ap 0.52 0.45 0.60 C 0.0 0.22 0.0 Mg # = 100(Mg/(Mg+0.85Fe+2) Figure 2.1: AFM diagram f o r volcanic rocks of I z t a c c i h u a t l and the northern S i e r r a Nevada. The s o l i d l i n e represents the s t a -t i s t i c a l discriminant of Irvine and Baragar (1971) that separates c a l c - a l k a l i n e (Ca) from t h o l e i i t i c suites (Th). 56 57 Figure 2.2: Histogram showing s i l i c a d i s t r i b u t i o n of analyzed samples (wt % recalculated to an anhydrous b a s i s ) . S i l i c a values used to subdivide the c a l c - a l k a l i n e series are those of Taylor (196 9) with the exception of a rhyodacite d i v i s i o n at 68-70 wt % S i 0 2 . B = basalt; BA = b a s a l t i c andesite; A = andesite; D = dacite; RD = rhyodacite; R = r h y o l i t e . 58 53 56 62 BA SIERRA NEVADA 68 70 RD 4 2 0 6 4 2 0 1 8 CO 16 CO = 14 1 2 1 0 8 6 4 I z t a l t e t l a c J Z J P a pa y o | P | B u e n a v i s t a | B| R i o F r i o | f | 1 T IZTACCIHUATL Younger Flank Activity 6 4 2 50 [TJ T e y o t l | J [ L a J o y a IZTACCIHUATL Younger Andesites and Dacites H Plug Dome ( Y a u t IZTACCIHUATL Older Andesites and Dacites Older Flank Activity — i — 55 T l a c u p a s o and La T rampa A n c e s t r a l P i e s L l a n o G r a n d e V o l c a n o 60 65 S i 0 2 Wt % 70 I— 75 80 59 s a l i c c a l c - a l k a l i n e volcanic rocks i n continental arcs (Ewart, 1979, 1982; Baker, 1982). A c l a s s i f i c a t i o n based primarily on s i l i c a content i s used to d i s -criminate members of the c a l c - a l k a l i n e suite (Figure 2.2). The boundaries of the f i e l d s i n t h i s diagram are e s s e n t i a l l y those of Taylor (1969) with the addition of a rhyodacite subdivision at 68-70 wt % SiO,_ (recalculated anhydrous b a s i s ) . From the frequency of s i l i c a abundances, i t i s cl e a r that the majority of I z t a c c i h u a t l lavas are d a c i t i c with an o v e r a l l mode of 62.5-65.5 wt % SiO x. The complete range of s i l i c a abundances extends from andesite to r h y o l i t e without any abrupt change i n mineralogy, texture, or other compositional v a r i a b l e s . Mineralogical c r i t e r i a that d i s t i n g u i s h p a r t i c u l a r lava types are used to augment t h i s c l a s s i f i c a t i o n . The name oli v i n e - b e a r i n g andesite (or simply o l i v i n e - b e a r i n g lava) i s used to d i s t i n g u i s h a n d e s i t i c lavas that contain d i s e q u i l i b r i u m mineral assemblages characterized by r e l a t i v e l y abundant phenocrysts of f o r s t e r i t i c o l i v i n e coexisting with quartz. This term i s not synonymous with b a s a l t i c andesite which has a lower s i l i c a content (<56 wt % S i O ^ . Because i t i s not always possible by microscopic observation alone to i d e n t i f y mineral phases that are out of equilibrium with coexisting glass, I r e f e r to a l l of the larger c r y s t a l s as e i t h e r phenocrysts (>0.4 mm) or microphenocrysts (0.4-0.2 mm); the term xenocryst i s not used. In c e r t a i n o l i v i n e - b e a r i n g lavas the e n t i r e phenocryst popu-l a t i o n may be out of equilibrium with t h e i r host volcanic glass. 2.5 IZTACCIHUATL VOLCANO The volcanic rocks of I z t a c c i h u a t l can be divided i n t o two p r i n c i p a l groups: lavas and p y r o c l a s t i c rocks that form the basal structure of the 60 Table 2.2: Vent Type, Exposed Surface Area and Volume of Eruptive Products f o r I z t a c c i h u a t l and the Northern S i e r r a Nevada Volcanic Unit S i e r r a Nevada: Papayo Dacite Buenavista Dacite I z t a l t e t l a c Cone Nature of Vent ce n t r a l vent concealed ?central vent c e n t r a l vent Surface Area (km 2) 84 4 Volume (km 3) 21 0.2 0.2 I z t a c c i h u a t l : Younger Flank A c t i v i t y Teyotl Dacite La Joya Lavas concealed ?fissure c e n t r a l vent and f i s s u r e Younger Andesites and Dacites Summit Series c e n t r a l vents Pies c e n t r a l vent Older Andesites and Dacites Ancestral Pies Llano Grande Volcano Older Flank A c t i v i t y Tlacupaso Rhyodacite La Trampa Lavas ce n t r a l vent c e n t r a l vent with caldera pre-caldera ri n g fracture? c e n t r a l vent? 24 160 52 31 115 1.6 0.15* 125 100 220 0.3 30 I z t a c c i h u a t l t o t a l volume * O r i g i n a l volume probably greater than 0.4 km3 445 61 volcano, informally named the Older Volcanic Series, and rocks that form the modern peak region which are referred to as the Younger Volcanic Ser-i e s . Within each of these main volcanic s e r i e s , a number of d i s t i n c t rock units may be recognized that form coalescing or superimposed volcanic cones and p a r a s i t i c lava f i e l d s . The legend to the geologic map i d e n t i f i e s the various volcanic units and Table 2.2 describes the nature of source vents and provides an estimate of the volume and exposed surface area of each u n i t . A summary of the volcanic evolution of I z t a c c i h u a t l presented below i s followed by detai l e d descriptions of the morphology and structure of the map u n i t s , arranged i n s t r a t i g r a p h i c order from oldest to youngest, and the mineralogy and texture of the lavas. P y r o c l a s t i c breccias, xenoliths, and hydrothermal a l t e r a t i o n are treated separately, and g l a c i a l and e p i c l a s t i c deposits are described thereafter. 2.5.1 K-AR GEOCHRONOMETRY AND ERUPTIVE HISTORY New K-Ar dates f o r I z t a c c i h u a t l lavas are presented i n Table 2.3 and enable a detai l e d reconstruction of the various stages of cone growth shown schematically i n Figure 2.3. The e a r l i e s t volcanic a c t i v i t y at I z t a c c i h u a t l began with the con-s t r u c t i o n of a large c e n t r a l volcano, named Llano Grande, formed by flows of p o r p h y r i t i c andesite and dacite. A glassy andesite sampled high (3600 m) on the western flank of t h i s structure yielded a K-Ar whole-rock age of approximately 0.9 Ma (LG-4, Table 2.3) by which time t h i s lava cone was well-developed (Figure 2.3). Llano Grande volcano remained active u n t i l at least 0.6 Ma when lavas at the western foot of the s h i e l d were emplaced. Flows of s i m i l a r age form the proturberance known as Pulpito del Diablo. It was a sample from t h i s l o c a l i t y that gave a K-Ar age of 13 Ma which, Table X.3: K-Ar Age Determinations for Iztaccihuatl Volcano No. Sample Volcanic Unit Younger Volcanic Series Younger Flank Activity: 1 IZ-254 Teyotl dacite 2 IZ-620 La Joya andesite Younger Andesites and Dacites: 3' IZ-1 Pies 4 P-6 Pies 5 R-12 Summit Series Older Volcanic Series Latitude (N) 19°ll-40" 19°07'32" 19°08'28" 19'08'19" 19«09'00" Longitude ; (w) 98°37'30" 98°38'56" 98°38'32" 9'8°38'12" 98°40'01" Material1 ,dated WR WR Plag Plag Plag 1.57 1.07 0.44 0.35 0.51 ""ArMxlO- 1 0 raol/gm) 0.00206 0.00510 0.00260 0.00249 0.00517 %»°Ar* 5.6 12.0 5.6 3.1 5.8 Date ± o 0.0810.02 0.27+0.02 0.34+0.01*. 0.4110.14 0.58+0.11 Older Andesites and Dacites: ' , < 6 LG-10 Llano Grande volcano 19'10'14" 98° 44'44" • ; WR 7 LG-4 Llano Grande volcano 19'13'30" 98°41'547 '', •' WR 1.49 1.37 0.01509 0.02154 4.2 12.9 0.58+0.12 0.90+0.07 1 The size fraction of material dated was -30+40 mesh in the case of\ whole-rock samples and -50+80 mesh for plagioclase separates. Plagioclase was washed In warm 5% HF for 20 minutes, rinsed in an ultrasonic bath of distilled water then dried prior to argon measurement. ' ; ' < ' , ' ' • 2 Average value of replicate analyses determined by atomic absorption^and XRF techniques. 3 Date provided by J . Hoover, U.S.G.S. laboratories, Menlo Park a = one standard deviation; WR = whole-rock; Plag = plagioclase; *°Ar* = radiogenic *°Ar Constants used: *°KX8 = 4.962xl0-10 yr-'; ""KAE = 0.581xl0-10 yr - 1 ; *°K/K = 0.01167 atom % 1. Hornblende dacite flow at base of sequence. Phenocrysts of plagioclase, hypersthene, hornblende, and minor augite set in a hyalopilitic groundmass. 2. Young La Joya aphanltic andesite. Microphenocrysts of orthopyroxene, plagioclase, and olivine enclosed in a pilotaxitic nearly holocrystalline groundmass. 3. Young Pies hornblende dacite overlying altered lavas and pyroclastlc breccias. Phenocrysts of plagioclase, hypersthene, and hornblende enclosed in a pale brown glassy groundmass. 4. Hornblende dacite exposed in south-facing cliffs of Pies. Phenocrysts of hypersthene, plagioclase and hornblende with trace amounts of biotite, olivine xenocrysts, and clinopyroxene microphenocrysts enclosed by very pale brown 5. Basal flow of the Younger Andesites and Dacites. Olivine-bearing andesite (mixed lava) carrying phenocrysts of plagioclase, hypersthene, hornblende, and quartz set in a dark grey oxide-rich glassy groundmass. 6. Glassy andesite flow at western foot of Llano Grande volcano. Olivine xenocrysts coexist with crystals of plagioclase, orthopyroxene, and minor augite enclosed In an oxide-charged glass. ON 7. Glassy andesite high on west flank of Llano Grande volcano. Olivine-bearing flow with crystals of plagioclase, 1 0 orthopyroxene, and trace hornblende enclosed in a seriate glassy groundmass. Figure 2.3: Diagrammatic cross-sections representing stages i n the evolution of I z t a c c i h u a t l volcano: A) Llano Grande volcano at an advanced stage of development p r i o r to caldera formation. B) Caldera collapse has taken place and postdates emplacement of Tlacupaso rhyodacite. The cones of La Trampa and Ancestral Pies have reached maturity. C) Extensive erosion of the Older V o l -canic Series has been followed by renewed volcanic a c t i v i t y and growth of cones belonging to the Younger Volcanic Series. Culmi-nating eruptions of Pies cone have produced a large summit crater and flank a c t i v i t y at La Joya has recently ended. D) Eruption of Teyotl lavas on the northern flank and construction of the summit region of the volcano came to an end shortly before Late P l e i s t o -cene (Wisconsin) g l a c i a t i o n . I z t a c c i h u a t l i s presently dormant. 64 VOLCANIC EVOLUTION OF IZTACCIHUATL a ) North Ma (vertical exaggeration x2) South 5500 m - - 4500 m 3 5 0 0 m - 1 - 2500 m 6 4 2 0 km b) c) 06 -°-2? ,^0Kk {{r''.'V^ >^ «s '" " d) 0.27 - Present Y O U N G E R A N D E S I T E S AND DACITES —1 Summit Series I 1 Pies Y O U N G E R FLANK ACTIVITY Teyotl Dacite La Joya Lavas O L D E R A N D E S I T E S AND DACITES Llano Grande ^-•fcl shield Ancestral Pies O L D E R FLANK ACTIVITY Tlacupaso Rhyodacite La Trampa Lavas 65 when combined with other anomalously old K-Ar dates of 8 Ma and 5 Ma r e -ported by Steele (1971) f o r s t r a t i g r a p h i c a l l y higher lavas of I z t a c c i h u a t l , supported the Miocene to Plio-Pleistocene chronology f o r the S i e r r a Nevada advocated by Mooser and others (1974). Culminating eruptions of Llano Grande volcano resulted i n collapse of the summit region to form Llano Grande caldera, named f o r the p l a i n of a l l u v i a l sediments ponded within i t s western rim. Late-stage a c t i v i t y on the northern flank of the cone produced a small p a r a s i t i c cone known as La Trampa, and a stubby viscous flow of rhyodacite that forms Cerro Tlacupaso. The source of t h i s flow may have been the s i t e of a pre-caldera r i n g -fracture i n the summit of Llano Grande. Penecontemporaneous with flank eruptions and waning a c t i v i t y of Llano Grande volcano, a major eruptive center, Ancestral Pies, was forming on the southern f l a n k . As t h i s lava cone increased i n height, p y r o c l a s t i c breccias became more common and f i l l e d narrow ravines eroded i n the flanks of the structure. Volcanoes that gave r i s e to the Older Volcanic Series probably became extinct about 0.6 Ma. A period of erosion followed u n t i l renewed volcanic a c t i v i t y produced the Younger Andesites and Dacites. A new group of vents aligned NNW-SSE breached the southeastern flank of the older volcanic structure. The southernmost vent extruded Pies lavas and p y r o c l a s t i c breccias which proceeded to bury much of the Ancestral Pies cone. Flows high i n the succession south of the type l o c a l i t y of Pies y i e l d K-Ar dates of 0.41 - 0.34 Ma. The f i n a l phase of a c t i v i t y ended with a series of powerful explosive (Plinian?) eruptions that formed the present summit crate r . P r i o r to t h i s event the cone may have attained an a l t i t u d e of some 5000 m. Lavas and p r y o c l a s t i c breccias that b u i l t the highest peaks of I z t a c c i h u a t l , rocks known as the Summit Series, also postdate erosion of Older Andesites and Dacites. A R o d i l l a s flow at the base of the succession has been dated at 0.58 ± 0.11 Ma (R-12, Table 2.3). Taking i n t o account the differences i n s t r a t i g r a p h i c s e t t i n g of dated flows and a n a l y t i c a l u n c e r t a i n t i e s , early Pies and Summit Series a c t i v i t y was probably syn-chronous . The e a r l i e s t recognizable g l a c i a t i o n of Pies lavas probably occurred p r i o r to flank eruptions of b a s a l t i c to a n d e s i t i c scoriae and flows of La Joya at approximately 0.27 Ma. Summit Series cones continued to grow a f t e r Pies a c t i v i t y had ceased and may have reached t h e i r present a l t i t u d e before the eruption of Teyotl dacite on the north flank of the volcano dated at ap-proximately 0.08 Ma. With the advent of Late Pleistocene (Wisconsin) g l a c i a t i o n , I z t a c c i h u a t l became dormant. The construction of I z t a l t e t l a c cone at the northwestern edge of the S i e r r a Nevada may predate Wisconsin g l a c i a t i o n . During retreat of the g l a c i e r s to t h e i r present locations, volcanism r e -sumed i n the northern S i e r r a Nevada with outpourings of d a c i t i c lavas from Cerro Papayo and more recent p y r o c l a s t i c flow deposits at Rio F r i o . Erup-tions at the southern margin of I z t a c c i h u a t l produced a small dacite flow at Buenavista. The t o t a l volume of s o l i d i f i e d lava produced by I z t a c c i h u a t l i s estim-ated at =445 km3. Llano Grande lava cone comprises 50% of t h i s estimate whereas only 28% i s accounted for by the imposing Summit Series (Table 2.2). 2.5.2 OLDER VOLCANIC SERIES: MORPHOLOGY AND STRUCTURE The Older Volcanic Series of I z t a c c i h u a t l i s subdivided i n t o a group of 67 Older Andesites and Dacites which form the major volcanic e d i f i c i e s of Llano Grande and Ancestral Pies, and a group of flows erupted from p a r a s i -t i c vents on the flanks of Llano Grande volcano. 2.5.2.1 Llano Grande Volcano The lavas of Llano Grande volcano represent the e a r l i e s t stages of cone-building at I z t a c c i h u a t l . They form a broad lava cone 24 km across at the base with an aspect r a t i o (cone height/basal diameter) of 0.05 and an estimated volume of 220 km3. The summit of the cone i s occupied by a collapse structure, Llano Grande caldera, measuring 4.5 km i n diameter. Its g l a c i a t e d western rim r i s e s 130 m above caldera f i l l to an a l t i t u d e of 3770 m, approximately 1300 m above the f l o o r of the Valley of Mexico. The eastern half of t h i s structure has been buried by younger flows from Teyotl, the Summit lavas of I z t a c c i h u a t l , and a veneer of g l a c i a l d r i f t . The surface of the volcano has been glaciated and dissected by streams to form deep V-shaped canyons with steep walls towering 100-250 m above v a l l e y f l o o r s that are separated by narrow t i l l - c o v e r e d i n t e r f l u v e s . A conspicu-ous topographic d i s c o n t i n u i t y between 3500 m and 3000 m on western and northern slopes corresponds p r e c i s e l y with the l i m i t of Younger I z t a c c i h u a t l lavas rather than the maximum extent of Hueyatlaco g l a c i e r s as proposed by White (1962). To the east, lavas of Llano Grande are exposed i n a 20 km 1 i n l i e r that i s capped at a l t i t u d e s greater than 3200 m by lavas that may represent part of the Ancestral Pies cone. Although erosion has revealed the i n t e r n a l structure of i n d i v i d u a l flows, contacts between successive flow units are only r a r e l y exposed. Centimeter-scale flow-banding i s well-developed and l o c a l l y d i s t o r t e d by flow folds and ramp structures which reach several meters i n amplitude. On 68 the limbs of these features flow f o l i a t i o n generally dips moderately to steeply, and s t r i k e s perpendicular to the d i r e c t i o n of t r a v e l . At flow margins the f o l i a t i o n i s usually uncontorted and i s oriented roughly para-l l e l to the d i r e c t i o n of t r a v e l . In the v i c i n i t y of the caldera, flow f o l i a t i o n generally dips r a d i a l l y away from the rim. Although r a r e l y exposed, t h i n (1-3 m) flow breccias i n d i c a t e f a i r l y shallow basal dips of 5-20°. Flow thickness, estimated i n the v a l l e y walls of Canada del Negro and Canada Cosa Mala j u s t east of San Rafael, i s t y p i c a l l y 20-50 m. 2.5.2.2 Ancestral Pies Two stages of cone construction separated by a period of repose can be recognized within lavas that form the southern part of I z t a c c i h u a t l . The f i r s t phase of a c t i v i t y b u i l t a large lava cone, Ancestral Pies, situated on the south flank of Llano Grande volcano. Later a c t i v i t y superimposed on the s i t e of Ancestral Pies produced Pies lavas and p y r o c l a s t i c deposits which are among the e a r l i e s t exposed members of the Younger Andesites and Dacites. Only the southwestern sector of Ancestral Pies i s well-exposed, ex-tending down to at least 2800 m where i t s lavas are buried by younger e p i c l a s t i c volcanic breccias and toba sediments. The dip of basal flow breccias decreases from moderate to shallow (20-9°) with increasing d i s -tance from the source, and flow-banding i s generally steep and contorted, e s p e c i a l l y i n outcrops between 3600 m and 3800 m. Northern and eastern portions of the cone are l a r g e l y covered by flows from the summit vents of I z t a c c i h u a t l and Pies r e s p e c t i v e l y . Small i n l i e r s representing the eroded flanks of Ancestral Pies are found 2.5 km northeast of Pies at 4200 m, and 3.5 km southwest of Pecho at 3800-3900 m. Other remnants may e x i s t i n the 69 walls of deep U-shaped v a l l e y s east of Pies or i n a whaleback ridge due east of Pecho at a l t i t u d e s of 3200-3800 m. The youngest flows of the ancestral cone occur at 4400-4600 m i n the western rim of a glaciated amphitheatre formed during the culminating eruptions of younger Pies a c t i -v i t y . Intense hydrothermal a l t e r a t i o n a f f e c t s older lavas i n the crater walls and f l o o r , i n the cirque at T l a l t i p i t o n g o v a l l e y head, and at Cerro Altzomoni, 4 km southwest of the vent. Lavas of Ancestral Pies may be grouped with those of Llano Grande volcano on the basis of s i m i l a r grain s i z e , texture, and morphological c h a r a c t e r i s t i c s . Overlying Pies flows have t e x t u r a l and mineralogical a t t r i b u t e s t r a n s i t i o n a l towards those of Summit Series lavas with which they are grouped ( i . e . Younger Andesites and Dacites). 2.5.3 MINERALOGY AND PETROGRAPHY OF THE OLDER VOLCANIC SERIES Phenocryst mineralogy and the petrographic c h a r a c t e r i s t i c s of Iz t a c c i h u a t l lavas are summarized i n Figures 2.4 and 2.5, and modal anal-yses are given i n Table 2.4. The most voluminous lavas of the Older V o l -canic Series are two-pyroxene andesites and dacites. They form Ancestral Pies and the majority of Llano Grande volcano. Minor proportions of horn-blende-bearing lavas appear to be i n t e r c a l a t e d w i t h i n the sequence and eventually become a common rock-type within the upper 500 m of Ancestral Pies cone (Figures 2.4 and 2.5). O l i v i n e phenocrysts occur i n lavas with complex t e x t u r a l and mineralogical r e l a t i o n s h i p s , and a sing l e ortho-pyroxene-phyric flow i s found at Trancas (3500 m) below the southwestern rim of Llano Grande caldera. Plagioclase i s the dominant phenocryst phase, representing 14-24% of the mode, although r e l a t i v e l y less abundant (5-13%) i n ol i v i n e - b e a r i n g Figure 2.4: Grain-size d i s t r i b u t i o n , coexisting phenocryst assemblages, and s t r a t i g r a p h i c range of phenocrysts i n I z t a c c i h u a t l lavas. Each measurement represents the mean of the f i v e largest grains observed i n a sing l e t h i n - s e c t i o n . PI plagioclase; Opx = orthopyroxene; Cpx = clinopyroxene; 01 = o l i v i n e ; Hb = hornblende; Bi = b i o t i t e ; Qz = quartz. Brackets indi c a t e trace amounts of the indicated phase. Plagioclase Orthopyroxene Hornblende Qz Bi Hb Opx Cpx 01 PI c CD E CD t_ 3 CO CO CP o z Cabeza Pecho Rodil las M IBI ran R E "FT! Pies Ancestral Pies Ml Llano Grande v o l c a n o Grain Size 0 (mm) _5L L B rl IQUI 1 5 •Ftilil HI •J • Eg Cpx-Opx-PI E Hb-Opx-PI-Cpx-(Bi) H Hb-Opx-Cpx-PI-(Qz-Bi) Coexisting Phenocryst Phases 0 Opx-PI S Opx H Hb-PI H Hb-Opx-PI H OI-Hb-Opx-PI-(Qz-Bi) • Hb-Opx-PI-(Qz-Bi) Stratigraphic Range QI OI-Opx-PI-Cpx-(Hb) [o] OI-Hb-Opx-Cpx-PI-(Qz-BI) 72 Figure 2.5: Mineralogical and petrographic c h a r a c t e r i s t i c s of I z t a c c i h u a t l lavas. Abbreviations are the same as those used i n Figure 2.4. 73 Altitude Sample Oz Bi Hb Opx Cpx 01 PI Comments CA8EZA 4820 m C-1 0 • • • 4700 C-2 0 • • • 4680 C-3 © • • • 4530 C-4 © o • • • 1 small meta-pelfte xenollth 4500 C-5 o • • • 4390 c-e © • • • aclcufar Mb mtcropftenocryata PECHO 4860 m PE-1 © • • • 4800 PE-2 © • • o o • small s k « l a t a l 01 and larga anhedral 01 lacking Px reaction rims 4700 PE-3 © o • • • Tweak compositional banding In glassy groundmass 4620 PE-4 • • • crystal-rich 4610 PE-5 o • • o A • soma akslatal Ol with Ops reaction rims 4190 PE-6 • • • collapsed vesicular texture 4080 PE-7 o • • • o • subhedral Ol lacking Px reaction rims 1 xenolith -01 with Opx rims. PI, Cpx, Hb. and minor brown glass 4010 PE-8 • • • 4000 PE-9 • • • 4530 PE-10 • • o • • 3600 PE-1 1 • • 0 • abundant mieronorite xenolitha with minor Cpx. Hb, and opaque oxides 3400 PE-12 o • • o • 3300 PE-13 • • • C l o s e d s y m b o l s : c r y s t a l s r e l a t i v e l y a b u n d a n t O p e n s y m b o l s : c r y s t a l s r e l a t i v e l y s c a r c e 0 Q u a r t z c r y s t a l s w i t h c l i n o p y r o x e n e r e a c t i o n rim Q H O l i v i n e w i t h w e l l - d e v e l o p e d o r t h o p y r o x e n e r e a c t i o n rim 0 H e t e r o g e n e o u s Q u a r t z p o p u l a t i o n : c r y s t a l s w i t h a n d w i t h o u t c l i n o p y r o x e n e r e a c t i o n rim £i. A H e t e r o g e n e o u s O l i v i n e p o p u l a t i o n : c r y s t a l s w i t h o r w i t h o u t o r t h o p y r o x e n e r e a c t i o n r i m • H e t e r o g e n e o u s O r t h o p y r o x e n e p o p u l a t i o n : c o l o u r l e s s to w e a k l y p l e o c h r o i c b r o n z i t e ( w i t h C r - s p i n e l i n c l u s i o n s ) a n d s t r o n g l y p l e o c h r o i c h y p e r s t h e n e ( w i t h m a g n e t i t e i n c l u s i o n s ) © A m p h i b o l e p s e u d o m o r p h e d b y o r t h o p y r o x e n e . c l i n o p y r o x e n e , p l a g i o c l a s e , a n d F e - T i o x i d e s a O l i v i n e p a r t l y or c o m p l e t e l y a l t e r e d to i d d i n g s i t e , c l a y m i n e r a l s , a n d c h l o r o p h a e i t e Altitude Sample Qz Bi Hb Opx Cpx Ol PI Comments WEST RODILLAS 4800 m R-1 © • • • 4780 R-2 © • • • 4650 R-3 • • • . abundant Opx mierolltea 4640 R-4 © • • • abundant Opx mierolltea and Hb microphenocrysts 4550 4520 R-5 R-6 © © o • • • • • • some skeletal Ol and adhering baaaltic glass weak compositional banding skeletal Ol with incipient or completely lacking PK rims 4500 R-7 © o • • • 4480 R-8 • o • Cpx microphenocrysts commonly in clots 4330 R-9 © • • • • few skeletal Ol - no Px rims 4200 R-10 • o • Cpx microphenocrysts commonly In clots 4190 R-11 0 o • • • • • Bronzile clots 3800 R-12 © • • • • skeletal Ol lacking Px reaction rima EAST ROD ILLAS 4910 m R-13 © • • • crystal-poor 4830 R-14 • • • crystal-poor 4770 R-15 © • • • o • subhedral Ol intergrown with Cpx 4690 R-1 6 • o • ?heterog»neous Opx population 4280 R-17 o • • • PIES 4630 m P-1 o • • o • • lew skeletal Ol with incipient Px reaction rims 4610 4600 P-2 P-3 o • • o • • xenollth comprising PI, Cpx, Opx. and vealculated brown glass PI mierolltea adhering to some Ol. No skeletal morphologies 4590 P-4 © o • • • • euhedral Ol with thin Px rims 4450 P -5 • • • • 4400 P-6 o • • •" • 4380 P-7 • • • some skeletal Ol 4220 P-8 • • • euhedral Ol lacking Px reaction rfma 4100 P-9 • • • • some skeletal Ol with tnin Px rims 4710 P -10 © o • • • 4700 P-11 • • o • 4400 P-12 © o • • • 4100 P-13 • • • • xenollth of Ol with Opx reaction rims, Opx, Cpx, PI and minor Hb and Oxides 4050 P -14 • • • aeleular Hb microphenocrysts 3800 P-15 © • • • • skeletal Ol lacking Px rims 3600 P-16 • • • aclcular Hb microphenocrysts Altitude Sample Bi Hb Opx Cpx Ol PI C o m m e n t s ANCESTRAL PIES 4490 m PA-1 © • • H b a n d O p x a l t e r e d t o F e O x i d e a n d c l a y m i n e 4400 PA-2 © • 4250 PA-3 o © • o © s o m e O l In g l a s s y d l a b a s l c c l o t s h a s O p x r i m s 4000 PA-4 o • • 3900 PA-5 © o • 3800 PA-6 • a • • s o m e s k e l e t a l 01. N o P K r e a c t i o n r i m e 3700 PA-7 • © • a c i c u l a r H b m i c r o p h e n o c r y s t a 3300 PA-8 • e • LLANO GRANDE VOLCANO 3700 m LG-1 • • • • r e v e r s e f . ? ) z o n i n g In O p x 3700 LG-2 • • • 3630 LG-3 • • • • 3620 LG-4 o • • • s o m e s k e l e t a l O l 3500 LG-5 • o e u h e d r a l O p x 3500 LG-6 • • • 3 100 LG-7 • • • 1 s m e l l m e l a - p e l l t e x e n o l l t h 28 10 LG-8 • • r e v e r s e ! ? ) z o n i n g In O p x 2800 LG-9 • • • • a c i c u l a r H b m l c r o p h e n o c r y a t e 2620 LG-10 • • A • s o m e Ot I n t e r g r o w n w i t h P I m l c r o l l l e s 1 e m a i l m e l a - p e l l t e x e n o l l t h TLACUPASO AND LA TRAMPA FLOWS 3295 m LG-1 1 • • c r y s t a l - r i c h <C. T t a c u p a a o ) 3600 LG-12 • • • a c i c u l a r H b m l c r o p h e n o c r y s i a ( C . La T r a m p s ) 3550 LG-13 • • c r y s t a l - r i c h <C. La T r a m p s ) Table 2.4: Modal Analyses (volume %, v e s i c l e - f r e e ) I z t a c c i h u a t l : Older Andesites and Dacites Llano Grande Volcano Sample LG- 2 LG-3 LG-4 LG-5 LG--6 LG-7 Plag 23. 