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Field relations and petrology of the Rainbow Range shield volcano, west-central British Columbia Bevier, Mary Lou 1978

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FIELD RELATIONS AND PETROLOGY OF THE RAINBOW RANGE SHIELD VOLCANO, WEST-CENTRAL BRITISH COLUMBIA by MARY LOU BEVIER .S. (Hons.)» University of C a l i f o r n i a , Santa Cruz, 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Department of Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1978 (c) Mary Lou Bevier, 1978 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish 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 representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of Brit ish Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date 2 8 A P r i l 1 9 7 8 - i i -FIELD RELATIONS AND PETROLOGY OF THE RAINBOW RANGE SHIELD VOLCANO, WEST-CENTRAL BRITISH COLUMBIA ABSTRACT The Rainbow Range i s a Late Miocene shield volcano (30 km diameter, 370 km^) whose stratiform flanks surround a complex central vent zone. Over a time span of 1-2 m.y., extrusion of highly f l u i d comendites and comenditic trachytes, along with minor mugearites and hawaiites, b u i l t up the gently sloping flanks. The v i s c o s i t y of the peralkaline lavas was so low that their eruption produced a sh i e l d volcano rather than a composite cone. Comenditic trachytes (65.5 percent S102) are the lowest flows exposed on the north flank of the Rainbow Range. Chemical t r a i t s include high Na20 + K/jO (11 percent), moderately high AI2O3 (15 percent), low t o t a l i r o n as Fe203 (5 percent), and high Ba (300-1000 ppm). Thin flows of mugearite (54.9 percent Si02) rest on the comenditic trachytes. Comendites (68.7 percent Si02) uncon-formably overlie the mugearites and account for at least 75 percent of the volume of flows within the flank zone. These lavas are distinguished by lower AI2O3 (13 percent), higher t o t a l iron as Fe203 (7 percent), and extremely depleted Sr (1-10 ppm) and Ba (10-100 ppm). The termination of flank volcanic a c t i v i t y i s recorded by the eruption of capping flows and related feeder dikes of hawaiite (50.1 percent Si02). Comenditic trachytes contain phenocrysts of anorthoclase (Or25_27)> heden-bergite, and iron-titanium oxides i n a groundmass of a l k a l i feldspar, quartz, acmite, iron-titanium oxides, aenigmatite, and arfvedsonite. Comendites bear the phenocryst assemblage sanidine (Or34_37) + hedenbergite + f a y a l i t e + arfvedsonite set i n a p i l o t a x i t i c groundmass of a l k a l i feldspar, quartz, acmite, iron-titanium oxides, aenigmatite, and arfvedsonite. ~ i i i -Continuous v a r i a t i o n i n major and trace element trends and feldspar compositions suggests that the hawaiite-mugearite-comenditic trachyte-comendite suite was derived from an a l k a l i basalt parent, tapped several times as i t underwent prolonged 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 an i n t r a c r u s t a l magma chamber. A b e s t - f i t mathematical model for the o r i g i n of the suite involves step-wise derivation of the lavas.in the order hawaiite > mugearite ? comenditic trachyte > comendite, with the main phases p r e c i p i t a t i n g out i n the order o l i v i n e , clinopyroxene, plagioclase, iron-titanium oxide, and a l k a l i feldspar. Strontium isotopic evidence indicates that the peralkaline lavas were erupted soon after d i f f e r e n t i a t i o n . The Rainbow Range and other peralkaline and alkaline volcanic centers of the Anahim volcanic belt are coeval.with calc-alkaline volcanic centers of the Pemberton volcanic b e l t . Together these belts outline the orientation and extent of the subducted Juan de Fuca plate during Late Miocene time. Volcanic a c t i v i t y i n the Anahim be l t may be related to a) an "edge e f f e c t " of the subducted Juan de Fuca plate, b) movement of the North American plate over a mantle hot spot at a rate of 2-3 cm/year, or c) an east-west trending r i f t zone. - i v -TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES AND PLATES v i i i ACKNOWLEDGEMENTS x INTRODUCTION 1 Location and Access 1 Purpose 1 Present Investigation 3 Previous Work 3 REGIONAL SETTING 4 Geology 4 Late Miocene Tectonic Setting of West-Central B r i t i s h Columbia 6 FIELD RELATIONS 8 North Flank of the Rainbow Range 9 Structure 9 Stratigraphy 9 Comenditic Trachyte Unit 9 Mugearite Unit 11 Comendite Unit 13 Hawaii te Unit 14 - V -TABLE OF CONTENTS (Continued) Page Anahim Peak 16 Structure 16 Stratigraphy 17 Flows 17 Plug Dome. 18 Age of Volcanism 18 PETROGRAPHY 19 Hawaii te 19 Mugearite 20 Comenditic Trachyte 20 Comendite 22 Trachyte 23 MINERAL CHEMISTRY 23 Introduction 23 Methods 24 Olivine 24 Pyroxene Phenocrysts 26 Groundmass Pyroxene 28 Amphibole 30 Aenigmatite 31 Feldspar 32 Iron-Titanium Oxides 36 Other Accessory Minerals 37 WHOLE ROCK CHEMISTRY 40 ' Introduction 40 - v i -TABLE OF CONTENTS (Continued) Page Normative Mineralogy 40 Rock C l a s s i f i c a t i o n 41 Compositional Changes i n S i l i c i c Peralkaline Rocks as a Result of Post-Eruptive Processes. 44 Chemical Variation Between Flows 45 Major Element Oxides 45 Trace Elements 47 Strontium Isotopic Data 50 MORPHOLOGY OF THE RAINBOW RANGE SHIELD VOLCANO 53 ORIGIN OF OVERSATURATED PERALKALINE ROCKS 55 ORIGIN OF.LAVAS FROM THE RAINBOW RANGE AND ANAHIM PEAK 58 ORIGIN OF THE ANAHIM VOLCANIC BELT 61 CONCLUSIONS 65 Eruptive History of the Rainbow Range Shield Volcano 65 Origin of Rainbow Range Lavas and the ' Anahim Volcanic Belt 66 TABLES 68 REFERENCES 93 - v i i -LIST OF TABLES Table Page I. Potassium-argon a n a l y t i c a l data for volcanic rocks from the Rainbow Range 68 I I . Mineral assemblages of volcanic rocks from the Rainbow Range 69 I I I . Microprobe analyses of o l i v i n e phenocrysts 70 IV. Microprobe analyses of pyroxene phenocrysts and groundmass grains 71 V. Microprobe analyses of amphibole grains 73 VI. Microprobe analyses of aenigmatite 74 VII. Microprobe analyses of feldspar phenocrysts 75 VII I . Microprobe analyses of titanomagnetite 78 IX. Microprobe analyses of ilmenite 79 X. Whole rock major- and trace-element analyses and normative mineralogies 80 XI. Strontium isotopic data 84 XII. Calculated and observed v i s c o s i t i e s 85 XII I . Least-squares mass balance calculation deriving mugearite from hawaiite 86 XIV. Least^squares" mass balance .calculation'' deriving comenditic trachyte from hawaiite 87 XV. Least-squares mass balance calculation deriving comendite from hawaiite 88 XVI. Least-squares mass balance calculation deriving comenditic trachyte from mugearite 89 XVII. Least-squares mass balance calculation deriving comendite from mugearite 90 XVIII. Least-squares mass balance calculation deriving comendite from comenditic trachyte 91 XIX. Least-squares mass balance calculation deriving Anahim Peak trachyte from Anahim Peak hawaiite 92 - v i i i -LIST OF FIGURES AND PLATES  Figure Page 1. Location map 2 2. Generalized geologic map of the western part of the Anahim volcanic belt 5 3. Present day plate tectonic setting of south-western B r i t i s h Columbia 7 4. Schematic stratigraphic section of volcanic rocks from the north flank of the Rainbow Range 10 5. Mugearite flows 12 6. Comendite flows 12 7. Hawaiite plugs near T s i t s u t l Peak 15 8. Anahim Peak 15 9. Photomicrograph of anorthoclase i n comenditic trachyte 21 10. Photomicrograph of phenocrysts phases i n comendite 21 11. Triangular diagram for o l i v i n e showing Mn enrich-ment 25 12. Pyroxene phenocrysts plotted on the pyroxene quadrilateral 27 13. Triangular diagram showing acmite, diopside, and hedenbergite components of pyroxene phenocrysts and groundmass grains 29 14. Triangular diagram showing anorthite, a l b i t e , and orthoclase components of feldspar phenocrysts 33 15. Liquidus diagram for the system Q-Ab-Or plus d i f -ferent percentages of additional acmite and nosean 35 16. Compositions of titanomagnetites and ilmenites i n molecular percent of FeO, Fe203, and Ti02 38 17. Log f 0 2 -T diagram i n which are plotted coexisting iron-titanium oxide pairs from Rainbow Range lavas (using curves of Buddington and Lindsley, 1964) 39 18. C l a s s i f i c a t i o n schemes for oversaturated, peralk-aline rocks 43 - i x -LIST OF FIGURES AND PLATES (Continued) Figure Page 19. Major element Harker diagrams for volcanic rocks from the north flank of the Rainbow Range 46 20. Trace element Harker diagrams for volcanic rocks from the north flank of the Rainbow Range 48 21. Trace element rat i o Harker diagrams for v o l -canic rocks from the north flank of the Rain-bow Range 49 22. S r 8 7 / S r 8 6 versus R b 8 7 / S r 8 6 isochron plot 52 23. Late Cenozoic volcanic belts of the Canadian Co r d i l l e r a 62 Plate I. Geology of the North Flank of the Rainbow Range and Anahim Peak, B r i t i s h Columbia....(in back pocket) I I . Map of Sample L o c a l i t i e s ( i n back pocket) - x -ACKNOWLEDGEMENTS I wish to thank Dr. Jack G. Souther of the Geological Survey of Canada for suggesting the project, and providing helicopter support during the fieldwork. I am g r a t e f u l to Drs. Richard L» Armstrong and Jack G. Souther for h e l p f u l discussions, suggestions, and comments on the improvement of the f i n a l manuscript. In addition, I benefited greatly from discussions with other f a c u l t y members and graduate students of the Department of Geological Sciences at the U n i v e r s i t y of B r i t i s h Columbia. K r i s t a Scott was of invaluable assistance with strontium i s o t o p i c measurements, and I thank Joe Harakal f o r running the argon analyses. Brian Apland of the Department of Archaeology, Simon Fraser University, was of great help with the fieldwork and enlightened the author as to the archaeological s i g n i f i -cance of the Rainbow Range. My thanks go out to the many people we met i n Anahim Lake and B e l l a Coola ( e s p e c i a l l y Darcy Christiansen, Cam Moxon, Bob Cohen, Frances Wilmeth, Darryl Hodson, Hank Veelbehr, and Odd Knutsen) who u n s e l f i s h l y provided lodging and shared t h e i r knowledge on l o c a l t e r r a i n and f o l k l o r e . National Research Council of Canada Grant 67-8841 to Dr. R« L. Arms-trong provided funding for the project. - 1 -INTRODUCTION Location and Access The Rainbow Range shield volcano l i e s i n the southern part of Tweeds-muir Provincial Park (Figure 1 ) , along the boundary between the Coast Moun-tains and the Inter i o r Plateau i n west-central B r i t i s h Columbia. A good gravel road (Highway 20) that runs between B e l l a Coola and Anahim Lake comes within 20 km of the f i e l d area, but no adequate access f o r person-nel and gear exists beyond this point. Helicopters provide the easiest means of entering the area. Transwest Airways maintains a small helicopter i n Bel-l a Coola during the summer months. Purpose This project was undertaken as a combined f i e l d and laboratory study of the Late Miocene volcanic rocks of the Rainbow Range. Specific objectives were: 1 ) to map i n d e t a i l the rock units comprising the north flank of the Rainbow Range shield volcano and work out the structure and eruptive history of the volcano, 2) to characterize the mineralogy and chemical composition of lavas from the Rainbow Range, 3) to determine the genetic relations between the lavas, and 4) to establish the connection, i f any, between volcanism and plate tectonic setting. The f i e l d work was conducted i n collaboration with Brian Apland, a grad-uate student i n Archaeology at Simon Fraser University, who was investigating s - 2 -Figure 1. Map of British Columbia showing location of the Rainbow Range. Lined area is Tweedsmuir Provincial Park. - 3 -under contract to the National Museum of Canada, the nature and extent of obsidian sources i n the region, as well as obtaining samples of source material used by p r e h i s t o r i c peoples (Apland, i n press). Present. Investigation F i e l d work was c a r r i e d out during part of July and August* 1976* Four and one-half weeks were spent mapping flows exposed on the northern flank of the s h i e l d volcano on 1:50,000 scale National Topographic Map Series Maps (No. 93G/12 (Tusulko River) and No. 93C/13 (Ulkatcho)). Laboratory work was done at the Uni v e r s i t y of B r i t i s h Columbia* Whole-rock chemical analyses of major-element oxides and selected trace elements were obtained by X-ray fluorescence spectrometry, and mineral chemistry was determined by electron microprobe analysis. K-Ar dates and Sr isotopie r a t i o s were determined by a combination of atomic absorption spectroscopy s X-ray fluorescence spectrometry, and mass spectrometry. Previous Work Volcanic rocks of the Rainbow Range were f i r s t studied by H. W. Tipper (1969) of the Geological Survey of Canada as he mapped the (1° x 2°) Anahim Lake sheet at a scale of 1" = 4 miles. Descriptive notes published with the map state that the c e n t r a l part of the Rainbow Range i s composed of v a r i -colored a n d e s i t i c to d a c i t i c flowsxand fragmental rocks, while the outer con-i c a l flanks are composed of l a t e r eruptions of basalt flows which f l a t t e n and merge with l a t e T e r t i a r y plateau lavas. Anahim Peak, a small i s o l a t e d moun-tain on the northeast flank of the Rainbow Range, i s described as a b a s a l t i c neck surrounded by f l a t - l y i n g flows. - 4 -REGIONAL SETTING Geology Late Miocene volcanic rocks that make up the Rainbow Range are underlain on the west by Jurassic Hazelton Group and on the east by Miocene plateau lavas (Figure 2). The Hazelton Group consists mainly of volcanic breccias, waterlain t u f f s , v a r i c o l o r e d andesites and basalts, and rare sedimentary rocks derived from the volcanic rocks (Baer, 1973). 2 South and east of the Rainbow Range l i e the extensive (>4750 km ) Miocene plateau lavas. These consist of f l a t - l y i n g basalts and b a s a l t i c andesites for which few vent areas are known. South of the Rainbow Range these flows have been upwarped to the west, i n d i c a t i n g that the Coast Moun-tains were s t i l l r i s i n g a f t e r the Miocene (Tipper, 1963). The Rainbow Range i s far enough east of the Coast Mountains that i t has not been noticeably t i l t e d or deformed. Peralkaline rocks are also exposed west and east of the Rainbow Ranget At Tanya Lakes and S i g u t l a t Lake, i n t r u s i v e bodies of syenite (10-^50 km2) are exposed i n north-east trending grabens. On King Island (100 km farther west), a 13+2 m.y. syenite, east-west elongate, pluton i s exposeds Numerous aegerine-augite and/or aenigmatite bearing dikes, along with small syenite bodies, and b a s a l t i c dikes, occur on islands west of King Island, Baer (1973) reports K-Ar age determinations of 14.5 and 12.5 m.y. for two of these dikes, Two other large, peralkaline s h i e l d volcanoes (Ilgachuz Range and Iteha Range) l i e east of the Rainbow Range. No d e t a i l e d mapping has been done i n the Ilgachuz Range; however, geomorphological evidence suggests that i t i s equivalent i n age and composition to the Rainbow Range. K-Ar dates from I Figure 2. Generalized geologic map of the western part of the Anahim v o l c a n i c b e l t , showing d i s t r i b u t i o n of Miocene-Quaternary v o l c a n i c centers (black), Coast Mountain p l u t o n i c rocks (uncolored), Mesozoic supracrustals ( h o r i z o n t a l bars), J u r a s s i c sedimentary rocks ( v e r t i c a l b a r s ) , and Miocene plateau lavas (diagonal b a r s ) . KI- King Island, RR- Rainbow Range, AP- Anahim Peak, IR-Ilgachuz Range, ItR- Itcha Range, TL- Tanya Lakes, SL- S i g u t l a t Lake. - 6 -the central part of the Itcha Range range from 3.2 to 0.78 m.y. (J.E. Hara-k a l , unpublished determinations). The central part of the Itcha Range con-s i s t s of flows, domes, breccias,and tuff breccias, plugs, and shallow intrusions of alkaline and peralkaline phonolites, trachytes, trachyande-s i t e s , and minor r h y o l i t e , which have been strongly dissected by g l a c i a l erosion.