42 23.68 5.11 t r 17 .50 20.85 Opx 5. 12 2.49 0.40 8.58 3 .61 4.07 Cpx 1. 12 0.64 - - 0 .42 0.49 01 - 3.75 - -Hb 0.40 t r - -Opaques 1 0. 24 0.24 0.32 t r 0 .11 0.08 Gmass 70. 10 72.55 90.42 91.42 78 .36 74.51 Llano Grande Volcano Ancestral Pies Sample LG-8 LG- 10 LG- •11 LG-13 PA-6 Plag 13.80 9. 57 14. 21 24.33 12.50 Opx 2.81 3. 46 3. 51 t r 3.25 Cpx 0.56 0. 32 - t r 01 - - 3.25 Hb - 5.70 -Opaques 1 0.08 0. 40 0. 37 t r 0.41 Gmass 82.75 84. 96 81. 91 69.97 80.59 I z t a c c i h u a t l : Younger Andesites and Dacites Pies Rodillas Sample P - l l P-15 P-16 R-3 R-6 R-9 Plag 25.08 9.45 33.65 14.35 16.97 11.10 Opx 7.75 3.46 2.86 2.33 3.15 2.24 Cpx t r - - - t r -01 - 4.58 2.18 - 0.85 4.79 Hb 2.32 0.65 2.10 3.59 1.79 2.04 Bi - - - - t r -Qz - - - - t r t r Opaques 1 . 0.81 0.37 0.57 0.36 0.51 0.20 Gmass 64.04 81.49 60.82 79.37 76.73 79.63 Table 2.4 (Cont'd): I z t a c c i h u a t l : Younger Flank A c t i v i t y La Joya Teyotl Sample IZ-839 IZ-10 IZ-86 IZ-177 IZ-105 Plag 5.961 2.17 0.32 — 5.33 Opx - 0.24 0.82 - 0.92 Cpx 1.77 1 - - t r -01 11.98 7.25 2.02 0.68 -Hb - - - - 2.81 Qz - t r - - -Opaques I _ - - - 0.40 Gmass 80.28 90.34 96.84 99.32 90.54 Si e r r a Nevada: I z t a l t e t l a c Papayo Buena- Rio Fr: v i s t a Sample IZ-1046B IZ-1042 IZ-1044 IZ-132 RF-3 2 Plag 3.42 17.99 6.39 5.67 16.79 Opx 0.62 2.78 1.36 1.39 -Cpx - 0.25 t r t r -01 3.37 1.49 - - -Hb - 0.74 1.11 1.94 -Bi - - - - 0.45 Qz - - 0.21 0.19 2.24 Opaques i _ 0.20 - - t r Gmass 92.59 76.55 90.93 90.81 80.52 1 microphenocrysts 2 obsidian fragment Plag = plagi o c l a s e ; Opx = orthopyroxene; Cpx = clinopyroxene; 01 = o l i v i n e ; Hb = hornblende; Bi = b i o t i t e ; Qz = quartz; Gmass = groundmass; t r = trace amount. Sample locations are given i n Table 2.1. 78 flows. Crystals are generally subequant with f i n e o s c i l l a t o r y zoning and r a r e l y form glomeroporphyritic aggregates. Plagioclase compositions i n two-pyroxene assemblages are generally sodic labradorite to c a l c i c andesine (An 5 g-An^ 5). Compositional zonation i n the phenocrysts of hornblende-bearing lavas t y p i c a l l y ranges An 1 ( 1 )-An 3 7 with microphenocrysts as c a l c i c as An.,5 zoned normally towards the composition of m i c r o l i t e s of sodic andesine i n the groundmass. Irregular pools of groundmass glass may be l o c a l l y concentrated just i n s i d e phenocryst margins. Inclusions of hypersthene, hornblende, and opaque oxides may also be present. The larger phenocrysts average 1-2.5 mm, although hornblende-bearing flows are generally coarser grained. It i s the si z e gradation of plagioclase that i s p r i m a r i l y r e -sponsible f o r strongly s e r i a t e textures i n these rocks. Hypersthene with c h a r a c t e r i s t i c pale pink and green pleochroism i s the next most abundant phase a f t e r p l a g i o c l a s e . It t y p i c a l l y forms 75-90% of modal pyroxene but may be absent i n hornblende-bearing r h y o l i t e s such as La Trampa and s i m i l a r lavas forming the northern wall of Canada del Negro and the narrow ridge separating Milpulco and T l a l t i p i t o n g o v a l l e y s . Crystals are generally euhedral to subhedral, and may contain inclusions of magnet-i t e and ilmenite (0.1-0.3 mm), or form c r y s t a l c l o t s 3 mm i n diameter incorporating plagioclase and minor hornblende. Strong reverse zoning marked by a strongly pleochroic core with a non-pleochroic colourless overgrowth i n o p t i c a l continuity with the core region i s evident i n c e r t a i n two-pyroxene andesites of Llano Grande. M i c r o l i t e s of prismatic hyper-sthene l o c a l l y intergrown with a u g i t i c pyroxene constitute a s i g n i f i c a n t proportion of the groundmass of most lavas. Large (2.5-3 mm) phenocrysts of weakly pleochroic bronzite form over 8% of the mode of a flow at Trancas that i s p r a c t i c a l l y devoid of plagioclase phenocrysts. 7 9 A u g i t i c pyroxene i s a r e l a t i v e l y common phase i n rocks of the Older Volcanic Series but r a r e l y exceeds 1% of the mode (Table 2.4). Crystals are colourless to pale green, subequant i n habit, and range 0.4-1 mm i n si z e , although a few prisms a t t a i n 4 mm i n length. Monomineralic aggreg-ates of microphenocrysts 1-2 mm i n diameter and intergrowths with p l a g i o -clase, hypersthene, and magnetite are common. Many c r y s t a l s e x hibit comb-ined simple and lamellar twins, uniform (?normal) zoning, and lack i n c l u -sions with the rare exception of opaque oxides. C a l c i c pyroxene i s es p e c i a l l y abundant as microphenocrysts and i n the groundmass of o l i v i n e -bearing lavas where i t l o c a l l y forms jackets on hypersthene phenocrysts. Equilibrium coexistance of hornblende and augite phenocrysts i n I z t a c c i h u a t l lavas appears to be rare. Where hornblende does occur i n pyroxene-dominated lavas, i t generally forms euhedral grains with dark olive-green to reddish-brown pleochroism and shows a s e r i a l gradation into a c i c u l a r c r y s t a l s (0.2-0.4 mm) that appear to have formed on quenching. Similar microphenocrysts are found within pale brown glass that occupies the i n t e r s t i c e s of cognate c r y s t a l c l o t s comprising plagioclase, hyper-sthene, and augite. Hornblende c r y s t a l s have l o c a l l y been alt e r e d to opaque oxide ± granular pyroxene. Small quantities of o l i v i n e phenocrysts make t h e i r f i r s t appearance i n early Llano Grande lavas and occur sporadically i n minor quantities throughout the rest of the Older Andesites and Dacites. Crystals are t y p i c a l l y euhedral or subhedral and s k e l e t a l morphologies are common, es p e c i a l l y i n the most o l i v i n e - r i c h rocks (3-4% modal o l i v i n e ) . I n dividual grains r a r e l y exceed 1.5 mm i n length though cl u s t e r s of microphenocrysts may reach 3 mm i n diameter. O l i v i n e compositions are f o r s t e r i t i c with uniformly high 2V regardless of the nature and abundance of coexisting 80 phases. Inclusions of dark brown chrome s p i n e l and b a s a l t i c glass up to several tens of microns across are common. Reaction rims of orthopyroxene may be well-developed (=0.2 mm i n width), i n c i p i e n t ( i . e . weak b i r e f r i g -ence at the margins of o l i v i n e grains), or completely lacking, but one v a r i e t y i s generally dominant within a s i n g l e t h i n - s e c t i o n . Associated phenocrysts of hypersthene and plagioclase (andesine) may be resorbed or reversely zoned and coexist with m i c r o l i t i c plagioclase ( c a l c i c andesine to l a b r a d o r i t e ) i n the groundmass. The rare occurrence of compositionally heterogeneous lavas provides d i r e c t evidence f o r mixing of magmas during eruption. In one flow-banded specimen, light-coloured layers of dacite several centimeters or less i n width contain subequant phenocrysts of andesine, hypersthene, and sparse hornblende set i n a colourless r h y o l i t i c glass carrying abundant p l a g i o -clase m i c r o l i t e s . Interlayered b a s a l t i c lava contains c r y s t a l s of f o r -s t e r i t i c o l i v i n e with tiny chrome-spinel inclusions enclosed i n dark brown glass charged with opaques and groundmass orthopyroxene, clinopyroxene, and c a l c i c p l a g i o c l a s e . Corroded phenocrysts derived from the dacite are found i n the b a s a l t i c groundmass but the dacite i s free of extraneous b a s a l t i c material. These textures appear to have formed by two d i s t i n c t stages of mixing of compositionally diverse magmas: the d i s e q u i l i b r i u m phenocryst assemblages within the mafic layers may represent rather thorough l o c a l i z e d pre-eruptive mixing within the magma chamber whereas the layered textures were formed on eruption by kneading of viscous hybrid and d a c i t i c l i q u i d s formerly coexisting at depth. 2.5.4 OLDER FLANK ACTIVITY 2.5.4.1 La Trampa Flows 81 The La Trampa lavas form a p a r a s i t i c structure about 9 km3 i n volume that covers 30 km2 of the northwestern flank of Llano Grande volcano. The vent has been removed by erosion but was located near an unnamed h i l l 0.7 km south of Cerro La Trampa (3730 m). These flows are bounded to the south and east by lavas of the underlying s h i e l d along Barranca e l Tomicoxco and Canada e l Guajito r e s p e c t i v e l y . They are covered by p o s t - g l a c i a l Papayo flows to the north, and loess and f l u v i a t i l e deposits to the east. Roadcuts and c l i f f s along the southern and eastern margins of the La Trampa f i e l d provide the best exposures. Lavas are medium-grained with s e r i a t e textures incorporating v a r i a b l e proportions of plagioclase, ortho-pyroxene, and hornblende. Fresh surfaces are medium grey to pale red (N5 to 10R6/2 using the Munsell Colour System code) and flow-banding i s i n -va r i a b l y present. Of the two samples c o l l e c t e d f or chemical a n a l y s i s , one i s a hornblende-hypersthene dacite, the other a hornblende r h y o l i t e (Table 2.1). Plagioclase phenocrysts i n these lavas are subequant, reach 3 mm i n length, and form up to 25% by volume of the rock (LG-13, Table 2.4). Spongy cores enclose pale brown glass and f i n e o s c i l l a t o r y zoning of r e l a -t i v e l y c a l c i c plagioclase (An s 2-An„ 0) occur i n both lavas. Dark brown horn-blende laths (0.3-2.8 mm i n length) are replaced or rimmed by opaque ox-ides. Euhedral hypersthene phenocrysts generally contain in c l u s i o n s of magnetite (0.25-0.1 mm) and r a r e l y form intergrowths with hornblende. Lavas mineralogically s i m i l a r to La Trampa r h y o l i t e are present i n Ancestral Pies (e.g. PA-5, Figure 2.5). 2.5.4.2 Tlacupaso Rhyodacite The Tlacupaso rhyodacite flow forms a subdued ridge trending northeast-southwest about 4 km northeast of Llano Chico. The northern end of t h i s 82 ridge i s formed by a glaci a t e d flow lobe 200 m high (Cerro Tlacupaso) whereas to the south t h i s feature i s overridden by younger lavas. The source of the flow may have been a pre-caldera r i n g - f r a c t u r e i n Llano Grande volcano. A d i s t i n c t i v e mesoscopic texture i s formed by dark s p h e r u l i t i c pods several centimeters across enclosed i n a l i g h t grey (N6-N7) d e v i t r i f i e d groundmass. In t h i n section phenocrysts of subequant plagioclase (2 mm) with f i n e o s c i l l a t o r y zoning (An^g bulk composition) and hypersthene en-clo s i n g euhedral magnetite microphenocrysts (0.04-0.1 mm) grade s e r i a l l y i nto a mesostasis r i c h i n m i c r o l i t e s and i n t e r s t i t i a l r h y o l i t i c glass. 2.5.5 YOUNGER VOLCANIC SERIES: MORPHOLOGY AND STRUCTURE Rocks of the Younger Volcanic Series are subdivided into a group of Younger Andesites and Dacites that form the highest peaks of I z t a c c i h u a t l , and lavas and p y r o c l a s t i c deposits erupted penecontemporaneously from vents at lower a l t i t u d e s on the northern (Cerro Teyotl) and southern (La Joya) flanks of the volcano. The Younger Andesites and Dacites are further subdivided into Summit Series and Pies rock groups. 2.5.5.1 Pies Erosion of Ancestral Pies by streams and g l a c i a l i c e (?) was followed by renewed cone growth i n the form of lava flows and p y r o c l a s t i c breccias that are well-exposed i n c l i f f sections below Pies (4703 m) and on ridges west and southwest', of the crater rim. Coarsely p o r p h y r i t i c Pies lavas r e s t i n g on the eroded surface of f i n e - to medium-grained Older Andesites and Dacites are found at 4300 m along the southwestern base of Pies, i n the north wall of the head of T l a l t i p i t o n g o v a l l e y , and i n Milpulco at ap-83 proximately 4150 m. At the l a t t e r l o c a l i t y older lavas have been p a r t i a l l y a l t e r e d to clay minerals and stained rusty brown by springs emanating from the contact. The majority of Pies lavas flowed east and south burying the older cone and covering an area of about 55 km2. Exposures at the base of flows may reveal a t h i n (1 m) oxidized flow breccia o v e r l a i n by a dense glassy zone with upright curving joint-planes spaced 0.5 m or less apart, succeeded above by h o r i z o n t a l platy j o i n t s oriented p a r a l l e l to the flow f o l i a t i o n . Flow thickness generally varies from about 10 m to 60 m. The vent i s located within an asymmetrical crater about 1.2 km across, open on i t s eastern side and somewhat enlarged by g l a c i a l erosion. Pre-ci p i t o u s crater walls tower 200-300 m above a sloping f l o o r l i t t e r e d with g l a c i a l t i l l , outwash, and loose t a l u s . Flows forming the northeast crater rim preserve g l a c i a l l y - s t r i a t e d pressure ridges and dip r e l a t i v e l y steeply (30°) away from the vent. 2.5.5.2 Summit Series: Cabeza, Pecho, R o d i l l a s A number of vents may be recognized within the high summit region of I z t a c c i h u a t l : the partly eroded "chin' region of Cabeza (5146 m); the ice-capped peak of Pecho (5286 m); and two centers spaced about 0.3 km apart at R o d i l l a s (5100 m). Lavas from each of these vent areas i n t e r -d i g i t a t e within the upper part of the succession, and young Ro d i l l a s flows rest with marked discordance upon the northern rim of the Pies cone. As these c e n t r a l e d i f i c i e s grew, t h e i r lavas moved east and northeast a d i s -tance of more than 20 km, crossing an uneven basement of denuded rocks of the Older Volcanic Series. Lobe and c l e f t features at the nose of some flows form abrupt scarps 80-100 m i n height protruding above thick toba 84 sediments of the Puebla Basin. To the west, these lavas inundated the eastern part of Llano Grande caldera and proceeded along major t r i b u t a r i e s towards the Valley of Mexico. For example, Younger lavas that followed the low divide between Llano Grande and Ancestral Pies volcanoes have terminal c l i f f s 40-50 m high at E l Salto (2600 m) about 3 km east of Amecameca. Further north i n Canada Cosa Mala, flows passed below 2 900 m where a narrow glaciated o u t l i e r of coarsely p o r p h y r i t i c lava i s found adhering to the steep canyon w a l l . In general, the thickness of i n d i v i d u a l flow units averages 20-30 m, ranging from 8-10 m on the lower flanks of Pecho to over 70 m i n flows l o c a l l y thickened by topographic c o n s t r i c t i o n s . The former extent of Summit lavas c e r t a i n l y exceeded the 160 km2 area presently ex-posed. The calculated volume of 125 km3 thus represents a minimum estim-ate. Despite d i s s e c t i o n by Late Pleistocene g l a c i e r s , s u r f i c i a l flow f e a t -ures are l o c a l l y preserved. For example, just outside the r i n g of Neogla-c i a l moraines, arcuate pressure ridges transverse to the d i r e c t i o n of flow have been swept bare of blocky flow breccias and polished and s t r i a t e d by g l a c i a l i c e . One such flow, a viscous dacite 0.7 km north of Ayoloco at 4200 m has been breached, exposing ramp structures beneath the pressure ridges that comprise a steeply dipping a p i c a l thrust plane bounded by near v e r t i c a l flow-banding. As a r u l e , flow f o l i a t i o n dips steeply only i n zones of flow f o l d i n g or at the margins of flows and i s formed by variably oxidized bands of dense glassy or weakly vesiculated lava 0.5-3 cm i n width. J o i n t i n g i s commonly blocky and rectangular i n plan, although l o c a l i z e d platy horizons are quite common. Basal flow breccias are gener-a l l y t h i n (0.5-1.5 m) and t h e i r angle of repose varies systematically from moderate (20°) on the flanks to r e l a t i v e l y steep (28-34°) i n the v i c i n i t y 85 of summit vents. Aside from the main vents, gla c i a t e d remnants of two small plug domes, Los Yautepemes (4250 m) and E l S o l i t a r i o (4210 m), r i s e 60-70 m above the northwestern mountain slopes. The southern plug known as Los Yautepemes (the "Needles') exhibits v e r t i c a l j o i n t columns curving away from a ce n t r a l keystone, the upper part of which had been removed by erosion. E l S o l i -t a r i o forms a more subdued knob that i s bordered by c l i f f s on i t s northern side. 2.5.6 MINERALOGY AND PETROGRAPHY OF THE YOUNGER VOLCANIC SERIES Petrographic differences between the Older and Younger Volcanic Series are summarized i n Figure 2.4. In the Younger lavas, hornblende j o i n s plagioclase and hypersthene as a ubiquitous phenocryst phase whereas c l i n o -pyroxene i s less common and reduced i n amount (Table 2.4). Trace propor-tions of b i o t i t e and quartz make a l a s t i n g appearance and d i s e q u i l i b r i u m mineral assemblages involving f o r s t e r i t i c o l i v i n e become extremely common. With the exception of orthopyroxene, phenocryst si z e i n the Younger V o l -canic Series i s s i g n i f i c a n t l y greater than encountered i n older lavas. In p a r t i c u l a r , plagioclase develops a h i a t a l texture and an average grain s i z e of about 3.5 mm as opposed to 2-2.5 mm i n the Older Volcanic Series. Mineralogical differences also e x i s t between the lavas of i n d i v i d u a l vents. Note, for example, the high proportion of oli v i n e - b e a r i n g andesites i n the lavas of Pies, the common occurrence of clinopyroxene i n Pecho flows, and the lack of both minerals i n Cabeza lavas (Figure 2.5). As i n e a r l i e r lavas, plagioclase i s the most abundant phenocryst, generally forming 10-25% of the mode (Table 2.4). A l l phenocrysts exhibit o s c i l l a t o r y zoning ( A n 3 0 - A n 6 5 ) , euhedral to subhedral o u t l i n e s , and l o c a l l y 86 form glomeroporphyritic c l u s t e r s . The largest grains reach lengths of over 5 mm. Phenocryst cores may show patchy zoning and in c l u s i o n s of pale brown glass that contain exsolved vapour bubbles or minute grains of Fe-Ti oxides which may have c r y s t a l l i z e d from trapped l i q u i d . Andesine m i c r o l i t e s i n the groundmass exhibit normal zoning and form a h y a l o p i l i t i c f a b r i c i n -volving very pale brown to colourless r h y o l i t i c glass. Hypersthene phenocrysts t y p i c a l l y form 2-8% of the mode. They occur as euhedral to subhedral c r y s t a l s 0.6-2 mm i n length with strong pleochroism and a s e r i a l s i z e gradation. Phenocryst cores are generally uniform i n composition whereas rims are weakly zoned. Inclusions of Fe-Ti oxide microphenocrysts (0.2 mm) or cores s c h i l l e r e d with opaque dust are especi-a l l y common i n monomineralic c l o t s . Colourless to weakly pleochroic mag-nesian orthopyroxene coexists with hypersthene i n several lavas but mutual intergrowths are not observed. The magnesian v a r i e t y may enclose dark brown chrome-spinel or form c r y s t a l c l o t s up to 2.5 mm i n diameter. Hete-rogeneous orthopyroxene populations of t h i s type are found i n both o l i v i n e -bearing and o l i v i n e - f r e e lavas (Figure 2.5). Pale green clinopyroxene i s r e l a t i v e l y sparse i n rocks of the Younger Volcanic Series. Microphenocrysts (0.2-0.3 mm) commonly form aggregates 0.4-0.8 mm across whereas i n d i v i d u a l phenocrysts may reach 1.4 mm i n length. C a l c i c pyroxene i n the groundmass (0.15 mm i n size) i s subordinate i n amount to orthopyroxene (except i n r e l a t i v e l y o l i v i n e - r i c h andesites) and l o c a l l y e x hibits hourglass e x t i n c t i o n or forms jackets on hypersthene phenocrysts or incomplete rims on m i c r o l i t e s . Hornblende phenocrysts are generally prismatic, average 2 mm i n length, and i n c e r t a i n lavas grade s e r i a l l y into a c i c u l a r microphenocrysts (0.2 mm) that formed on quenching. Pale olive-green to brownish-green pleochroism 87 i n the freshest c r y s t a l s acquires an orange-brown colour as the oxidation state of the rock increases. A t h i n opaque rim or complete replacement of hornblende by Fe-Ti oxides i s encountered i n more intensely oxidized lavas. A f i n e l y c r y s t a l l i n e aggregate of clinopyroxene, orthopyroxene, opaque oxide, and minor plagioclase forms a c h a r a c t e r i s t i c breakdown product of amphibole. Pyroxene prisms wi t h i n these intergrowths are commonly arranged e p i t a x i a l l y with respect to the c r y s t a l l o g r a p h i c c-axis of t h e i r host. Cognate c r y s t a l c l o t s i n v o l v i n g hypersthene, plagioclase, and Fe-Ti oxides i n addition to hornblende are common, and inclusions of b i o t i t e , apatite, and rare z i r c o n have been observed. Quartz and b i o t i t e i n v a r i a b l y occur as rounded c r y s t a l s measuring 0.3-1.2 mm i n diameter. Quartz i s ei t h e r resorbed or surrounded by micro-c r y s t a l l i n e clinopyroxene needles, and several lavas contain both v a r i e -t i e s . B i o t i t e e x hibits pale brown or pale green to dark coffee-brown pleochroism, although more oxidized c r y s t a l s appear reddish-orange. Most grains are surrounded by a t h i n opaque oxide rim or have reacted to form a corona of granular orthopyroxene, ilmenite, magnetite, and feldspar ( s a n i -dine?). F o r s t e r i t i c o l i v i n e i n a n d e s i t i c lavas i s almost as common as quartz and b i o t i t e but reaches modal concentrations (5%) f a r i n excess of ei t h e r of the l a t t e r phases. O l i v i n e morphology varies from anhedral to euhedral or s k e l e t a l , and i n d i v i d u a l c r y s t a l s range 0.2-2.4 mm i n size with most measuring less than 1 mm. Sparse p o r p h y r i t i c intergrowths comprising two to f i v e c r y s t a l s seldom exceed 2.5 mm i n diameter. A t h i n corona of minute orthopyroxene prisms generally separates o l i v i n e from host glass although t h i s reaction product i s o p t i c a l l y i n d i s t i n c t on some grains. Other crys-t a l s are surrounded by m i c r o l i t e s of c a l c i c plagioclase with i n t e r s t i t i a l 88 b a s a l t i c glass, a texture s i m i l a r to that observed within o l i v i n e - b e a r i n g xenoliths found i n these lavas (described below). In addition to o l i v i n e , these lavas contain other important clues to t h e i r o r i g i n i n the form of corroded c r y s t a l s of quartz, b i o t i t e , p l a g i o -clase, hornblende, and hypersthene. The cores of plagioclase phenocrysts are compositionally s i m i l a r to c r y s t a l s i n o l i v i n e - f r e e lavas i n t e r c a l a t e d throughout the stratigraphy, yet c r y s t a l margins may be resorbed or support a narrow overgrowth of labradorite ( A n S 0-An 6 5) zoned normally towards andesine. Quartz i n o l i v i n e - r i c h lavas has a clinopyroxene reaction rim. Hydroxylated minerals are l o c a l l y pseudomorphed by t h e i r respective reac-t i o n products and hypersthene may be jacketed by c a l c i c pyroxene. Local heterogeneities i n the composition of glass i n these lavas have been detected and may be accompanied by subtle differences i n the propor-tions of m i c r o l i t i c plagioclase and opaque oxides. Compositionally hetero-geneous lavas are comparatively rare but have been encountered among the flows of R o d i l l a s and Pecho. Like s i m i l a r textures i n Older Andesites and Dacites, centimeter-scale domains of r h y o l i t i c and b a s a l t i c glass support d i s t i n c t phenocryst populations. A d a c i t i c component contains e s s e n t i a l phenocrysts of plagioclase, hornblende, and hypersthene, together with sparse quartz and b i o t i t e set i n a colourless glass. B a s a l t i c domains enclose c r y s t a l s of a l l of the above i n addition to euhedral o l i v i n e pheno-crysts enclosed by oxide-charged dark grey glass containing m i c r o l i t i c plagioclase and clinopyroxene. Reaction rims of orthopyroxene on o l i v i n e are e i t h e r extremely t h i n or completely lacking; quenching presumably i n h i b i t e d t h e i r development. Textures observed within the b a s a l t i c compo-nent resemble the more or less homogeneous mix of c r y s t a l s and l i q u i d found throughout e n t i r e flows of o l i v i n e - b e a r i n g andesite. In the d i s t i n c t i v e l y 89 heterogeneous lavas, volcanic eruption appears to have arrested the mixing process which c l e a r l y involves a p a r t i a l l y c r y s t a l l i n e dacite magma and ol i v i n e - b e a r i n g b a s a l t i c melt. 2.5.7 PIES DACITE PLUG On the southeastern flank of Pies near Buenavista, loose angular blocks of o l i v i n e - b e a r i n g hornblende dacite (IZ-134, Table 2.1) measuring up to 2 m across have been disturbed by road construction. The source of the debris can be traced to small i s o l a t e d concentrations of blocks that r e -present the g l a c i a t e d remnants of underlying outcrops. The rock i s l i g h t grey (N7) and contains phenocrysts of andesine (2-3.6 mm), o l i v i n e (1-4 mm), hypersthene (<5 mm), hornblende (<1.6 mm), and accessory opaque oxides and apatite enclosed i n p a r t l y d e v i t r i f i e d r h y o l i t i c g l a s s . A l l c r y s t a l s of f o r s t e r i t i c o l i v i n e exhibit well-developed orthopyroxene reaction rims. Rounded dark grey xenoliths several centimeters i n diameter enclose a s i m i l a r phenocryst population but appear to be accidental i n o r i g i n . The abundance of xenoliths and r e s t r i c t e d outcrop suggest that the rock may form the eroded root of a plug dome. 2.5.8 YOUNGER FLANK ACTIVITY 2.5.8.1 Teyotl Dacite Volcanic a c t i v i t y on the north flank of I z t a c c i h u a t l occurred at ap-proximately 0.08 Ma and produced about 5 km3 of stubby dacite flows that cover an area of approximately 24 km*. G l a c i a l erosion has modified pre-e x i s t i n g volcanic morphology making i t d i f f i c u l t to recognize the exact l o c a t i o n and number of source vents. However, i t i s clear that most i f not a l l of these lavas originated at vents concealed beneath Cerro Teyotl (4660 90 m), the summit region of an east-west trending asymmetrical ridge bordered by c l i f f s 160 m high to the south and descending gradually to the north v i a crags and talus slopes. Teyotl lavas t r a v e l l e d east and west a distance of 3 km across underlying Cabeza and Pecho flows, and northwards about 6.5 km. Most impressive are the huge f r o n t a l lobes of i n d i v i d u a l flows that form abrupt gla c i a t e d scarps 100-200 m i n height. The phenocryst-poor character of Teyotl dacites (less than 10% t o t a l phenocrysts) distinguishes them from underlying andesites and dacites (generally 15-30% phenocrysts). Lavas are l i g h t grey to pale red (N5.5 to 5R6/2) and contain phenocrysts of plagioclase, hypersthene, hornblende, and minor augite enclosed i n a glassy m i c r o l i t i c groundmass. Large (4.5 mm) c r y s t a l s of subequant plagioclase with o s c i l l a t o r y zoned cores (An^ 0-An 5 S) enclose very pale brown to colourless r h y o l i t i c glass. Prismatic horn-blende (0.3-2.5 mm) commonly exhibits complete a l t e r a t i o n to opaque oxides or f i n e intergrowths of hypersthene, augite, magnetite, rare plagioclase, and ilmenite. Hypersthene phenocrysts contain opaque incl u s i o n s and s c h i l -lered cores, and some flows contain glomeroporphyritic intergrowths of hypersthene, plagioclase, and hornblende. 2.5.8.2 La Joya Lava F i e l d A small but d i s t i n c t i v e lava f i e l d comprising p o r p h y r i t i c o l i v i n e basalt, b a s a l t i c andesite, and aphanitic andesite (52.5-62 wt % S i 0 2 ) occupies an area of 3 km' on the southwestern flank of Pies at an a l t i t u d e of 4000 m. These flows can be traced southwards to i s o l a t e d outcrops surrounded by Recent e p i c l a s t i c and p y r o c l a s t i c f a l l deposits, and westw-ards to exposures at 3300 m along the Amecameca-Paso de Cortes road. The former extent of t h i s lava f i e l d appears to have been at least 8 km2 with a 91 t o t a l volume probably i n excess of 0.4 km3. Late Pleistocene g l a c i a t i o n has polished and scoured flow surfaces and removed a l l trace of source vents; t h e i r l o c a t i o n may be i n f e r r e d , howev-er, from dyke i n t r u s i o n , s c o r i a beds, and r e l i c t flow features. The lack of a hiatus within the volcanic stratigraphy suggests a f a i r l y short erup-t i v e episode, perhaps only tens of years i n duration. A young La Joya andesite y i e l d e d a K-Ar age of approximately 0.27 Ma (Table 2.3) i n d i c a t i n g that flank a c t i v i t y began shortly a f t e r the construction of a mature Pies cone. Some of the e a r l i e s t volcanic a c t i v i t y was Strombolian i n character, as evidenced by b a s a l t i c scoriae exposed i n a stream v a l l e y 0.9 km south of Cerro Altzomoni. Pale to moderate reddish brown (10R5/4-4/6) scoriae, fusiform bombs, and angular lava blocks up to 1 m across form a bed of non-agglutinated fragmental material about 15 m t h i c k . The lower part of the unit i s crudely s t r a t i f i e d , poorly sorted, and stained dusky yellow (5Y6/4) by c l a y - s i z e d material derived from loosely cemented t i l l - l i k e sediments below. Higher i n the stratigraphy, bedding planes become pro-minent at i n t e r v a l s of 15 cm to 1 m, and oxidation i s more intense. The thickness of scoriae at t h i s l o c a l i t y and s i z e of some of the blocks sug-gest proximity to a source vent. Flows d i r e c t l y overlying t h i s scoriae deposit, however, appear to have originated from vents near La Joya. Penecontemporaneous eruptions of b a s a l t i c andesite and andesite flows issued from source vents that have since been removed by erosion but were situated near the type l o c a l i t y of La Joya, a parking l o t 1.5 km northeast of Cerro Altzomoni. C l i f f s south and east of La Joya are formed by Pies lavas capped by oxidized b a s a l t i c scoriae and flows of b a s a l t i c andesite. Here, as w e l l as i n the eastern part of the lava f i e l d , many primary flow 92 features are preserved, including p a r t i a l l y plugged lava tubes, l o n g i t u -d i n a l gutters, t h i n rinds df l a t e r a l and upper flow breccias, and trans-verse pressure ridges. Except f o r a si n g l e small lobe that flowed northw-ards, La Joya lavas flowed south and southwestward for at least 7 km, from vents that were o r i g i n a l l y located north of La Joya. Early flows were fed i n part by open f i s s u r e s ; one such feeder, a v e r t i c a l dyke 1 m wide with h o r i z o n t a l columnar j o i n t i n g , cuts older Pies lavas j u s t south of La Joya. The youngest flows are found at the western and southern margins of the f i e l d . They are re a d i l y recognized by t h e i r aphanitic character and pro-nounced platy j o i n t i n g developed p a r a l l e l to a strong p i l o t a x i t i c f a b r i c most conspicuous i n t h i n - s e c t i o n . Weathering imparts a tan colour (5YR6/4) to these rocks but fresh surfaces are dark to medium grey (N3-N5.5). Flow thickness i s t y p i c a l l y 10 m or less but l o c a l l y exceeds 40 m where lava has ponded due to topographic c o n s t r i c t i o n . Thick intracanyon flows l i e i n contact with older lavas of Volcan Popocatepetl at the southern edge of the map area. The mineralogy of La Joya lavas i s complex considering t h e i r r e l a t i v e l y small volume and b r i e f eruptive h i s t o r y . F o r s t e r i t i c o l i v i n e p e r s i s t s throughout the e n t i r e compositional range and orthopyroxene becomes a more important phenocryst phase i n andesitic lavas (IZ-86, Table 2.4). A var i e t y of extraneous phenocrysts (quartz, andesine, hypersthene, and amphibole) occur i n a l l rock types except s c o r i a beds south of Cerro A l t z o -moni. B a s a l t i c scoriae near Cerro Altzomoni contain euhedral to subhedral phenocrysts of f o r s t e r i t i c o l i v i n e (2V * 80-90°) less than 1 mm i n length set i n an almost black oxide-charged v e s i c u l a r glass that contains micro-l i t i c p l a g i oclase and i n t e r s t i t i a l clinopyroxene. Laths of labradorite 93 (0.4 mm) and pale green to colourless granular augite (0.3 mm) with hour-glass e x t i n c t i o n grade s e r i a l l y i n t o the groundmass. Lava blocks taken from upper and lower s c o r i a horizons are i d e n t i c a l i n composition (Table 2.1). Quartz-normative b a s a l t i c andesites at La Joya contain s k e l e t a l o l i v i n e c r y s t a l s measuring up to 2 mm i n length. Lavas with h y a l o p i l i t i c textures formed by plagioclase laths (0.25-0.1 mm) enclosed i n brown glass may lack groundmass orthopyroxene, although h o l o c r y s t a l l i n e flow i n t e r i o r s possess o l i v i n e phenocrysts with orthopyroxene reaction rims, normally zoned micro-l i t e s of c a l c i c plagioclase (Na-labradorite to andesine), intergranular augite, magnetite, ilmenite, and minor i n t e r s t i t i a l tridymite. Inclusions within o l i v i n e phenocrysts consist of brown glass, and minute cubes of dark brown chrome-spinel. In more s i l i c e o u s compositions, anhedral micro-phenocrysts of o l i v i n e a l t e r e d to i d d i n g s i t e coexist with weakly pleochroic orthopyroxene phenocrysts (0.8-1.5 mm) enclosed i n a groundmass of andesine m i c r o l i t e s , granular oxide, and minor colourless glass. Strongly resorbed or reacted phenocrysts of quartz, hypersthene, amphi-bole, and plagioclase are extremely common i n early b a s a l t i c andesite flows at La Joya. Subequant plagioclase c r y s t a l s up to 4 mm i n length are abundant. C r y s t a l cores have f i n e o s c i l l a t o r y zoning (An^-An^) sur-rounded by a dark i n c l u s i o n - r i c h outer core overgrown by a t h i n c l e a r labradorite rim zoned externally towards groundmass andesine. Rarely, more complex resorption and overgrowth textures are developed i n which one plagioclase grain i s completely enveloped by another. Rounded quartz c r y s t a l s 1.2 mm i n diameter i n v a r i a b l y exhibit a t h i n reaction corona of prismatic to a c i c u l a r clinopyroxene c r y s t a l s that are p r e f e r e n t i a l l y elon-gated perpendicular to quartz grain boundaries. Corroded, strongly 94 pleochroic hypersthene (0.4-1.2 mm) may posses a t h i n overgrowth of colour-less c a l c i c pyroxene; inclusions of octahedral magnetite (0.1 mm) are present i n several c r y s t a l s . Amphiboles are replaced by granular pyroxene and opaque oxide which preserve subhedral grain o u t l i n e s . This extraneous phenocryst s u i t e i s found as a stable mineral assemblage i n the d a c i t i c lavas of I z t a c c i h u a t l . The presence of such d i s e q u i l i b r i u m phenocryst assemblages i n La Joya lavas i s due to ei t h e r bulk a s s i m i l a t i o n of s o l i -d i f i e d magma, or more probably, to mixing of p a r t l y c r y s t a l l i n e magmas at depth. 2.5.7 PYROCLASTIC BRECCIAS P y r o c l a s t i c deposits are r e l a t i v e l y scarce on I z t a c c i h u a t l , occupying a surface area of only 0.5 km2 and comprising a t o t a l volume of about 0.01 km3. The main outcrops are found at the head of Ayoloco v a l l e y (4700-4900 m), at the northern and western crater rim of Pies, along the eastern headwall of T l a l t i p i t o n g o , and among lava b l u f f s south of Pies (4200-4300 m). Most of these deposits occupy narrow canyons cut into older flows, although volcanic breccias proximal to the higher c e n t r a l vents form t h i n aprons interbedded with Summit lavas. D i f f e r e n t i a l erosion along j o i n t planes commonly leaves narrow walls and pinnacles crowned by lava blocks. The most c h a r a c t e r i s t i c deposit consists of angular to subangular blocks of po r p h y r i t i c dacite, generally less than 1.3 m i n length, enclosed i n a very l i g h t grey (N8) to yellowish orange (10YR8/6-6/6) f i n e l y comminuted matrix. The l a t t e r contains angular rock fragments up to 1 cm i n size and t y p i c a l l y constitutes 5-10% of the deposit. Thin (2-8 cm) discontinuous lenses of si m i l a r material form crude bedding planes 3-5 m apart that separate i n -di v i d u a l u n i t s . Lava blocks within are monolithologic and flow-banded, and 95 rare fragments of j o i n t columns up to 3 m i n length may be recognized. L o c a l l y these deposits pass upwards into the basal breccia of an overlying flow i d e n t i c a l with respect to the si z e and proportion of enclosed pheno-c r y s t s . Intercalated within these p y r o c l a s t i c breccias are t h i n l y bedded (0.1-3 m) water-washed deposits, l i g h t brown (5YR5/6) to pale red (5R7/4-6/6) i n colour, comprising a polymictic assemblage of variably a l t e r e d rock f r a g -ments (0.5-2.5 cm) enclosed i n a coarse volcanic sand. The matrix general-ly forms 80% or more of the deposit and may be cemented l o c a l l y by chalce-dony. Bimodal gr a i n - s i z e d i s t r i b u t i o n , chaotic i n t e r n a l structure, and the monolithologic character of these p y r o c l a s t i c breccias suggests f a i r l y rapid emplacement i n a high energy environment. They may have been depos-i t e d from a hot p y r o c l a s t i c flow i n the form of a lag concentrate (Wright and Walker, 1977), or from a hot avalanche of blocky debris o r i g i n a t i n g from the collapse of an advancing flow lobe or dome. A l l gradations between small nuees ardentes and hot avalanches were observed during the collapse of a n d e s i t i c flow fronts i n the December 1975 - A p r i l 1976 a c t i v -i t y of Volcan Colima (Thorpe and others, 1977). 2.5.8 XENOLITHS Apart from small c r y s t a l c l o t s of cognate o r i g i n , two types of a c c i d -e n t a l x e n o l i t h are found i n the lavas of I z t a c c i h u a t l . Rare metapelite inc l u s i o n s occur i n two-pyroxene andesites and dacites that form the nor-thern and southwestern flanks of Llano Grande volcano. They are pale to medium grey (N4-N4.5) i n colour, exhibit sharp contacts with t h e i r host, and have elongate cross-sections (3 mm x 1.5 mm). A m i c r o c r y s t a l l i n e base 96 of granular plagioclase and subhedral magnetite encloses hypersthene, b i o t i t e , and accessory corundum. Thin (0.2 mm) discontinuous b i o t i t e - r i c h zones may represent o r i g i n a l compositional l a y e r i n g . Similar xenoliths are found i n the lavas of La Malinche volcano and l i k e l y represent fragments of Cretaceous basement rocks. One common va r i e t y of igneous xenolith i s p a r t i c u l a r l y abundant i n Younger Andesites and Dacites of the Pies peak region. Angular to sub-angular blocks 10 cm or less i n size contain phenocrysts of plagioclase (andesine), hypersthene, hornblende, and sparse o l i v i n e set i n a mesostasis that i s r i c h i n m i c r o l i t e s of hypersthene, a c i c u l a r hornblende, and pl a g i o -clase with minor proportions of augite, Fe-Ti oxides and deep brown v e s i -cular glass. The larger c r y s t a l s are generally corroded or rimmed by reaction products i d e n t i c a l to those described i n ol i v i n e - b e a r i n g lavas which they resemble mi n e r a l o g i c a l l y . These xenoliths apparently represent material plucked from the walls of subvolcanic conduits. 2.5.9 HYDROTHERMAL ALTERATION Erosion has revealed l o c a l i z e d areas of intense hydrothermal a l t e r a t i o n a f f e c t i n g lavas exposed i n the crater walls and f l o o r of Pies as we l l as flows at the western rim of Llano Grande caldera. Altered rocks are l i g h t grey to dusky yellow or yellowish orange (N7 to 5Y6/4 or 10YR6/6) and exhibit p i t t e d erosion surfaces. Joint planes are l o c a l l y l i n e d with disseminated p y r i t e and rar e l y stained with native sulphur. Phenocrysts and volcanic glass are replaced to varying degrees by carbonate, clay, opaque oxides, and chalcedonic s i l i c a . O l i v i n e phenocrysts i n Pies flows are l o c a l l y a l t e r e d to a mesh of pale green serpentine, clays, and orange i d d i n g s i t e . 97 2.6 VOLCANIC ROCKS OF THE SIERRA NEVADA Lavas and p y r o c l a s t i c deposits that erupted from vents located north and south of Iztaccxhuatl are described below. L i s t e d from north to south, these rocks include pumice deposits at Rio F r i o , lavas at Cerro Papayo, a glac i a t e d volcanic cone forming Cerro I z t a l t e t l a c , and a dacite flow i n Paso de Cortes near the v i l l a g e of Buenavista. 2.6.1 Rio F r i o Pumice Deposit A pumiceous p y r o c l a s t i c flow deposit (not mapped) i s exposed at the northern edge of the map area i n a quarry just south of the highway at Rio F r i o . The source vent has not been i d e n t i f i e d . Non-welded r h y o l i t i c pumice with tube v e s i c l e s forms a massive basal layer over 15 m thick (base not exposed) o v e r l a i n by a t h i n l y bedded reworked a i r f a l l ash. Light grey (N8) to pale yellowish orange (10YR8/6) ash, l a p i l l i , and pumice blocks up to 20 cm across contain resorbed phenocrysts of quartz (3 mm), sodic o l i g o -clase (3.5 mm), and b i o t i t e (0.3-2 mm). Angular blocks of obsidian up to 7 cm i n length contain s p h e r u l i t i c d e v i t r i f i c a t i o n structures and the same phenocryst population (Table 2.4). Carbonized wood fragments several centimeters i n length are dispersed throughout the flow and a i r f a l l l a y e r s . The chemical composition of pumice and obsidian l a p i l l i are s i m i l a r except for a greater degree of hydration of the pumice accompanied by s l i g h t leaching of sodium (Table 2.1). The l a t t e r phenomenon appears to be common i n Recent pumice deposits of Central Mexico (G.T. Nixon, unpublished data). 2.6.2 Papayo Dacite 98 The northern extremity of the S i e r r a Nevada i s capped by approximately 84 km2 of p o s t - g l a c i a l dacite with a t o t a l volume of about 21 km3. The source of these flows i s located 2.5 km south of the Puebla-Mexico City highway at the crest of the mountain range. The vent i s marked by a steep-sided dacite dome measuring 1 km i n diameter and r i s i n g 230 m above sur-rounding flow surfaces. Early lavas descended 1000 m i n t o the Valley of Mexico, a distance of 9.5 km west of the vent, and t r a v e l l e d eastwards at lea s t 5 km towards the Puebla Basin. Later eruptions produced a series of flows extending 10 km to the east that form an elevated platform over 2 km i n width which dips gently eastwards between 3400-3200 m. At high a l t i -tudes lava surfaces are covered by mature pine forest whereas at low elev-ations flows extend beneath an encroaching blanket of yellow-brown loess. Numerous topographic features may be recognized as a product of primary flowage, notably transverse arcuate pressure ridges and l a t e r a l levees; narrow l o n g i t u d i n a l troughs and elongate depressions due to draining of lava at depth and consequent subsidence; and steep scarps formed around l a t e r a l and f r o n t a l flow lobes. Southwest of Cerro Papayo, flows en-countered a sharp break i n slope at an elevation of about 3100 m. Fro n t a l lobes were breached at t h e i r base and dacite lava poured from these root-less vents down stream v a l l e y s to inundate the northern and southern p e r i -meters of I z t a l t e t l a c cone. Exposures of early dacite flows are found at the southeastern margin of the lava f i e l d about 6 km from Cerro Papayo. Lava crusts are moderately oxidized, pale red (5R6/2) to brownish grey (5YR4/1) and are alte r e d s l i g h t l y to clay minerals. The rocks are p o r p h y r i t i c with a s e r i a t e t e x t -ure and contain r e l a t i v e l y large proportions of c r y s t a l s (30-50% by volume) set i n a pale to moderate brown m i c r o l i t i c glass. The larger phenocrysts 99 of plagioclase are less than 2 mm i n length, subequant and r a r e l y glomero-p o r p h y r i t i c , and have oscillatory-zoned cores (An. 8 -An^ g). Hypersthene i s strongly pleochroic and commonly encloses subhedral magnetite grains less than 0.04 mm across. Pale green augite i s rare but may form t h i n over-growths on hypersthene phenocrysts and incomplete rims on groundmass grains. Orange-brown to dark, brown hornblende i s present i n trace amounts and i s l o c a l l y intergrown with hypersthene. Subhedral c r y s t a l s of f o r -s t e r i t i c o l i v i n e form several percent by volume of the rock, occurring as ei t h e r s i n g l e grains or with adhering c a l c i c plagioclase m i c r o l i t e s and minor dark brown glass. Orthopyroxene, augite, opaque oxide, and minor plagioclase form c r y s t a l c l o t s 3 mm i n diameter. In contrast, younger Papayo flows and the c e n t r a l dome contain fewer phenocrysts (20% or less i n the mode), lack o l i v i n e , and carry c r y s t a l s of quartz, b i o t i t e , hornblende, and plagioclase. Large plagioclase c r y s t a l s 3-5 mm i n length have resorbed cores with o s c i l l a t o r y zoning (An^-An^ g ) surrounded by a zone of deep brown glass containing annular concentrations of. opaque oxide granules overgrown by a clear subhedral rim of normally zoned c a l c i c p l a g i o c l a s e . Microphenocrysts (0.6-0.3 mm) commonly ex h i b i t pronged terminations due to rapid growth and some c r y s t a l s are hollow i n cross-section. Euhedral c r y s t a l s of orthopyroxene (0.7 mm) may possess strongly pleochroic cores surrounded by weakly pleochroic rims (reverse zoning?) and monomineralic c r y s t a l c l o t s up to 1.5 mm across are r e l a t i v e l y common. Pale green augite forms sparse subhedral c r y s t a l s with inclusions of brown glass. Dark brown hornblende phenocrysts are mantled by granular plagioclase, pyroxene, and magnetite; ilmenite and hornblende may ac-company these reaction products i n the rims of orange-brown b i o t i t e . Quartz c r y s t a l s with coronas of clinopyroxene 0.3 mm i n width enclosing 100 deep brown glass are p a r t i c u l a r l y common i n these lavas. The proportion of glass enclosed w i t h i n these rims t y p i c a l l y exceeds that of s i m i l a r quartz phenocrysts found i n I z t a c c i h u a t l lavas. The age of Papayo volcanism has been estimated using the chronology of g l a c i a l t i l l s established for I z t a c c i h u a t l (described below). Papayo flows are younger than t i l l and g l a c i a l outwash occuring at an a l t i t u d e of 3300 m along the southern perimeter of the lava f i e l d . Since these deposits occur above Nexcoalango terminal moraines, they place a maximum age of approxim-ately 0.012 Ma on Papayo dacite. 2.6.3 I z t a l t e t l a c Cone A Late Pleistocene lava and s c o r i a cone named Cerro I z t a l t e t l a c occurs near the northwestern margin of the S i e r r a Nevada approximately 1.5 km south of the Mexico City-Puebla highway. The cone has a maximum basal diameter of 2.5 km and forms a kipuka vent surrounded by younger dacite flows from Cerro Papayo 4 km to the northeast. A shallow summit crater 0.4 km i n diameter reaches an elevation of 3290 m along i t s southern rim. Here, g l a c i a l erosion has produced precipitous c l i f f s over 80 m high that reveal an i n t e r n a l structure of b l u i s h grey (5B5/1) to moderate red (5R5/4) oxidized lava, spatter, and scoriae of b a s a l t i c andesite to andesite compo-s i t i o n (Table 2.1). The petrographic character of these lavas compares w e l l with that of La Joya flows and a s i m i l a r extraneous phenocryst population i s observed. Crystals of f o r s t e r i t i c o l i v i n e (0.5-3.0 mm) enclosing chrome s p i n e l occur as euhedral, s k e l e t a l , or anhedral grains some of which exhibit ortho-pyroxene reaction rims. Plagioclase forms subhedral to resorbed c r y s t a l s up to 2.8 mm i n length. O s c i l l a t o r y zoned cores (An,, 8 bulk composition) 101 r i d d l e d with groundmass glass are overgrown by rims of more c a l c i c compo-s i t i o n ( l a b r adorite) strongly zoned towards Na-andesine. A few cores are i n c l u s i o n - f r e e or contain t h i n zones of inclusions arranged p a r a l l e l to c r y s t a l margins. Colourless b r o n z i t i c pyroxene i s the most abundant micro-phenocryst phase accompanied by some subhedral augite. Dark grey to brow-nish i n t e r s t i t i a l glass i s crowded with opaque oxides and p i l o t a x i t i c p l a g i o c l a s e . Quartz phenocrysts less then 0.6 mm i n diameter are sur-rounded by coarsely c r y s t a l l i n e rims of augite with i n d i v i d u a l prisms measuring 0.15 mm i n length. Subhedral amphibole i s pseudomorphed by Fe-Ti oxides and minute ro d - l i k e c r y s t a l s of mildly pleochroic orthopyroxene. 