(B. P r o f f e t t , personal communication, 1978). Cinder cones and flows of trachybasalt and basanite cap the differentiated volcanic suite.(Nicholls et. a l . , 1976). The basanites contain l h e r z o l i t e nodules whereas the trachybasalts contain large plagioclase xenocrysts. These three shield volcanoes, along with many isolated cinder cones and small flows, define a belt of Late Miocene-Quaternary volcanic centers known as the Anahim volcanic belt*(Souther, 1977) that runs east-west across o B r i t i s h Columbia at approximately l a t i t u d e 52°N. Glaciation has p a r t i a l l y modified the form of a l l these volcanic centers. Few documented faults are associated with these volcanic centers. Tipper (1969) mentions east-west faults i n the Ilgachuz and Itcha Ranges and depicts northwest-trending faults i n the southwestern quadrant of the Rainbow Range. Late Miocene Tectonic Setting of West-Central B r i t i s h Columbia During Late Miocene time, plate geometry off the coast of B r i t i s h Columbia was si m i l a r to the present configuration (Figure 3); however, some doubt exists as to the exact subdivisions between the P a c i f i c , North American, and Juan de Fuca plates and position of the t r i p l e junction at that time. Re-views of the plate geometry of southwestern B r i t i s h Columbia have been given by Barr and Chase (1974), Chase, T i f f i n , and Murray (1976), Riddihough and Hyndman (1976), and Riddihough (1977). - 7 -Figure 3. Present p l a t e t e c t o n i c s e t t i n g of southwestern B r i t i s h Columbia, showing extent of Anahim v o l c a n i c b e l t ( c i r c l e s ) , Pemberton v o l c a n i c b e l t (squares), A l e r t Bay v o l c a n i c b e l t ( s t a r s ) , G a r i b a l d i v o l c a n i c b e l t ( t r i a n g l e s ) , and K-Ar ages i n m.y. Lined area i s the extent of Mio-cene p l a t e a u l a v a s , ( S o u t h e r , 1977). PP- P a c i f i c p l a t e , EP- E x p l o r e r p l a t e , NAP- North America p l a t e , JdFP- Juan de Fuca p l a t e , Bp- Brooks P e n i n s u l a , PRfz- Paul Revere f r a c t u r e zone, S f z - Sovanco f r a c t u r e zone. Dashed l i n e i s p o s s i b l e EP-JdFP boundary (R. Hyndman, o r a l communi-c a t i o n , 1978) . Diagram m o d i f i e d from Riddihough and Hyndman (1976) . - 8 -A r i g h t - l a t e r a l transform f a u l t (Queen Charlotte f a u l t ) separates the P a c i f i c and North American plates, while a subduction zone forms the boundary between the North American and Juan de Fuca plates. The Juan de Fuca-Pacific plate boundary i s a spreading ridge system (Barr and Chase, 1974)* Subduc-t i o n of the Juan de Fuca plate beneath the North American plate gives r i s e to arc and back-arc volcanism. The Anahim volcanic b e l t l i e s near the projected trace of the northern edge of the subducted Juan de Fuca plate, the p o s i t i o n of which i s not w e l l constrained during l a t e Miocene time. Movement i n the p o s i t i o n of the active spreading ridge makes magnetic anomaly patterns d i f f i c u l t to i n t e r p r e t , and i t i s these patterns which are used to determine r e l a t i v e motion between plates, and hence t r i p l e junction position. During the l a s t 10 m i l l i o n years the active ridge between the Juan de Fuca and P a c i f i c plates has repeatedly jumped between the Juan de Fuca, Explorer, Dellwood Kno l l s , and J . Tuzo Wilson Knolls sections (Riddihough and Hyndman, 1976; R. Hyndman, o r a l communication, 1978). The l a t e s t c a l c u l a t i o n s (Riddihough, 1977) place the t r i p l e junction off Brooks Peninsula on northwes-tern Vancouver Island for the period 5 to 10 m i l l i o n years ago, FIELD RELATIONS 2 Approximately 30 km of the northern flank of the Rainbow Range s h i e l d volcano was mapped i n d e t a i l (Plate I ) . This area i s dissected by r a d i a t i n g g l a c i a l v a l l e y s which a f f o r d excellent exposure of the i n t e r n a l structure of the volcano, and the t e r r a i n i s s u i t a b l e f o r backpacking. In addition,- Anahim Peak, on the northeast flank of the volcano, can be reached on foot. - 9 -North Flank of the Rainbow Range .Structure, The Rainbow Range s h i e l d volcano has a diameter of 30 km and 3 volume of approximately 370 km. Over a timespan of approximately 1 to 2 m.y., extrusion of highly f l u i d lavas b u i l t up the flanks of the s h i e l d volcano. Individual flows can be traced for up to 3 km. Primary dips range from 5° - 8°. Flow thickness i s r e l a t e d to chemical composition: basic rocks form 1 - 4 m thick flows, while s i l i c i c flows range from 30 - 60 m thick. The s t r a t i f o r m flank zone surrounds a ce n t r a l complex of small domes, short, thick flows, and small i n t r u s i v e bodies (Souther, 1977) that was not mapped i n t h i s study. No f a u l t i n g or deformation has occurred on the north flank of the volcano since the cessation of volanic a c t i v i t y . As with other p e r a l k a l i n e volcanic complexes i n B r i t i s h Colum-bia-, the morphology and structure of t h i s volcano are s i m i l a r to those of trachyte s h i e l d volcanoes i n the South Turkana region of the Kenya r i f t v a l l e y (Webb and Weaver, 1975). Stratigraphy. A l k a l i n e and p e r a l k a l i n e lava flows from four volca-ni c episodes make up the 845 m section exposed on the north flank. They are informally divided into the comenditic trachyte u n i t , the mugearite unit, the comendite un i t , and the hawaiite unit (Figure 4). With the exception of the hawaiite u n i t , no source vents are found within the map area. Comenditic Trachyte Unit. Outcrops of the lowest unit exposed, the comenditic trachyte u n i t , are r e s t r i c t e d to the sides of the westernmost v a l l e y i n the f i e l d area. Much of the area of exposure i s covered with t a l u s , and i n d i v i d u a l flows can be traced from as l i t t l e as 10 m to up to 2 km. A minimum of eight i n d i v i d u a l flows - 10 -3 to (J) US z o X I- LITHOLOGY DESCRIPTION Grey-greenorange-brown weathering, comendite flows (30-60 m thick) with well-developed co-lumns, chisel marks developed perpendicular to columnar joints, glassy selvage (0.5-1 m thick) at the base of some flows, flows stacked in layers, some f i l l valleys, holocrystalline with 10% phenocrysts of a l k a l i feldspar and clinopyroxene K-Ar date 7.9 section ± 0.3 m.y. at top of exposed UNCONFORMITY Dark grey, red-brown weathering, mugearite flows and flow-breccia; 30-50% flows with 0.5m columns, holocrystalline with 10-15% phenocrysts of plagioclase,clinopyroxene, and olivine; 50-70% breccia, angular to sub-angular clasts of UNCONFORMITY r e d " b r o ™ s c o r i a . *f grey mugear-xte; mxnor a i r - f a l l tuff with an-gular pumice; some comendite s i l l s Light green-grey, orange-brown weathering, comenditic trachyte flows(30-60m thick), well-developed columns that weather into 1-5 cm plates perpendicular to columnar joints, 5-10% pheno-crysts of a l k a l i feldspar and clinopyroxene. K-Ar date 8.7 ± 0.3 m.y. . exposed at base of section SCALE : 5000 Figure A. Schematic stratigraphic section of volcanic rocks from the north flank of the Rainbow Range. - 11 -are present. The base of the >245 m section i s not exposed. The unit consists of a sequence of 30 - 60 m thick flows of green-grey comenditic trachyte that display well-developed columnar j o i n t i n g . Primary dips are 5° - 8° to the north. Columns are 1 - 2 m i n dia-meter and they weather into thin ( 1 - 4 cm) plates that form per-pendicular to the cooling j o i n t s . In many places up to 1 m of brec-c i a i s present at the base of each flow. Flows are hol o c r y s t a l l i n e throughout the un i t . Pnenocrysts of a l k a l i feldspar ( 1 - 2 mm) and clinopyroxene (0.5 - 1 mm) make up 10 percent of the rocks. Mugearite Unit. The second cycle of volcanic a c t i v i t y pro-duced a 140 m thick series of thin (1 - 4 m) mugearite flows and discontinuous lenses of flow breccia that unconformable overlie the thicker comenditic trachyte flows (Figure 5). Dips range from 158 -25° to the north. This unit i s exposed only along the sides of the westernmost valley i n the f i e l d area. Flow breccia accounts for up to 70 percent of the section. Individual flows are hard to follow for more than 0.5 km due to the large proportion of discontinuous breccia zones. Some breccia zones are incorporated within flows. A l l the flows are columnar-jointed. At 64 m above the base of the unit a 2.5 m thick, medium to dark yellow-grey, banded a i r - f a l l t u f f i s present. The layer i s not traceable for more than 20 m i n any d i r e c t i o n ; i t lenses i n and Out and i s therefore not suitable for use as a marker bed. The tu f f i s composed of sub-angular pumice frag-ments that range i n size from 0.1 - 10 mm. Slight rounding of the pumice fragments i s possibly due to abrasion during and after depo- . s i t i o n . The l i g h t to medium-grey mugearite lavas are characterized by thei r predominantly porphyritic nature. Phenocryst content increases up section from 10 to 25 percent, with plagioclase feldspar (0.5 -Figure 5. A series of mugearite flows unconformably overlies comenditic trachyte flows i n the western-most part of the f i e l d area. Scale - 1 km across the photo Figure 6. Thick (30-60 m) comendite flows crop out on a ridge northwest of T s i t s u t l Peak. Scale - 1 km across the photo. - 13 -1 cm) greatly predominant over clinopyroxene ( 1 - 4 mm). The brec-c i a consists of c l a s t s of red s c o r i a and pahoehoe fragments of mugearite that range i n s i z e from 5 - 2 0 cm. Comendite Unit. Voluminous eruption of comendite lava flows was the next phase of volcanism on the north flank of the s h i e l d volcano (Figure 6). The s t r a t i g r a p h i c section i s a minimum of 460 m thick. These flows account for 75 percent of the volume of lava exposed within the flank zone. The flows spread out unconformably over the mugearite unit at slopes of 5° - 8° and b u i l d up the s h i e l d as i t i s seen today. S i l l s of comendite are found within the mugearite u n i t . The flows are r e l a t i v e l y thick (30 - 60 m) compared to basic flows from the volcano. Even so, they were em-placed i n a highly f l u i d condition compared to most s i l i c i c flows since i n d i v i d u a l flows as much as 1 km wide can be traced down a slope of 5° for at l e a s t 3 km. Forty comendite flows have been mapped on the north flank of the volcano, and more are thought to e x i s t under t a l u s . I t i s possible that some of the flows may be c o r r e l a t i v e across v a l l e y s , but since they are i d e n t i c a l i n morpho-logy, mineralogy, and chemical composition, attempts at c o r r e l a t i o n have been unsuccessful. The maximum measurable section i s 460 m thick and consists of approximately t h i r t e e n flows. T s i t s u t l Peak (\2973 m) , the highest peak i n the Rainbow Range, i s at the top of t h i s section. I t i s unknown how much of the stratigraphy has been stripped o f f by g l a c i e r s , but because the o r i g i n a l s h i e l d - l i k e shape of the volcano i s s t i l l preserved, i t appears that not much of the section i s missing. Some flows as much as 90 m thick f i l l v a l l e y s , but for the most part the flows are stacked i n layer-cake fashion. A l l of the flows show well-developed columnar j o i n t i n g - 14 -(2 - 4 m wide) with c h i s e l marks perpendicular to the j o i n t i n g . At the base of most flows the contact with the underlying flow i s abrupt, l i t t l e b r e c c i a t i o n i s present, and a i m thick glassy s e l -vage i s well-developed. This black vitrophyre bears a l k a l i f e l d s -par and clinopyroxene phenocrysts. The comendites are green-grey massive lavas, some v e s i c u l a r i n nature, that bear phenocrysts of a l k a l i feldspar ( 1 - 3 mm) and red weathering clinopyroxene (0.1 - 0.5 mm). Except f o r lavas at the very base of the section, most comendites also contain sparse phenocrysts of o l i v i n e (0.1 - 0.5 mm). Hawaiite Unit. During the waning stages of volcanic a c t i v i t y , hawaiite lavas erupted from at l e a s t f i v e centers scattered across the north flank of the s h i e l d volcano. The hawaiites crop out as plugs, dikes, and flows that cap the comendite un i t . On the western slope of T s i t s u t l Peak at le a s t seven plugs appear as "hoodoos" or pinnacles (Figure 7). The plugs are spher-o i d a l knobs approximately 20 m i n diameter, composed of 1 m wide columns that fan out i n a l l d i r e c t i o n s . At the base of these plugs are breccias consisting of angular to subangular fragments of hawaiite, comendite, and alte r e d red, green, yellow, and grey v o l -canic rock fragments. The breccia does not show any bedding and i t i s poorly sorted, with fragments ranging i n s i z e from 0.5 - 500 cm. Underlying comendite lavas are alte r e d f o r 0.5 km i n a l l d i r e c t i o n s from these plugs. At two l o c a l i t i e s , hawaiites crop out as v e r t i c a l to near v e r t i c a l dikes traceable l a t e r a l l y f o r 10 - 20 m. One of these dikes contains v e s i c l e s f i l l e d with z e o l i t e s . Erosional remnants of capping flows can be seen i n two places. - 15 -Figure 7. Close-up view of hawaiite plugs that crop out on the west side of T s i t s u t l Peak. The large plug on the right i s 20 m across. Figure 8. West side of Anahim Peak (2 km long), showing f l a t - l y i n g flows and plug dome. - 16 " One 10 m thick flow crops out over an area of approximately 1 km . At the base of the flow i s a zone of oxidized lava 1 - 3 m i n thick-ness. Columnar j o i n t i n g i s well-developed throughout the flow. Most hawaiites are p o r p h y r i t i c , containing 15 percent euhedral plagioclase feldspar (0.3 - 1 cm long) and clinopyroxene ( 1 - 2 mm) phenocrysts i n a dark grey aphanitic matrix. At the base of a plug situated one ridge west of T s i t s u t l Peak, c l o t s of xenocrystic plagioclase feldspar and orthopyroxene up to 8 cm across are present i n the hawaiite. Anahim Peak Structure. Anahim Peak (Plate I) i s situated on the flank of the Rainbow Range s h i e l d volcano 10 km northeast of T s i t s u t l Peak. Total 2 area covered by the peak i s s l i g h t l y more than 2 km . The vent for the Anahim Peak flows appears to have opened up through a topographic high of older volcanic rocks. Below present tree-l i n e on Anahim Peak, vegetation i s dense and only where trees have been uprooted does one see pieces of a white flow-banded " r