2.6.4 Buenavista Dacite A small p o s t - g l a c i a l lava flow named Buenavista dacite i s exposed i n roadcuts between Paso de Cortes and Buenavista. The flow extends eastwards 5 km from a concealed vent at i t s northwestern extremity to a flow terminus located 2.4 km east of Buenavista. Assuming an average thickness of 40 m, the flow has an estimated volume of about 0.2 km3. Although obscured by mature forest and tephra from Volcan Popocatepetl, i t s clinkery surface l o c a l l y protrudes from beneath the vegetation cover. It may be even younger than the dacites of Cerro Papayo. The lava i s glassy, crystal-poor, and dark grey to greyish black (N4-N2). Large plagioclase phenocrysts ( c a l c i c andesine) occur as sparse o s c i l l a t o r y zoned c r y s t a l s up to 5 mm i n length and contain spongy g l a s s -charged cores and normally zoned rims. Hypersthene with s c h i l l e r e d cores occurs intergrown with magnetite but i s more abundant as l a t h - l i k e pheno-crysts 0.2-0.6 mm i n length. A u g i t i c pyroxene i s present i n small quanti-t i e s i n the groundmass or forming coronas around rare quartz c r y s t a l s . 102 Dark brown oxyhornblende has reacted to form hypersthene, augite, Fe-Ti oxides, and p l a g i o c l a s e . Residual brown glass contains plagioclase and hypersthene m i c r o l i t e s which exhibit a weak h y a l o p i l i t i c texture. 2.7 VOLCANIC ROCKS OF THE VALLEY OF MEXICO : CHICHINAUTZIN GROUP The southwestern corner of the map area contains three small volcanic cones that belong to the Chichinautzin Group (Mooser, 1957). A l l three cones have been breached by erosion on t h e i r southern or western sides and blanketed by wind-blown loess or e p i c l a s t i c volcanic d e t r i t u s from the S i e r r a Nevada. Cone heights vary between 105 m and 145 m above t h e i r base, cone height/basal width r a t i o s range 0.11-0.16 and crater diameter to cone basal width i s uniform at 0.69. A comparison of these values with those of s i m i l a r geomorphological parameters correlated with radiocarbon dates by Bloomfield (1975) suggests that cones examined i n t h i s study have apparent ages of 0.02-0.04 Ma. Quarries i n the Amecameca and A l c a l i c a n cones reveal t h e i r i n t e r n a l structure: t h i n l y bedded layers of dense black glassy to vesiculated lava blocks and l a p i l l i i n c l i n e d 20-23° away from the vent. Breadcrust bombs 15-25 cm i n length with t h i n (2-3 mm) oxidized rinds are r e l a t i v e l y common. Chemical analyses of three flow-banded lava blocks are given i n Table 2.1. S i l i c a concentrations range 56.8-65.7 wt % with extreme values obtained on two blocks from the Amecameca cone. Such compositional d i v e r s i t y within a sin g l e cone may i n d i c a t e a complex or protracted petrogenetic h i s t o r y that has been incompletely sampled by eruptions at the surface. In such cases, the term 'monogenetic', adopted by some authors (e.g. Bloomfield, 1975) to describe cones and associated flows of the Chichinautzin Group, may be misleading. 103 2.8 TEPHRA DEPOSITS Layers of r h y o l i t i c to d a c i t i c ash and pumice i n the S i e r r a Nevada are commonly observed interbedded with s o i l s and reworked v o l c a n i c l a s t i c material. A ser i e s of measured s t r a t i g r a p h i c sections examined i n the southern part of the map area near Paso de Cortes are presented i n Figure 2.6. The ages of the youngest pumice horizons of Popocatepetl have been estimated at 430-500 and 800-965 ± 60 Carbon-14 years B.P. by Heine and Heide-Weise (1973). These dates appear to correspond to units 2 through 5 i n Figure 2.6 but unfortunately exact c o r r e l a t i o n between horizons dated by Heine and Heide-Weise and units i n the measured sections cannot be made with c e r t a i n t y . Unit 1, the topmost layer, forms a black humic horizon up to 1 m i n thickness with d a c i t i c pumice from the underlying l a p i l l i layer incorpo-rated into i t s base. A trace of black ash i s found i n the uppermost few centimeters of t h i s s o i l at Paso de Cortes, and White (1962) noted a s i m i l a r deposit coating Milpulco moraines on the west side of I z t a c c i h u a t l . The source of t h i s ash may be re l a t e d to cinder cone a c t i v i t y i n the Valley of Mexico or to a Recent ( h i s t o r i c a l ? ) eruption of Volcan Popocatepetl whose crater rim i s draped with s i m i l a r material (Demant, 1981). Underlying units 2, 3, and 5 (unit 4 i s a t h i n loess horizon that i s l o c a l l y absent) form a d i s t i n c t i v e l a p i l l i - a s h - l a p i l l i sequence that i s well-exposed i n roadcuts and stream gulleys throughout the southern part of the map area. Both upper and lower l a p i l l i units comprise l i g h t grey to greyish yellow dacite pumice and the lower unit i s d i s t i n c t l y more enriched i n rock fragments. The l i t h i c component i n both l a p i l l i units i s dominated by angular fine-grained andesite to dacite c l a s t s less than 1 cm across 1 0 4 Figure 2.6: St r a t i g r a p h i c sections and c o r r e l a t i o n of Recent sediments and tephra deposits from Popocatepetl. Sample s i t e s A to H are shown on the geologic map (rear pocket). Generalized Section (not to scale) West East o 106 that form up to 5% by volume of the deposit. In roadcuts at Paso de Cor-tes, pumice i n both units i s angular to rounded with the largest blocks reaching 8-10 cm i n diameter. A bed of f i n e a i r f a l l ash showing d e l i c a t e i n t e r n a l laminae occurs at the base of the upper pumice u n i t . It i s gene-r a l l y separated from the lower l a p i l l i layer by a black s o i l or yellow-brown loam (0-90 cm thick) that encloses sporadic pumice l a p i l l i , rock fragments, and root systems (Unit 4). A l l three tephra horizons appear to have been eroded or were not deposited west of Paso de Cortes on slopes below about 3300 m. The upper pumice unit thickens considerably from 10 cm, a distance of 4 km west of Paso de Cortes, to 60 cm near Buenavista, and becomes even thicker to the southeast i n exposures along the road to Puebla. The same layer can be traced north to I z t a c c i h u a t l where at 3500 m on the western flank ( l a t i t u d e 19° 10.5'N) i t forms a layer of f i n e pumice l a p i l l i 4 cm thick res t i n g on Hueyatlaco t i l l . At the same a l t i t u d e on eastern flanks i t occurs as a coarse l a p i l l i horizon 7 cm i n thickness that i s underlain by a trace of lightcoloured ash belonging to Unit 3. A water-washed pumice horizon below probably represents the lower l a p i l l i u n i t . Thin bands of orange-brown weathering d a c i t i c pumice that occur further north i n s o i l s and f l u v i a l deposits probably represent these and e a r l i e r tephra layers of Popocatepetl. Examples are found i n roadcuts through l a t e r a l moraines 2.5 km northeast of Llano Chico and along the southwestern perimeter of Papayo lavas. Unit 6, a layer of pale yellowish brown loess containing scattered pumice and rock l a p i l l i , separates tephra of Unit 5 from a thick sequence of p y r o c l a s t i c f a l l deposits and interbedded f l u v i a l d e t r i t u s below. Units 7, 9, and 10 (unit 8 represents the reworked top of unit 9) are w e l l -exposed i n roadcuts and stream gulleys near Buenavista (sections E, G, and 107 H i n Figure 2.6). They comprise l i g h t grey to pale yellowish orange dacite pumice that exhibits i n t e r n a l l y graded layers so t y p i c a l of a i r f a l l mater-i a l . Rock fragments form about 5% of the deposit and range 0.2-3 cm i n s i z e . The. upper unit i n section G, located on the h i l l s i d e north of Buena-v i s t a , i s represented by a coarse pumiceous breccia with a high l i t h i c concentration (10-20% by volume) and rounded pumice up to 10 cm i n diamet-er. In section E the lower unit exhibits a s i m i l a r texture with blocks of angular andesite commonly a t t a i n i n g 8 cm, and r a r e l y 70 cm, i n s i z e ; pumice l a p i l l i , however, are less than 7 cm across. These beds may r e -present a concentrate formed by downhill movement or incomplete winnowing of the f i n e r c l a s t i c f r a c t i o n . The l i t h i c population i s dominated by augite-hypersthene microdiorite that may have formerly plugged the vent, and c l a s t s of fine-grained f e l d s p a t h i c sandstone with an epidotized matrix that probably represent hydrothermally altered material ripped from the walls of the conduit. Oxidation and weathering generally produce an orange-brown colour, and reworking by water formed Unit 8, a t h i n l y bedded layer of well-rounded pumice and angular rock fragments i n a coarse sandy matrix. F l u v i a l sands and gravels i n this unit l o c a l l y exhibit c r o s s - s t r a t i f i c a t i o n and channel-ing occurs at the top of the underlying u n i t . The source of most Recent tephra deposits of the S i e r r a Nevada i s Volcan Popocatepetl. The a r e a l extent, thickness, and pyroclast s i z e d i s t r i b u t i o n of the upper l a p i l l i and ash horizons (Units 2 and 3) suggest deposition from an eruptive plume that moved eastwards from an open summit crate r . The dispersion pattern within older tephra layers i s not known. 2.9 EPICLASTIC VOLCANIC BRECCIAS, LOESS, AND ALLUVIAL DEPOSITS 108 E p i c l a s t i c volcanic breccias that form the low h i l l y topography at the western base of I z t a c c i h u a t l belong to a sequence of Late Pleistocene t i l l - l i k e deposits described b r i e f l y below i n reference to the ^older a l l u v i a l sediments'. Thickly bedded yellow-brown loess with no i n t e r n a l s t r a t i f i c a t i o n , r h y o l i t i c tephra, and l e n t i c u l a r horizons of stream sands and gravels blanket the lower slopes of the S i e r r a Nevada and reach great thicknesses i n the Valley of Mexico and Puebla Basins. These sediments are known as ^toba' i n t h i s part of Mexico. Heine and Schonhals (1973) con-sidered that these deposits began accumulating i n the Middle Pleistocene and have since been reworked by wind and water during Wisconsin and Recent times. 2.10. GLACIATION AND GLACIAL DEPOSITS The products of Late Pleistocene g l a c i a t i o n s and the a c t i v i t y of modern g l a c i e r s i n the summit regions of I z t a c c i h u a t l have been w e l l documented (for example, Ordonez, 1894; Farrington, 1897; Robles Ramos, 1944; De Terra and others 1949; Blasquez, 1961; and White, 1956, 1962). Most previous work i s summarized by White (1962) who provided the f i r s t d e t a i l e d account of Late Pleistocene to Recent g l a c i a l deposits on the west side of I z t a c c i h u a t l . White recognized a sequence of progressively younger moraine complexes located at successively higher elevations on the flanks, and established the r e l a t i v e a n t i q u i t y of the t i l l s on the basis of t h e i r degree of weathering, s o i l p r o f i l e s , and sedimentary c h a r a c t e r i s t i c s . The r o c k - s t r a t i g r a p h i c units established by White are shown i n Figure 2.7 together with c o r r e l a t i v e moraine complexes on La Malinche that have been dated by radiocarbon techniques (Heine, 1973). At elevations above t r e e - l i n e a v a r i e t y of mountain g l a c i a l features 109 Figure 2.7: G l a c i a l chronology of I z t a c c i h u a t l (west side a f t e r White, 1962) and c o r r e l a t i o n with radiocarbon-dated moraines on La Malinche volcano (Heine, 1973). Iztaccihuatl W. s ide Unit Ayoloco Till Milpulco Til l Hueyat laco Till Nexcoa lango Till T i l l - l i k e sediments (within older al luvial deposi ts ) E. s ide La Mal inche Lower limit of t i l l/moraines(m) 4 4 1 0 3 6 3 0 -3 7 6 0 3 1 3 5 2 7 5 0 2 4 5 0 4 3 8 0 3 8 5 0 1. 3 2 0 0 2. 3 4 0 0 - 3 5 0 0 2 8 0 0 < 2 7 0 0 not present 3 9 5 0 -4 2 0 0 3 0 0 0 2 6 3 0 -2 7 5 0 2 5 0 0 -2 5 5 0 Unit M IV M III M II M C a r b o n - 14 years B .P . N e o g l a c i a l < 8 0 0 0 9 0 0 0 -1 ° . ° 0 0 Late 1 2 , 1 0 0 W i s c o n s i n 2 1 , 0 0 0 - M i d d , e 3 9 , 0 0 0 Wiscons in I l l e x i s t , including s t r i a t e d and polished bedrock surfaces, horns, cirques, roches moutonnees fashioned from transverse pressure ridges at the surface of lava flows, and rock steps whose general morphology i s controlled by flow thickness and i n part by o r i g i n a l flow geometry such as the p o s i t i o n of l a t e r a l levees and f r o n t a l lobes. Aretes and U-shaped v a l l e y s extend w e l l below t r e e - l i n e to a l t i t u d e s of 3200-3400 m, the l i m i t of Hueyatlaco g l a c i e r s , below which deep V-shaped canyons drain the lower flanks. Gla-c i a l t i l l covers much of the t e r r a i n near t r e e - l i n e , forming a discontinu-ous veneer that extends down to about 3000 m on high ridges but becomes buried by brown forest s o i l s and yellowish brown loam at lower a l t i t u d e s . The t i l l i s composed of angular to subrounded block- and pebblesized v o l -canic rocks i n v a r i a b l e states of degradation set i n a matrix composed of f i n e sand, s i l t , and clay. On f r e s h surfaces the matrix i s generally f i r m and uniform i n colour ranging from very l i g h t grey to white (N8-N9) or greyish orange to dark yellowish orange (10YR7/4-6/6). Localized mottling i n shades of l i g h t to moderate red (5R6/6-5/4) and pale red-purple (5RP6/2) i s due to oxidation of enclosed c l a s t s or underlying lava flows. Descrip-tions of the morphology and extent of I z t a c c i h u a t l moraines given below supplement those provided by White (1962). 2.10.1 Older T i l l s and T i l l - L i k e Sediments At the west base of I z t a c c i h u a t l , White ( i b i d . ) recognized a p a r t i a l l y eroded apron of poorly consolidated v o l c a n i c l a s t i c debris that he i n -formally termed the "older a l l u v i a l deposits'. This material i s e s s e n t i a l -l y equivalent to that described as e p i c l a s t i c volcanic breccias i n t h i s report, and contains diamictons of g l a c i a l or mudflow o r i g i n , as w e l l as stream sands and gravels. Deposits of possible g l a c i a l provenance are 112 found within volcanic breccias 5 km east of Amecameca at the mouths of A l c a l i c a n and Diamantes canyons, and i n an i s o l a t e d h i l l at San Rafael (2450 m). T i l l presumably older than that forming Nexcoalango moraines i s exposed at a l t i t u d e s of 2550-3050 m i n roadcuts along the western flanks of I z t a c c i h u a t l and at approximately 2700 m on eastern slopes. Fresh surfaces on these older t i l l s resemble t i l l s higher on the mountain; weathering produces a dark yellowish brown to reddish-brown colour (10YR4/2-10R3/4). Deposits interpreted as ground moraine have been i d e n t i f i e d by Heine (1973) at 2500 m on the east slope of I z t a c c i h u a t l and underlie a considerable thickness (45 m) of "toba' sediments. White (1962) believed that these older t i l l - l i k e deposits represented a pre-Wisconsin g l a c i a t i o n i n c e n t r a l Mexico, but the c o r r e l a t i o n with M-I moraines on La Malinche (Heine, 1973) suggests that they are Middle Wisconsin i n age. Evidence f o r even e a r l i e r g l a c i a t i o n of I z t a c c i h u a t l i s represented by a diamicton, probably a lodgement t i l l , exposed i n a stream v a l l e y 0.9 km south of Cerro Altzomoni at 3790 m. This deposit i s at l e a s t 3 m thick and underlies a layer of b a s a l t i c scoriae erupted from a proximal vent. Rotten angular to subangular blocks of dark greenish grey (5G4/1) p o r p h y r i t i c andesite up to 1 m across are contained i n a f i n e r grained c l a s t i c matrix ranging i n colour from dark to yellowish brown (5YR3/6 to 10YR5/4). No s t r i a t i o n s were observed on the surfaces of constituent blocks and no s o r t i n g i s evident. A conspicuous subhorizontal i n t e r n a l f o l i a t i o n i s defined by platy andesite c l a s t s and a weak planar parting i s l o c a l l y v i s i b l e i n the clayey matrix. The grain size d i s t r i b u t i o n of phenocrysts i n the blocks i s t y p i c a l of rocks that belong to the Older Andesites and Dacites of Llano Grande volcano and Ancestral Pies. The minimum age of this diamicton i s bracketed by overlying La Joya lavas at approximately 113 0.27 Ma (Table 2.3). If Its g l a c i a l o r i g i n i s confirmed i t w i l l represent the oldest Pleistocene t i l l deposit yet documented i n Central Mexico. 2.10.2 Nexcoalango T i l l Nexcoalango t i l l forms the oldest g l a c i a l deposit of wide extent on I z t a c c i h u a t l . Morainal topography i s s t i l l recognizable although best preserved on broad i n t e r f l u v e s between stream v a l l e y s . The type l o c a l i t y i s found at Nexcoalango s t a t i o n located at an a l t i t u d e of 3410 m between Canada del Negro and Canada Cosa Mala (referred to as San Rafael Canyon by White, 1962) about 6 km east of the v i l l a g e of San Rafael. Moraine crests are low and broad, t h e i r flanks are severely eroded, and terminal positions are marked by moraine remnants on canyon f l o o r s at a l t i t u d e s of 3015-2750 m. On eastern slopes, fresh exposures of Nexcoalango t i l l are found i n roadcuts between about 3100 m and 2800 m. These deposits are commonly ove r l a i n by yellow brown loess and more rarely by coarse g l a c i a l outwash. In the north, moraines of Nexcoalango t i l l extending down to 2800 m are p a r t i c u l a r l y w e l l preserved at the mouth of Canada Tamascatitla, 2.5 km south of the eastern extremity of Papayo lavas. Nexcoalango moraines on western slopes extend within the belt of younger Hueyatlaco moraines at a l t i t u d e s of 3800-3700 m, and two sets occur i n Barranca e l Tomicoxco 2.5 km north of San Rafael (White, 1962). C o r r e l a t i o n of the Nexcoalango t i l l with M-II moraines on La Malinche places a Late Wisconsin age on these deposits (Figure 2.7). 2.10.3 Hueyatlaco T i l l Moraines of Hueyatlaco t i l l impressed White (1962, p. 948) as "the most spectacular moraines on I z t a c c i h u a t l ' . Indeed, t h e i r morphology, 114 s i z e , and e f f e c t on p o s t - g l a c i a l drainage i s a dominant feature of the landscape between 3200 m and t r e e - l i n e . They form a zone averaging 2-4 km i n width that i s concentric about the summit region. The best preserved moraines comprise narrow winding ridges 40-50 m i n width with sharp to rounded crests that r i s e about 10-50 m above surrounding bedrock. L a t e r a l moraines show moderate g u l l y i n g of t h e i r flanks whereas terminal moraines are strongly eroded. T y p i c a l examples are found at Trancas, 6.5 km east of San Rafael; at Hueyatlaco, the type l o c a l i t y , situated i n the upper r e -aches of Diamantes canyon; at Llano Grande within the caldera collapse structure and v a l l e y rim east of Llano Chico; along the eastern perimeter of Teyotl lavas; and at the northern and southern margins of Pecho and R o d i l l a s flows that moved due east towards the Puebla Basin. At l e a s t two advances of narrow v a l l e y g l a c i e r s can be recognized: i n the west, Hueya-t l a c o t i l l extends down to approximately 3135 m i n Milpulco v a l l e y ; i n the east, terminal moraines consistently reach 3200 m and 3400-3500 m i n the older and younger advances r e s p e c t i v e l y . Outwash gravels associated with t h i s g l a c i a t i o n occur i n the lower part of Barranca E l Tomicoxco, and are l o c a l l y interbedded with finer-grained e p i c l a s t i c d e t r i t u s i n A l c a l i c a n v a l l e y at approximately 3100 m. A thick blanket of Hueyatlaco t i l l occup-ies an area of 11.5 km2 w i t h i n Llano Grande caldera. P e r i g l a c i a l lakes ponded between the caldera wall and these morainal complexes were i n f i l l e d during t h i s period with coarse stream gravels. Hueyatlaco t i l l and g l a c i a l outwash i s also found i n roadcuts near the southern margin of Papayo flows at an a l t i t u d e of 3300 m. At the southern margin of Llano Grande and southeast of Llano Chico, White (1962) proposed that Hueyatlaco moraines i n t e r f i n g e r e d with lava flows extruded from Cerro Teyotl. However, de-t a i l e d mapping reveals that although a l l Teyotl lavas have indeed been 1 1 5 g l a c i a t e d , none are interbedded with Hueyatlaco t i l l . Since K-Ar dating of Teyotl lavas indicates that these rocks are approximately 0.08 ± 0.02 Ma i n age they consequently could not have escaped older g l a c i a t i o n s at these a l t i t u d e s . C o r r e l a t i o n of M-III moraines on La Malinche with Hueyatlaco t i l l provides an age of deposition of 0.01-0.009 Ma. 2.10.4 Milpulco T i l l Moraines of Milpulco t i l l form low inconspicuous mounds with sharp to rounded crests at elevations between 3600 m and 4300 m. Angular lava blocks are r e l a t i v e l y abundant on moraine flanks which have suffered l i t t l e erosion compared to the trenching that has taken place at terminal pos-i t i o n s . An inner and outer set of terminal moraines e x i s t i n Milpulco v a l l e y , the type l o c a l i t y , at 4100 m and 3650 m re s p e c t i v e l y ; and i n T l a l t i p i t o n g o at 3800 m and 3735 m. Other moraines believed to be cor-r e l a t i v e with Milpulco t i l l are found within the glaciated crater of Pies at 4320 m, on a rock ledge (4300 m) at the contact of Pies and R o d i l l a s lavas located 2 km due east of R o d i l l a s , and on the northern slopes of Cerro T e y o t l . Moraines b u i l t on Teyotl lavas are e s p e c i a l l y w e l l preserved and occur at the northern edge of an area of hummocky ground that repres-ents the s t a b i l i z e d deposits of s o l i f l u c t i o n processes. Terminal positions of the western moraine complex l i e between 4120-4050 m whereas to the east s i m i l a r moraines extend down to 3850 m. At l e a s t one recessional moraine has been b u i l t against the inner slope of the western terminal moraine; i n the eastern complex two recessional features occur between 3 980 m and 4100 m. Stream erosion of Milpulco t i l l commonly produces a boulder concentrate such as that found at the head of T l a l t i p i t o n g o v a l l e y . Milpulco g l a c i a t i o n was o r i g i n a l l y placed i n the Late Wisconsin by 116 White (1962) but subsequent work by Heine (1973) places t h i s g l a c i a l ad-vance younger than 0.008 Ma. 2.10.5 Ayoloco T i l l Huge terminal moraines of Ayoloco t i l l and small recessional moraines behind them represent the youngest g l a c i a l deposits on I z t a c c i h u a t l . They e n c i r c l e the summit region at a l t i t u d e s of 4800-4400 m but are not found on the steep south-facing slopes of R o d i l l a s or c l i f f s east of Pecho. The largest moraines occur i n the type l o c a l i t y of Ayoloco, at the head of Ayolotepito v a l l e y 1.7 km further north; and 1 km east of Cabeza. The moraine at Ayoloco, f o r example, forms a steep-sided ridge 30-60 m high composed of pale yellowish brown to pale red (10YR6/2-10R6/2) boulder-strewn t i l l that i s completely barren of vegetation. Moraine crests are very sharp and two c r e s t l i n e s are l o c a l l y apparent. Their flanks bear d i s t i n c t i v e stone s t r i p e s but lack erosional features. Terminal positions notched by meltwater streams l i e between 4410 m and 4270 m on the west side of the volcano and at approximately 4380 m on northeastern slopes. Re-cessional moraines occur i n Ayoloco and Ayolotepito between 4500 m and 4560 m, and at s i m i l a r a l t i t u d e s west of R o d i l l a s . Ayoloco moraines were regarded as Neoglacial by White (1962) and equi-valent to the Gannett Peak stage of g l a c i a t i o n of the Rocky Mountain r e -gion. Neoglacial moraines on La Malinche have not been developed. 2.11 SUMMARY I z t a c c i h u a t l ( l a t . 19° 10.7"; long. 98° 38.5'), a major volcano i n the Trans-Mexican Volcanic Belt, i s located approximately 60 km southeast of Mexico C i t y i n the c e n t r a l part of the S i e r r a Nevada, a north-south 117 oriented mountain range that forms the southeastern margin of the Valley of Mexico. The g l a c i a t e d summit of I z t a c c i h u a t l a t t a i n s an a l t i t u d e of 5286 m, ranking i t t h i r d highest of the Mexican volcanoes a f t e r her neighbour, Volcan Popocatepetl (5452 m), situated at the southern extremity of the S i e r r a Nevada, and Pico de Orizaba (5600 m) i n the east. The base of the volcano i s not exposed but according to Schlaepfer (1968) o v e r l i e s T e r t i a r y sedimentary and volcanic formations and Cretaceous limestones. Geologic mapping of I z t a c c i h u a t l has revealed a complex his t o r y of cone construction comprising a number of coalescing and superimposed c e n t r a l volcanoes. The p r i n c i p a l vent systems are aligned NNW-SSE and suggest co n t r o l by basement lineaments. The eruptive products have a t o t a l volume of about 450 km3 and throughout the exposed part of the structure are composed of viscous lava flows with minor amounts (<1%) of p y r o c l a s t i c breccias. K-Ar dating indicates that cone construction began p r i o r to 0.9 Ma and continued u n t i l s h o r t l y before Late Pleistocene (Wisconsin) g l a c i -ation when I z t a c c i h u a t l became dormant. K-Ar dates reported by Cantagrel et a l . (1981) on volcanic rocks from Nevado de Toluca, 100 km east of I z t a c c i h u a t l , suggest that the growth of major volcanoes within t h i s part of the Trans-Mexican Volcanic Belt commenced as early as 1.7 Ma (Nixon et a l . , i n press). Chemical analyses show that the lavas are predominantly c a l c - a l k a l i n e andesites and dacites (58-66 wt % SiO,) and distinguished by r e l a t i v e l y high Mg/(Mg+i;Fe). The volcanic stratigraphy of I z t a c c i h u a t l may be subdivided into two major rock groups named the Older Volcanic Series (OVS) and the Younger Volcanic Series (YVS) which are distinguished on the basis of phenocryst mineralogy and textures. Fine- to medium-grained augite-hypersthene ande-s i t e s and dacites with s e r i a t e textures characterize OVS rocks whereas 118 hornblende dacites with h i a t a l textures and minor amounts of b i o t i t e and quartz are volumetrically dominant within the YVS. In p a r t i c u l a r , the mean grain s i z e of plagioclase phenocrysts increases from 2-2.5 mm i n the OVS to 3.5 mm i n YVS rocks. Hornblende-bearing lavas are also more abundant i n the l a t t e r stages of OVS a c t i v i t y but phenocrysts of quartz and b i o t i t e are lacking. A n d e s i t i c lavas with d i s e q u i l i b r i u m phenocryst assemblages com-p r i s i n g f o r s t e r i t i c o l i v i n e + hypersthene + augite + hornblende + p l a g i o -clase + ilmenite + magnetite + b i o t i t e + quartz are f a i r l y common i n the YVS and appear to owe t h e i r o r i g i n to pre-eruptive mixing of hornblende dacite and b a s a l t i c magmas. Compositionally heterogeneous lavas with centimeter-scale layering of mafic and s i l i c i c groundmasses occur within each of the major rock groups but are most abundant i n the YVS succession. OVS lavas and p y r o c l a s t i c rocks constitute approximately two-thirds (300 km 3) of the volume of I z t a c c i h u a t l . They may be subdivided i n t o a group of older Andesites and Dacites (59-66 wt % S i 0 2 ) that form the major volcanic e d i f i c e s of Llano Grande volcano (220 km 3) i n the north and Ancestral Pies (<100 km 3) i n the south, and a group of younger lavas em-placed on the flank of these structures during the waning stages of cone growth (Older Flank A c t i v i t y ) . The summit of Llano Grande volcano i s occupied by a 4.5 km diameter caldera whose glaciated western margin i s well-exposed and eastern part buried by younger lavas. The f l o o r of the caldera i s formed by ponded g l a c i a l outwash and alluvium that form a p l a i n (Llano Grande) for which the volcano i s named. The Ancestral Pies cone i s b u i l t on the southern flank of Llano Grande volcano and i t s lavas are best exposed at the southwestern margin of I z t a c c i h u a t l and west of La Joya. This cone formerly reached an a l t i t u d e i n excess of 4500 m, some 500 m higher than the maximum probable a l t i t u d e of Llano Grande p r i o r to caldera 119 c o l l a p s e . Late stage p a r a s i t i c vents opened on the northwestern flank of Llano Grande near Cerro La Trampa and produced 9 km3 of hornblende dacite to r h y o l i t i c (65-72 wt% SiO z) lavas which flowed northwards and westwards towards the Valley of Mexico. A viscous hypersthene rhyodacite (68.2 wt% S i 0 2 ) flow (0.3 km 3) that forms Cerro Tlacupaso was extruded on the nor-theastern flank and may have originated form a vent located on pre-caldera r i n g - f r a c t u r e s . An unusual bronzite-phyric andesite (59.6 wt% S i 0 2 ) flow exposed at Trancas high on the western flank of Llano Grande volcano i s grouped with Older Andesites and Dacites but may have erupted from a nearby p a r a s i t i c vent. K-Ar dates of 0.90-0.58 Ma f o r the lavas of Llano Grande volcano correspond with a mature stage of cone development. YVS lavas and p y r o c l a s t i c rocks mark a second major phase of cone construction which began at approximately 0.6 Ma. Mu l t i p l e vents oriented along a NNW-SSE - trending lineament became active during the early stages of cone growth. The stratigraphy may be subdivided i n t o a volumetrically dominant (=150 km 3) group of Younger Andesites and Dacites (58-66 wt% S i 0 2 ) associated with the p r i n c i p a l vent systems, and p a r a s i t i c cones and flows belonging to a phase of Younger Flank A c t i v i t y . The younger Andesites and Dacites are further subdivided according to vent l o c a t i o n into Summit Series lavas and p y r o c l a s t i c rocks (125 km 3) that form the modern peaks of Cabeza (5146 m), Pecho (5286 m), and Ro d i l l a s (5100 m) from north to south r e s p e c t i v e l y , and s i m i l a r l i t h o l o g i e s further south named for the lower peak of Pies (4703 m). A basal flow of the Summit Series has been dated at 0.58 Ma and young Pies lavas y i e l d dates of 0.41-0.34 Ma. Summit Series flows inundated the caldera of Llano Grande and flowed east and west across the eroded flanks of Llano Grande volcano to the 120 Puebla Basin and Valley of Mexico r e s p e c t i v e l y . The g l a c i a t e d remnants of two i s o l a t e d plug domes, Los Yautepemes (4250 m) and E l S o l i t a r i o (4210 m), occur on the northwestern flanks of Cabeza. Pies lavas and p y r o c l a s t i c breccias marked renewed growth of Ancestral Pies and buried the eastern ha l f of t h i s ancient structure. This e f f u s i v e episode was culminated by a cataclysmic ( P l i n i a n ? ) eruption that produced a 1.5 km diameter summit crater open on i t s eastern s i d e . This feature may have been formed by sector collapse of the summit region s i m i l a r to the May 1980 eruption of Mt. St. Helens. Products of t h i s eruption have since been removed by g l a c i a t i o n and covered by thick loess deposits at the eastern margin of the Si e r r a Nevada. Late stage a c t i v i t y on the northern flank of I z t a c c i h u a t l resulted i n the extrusion of viscous dacite flows (5km 3) from vents located at Cerro Teyotl (4660 m), and the southern flank was covered by b a s a l t i c to andesi-t i c (52-62 wt% S i 0 2 ) lavas and s c o r i a deposits comprising the La Joya lava f i e l d (>0.15 km 3). The o l i v i n e - p h y r i c La Joya lavas dated at 0.27 Ma erupted from vents located near the type l o c a l i t y of La Joya, and another vent produced bedded scoriae and bombs preserved 2.5 km further south near Altzomoni. La Joya lavas with t h e i r d i s t i n c t i v e platy j o i n t i n g can be traced southwards where they form thick intracanyon flows ponded against a n d e s i t i c lavas of Popocatepetl. Teyotl hornblende dacites (63-65 wt% S i 0 2 ) dated at 0.08 Ma rest on flows from Cabeza and Pecho. They are e a s i l y distinguished from older lavas by t h e i r steep-sided flow lobes and phenocryst-poor glassy textures. The youngest lavas of Pecho and Ro d i l l a s conceivably post-date the eruption of Teyotl dacite. The p y r o c l a s t i c breccias of I z t a c c i h u a t l are commonly preserved i n palaeovalleys and consist of angular monolithologic blocks and j o i n t 121 columns (>2 m i n length) loosely cemented by f i n e l y comminuted rock (<10%). Their o r i g i n i s att r i b u t e d to collapse of advancing viscous flows fronts such as witnessed i n recent eruptions of Volcan Colima. Hydrothermal a l t e r a t i o n of I z t a c c i h u a t l lavas i s l o c a l i z e d and most pronounced i n the crater and cirque f l o o r s north and west of Pies. Volcanic rocks of the S i e r r a Nevada not associated with the development of I z t a c c i h u a t l include the p o s t - g l a c i a l lavas of Cerro Papayo, the g l a c i -ated cone of Cerro I z t a l t e t l a c , and the Rio F r i o pumice deposit at the northern edge of the map area, and a p o s t - g l a c i a l flow at Buenavista oc-cupying the divide between the lavas of I z t a c c i h u a t l and Popocatepetl. The Rio F r i o pumice deposit i s exposed i n a quarry at the crest of the Si e r r a Nevada. It comprises a lower p y r o c l a s t i c flow unit o v e r l a i n by t h i n l y bedded a i r f a l l material. Pumice l a p i l l i and obsidian fragments contain phenocrysts of quartz, b i o t i t e , and plagioclase and have r h y o l i t i c compositions (=76 wt% S i 0 2 v o l a t i l e - f r e e ) . This deposit i s c l e a r l y post-g l a c i a l but the l o c a t i o n of the source vent i s not known. The hornblende-bearing lavas of Papayo are d a c i t i c (62-66 wt% S i 0 2 ) and form an extensive flow complex (21 km 3) at the northern margin of the Si e r r a Nevada. The 1 km-diameter dacite dome of Cerro Papayo (3630 m) marks the vent. Early lavas are strongly p o r p h y r i t i c and become more g l a s s - r i c h with decreasing age. Disequilibrium phenocryst asemblages s i m i l a r to those occurring i n I z t a c c i h u a t l lavas are also found i n the eruptive products of Papayo. According to co r r e l a t i o n s made between the g l a c i a l t i l l deposits covered by Papayo flows and 1''C-dated moraine sequ-ences of La Malinche, the eruption of Papayo dacite i s younger than 0.012 Ma. Cerro I z t a l t e t l a c (3290 m) forms the glaci a t e d remnant of an an d e s i t i c 122 (=56 wt% S i 0 2 ) cone enveloped by younger Papayo lavas. Disequilibrium phenocryst assemblages are also conspicuous i n these o l i v i n e - p h y r i c lavas. The Buenavista dacite (62.4 wt% S i 0 2 ) flow (0.2 km 3) erupted from a concealed vent located at i t s western end and moved eastwards towards the Puebla Basin. The lava i s glassy and contains sparse phenocrysts of plagioclase, hypersthene, oxyhornblende, and quartz with coronas of augite. Late Quaternary volcanic rocks of the Valley of Mexico, known as the Chichinautzin Group, occur i n the southwestern part of the map area near Amecameca. The three small cones investigated are composed of t h i n l y bedded glassy scoriae and blocks with chemical compositions ranging from andesite to dacite (57-66 wt % S i 0 2 ) . Their craters have been breached by erosion and the degree of degradation implies apparent ages of 0.04-0.02 Ma (Bloomfield, 1975). Recent deposits of r h y o l i t i c to d a c i t i c ash and pumice that blanket the slopes of I z t a c c i h u a t l were sampled i n s t r a t i g r a p h i c sections at the sou-thern margin of the map area. A i r f a l l tephra layers are interbedded with palaeosols, loess, coarse f l u v i a l deposits, and reworked pumice beds. The source of the tephra can be traced to Volcan Popocatepetl. Pumice also forms a minor constituent of e p i c l a s t i c volcanic breccias of l a h a r i c or g l a c i a l o r i g i n best exposed along the western margin of I z t a c c i h u a t l . Thickly bedded loess deposits (or 'toba') form a more extensive blanket at the margins of the S i e r r a Nevada. The chronology of Late Pleistocene g l a c i a l moraines and t i l l s i d e n t i f -i e d by White (1962) on the western flanks of I z t a c c i h u a t l has been extended i n t h i s study to g l a c i a l deposits mapped on the eastern side of the v o l -cano, and correlated with 1''C-dated moraines of La Malinche (Heine, 1973). The oldest t i l l - l i k e deposit i s found near Altzomoni, r e s t i n g d i r e c t l y 123 beneath La Joya lavas and scoriae, and i s thus older than 0.27 Ma according to a K-Ar date on La Joya andesite. The oldest t i l l s and t i l l - l i k e s e d i -ments recognized by White (1962) occur at the entrances to A l c a l i c a n and Diamantes canyons at the southwestern perimeter of I z t a c c i h u a t l , and cor-r e l a t e with Middle Wisconsin (0.039-0.021 Ma) M-I moraines on La Malinche. Nexcoalango t i l l i s the oldest g l a c i a l deposit of wide extent on I z t a c c i h u a t l and correlates with Late Wisconsin (0.012 Ma) M-II moraines. Hueyatlaco t i l l forms the largest and most extensive terminal moraine complexes i n which at least two advances can be recognized. They are represented on La Malinche by M-III (0.010-0.009 Ma) moraines. Milpulco t i l l forms low moraine complexes equivalent to the youngest M-IV moraines of La Malinche. Neoglacial moraines of Ayoloco t i l l e n c i r c l e the summit region of I z t a c c i h u a t l . 124 CHAPTER 3 PETROLOGY OF THE YOUNGER ANDESITES AND DACITES OF IZTACCIHUATL  VOLCANO, MEXICO. 1. DISEQUILIBRIUM PHENOCRYST ASSEMBLAGES AS INDICATORS OF MAGMA CHAMBER PROCESSES 3.1 Introduction Detailed p e t r o l o g i c studies of orogenic volcanoes have recognized that many of the mineralogical and chemical a t t r i b u t e s of c a l c - a l k a l i n e rocks appear to require mixing of magmas of diverse composition (Larsen et a l . , 1938; Eichelberger, 1975; Heiken and Eichelberger, 1980; Anderson, 1976; Sakuyama, 1979, 1981; Gerlach and Grove, 1982; Kay and Kay, 1985). However, i t i s s t i l l unclear how magmas of contrasting composition and physic a l properties manage to mix i n view of recent advances i n f l u i d dynamic modelling of magma chambers (Sparks et a l . , 1984; McBirney et a l . , 1985). This study presents the mineralogical evidence f o r mixing of c a l c -a l k a l i n e b a s a l t i c and d a c i t i c magmas at I z t a c c i h u a t l volcano, Mexico, and evaluates the nature and timing of the processes involved. A complementary i n v e s t i g a t i o n (Chapter 4) addresses the geochemical evolution of Iz t a c c i h u a t l magma chambers. Disequilibrium phenocryst assemblages i n the Younger Andesites and Dacites of I z t a c c i h u a t l provide an excellent record of episodic r e p l e n i s h -ment, mixing, and c r y s t a l l i z a t i o n events occurring i n c r u s t a l magma chamb-ers. The compositions and textures of phenocrysts i n mixed lavas are used to determine the mineralogical and thermal c h a r a c t e r i s t i c s of end-member 125 components and the chemical and physical interactions that take place on mixing. Magma mixing i s driven by convective turbulence generated by large differences i n temperature and composition between end-members. The mixing mechanism involves: 1) rapid homogenization of r e s i d u a l melt compositions by thermal erosion and d i f f u s i v e t r a n s f e r ( l i q u i d blending); 2) a s s i m i l -ation of phenocrysts derived from the low-temperature end-member; and 3) dynamic f r a c t i o n a l c r y s t a l l i z a t i o n of rapidly evolving hybrid l i q u i d s i n the turbulent boundary layer separating b a s a l t i c and d a c i t i c magmas. Calculations of phenocryst residence times i n mixed magmas ind i c a t e that i n f l u x of b a s a l t i c magma from depth l i k e l y t riggers e f f u s i v e a c t i v i t y . 3.2 General Geology and Sampling I z t a c c i h u a t l (5286 m), a major Quaternary volcano i n the c e n t r a l part of the Trans-Mexican Volcanic Belt, i s located 60 km southeast of Mexico Cit y i n the S i e r r a Nevada, a volcanic mountain range that forms the southeastern margin of the Valley of Mexico. The volcano i s composed p r i n c i p a l l y of lava flows and minor amounts of p y r o c l a s t i c breccias (<1%) of c a l c - a l k a l i n e andesite and dacite (58-66 wt% Si-C^ with an estimated 3 t o t a l volume of approximately 450 km . The geology of I z t a c c i h u a t l and the northern S i e r r a Nevada has been described i n Chapter 1 and i s summarized below. A generalized geologic map and cross-section i s presented i n Figure 3.1. The volcanic stratigraphy of I z t a c c i h u a t l i s composed of two main eruptive sequences. The e a r l i e s t lavas and p y r o c l a s t i c rocks, or Older Volcanic Series, comprise f i n e - to medium-grained augite-hypersthene ande-s i t e s and dacites, the Older Andesites and Dacites, that form Llano Grande Figure 3.1: Generalized geologic map and cross-section of I z t a c c i h u a t l volcano and the northern S i e r r a Nevada, Mexico. Black squares represent K-Ar sample l o c a l i t i e s ; dates are given i n Ma. Key to map units: Older Volcanic Series of I z t a c c i h u a t l : Older Andesites and Dacites: Ga = Llano Grande volcano; APa = Ancestral Pies cone; APGa = un d i f f e r e n t i a t e d lavas of Llano Grande and Ancestral Pies; Older Flank A c t i v i t y : LTa = La Trampa flows; Trd = Tlacupaso rhyodacite flow; Younger Volcanic Series of I z t a c c i h u a t l : Younger Andesites and Dacites: Pa = Pies lavas and p y r o c l a s t i c breccias; Sa = Summit Series lavas and p y r o c l a s t i c breccias; Younger Flank A c t i v i t y : Jba = La Joya b a s a l t i c to an d e s i t i c flows and scoriae; Td = Teyotl dacite flows; S i e r r a Nevada and Valley of Mexico: Zba = I z t a l t e t l a c b a s a l t i c andesite flows and scoriae (Pre-Wisconsin g l a c i a t i o n ) ; Yd = Papayo dacite flows ( p o s t - g l a c i a l ) ; Bd = Buenavista dacite flow ( p o s t - g l a c i a l ) ; Vbr = e p i c l a s t i c volcanic breccias; Qal = alluvium, f l u v i o g l a c i a l deposits, loess, and a i r f a l l pumice; Ch = Chichinautzin Group cones. Stars i n d i c a t e source vents and dashed l i n e s volcanic craters or calderas. 19' OS' \-IZTACCIHUATL Pecho Cabaza Rodl l lae B m Popocatepet l - . 5 9 0 0 128 and Ancestral Pies volcanoes (Figure 3.1), and d a c i t i c to r h y o l i t i c flows erupted from p a r a s i t i c vents located on the northern flank of Llano Grande (Older Flank A c t i v i t y ) . The summit of Llano Grande i s crowned by a 4.5 km diameter caldera floored by g l a c i a l outwash and alluvium, and p a r t l y buried by younger lavas. K-Ar dating has established that these cones had already reached maturity by 0.9 Ma, and eruptions continued i n t e r m i t t e n t l y u n t i l approximately 0.6 Ma. C o r r e l a t i o n of K-Ar dated volcanic rocks forming the substructure of Nevado de Toluca, 100 km east of I z t a c c i h u a t l , suggests that the construction of major c a l c - a l k a l i n e volcanoes i n t h i s part of the Trans-Mexican Volcanic Belt began at about 1.7 Ma (Cantagrel et a l . , 1981; Nixon et a l . , i n press). Lavas and p y r o c l a s t i c breccias of the Younger Volcanic Series mark a second major phase of cone construction that commenced at approximately 0.6 Ma following a period of erosion of the older volcanic e d i f i c e s . The p r i n c i p a l vents l i e along a NNW-SSE oriented lineament. The Younger Volcanic Series i s composed of a group of Younger Andesites and Dacites, the subject of t h i s i n v e s t i g a t i o n , which erupted from the main vent system, and p a r a s i t i c cones and flows (Younger Flank A c t i v i t y ) emplaced on the northern (Teyotl) and southern (La Joya) flanks of I z t a c c i h u a t l . The Younger Andesites and Dacites are coarsely p o r p h y r i t i c lavas d i s -tinguished from most older eruptive products by the presence of amphibole, b i o t i t e , and quartz phenocrysts. They are subdivided i n t o Summit Series rocks that form the modern peaks of Cabeza (5146 m), Pecho (5286 m), and R o d i l l a s (5100 m), and s i m i l a r lavas and p y r o c l a s t i c rocks that form the more southerly peak of Pies (4703 m). Summit Series flows buried much of Llano Grande volcano and Pies lavas inundated the eastern half of the Ancestral Pies substructure. A powerful Figure 3.2: Geologic map of the summit region of I z t a c c i h u a t l showing sample l o c a t i o n s . Sample s i t e s within s t r a t i g r a p h i c traverses are connected and numbered sequentially i n order of increasing age. Not shown are sample l o c a l i t i e s f o r PE-11, c o l l e c t e d 3.5 km northeast of Cerro Teyotl ( l a t . 19° 13' 38"; long. 98° 36' 57"); b a s a l t i c s c o r i a (IZ-839) c o l l e c t e d j u s t south of Altzomoni at the northern t i p of an o u t l i e r of andesite flows ( l a t . 19° 07' 25"; long. 98° 39' 30") shown i n Figure 3.1; and a b a s a l t i c flow (MXC-7) from Dos Cerros ( l a t . 19° 08' 15"; long. 98° 56' 40") 16 km west of Amecameca. 19° 12' 19° 10' 19° 08' -Vbr V 4 4 C.t> > 98' 42' Glacial till Rock talus Glaciofluvial deposits and peat 98" 40' Plug dome Sample localities 98° 38' 98° 36 Glacial moraines (and crest lines) Neoglacial (Ayoloco) (<77> Late Wisconsin (Milpuico) Late Wisconsin (Hueyatlaco) explosive eruption demolished the upper part of the Pies cone leaving a 1.5 km diameter eastward-gaping summit crat e r . This eruption post-dates young Pies flows that have yielded K-Ar dates of 0.41-0.34 Ma. Emplacement of the Summit Series continued u n t i l shortly before Late Pleistocene (Wisconsin) g l a c i a t i o n when I z t a c c i h u a t l became dormant. Samples used i n th i s study and i n a complementary i n v e s t i g a t i o n of the geochemistry of the Younger Andesites and Dacites (Chapter 4) were co l l e c t e d s t r a t i g r a p h i c a l l y from each of the main vent sequences. Sample locations and s t r a t i g r a p h i c sections are shown i n Figures 3.2 and 3.3. Additional samples relevent to these studies include b a s a l t i c scoriae preserved beneath outcrops of La Joya flows, erupted at about 0.27 Ma on the southwestern flank of the Pies cone, and b a s a l t i c lavas of the Chichinautzin Group i n the Valley of Mexico. The l a t t e r rocks are important because they represent the closest natural analogs, i n terms of phenocryst mineralogy and bulk composition, of the b a s a l t i c component involved i n magma mixing processes occurring within I z t a c c i h u a t l magma chambers. 3.3 Lava Petrography The petrographic c h a r a c t e r i s t i c s of the Younger Andesites and Dacites of I z t a c c i h u a t l are summarized i n Figure 3.3 and modal analyses are presented i n Table 3.1. The lavas may be grouped into o l i v i n e - b e a r i n g and o l i v i n e - f r e e v a r i e t i e s depending on the presence or absence of o l i v i n e phenocrysts. O l i v i n e - f r e e lavas include hornblende (Hb) dacites (63-66 wt% SiO^) and clinopyroxene-orthopyroxene (Cpx-Opx) dacites (62-64 wt% SiO^) with minor amphibole. Olivine-bearing andesites and dacites (58-64 wt % SiO„) are characterized by d i s e q u i l i b r i u m phenocryst assemblages inv o l v i n g Figure 3.3: Phenocryst mineralogy and petrographic characteris t i c s of the Younger Andesites and Dacites of I z t a c c i h u a t l v o l cano. 133 Altitude Sample Qz Bi Hb Opx Cpx Ol PI Comments CABEZA 4820 m C - 1 © • • • 4700 C - 2 0 • • • 4680 C - 3 © • • • 4530 C - 4 © o • • • 1 smal l m e t a - p e l i t e xanol i th 4500 C - 5 o • • e 4390 C - 6 © • • • ac icu la r Hb m i c r o p h e n o c r y s t s PECHO 4860 m P E - 1 © • e • 4800 P E - 2 © • • o o small s k e l e t a l Ol and large a n h e d r a l Ol lack ing Px r e a c t i o n rims 4700 P E - 3 © o • • • ?weak c o m p o s i t i o n a l banding in g l a s s y g r o u n d m a s s 4620 P E - 4 • • 9 c r y s t a l - r i c h 4610 P E - 5 o • o A 9 soma s k e l e t a l Of with Opx r e a c t i o n rims 4190 P E - 6 • • • c o l l a p s e d ves icu la r texture 4080 P E - 7 o • • • o • s u b h e d r a l Ol lack ing Px r e a c t i o n rims 1 xenol l th -Ol with Opx r ims, PI. Cpx . Hb. and minor brown g l a s s 4010 P E - 8 © • • • 4000 P E - 9 ® • • • 4530 P E - 1 0 • • o • © 3600 P E - 1 1 • • o • abundant micronor i te xenol i ths with minor C p x , Hb. and opaque o x i d e s 3400 P E - 1 2 o • T o 9 3300 P E - 1 3 • • 9 Closed symbols: crystals relatively abundant Open symbols: crystals relatively scarce © Quartz crystals with clinopyroxene reaction rim O fl Olivine with wel l -developed orthopyroxene reaction rim Q Heterogeneous Quartz population: crystals with and without clinopyroxene reaction rim A A Heterogeneous Olivine population: crystals with or without orthopyroxene reaction rim • Heterogeneous Orthopyroxene population: colourless to weakly pleochroic bronzite(with Cr -sp ine l inclusions) and strongly pleochroic hypersthene(with magnetite inclusions) © Amphibole pseudomorphed by orthopyroxene, c l inopyroxene, p lag ioc lase , and F e - T i ox ides g Olivine partly or completely altered to iddingsite, clay minerals, and chlorophaeite Altitude Sample Qz Bi Hb Opx Cpx Ol PI Comments WEST RODILLAS 4800 m R-1 © • • • 4780 R-2 © • • • 4650 R-3 • • • abundant Opx microlites 4640 R-4 © • • • abundant Opx microlites and Hb microphenocrysts 4550 4520 R-5 R-6 © © o • • 9 9 • • • • I some skeletal Ol and adhering basalt ic glass weak compositional banding skeletal Ol with incipient or completely lacking Px rims 4500 R-7 © o • 9. 