h y o l i t e " that bears quartz and a l k a l i feldspar phenocrysts. Tipper (1969) mapped t h i s unit as being l i t h o l o g i c a l l y i d e n t i c a l to the Cretaceous (?) - T e r t i a r y Ootsa Lake Group that l i e s to the northwest. Before the f i r s t flow erupted from the Anahim Peak vent, a f r i a b l e v o l c a n i c l a s t i c sandstone covered the white r h y o l i t e . The a l t i t u d e of th i s sandstone i s not known. The contact between these two rock types i s not exposed, and i n fac t i t i s not known i f there are other units present. The presence of broken c r y s t a l s of sodic hedenbergite (a clinopyroxene c h a r a c t e r i s t i c of pe r a l k a l i n e rocks from the Rainbow Range) i n the sand-stone indicates that erosion of the flanks of the s h i e l d volcano was - 17 -taking place before Anahim Peak became a center for volcanic a c t i v i t y . At the beginning of the f i r s t eruption, cinders and spatter spewed out and disrupted the top 20 cm of the sandstone near the vent p r i o r to a lava flow spreading out over the area. Subsequent to this flow at l e a s t s i x other flows were erupted from the vent, a l l preceded by spatter (and minor bombs) thrown from lava fountains. Near the vent, a layer of agglutinate up to 2 m thick i s present at the base of most flows. A l l the flows are f l a t - l y i n g and appear to have been constrained i n a r e a l extent when erupted. Anahim Peak hawaiites are almost i d e n t i c a l i n chemical composition to Rainbow Range hawaiites (Table X) yet Anahim Peak flows are four to eight times as thick as Rainbow Range hawaiite flows. Examination of t h i n sections from the base land upper portion of several Anahim Peak hawaiite flows shows that the lower portions of flows have a s i g n i f i c a n t l y coarser-grained groundmass and higher percentage of phenocrysts than the upper portions of flows. These two l i n e s of evidence suggest that the flows were ponded during eruption and cooling, probably as lava lakes within a p y r o c l a s t i c cone, a l l trace of which has now been removed by erosion. The volcanic conduit i s plugged by a trachyte dome which brecciated and a l t e r e d the immediately surrounding lavas. Strat igraphy. Flows,. Seven hawaiite flows with an outcrop area of approxi-2 mately 1 km . each make up the 335 m thick section exposed on Anahim Peak (Figure 8). Flows range i n thickness from 30 - 80 m but an average thickness i s 45 m. Columnar j o i n t i n g i s well-devel-oped^ i n a l l the flows. The j o i n t faces commonly have c h i s e l marks - 18 -on them. Spheroidal weathering i s seen on the sides of some of the columns* The hawaiites contain phenocrysts of plagioclase feldspar and clinopyroxene, both 0.1 - 0.8 mm long. The rocks are commonly coarser-grained than most b a s a l t i c lava flows. This c h a r a c t e r i s t i c , along with the f a c t that the flows are rather thick f or basic lavas, suggests that the flows were ponded during cooling. Plug Dome. The plug dome crops out over an area of about 1/8 km^. The plug i s a light-grey metaluminous trachyte with sodic plagioclase feldspar and clinopyroxene phenocrysts. Thick, poorly developed columns (4 - 5 m diameter) are present and fan out toward the margins of the plug. Where the plug contacts the surrounding flows, there i s a zone of a l t e r a t i o n and b r e c c i a t i o n . The breccia i s yellow and contains angular to sub-angular fragments of massive hawaiite, s c o r i a , white " r h y o l i t e " , and rare granite. Age of, Volcanism Four lavas from the north flank of the Rainbow Range and Anahim Peak were dated by the potassium-argon method. Samples include a comenditic trachyte from the westernmost v a l l e y i n the f i e l d area, a comendite from T s i t s u t l Peak, a hawaiite from 2.5 km north-east of T s i t s u t l Peak, and a hawaiite from Anahim Peak. Location of the samples and th e i r calculated dates are given i n Table T* S t r a t i g r a p h i c a l l y the comenditic trachyte represents the lowest flow that has been mapped on the north flank of the volcano, while the comendite from T s i t s u t l Peak i s topographically the highest flow exposed on the north flank* The hawaiite from 2.5 km northeast of T s i t s u t l Peak i s from a flow that caps the comendite u n i t . The hawaiite from Anahim Peak i s - 19 -from the f i r s t flow that issued out of the vent. Calculated K-Ar dates indicate that the Rainbow Range shield v o l -cano was active during Late Miocene time. The hawaiite flows that were extruded from the Anahim Peak vent are younger than the flows on the north flank of the Rainbow Range. PETROGRAPHY Thin sections of samples from a l l units exposed on the north flank of the Rainbow Range shield volcano and Anahim Peak were examined. A summary of the mineral assemblages i s given i n Table I I . Hawaiite Hawaiites from the north flank of the Rainbow Range and from Anahim Peak are petrographically s i m i l a r . Most samples are highly porphyritic (15 per-cent phenocrysts) but a few from the shield volcano have only 1 - 2 percent phenocrysts. Plagioclase, pyroxene, and o l i v i n e are found as phenocrysts; micro-phenocrysts of magnetite, ilmenite, apatite, and b i o t i t e occur i n trace amounts. Plagioclase (An ). i s found as fragments of euhedral c r y s t a l s , some 59 of which exhibit normal zoning. Most crystals are a l b i t e twinned but a few crystals have p e r i c l i n e and Carlsbad twins. Commonly the phenocrysts are poik-i l i t i c , enclosing small crystals of augite and opaque oxides. Glomeroporphyri-t i c . c l o t s occur i n most .samples. Pink to pale brown phenocrysts of t i t a n -augite have rims of opaque granules and fibrous orthopyroxene (?), showing that they are out of equilibrium with the rock. The clinopyroxene p o i k i l i t i c a l -l y encloses magnetite. In some samples (R-36, R-117), xenocrystic ortho-pyroxene i s present as anhedral macrophenocrysts surrounded by a fibrous rim of clinopyroxene. Forsterite occurs as euhedral phenocrysts i n the Anahim - 20 -Peak hawaiites but only as alte r e d microphenocrysts i n the Rainbow Range hawaiites. The groundmass i s a c r y s t a l l i n e mosaic of twinned plagioclase laths with intergranular augite and opaque oxides. In some samples (R-36, R-117), the groundmass i s h y a l o p i l i t i c . Mugearite The mugearites contain 10 - 25 percent phenocrysts (plagioclase, o l i v i n e , and clinopyroxene) set i n a p i l o t a x i c or t r a c h y t i c groundmass of twinned plagioclase and a l k a l i feldspar laths which enclose equant grains of augite and granules of opaque oxides. Plagioclase phenocrysts (An^^) occur i n gglpmer.oporphyritic c l o t s , o l i v i n e phenocrysts have been p a r t i a l l y to t o t a l l y a l t e r e d , and pinkish augite occurs as rare subhedral phenocrysts. Magnetite and ilmenite phenocrysts are very scarce. One sample (R-107) has a trace of quartz i n the groundmass. Comenditic Trachyte In a t y p i c a l comenditic trachyte, phenocrysts of g r i d i r o n and C a r l s -bad twinned anorthoclase, and hedenbergite compose 5 - 10 percent of the rock, with anorthoclase by f a r the most abundant phenocryst type (Figure 9). Heden-bergite phenocrysts are weakly pleochroic ( a = green; 6 = pale green; x = brownish-yellow)* Sparse ilmenite and magnetite grains are also present. The groundmass consists of s u b - p a r a l l e l laths of a l k a l i feldspar and varying proportions of intergranular acmitic pyroxene, opaque oxide, aenig-matite (, „ arfvedsonite, and quartz. In some samples, acmitic pyroxene occurs as p o i k i l i t i c grains. Aenigmatite (pleochroic dark-brown to opaque) i s found Figure 9. Photomicrograph of gridiron twinned anortho-clase phenocryst i n comenditic trachyte. Groundmass i s mainly a l k a l i feldspar laths with ferromagnesian minerals and quartz i n the i n t e r s t i c e s . Scale - 3 mm across the photo. Figure 10. Photomicrograph of hedenbergite, f a y a l i t e and sanidine phenocrysts i n comendite. Groundmass i s composed of a l k a l i feldspar and quartz with acmitic pyroxene, arfved-sonite, and aenigmatite f i l l i n g the i n t e r s t i c e s between grains. Scale - 3 mm across the photo. _ 22 _ i n c l o t s surrounding magnetite and i l m e n i t e grains where i t has grown as a r e a c t i o n product of these oxides. Sparse c l u s t e r s of a r f v e d s o n i t e (g = deep green - blue; 3 = g r e y i s h v i o l e t ; y = blue green) are present i n some samples. Comendite Comendites from the north f l a n k of the Rainbow Range are h o l o c r y s t a l -l i n e and conta i n approximately 10 percent phenocrysts. Groundmass mineralogy and textures remain constant whereas two phenocryst assemblages are found. Sanidine and hedenbergite phenocrysts are found i n a l l comendites while faya-l i t e and a r f v e d s o n i t e are found i n comendites from the top two-thirds of the s t r a t i g r a p h i c s e c t i o n (Figure 10). Sanidine phenocrysts are g e n e r a l l y euhedral and commonly e x h i b i t Carlsbad twinning. Glomeroporphyritic c l o t s of sanidine are present. EuhedT-r a l to subhedral hedenbergite phenocrysts ( a = green; g = pale green; y = pale yellow-green) are i n c l u d e d w i t h i n sanidine l a t h s and mechanically i n c l u -deded w i t h i n the c l o t s . In the lavas that c o n t a i n a r f v e d s o n i t e phenocrysts, some hedenbergite c r y s t a l s are mantled by a r f v e d s o n i t e . I l m e n i t e and magne-t i t e phenocrysts are r a r e . Most f a y a l i t e ( a = y = pale yellow; g = yellow) has an op a c i t e rim or i s surrounded by a t h i c k corona of a r f v e d s o n i t e . Rare a r f v e d s o n i t e phenocrysts (oi = deep green-blue; g = greyish-white; y = brown-blue) are always corroded. Laths of a l k a l i f e l d s p a r and patches of anhedral quartz grains form over h a l f the groundmass. Ferromagnesian minerals ( a c m i t i c pyroxene, aenigmatite, and arfvedsonite) i n the f e l t e d groundmass occur as mossy, f e r n - l i k e aggregates that envelope a l k a l i f e l d s p a r and quartz. - 23 -Trachyte Trachyte (AP-12) that forms a plug dome at Anahim Peak contains 8 -10 percent phenocrysts of plagioclase and pyroxene. C a l c i c o l i g o c l a s e (A^g) occurs as p o i k i l i t i c phenocrysts that show patchy e x t i n c t i o n and enclose hedenbergitic pyroxene and opaque oxides. Titan-augite phenocrysts are pink to pale brown, show hourglass zoning, and are e n c i r c l e d by opacite rims. Augite, magnetite, and ilmenite are found i n the i n t e r s t i c e s between plagioclase and a l k a l i feldspar m i c r o l i t e s of the intergranular groundmass. Rare, l i g h t blue hexagonal grains of an i s o t r o p i c mineral with streaks of minute i n c l u s i o n s , probably haiiyne, are found i n the trachyte. MINERAL CHEMISTRY In t roduet i on Along the north flank'of the Rainbow Range s h i e l d volcano, four d i s t i n c t s t r a t i g r a p h i c episodes of volcanic a c t i v i t y produced a sui t e of lavas ranging i n composition from hawaiite to comendite. These lavas can be distinguished on the basis of mineralogy and chemical composition. In the oversaturated p e r a l k a l i n e rocks most phases are high i n Na and Fe and low i n Ca and Mg, whereas mineral phases found i n the basic rocks do not show such extremes i n composition. The minerals are examined with respect to the insig h t they provide into petrogenesis of the lavas. Reviews of the mineralogy of oversaturated peralkaline rocks include those by Carmichael (1962), N i c h o l l s and Carmichael (1969), Sutherland (1974), and Bryan (1976). - 24 -Methods An ARL-SEMQ electron microprobe was used for approximately eighty anal-yses of minerals from fourteen rocks. The accelerating voltage used was 15 KV; the sample current was about 40 nannoamperes. Spot size was 10 - 15 mic-rometers for phenocryst analyses and 1 - 3 micrometers for groundmass grain analyses. The correction procedure was that of Bence and Albee (1968) and Albee and Ray (1970). Ferric iron contents of pyroxenes and amphiboles were calculated following Papike et. a l . (1974). Olivine F a y a l i t i c o l i v i n e i s present i n minor amounts as phenocrysts i n the co-mendites, while f o r s t e r i t i c o l i v i n e occurs as phenocrysts i n mugearites and microphenocrysts i n hawaiites. Representative analyses of ol i v i n e s are presented i n Table I I I . Fayalite has been reported as phenocrysts i n many peralkaline, s i l i c i c rocks (e.g.Carmichael, 1962; Becker, 1976; Bryan, 1976). Compositionally, the fa y a l i t e that occurs i n comendites from the Rainbow Range i s extremely ir o n -r i c h , Fa... ,-. The manganese oxide content of the f a y a l i t e averages about 3 y 4. J weight percent (Figure 11) which i s much higher than the whole-rock manganese oxide content. Becker (1976, Table II) reports f a y a l i t e from a comendite i n Big Bend National Park, Texas, that i s almost i d e n t i c a l i n composition to that from the Rainbow Range. Forsterite from hawaiites and mugearites has an average composition of Fo^^- Phenocrysts are usually zoned, have half the calcium and one-tenth the manganese of the f a y a l i t e s , and the range of compositions probed i s from Fo-.0-Fo . . Basaltic rocks associated with oversaturated peralkaline rocks / o 5 4 on I s l a Socorro i n the east P a c i f i c (Bryan, 1976, Table 5) contain f o r s t e r i t e Mg2Si04 Fe2Si04 Figure 11. Triangular diagram for olivine with end members tephroite (Tph) , forsterite (Fo), and fayalite (Fa) (mol. %), showing Mn enrichment of fayalite. Symbols: hawaiite- • , mugearite- A » Anahim Peak tra-chyte- [J , comendite- 9 . - 26 -s i m i l a r i n composition. One f o r s t e r i t e c r y s t a l (1 mm diameter) analyzed from the trachyte plug at Anahim Peak shows an extreme compositional v a r i a -t i o n , from Fo72 - F o37 (normal zoning). The c r y s t a l occurs i n a c l o t of plag-i o c l a s e and pyroxene m i c r o l i t e s that appears foreign i n o r i g i n . Pyroxene Phenocrysts. C a l c i c pyroxenes are t y p i c a l of the Rainbow Range lavas, and range from aluminous augite through ferro-augite to hedenbergite and sodic hedenbergite (Table IV). When plotted i n the pyroxene q u a d r i l a t e r a l (Figure 12) they l i e on. a trend of iron-enrichment close to that of Nandewar volcano i n A u s t r a l i a , a center of m i l d l y a l k a l i n e lavas with associated oversaturated d i f f e r e n t i a -t i o n products (Abbott, 1969). Bryan (1976) also reports a su i t e of pyroxenes from I s l a Socorro i n the east P a c i f i c that embodies the same compositional range as those from the Rainbow Range. Diopsi d i c augites from the hawaiites are distinguished by t h e i r moderately high T i 0 2 (1.61-2.45 percent), Na 20 (0.39-0.65 percent), and A1 20 3 (3.02-5,68 percent) contents. Comparatively, ferro-augite (found only i n the Anahim Peak trachyte) i s lower i n these elements and contains about 5 weight percent more t o t a l i r o n . Highly aluminous (6.72 weight percent AI2O3) hypersthene (Enyn.) i s found as megacrysts i n some hawaiites. Most orthopyroxene megacrysts reported i n the l i t e r a t u r e are aluminous bronzite (EngQ-En^g) (Binns, et. a l . , 1970). High aluminum content i n orthopyroxene i s a phenomenon that i s favored at high pressure (Boyd and England, 1960, 1963). Experimental studies of basal-t i c compositions (for example, Green and Ringwood, 1967; Green and Hibberson, 1970) have shown that orthopyroxene + clinopyroxene with compositions s i m i l a r to those of megacrysts are near-liquidus phases at pressures of approximately Figure 12. Compositions of pyroxene phenocrysts plotted on the pyroxene quadrilateral (mol. %). Trend from Nandewar volcano shown for comparison (Abbott, 1969). Symbols: hawaiite- • , mugearite- A > Anahim Peak trachyte- SA > comenditic trachyte-Q , comendite- £ . - 28 -10 - 20 kbi The Rainbow Range orthopyroxene megacrysts have a vitreous appearance i n hand sample, a tendency toward conchoidal fr a c t u r e rather than cleavage, and an absence of exsolution lamellae, t y p i c a l of megacrysts (Irving, 1974)* Reaction of megacryst rims with the host rock indicates that the orthopyroxene and host rock were not i n equilibrium at the time of extrusion. Clinopyroxenes found as phenocrysts i n the peralkaline rocks are sodic hedenbergites, with compositions close to Wo^ 5 F s47 5 E n7 ^ n c o m e n d i t i c t r a -chyteseand Wo^Fs^En-^ i n ccomendites. V a r i a t i o n i n composition of pyroxenes from comenditic trachytes and comendites p a r a l l e l s trends seen i n whole-rock composition of these rocks: Na^O and Fe^O^ are high while MgO and CaO are low. Compared to a u g i t i c pyroxenes found i n basic a l k a l i n e rocks, the sodic hedenbergites are high i n Na^O and t o t a l i r o n , and low i n Ti02» A^O^, and MgO. Groundmass Pyroxene Groundmass pyroxenes from the p e r a l k a l i n e rocks are near pure end-member +3 acmite (NaFe S±20^) i n composition, with small amounts of hedenbergite i n s o l i d solution^ They are chemically d i s t i n c t from the pyroxene phenocrysts, being higher i n Fe20^and ^ £ 0 , and lower i n CaO (Figure 13). The pyroxene c r y s t a l l i z a t i o n trend i n these rocks i s toward extreme i r o n enrichment before sodium becomes enriched. Because the groundmass pyroxenes are very small, r e l i a b l e analyses were d i f f i c u l t to obtain. For comparison, one analysis of a groundmass pyroxene from the Burro Mesa "Riebeckite" Rhyolite (Becker, 1976), a c r y s t a l l i n e comendite flow from Big Bend National Park, Texas, i s l i s t e d i n Table IV and plotted i n Figure 13. One q u a l i t a t i v e analysis of a ground-mass pyroxene from a Rainbow Range comendite shows the same r e l a t i v e amounts ACMITE Na Groundmass pyroxene analysis from Becker (1976) Mg DIOPSIDE Groundmass pyroxene, this study Fe2++Mn HEDENBERGITE Figure 13. Triangular diagram f o r pyroxene i n terms of acmite, diop-side, and hedenbergite (mol. % ) . One groundmass pyroxene analysis from the Rainbow Range i s only semi-quantitative; one a n a l y s i s from Becker (1976) i s shown for comparison. Symbols as i n Figure 12. - 30 -of oxide components. Ti02 and A^O^ contents are higher i n the groundmass pyroxenes, while MnO i s higher i n the phenocrysts. N i c h o l l s and Carmichael (1969) note the same inverse r e l a t i o n s h i p between T i and Mn i n pyroxenes from New Zealand comendites. The p o i k i l i t i c habit of the acmitic pyroxene i s evidence for a low pres-sure o r i g i n , i . e . a f t e r extrusion of the lava, at 1 atm. pressure. Experi-mental work shows that acmite has a low thermal s t a b i l i t y with respect to other s i l i c a t e minerals. Bailey and Schairer (1966) found that pure acmite melts at 975? + 5°C at 1 atm. The presence of acmite does i n d i c a t e that the l i q u i d was highly p e r a l -kaline at the time of extrusion. In the system Na^O-Al^O^- Fe^O^-SiO^, Bailey and Schairer (1966) found that acmite c r y s t a l l i z e s i n the presence of quartz + l i q u i d only from a l i q u i d bearing normative sodium s i l i c a t e . Amphibole, Amphibole i s found only i n peralkaline s i l i c i c rocks from the Rainbow ;Kange. I t occurs p r i m a r i l y as a groundmass mineral and r a r e l y as phenocrysts or as mantles around f a y a l i t e phenocrysts. Microprobe analyses show that two f +2 +3 amphiboles are present: arfvedsonite (Na K ) (Ca Na )(Mn Fe ,-Fe O.o 0*2 0.5 1.5 0*2 4*6 0. T i ) S i 0 (OH.F) land katophorite f(Na K ) (Ca Na )'(Mn F e + 2 F e + 3 0.1 8 22 2J L 0.9 0.1 1.5 0.5 ' 0.1 2.8 1.6 A I Q $T1Q^1)Sig022(OH,F)2J (Table V ) . The one analyzed katophorite i s from the groundmass. Arfvedsonite i s the amphibole .expected i n pe r a l k a l i n e s i l -i c i c volcanic rocks because i t i s stable to a higher temperature than r i e -beckite (Ernst, 1962). Arfvedsonite has been reported as a groundmass com-ponent i n many peralkaline s i l i c i c v olcanic rocks (e.g. N i c h o l l s and Carmi-chael, 1969; Sutherland, 1974; Becker, 1976). The amphibole forms i r r e g u l a r patches of p o i k i l i t i c grains i n the _ 31 -groundmass, suggesting that i t formed after extrusion, as did the acmite. Hydroxyl amphibole i s not stable as magmatic liquidus temperatures and low pressure, but fluorine has been found to s t a b i l i z e amphibole at low pressure ( T r o l l and Gil b e r t , 1974). Although fluorine was not determined i n Rainbow Range amphiboles, i t has been reported i n most arfvedsonites from peral-kaline s i l i c i c volcanic rocks (e.g. Nicholls and Carmichael, 1969; Sutherland, 1974; Becker, 1976), and i t seems l i k e l y that fluor-arfvedsonite i s the amphi-bole 1 present i n the Rainbow Range peralkaline lavas. Aenigmatite Aenigmatite (Na2Fe^TiSigO^Q) i s a common constituent of the comenditic trachyte and comendite lavas from the Rainbow Range, where i t i s found exclu-sivelylyas£ an i n t e r s t i t i a l groundmass phase. The mineral has been found i n a number of peralkaline volcanic rocks (e.g. Abbott, 1967; Nash et. a l . , 1969; Nicholls and Carmichael, 1969; Sutherland, 1974; Yagi and Souther, 1974; Marsh, 1975; Becker, 1976; Self and Gunn, 1976; Barker and Hodges, 1977; and Larsen, 1977). Two analyses of groundmass aenigmatite grains from Rainbow Range comen-dites are given i n Table VI* One of these (R-44-1) was provided by B. Prof-f e t t of the University of Calgary. An analysis of aenigmatite from Mt* Edziza (Yagi and Souther, 1974, Table 4, #2) i s shown for comparison. Because the mineral grains are very small r e l i a b l e analysis were d i f f i c u l t to obtain. The composition of the Rainbow Range aenigmatite i s simi l a r to those reported for groundmass aenigmatite i n other peralkaline volcanic rocks (e.g. Nicholls and Carmichael, 1969)* In the Rainbow Range peralkaline lavas aenigmatite c r y s t a l l i z e d after extrusion, as evidenced by i t s i n t e r s t i t i a l nature. This indication of late - 32 -stage formation i s consistent with experimental results (Lindsley, 1971) that show the maximum thermal s t a b i l i t y of synthetic aenigmatite to be below 900°C. Aenigmatite always surrounds small grains of Fe-Ti oxide, and appears to have formed by a reaction that consumed the oxide phase. This petrographic feature has been observed by Lindsley et. a l . (1971), Marsh (1975), and Becker (1976). Becker (1976) suggests that a possible reaction for the formation of aenig-matite i s : FeTi0 o + Na oSi0 c + 2Fe oSi0, + 2Si0 o = NaoFecTiSi,0o„ 3 2 5 2 4 2 z 5 6 20 ilmenite + l i q u i d + f a y a l i t e + s i l i c a = aenigmatite Fayalite i s not necessarily s p e c i f i c a l l y involved i n the reaction; however, the l i q u i d reacting with ilmenite must be r i c h i n iron and s i l i c a . Feldspar Feldspar i s the most prevalent phenocryst phase found i n Rainbow Range lavas. A suite of analyzed feldspar phenocrysts ranges from plagioclase i n basic rocks to a l k a l i feldspar i n peralkaline s i l i c i c rocks (Figure 14). The compositional range covered by these feldspar phenocrysts i s similar to that of analyzed feldspar suites from two oceanic islands, I s l a Socorro (Bryan, 1976) and Terceira i n the Azores (Self and Gunn, 1976). Microprobe analyses of 17 feldspar phenocrysts are given i n Table VII. Labradorite i s the plagioclase feldspar most often found i n hawaiites and and mugearites, although one hawaiite (AP-11) contains c a l c i c andesine. The trachyte plug at Anahim Peak bears large phenocrysts of oligoclase. Zoning i n plagioclase phenocrysts can be either normal or reverse, and nowhere exceeds 3 mol* percent. Anorthoclase from comenditic trachytes ranges from Or22~Or2g (mol. percent) which i s s l i g h t l y more sodic than the range for Figure 14. Triangular diagram for feldspar i n terms a l b i t e (Ab) , anorthite (An), and orthoclase (Or).(mo Symbols as i n Figure 12. - 34 -peralkaline trachytes cited i n Sutherland (1974), but corresponds well to one analysis from P a n t e l l e r i a ( V i l l a r i , 1970, p. 361). A l k a l i feldspar from Rainbow Range comendites ranges from Or ^-0r ^, within the observed range from other reported l o c a l i t i e s (Sutherland, 1974). A l l a l k a l i feldspars anal-yzed have less than 1 weight percent CaO. The maximum zonation found between end members i n any c r y s t a l i s 1 mol. percent. Experimental work on a l k a l i feldspar i n peralkaline li q u i d s has concen-trated on determination of the location of the thermal valley under varying experimental conditions (e.g. Carmichael, 1962; Carmichael and MacKenzie, 1963; Bailey and Schairer, 1964; Thompson and MacKenzie, 1967; Bailey and Macdonald, 1969; and Roux and Varet, 1975). Bailey (1974) presents an over-view of experimental work on feldspars i n oversaturated, peralkaline systems. A l k a l i feldspar^phenocrysts from Rainbow Range peralkaline lavas are pre-sumed to represent the f i r s t feldspar to precipitate from a l i q u i d of the rock composition, because there i s l i t t l e range i n composition of a l k a l i f e l d -spar from any one rock, and the phenocrysts are not obviously xenocrystic. In order to determine where the Rainbow Range peralkaline l i q u i d s and feldspars l i e with respect to a "thermal val l e y " and a quartz-feldspar f i e l d boundary, the normative constituents Q + Ab + Or were recalculated to 100 percent and plotted, along with coexisting feldspar compositions, on Carmi-chael and MacKenzie's (1963, Figure 4) composite liquidus diagram for the sys-tem NaAISi 0 -KAlSi.O -SiO -H 0 with di f f e r e n t percentages of additional acmite and nosean (Figure 15). The absence of quartz phenocrysts i n the rocks indicates that the intersection of the thermal valley with the quartz-feldspar f i e l d boundary was not reached by the l i q u i d s while phenocrysts were s t i l l c r y s t a l l i z i n g . The bulk rock compositions must have been near the bot-tom of the thermal valley to produce the r e s t r i c t e d range of feldspar B P^cr 1 0 0 0 k 9 / c r r | 2 n nn r u Figure 15. Compositions of Rainbow Range comendUic trachytes (open c i r c l e s ) and comendites ( s o l i d c i r c l e s ) and their coexisting feldspars (open and s o l i d squares, respectively) (mol. X) plotted on Carmichael and MacKenzie 1s (1963, Figure 4) composite liquidus diagram for the sys-tem NaAlSi3Og-KAlSi30Q-SiO2-H20 plus different percentages of additional acmite and nosean. A, B, and C represent the thermal minimum for A- no acmite or nosean, B- 4.5% acmite + 4.5% nosean, and C- 8.3% acmite + 8.3% nosean. - 36 -compositions. Experimentally, the minimum has been seen to move towards the Or-Q j o i n with increased p e r a l k a l i n i t y (Carmichael, 1962). The Rainbow Range comendite trend l i e s p a r a l l e l to Carmichael and MacKenzie's curve C for 8.3 percent nosean. Rainbow Range comendites have about 5 percent normative acmite and 1 percent normative nosean. While the Rainbow Range comenditic trachyte trend i s not as d i s t i n c t , i t i s displaced towards a lower Or content. The comen-d i t i c trachytes have approximately 1 percent normative acmite and lack norma-ti v e nosean. Carmichael and MacKenzie's data are for P = 1 kb,-yet oversaturated, peralkaline rocks are generally thought to be dry (anhydrous) and r i c h i n halogens (Nicholls and Carmichael, 1969; Roux and Varet, 1975). The variations of the liquidus surface under varying confining pressures and vapor composi-tions are not knownt Preliminary experiments by J. T. Iiyama (see Roux and Varet, 1975) show that the minimum i s shifted towards K-rich compositions i n the presence of an a l k a l i chloride vapor at 1 kb. The Rainbow Range data are consistent with this observation. Iron-Titaniurn Oxides Iron-titanium oxides are found i n a l l Rainbow Range lavas. In hawaiites, mugearites, and the Anahim Peak trachyte, titanomagnetite and ilmenite are found as discrete phenocrysts, microphenocrysts, groundmass granules, and occasionally as inclusions i n plagioclase and clinopyroxene, Titanomagnetite and ilmenite i n the comenditic trachytes and comendites occur-primarily as groundmass granules and microphenocrysts; i n comendites the oxide phase has always p a r t i a l l y reacted to form aenigmatite. Analyses are given i n Tables VIII and IX. Composition of the oxide - 37 -phases varies with rock type. MgO i s enriched i n oxide phases from the basic lavas and depleted i n oxide phases from the peralkaline lavas, while MnO displays the opposite trend. Analysed oxide phases from peralkaline acid lavas i n P a n t e l l e r i a and eastern Au s t r a l i a also show high MnO and low MgO concentrations (Carmichael, 1967; Ewart et. a l . , 1976). Analyses have been recalculated according to the method described by Carmichael (1967), and p l o t -ted i n terms of FeO, Fe^O^, and TIO^ i n Figure 16. Ulvospinel content i n t i t -anomagnetites ranges from 50 to 87 percent, while coexisting ilmenites contain 2 to 7 percent R2^3* Iron-titanium oxides from Rainbow Range lavas vary i n composition with coexisting ferromagnesian s i l i c a t e s , a characteristic noted by Carmichael (1967). Oxide phases from Rainbow Range peralkaline lavas are lower i n a n d higher i n ulvospinel than oxide phases from Rainbow Range alkaline lavas. The temperature and log f of e q u i l i b r a t i o n of coexisting titanomag-netite and ilmenite can be obtained using the methods•of Buddington. tand Lirid-sley (1964). The analyzed oxide phases are a l l from the groundmass and hence eq u i l i b r a t i o n temperatures should represent solidus temperatures. In Figure 17, coexisting oxide pairs from s i x Rainbow Range lavas ranging from hawaiite to comenditic trachyte are plotted on a log f. -T diagram according to the °2 curves of Buddington and Lindsley (1964). Estimated temperatures and oxygen fugacities of Rainbow Range lavas f a l l along or close to the QFM buffer. This i s consistent with the mineralogy of the rocks. Other Accessory Minerals Apatite i s commonly present as small needles i n the matrix of a l l Rain-bow Range lavas, whereas stout microphenocrysts f i l l e d with p a r a l l e l inclusion trains occur i n the Anahim Peak hawaiites. Rare groundmass red-brown b i o t i t e T i0 2 Figure 16. Compositions of titanomagnetite and ilmenite (mol. %) plotted i n terms of TiC^, FeO, and F e z O s . T i e - l i n e s connect coexisting oxide pairs. Symbols