9 4480 R-8 ® 9 O 9 Cpx microphenocrysts commonly in clots 4330 R-9 © • 9 • 9 few skeletal Of - no Px rims 4200 R-10 © 9 o 9 Cpx microphenocrysts commonly in clots 4190 R-1 1 0 o e T • 9 Bronzite clots 3800 R-12 © • • • 9 skeleta l Ol lacking Px react ion rims EAST ROD ILLAS 4910 m R-13 © • • 9 crys ta l -poor 4830 R-14 • • crysta l -poor 4770 R-1 5 © • • 0 o 9 subhedral Ol intergrown with Cpx 4690 R-1 6 o T o 9 ?heterogeneous Opx population 4280 R-1 7 o • • 9 PIES 4630 m P-1 o 9 • o • 9 few skeletal Ol with incipient Px reaction rims 4610 4600 P-2 P-3 o © • • • o • * xenolith comprising PI, Cpx, Opx, and vesiculated brown glass PI microlites adhering to some Ol. No skeletal morphologies 4590 P-4 © o • • • euhedral Ol with thin Px rims 4450 P-5 • • • • 4400 P-6 o • • • • 4380 P-7 • • • some skeletal Ol 4220 P-8 • • a • • euhedral Ol lacking Px reaction rims 4100 P-9 • • • • some skeletal Ol with thin Px rims 4710 P-10 © o • • • 4700 P-1 1 • • o • 4400 4100 P-12 P-13 © o • • • © • I xenolith of Ol with Opx reaction rims, Opx, Cpx. PI and minor Hb and Oxides 4050 P-14 • 9 9 acicular Hb microphenocrysts 3800 P-1 5 © • 9 • 9 skeletal Ol lacking Px rims 3600 P-16 • 9 9 acicular Hb microphenocrysts Table 3 . 1 : Modal Analyses o f I a t a c c i h u a t l Lavas (volume %, v e s i c l e - f r e e ) Lava Sample Phenocrysts Type Plag Hb Opx Cpx 01 Bi Qz Ox Gmass +Mph Pecho HbD PE-1 13. .5 2.2 1.8 - t r t r 82.5 1398 HbD PE-6 30. .0 4.7 5.4 - - 0.4 59.5 1162 PxD PE-8 13. .3 1.8* 2.7 1.0 - 0.2 81.0 1230 PxD PE-9 9. ,3 1.0* 2.5 1.6 - 0.5 85.1 1418 R o d i l l a s HbD R-1 13.1 1.5 1.6 - - - t r 0.3 83.5 972 HbD R-2 14.4 4.2 2.6 - - - t r t r 78.8 1131 HbD R-3 14.4 3.6 2.3 - - - - 0.4 79.3 1115 HbD R-4 22.7 3.1 1.5 - _ - t r 0.2 72.5 1010 MLT-I R-5 13.2 3.7 1.3 - 4.0 - t r 0.4 77.4 916 MLT-I R-6 17.0 1.8 3.1 t r 0.9 t r t r 0.5 76.7 1173 MLT-I R-9 11.1 2.0 2.2 - 4.8 - t r 0.2 79.7 982 MLT-I R-12 13.1 3.4 4.0 - 2.2 0.3 0.3 76.7 1229 HbD R-13 5.7 0.6 0.7 - - - t r 0.1 92.9 828 Hbd R-14 7.8 1.4 1.4 - - - - 0.1 89.3 958 HbD R-17 11.4 1.7 0.9 - - - t r 0.2 85.8 1235 MLT- I P-1 12.7 2.9 MLT- I P-3 16.1 1.3 MLT- I P-4 18.7 2.8 MLT- II P-7 + 23.5 3.5 MLT- II P-8 + 21.2 5.2 MLT- I P-9 19.6 1.1 PxD P - l l 25.1 2.3 MLT- I P-15 9.5 0.7 HbD P-16 33.6 2.1 P i e s 1.7 t r 2.2 t r 1.6 - 0.6 t r 1.7 - 1.6 t r 3.4 - 1.1 2.6 - 0.9 4.8 - 1.7 7.7 t r 3.5 - 4.6 2.9 0.3 80.2 1283 _ 0.2 80.2 1116 t r 0.1 75.1 1268 _ 0.3 68.2 1217 0.3 69.8 1051 _ 0.6 72.2 1315 0.8 64.1 993 t r 0.4 81.3 1069 - 0.6 60.8 1572 Abbreviations f o r lava types are as f o l l o w s : HbD = hornblende d a c i t e ; PxD = clinopyroxene-orthopyroxene d a c i t e ; MLT-I = Mixed Lava Type I ; MLT-II = Mixed Lava Type I I . Plag = p l a g i o c l a s e ; Hb = hornblende; Opx = orthopyroxene; Cpx = clinopyroxene; 01 = o l i v i n e ; Bi = b i o t i t e ; Qz = quartz; Ox = Fe-Ti oxides (0.4-0.2mm); Gmass+Mph = groundmass plus microphenocrysts (<0.4inm); t r = trace amount; n = No. of points counted. * Hb phenocrysts pseudomorphed by Cpx + Opx + Fe-Ti oxides. "•"Olivine - r i c h m i c r o x e n o l i t h s . coexisting quartz and f o r s t e r i t i c o l i v i n e . These rocks are referred to as Mixed Lavas because they contain an unequilibrated mixture of phenocrysts derived from p a r t i a l l y c r y s t a l l i n e Hb dacite and b a s a l t i c magmas. A com-plete gradation i n texture, mineralogy, and composition e x i s t s among these lava types but petrographic features t y p i c a l of each group are given below. 3.3.1 Hornblende Dacites Hb dacites contain phenocrysts of plagioclase, hypersthene, and hornblende, trace amounts of b i o t i t e and quartz, and microphenocrysts of ilmenite and magnetite set i n a colourless to pale brown r h y o l i t i c glass with m i c r o l i t e s of plagioclase, orthopyroxene, clinopyroxene, and opaque oxides. Groundmass textures are h y a l o p i l i t i c i n which a u g i t i c pyroxene i s modally subordinate to orthopyroxene. Plagioclase i s the dominant phenocryst, forming 7-40 v o l . % of the mode (Table 3.1), and i s primarily responsible f o r the c h a r a c t e r i s t i c h i a t a l texture of these rocks. Modal proportions of hypersthene (0.7-5.4 v o l . %) and hornblende (0.6-4.7 v o l . %) are s i m i l a r and Opx/Hb ra t i o s range from 0.5 to 1.4 with a mean of 0.9. The large modal v a r i a t i o n of plagioclase phenocrysts i n these lavas cannot represent d i f f e r e n t i a l accumulation since condensed bulk compositions are uniform. No systematic v a r i a t i o n s are evident between the r e l a t i v e proportions of phenocrysts and the degree of c r y s t a l l i z a t i o n but a strong negative c o r r e l a t i o n (r=-0.93) exists between the modal abundance of plagioclase and amphibole, and a weaker c o r r e l a t i o n (r=-0.57) i s evident between plagioclase and orthopyroxene. These co r r e l a t i o n s probably r e f l e c t the s e n s i t i v i t y of plagioclase to varia t i o n s i n PH 20' Hb dacites occur i n each of the main vent sequences (Figure 3.3) 137 i n c l u d i n g Cabeza (CI to C6), Pecho (PE-1, -3, -4, -6, -13), Rodillas (R-1 to R-4, R-7, -13, -14, -17), and Pies (P-10, -12, -14, -16). 3.3.2 Cpx-Opx Dacites Cpx-Opx dacites are p o r p h y r i t i c lavas that share the h i a t a l textures of Hb dacites. They contain phenocrysts of plagioclase, hypersthene, augite, and minor amphibole, and Fe-Ti oxide microphenocrysts enclosed i n a pale brown to grey h y a l o p i l i t i c groundmass studded with m i c r o l i t e s of plagioclase, orthopyroxene, clinopyroxene, and opaques. These lavas are distinguished petrographically from t y p i c a l Hb dacites by the absence of quartz and b i o t i t e , the occurrence of augite phenocrysts, and modal r a t i o s of (0px+Cpx)/Hb > 2. Amphibole i s present i n a l l these lavas but i s gene-r a l l y pseudomorphed by pyroxene-rich breakdown products or opaque oxides. T y p i c a l Cpx-Opx dacites occur among older Pecho lavas (PE-8, -9, -11, -12) and In R o d i l l a s (R-8, -10, -16) and Pies (P-2 and P - l l ) eruptive sequences. Included within this group are lavas containing sparse augite and un-decomposed oxyhornblende and quartz phenocrysts (PE-11, -12, R-16, and P - l l ) . The l a t t e r rock-types are t r a n s i t i o n a l towards Hb dacite i n that pyroxenes remain dominant i n the mode. 3.3.3 Mixed Lavas The d i s e q u i l i b r i u m phenocryst assemblages of mixed lavas comprise f o r s t e r i t i c o l i v i n e + orthopyroxene + augite + plagioclase + amphibole + b i o t i t e + quartz plus microphenocrysts of ilmenite and magnetite. A var i e t y of reaction textures described below occur at grain boundaries. The abundance of o l i v i n e phenocrysts and degree of development of reaction textures i s used to a r t i f i c i a l l y subdivide mixed lavas into two basic 138 categories: Mixed Lava Type I (MLT-I) contain r e l a t i v e l y abundant o l i v i n e phenocrysts (0.5-5 v o l . %; Table 3.1) which commonly lack evidence of reaction with host l i q u i d or support very t h i n reaction rims of ortho-pyroxene; Mixed Lava Type II (MLT-II) has trace amounts of o l i v i n e (one or two c r y s t a l s per t h i n section except for samples with o l i v i n e - r i c h micro-xenoliths) and orthopyroxene reaction rims are well-developed. It i s useful to further subdivide MLT-I in t o lavas that contain greater than 2 v o l . % (MLT-IA) and less than 2 v o l . % (MLT-IB) o l i v i n e . The groundmass of MLT-I generally constitutes an o p t i c a l l y uniform dark grey oxide-charged glass r i c h i n m i c r o l i t i c plagioclase and granular to prismatic augite, orthopyroxene, and Fe-Ti oxides. Rarely, MLT-I's exhibit subtle but d i s t i n c t compositional heterogeneity i n the groundmass (e.g. P-4 and R-5) yet there i s no d i s c e r n i b l e difference i n micro-phenocryst or phenocryst populations. One spectacular example of compo-s i t i o n a l l y heterogeneous lava occurs at the flow-banded margin of a young Pecho flow where Hb dacite and mixed lava form centimeter-scale l a y e r i n g . Both the abundance of opaques and the Cpx/Opx r a t i o of the groundmass decrease concomitantly with the proportion of o l i v i n e i n mixed lavas. MLT-I's have a n d e s i t i c (58-62 wt% SiO^) compositions whereas MLT-II are a n d e s i t i c to d a c i t i c i n composition (61-64 wt % SiO^) and thus t r a n s i t i o n a l i n mineralogy, texture, and bulk composition between MLT-I and Hb da c i t e s . An important difference between these mixed lavas i s that the composition of MLT-I's l i e along binary mixing l i n e s between Hb dacite and b a s a l t i c magmas whereas MLT-II's exhibit no such control (Chapter 4). The d i s t r i b u t i o n of mixed lavas among the p r i n c i p a l vent sequences i s not uniform. MLT-I's are r e s t r i c t e d to Pies (P-1, -3, -4, -9, -15) and Rodillas (R-5, -6, -9, -12) flow sequences whereas MLT-II's are more 139 widespread occurring within Pies (P-5 to P-8 and P-13), Rod i l l a s ( R - l l , -15) and Pecho (PE-2, -5, -7) stratigraphy. 3.3.4 B a s a l t i c Rocks La Joya s c o r i a sample IZ-839 contains o l i v i n e phenocrysts (12 v o l . %) up to 1.5 mm i n length set i n a dark grey-brown v e s i c u l a r glassy groundmass enclosing flow-aligned plagioclase laths, pale green to brownish granular augite, and fine-grained opaques p a r t i a l l y oxidized to hematite. O l i v i n e phenocrysts are generally euhedral and may exhibit i n c i p i e n t s k e l e t a l outgrowths. Sample MXC-7 from a flow i n the Valley of Mexico i s r i c h e r i n o l i v i n e phenocrysts (16 v o l . %) which are subhedral (<2.5 mm i n length) with i d d i n g s i t i z e d margins and reaction rims of granular orthopyroxene. Microphenocrysts of plagioclase and augite are enclosed i n a nearly holo-c r y s t a l l i n e groundmass composed of plagioclase m i c r o l i t e s and intergranular augite, orthopyroxene, and opaque oxides. Inclusions of euhedral C r - s p i n e l and rare brown glass are l o c a l l y observed within o l i v i n e phenocrysts i n both samples. Glomeroporphyritic textures are rarely observed. 3.4 Mineralogy and Phase Chemistry The phenocryst mineralogy of the Younger Andesites and Dacites i s summarized i n Figure 3.3 and representative chemical analyses of pheno-crysts and reaction products are given i n Tables 3.2 to 3.9 and l i s t e d i n Appendix B. Mineral analyses were performed using an ARL-SEMQ e l e c t r o n -microprobe f i t t e d with automated wavelength-dispersive spectrometers. An accelerating voltage of 15 k i l o v o l t s and specimen current of 40 nan-noamperes were used with beam diameters of 5 um for s p i n e l and 10 um for Table 3 . 2 : Representa t i ve Microprobe P o i n t Ana lyses o f O l i v i n e Phenocrysts and Microphenocrysts Sample Analysis Spot S i 0 2 FeO MnO MgO CaO Sum Mixed Lavas Type I --R-5 R-5 R-6 R-6 R-9 R-9 R-12 R-12 7pc 7pr 2ac 2ar 2pc 2pr G P 0L p-core p - r i m p-core p-rim p-core p - r i m gp-core p - r i m 40.28 39.35 40.63 39.73 41.24 40.65 40.81 40.17 9.80 16.29 11.74 14.93 9.73 11.58 10.07 11.15 0.14 0.30 0.16 0.23 0.14 0.15 0.22 0.22 48.38 43.44 47.83 45.41 49.30 48.32 48.55 47.88 0.14 0.15 0.12 0.12 0.13 0.16 0.20 0.19 98.74 99.53 100.48 100.42 100.54 100.86 99.85 99.61 Cations per 4 Oxygens Si 0.9999 0.9994 Fe 0.2034 0.3460 Mn 0.0030 0.0065 Mg 1.7901 1.6445 Ca 0.0037 0.0041 XY 2.00 2.00 Z 1.00 1.00 Fo 89.8 82.6 Fa 10.2 17.4 0.9993 0.9935 1.0037 0.2415 0.3122 0.1980 0.0034 0.0049 0.0029 1.7534 1.6926 1.7884 0.0032 0.0032 0.0034 2.00 2.01 1.99 1.00 0.99 1.00 87.9 84.4 90.0 12.1 15.6 10.0 0.9957 1.0026 0.9954 0.2372 0.2069 0.2311 0.0031 0.0046 0.0047 1.7641 1.7779 1.7685 0.0042 0.0053 0.0050 2.01 2.00 2.01 1.00 1.00 1.00 88.2 89.6 88.4 11.8 10.4 11.6 Table 3 . 2 ( C o n t ' d ) : Mixed Lavas Type I Mixed Lava Type II Sample Analysis Spot P-1 lpc p-core P-1 3pr p-rim P-3 2mp m-core P-3 8mpr m-rim P-4 OL p-core P-4 OLR p-rim P-9 lc p-core P-9 l r p-rim P-15 ac p-core P-15 ar p-rim R-11 4c p-core R-11 4r p-rim SiO? 40. .75 40.17 40.34 39.67 40.71 39.47 40. .84 40.26 39.92 39.44 40.84 38.00 FeO 10. .37 12.06 12.22 12.17 10.03 15.58 11. .11 12.68 11.33 18.21 12.64 24.78 MnO 0. .12 0.20 0.16 0.23 0.17 0.31 0. .12 0.17 0.15 0.26 0.17 0.35 MgO 48. .96 47.76 48.11 47.54 48.52 44.76 47. .65 45.92 47.57 42.37 47.66 37.94 CaO 0. .12 0.15 0.09 0.13 0.19 0.14 0. .17 0.17 0.15 0.18 0.13 0.11 Sum 100. .32 100.34 100.92 99.74 99.62 100.26 99. .89 99.20 99.12 100.46 101.44 101.18 Cations per 4 Oxygens Si 0.9977 0.9922 0.9909 0.9873 1.0022 0.9923 1.0067 1.0071 0.9948 1.0005 0.9987 0.9890 Fe 0.2123 0.2491 0.2510 0.2533 0.2065 0.3276 0.2290 0.2653 0.2361 0.3863 0.2585 0.5359 Mn 0.0025 0.0042 0.0034 0.0049 0.0036 0.0066 0.0025 0.0036 0.0032 0.0056 0.0035 0.0078 Mg 1.7867 1.7583 1.7615 1.7636 1.7805 1.6773 1.7507 1.7122 1.7670 1.6021 1.7372 1.4718 Ca 0.0031 0.0040 0.0024 0.0035 0.0050 0.0038 0.0045 0.0046 0.0040 0.0049 0.0034 0.0031 XY 2.00 2.02 2.02 2.02 2.00 2.01 1.99 1.99 2.01 2.00 2.00 2.02 Z 1.00 0.99 0.99 0.99 1.00 0.99 1.01 1.01 0.99 1.00 1.00 0.99 Fo 89.4 87.6 87.5 87.4 89.6 83.7 88.4 86.6 88.2 80.6 87.0 73.2 Fa 10.6 12.4 12.5 12.6 10.4 16.3 11.6 13.4 11.8 19.4 13.0 26.8 Spot designation p-core = phenocryst core ; p-rim = phenocryst rim ; m-core = microphenocryst core m-rim = microphenocryst rim ; gp = phenocryst within glomeroporphyritic clot 142 a l l other minerals. Natural and synthetic minerals were used as standards. Analyses were reduced using the correction procedures of Bence and Albee (1968) and Albee and Ray (1970). A n a l y t i c a l uncertainties were determined by r e p l i c a t e analyses of standards at the beginning and end of each exper-imental run. Repro d u c i b i l i t y (operator plus instrumental errors) f o r the 2a l e v e l of confidence i s : S i 0 2 (1.4% r e l a t i v e ) , T i 0 2 (10%), A l ^ (3.5%), FeO (1.4%), MnO (60%), MgO (2,0%), CaO (1.4%), Na 20 (4.5%), and K 20 (10.0%). 3.4.1 O l i v i n e O l i v i n e occurs as i n d i v i d u a l c r y s t a l s 0.15-2.0 mm i n length or monomineralic aggregates less than 3 mm i n diameter. Several d i s t i n c t o l i v i n e morphologies may be recognized: 1) equant c r y s t a l s with euhedral outl i n e ; 2) anhedral to subhedral c r y s t a l s whose margins are scalloped by fine-grained reaction products; and 3) subequant s k e l e t a l c r y s t a l s with accentuated edge growth at the i n t e r s e c t i o n of the common dome and prism forms. Morphologies 1 and 3 above belong to the polyhedral and hopper categories of Donaldson (1976). Representatives of each morphological category may occur i n a si n g l e t h i n - s e c t i o n and no c o r r e l a t i o n i s evident between o l i v i n e habit and pro-ximity to flow margins such as encountered i n c e r t a i n k o m a t i i t i c lavas (Arndt et a l . , 1977). However, relationships do e x i s t between the frequency d i s t r i b u t i o n of each morphological type, associated rim textures, and the modal abundance of o l i v i n e i n mixed lavas. Euhedral and hopper shapes are most abundant i n MLT-IA, mixed lavas containing >2 v o l . % o l i v i n e . Grain margins are commonly i n sharp contact with host glass although some c r y s t a l s are surrounded by a zone of weakly bir e f r i n g e n t material several microns i n width that marks the i n i t i a l stages of reaction between o l i v i n e and r e s i d u a l l i q u i d . Neighbouring phenocrysts may exhibit a narrow (5-20 um) corona of granular orthopyroxene although t h i s texture i s more commonly developed i n MLT-IB, lavas with reduced proportions of o l i v i n e (<2 v o l . % ) . Anhedral to subhedral o l i v i n e s with broad (0.05-0.15 mm) reaction rims of orthopyroxene, r a r e l y incorporating plagioclase and sparse titanomagnetite, characterize the olivine-poor MLT-II flows. The l a t t e r lavas commonly contain microxenoliths (1-4 mm) of corroded o l i v i n e surrounded by orthopyroxene and plagioclase m i c r o l i t e s (0.1-0.2 mm i n length) with intergranular opaques, clinopyroxene, and rare amphibole. Intergrowths of o l i v i n e and clinopyroxene are rare but have been observed i n Pecho flows. Localized hydrothermal a l t e r a t i o n of c e r t a i n Pies lavas has s e l e c t i v e l y replaced o l i v i n e by i d d i n g s i t e and chlorophaeite. Representative microprobe analyses of o l i v i n e phenocrysts are presented i n Table 3.2. The compositional range extends from F O ^ Q (core analysis) to Fo (rim) with most analyses f a l l i n g i n the range Fo. to Fo . Zoning / J y(j 87 within i n d i v i d u a l c r y s t a l s i s i n v a r i a b l y normal and var i a t i o n s i n minor element abundances are s l i g h t . CaO (0.09-0.22 wt %) shows no systematic c o r r e l a t i o n with f o r s t e r i t e content or c r y s t a l habit, whereas MnO (0.12-0.37 wt %) increases predictably from core to rim where zoning i s well-developed (Simkin and Smith, 1970). 3.4.2 Pyroxenes The mineralogy of pyroxenes i n I z t a c c i h u a t l lavas i s t y p i c a l of c a l c -a l k a l i n e rock s e r i e s . Orthopyroxenes range from hypersthene to bronzite i n composition, c a l c i c pyroxenes are a u g i t i c , and pigeonite i s absent e i t h e r as a phenocryst or i n the groundmass. Pyroxene zoning patterns and reac-t i o n textures are rather diverse and consequently f u r n i s h a great deal of information concerning the c r y s t a l l i z a t i o n h i s t o r i e s of various lava types. Clinopyroxene occurs as colourless to pale green c r y s t a l s usually free of i n c l u s i o n s . Phenocrysts (0.4-1.5 mm) are generally subhedral, weakly zoned, and commonly exhibit simple or lamellar twins, or twin combinations i n the larger c r y s t a l s . Microphenocrysts (0.2-0.4 mm) i n MLT-I may e x h i b i t a weak hourglass e x t i n c t i o n superimposed on strong continuous core to rim zoning. Monomineralic aggregates (<3 mm across) of up to f i v e i n d i v i d u a l s are quite common and c r y s t a l c l o t s of clinopyroxene + orthopyroxene ± plagioclase ± Fe-Ti oxides are also observed. Groundmass clinopyroxenes (<0.2 mm) are granular or form p a r a l l e l intergrowths with hypersthene i n which c r y s t a l l o g r a p h i c c-axes share a common or i e n t a t i o n . Reaction rims of clinopyroxene up to 0.2 mm i n width surrounding quartz phenocrysts are found i n a l l lava types (Figure 3.3). T y p i c a l coronas extend 0.1-0.2 mm from c r y s t a l margins and comprise an inner zone of glass and stubby augite prisms elongated perpendicular to quartz grain boundaries which i s succeeded outwards by a c i c u l a r c r y s t a l s arranged more haphazardly and protruding i n t o the groundmass. These textures are i d e n t i c a l to the inner zones of clinopyroxene coronas (reaction zones I and II) on quartz xenocrysts described by Sato (1975) i n c a l c - a l k a l i n e lavas of Japan. Hypersthene i s the dominant pyroxene phase i n a l l lavas. Phenocrysts (0.4-2 mm i n length) have euhedral to resorbed outlines with pronounced pale green to pink pleochroism and broad o p t i c a l l y homogeneous cores sur-rounded by narrow normally-zoned rims. Phenocryst cores are l o c a l l y s c h i l -lered with opaque oxide dust and r a r e l y enclose a blebby monoclinic phase, probably augite, exsolved during subsolidus cooling. Inclusions comprise opaque oxides, plagioclase, apatite, glass, and rare z i r c o n i n decreasing 1 4 5 order of abundance. Micronorite c r y s t a l c l o t s (<3 mm) with subordinate hornblende and opaque oxides, and glomeroporphyritic aggregates of hyper-sthene and Fe-Ti oxides, are considerably more abundant than clinopyroxene-bearing intergrowths. Monomineralic clusters of colourless to weakly pleochroic Mg-rich orthopyroxene with strong normal zoning are p a r t i c u l a r l y common i n c e r t a i n R o d i l l a s lavas (R-11 and R-16; Figure 3.3). Reaction textures exhibited by orthopyroxenes are complex. In mixed lavas and Cpx-Opx dacites, magnesian orthopyroxene l o c a l l y forms overgrowths on hypersthene phenocrysts and microphenocrysts (0.2-0.4 mm). A colourless non-pleochroic rim t y p i c a l l y 30-60 um but ranging 5-70 pm i n width completely envelops a pleochroic core that i s rar e l y faceted and commonly rounded as a re s u l t of corrosion p r i o r to rim growth. The sharp i n t e r n a l contact marks a d i s t i n c t compositional hiatus between homogeneous core and normally-zoned b r o n z i t i c rim. Analogous textures have been i l l u s t r a t e d i n the tephra of Mt. Shasta by Anderson (1976) who related t h e i r o r i g i n to the i n t e r a c t i o n of b a s a l t i c and d a c i t i c magmas. In a related though more incongruous texture, hypersthene phenocrysts exhibit ghost outlines of successive stages of c r y s t a l growth which may represent bronzite overgrowths that have subsequently been annealed. An unusual but not uncommon texture encountered i n MLT-I consists of orthopyroxene c r y s t a l s jacketed by augite. An inner phenocryst or micro-phenocryst of pleochroic hypersthene (± Fe-Ti oxide inc l u s i o n s ) with sharp c u r v i l i n e a r or d e l i c a t e l y cuspate margins supports a euhedral to subhedral clinopyroxene overgrowth varying from 20 to 80 um i n width. These jackets are composed of a single o p t i c a l l y uniform c r y s t a l with well-developed lamellar twinning and smooth core to rim compositional zoning. Similar textures in v o l v i n g hypersthene cores mantled by pigeonite (± o l i v i n e ) Table 3.3: Representative Microprobe Analyses of Clinopyroxene Phenocrysts, Microphenocrysts, and Discrete Reaction Products Hb Dacite Sample R-7 R-5 R-5 Analysis xpc 3pc 3pr Spot p-core p-core p-rim Mixed Lavas Type I R-5 R-5 R-6 R-6 P-1 P-1 P-9 P-9 P-9 4 4 5pc 5 8pc 8pr 9mc lmc 5 (Qz) (Qz) p-core (Opx) p-core p-rim m-core m-core (Opx) Si0 2 52.99 51.23 51.76 52.39 49.68 53.03 53.40 52.76 51.73 52.98 51.28 52.84 Ti05 0.29 0.44 0.56 0.31 1.39 0.33 0.30 0.59 0.69 0.53 0.42 0.43 Al , 6 \ 1.12 2.13 1.56 1.62 4.18 2.59 2.17 2.97 3.14 1.92 1.61 1.67 FeO 8.38 6.28 8.40 4.76 5.64 5.04 4.94 6.51 6.31 6.24 8.73 6.67 MnO 0.18 0.32 0.12 0.23 0.28 0.13 0.14 0.15 0.16 0.16 0.27 0.23 MqO 14.91 17.15 14.88 17.42 15.43 17.74 17.28 17.68 17.18 18.68 14.65 17.86 CaO 22 00 20.27 20.65 21.53 21.56 20.96 21.58 19.24 19.76 18.33 21.05 19.17 NapO 0.29 0.36 0.40 0.34 0.33 0.43 0.39 0.38 0.39 0.31 0.34 0.28 KJ) 0.01 0.04 0.0 0.01 0.01 0.03 0.02 0.02 0.04 0.01 0.02 0.0 Sum 100.17 98.22 98.33 98.61 98.50 100.28 100.22 100.30 99.40 99.16 98.37 99.15 Si Cations per 6 Oxygens 1.9669 1.9203 1.9539 1.9445 1.8604 1.9307 1.9464 1.9235 1.9083 1.9466 1.9436 1.9512 Al IV 0.0331 0.0797 0.0461 0.0555 0.1396 0.0693 0.0536 0.0765 0.0917 0.0534 0.0564 0.0488 Ti 0.0081 0.0124 0.0159 0.0087 0.0391 0.0090 0.0082 0.0162 0.0191 0.0146 0.0120 0.0119 Al VI 0.0159 0.0144 0.0234 0.0154 0.0449 0.0419 0.0397 0.0511 0.0448 0.0298 0.0155 0.0239 Fe 0.2601 0.1969 0.2652 0.1478 0.1766 0.1535 0.1506 0.1985 0.1947 0.1917 0.2767 0.2060 Mn 0.0057 0.0102 0.0039 0.0073 0.0089 0.0040 0.0044 0.0047 0.0050 0.0050 0.0087 0.0072 Mg 08249 0.9582 0.8373 0.9637 0.8613 0.9627 0.9388 0.9607 0.9447 1.0231 0.8277 0.9830 Ca 0.8750 0.8141 0.8352 0.8562 0.8651 0.8176 0.8428 0.7515 0.7810 0.7216 0.8548 0.7585 Na 0 0209 0.0262 0.0293 0.0245 0.0240 0.0304 0.0276 0.0269 0.0279 0.0221 0.0250 0.0200 K 0 0005 0 0019 0.0 0.0005 0.0005 0.0014 0.0009 0.0009 0.0019 0.0005 0.0010 0.0 XY Z 2.01 2.00 2.03 2.00 2.01 2.00 2.02 2.00 2.02 2.00 2.02 2.00 2.01 2.00 2.01 2.00 2.02 2.00 2.01 2.00 2.02 2.00 2.01 2.00 Ca Mg Fe 44.6 42.1 13.3 41.3 48.7 10.0 43.1 43.2 13.7 43.5 49.0 7.5 45.4 45.3 9.3 42.3 49.8 7.9 43.6 48.6 7.8 39.3 50.3 10.4 40.7 49.2 10.1 37.3 52.8 9.9 43.6 42.3 14.1 38.9 50.5 10.6 Mg # 76.0 83.0 76.0 86.7 83.0 86.2 86.2 82.9 82.9 84.2 74.9 82.7 (Opx) = reaction rim on orthopyroxene; (Qz) = reaction rim on quartz; other abbreviations as in Table 3.2. Mg # = 100Mg/(Mg+Fe). Table 3.