as i n Figure 12. - 39 -8 0 0 9 0 0 1 0 0 0 T (°C) Figure 17. Log fo 2 - T diagram i n which are p l o t t e d maximum values of T ( C) and for e q u i l i b r a t i o n of oxide phases i n Rainbow Range and Anahim Peak lavas, using the curves of Buddington and Lindsley (1964). HM- hematite-magnetite b u f f e r (Eugster and Wones, 1962), Ni-NiO - n i c k e l - n i c k e l oxide b u f f e r (Eugster and Wones, 1962), WM - wiistite-magnetite b u f f e r (Eugster and Wones, 1962), QFM - quartz-fayalite-magnetite b u f f e r (Wones and G i l b e r t , 1968). Symbols as i n Figure 12. - 40 -and l i g h t blue hatiyne (?) are also present i n the Anahim Peak trachyte. Quartz occurs as minute anhedral grains throughout the groundmass of the comendites and comenditic trachytes. WHOLE ROCK CHEMISTRY Introdue tion Major and trace element data are used to interpret the nature of source rocks for the Rainbow Range lavas, and the role of various processes i n magma genesis and d i f f e r e n t i a t i o n . Thirty-two samples of Late Miocene volcanic rocks from the Rainbow Range and Anahim Peak (Plate II) were analyzed by X-ray fluorescence for major element oxides Si02» Ti02» AI2O3, Fe203, MnO, MgO, CaO, Na20, K 20, and P2C>5» an.d the trace elements Nb, Rb, Sr, Ba, and Ni« Anhydrous analyses, recalculated to 100 percent, are presented i n Table X. Samples were prepared following the method of Norrish and Hutton (1969). Concentrations of major element oxides, except Na20, were determined from glasses diluted with a commercially prepared f l u x (Chemplex Grade I I I , #925, composition: 16 percent lanthanum oxide, 47 percent lithium tetraborate, and 37 percent lithium carbonate); trace elements and Na20 concentrations were determined from undiluted pressed powder p e l l e t s . Data were reduced using the procedures of Watters (1976). N o rmative Miner a 1 ogy. Normative minerals (Table X) for a l l samples were calculated using a computer program. The Fe203/Fe0 r a t i o of the volcanic rocks has not been determined a n a l y t i c a l l y . In order to calculate normative mineral compositions, - 4 1 -iron was partitioned according to the method of Carman et. a l . (1975). A factor (R) for each rock type was determined from Fe203/FeO ratios i n the same rock types found i n the l i t e r a t u r e . The formulas used for the p a r t i t i o n of iron are F e ^ ' = Fe^/(1+1.1113R) and FeO' = Fe^'/R, where R = F e ^ / FeO of the same rock type. The R values used were: .32 for hawaiites (Nock-olds, 1954), .38 for mugearites, metaluminous trachytes, and comenditic t r a -chytes (Irvine and Baragar, 1971; Macdonald, 1974), and .43 for comendites (Macdonald, 1974). The proportions of ferromagnesian minerals i n the norm are dependent on the oxidation state of the iron. For example, when a l l the iron i n a peral-kaline rock i s i n the +3 state, sodium i s used to form acmite, and sodium metasilicate i s absent. F e r r o s i l i t e and nosean become important phases when ferrous iron i s added, and hematite and sphene disappear while ilmenite increases. Rock C l a s s i f i c a t i o n Lava flows from the Rainbow Range represent a compositionally bimodal suite of basic alkaline and s i l i c i c peralkaline rocks. Peralkaline rocks account for at least 80 volume percent of the rocks exposed on the north flank of the shield volcano. Basic to intermediate rocks are c l a s s i f i e d according to Thompson et. a l . (1972) using the Thornton and Tuttle (1960)differentiation index (D.I.). A l k a l i o l i v i n e basalts have D.I. < 35, hawaiites 35-45, mugearites 45-65, benmoreites 65-75, and trachytes> 75. By, d e f i n i t i o n (Shand, 1951), peralkaline rocks have a molecular excess of (Na?0+K?0) over A1?0^. Oversaturated peralkaline rocks have been divided - 42 -into two groups, pa n t e l l e r i t e s and comendites. The two types d i f f e r mainly i n chemistry and not i n mineralogy. Comendites are mildly peralkaline, while pantellerites are more extremely peralkaline. O r i g i n a l l y the d i s t i n c t i o n was based on':the fact that pa n t e l l e r i t e s have a higher proportion of femic min-erals. Lacroix (1927) chose 12.5 percent normative femic minerals as his d i v i s i o n between the two, aware that since a complete chemical t r a n s i t i o n existed between the two groups any d i v i s i o n would be arbitrary. A c l a s s i -f i c a t i o n scheme based on normative femic minerals and normative quartz was used by Macdonald and Bailey (1973). This scheme included the peralkaline trachytes as those rocks with less than 10 percent normative quartz. Sub-sequently Macdonald (1974) proposed a c l a s s i f i c a t i o n scheme based on iron and aluminum contents. These elements were chosen because they appeared to be largely unaffected by post-eruptive d e v i t r i f i c a t i o n and hydration. In Figure 18,oversaturated, peralkaline volcanic rocks from theTRainbow Range are plotted on both of the c l a s s i f i c a t i o n schemes. Macdonald and Bailey's (1973) scheme separates the lower unit and upper unit of Rainbow Range lava flows into comenditic trachytes and comendites, respectively. In Macdonald's l a t e r scheme, analyses that previously plotted as comendites are scattered around the intersection of the f i e l d boundaries, p l o t t i n g as comenditic t r a -chytes, comendites, and p a n t e l l e r i t e s . Peralkaline volcanic rocks from the Rainbow Range do not f i t unambig-uously into either c l a s s i f i c a t i o n scheme. They are high i n Al^O^ (indicating trachytic a f f i n i t i e s ) and low i n normative femic minerals compared to crys-t a l l i n e p a n t e l l e r i t e s (Macdonald, 1974). Based on a combination of petro-graphic and chemical variations, volcanic rocks from the lower unit are named comenditic trachytes, and volcanic rocks from the upper unit are named comendites. - 43 -(92) COMENDITE •V PANTELLERITE \ # COMENDITIC \ PANTELLERITIC TRACHYTE \ TRACHYTE 0 10 20 (22) 30 40 S normative femic minerals (%) B. (10.98) Iron as FeO (wt. percent) Figure 18. Classification schemes for silicic peralkaline volcanic rocks. A. From Macdonald and Bailey (1973). B. From Macdonald (1974). Symbols: comenditic trachyte- Q , comendite- Q . - 44 -Compositional Changes i n S i l i c i c Peralkaline Rocks as a Result of Post- Eruptive Processes S i l i c i c peralkaline volcanic rocks are p a r t i c u l a r l y susceptible to changes i n chemical composition due to a variety of post-eruptive processes, because of their peralkaline nature. The most common change i s a loss of sodium from c r y s t a l l i n e rocks or hydrated glasses. This has been attributed to d e v i t r i f i c a t i o n (Lipman, 1965; Noble, 1965; Ewart et. a l . , 1968), primary c r y s t a l l i z a t i o n (Noble, 1968; Macdonald and Bailey, 1973), and ground water leaching (Lipman, 1965; Macdonald and Bailey, 1973). Macdonald and Bailey (1973) show that 2 wt. percent Na^O can be l o s t from the c r y s t a l l i n e i n -t e r i o r of a flow. K^ O i s usually l o s t or gained (Lipman, 1965; Noble et. a l . , 1967). Halogens can be l o s t during primary c r y s t a l l i z a t i o n , d e v i t r i f i c a t i o n , and hydration (Noble, 1965, 1968; Noble et. a l . , 1967; Macdonald and Bailey, 1973). In fact Noble et. a l . (1967) report that half the fluorine and four-f i f t h s of the chlorine i n peralkaline volcanic rocks can be l o s t during primary c r y s t a l l i z a t i o n . Fluorine and chlorine contents of Rainbow Range peralkaline volcanic rocks were not determined; however, thei r presence has been well documented i n many s i l i c i c , peralkaline volcanic rocks (Nicholls and Carmichael, 1969; Macdonald and Bailey, 1973) and i t i s probable that some halogens were l o s t from Rainbow Range peralkaline lavas after eruption. I t has been suggested that only non-hydrated obsidians represent the true composition of the o r i g i n a l peralkaline l i q u i d (Noble, 1966; Macdonald and Bailey, 1973). A l l analyzed peralkaline volcanic rocks from the Rainbow Range are from ho l o c r y s t a l l i n e lava flows. None of the samples analyzed are s i g n i f i c a n t l y hydrated (maximum loss on i g n i t i o n was 1.88 weight percent). There i s good agreement between sodium and potassium contents from analyzed - 45 -samples of any one rock type, indicating that a l k a l i s were not lost i n great amounts* Rainbow Range peralkaline lavas do not appear to have changed enough i n chemical composition since extrusion to have o r i g i n a l l y been pantellerites * Chemical Variation Between Flows Major Element Oxides * Concentrations of major element oxides are shown i n Harker diagrams (Figure 19). These diagrams show important chemical variations between different rock types from the Rainbow Range. A l l oxides except ^£0 and K^ O decrease with increasing s i l i c a content. Although iron displays a decreasing trend, i t i s enriched i n the most s i l i c i c rocks. For lavas from the Rainbow Range there i s a complete gap i n s i l i c a content between 56.30 and 64.80 weight percent. Only the Anahim Peak trachyte has a s i l i c a content within this gap. Comenditic trachyte and comendite analyses from the Rainbow Range are sim i l a r i n major element chemistry to Macdonald's (1974. Table 1) "average c r y s t a l l i n e comenditic trachyte" and "average c r y s t a l l i n e comendite*" These s i l i c i c , peralkaline rocks have higher contents of t o t a l iron and Na20, and extremely lower contents of MgO and CaO than the average calc-alkaline rhyolite (Carmichael et. a l * , 1974). Suites of volcanic rocks with si m i l a r major element oxide trends include those from Aden (Cox et* a l * , 1970), Erta Ale i n the northern Afar (Barberi and Varet, 1974), Mt* Edziza i n northwestern B r i t i s h Columbia (Souther and Symons, 1974), eastern A u s t r a l i a (Ewart et. a l . , 1976), I s l a Socorro i n the eastern P a c i f i c (Bryan* 1976), Terceira i n the Azores (Self and Gunn, 1976), and the Trans-Pecos of west Texas and Kenya (Gregory) r i f t i n east A f r i c a (Barker, 1977). - 46 -T—i—i—i—r—i—i—i—i—i—i—i—i—i—i—i—i—r~~i—i—r~i—r II r 8 4 0 • a? 4. 2 0 7 5 3 % A 1 • * • Total Fe as F e 2 0 3 n i l B !5 • ft] - . « CaO Q » c » » C D « — 4 MgO * A T I0 2 " O ' ft • S 3 A A H P 2°5 o. ° • • • N a 2 0 E ° 4 A K 2 ° E I » • 1 « t • I • I I 1 | 1 i 1 1 1 1 1 1 JL 10 6 2 6 4 2 5 0 55 6 0 65 7 0 sio 2 Figure 19. Major-element Harker diagrams f or volcanic rocks from the Rainbow Range and Anahim Peak. Symbols: hawaiite- • , mugearite-A\ , Anahim Peak trachyte- B » comenditic trachyte- Q , comendite-- 47 -Trace Elements. Samples from the Rainbow Range volcanic suite were analyzed for f i v e trace elements- Nb, Rb, Sr, Ba, and Ni (Table X). The f i v e show a variety of trends and serve to emphasize the relevance of trace element data to problems of magma genesis. Trace elements and trace element ratios are ploted i n Harker diagrams (Figures 20 and 21) analogous to those used for major element oxides. Rainbow Range peralkaline rocks have trace element contents within the range of values compiled by Macdonald and Bailey (1973) for peralkaline oversaturated obsidians, although rubidium contents are somewhat low. In addition, the trace element contents of the Rainbow Range peralkaline rocks show the characteristic pattern of extreme enrichment i n some trace elements and p a r t i c u l a r l y low contents of others. Nickel content decreases with increasing s i l i c a content. This i s an expected trend, indicating incorporation of Ni into the l a t t i c e s of pyroxene, o l i v i n e , i l m e n i t e a n d magnetite i n the more basic rocks. Niobium i s con-centrated i n the s i l i c e o u s peralkaline rocks where i t i s thought to follow zirconium (Cox et, a l . , 1970). Rubidium, strontium, and barium are a l l related and w i l l be discussed together. These three elements have large ionic r a d i i ( 1.16&) and are partitioned into the feldspar phase. Rb/Sr ratios increase r a d i c a l l y with d i f f e r e n t i a t i o n . Rubidium steadily increases with increasing a c i d i t y , f o l -lowing calcium* Strontium values for comendites are extremely low, ranging from 1,4 to 10.9 ppm, s i g n i f i c a n t l y lower than the 450 ppm average for mugear-i t e s . This may indicate a different parent magma for the comendites, although a common primary magma i s implied by the smooth rubidium trend (Self and Gunn, 1976). K/Rb ratios appear unaffected by d i f f e r e n t i a t i o n , except at the s i l i c a - r i c h end where they show a s l i g h t decrease. The average K/Rb- ratio. - ,48.-100 5 0 1200 8 0 0 4 0 0 0 8 0 4 0 0 Tn—i—i—i—m—i—r—1—i—i—i—!—i—i—i—i—i— L I i—r Rb • E f f • TT Ba • Ni A 4 A E I 1 I L l o o Sr O Nb J • L P t i i i i i 4 At . . . iP, . . O i ^ C h f t J 5 0 55 6 0 65 7 0 SiCL 6 0 0 4 0 0 2 0 0 0 8 0 4 0 0 Figure 20. Trace element Harker diagrams for volcanic rocks from the Rainbow Range and Anahim Peak. Symbols as i n Figure 19. _ A? -800| i [ i i i i ' ) i i i i i i i i i i i i » 600 400 IO4 IO2 K/Sr 20 0 m K/Rb A * I O 4 K/Ba _ -I I O 2 • CTJI A A H o o. 0 Rb/Sr # • 80 40 • LTD * A E c r ^ < ? f * • o B A / S F E 0 « l f .mrfTJ - A A 2 « # • ^ - 1400 # -j iooo Ca/Sr ^ « e o o n^r, m H200 n^Cffl i i iJk A t . i t IEI I i Q I i I I I I s i o 2 Figure 21. Trace-element ratio Harker diagrams for volcanic rocks from the Rainbow Range and Anahim Peak. Symbols as i n Figure 19. - 50 -of Rainbow Range comendites i s 594, unusually high for s a l i c igneous rocks (Ferrara and T r e u i l , 1974). Korringa and Noble (1972) noted similar high K/Rb ratios and (and low Rb contents) i n s i l i c i c peralkaline rocks from P a n t e l l e r i a . Barium builds up to a maximum and f a l l s sharply off i n the most s i l i c i c members of the suite. This i n f l e c t i o n point i n the barium trend i s due to the onset of a l k a l i feldspar c r y s t a l l i z a t i o n (Ferrara and T r e u i l , 1974). Enormous'increases i n K/Ba and Rb/Ba at the i n f l e c t i o n point-also support this conclusion. Other basic a l k a l i n e - s i l i c i c peralkaline volcanic rock suites that show these same trends are from San Pietro and P a n t e l l e r i a (Barberi, et. a l . , 1970), eastern Aus t r a l i a (Ewart et. a l . , 1976). and I s l a Socorro (Bryan, 1976) . Strontium Isotopic Data Sr^ 7/Sr86 ratios for eight Rainbow Range volcanic rocks, covering the range of compositions described from the shield volcano and including two samples from Anahim Peak, range from .7031 to .727 (Table XI). The ratios f a l l into two d i s t i n c t groups: ratios from basic rocks cluster around .7032 (Rb/Sr = .028 to *204) while ratios from s i l i c i c peralkaline rocks range from .7042 to .727 (Rb/Sr = 2 828 to 74.4). Other basic a l k a l i n e - s i l i c i c peralka-l i n e suites that display similar spreads i n strontium isotopic ratios include those from Aden (.7038-.7072), Cox et. a l . , 1970; Erta Ale (.702-704), Bar-b e r i and Varet, 1970; eastern A u s t r a l i a (.7038-^.71), Ewart et. a l . , 1976; the Trans-Pecos f i e l d of west Texas (.7032-.7254), Barker et. a l . , 1977; and and the Stikine volcanic belt of northwestern B r i t i s h . Columbia (.7026->.714), Armstrong et. a l * , 1977. - 51 -An isochron plot of a l l eight samples (Figure 22) shows a very good cor-r e l a t i o n between S r ^ / S r ^ and R b ^ / S r ^ r a t i o s , and regression of the data according to the method of York (1967) gives an age of 6.9 + 1.0 m.y. and 87 86 i n i t i a l Sr /Sr r a t i o of .7032 + .0001 for the su i t e . The age i s i n good agreement with K-Ar age determinations from the shield volcano (Table I ) . The calculated i n i t i a l r a t i o i s equivalent to that of the most primitive lavas known from the shield volcano, hawaiites, whose S r ^ / S r ^ ratios have not changed s i g n i f i c a n t l y since eruption because of their low Rb/Sr r a t i o s . 