^Representative Microprobe Point Analyses of Orthopyroxene Phenocrysts, Microphenocrysts, and Discrete Reaction Products Sample Analysis Spot R-1 5pc p-core R-1 5pr p-rim R-1 5-A (Hb) R-1 2-A (Hb) Hb Oacites R-2 9pc p-core R-3 5mc m-core • 3$ 5mr m-rim R-3 8pc p-core R-3* 8pr p-rim R-7 2pc p-core S i 0 2 T i 0 2 FeO J MnO MgO CaO Na20 K20 Sum 52.69 0.13 0.52 21.44 0.70 23.63 0.86 0.0 0.01 53.24 0.24 0.63 21.69 0.63 23.37 1.00 0.05 0.0 53.49 0.17 0.40 20.56 0.65 24.58 0.98 0.02 0.0 53.16 0.25 0.91 21.73 0.59 23.10 1.21 0.02 0.0 53.35 0.06 0.34 22.65 1.11 22.64 0.61 0.0 0.0 54.60 0.16 3.14 8.24 0.20 31.84 1.47 0.05 0.0 52.58 0.13 0.38 21.54 0.75 23.00 0.78 0.03 0.01 55.55 0.12 1.42 9.78 0.21 32.20 1.13 0.01 0.01 52.74 0.09 0.52 22.84 0.98 22.71 0.70 0.02 0.02 52.56 0.07 0.77 23.31 1.16 21.78 0.68 0.03 0.01 99.98 100.85 100.85 100.97 100.76 99.70 99.20 100.43 100.62 100.37 Cations per 6 Oxygens Si Al IV Ti Al VI Fe Mn Mg Ca Na K 1.9615 0.0228 0.0036 0.0 0.6675 0.0222 1.3112 0.0343 0.0 0.0005 1.9648 0.0274 0.0067 0.0 0.6694 0.0198 1.2855 0.0395 0.0036 0.0 1.9642 0.0173 0.0047 0.0 0.6314 0.0204 1.3453 0.0386 0.0014 0.0 1.9601 0.0395 0.0069 0.0 0.6701 0.0186 1.2695 0.0478 0.0014 0.0 1.9798 0.0149 0.0017 0.0 0.7030 0.0351 1.2523 0.0243 0.0 0.0 1.9143 0.0857 .0042 .0440 ,2416 .0060 .6639 .0552 ,0034 0.0 1.9738 0.0168 0.0037 0.0 0.6762 0.0240 1.2869 0.0314 0.0022 0.0005 1.9449 0.0551 0.0032 0.0035 0.2864 0.0063 1.6804 0.0424 0.0007 0.0004 1.9642 0.0228 0.0025 0.0 0.7114 0.0311 1.2607 0.0279 0.0014 0.0010 1.9673 0.0327 0.0020 0.0013 0.7297 0.0370 1.2151 0.0273 0.0022 0.0005 XY Z 2.04 1.98 2.02 1.99 2.04 1.98 2.01 2.00 2.02 1.99 2.02 2.00 2.02 1.99 2.02 2.00 2.04 1.99 2.02 2.00 Ca Mg Fe 1.7 65.1 33.2 2.0 64.4 33.6 1.9 66.8 31.3 2.4 63.9 33.7 1.2 63.3 35.5 2.8 84.9 12.3 1.6 64.5 33.9 2.1 83.6 14.3 1.4 63.0 35.6 1.4 61.6 37.0 Mg # 66.3 65.8 68.1 65.5 64.0 87.3 86.2 85.4 63.9 62.5 Table 3.4 (Cont'd): Hb Dacite Mixed Lavas Type I ; Sample R-7 R-7 R-7 R-5 R-6 R - 6+ R-6 R - 6+ R-9 R-12 P-1 P-1 Analysis 2pr XLmc Xmr 9pc llpc l l p r 13pc 13pr 8pc pc lpc lpr Spot p-rim m-core m-rim p-core p-core p-rim p-core p-nm p-core p-core p-core p-rim SiO, 53.34 54.46 52.58 53.15 51.41 54.59 52.57 53.58 53.81 53.42 53.44 53.07 TiO? 0.09 0.12 0.13 0.15 0.11 0.19 0.14 0.20 0.18 0.0 0.17 0.09 A l X 0.39 2.17 0.38 0.43 0.61 0.91 0.70 1.29 0.46 0.50 0.65 Peg 3 21 34 9 26 21 54 21.03 23.31 11.60 21.85 13.45 21.70 21.66 22.37 22.38 MnO 0.68 0.16 0.75 0.72 1.37 0.24 0.58 0.28 0.47 0.71 0.77 0.68 B 24 10 30.81 23.00 23.25 20.56 28.58 21.74 27.47 24.04 23.21 22.50 22.65 CaO 0 80 1.33 0.78 0.75 0.75 1.70 1.02 1.93 0.63 0.72 0.81 0.79 Na,0 0 01 0 09 0 03 0.04 0.04 0.0 0.04 0.04 0.0 0.04 0.0 0.03 $ 0.0 0.0 0.01 0.0 0.02 0.01 0.03 0.01 0.0 0.05 0.02 0.02 100.75 98.40 99.20 99.52 98.18 97.82 98.67 98.25 101.29 100.31 100.73 100.18 Sum Cations per 6 Oxygens c, i ofifio i 9421 1 9738 1.9809 1.9744 1.9802 1.9849 1.9568 1.9715 1.9800 1.9784 1.9771 Al IV 0:0169 0!0579 oioiee oioiW 010256 0^0198 0.0151 0.0432 0.0199 0.0200 0.0216 0.0206 Ti 0.0025 0.0032 0.0037 0.0042 0.0032 0.0052 0.0040 0.0055 0.0050 0.0 0.0047 0.0025 A VI 0 0 0.0333 0.0 0.0 0.0020 0.0191 0.0160 0.0123 0.0 0.00 8 0.0068 0.0 Fe 0 6581 0 2762 0.6762 0.6555 0.7487 0.3519 0.6899 0.4108 0.6649 0.6714 0.6926 0.6973 Mn 0iu214 0 0049 0 0240 0^0229 0.0449 0.0074 0.0187 0.0087 0.0147 0.0224 0.0243 0.0216 Mn 1 3246 1 6377 1 2869 1.2916 1.1770 1.5453 1.2235 1.4954 1.3129 1.2823 1.2416 1.2578 cl l o l i l oiuSOS 00314 0 0300 0)0309 0.0661 0.0413 0.0755 0.0247 0.0286 0.0321 0.0315 Na 0 0007 0.0062 0.0022 0.0029 0.0030 0.0 0.0029 0.0028 0.0 0.0029 0.0 0.0022 K o!o 0.0 0.0005 0.0 0.0010 0.0005 0.0014 0.0005 0.0 0.0024 0.0009 0.0010 XY 2 04 2 01 2.02 2.01 2.01 2.00 2.00 2.01 2.02 2.01 2.00 2.01 Z l!98 2!00 199 2.00 2.00 2.00 2.00 2.00 1.99 2.00 2.00 2.00 Ca Mg Fe Mg # 1.6 65.7 32.7 2.6 83.3 14.1 1.6 64.5 33.9 1.5 65.3 33.2 1.6 60.1 38.3 3.4 78.7 17.9 2.1 62.6 35.3 3.8 75.5 20.7 1.2 65.6 33.2 1.4 64.7 33.9 1.6 63.2 35.2 1.6 63.3 35.1 66.8 85.6 65.6 66.3 61.1 81.5 63.9 78.5 66.4 65.6 64.2 64.3 Table 3.4 (Cont'd): Sample Analysis Spot P-1 P-3 lgp 12mc gp-core m-core Mixed Lavas Type I P-3 5apc p-core P-3 5apr p-rim P-4 P-4 gp 5 gp-core (01) P-4 5 (01) Mixed Lava Type II R - l l * R - l l * 4mc m-core 4mr m-rim R - l l 4 (01) R - l l 4 (01) SiO? TiO? A1203 Fe6 MnO MgO CaO Na20 K20 52.98 0.12 0.81 22.92 0.83 22.10 0.84 0.03 0.03 53.73 0.32 2.44 14.41 0.38 25.39 1.56 0.09 0.04 53.44 0.12 0.63 21.57 0.65 23.37 1.00 0.03 0.01 53.15 0.07 0.51 22.30 0.83 23.09 0.70 0.0 0.01 53.02 0.0 0.66 20.90 0.58 23.46 0.90 0.07 0.04 52.64 0.29 2.76 13.06 0.30 28.26 2.02 0.02 0.0 54.21 0.20 1.85 10.03 0.24 30.62 2.06 0.04 0.0 55.96 0.11 1.61 7.70 0.15 32.70 0.93 0.08 0.01 56.01 0.11 1.57 7.63 0.17 32.86 0.85 0.05 0.0 53.76 0.32 1.19 17.24 0.37 26.27 42 03 01 54.04 0.37 1.60 11.95 0.29 28.42 1.54 0.04 0.0 Sum 100.66 99.36 100.82 100.66 99.63 99.35 99.25 99.25 99.25 100.61 98.25 Cations per 6 Oxygens Si Al IV 1.9703 0.0297 1.9450 0.0550 1.9706 0.0274 1.9705 0.0223 1.9734 0.0266 1.9017 0.0983 1.9307 0.0693 1.9598 0.0402 1.9604 0.0396 1.9484 0.0508 1.9561 0.0439 Ti Al VI Fe Mn Mg Ca Na K 0.0034 0.0058 0.7128 0.0263 1.2250 0.0335 0.0022 0.0014 0.0087 0.0491 0.4362 0.0117 1.4239 0.0605 0.0063 0.0018 0.0033 0.0 0.6652 0.0204 1.2845 0.0395 0.0021 0.0005 0.0020 0.0 0.6914 0.0262 1.2760 0.0278 0.0 0.0005 0.0 0.0023 0.6506 0.0184 1.3015 0.0359 0.0051 0.0019 0.0079 0.0193 0.3946 0.0092 1.5218 0.0782 0.0014 0.0 0.0054 0.0083 0.2987 0.0073 1.6255 0.0786 0.0028 0.0 0.0029 0.0263 0.2255 0.0045 1.7070 0.0349 0.0054 0.0004 0.0029 0.0252 0.2233 0.0051 1.7143 0.0319 0.0034 0.0 0.0087 0.0 0.5225 0.0114 1.4191 0.0551 0.0021 0.0005 0.0101 0.0244 0.3618 0.0090 1.5334 0.0597 0.0028 0.0 XY I 2.01 2.00 2.00 2.00 2.02 2.00 2.02 1.99 2.02 2.00 2.03 2.00 2.03 2.00 2.01 2.00 2.01 2.00 2.02 2.00 2.00 2.00 Ca Mg Fe 1.7 62.1 36.2 3.2 74.1 22.7 2.0 64.6 33.4 1.4 64.0 34.6 1.8 65.5 32.7 3.9 76.3 19.8 3.9 81.2 14.9 1.8 86.7 11.5 1.6 87.1 11.3 2.8 71.0 26.2 3.1 78.4 18.5 Mg # 63.2 76.5 65.9 64.9 66.7 79.4 84.5 88.3 88.5 73.1 80.9 (Hb) = amphibole breakdown product; (01) = reaction rim on olivine; other abbreviations as in Tables 3.2 and 3.3. »bronziteovergrowth with sharp internal contact •hypersthene overgrowth on corroded bronzite core; sharp or gradational contact * bronzite clot Table 3.5: Representative Microprobe Point Analyses of Coexisting Orthopyroxene and Clinopyroxene i n Crystal Clots and Aaphibole Breakdown Products Mixed Lavas Type I Crystal Clot Hb breakdown products Sample R-6 R-6 R-6 R-6 R-9 R-9 R-9 Analysis 2gpc Opx 2gpc Opx 9gpc Cpx 9gpc Cpx 9-A Opx 9-A Opx 9-A Cpx Si0 2 Ti0 2 lift MnO MgO CaO Na20 K20 56.34 0.13 0.97 9.48 0.19 31.00 1.54 0.06 0.03 55.39 0.28 2.04 10.57 0.28 30.95 1.43 0.09 0.03 53.30 0.38 2.37 4.98 0.15 17.30 21.41 0.39 0.01 54.28 0.30 1.76 5.16 0.19 18.11 20.80 0.33 0.01 53.58 0.27 1.02 19.12 0.43 24.62 1.45 0.08 0.01 53.79 0.13 0.55 21.27 0.65 23.80 0.74 0.0 0.0 53.59 0.31 1.02 9.12 0.28 15.77 21.23 0.28 0.0 Sum 99.74 101.06 100.29 100.94 100.58 100.93 101.60 Cations per 6 Oxygens Si Al IV 1.9813 0.0187 1.9361 0.0639 1.9410 0.0590 1.9595 0.0405 1.9595 0.0405 1.9757 0.0238 1.9625 0.0375 Ti Al VI Fe Mn Mg Ca Na K 0.0034 0.0215 0.2788 0.0057 1.6250 0.0580 0.0041 0.0013 0.0074 0.0201 0.3090 0.0083 1.6125 0.0536 0.0061 0.0013 0.0104 0.0427 0.1517 0.0047 0.9390 0.8354 0.0275 0.0005 0.0081 0.0344 0.1558 0.0059 0.9745 0.8045 0^ 0231 0.0005 0.0074 0.0035 0.5848 0.0134 1.3421 0.0568 0.0057 0.0005 0.0036 0.0 0.6533 0.0204 1.3030 0.0291 0.0 0.0 0.0085 0.0066 0.2793 0.0087 0.8608 0.8330 0.0199 0.0 XY Z 2.00 2.00 2.02 2.00 2.01 2.00 2.01 2.00 2.01 2.00 2.01 2.00 2.02 2.00 Ca Mg Fe 3.0 82.8 14.2 2.7 81.7 15.6 43.4 48.7 7.9 41.6 50.4 8.0 2.8 67.7 29.5 1.5 65.6 32.9 42.2 43.6 14.2 Mg I 85.3 83.9 86.1 86.2 69.6 66.6 75.5 Table 3.5 (Cont'd): Mixed Lavas Type I Mixed Lava Cpx-Opx Dacite Type II — Hb breakdown products — Crystal Clot Sample P-4 P-4 P-4 R-11 R-11 PE-8 PE-8 PE-8 PE-8 Analysis 3-A Opx 3-A Cpx 3-A Cpx A Opx A Cpx Ape Opx Bpc Opx Ape Cpx Ape Cpx S i 0 2 TiO? AI2&3 FeO MnO MgO CaO Na?0 K20 52.58 0.77 1.54 18.46 0.39 25.20 1.44 0.04 0.0 50.93 0.78 2.27 10.22 0.19 14.76 20.62 0.49 0.0 51.45 0.44 1.83 11.27 0.25 14.00 20.45 0.28 0.0 53.22 0.37 2.11 18.09 0.43 25.06 1.39 0.03 0.01 50.85 0.83 3.50 9.67 0.24 14.47 20.25 0.40 0.02 54.32 0.39 3.49 12.29 0.28 27.63 1.92 0.04 0.0 55.47 0.20 2.01 10.29 0.20 30.07 1.63 0.04 0.03 52.67 0.60 3.06 7.16 0.17 15.93 20.89 0.43 0.0 51.29 0.74 4.27 8.00 0.21 14.97 20.88 0.44 0.01 Sum 100.42 100.26 99.97 100.71 100.23 100.36 99.94 100.91 100.81 Cations per 6 Oxygens Si Al IV 1.9242 0.0664 1.9060 0.0940 1.9356 0.0644 1.9341 0.0659 1.8939 0.1061 1.9263 0.0737 1.9555 0.0445 1.9231 0.0769 1.8853 0.1147 Ti Al VI Fe Mn Mg Ca Na K 0.0212 0.0 0.5650 0.0122 1.3746 0.0565 0.0028 0.0 0.0220 0.0061 0.3199 0.0061 0.8234 0.8268 0.0356 0.0 0.0124 0.0167 0.3546 0.0080 0.7851 0.8243 0.0204 0.0 0.0101 0.0245 0.5498 0.0133 1.3575 0.0541 0.0021 0.0005 0.0232 0.0476 0.3012 0.0076 0.8033 0.8081 0.0289 0.0010 0.0104 0.0722 0.3645 0.0085 1.4605 0.0730 0.0028 0.0 0.0053 0.0390 0.3034 0.0060 •1.5801 0.0616 0.0027 0.0013 0.0165 0.0548 0.2186 0.0053 0.8670 0.8172 0.0304 0.0 0.0205 0.0703 0.2459 0.0066 0.8202 0.8223 0.0314 0.0005 XY Z 2.03 1.99 2.04 2.00 2.02 2.00 2.01 2.00 2.02 2.00 1.99 2.00 2.00 2.00 2.01 2.00 2.02 2.00 Ca Mg Fe 2.8 68.9 28.3 42.0 41.8 16.2 . 42.0 40.0 18.0 2.8 69.2 28.0 42.2 42.0 15.8 3.8 77.0 19.2 3.2 81.2 15.6 42.9 45.6 11.5 43.6 43.4 13.0 Mg # 70.9 72.0 68.9 71.2 72.7 80.0 83.9 79.9 76.9 Abbreviations as in Tables 3.2 and 3.3. 152 appear i n c l a s s i c a l petrographic descriptions of the Hakone region, Japan (Kuno, 1950). Representative analyses of pyroxenes i n I z t a c c i h u a t l lavas are given i n Tables 3.3 to 3.5. Analyses were obtained for phenocrysts, micro-phenocrysts, and phenocryst-meIt reaction products, many of which are out of equilibrium with each other and with t h e i r host rock compositions. Despite the r e s t r i c t e d range of whole-rock compositions, pyroxene s o l i d solutions are almost as extensive as the ent i r e spectrum of phenocryst compositions compiled by Ewart (1979, 1982) for c a l c - a l k a l i n e basalts, andesites, dacites, and r h y o l i t e s . Clinopyroxenes are predominantly augite with some scatte r towards endiopside (Ca.,. Mg,.,. Fe„ to Ca.. Mg.„ Fe, r ) . 42 50 8 43 42 15 Orthopyroxene compositional v a r i a t i o n i s more extensive, ranging from Mg-bronzite to Fe-hypersthene (Ca„ Mg Fe,„ to Ca, Mg,, F e „ Q ) . J o j 1Z 1 Dl JO The concentrations of minor elements ( T i , A l , Mn, Na) i n clinopyroxene exceeds abundances i n orthopyroxene with the exception of MnO which com-monly reaches le v e l s i n excess of 1.0 wt % i n hypersthene phenocrysts and exhibits an o v e r a l l decrease i n abundance with decreasing Fe/Mg. ^ 2 ^ 3 a n c * TiO^ a t t a i n greater concentrations i n bronzites (<3.5 wt % and <0.4 wt % res p e c t i v e l y ) than i n hypersthenes (usually <1.6 wt % and <0.2 wt % r e -spectively) although considerable v a r i a b i l i t y e x i s t s . The proportion of non-quadrilateral components or 'Others' (Papike et a l . , 1974; Cameron and Papike, 1981) averages 3-4 % i n hypersthenes, 5.5-8.5 % i n bronzites, and i s greatest i n augites intergrown with bronzite i n c r y s t a l c l o t s (=11.5 %) and reaction rims on quartz (=14 % ) . 3.4.3 Amphibole Amphibole i s an important phenocryst i n Hb dacites and also occurs as a 153 minor constituent i n Cpx-Opx dacites. Phenocrysts commonly reach 2 mm i n length and i n some cases grade s e r i a l l y into a c i c u l a r microphenocrysts (0.3 mm) that have l o c a l l y developed hollow terminations suggestive of rapid growth during eruptive quenching. Inclusions observed within amphibole phenocrysts are plagioclase, orthopyroxene, magnetite, ilmenite, b i o t i t e , apatite, and rare z i r c o n . Olive-green to pale brown pleochroism changes to the orange-brown hues of oxyhornblende as whole-rock oxidation becomes more intense. Thin opaque rims, presumably formed by oxidation during extrusion, characterize many cr y s t a l s and i n some mixed lavas and Cpx-Opx dacites Fe-Ti oxides appear to have replaced e n t i r e phenocrysts. The l a t t e r texture i s s i m i l a r to the 'black-type' amphiboles i n Cascade andesites described by Garcia and Jacobson (1979). The most common amphibole breakdown products consist of a fine-grained intergrowth of clinopyroxene + orthopyroxene + plagioclase + magnetite + ilmenite. In sections cut p a r a l l e l to the length of the prism, pyroxene c r y s t a l s form an e p i t a x i a l structure i n which the c-axis i s concordant with that of t h e i r host. In sections oriented at a high angle to [001] equigranular textures are apparent and pyroxenes may f a i t h f u l l y reproduce simple twins formerly e x i s t i n g i n amphibole phenocrysts. These textures resemble the 'gabbroic-type' amphibole replacement structure described by Garcia and Jacobson (1979) and are e a s i l y distinguished from synneusis intergrowths of s i m i l a r minerals (± amphibole) which involve o s c i l l a t o r y zoned plagioclase and are c l e a r l y primary (Garcia and Jacobson, 1979; Stewart, 1975). The modal abundances of c l o t minerals vary s u b s t a n t i a l l y , e s p e c i a l l y clinopyroxene/orthopyroxene ra t i o s and i n rare cases clinopyroxene appears to be absent. The l a t t e r texture may be due to Table 3.6: Representa t i ve Microprobe P o i n t Analyses o f Amphiboles Hb Dacite Sample R-2 R-3 R-3 R-3 R-3 R-7 R-7 Analysis Spot 8mc m-core 7c p-core 6c p-core 6ir pi-rim 6r p-rim 7c p-core 7c p-rim SiO? TiO? Al ? th FeO MnO MgO CaO Na20 K20 Ca Mg Fe Mg 46.45 2.01 7.42 12.44 0.17 14.54 11.02 1.74 0.52 45.83 2.11 8.31 12.99 0.19 14.75 11.13 1.96 0.63 45.95 1.77 8.05 13.82 0.35 13.98 11.03 2.24 0.67 47.19 1.54 7.00 13.35 0.35 14.66 11.18 1.94 0.55 47.59 1.39 7.05 13.98 0.37 14.52 10.91 1.82 0.48 45.33 1.78 7.86 13.11 0.18 14.28 11.11 1.98 0.70 47.17 1.33 6.40 12.53 0.19 15.44 11.10 1.56 0.51 Sum 96.31 97.90 97.86 97.76 98.11 96.33 96.23 Cations per 23 Oxygens Si 6 .8887 6.7241 6.7788 6.9310 6.9654 6.7742 6.9976 Al IV 1 .1113 1.2759 1.2212 1.0690 1.0346 1.2258 1.0024 Ti 0 .2242 0.2328 0.1964 0.1701 0.1530 0.2000 0.1484 Al VI 0 .1856 0.1611 0.1785 0.1428 0.1815 0.1587 0.1166 Fe 1 .5429 1.5939 1.7051 1.6398 1.7112 1.6385 1.5545 Mn 0 .0215 0.0238 0.0440 0.0439 0.0462 0.0229 0.0240 Mg 3 .2141 3.2257 3.0741 3.2094 3.1677 3.1809 3.4141 Ca 1 .7511 1.7496 1.7435 1.7594 1.7109 1.7789 1.7643 Na 0 .5003 0.5576 ' 0.6407 0.5525 0.5165 0.5737 0.4487 K 0 .0984 0.1179 0.1261 0.1030 0.0896 0.1334 0.0965 T 8.00 8.00 8.00 8.00 8.00 8.00 8.00 C 5.00 • 5.00 5.00 5.00 5.00 5.00 5.00 B 2.00 2.00 2.00 2.00 2.00 2.00 2.02 A 0.54 0.66 0.71 0.62 0.58 0.69 0.55 (Na)B 0.06 0.01 0.06 0.03 0.03 0.02 0.0 (Na+Ca)B 1.81 1.76 1.80 1.79 1.74 1.80 1.76 26.9 26.6 26.7 26.6 26.0 27.0 26. .2 49.4 49.1 47.1 48.6 48.0 48.2 50. .7 23.7 24.3 26.2 24.8 26.0 24.8 23. ,1 67.6 66.9 64.3 66.2 64.9 66.0 68. .7 155 amphibole breakdown outside the s t a b i l i t y f i e l d of clinopyroxene, but whatever the cause i t i s evident that amphibole decomposition reactions are complex (Eggler, 1972a, Helz, 1973). Representative analyses of amphibole phenocrysts are given i n Table 3.6. S t r u c t u r a l formulae are calculated on the basis of 23(0) taking ZFe = +2 Fe . Cation s i t e a l l o c a t i o n s and amphibole nomenclature follow Leake (1978). Since amphibole compositions have (Ca + Na) >1.34 and (Na) <0.67 B B a l l belong to the c a l c i c amphibole group (Leake, 1978). The analyzed range of s o l i d solutions i s continuous with (Na + K)^ >0.50 and extends from edenite ( i . e . Si per 23(0) >6.75) and e d e n i t i c hornblende (Si = 6.75-6.50) to ferroan p a r g a s i t i c hornblende ( S i = 6.50-6.25). The majority of amphi-boles are edenite or e d e n i t i c hornblende and characterized by r e l a t i v e l y high S i and low to moderate A l and T i . Amphibole cores have a r e s t r i c t e d +2 range of Mg #'s (lOOMg/(Mg+zFe ) = 0.63-0.70) and rims may exhibit a weak increase or decrease i n Fe/Mg r e l a t i v e to core compositions. Phenocrysts of ferroan p a r g a s i t i c hornblende i n mixed lava P-4 are t i t a n i a n v a r i e t i e s with T i >0.30 (Table 3.6). The composition of I z t a c c i h u a t l hornblendes i s t y p i c a l of amphiboles occurring i n orogenic dacites and s i l i c i c andesites at continental margins (Jakes and White, 1972; Ewart, 1979, 1982). 3.4.4 Plagioclase Plagioclase i s the p r i n c i p a l mineral phase i n I z t a c c i h u a t l lavas. Crystals are described i n terms of three a r b i t r a r y s i z e categories: 1) phenocrysts average 2-4 mm i n length and rarely a t t a i n 7 mm; 2) microphenocrysts ranging 0.2-0.4 mm; and 3) groundmass grains less than 0.2 mm i n length. O s c i l l a t o r y zoning i s present i n a l l phenocrysts; microphenocrysts display both normal and o s c i l l a t o r y zoning; and groundmass grains are usually 1 5 6 normally zoned. Groundmass c r y s t a l s generally have subequant outlines i n Hb dacites whereas l a t h - l i k e m i c r o l i t e s are r e l a t i v e l y abundant i n MLT-I. Some degree of heterogeneity i s in v a r i a b l y present among the phenocrysts of a given sample, but despite these complexities, systematic v a r i a t i o n s i n the d i s t r i b u t i o n of plagioclase morphologies and textures can be d i s -tinguished among the d i f f e r e n t lava types. Plagioclase compositional v a r i a t i o n s were examined o p t i c a l l y using the univ e r s a l stage and determinative curves of Tobi and K r o l l (1975). O p t i c a l determinations provide An content and complement the microprobe data reported below as An-Ab-Or s o l i d s o l u t i o n s . In Hb dacites and Cpx-Opx dacites, o s c i l l a t o r y zoning i n plagioclase phenocrysts varies from about An 0_ to An,. , c . Most zoning l i e s within the _>0 bO—OJ range An^-An^,. and bulk phenocryst compositions average ^ ^ Q ' a s deter-mined o p t i c a l l y . Narrow c a l c i c zones (An^-An^) may be encountered i n phenocryst cores, or more r a r e l y near c r y s t a l margins, and usually o v e r l i e resorption surfaces. The width of zones of uniform e x t i n c t i o n varies from less than 5 pm to 100 um. Some phenocrysts exhibit a broad (100-200 pm) c e l l u l a r region charged with inclusions of pale brown r h y o l i t i c glass (<120 pm across) and mantled by an i n c l u s i o n - f r e e rim commonly 15-50 pm i n width. The walls of these inc l u s i o n s are normally smooth and i r r e g u l a r , and l o c a l l y form narrow channelways connecting larger pools of quenched melt. Similar textures produced by d e n d r i t i c growth i n synthetic feldspars have been described by Lofgren (1974). Some of these g l a s s - r i c h zones may have a s i m i l a r o r i g i n since t h e i r width mimics thickness v a r i a t i o n s within i n d i v i d u a l o s c i l l a t o r y zones, becoming s i g n i f i c a n t l y broader i n the d i r e c t i o n of c r y s t a l elong-ation than i n [010], the d i r e c t i o n of slowest growth i n plagioclase (Kirk-157 patrick, 1977). However, other zones display corroded inner margins and appear to be resorption phenomena. Under crossed-nicols, plagioclase within t h i s zone exhibits a mottled e x t i n c t i o n or patchy zoning (Vance, 1965) caused by small v a r i a t i o n s i n composition (AAn <5 mol. % ) . Zoning of t h i s type i s also observed i n phenocrysts v i r t u a l l y devoid of i n c l u -sions. At least two post-entrapment events may be recognized w i t h i n the glass i n c l u s i o n s : 1) a more a l b i t i c plagioclase may have nucleated on i n c l u s i o n walls and quenched as ragged outgrowths into trapped l i q u i d ; and 2) exsolution of a vapour phase resulted i n the formation of s p h e r i c a l to ovoid v e s i c l e s measuring up to 80 um i n diameter. Effervescence may have been triggered by decompression accompanying eruption or by heating of hydrous melt. Plagioclase phenocrysts and glomerocrysts i n mixed lavas have zoning patterns and bulk compositions s i m i l a r to those observed i n Hb dacites. However, these c r y s t a l s commonly exhibit t h i n c a l c i c overgrowths whose composition varies from c r y s t a l to c r y s t a l but peaks at about An -An . 60 65 The rim usually has normal or normal-oscillatory zoning and o v e r l i e s a subhedral to resorbed sodic core. The boundary between the core and the inner margin of the c a l c i c rim i s compositionally abrupt (AAn commonly 10-20 mol. % ) , planar to c u r v i l i n e a r or i r r e g u l a r i n d e t a i l , and generally corrodes inner o s c i l l a t o r y zones. This outer c a l c i c s h e l l varies i n width from 0-120 um, r a r e l y within the same thi n - s e c t i o n . In general, c a l c i c overgrowths are better developed on phenocrysts i n MLT-IB's which contain abundant An-rich microphenocrysts. In contrast, MLT-IA's contain fewer plagioclase microphenocrysts and c a l c i c overgrowths on andesine phenocrysts are e i t h e r very t h i n (<10 um) or absent altogether, and where corrosion i s well-advanced i n t r i c a t e embayments have developed. Tab le 3.7: Representa t i ve Microprobe P o i n t Ana lyses o f P l a g i o c l a s e F e l d s p a r s Hb Dacite Sample Analysis Spot Si 0 2 TiO? A I 9 O 3 FeO MnO MgO CaO Na20 K20 Sum R-1 R-1 R-1 R-1 R-1 R-2 R-2 R-3 R-3 R-3 Pc Ir Or E OA 8Lmc 8Lmr 2c 2ir 2r p-core pi-rim p-rim gm (Hb) m-core m-rim p-core pi-rim p-rim 59. ,45 58.60 59.77 60.83 54.80 55. .57 58.83 57. .10 57.75 58.72 0. .06 0.03 0.0 0.01 0.02 0. .0 0.0 0. .0 0.01 0.0 25. .63 25.62 25.17 23.35 28.27 27. .52 25.27 26, .35 25.85 25.57 0. ,25 0.25 0.31 0.24 0.49 0. .28 0.28 0. .21 0.30 0.34 0. ,0 0.0 0.0 0.0 0.02 • 0. .0 0.0 0, .0 0.0 0.01 0. ,0 0.02 0.01 0.01 0.03 0. .05 0.03 0. .01 0.05 0.02 7. .63 7.85 7.25 5.85 10.68 9. .71 7.67 9, .09 8.41 8.29 6. ,95 6.64 7.16 7.44 5.38 5. .99 7.13 6. .20 6.44 6.77 0. ,57 0.67 0.79 0.67 0.43 0. .32 0.53 0. .37 0.40 0.44 100. .54 99.68 100.46 98.40 100.12 99. .44 99.74 99. .33 99.21 100.16 Cations per 8 Oxygens Si 2.6454 2.6329 2.6635 2.7481 2.4767 2.5185 2.6428 2.5814 2.6094 2.6289 Al 1.3442 1.3567 1.3220 1.2433 1.5058 1.4700 1.3380 1.4040 1.3767 1.3492 Ti 0.0020 0.0010 0.0 0.0003 0.0007 0.0 0.0 0.0 0.0003 0.0 Fe 0.0093 0.0094 0.0116 0.0091 0.0185 0.0106 0.0105 0.0079 0.0113 0.0127 Mn 0.0 0.0 0.0 0.0 0.0008 0.0 0.0 0.0 0.0 0.0004 Mg 0.0 0.0013 0.0007 0.0007 0.0020 0.0034 0.0020 0.0007 0.0034 0.0013 Ca 0.3638 0.3779 0.3462 0.2832 0.5172 0.4715 0.3692 0.4403 0.4072 0.3977 Na 0.5996 0.5785 0.6186 0.6517 0.4714 0.5264 0.6210 0.5435 0.5642 0.5877 K 0.0324 0.0384 0.0449 0.0386 0.0248 0.0185 0.0304 0.0213 0.0231 0.0251 XY 1.01 1.01 1.02 0.98 1.03 1.03 1.03 1.01 1.01 1.02 Z 3.99 3.99 3.99 3.99 3.98 3.99 3.98 3.99 3.99 3.98 An 36.5 38.0 34.3 29.1 51.0 46.4 36.2 43.8 40.9 39.3 Ab 60.2 58.1 61.3 66.9 46.5 51.8 60.8 54.1 56.8 58.2 Or . 3.3 3.9 4.4 4.0 2.5 1.8 3.0 2.1 2.3 2.5 03 Table 3.7 (Cont'd): Mixed Lavas Type I Sample Analysis Spot R-5 R-5 4mc 2gm m-core gm R-6 8c p-core R-6 14c p-core R-6 14c p-core R-9 9mc m-core R-9 60A (Hb) P-1 8c p-core P-1 8c p-core P-1 8r p-rim Si0 2 T i 0 2 Al 2(h Fe6 MnO MgO CaO Na20 K20 Sum 57.44 55.36 60.03 57.90 53.25 54 .39 57.94 57.21 59.12 58.62 0.18 0.03 0.0 0.0 0.05 0 .01 0.06 0.02 0.01 0.01 24.98 26.55 25.16 24.95 27.52 29 .04 27.39 27.02 25.48 26.58 0.24 0.26 0.23 0.29 0.25 0 .33 0.52 0.24 0.26 0.38 0.09 0.05 0.0 0.0 0.0 0 .0 0.01 0.01 0.02 0.01 0.0 0.0 0.02 0.0 0.01 0 .02 0.02 0.03 0.01 0.01 8.06 9.62 7.49 8.06 10.52 11 .60 8.87 9.34 7.65 8.30 6.38 5.82 7.23 6.35 4.99 4 .71 6.03 6.04 6.92 6.63 0.51 0.42 0.53 0.47 0.30 0 .28 0.58 0.40 0.55 0.49 97.88 98.11 100.69 98.02 96.89 100 .38 101.42 100.31 100.02 101.03 Cations per 8 Oxygens Si Al Ti Fe Mn Mg Ca Na K XY Z An Ab Or 2.6298 2.5427 2.6660 2.6432 2.4803 2.4498 2.5668 2.5627 2.6451 2.6022 1.3479 1.4372 1.3170 1.3424 1.5108 1.5416 1.4301 1.4265 1.3436 1.3906 0.0062 0.0010 0.0 0.0 0.0018 0.0003 0.0020 0.0007 0.0003 0.0003 0.0092 0.0100 0.0085 0.0111 0.0097 0.0124 0.0193 0.0090 0.0097 0.0141 0.0035 0.0020 0.0 0.0 0.0 0.0 0.0004 0.0004 0.0008 0.0004 0.0 0.0 0.0013 0.0 0.0007 0.0013 0.0013 0.0020 0.0007 0.0007 0.3954 0.4734 0.3564 0.3942 0.5250 0.5598 0.4210 0.4483 0.3667 0.3948 0.5664 0.5183 0.6226 0.5621 0.4507 0.4113 0.5179 0.5246 0.6003 0.5706 0.0298 0.0246 0.0300 0.0274 0.0178 0.0161 0.0328 0.0229 0.0314 0.0277 1.01 1.03 1.02 1.00 1.01 1.00 0.99 1.01 1.01 1.01 3.98 3.98 3.98 3.99 3.99 3.99 4.00 3.99 3.99 3.99 39.9 46.6 35.3 40.1 52.8 56.7 43.3 45.0 36.7 39.7 57.1 51.0 61.7 57.1 45.4 41.7 53.3 52.7 60.2 57.5 3.0 2.4 3.0 2.8 1.8 1.6 3.4 2.3 3.1 2.8 Table 3.7 (Cont'd): Mixed Lavas Mixed Lava Cpx - Opx - Type I Type II Dacite Sample P-3 P-3 P-4 R - l l R - l l PE-8* PE-8 Analysis 4c 4c lgm 11c 3c Ac Ar Spot p-core p-core gm p-core p-core p-core p-rim SiO? 60.36 59.18 52.68 60.67 59.95 52.94 55.66 TiO? 0.05 0.0 0.05 0.01 0.02 0.04 0.06 A1?0 3 24.35 25.97 29.00 24.54 25.18 29.38 27.62 Fe6 0.39 0.28 0.56 0.22 0.28 0.35 0.41 MnO 0.0 0.01 0.0 0.0 0.02 0.02 0.0 MgO 0.03 0.03 0.10 0.0 0.03 0.08 0.10 CaO 6.45 7.90 12.39 6.27 7.48 12.90 10.59 Na?0 7.29 7.01 4.77 7.34 7.14 4.14 5.07 K20 0.74 0.54 0.19 0.96 0.54 0.15 0.30 Sum 99.66 100.92 99.74 100.01 100.64 100.00 99.81 Cations per 8 Oxygens Si 2.7027 2.6278 2.4035 2.7059 2.6641 2.4031 2.5131 Al 1.2851 1.3591 1.5594 1.2900 1.3188 1.5718 1.4698 Ti 0.0017 0.0 0.0017 0.0003 0.0007 0.0014 0.0020 Fe 0.0146 0.0104 0.0214 0.0082 0.0104 0.0133 0.0155 Mn 0.0 0.0004 0.0 0.0 0.0008 0.0008 0.0 Mg 0.0020 0.0020 0.0068 0.0 0.0020 0.0054 0.0067 Ca 0.3094 0.3759 0.6057 0.2996 0.3562 0.6274 0.5123 Na 0.6329 0.6035 0.4220 0.6347 0.6152 0.3644 0.4438 K 0.0423 0.0306 0.0111 0.0546 0.0306 0.0087 0.0173 XY 1.00 1.02 1.07 1.00 1.02 1.02 1.00 Z 3.99 3.99 3.96 4.00 3.98 3.98 3.98 An 31.4 37.2 58.3 30.3 35.5 62.7 52.6 Ab 64.3 59.8 40.6 64.2 61.4 36.4 45.6 Or 4.3 3.0 1.1 5.5 3.1 0.9 1.8 Spot designation: p-rim = phenocryst rim(10-30 microns from margin); pi-rim = inner rim of phenocryst(30-50 microns from margin); gm = groundmass cr y s t a l ; other abbreviations as in Tables 3.2 and 3.4. * Cpx - Opx crystal clot CTi o 161 C a l c i c rims may be separated from sodic cores by a dark grey to almost colourless zone of 'clouded' plagioclase composed of micron-sized melt inclusions variably enriched i n c r y p t o c r y s t a l l i n e opaque material. At i n t e r v a l s within t h i s zone a feldspar base may become so i n d i s t i n c t that the region resembles oxide-charged groundmass adjacent to the i n c l u s i o n -free rim. The i n c l u s i o n zone has a honeycomb structure and an inner boun-dary that commonly exhibits p r e f e r e n t i a l corrosion i n the [010] d i r e c t i o n . These textures bear a s t r i k i n g resemblance to the sieve-textured or 'man-t l e d ' c r y s t a l s produced experimentally during d i s s o l u t i o n of plagioclase i n the system Di-An-Ab (Tsuchiyama, 1985). Similar textures occur less frequently i n plagioclase phenocrysts i n MLT-II's. Rarely, phenocrysts display a curious asymmetry i n which part of the c r y s t a l has suffered s u b s t a n t i a l corrosion whereas other margins are faceted and barely a f f e c t e d . In one case, a mafic groundmass adhering to the resorbed margin leaves l i t t l e doubt that such assymmetry developed when compositional gradients i n the melt were s u f f i c i e n t l y sharp as to allow portions of a single phenocryst to be exposed to attack by b a s a l t i c magma. In general, the groundmass adjacent to such c r y s t a l s i s quite uniform i n composition and any inhomogeneities that may have formerly existed have since disappeared. In other cases, t h i s texture may have been caused by disaggregation of synneusis intergrowths (Vance, 1969). Representative analyses of plagioclase feldspars are given i n Table 3.7. Plagioclase compositions determined by microprobe analysis are i n good agreement with those determined o p t i c a l l y . The most c a l c i c p l a g i o -clase compositions are encountered i n two-pyroxene c r y s t a l c l o t s (An 6 j Or ) and amphibole breakdown products (An Or ); i n t h i n c a l c i c zones 0.8 5 1 2 . 