87 86 Many explanations have been given for the differences i n Sr /Sr ratios i n suites of oogenetic volcanic rocks...' The . f i r s t.Cls thati the isotopic v a r i -87 86 ations r e f l e c t variations i n the Rb/Sr and Sr /Sr ratios of magma source material i n the mantle. Mantle Rb/Sr r a t i o may vary l a t e r a l l y and with 87 86 depth. A second explanation i s that different Sr /Sr ratios w i l l be generated as a resul t of diff e r e n t Rb/Sr ratios i n different portions of a long-lived magma chamber (Artemov and Yaroshevskiy, 1965). The th i r d ex-planation i s that the isotopic variations were caused by contamination of thenmagmas with radiogenic strontium by bulk assimilation or wall-rock reaction (Green and Ringwood, 1967). In order to assess which of these explanations appears to account for the high strontium isotopic ratios of the differentiated members of the 87 86 Rainbow Range volcanic s u i t e , i n i t i a l Sr ••/.Sr ratios were calculated (Table 87 86 XI) using measured Sr /Sr andLRb/Sr ratios and known K-Ar ages or e s t i -mated ages based on r e l a t i v e stratigraphic position. With the exception of 87 86 one sample (R-29), calculated i n i t i a l Sr /Sr ratios are, within la error l i m i t s , equal to the i n i t i a l r a t i o calculated for the isochron, and a l l 87 87 agree within 2a. Decay of Rb to Sr since the time of eruption can ac-count for the higher Sr^^/Sr^^ ratios of the peralkaline lavas. Figure 22. Rb/Sr whole rock isochron for volcanic rocks from the Rainbow Range and Anahim Peak. See Table XI for meaning of error bars. - 53 -MORPHOLOGY OF THE RAINBOW RANGE SHIELD VOLCANO The Rainbow Range shield volcano i s unique i n that even though a high proportion of the lava i s s i l i c i c , ranging from 64 to 71 weight percent Si02» the volcanic e d i f i c e i s a shield volcano, intermediate i n size between Ice-landic and Hawaiian shield volcanoes. The broad, low slopes of the volcano indicate that the v i s c o s i t y of these lavas was unusually low during eruption. Lavas with the same s i l i c a content and average v i s c o s i t y would normally build a composite cone. Peralkaline shield volcanoes have been reported i n the li t e r a t u r e only from one other l o c a l i t y outside of B r i t i s h Columbia - the South Tiirkana region cof the Kenya r i f t v a lley (Webb and Weaver, 1975). The v i s c o s i t y of a s i l i c a t e l i q u i d i s dependent on the extent of deve-lopment of chains and networks of s i l i c a tetrahedra (Ringwood, 1955; Barth, 1962). Networking forming ions (e.g. S i + ^ , A l + ^ , T i + ^ ) increase polymeriza-tion of s i l i c a tetrahedra i n the magma i f oxygen atoms are available to bond with them. When present i n large quantities, network modifying ions (e.g. Na +, K +) free oxygen atoms so that s i l i c a tetrahedra form independently from one another. An increase i n independent s i l i c a tetrahedra corresponds to a decrease i n v i s c o s i t y . Schminke (1974) discusses the factors influencing the vi s c o s i t y of per-alkaline magmas and proposes that the combination of peralkaline composition and high temperature together reduce the v i s c o s i t y of peralkaline s i l i c i c lavas below that of an average calc-alkaline s i l i c i c lava. Peralkaline lavas are high i n Na +, K +, Fe +^, and F~ ( s i l i c a t e network modifiers) and low i n +3 Al .. (a s i l i c a t e network former) . Many peralkaline lavas are reported to have low v i s c o s i t i e s (Schminke and Swanson, 1967; Walker and Swanson, 1968; Sch-minke, 1974; Scarfe, 1977). Liquidus temperatures calculated for s i l i c i c - 54 -peralkaline lavas range from 900 C to 1050 C (Carmichael, 1967; Bailey et. a l . , 1974; Barberi et. a l . , 1975). In contrast, Carmichael et. a l . (1974) report extrusion temperatures for " r h y o l i t e s " ranging from 735°C to 925°C. Using the method of Shaw (1972), v i s c o s i t i e s were calculated for peral-kaline lavas from the Rainbow Range and compared with calculated and exper-imentally determined v i s c o s i t i e s of calc-alkaline r h y o l i t e , p a n t e l l e r i t e , tho-l e i i t i c basalt, and a l k a l i basalt (Table XII). Results show that Rainbow Range peralkaline lavas are 10 to 30 times less viscous than calc-alkaline rhyolite at the same temperature and water content. In addition, the results are i n agreement with experimentally determined v i s c o s i t i e s of a p a n t e l l e r i t e from Fantale i n Ethiopia (Scarfe, 1977). T h o l e i i t i c and a l k a l i c basalts are 1000 times less viscous than peralkaline s i l i c i c lavas at 1200°C (approxi-mate liquidus temperature of basalt). During eruption, the actual v i s c o s i t y of the Rainbow Range peralkaline lavas would have differed from the calculated values for a number of reasons. The presence of crystals i n a melt would raise i t s v i s c o s i t y (Shaw, 1965, 1969; Murase and McBirney, 1973), whereas dissolved v o l a t i l e s would reduce i t s v i s -cosity (Shaw, 1963, 1972). Rainbow Range peralkaline lavas contain 5 - 1 0 percent c r y s t a l s , the presence of which would be unlikely to increase the vi s c o s i t y of the melt more than a factor of 1.3 - 1.8 (Shaw, 1965). Peralka-l i n e lavas are known to be r i c h i n halogens (Nicholls and Carmichael, 1969; Bailey and Macdonald, 1975). The effect of F and Cl on the v i s c o s i t y of melts of geologic interest i s not known, but studies of glass systems sug-gest that F i s an important melt depolymerizer (Hirayama and Camp, 1969). Therefore, the p o s s i b i l i t y exists that upon eruption, Rainbow Range peralkaline lavas were considerably less viscous than the calculated values. _ 55 _ ORIGIN OF OVERSATURATED, PERALKALINE VOLCANIC ROCKS Several theories have been proposed for the o r i g i n of oversaturated, peralkaline volcanic rocks. The most common of these are p a r t i a l fusion of lower crustal material 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 or p a r t i a l fusion of a basic parent. Another process currently being discussed i n the l i t e r a t u r e i s the modification of the above mechanisms by v o l a t i l e transfer i n an open system. In a study of non-hydrated, aphyric, peralkaline obsidians ( i . e . rocks most representative of peralkaline l i q u i d compositions) from different con-tin e n t a l and oceanic l o c a l i t i e s , Bailey and Macdonald (1970) found that con-tin e n t a l obsidians clustered i n composition around the quartz-feldspar minima, while oceanic obsidians scattered along the thermal valley between Ab and Or. They concluded that continental peralkaline magmas must form by p a r t i a l melting of lower crustal material i n order to have such t i g h t l y clustered compositions at the quartz-feldspar minima, while oceanic peralkaline magmas form from 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 trachytes. However, i n i t i a l S r ^ / S r ^ r a t i o s of peralkaline lavas indicate that these rocks are mantle-derived. Peralkaline volcanic rocks from P a n t e l l e r i a have strontium isotopic ratios ranging from .7029 to .7032 (Korringa and Noble, 1972). Ferrara and T r e u i l (1974) report a range of .703 to .707 for most young peralkaline volcanic rocks from the Kenya r i f t . In his c l a s s i c paper, Yoder (1973) showed that hydrous melting of a rock composed of o l i v i n e and clinopyroxene at high pressure (10-15 kb) could pro-duce contemporaneous basalt and " r h y o l i t e " . This " r h y o l i t e " could differen-t i a t e towards peralkaline composition by a l k a l i feldspar fractionation. The most commonly invoked mechanism for o r i g i n of oversaturated, peralka-- 56 " li n e magmas i s 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 an a l k a l i c basalt. Lindsey and others (1971) showed unequivocally that a peralkaline residuum could form from 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 basalt, i n their study of pegmatitic seg-regations found i n a flow of Columbia River basalt. Indeed, a l k a l i c basalts are often s p a t i a l l y related to areas of peralkaline volcanic a c t i v i t y (Bar-ber! and Varet, 1970; Barberi et. a l . , 1975; Bryan, 1976; Self and Gunn, 1976). This process i s envisaged i n several steps. Fractionation of plagio-clase and ferromagnesian minerals depletes the residual l i q u i d i n Ca, Mg, A l , Fe, Cr, Sc, V, and Sr, and enriches the residual l i q u i d i n S i , a l k a l i s , Nb, Th, Y, and Zr. When the magma becomes low i n Ca, continued c r y s t a l l i z a t i o n of plagioclase (the "plagioclase effect" of Bowen, 1945) drives the l i q u i d toward peralkaline compositions by depleting the aluminum content of the magma with respect to a l k a l i content. Subsequent c r y s t a l l i z a t i o n of a l k a l i feldspar increases the excess of a l k a l i s over aluminum. The Na/K ra t i o of a l k a l i feldspar i s lower than that of the l i q u i d , so a l k a l i feldspar f r a c -tionation raises the Na/K ra t i o of the residual l i q u i d (the "orthoclase effect" of Bailey and Schairer, 1964), Experimental studies (Carmichael and MacKenzie, 1963; Bailey, 1967; Bailey and Macdonald, 1969; Roux and Varet, 1975) also support the theory that oversaturated peralkaline magmas are derived by feldspar fractionation. Numerous experiments document that peralkaline rock compositions l i e i n the low temperature thermal valley between Ab and Or i n the system Q-Ab-Or, i n d i -cating that the magmas were derived by feldspar fractionation from l i q u i d s of trachytic compositions Many f i e l d and petrologic studies also point towards 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 an a l k a l i c basalt parent for the derivation of oversaturated, peralkaline lavas. Barberi et., a l , (1975) describe a most convincing example-s' basalt to p a n t e l l e r i t e suite with continuous v a r i a t i o n i n major and trace - -57 _ element composition, from the Boina Center i n Ethiopia. Noble (1965, 1968) argues for prolonged 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 based on the r a d i c a l l y enriched and extremely depleted trace element contents found i n oversaturated, peralkaline rocks* Similar reasoning i s employed by Gass and Mallick (1968), Noble et a l , (1969),.Barberi and Varet C!9 70), Ewart et. a l , (la?*),, and Self and Gunn (1976). As more precise trace element data become available for oversaturated, peralkaline rocks, increasing evidence suggests that c r y s t a l fractionation or p a r i a l fusion cannot t o t a l l y explain observed trace element trends. Presence of an a l k a l i and/or halogen-rich v o l a t i l e phase i s claimed necessary for the deviation of trace element contents from those predicted by c r y s t a l - l i q u i d e q u i l i b r i a (Bailey and Macdonald, 1969, 1975; Bailey, 1973; Bailey et. a l , 1975). In a study of peralkaline obsidians from Eburru volcano i n Kenya. Bailey and Macdonald (1975) found that trace element abundances were not compatible with a closed system evolutionary process controlled by c r y s t a l - l i q u i d equi-l i b r i a . However, they found high correlation of F with Zr and Rb, and CI with Nb and Yt, and suggested that i n an open system p a r t i t i o n i n g of metal-halogen complexes between a v o l a t i l e phase and s i l i c a t e melt would depend on condi-tions at the time of d i f f e r e n t i a t i o n , such as volume of vapor and melt, variations i n composition of vapor and melt, pressure, and temperature. Martin (1970) showed experimentally that the presence of water as a v o l a t i l e phase was responsible for the separation of a peralkaline l i q u i d f r a c t i o n from a ba s a l t i c parent, especially through the mobilization of s i l i c a and a l k a l i s . O r v i l l e (1963) also demonstrated a l k a l i transfer between a l k a l i feldspars and a l k a l i chloride-water solutions experimentally. The derivation of oversaturated, peralkaline volcanic rocks i s by no -"58 _ means a simple processt Their close association i n space and time with basic alkaline rocks indicates that 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 or p a r t i a l fusion of a mafic parent i s almost assuredly necessary for their genesis. Several studies show the importance of a v o l a t i l e phase i n a l t e r i n g the f i n a l com-position of these rocks, and emphasize the need for additional experiments i n the presence of a v o l a t i l e phase to determine the extent of i t s influence on evolution of the peralkaline condition. ORIGIN OF LAVAS FROM THE RAINBOW RANGE AND ANAHIM PEAK. Continuous va r i a t i o n i n major and trace element trends and feldspar com-positions for the hawaiite-mugearite-comenditic trachyte-comendite suite from the Rainbow Range suggests that the differentiated rocks were derived from the same parent magma, tapped several times as i t was undergoing 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 a high-level magma chamber. In addition, the magma chamber may have been compositionally zoned prior to eruption to produce the sequence of alternating basic and s i l i c i c , peralkaline lava flows. The Rainbow Range shield volcano l i e s near the western edge of the Late Miocene plateau basalt province of south-central B r i t i s h Columbia, suggesting that these basalts are perhaps the ultimate source for the differentiated suite seen i n the Rainbow Range. Least squares mass balance equations were made using the computer pro-gram MAGMIX (modified from Bryan and others, 1969) i n order to determine whether the major element composition of one lava could be derived from the composition of another by fractionation of the phenocryst phases present. Compositions of fractionated phases are actual phenocryst compositions from the parent lavas. Mass balance calculations for trace elements were made using the results of major element models and equation 1 of Gast (1968) for rr 59/ -Rayleigh (surface) equilibrium. Data of Drake and Wei l l (1975), Ringwood (1975), and Arth (1976) have been used to estimate p a r t i t i o n coefficients for trace elements. Because no obsidians were found within the f i e l d area compositions of porphyritic volcanic rocks were used i n the models. I t i s not known whether the phenocrysts (1 to 2 percent i n basic rocks and up to 10 percent i n s i l i c i c rocks) represent cumulate material or whether they c r y s t a l l i z e d from the differentiated l i q u i d . Disequilibrium features such as reaction rims are rarely seen i n the rocks, suggesting that the phenocrysts are i n equilibrium with the bulk rock composition. Seven models were tested, Hawaiite, the most primitive rock type found on the north flank of the Rainbow Range, was used as parental material to a l l the more differentiated rock types. I t i s presumed to have been derived from approximately 30 percent 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 a l k a l i o l i v i n e basalt i n a manner similar to that discussed by Fiesinger and Nicholls (1977, pages 34-35). Mugearite was tested as parental material to comenditic trachyte and comendite, and comenditic trachyte was evaluated as a possible parent of comendite. In addition, hawaiite from Anahim Peak was modelled as a parent to the Anahim Peak trachyte. The phases subtracted from basic parents were o l i v i n e , plagioclase, augite, magnetite, ilmenite, and apatite, while anor-thoclase and hedenbergite were subtracted from comenditic trachyte. Results of calculations yielding the lowest residuals are shown i n Tables XIII through XIX, Increasing degrees of c r y s t a l l i z a t i o n (71, 86, and 87 percent) of a hawaiite parent can account for the major element composition of mugearite, comenditic trachyte, and comendite, respectively. Major element compositions of comenditic trachyte and comendite can also be matched by f r a c t i o n a l - 60 -c r y s t a l l i z a t i o n of a mugearite parent. However, trace element calculations bring to l i g h t serious defects i n some of these models. I f hawaiite or mugearite are used as parental l i q u i d for the derivation of comendite, calculated Ba concentrations are unreasonable. There i s no satisfactory way to resolve this discrepancy except by the fractionation of a Ba-rich phase, presumably a l k a l i feldspar. Comendite can be derived from comenditic t r a -chyte by c r y s t a l l i z a t i o n of 40 percent of the parent as anorthoclase and hed-enbergite, and the Ba discrepancy i s reduced. A small center on the northeast flank of the shield volcano, Anahim Peak, produced seven hawaiite flows before a metaluminous trachyte plug f i l l e d the vent, Major and trace element contents of the trachyte, except for Sr, can be derived by 59 percent c r y s t a l l i z a t i o n of hawaiite. The strontium d i s -crepancy can be r e c t i f i e d by assuming a of 6.0 for strontium i n plagioclase. Mathematically, the series of models which most accurately reproduce the compositions of the more differentiated lavas i s hawaiite —> mugearite — } comenditic trachyte — * comendite. In order of appearance, the main phases c r y s t a l l i z i n g are o l i v i n e , clinopyroxene, plagioclase, iron-titanium oxides and a l k a l i feldspar. In this manner, c r y s t a l l i z a t i o n of a l k a l i feldspar from a l i q u i d of comenditic trachyte composition can account for the trace element contents of comenditic l i q u i d , for they are incompatible with frac-tionation of phases found i n more basic lavas. The same c r y s t a l l i z a t i o n sequence was reported by Barberi et. a l . (1975) for the Boina center i n Ethiopia. From oldest to youngest, the stratigraphic sequence of lavas erupted from the Rainbow Range center i s comenditic trachyte-mugearite-comendite-hawaiite. Unfortunately the base of the section i s not exposed within the study area. Two explanations are offered for the production of this sequence: _ 61 _ a) each pulse of basic magma introduced beneath the volcano differentiated for a different amount of time before erupting, or b) a zoned magma chamber existed beneath the volcano, and li q u i d s at different depths were tapped at different times* Derivation by cry s t a l fractionation provides a simple and reasonable model for generation of the differentiated rock suite found i n the Rainbow Range. However, trace element contents become harder to model as the rocks grow progressively more diffe r e n t i a t e d . This i s partly due to the lack of accurately known d i s t r i b u t i o n c o e f f i c i e n t s . I t may also indicate open system behavior of trace elements by a v o l a t i l e transfer mechanism when only small amounts of residual l i q u i d remain. Data on halogen contents of the differen-tiated lavas and the relationship of residual trace elements (Rb, Nb, Zr, Y, Zr) to halogens are needed to examine this process. ORIGIN OF THE ANAHIM VOLCANIC BELT The Anahim volcanic belt runs east-west along approximately latitude 52° N. and consists of at least 37 Quaternary volcanic centers plus some Miocene and Pliocene centers (Souther, 1977). Compositionally the volcanic rocks range from a l k a l i basalts through peralkaline, s i l i c i c d i f f e r e n t i a t e s . This east-west trend of highly alkaline volcanic centers i s at a high angle to the Pemberton volcanic b e l t , a group of middle to late Miocene sub-vol-canic plutons of calc-alkaline composition which p a r a l l e l the continental margin of southwestern B r i t i s h Columbia. Together these belts outline the orientation and extent of the subducted Juan de Fuca plate i n middle to Late Miocene time (Figure 3). This pattern of paired volcanic belts of contrasting chemical types i s repeated two other times within the late Cenozoic record i n the Canadian Co r d i l l e r a (Figure 23). The calc-alkaline Wrangell volcanic b e l t , generated Fxgure 23 Late Cenozoic volcanic belts of the Canadian Cordillera. The Wrangell, Pemberton, and Garibaldi belts were the site of calc-alkaline volcanism, whereas the Stikine,.Anahim, and Alert Bay belts were the site of alkaline to peralkaline volcanism. - 63 " by subduction of the P a c i f i c plate beneath Alaska, i s at an acute angle to, and separated from the alkaline to peralkaline Stikine volcanic belt of north-western B r i t i s h Columbia, Volcanoes from both of these belts range i n age from Miocene through Quaternary. The.calc-alkaline Garibaldi volcanic b e l t , formed from subduction of the Juan de Fuca plate beneath southwestern B r i t i s h Columbia (Riddihough and Hyndman, 1976; Green, 1977),. l i e s at a high angle to the A l e r t Bay volcanic belt on Vancouver Island, and the Anahim volcanic b e l t . Continental alkaline to peralkaline volcanic suites are found i n ten-sional settings, for example, the Stikine volcanic belt of northwestern B r i t i s h Columbia (Souther, 1970, 1977), the East A f r i c a r i f t , Great Basin of the western United States, and Trans-Pecos of west Texas. There i s l i t t l e evidence for major f a u l t i n g and offset along the trace of the Anahim volcanic b e l t , although east-west trending normal faults have been documented from the Ilgachuz Range (Tipper, 1969) and Itcha Range (Tipper, 1957). Souther (1970, 1977) proposed that volcanic rocks of the Anahim volcanic belt were erupted from magma r i s i n g i n deep fractures along the northern edge of the subducted Juan de Fuca plate. In Late Miocene time, while calc-alkaline volcanism was occuring i n a belt p a r a l l e l to the ancient continental margin (Pemberton volcanic belt) and indicating subduction of the Juan de Fuca plate beneath the North American plate, the northern edge of the subducted Juan de Fuca plate was s u f f i c i e n t l y competent to disrupt the upper mantle, causing diapirism and subsequent p a r t i a l melting of peridotite to produce a l k a l i basalt. Thus, volcanic a c t i v i t y along the Anahim volcanic belt i s an "edge effect" related to subduction of the Juan de Fuca plate. The tectonic setting i s si m i l a r i n many ways to that of the Samoan islands i n the south-west P a c i f i c , i n which the P a c i f i c plate i s being buckled beneath the Australian plate along a transform fa u l t at right angles to the Tonga trench. _ 64 _ 1 /• The Samoan islands are constructed of alkaline lavas (thought to be derived from viscous shear melting of the upper mantle) which are erupted along the trace of th i s transform f a u l t (Hawkins and Natland, 1975). A second possible tectonic o r i g i n for the Anahim volcanic belt involves the movement of the North American plate over a mantle hot spot. Available dates from the Anahim belt (Figure 3) record eastward time transgression of volcanic a c t i v i t y , suggesting that volcanism i n the Anahim volcanic belt i s related to a mantle hot spot beneath B r i t i s h Columbia. Using the oldest date known from various volcanic centers i n the b e l t , one can calculate that volcanic a c t i v i t y has moved eastward with time at a rate of 2.0 - 3.3 cm/year. This compares well with the 2.7 cm/year rate calculated for move-ment of hot spots beneath Yellowstone, Wyoming, and Raton, New Mexico (Min-ster et. a l . , 1974)* In addition, the trend of the Anahim volcanic belt along approximately latitude 52° N. i s a trend consistent with i t s being along a small c i r c l e to Minster et. al.'s (1974) pole to the other North American hot spot traces. The common association of continental alkaline to peralkaline volcanic rocks with tensional environments leads one to speculate that lavas of the Anahim volcanic belt may have originated i n a r i f t zone. The lack of normal fa u l t i n g (such as i s wel l documented i n the Stikine belt (Souther, 1970, 1977)) casts some doubt on this p o s s i b i l i t y . Furthermore, i t i s d i f f i c u l t to relate an east-west trending r i f t zone with the probable tectonic setting at the time the Anahim volcanic rocks were being erupted. F i e l d relations show that along the entire length of the Anahim volcanic b e l t , basalt rose rapidly to the surface (as evidence by i t s primitive com-position) through narrow conduits (as evidenced by the abundance of small one-pulse centers) (Souther, 1977). Where batches of a l k a l i basalt were - 65 -trapped i n long-lived magma reservoirs, s a l i c d i f ferentiates had time to deve-lop. This resulted i n an evolutionary trend towards the establishment of more complex central volcanoes, b u i l t up of strongly differentiated lavas. As a whole, the age, petrologic, and isotopic variations along the belt have not been systematically studied and are not well known. A l k a l i basalts are found along the length of the b e l t , while s a l i c d i f f e r e n t i a t e s are reported only from the larger volcanic centers. In addition to oversaturated peralkaline volcanic rocks found i n the Rainbow Range shield volcano, under-saturated v a r i e t i e s have been reported from the central part of the Itcha Range (B. P r o f f e t t , personal communication, 1977). Other peralkaline rocks are found on King, Lake, Campbell, Price, and Bardswell Islands, and near Tanya and Sigutlat Lakes. Strontium isotopic studies suggest that the o r i g i n of these lavas i s related to upper mantle processes. In fact, strontium isotopic studies combined with petrologic data show that volcanic rocks from the Anahim volcanic b e l t are consistent with an o r i g i n by p a r t i a l melting of the upper mantle to produce the a l k a l i basalts (Fiesinger and Nic h o l l s , 1977), while modification by low pressure crystal'fractionation of the a l k a l i basalt magma, trapped i n in t r a c r u s t a l magma chambers, produced the s a l i c d i f f e r e n t i a t e s . CONCLUSIONS Eruptive History of the Rainbow Range Shield Volcano Four p e t r o l o g i c a l l y d i s t i n c t units make up the exposed stratigraphic sec-tion on the north flank of the Rainbow Range shield volcano: (1) Comenditic trachyte unit, characterized by thick greenish-grey flows, s l i g h t l y peralkaline chemistry, and phenocrysts of grid -twinned anorthoclase ( 0 r O I - _ 9 7 ) , and hedenbergite. - 66 -(2) Mugearite unit, distinguished by th i n dark flows with intercalated flow breccia, and phenocrysts of plagioclase, o l i v i n e , and augite. (3) Comendite unit, the most voluminous unit on the north flank of the shiel d volcano, characterized by thick greenish-grey flows, peral-kaline chemistry (including strongly enriched or depleted trace element contents), and phenocrysts of sanidine (Or34_37) a n <^ hedenbergite, + f a y a l i t e and arfvedsonite. (4) Hawaiite unit, notable for i t s r e s t r i c t e d occurrence as capping flows and related feeders, primitive chemistry, and sparse phenocrysts of plagioclase, o l i v i n e , and augite. The Anahim Peak lava flows are hawaiites which erupted from a vent now plugged by trachyte. These lavas are younger than those from the north flank of the shield volcano. Basaltic lavas found along the Anahim volcanic belt t y p i c a l l y were f i s s u r e -fed. A major constructional volcanic e d i f i c e formed only when a long-lived magma reservoir was established at depth.- Magma stagnating i n the reservoir had time to d i f f e r e n t i a t e , and repeated eruption of evolved lavas i n one area b u i l t the Rainbow Range shiled volcano. Oversaturated, peralkaline lavas are less viscous than calc-alkaline s i l i c i c lavas; hence the volcano has a sh i e l d - l i k e morphology. Origin of Rainbow Range Lavas and the Anahim Volcanic Belt Lava flows that b u i l t up the north flank of the Rainbow Range l i k e l y originated 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 a l k a l i basalt, a rock type common along the entire length of the Anahim volcanic belt* Data on v o l a t i l e and other residual trace element (e.g. Zr, Y) contents are needed to evaluate the influence of a v o l a t i l e phase on the d i s t r i b u t i o n of trace elements between cumulate and residual l i q u i d , when the mass of the l i q u i d i s very small. The Anahim and Pemberton volcanic belts outline the extent of the subducted Juan de Fuca plate during Late Miocene time. The Pemberton volcanic belt p a r a l l e l s the continental margin of southwestern B r i t i s h Columbia, whereas the Anahim volcanic belt l i e s along the probable trace of the northern edge of the subducted Juan de Fuca plate. Three possible models have been suggested for the o r i g i n of the Anahim volcanic b e l t . These are: 1) the volcanic rocks are derived from magmas generated along the northern edge of the subducting Juan de Fuca plate. 