5 within o s c i l l a t o r y zoned phenocrysts; and i n microphenocrysts and ground-162 mass laths i n MLT-I's. Minor elements i n plagioclase are near the l i m i t s of a n a l y t i c a l un-certa i n t y with the exception of FeO which commonly exceeds 0.2 wt. %, abundance lev e l s common to most other volcanic plagioclase (Smith, 1974). Within i n d i v i d u a l samples Fe i s generally higher i n phenocryst rims r e l a -t i v e to cores, and microphenocrysts and groundmass c r y s t a l s r e l a t i v e to phenocrysts. No c o r r e l a t i o n can be recognized between Fe content and eith e r plagioclase or host rock composition, suggesting some degree of metastable enrichment of Fe during quenching. 3.4.5 Oxides Opaque oxides occur i n a l l the lavas of I z t a c c i h u a t l . Titanomagnetite and ilmenite form subhedral to anhedral microphenocrysts (0.20-0.35 mm) that grade s e r i a l l y i n t o dust-size p a r t i c l e s i n the groundmass. Fe-Ti oxide microphenocrysts are commonly intergrown with deeply pleochroic s c h i l l e r e d hypersthene and amphibole, but are generally excluded from p l a g i o c l a s e . Minute opaque granules (<2 um) are a major constituent of the glassy inclusion-charged margins of many plagioclase phenocrysts i n mixed lavas. The modal abundance of s p i n e l r e l a t i v e to the rhombohedral phase i n the breakdown products of hydroxylated minerals appears to be rela t e d to the T1O2 content of the host mineral: ilmenite i s more abundant than titanomagnetite among the breakdown products of b i o t i t e whereas these roles are reversed on amphibole s i t e s . Dark brown cubes or octahedra of chrome s p i n e l (5-15 um across) occur as i s o l a t e d c r y s t a l s or monomineralic ag-gregates (<50 um) i n the cores of o l i v i n e phenocrysts i n mixed lavas. Although d i s c r e t e homogeneous oxide phases are found i n these lavas, intense oxidation of groundmass c r y s t a l s and microphenocryst rims, r e s u l t -163 Table 3.8; Microprobe Point Analyses of Oxides Sample R-1 R-1 R-1 Hb Dacite R-1 R-1 R-1 R-1 R-1 Analysis A1A A2 A1B A3 A1C A4 2F 2F Spot m-core m-core m-core m-core m-core m-core gm gm Phase Mt Ilm Mt Ilm Mt Ilm Mt Ilm Si0 2 0.34 0.10 0.33 0.09 0.34 0.09 0.24 0.12 TiO, 11.61 44.63 11.59 44.85 11.51 44.30 9.99 44.20 Al.O, 1.09 0.03 1.05 0.07 1.11 0.09 0.96 0.34 Cr.O, 0.39 0.07 0.34 0.06 0.35 0.06 0.86 0.14 FeO 79.56 50.41 79.67 49.85 79.57 50.64 81.85 48.15 MnO 0.31 0.49 0.30 0.50 0.32 0.42 0.41 0.66 MgO 0.96 1.67 1.02 1.66 1.01 1.73 0.53 1.22 CaO 0.06 0.00 0.07 0.05 0.07 0.04 0.07 0.10 NiO — — - - - - -Sum 94.32 97.40 94.37 97.13 94.28 97.37 94.91 94.93 Recalculated Analyses FeO 40.21 36.78 40.10 36.91 40.02 36.38 39.40 36.92 Fe^O, 43.72 15.15 43.97 14.38 43.95 15.84 47.17 12.48 Sum 98.70 98.91 98.77 98.57 98.68 98.95 99.63 96.18 Usp(mol %) 35.01 34.66 34.60 30.73 RjO,(mol %) 14.83 14.16 15.54 13.24 T ( (C) 876 868 878 822 1 o * f o, -12.0 -12 .2 -11 .9 -13 .0 Table 3.8 (Cont'd): Hb Dacite Mixed Lava Valley of Mexico Type I b a s a l t i c lava Sample R-1 P-12 R-6 R-6 MXC-7 MXC-7 Analysis 2E NG 1 2 B Cl Spot m-core 1 m-core* m-core 1 m-core 1 m-core 5 m-core' Phase Mt Ilm Ilm Ilm Cr-Sp Mt S i 0 2 0.10 0.14 0.08 0.09 0.22 0.21 TiOj 5.66 41.18 41.98 42.25 0.17 9.26 Al-O, 1.05 0.26 0.44 0.34 14.26 3.09 Cr 20, 0.74 - - - 46.38 12.41 FeO 85.56 52.58 49.50 50.41 25.08 67.85 MnO 0.40 0.02 0.38 0.33 0.27 0.32 MgO 0.48 2.95 3.63 3.02 11.40 3.62 CaO 0.02 0.03 0.02 0.03 0.04 0.06 NiO - - - - 0.21 0.16 Sum 94.01 97.16 96.03 96.47 98.03 96.98 Recalculated Analysis FeO 35.33 31.88 30.96 32.34 16.33 34.74 55.81 23.00 20.60 20.08 9.72 36.79 Sum 99.60 99.46 98.09 98.48 99.00 100.66 Usp(mol %) 18.32 R 20j(mol %) 22.42 20.40 19.80 Recalculation procedures, temperatures and oxygen f u g a c i t i e s according to Ghiorso and Carmichael (1981). Mt = titanomagnetite; Ilm = ilm e n i t e ; Cr-Sp •= chrome s p i n e l ; m = microphenocryst; gm = groundmass. 1 Intergrown with hypersthene i n bim i n e r a l i c c r y s t a l c l o t 1 B i o t i t e breakdown product ' Enclosed w i t h i n core of o l i v i n e phenocryst * Enclosed within rim of o l i v i n e phenocryst 165 Ing i n the formation of hematite, i s e s p e c i a l l y common i n the younger Hb dacites, and microphenocryst cores may exhibit t r e l l i s e d oxidation-'exsolution' textures. From detai l e d observations i n r e f l e c t e d l i g h t , Steele (1971) reported the presence of maghemite i n I z t a c c i h u a t l lavas, and i n a more comprehensive study of the S i e r r a Nevada and Valley of Mexico, Negendank (1972) recognized a complete range of oxidation textures, i n -volving pseudobrookite and r u t i l e at the most advanced stages of a l t e -r a t i o n . The compositions of homogeneous oxide phases have been determined for two Hb dacites, a mixed lava (MLT-I), and a b a s a l t i c flow (MXC-7) (Table 3.8). The amount of Fe^O^ i n the analysis, and u l v o s p i n e l and ^ 2 ^ 3 c o m P ° -nents i n the cubic and rhombohedral phases respectively, were calculated according to the procedures of Ghiorso and Carmichael (1981). Abundances of V, Zn, and Ni i n Fe-Ti oxides have not been determined but probably contribute no more than an a d d i t i o n a l 1 wt % to the oxide sum, assuming that t h e i r concentrations are comparable to those of s i m i l a r phases occur-r i n g i n andesitic lavas of Ceboruco and Colima volcanoes further west (Nelson, 1980; Luhr and Carmichael, 1980). Discrete titanomagnetite microphenocrysts (34.6-35.0 mol. % Usp) i n Hb dacite (R-1) are r i c h e r i n TiO^ and MgO but poorer i n Cr^O^ and MnO than groundmass titanomagnetite (30.7 mol. % Usp). Coexisting ilmenite micro-phenocrysts (14.2-15.5 mol. % R2^3^ a r e e n r i c h e d i n MgO but poorer i n MnO, A^O^, and C^O^ than groundmass ilmenite (13.2 mol % R^O^). Titanomagne-t i t e c oexisting with hypersthene i n bimin e r a l i c c l o t s i s impoverished i n TiO^ (18.3 mol. % Usp) r e l a t i v e to disc r e t e microphenocrysts i n the same rock and has MgO, MnO, and Or 0 abundances s i m i l a r to those i n groundmass grains. Ilmenite intergrown with hypersthene i n mixed lava R-6 i s enriched i n ^ 2 ^ 3 mol. %) and has d i s t i n c t l y higher MgO than microphenocrysts i n Hb dacite. Ilmenite formed by the breakdown of b i o t i t e has even higher R 2 0 3 (22.4 mol. %) and i s likewise high i n MgO. No r e l i a b l e microprobe determinations were obtained on the small s p i n e l i n c l u s i o n s within o l i v i n e phenocrysts i n I z t a c c i h u a t l lavas. However, analyses were successfully performed on larger c r y s t a l s (20-30 pm) i n o l i v i n e phenocrysts of i d e n t i c a l composition from one of the b a s a l t i c flows i n the Valley of Mexico (MXC-7, Table 3.8). Euhedral s p i n e l i n o l i v i n e cores (Fo ) i s r i c h i n chrome (46 wt % Cr 0 ), contains r e l a t i v e l y high oo z 3 +2 abundances of A l ^ and MgO, and has high Mg/(Mg+zFe ) (0.58). The Cr/(Cr+Al) r a t i o of 0.68 i s s i g n i f i c a n t l y higher than t y p i c a l s p i n e l compo-s i t i o n s i n M0RB and abyssal p e r i d o t i t e s but s i m i l a r to r a t i o s observed i n spinels of a r c - r e l a t e d lavas (Dick and Bullen, 1985). A s p i n e l c r y s t a l p a r t i a l l y enclosed i n the rim of an o l i v i n e phenocryst (=Fo ) i s a chrom-o0 ian titanomagnetite with lower Cr^O-j (12 wt % ) , Al^O^, MgO, and NiO, and higher TiO^ and ZFe than s p i n e l i n the core region. The change i n s p i n e l composition with decreasing f o r s t e r i t e content of the o l i v i n e host i s as expected over the normal course of c r y s t a l l i z a t i o n ( H i l l and Roeder, 1974). 3.4.6 B i o t i t e and Quartz Phenocrysts of b i o t i t e and quartz occur i n approximately equal propor-tions i n Hb dacites but quartz appears to be much more common i n mixed lavas on account of i t s highly refractory nature. B i o t i t e occurs as subhedral laths or resorbed c r y s t a l s less than 1.3 mm i n diameter. Pleochroism i s t y p i c a l l y dark coffee brown to pale brown or green, or reddish-orange when oxidized. Reaction at the rims of b i o t i t e phenocrysts forms concentrations of opaque oxides or a granular intergrowth Table 3.9: Representative Microprobe Point Analyses of Biotites Sample Analysis Spot S i 0 2 T i 0 2 A1 20 3 FeO MnO MgO CaO Na20 K20 Hb Dacite P-12 11c p-core 36.87 4.79 13.20 14.42 0.08 14.95 0.01 0.83 8.26 Mixed Lavas Type I P-1 lc p-core 37.95 4.92 13.85 14.42 0.07 15.39 0.02 0.93 8.71 P-3 7c p-core 37.96 4.98 13.42 13.74 0.06 15.67 0.0 1.01 8.65 Mixed Lava Type II R-ll 7c p-core 37.62 4.41 14.03 17.13 0.13 13.83 0.0 0.54 8.93 Sum 93.41 96.26 95.49 96.62 Cations per 22 Oxygens Si 5.6027 5.5913 5.6225 5.5942 Al 2.3641 2.4050 2.3428 2.4589 Ti 0.5474 0.5451 0.5547 0.4932 Fe 1.8326 1.7768 1.7020 2.1303 Mn 0.0104 0.0088 0.0076 0.0165 Mg 3.3862 3.3797 3.4596 3.0654 Ca 0.0016 0.0032 0.0 0.0 Na 0.2445 0.2657 0.2901 0.1557 K 1.6012 1.6370 1.6344 1.6940 Z 7.97 8.00 7.97 8.05 Y 5.78 5.71 5.72 5.71 X 1.85 1.91 1.92 1.85 Mg # 64.9 65.5 67.0 59.0 Abbreviations as in Tables 3.2 and 3.3. 169 of orthopyroxene + ilmenite + magnetite ± K-feldspar. The lack of clinopy-roxene and plagioclase, and increased abundance of ilmenite, distinguishes these breakdown products from amphibole pseudomorphs. Apatite and i r o n -titanium oxides are found as i n c l u s i o n s . Quartz phenocrysts (<1 mm) are i n v a r i a b l y rounded and may possess i n t r i c a t e embayments and inclusions of colourless to pale brown r h y o l i t i c glass or rare hornblende. Reaction coronas of clinopyroxene are most commonly found i n mixed lavas but not r e s t r i c t e d to them (Figure 3.3). The densest c l u s t e r s of clinopyroxene c r y s t a l s i n these reaction rims generally occur a short distance away from the edge of the quartz c r y s t a l suggesting nucleation within the boundary layer melt. Both Sato (1975) and Watson (1982) have recognized the complex i n t e r d i f f u s i o n e f f e c t s accompanying d i s s o l u t i o n of quartz i n b a s a l t i c to intermediate melt compositions. Representative microprobe analyses of b i o t i t e phenocrysts i n mixed lavas and Hb dacites are given i n Table 3.9. B i o t i t e compositions exhibit uniformly high TiO^ (4.4-5.0 wt %) and are r e l a t i v e l y magnesian with l i m -i t e d v a r i a t i o n i n Mg #'s (0.67-0.59), suggesting that they c r y s t a l l i z e d from l i q u i d s of r e s t r i c t e d composition. Variations i n Al-Mg-Fe are t y p i c a l of b i o t i t e s occurring i n orogenic rock suites (Ewart, 1979, 1982). +2 I z t a c c i h u a t l b i o t i t e s have higher Mg/(Mg+ZFe ) than phenocrysts i n rhyo-l i t i c lavas of Michoacan ( S i l v a , 197 9) or b i o t i t e s i n r h y o l i t i c pumice (76 wt % Si02) exposed north of I z t a c c i h u a t l at Rio F r i o (Appendix B). Their o v e r a l l chemistry i s very s i m i l a r to b i o t i t e phenocrysts coexisting with plagioclase + pyroxene + amphibole ± quartz i n rhyodacitic lavas (6 9-71 wt % SiO ) of the Lassen Peak region (Carmichael, 1967). 3.4.7. Accessory Minerals 169 Accessory phases i n I z t a c c i h u a t l lavas include z i r c o n and ap a t i t e . Zircon occurs rarely as clear euhedral c r y s t a l s (<80 um) enclosed within hornblende, hypersthene, and ilmenite. Apatite forms euhedral prisms (<0.1 mm) commonly included i n hypersthene, hornblende, plagioclase, and i r o n -titanium oxides, l i s t e d i n order of decreasing frequency of occurrence. Some hypersthene phenocrysts contain annular concentrations of apatite needles with c-axes oriented i n the plane of former growth surfaces i n t h e i r host. Luhr and Carmichael (1980) observed s i m i l a r textures i n plagioclase and orthopyroxene phenocrysts of hornblende andesites from Volcan Colima. Green and Watson (1982) att r i b u t e d concentric zonation of apatite inclusions i n the phenocrysts of orogenic volcanic rocks to a boundary layer phenomenon whereby the slow d i f f u s i v i t y of phosphorus i n s i l i c a t e melts leads to l o c a l i z e d concentration of r^O *~n t n e m e ^ t bound-ary layer ahead of an advancing c r y s t a l i n t e r f a c e , r e s u l t i n g i n c r y s t a l l i -zation of apatite which i s trapped within the growing c r y s t a l . 3.5 O l i v i n e - L i q u i d E q u i l i b r i a Maximum and minimum f o r s t e r i t e contents of o l i v i n e s i n mixed lavas and b a s a l t i c reference samples are plotted against whole-rock Mg # +2 (100Mg/(Mg+0.85zFe )) i n Figure 3.4 assuming an a r b i t r a r y oxidation state. The l i m i t e d compositional v a r i a t i o n shown by the most magnesian o l i v i n e cores (Fo -Fo ) and lack of c o r r e l a t i o n between core or rim compositions yo oo and host rock Mg # (or any other whole-rock compositional variable) rules out r e l a t i n g these lavas simply by f r a c t i o n a l c r y s t a l l i z a t i o n of o l i v i n e -bearing extracts. A l t e r n a t i v e l y , Mg-rich o l i v i n e cores consistently f a l l w ithin the spectrum of compositions ( F o n -Fo ) t y p i c a l l y associated with y4 oo mantle l h e r z o l i t e s and harzburgites(Ringwood, 1975; Yoder, 1976; B a s a l t i c Figure 3.4: Plot of o l i v i n e phenocryst compositions (Fo mol. %) vs. host rock Mg-number calculated at a f i x e d oxidation state +3 +2 (Fe =0.15zFe ) for mixed lavas and b a s a l t i c reference samples. Olivines i n mixed lavas (MLT-I's) from Rodillas and Pies are denoted by t r i a n g l e s and c i r c l e s r e s p e c t i v e l y , and diamonds are phenocrysts i n b a s a l t i c lavas and scoriae. Closed and open symbols represent core and rim compositions r e s p e c t i v e l y . Curves denote values (Roeder and Emslie, 1970). The shaded region shows the range of values f o r l i q u i d s e q u i l i b r a t e d with F o g Q to F o Q O at v a r i a b l e oxidation states (Fe + 3=10-15%2:Fe + 2). 1 0 0 9 0 CD c O o 8 0 7 0 L -6 0 Mixed Lavas / Kd = (Fe/Mg) Olivine (Fe/Mg) Host Rock Basaltic Rocks 6 5 7 0 + 2 100 M g / ( M g + 0 . 8 5 E F e + 2 ) H o s t R o c k 7 5 Volcanism Study Project, 1981) so that these lavas could conceivably r e -present a series of mantle-derived p a r t i a l melts that have undergone l i t t l e modification by low pressure c r y s t a l f r a c t i o n a t i o n . In f a c t , experiments designed to investigate the composition of l i q u i d s produced during p a r t i a l melting of model p e r i d o t i t e s under water-saturated conditions and moderate to high pressures have demonstrated that quartz-normative l i q u i d s may be generated (Kushiro, 1972; Nicholls and Ringwood, 1973; N i c h o l l s , 1974). Ignoring the obvious d i f f i c u l t i e s of accounting for the d i s e q u i l i b r i u m phenocryst assemblages i n mixed lavas, a n d e s i t i c (58-60 wt % SiO^) l i q u i d s that p r e c i p i t a t e o l i v i n e on cooling are expected to have a p a r t i t i o n coef-melt (Roeder and Emslie, 1970) approaching 0.40 ( N i c h o l l s , 1974). Hence values <0.30 defined by o l i v i n e cores and a n d e s i t i c to d a c i t i c whole-rock compositions (64-58 wt % SiO^) are u n l i k e l y to r e f l e c t equilibrium. Fur-thermore, o l i v i n e phenocrysts of i d e n t i c a l composition are found i n un-contaminated b a s a l t i c lavas i n the Valley of Mexico and at La Joya on the southern flank of I z t a c c i h u a t l , and i t i s these bulk compositions that constitute the most l i k e l y candidates for olivine-melt e q u i l i b r a t i o n ( F i g -+2 ure 3.4). Assuming a range of magmatic oxidation states (10-15% ZFe as +3 Fe ), values for liquidus o l i v i n e (core) - bulk rock pairs i n the b a s a l t i c compositions range from 0.32-0.40, implying Mg values of 70-78 for b a s a l t i c melts i n equilibrium with Fo_ - F o Q O (the shaded region i n Figure yo oo 3.4). Assuming that the quickly quenched sc o r i a sample (IZ-839) i s r e -+3 +2 presentative and standardizing Fe to 15%ZFe defines a of 0.34. This value i s s i g n i f i c a n t l y higher than K-^ 's of 0.27-0.28 which appear ap-propriate f or ocean f l o o r t h o l e i i t e s at comparable oxidation states (Dungan et a l . , 1978; B a s a l t i c Volcanism Study Project, 1981) and f a l l s j u s t f o r equilibrium exchange of Fe +2 and Mg between o l i v i n e and outside the range of K Q ' S ( 0 . 3 0 ± 0 . 0 3 ) recommended by Roeder and Emslie ( 1 9 7 0 ) for a wide range of t e r r e s t r i a l basalts. Allowing for a l l Fe as +2 Fe y i e l d s a minimum of 0.30 f o r these Mexican b a s a l t i c compositions. The cause of t h i s discrepancy i s a t t r i b u t e d to v a r i a t i o n s i n melt composition since i s v i r t u a l l y independent of temperature, pressure, and oxygen fugacity (Roeder and Emslie, 1 9 7 0 ; Roeder, 1 9 7 4 ; Leeman, 1 9 7 8 ; Bender et a l . , 1 9 7 8 ; Longhi et a l . , 1 9 7 8 ; Ford et a l . , 1 9 8 3 ) . The compo-s i t i o n a l l y corrected equations of Ford et a l . ( 1 9 8 3 ) , based on synthetic and natural olivine-melt e q u i l i b r a t e d p a i r s , y i e l d s i m i l a r K Q ' S to those derived using Roeder and Emslie's equations; likewise a pressure increase of up to 1 0 - 1 5 kb also has l i t t l e e f f e c t on o l i v i n e - l i q u i d p a r t i t i o n i n g , creating uncertainties much less than those surrounding the exact oxidation state. Longhi et a l . ( 1 9 7 8 ) , for example, at t r i b u t e d differences i n of 0.28 and 0.33 between high-Ti and low-Ti lunar basalts respectively to the e f f e c t of composition on melt structure and noted a strong p o s i t i v e cor-r e l a t i o n between and bulk rock SiO^ which they considered a p r i n c i p a l factor i n determining values f o r t e r r e s t r i a l b asalts. The r e l a t i v e l y high K J J ' S i n f e r r e d for these Mexican b a s a l t i c compositions deviate i n the sense predicted by t h e i r r e l a t i v e l y s i l i c e o u s and a l k a l i - r i c h compositions. 3.6 O l i v i n e Morphology - Composition Relations Chemical zonation coupled with euhedral to s k e l e t a l phenocryst morpho-logies point unequivocally to o l i v i n e c r y s t a l l i z a t i o n from a s i l i c a t e melt rather than inheritance of phenocrysts from previously consolidated magma. Oli v i n e morphology and chemistry may be used to determine the conditions of c r y s t a l l i z a t o n and composition of equilibrium l i q u i d s . Dynamic c r y s t a l l i z a t i o n experiments on lunar and t e r r e s t r i a l basalts, 174 summarized by Lofgren (1980), have demonstrated systematic v a r i a t i o n s i n o l i v i n e shape such that the t r a n s i t i o n from a well-faceted equilibrium habit to a non-equilibrium s k e l e t a l growth form commonly r e f l e c t s an i n -crease i n cooling rate or degree of undercooling (AT). At a f i x e d cooling rate or AT the development of a p a r t i c u l a r o l i v i n e morphology depends to some extent on sample-specific properties such as v i s c o s i t y and normative o l i v i n e composition, but remains e s s e n t i a l l y independent of oxygen fugacity or phase r e l a t i o n s h i p s (Donaldson, 1976, 1977, 1979; Lofgren and Donalds-on, 1975). Quantitative estimates of these variables are thus uncertain where bulk compositions deviate s i g n i f i c a n t l y from those studied exper-imentally, but given an appropriate suite of samples r e l a t i v e cooling h i s t o r i e s may be deduced. Suitable reference material f o r the i n t e r p r e t -ation of t e x t u r a l and compositional v a r i a t i o n s of o l i v i n e phenocrysts i n the mixed lavas of I z t a c c i h u a t l i s provided by b a s a l t i c scoriae erupted from p a r a s i t i c vents on the southern flank of the volcano and flows of s i m i l a r composition i n the Valley of Mexico. Relevant compositional data are presented i n Figure 3.5 which shows the various morphological and s i z e categories of o l i v i n e s and host-rock SiO^ concentration. O l i v i n e phenocrysts and microphenocrysts i n rapidly c h i l l e d b a s a l t i c s c o r i a (IZ-839) exhibit l i t t l e zoning and lack pronounced s k e l e t a l out-growths. Their counterparts within the slowly cooled i n t e r i o r of a holo-c r y s t a l l i n e b a s a l t i c flow (MXC-7) are more extensively zoned, groundmass o l i v i n e i s Fe-enriched, and d e n d r i t i c protrusions are also rare. O l i v i n e compositional v a r i a t i o n i n scoriae (AFo = 2 mol. %) versus flow i n t e r i o r (AFo = 11 mol. %) evidently r e f l e c t s differences i n cooling rate since bulk compositions and near-liquidus phase assemblages are very s i m i l a r . At the horizon sampled, t h i s flow cooled s u f f i c i e n t l y slowly to allow o l i v i n e -Figure 3.5: D i s t r i b u t i o n of o l i v i n e morphologies and composition i n the mixed lavas of I z t a c c i h u a t l and b a s a l t i c reference samples at La Joya and i n the Valley of Mexico. MLT-I = Mixed Lava Type I; MLT-II = Mixed Lava Type I I ; and SiO^ abundances r e f e r to host rock compositions. M L T - U SiO c R-11 • r r • a ID r r 64.33 • • wt.% i 90 M L T -i -I \ \J J 88 88 1 82 86 i 80 84 ^ r 1 : 1 73 82 P-9 i • • 1 1 • • OO I o i i 62.73 I P-3 i 1 1 • A T A AA 1 1 i i 62.57 I P-4 I T T T i i V • 1 1 O i i ' 62.14 R-6 i i i • • 1 T 0 1 i 62.06 i P-1 1 • . T • • i i V O V 0 i 1 i I 61.53 i R-5 i • • • T 1 1 V • O i 0 1 60.35 0 R-12 • i • • o i i O 1 1 i 60.30 l P-15 1 1 1 • • • mo o 1 0 i i 59.82 O I R-9 T 1 • o o 1 o 1 J J i 81 58.32 90 88 B a s a l t i c R o c k s 1 86 1 84 I i 82 MXC-7 • • • • • A o 0 A O 53.88 A A i 88 lZ-839 i 1 A i i J J i J J Q 86 84 • 1 I i 82 I 1 80 T— 1 1 52.71 i •—' 90 88 86 Fo (mo! %) 84 82 Phenocryst • core Microphenocryst A core Glomerocryst T core O rim A rim v rim Hopper • core Microxenolith 0 rim Olivine • core Groundmass A • rim Olivine 177 melt reaction and complete s o l i d i f i c a t i o n of r e s i d u a l l i q u i d whereas the s c o r i a sample was quenched to glass plus c r y s t a l s before reaction to ortho-pyroxene could take place. B a s a l t i c microxenoliths i n MLT-II sample R-11 apparently experienced cooling h i s t o r i e s comparable to flow i n t e r i o r s judging from t h e i r r e l a -t i v e l y coarse groundmass, strongly zoned o l i v i n e phenocrysts (AFo = 14 mol. % ) , and well-developed orthopyroxene reaction rims. They are most l i k e l y cognate i n o r i g i n and may represent material plucked from conduit walls. Olivine phenocrysts i n MLT-II free of adhering c r y s t a l l i z a t i o n products have o p t i c a l l y s i m i l a r zoning patterns and reaction textures. Glom