2) the belt represents a surface trace marking the passage of the North American plate over a mantle hot spot. 3) the magmas were generated i n an east-west trending r i f t zone. Insufficient data i s available at present to favor any one of these models. TABLE I. POTASSIUM-ARGON ANALYTICAL DATA FOR VOLCANIC ROCKS FROM THE RAINBOW RANGE. Sample Location Rock Type 40 % K° Ar rad (xl0" 6cc/g) 100 4 0Ar rad Calculated 40, Date (m.y.) t o t a l R-97 52 44'N, 125 52'W Northwest flank, Rainbow Range R-29 52 47'N, 125 43'W T s i t s u t l Peak R-25 52 45'N, 125 45'W Northeast flank, Rainbow Range AP-15 52 45'N, 125 38'W Anahim Peak comenditic 4.20, 4.24 trachyte comendite 4.21, 4.26 hawaiite hawaiite 1.26, 1.28 1.37, 1.37 1.461 1.224 0.402 0.368 60.0 30.6 68.0 53.5 8.7:+ 0.3 7.2-; + 0.3 7.9_: + .0.3 6.7 + 0.3 a. Potassium analyses by M.L. Bevier using atomic absorption techniques; duplicate analyses are l i s t e d . b. Argon analyses by J. Harakal and M.L. Bevier using MS10 spectrometer; constants used i n calculations are: X ••=  0.585 x 10_1°/year; X0 = 4.72 x 10_1°/year; 4°K/K = 1.19 x 10~ 4 (atom, ratio)/.;,. e P TABLE II. MINERAL ASSEMBLAGES OF VOLCANIC ROCKS FROM THE RAINBOW RANGE. Sample Phenocrysts Groundmass Phases Number : Plag. Aug. Fo. Fa. Hed. Alk. Fs. Opaques Plag. Aug. Arf. Aenig. Alk. Fs. Qtz. Opaques Glass Hawaiite R-25 X X 'tr X X tr X X R-36 X X X X X X X R-73 X X tr X X x X R-77 ' X X X X X X Mugearite R-99 X X X X X X X X R-107 X tr X X X X X tr X R-108 X X X X X x X X R-109 X X X X X X X Comenditic Trachyte R-96 X X X X tr X X X X R-97 X X tr X X X X X R-l 02 X X tr X X X R-106 X X tr X X X Comendite R-23 X X X X X X X X X R-29 X X X X X X X X X R-44 X X X X X X X X X X R-54 X X X X X X X R-68 X X X X X X X X X X Anahim Peak Hawaiite AP-11 AP-15 Anahim Peak X X X X X X X X tr tr tr X X X X X X X X X tr X X X Trachyte AP-12 - 70 -TABLE I I I . MICROPROBE ANALYSES OF OLIVINE PHENOCRYSTS. AP-11 R-25 AP-12 R-108 R-44 R-68 R-68 8 14 11 16 9 9 12 S i 0 2 37.04 37.10 37.52 37.42 26.43 28.77 29.28 T i 0 2 - 0.02 0.04 0.05 - 0.07 0.06 -AI2O3 0.06 0.08 0.05 0.04 0.05 0.03 0.02 FeO 27.73 29.90 25.08 27.20 65.69 66.15 65.92 MnO 0.37 0.41 0.30 0.36 3.03 2.89 2.80 MgO 33.84 32.57 36.02 35.59 0.05 0.15 0.15 CaO 0.18 0.27 0.20 6.25 0.54 0.52 0.52 Na20 0.02 0.02 0.02 0.01 0.02 0.05 0.06 K 20 - 0.01 0.01 - 0.03 0.03 -SUM 99.24 100.38 99.24 100.92 95.91 98.65 98.75 Cations based on 4 oxygens Si 0.9987 0.9985 0.9983 0.9889 0.9503 0.9895 1.0015 T i - 0.0004 0.0008 0.0009 0.0019 0.0015 -Al 0.0020 0.0026 0.0015 0.0013 0.0022 0.0011 0.0009 F e 2 + 0.6253 0.6731 0.5580 0.6012 1.9755 1.9030 1.8858 Mn 0.0085 0.0094 0.0068 0.0080 0.0923 0.0842 0.0811 Mg 1.3600 1.3066 1.4282 1.4018 0.0026 0.0077 0.0077 Ca 0.0052 0.0077 0.0058 0.0070 0.0209 0.0190 0.0191 Na 0.0011 0.0013 0.0009 0.0007 0.0012 0.0034 0.0039 K - 0.0003 0.0003 0.0002 0.0012 0.0013 -EY 2.0021 2.0010 2.0015 2.0202 2.0959 2.0197 1.9985 ZZ 0.9987 0.9989 0.9991 0.9898 0.9522 0.9910 1.0015 (atomic,'%) Fo 68.03 65.43 71.45 69.46 0.12 0.38 0.39 Fa 31.28 33.71 27.92 29.79 94.46 94.49 94.59 Tph 0.43 0.47 0.34 0.40 4.41 4.18 4.07 La 0.26 ,0.39 0.29 0.35 1.00 0.94 0.96 68.21 65.69 71.66 69.70 0.12 0.38 0.39 F e 2 + 31.36 33.84 28.00 29.89 95.42 95.40 95.50 Mn 0.43 0.47 0.34 0.40 4.45 4.22 4.11 - 71 -TABLE IV. MICROPROBE ANALYSES OF PYROXENE PHENOCRYSTS AND GROUNDMASS GRAINS. R-79 R-36 R-25 AP-11 AP-15 AP-12 R-97 3 9 22 5 21 29 12 S i 0 2 51.85 51.65 51.62 49.21 49.79 50.97 48.30 T i 0 2 0.64 0.61 1.61 1.91 1.71 0.49 0.52 A1 20 3 3.36 6.72 3.02 5.68 3.89 1.35 0.54 Fe 20 3* - - - 0.08 0.72 0.42 0.76 FeO 19.04 15.52 10.52 9.43 9.80 15.88 26.10 MnO 0.34 0.22 0.28 0.16 0.20 0.56 0.95 MgO 22.00 23.18 12.12 12.26 13.11 9.22 2.28 CaO 1.82 1.58 20.24 20.37 20.00 20.56 20.22 Na20 0.09 0.12 0.39 0.65 0.52 0.50 0.37 K 20 0.02 0.01 0.03 0.02 0.02 0.03 0.01 SUM 99.16 99.61 99.83 99.77 99.76 99.98 100.05 Cations based on 6 oxygens S i 1.9236 1.8706 1.9331 1.8434 1.8741 1.9677 1.9718 Al 0.0764 0.1294 0.0669 0.1566 0.1259 0.0323 0.0260 Al 0.0704 0.1574 0.0663 0.0942 0.0466 0.0293 -T i 0.0179 0.0165 0.0454 0.0539 0.0484 0.0142 0.0160 F e 3 + - - - 0.0021 0.0203 0.0122 0.0232 Fe 2+ 0.5909 0.4700 0.3294 0.2955 0.3083 0.5129 0.8912 Mn 0.0106 0.0067 0.0090 0.0050 0.0064 0.0182 0.0329 Mg 1.2164 1.2514 0.6767 0.6847 0.7354 0.5306 0.1387 Ca 0.0725 0.0614 0.8123 0.8177 0.8065 0.8506 0.8845 Na 0.0062 0*0084 0.0282 0.0475 0.0378 0.0376 0.0292 K 0.0007 0.0064 0.0015 0.0007 0.0009 0.0013 0.0006 ZX4Y 1.9856 1.9782 1.9688 2.0013 1.9640 1.9776 2.0163 ZZ 2.0000 2.0000 2.0000 2.0000 2.0000 2.0000 1.9978 (atomic %) Ca 3.84 3.43 44.45 45.35 43.44 44.48 45.42 Mg 64.35 69.93 37.03 37.98 39.61 27.75 7.12 F e 2 + + Mn 31.82 26.64 18.52 16.67 16.95 27.77 47.46 Na 0.34 0.48 2.70 4.60 3.47 3.42 2.67 Mg 66.68 72.06 64.86 66.30 67.60 48.27 12.70 F e 2 + + Mn 32.98 27.45 32.44 29.10 28.93 48.31 84.62 Iron partitioned according to the method of Papike et. a l . (1974). - 72 -TABLE IV. CONTINUED Groundmass ** R-23 R-44 R-44 R-54 R-lOOb R-44 1504 15 18 20 16 14 2 S i 0 2 47.84 47.29 48.67 48.13 47.94 62.15 52.58 T i 0 2 0.60 0.39 0.40 0.46 0.60 0.72 4.56 AI2O3* 0.38 0.09 0.13 0.14 0.35 0.93- 0.70 Fe 20 3 0.59 3.52 2.88 3.01 1.47 6.16 25.46 FeO 28.45 27.49 27.81 27.83 28.08 12.16 2.89 MnO 1.17 1.10 1.05 1.01 1.15 0.31 0.42 MgO 0.35 0.03 0.04 0.05 0.48 0.06 0.04 CaO 20.06 17.60 18.37 17.57 19.06 4.84 0.18 Na20 0.47 1.61 1.51 1.61 0.50 3.51 13.65 K 20 0.01 0.02 0.01 - - 0.41 -SUM 99.92 99.14 100.87 99.81 99.63 91.25 100.48 Cations based on 6 oxygens S i 1.9802 1.9930 2.0043 2.0048 1.9885 2.4691 1.99 Al 0.0185 0.0044 - - 0.0115 - 0.01 Al - - 0.0065 0.0069 0.0055 0.0436 0.03 Ti 0.0188 0.0123 0.0125 0.0143 0.0187 0.0215 0.13 Fe 3+ 0.0185 0.1116 0.0892 0.0942 0.0092 0.1841 0.73 Fe 2+ 0.9849 0.9691 0.9577 0.9692 1.0113 0.4039 0.10 Mn 0.0411 0.0391 0.0365 0.0356 0.0404 0.0103 0.01 Mg 0.0185 0.0016 0.0024 0.0034 0.0299 0.0036 • -Ca 0.8895 0.7949 0.8106 0.7841 0.8472 0.2060 0.01 Na 0.0376 0.1318 0.1207 0.1297 0.0406 0.2707 1.00 K 0.0004 0.0011 0.0003 - - 0.0206 -EX+Y 2.0093 2.0615 2.0364 2.0374. 2.0028 1.1643 2.01 EZ 1.9987 1.9974 2.0043 2.0048 2.0000 2.4691 2.00 (atomic %) Ca 45.99 44.05 44.85 43.75 43.92 33.02 8.33 Mg 0.96 0.09 0.13 0.19 1.55 0.58 8.33 F e 2 + + Mn 53.05 55.87 55.01 56.06 54.53 66.40 83.33 Na 3.47 11.55 10.80 11.40 3.62 39.32 90.09 Mg 1.71 0.14 3.27 0.30 2.66 0.52 -F e 2 + + Mn 94.82 88.31 88.98 88.30 93.72 60.16 9.91 Iron partitioned according to the method of Papike et. a l . (1974). Analysis from Becker (1976). - 73 -TABLE V. MICROPROBE ANALYSES OF AMPHIBOLE GRAINS. R-54 R-54 R-54 R-68 23 24 28 19 S i 0 2 46.57 47.62 49.80 48.85 T i 0 2 0.57 0.46 0.46 0.33 A1 20 3 0.15 0.18 2.01 0.17 Fe 20 3* - 1.42 10.29 1.25 FeO'c 35.04 33.76 16.08 34.04 MnO 1.23 1.20 0.53 1.14 MgO 0.10 0.08 0.06 0.05 CaO 2.09 2.07 8.27 2.44 Na20 7.64 6.65 7.59 7.06 K 20 1.48 1.32 0.72 1.23 SUM 94.87 94.76 95.81 96.56 Cations based on 23 oxygens S i 7.9144 8.0407 8.0516 8.0721 Al 0.0305 - - -Al - 0.0352 0.3838 0.0339 T i 0.0724 0.0586 0.0561 0.0407 F e 3 + - 0.1847 1.2515 0.1552 Fe 2+ 4.9799 4.7676 2.1747 4.7036 Mn 0.1765 0.1713 0.0730 0.1601 Mg 0.0251 0.0193 0.0140 0.0117 Ca 0.3809 0.3746 •1.4321 0.4326 Na 1.4175 1.3996 0.4809 1.3956 Na 1.0988 0.7785 1.8970 0.8656 K 0.3202 0.2840 0.1495 0.2595 EZ 7.9449 8.0407 8.0516 8.0721 EX 5.0523 5.0109 3.8660 4.9334 EY 2.0000 2.0000 2.0000 2.0000 EW 1.4190 1.0625 2.0465 1.1251 (atomic %) Mg 0.86 0.75 0.37 0.43 Ca 13.03 14.56 37.45 15.99 Na 86.11 84.69 62.18 83.58 *Iron partitioned according to the method of Papike et. a l . (1974). - 74 -TABLE VI. MICROPROBE ANALYSES OF AENIGMATITE. * R-44 R-54 2 1 26 S i 0 2 40.92 39.59 41.70 T i 0 2 8.18 7.91 6.95 A1 20 3 0.23 0.40 0.29 FeO 42.30 39.93. 42.30 MnO- 0.79 1.10 0.71 MgO - ''• 0.06 0.11 CaO 0.16 0.23 0.32 Na20 7.67 4.78 7.20 K 20 - - 0.11 SUM 100.08 94.00 99.69 Cations based on 20 oxygens S i 5.827 6.0409 5.97 Ti 0.876; 0.9072 0.75 Al 0.040 0.0718 0.05 F e 2 + 4.952 5.0955 5.01 Mn 0.096 0.1422 • 0.09 Mg - 0.0131 0.03 Ca 0.025 0.0374 0.05 Na 2.119 1.4153 2.00 K - - 0.02 (atomic %) 100 x T i Fe + T i 0 ' U J 15.11 13.02 Analysis supplied by B. Pr o f f e t t (University of Calgary). **Analysis from Yagi and Souther (1974). TABLE VII. MICROPROBE ANALYSES OF FELDSPAR PHENOCRYSTS. AP-11 AP-15 R-25 R-25 R-36 R-108 R-108 1 5 lb 7 5 1 3 S i 0 2 57.96 53.02 54.21 55.65 55.86 56.09 54.78 T i 0 2 0.06 0.12 0.14 0.11 0.09 0.06 0.10 A1 20 3 26.45 29.62 29.86 28.20 27.95 27.76 27.94 FeO 0.26 0.55 0.56 0.57 0.32 0.50 0.52 MnO 0.04 - - 0.03 - 0.03 -MgO 0.03 0.05 0.08 0.09 0.07 0.06 0.07 CaO 9.03 12.45 11.24 11.26 10.49 10.31 10.58 Na20 5.87 4.40 4.27 4.58 4.68 5.31 5.47 K 20 0.46 0.36 0.27 0.35 0.41 0.37 0.41 SUM 100.16 100.57 100.63 100.84 99.87 100.49 98.87 Cations based on 8 oxygens S i 2.5942 2.3971 2.4309 2.4915 2.5148 2.5162 2.4822 Ti 0.0019 0.0041 0.0046 0.0039 0.0031 0.0022 0.0034 Al 1.3955 1.5785 1.5780 1.4884 1.4829 1.4680 1.4919 Fe 0.0097 0.0209 0.0209 0.0212 0.0119 0.0189 0.0197 Mn 0.0017 0.0001 - 0.0013 - 0.0012 -Mg 0.0022 0.0003 0.0056 0.0059 0.0048 0.0041 0.0049 Ca 0.4330 0.6021 0.5339 0.5400 0.5060 0.4955 0.5138 Na 0.5099 0.3853 0.3712 0.3976 0.4083 0.4618 0.4810 K 0.0262 0.0206 0.0156 0.0199 0.0234 0.0212 0.0239 EZ'- 3.9916 3.9797 4.0135 3.9838 4.0008 3.9864 3.9775 EX 0.9827 1.0293 0.9472 0.9859 0.9544 1.0027 1.0433 (atomic %) An 44.68 59.73 57.99 56.40 53.96 50.64 50.44 Ab 52.62 38.22 40.32 41.52 43.54 47.19 47.22 Or 2.70 2.04 1.69 2.08 '2.50 2.17 2.35 - 76 -TABLE VII. CONTINUED AP-12 R-97 R-97 R-97 R-44 R-44 R-54 9 1 3 7 3 4 5 S i 0 2 62.17 67.46 66.64 66.41 69.17 68.98 67.64 T i 0 2 0.01 - - 0.02 0.01 - -A1 20 3 22.90 19.36 18.76 19.45 17.54 17.25 18.29 FeO 0.56 0.22 0.20 0.20 0.44 0.70 0.31 MnO 0.01 - 0.03 - 0.02 - -MgO 0.01 - - - 0.02 - -CaO 4.61 0.89 0.85 1.35 - - 0.15 Na20 8.22 8.12 7.96 8.24 7.37 7.32 7.19 K 20 0.96 4.66 4.91 3.73 6.00 5.98 6.11 SUM 99.45 100.71 99.35 99.40 100.57 100.23 99.69 Cations based on 8 oxygens S i 2.7807 2.9794 2.9885 2.9645 3.0615 3.0662 3.0246 Ti 0.0004 - - 0.0007 0.0004 0.0001 -A l 1.2074 1.0079 0.9913 1.0232 0.9147 0.9037 0.9641 Fe 0.0208 0.0081 0.0074 0.0074 0.0164 0.0262 0.0116 Mn 0.0003 - 0.0011 0.0002 0.0009 0.0001 -Mg 0.0009 - 0.0003 - 0.0015 0.0002 -Ca 0.2208 0.0421 0.0408 0.0645 - 0.0002 0.0073 Na 0.7129 0.6953 0.6918 0.7130 0.6325 0.6310 0.6230 K 0.0547 0.2627 0.2811 0.2125 0.3386 0.3393 0.3484 EZ 3.9885 3.9873 3.9798 3.9884 3.9766 3.9700 3.9887 EX 1.0104 1.0082 1.0225 0.9976 0.9899 0.9971 0.9903 (atomic %) An 22.34 4.21 4.02 6.52 - 0.02 0.75 Ab 72.13 69.52 68.25 72.02 63.22 65.02 63.66 Or 5.53 26.27 27.73 21.46 34.87 34.96 35.60 - 77 -TABLE VII. CONTINUED R-68 R-68 RIOOb 1 5 6 S i 0 2 68.46 67.56 68.27 T i 0 2 0.04 0.04 0.03 A1 20 3 18.39 18.33 17.54 FeO 0.46 0.24 0.29 MnO - 0.02 -MgO - - -CaO 0.11 0.02 0.17 Na20 7.15 7.02 7.05 K 20 6.00 6.28 6.41 SUM 100.61 99.51 99.76 Cations based on 8 oxygens S i 3.0300 3.0254 3.0513 Ti 0.0014 0.0015 0.0008 Al 0.9591 0.9675 0.9240 Fe 0.0171 0.0009 0.0108 Mn - 0.0007 -Mg - 0.0001 -Ca 0.0052 0.0009 0.0082 Na 0.6139 0.6098 0.6110 K 0.3389 0.3590 0.3655 EZ 3.9905 3.9944 3.9761 EX 0.9751 0.9714 0.9955 (atomic %) An 0.54 0.09 0.83 Ab 64.08 62.89 62.05 Or 35.38 37.02 37.12 TABLE VIII. MICROPROBE ANALYSES OF TITANOMAGNETITE. AP- 11 AP- 15 R-117 R-108 AP- 12 R-97 R- 23 12 24 20 21 18 17 9 S i 0 2 0. 07 0. 14 0. 34 0. 11 0. 12 0. 24 0 .27 T i 0 2 17. 26 19. 50 24. 53 21. 32 21. 30 23. 67 28 .39 A1 20 3 1. 79 2. 73 : 3. 81 2. 09 0. 46 0. 20 0 .06 FeO 73. 79 70. 29 65. 10 70. 83 74. 29 67. 45 61 .50 MnO 0. 38 0. 48 ..0. 41 0. 54 0. 88 1. 07 1 .25 MgO 1. 32 2. 13 3. 23 1. 01 0. 28 0. 06 0 .01 CaO 0. 06 0. 01 0. 04 0. 05 SUM 94. 66 95. 25 97. 46 95. 91 97. 38 92. 70 91 .48 Recalculated Analyses, Ulvospinel basis FeO 44. 37 45. 62 49. 30 48. 76 49. 74 50. 54 54 .14 Fe 20 3 32. 70 27. 97 17. 56 24. 53 27. 28 18. 79 8 .17 TOTAL 97. 94 98. 55 99. 22 98. 36 100. 11 94. 58 92 .29 Mol. % Usp 50. 08 56. 08 69. 86 61. 62 60. 71 71. 61 87 .46 - 79 --TABLE IX. MICROPROBE ANALYSES OF ILMENITE. AP- 11 AP- 15 R-117 R-108 AP- 12 R-97 R-54 11 26 14 22 20 16 9 S i 0 2 0. 03 0. 09 0 .08 - 0. 08 0. 08 0 .04 T i 0 2 49. 81 50. 97 46 .81 48. 18 50. 28 46. 55 51 .04 A1 20 3 0. 14 0. 17 1 .43 0. 33 0. 05 0. 12 0 .01 FeO 47. 75 44. 53 42 .08 47. 47 48. 08 44. 22 46 .48 MnO 0. 48 0. 61 0 .58 0. 37 0. 63 1. 10 2 .05 MgO 2. 00 3. 47 3 .41 2. 87 0. 92 0. 05 0 .01 CaO 0. 05 0 .23 0. 03 0. 03 0. 88 0 .02 SUM 100. 28 99. 85 94 .81 99. 25 100. 08 93. 00 99 .65 Recalculated Analyses, Ilmenite basis FeO 43. 12 41. 02 38 .18 41. 18 44. 68 41. 21 44 .72 Fe 20 3 5. 14 3. 91 4 .33 6. 99 3. 74 3. 34 1 .96 TOTAL 100. 79 100. 25 95 .24 99. 95 100. 42 93. 33 99 .85 Mol. % R 20 3 4. 89 3. 74 5 .32 6. 76 3. 55 3. 49 1 .87 - 80 -TABLE X. WHOLE ROCK MAJOR-ELEMENT VOLATILE-FREE ANALYSES RECALCULATED TO 100 PERCENT, VOLATILE-INCLUDED ORIGINAL SUMS, TRACE-ELEMENT ANALYSES, AND NORMATIVE MINERALOGIES. A Hawaiite Mugearite Precision AP-11 AP-15 R-25 R-36 R-73 R-77 R-99 R-107 Si0 2 ±1% 49.74 50.33 50.30 Ti0 2 ±2% 1.76 2.29 1.98 A1 20 3 ±1% 15.29 16.17 16.54 Fe 20 3 ±1% 12.91 12.84 12.89 MnO ±1% 0.15 0.14 0.19 MgO ±4% 7.03 4.32 3.18 CaO ±1% 8.19 8.01 8.52 Na20 ±5% 3.41 4.16 4.43 K20 ±5% 1.13 1.57 1.46 P 20 5 ±30% 0.38 0.17 0.51 L.O.I. 1.23 0.74 0.00 ORIGINAL SUM 98.83 100.29 99.40 Rb ±5% 18.4 24.9 17.5 Sr ±5% 497.0 568.2 619.1 Ba ±5% 329.0 431.9 1154.4 Nb ±15% 24.7 33.5 23.8 Ni ±5% 96.1 47.0 21.5 Rb/Sr 0.037 0.044 0.028 K/Rb 504 522 686 K/Ba 28.2 30.1 10.4 K/Sr 18.8 22.9 19.5 Ba/Sr 0.7 0.8 1.9 Ca/Sr 117.8 100.8 98.4 Q - - -Or 6.68 9.28 8.63 Ab 28.85 34.62 35.61 An 23.08 20.81 20.93 Ne - 0.31 1.02 Ac - - -Ns -fWo 6.29 7.44 7.52 Di En 3.62 3.68 3.12 IFS 2.39 3.62 4.43 „ ,En 6.37 \Fs 4.20 -rfo 5.27 4.96 3.36 0 1\Fa 3.83 5.38 5.26 Mt 4.19 4.16 4.18 Ilm 3.34 4.35 3.76 Ap 0.88 0.39 1.18 Cor -48.85 50.62 49.83 54.92 56.30 2.18 2.79 2.37 1.71 1.63 17.90 15.76 16.45 15.69 16.02 12.60 14.61 13.25 11.41 11.00 0.14 0.17 0.15 0.14 0.13 5.18 3.51 4.87 2 .69 2.14 8.12 6.79 7.68 5.75 5.35 3.51 3.52 3.58 4.57 4.21 1.15 1.80 1.42 2.60 2.80 0.36 0.44 0.40 0.52 . 0.44 0.00 0.00 0.90 1.88 1.97 98.73 102.88 101.19 99.03 102.67 15.9 25.5 18.4 31.1 40.5 649.1 451.8 527.7 468.7 432.7 290.0 613.3 397.9 663.3 618.6 26.9 27.6 28.2 42.7 46.0 39.2 21.7 36.6 10.9 10.0 0.024 0.056 0.035 0.066 0.094 594 601 648 681 587 32.6 25.0 30.0 32.0 38.4 14.7 33.0 22.3 45.9 53.6 0.5 1.4 0.8 1.4 1.4 89.4 107.4 104.0 87.7 88.4 _ _ 1.66 5.22 6.80 8.39 8.39 15.36 16.55 29.70 29.79 30.29 38.67 35.62 29.69 23.01 24.62 14.62 16.55 3.44 3.37 4.54 4.39 2.97 1.84 1.44 2.33 1.87 1.14 1.49 1.93 2.10 2.53 1.88 4.73 7.30 6.54 4.83 4.19 3.84 9.82 5.89 6.55 6.91 4.44 - 2.28 - -3.98 - 2.27 - -4.09 4.74 4.29 4.22 4.06 4.14 5.30 4.50 3.25 3.10 0.83 0.93 0.93 1.20 1.02 _ _ — Precision i s based on replicate analyses of U.S.G.S. rock standard AGV-1 * o Loss on ignition at 1000 C. - 81 -TABLE X. CONTINUED Mugearite Trachyte Comenditic Trachyte R-108 R-109 AP-12 R-96 R-97 R-102 R-106 S i 0 2 54.93 54.64 6-1.33 67.81 68.32 66.07 64.82 T i 0 2 1.44 1.92 0.64 0.32 0.34 0.39 0.45 A1 20 3 18.69 15.25 17.31 14.51 14.97 15.00 16.25 Fe 20 3 9.03 11.62 6.39 4.63 4.39 6.16 5.79 MnO 0.12 0.15 0.11 0.10 0.07 0.14 0.14 MgO 1.86 2.46 0.56 0.00 0.03 0.00 0.01 CaO 6.71 5.80 2.83 0.95 1.00 1.27 1.82 Na20 4.66 5.04 6.18 6.61 5.88 5.70 6.19 K 20 2.14 2.61 4.24 5.00 4.92 5.20 4.43 P 2 0 5 0.42 0.50 0.42 0.07 0.07 0.08 0.10 L.O.I. 0.00 0.00 0.00 0.00 0.75 1.15 0.54 ORIGINAL SUM Rb Sr Ba Nb Ni Rb/Sr K/Rb K/Ba K/S.r Ba/Sr Ca/Sr Q Or Ab An Ne Ac Ns /Wo |DiJEri VFs fEn \Fs iFa Mt Ilm Ap Cor Hy(5 98.34 97.95 99.07 97.13 96.92 100.43 97.92 32.5 34.7 54.7 75.7 78.4 68.1 49.2 624.7 491.1 267.9 13.0 34.4 7.4 155.5 579.1 616.0 1050.9 346.9 781.8 46.9 1002.5 39.0 42.3 44.5 79.9 73.6 55.5 65.6 13.5 11.1 12.6 10.7 8.9 15.0 9.0 0.052 0.071 0.204 5.823 2.279 9.203 0.316 535 611 635 532 504 635 730 30.0 34.4- 33.1 116,0 50.5 921.6 35.9 28.4 44.0 131.1 3184.6 1184.2 5818.4 235.9 0.9 1.3 3.9 26.7 22.7 6.3 6.5 76.8 84.4 75.5 522.3 207.8 1226.6 83.7 1.91 _ 1.80 11.66 12.48 9.09 6.82 12.65 15.42 25.06 29.55 29.07 30.73 26.18 39.43 42.65 52.29 46.80 49.61 48.21 52.38 23.76 11.28 6.97 - - - 3,47 _ „ 3.41 0.13 0.02 — - - - 1.22 - - -2.83 5.94 1.81 1.78 1.88 2.41 2.05 1.13 2.43 0.38 - 0.04 - 0.01 1.73 3.56 1.55 2.02 2.09 2.74 2.31 3.50 3.13 1.01 - 0.04 - 0.01 5.34 4.58 4.11 3.33 1.99 3.17 3.11 - 0.40 - - - -- 0.64 - - - - -3.33 4.29 2.36 - 1.56 2.26 2.15 2.73 3.65 1.22 0.61 0.65 0.74 0.85 0.97 1.16 0.97 0.16 0.16 0.19 0.23 - - - - - - -- 82 -TABLE X. CONTINUED Comendite S i 0 2 T i 0 2 A1 20 3 Fe 20 3 MnO MgO CaO Na20 K 20 L.O.I. ORIGINAL SUM Rb Sr Ba Nb Ni Rb/Sr K/Rb K/Ba K/Sr Ba/Sr Ca/Sr Q Or Ab An Ne Ac Ns (Wo Di^En iFs |Hy'En Fs Fo Fa Mt Ilm Ap Cor R-18 R-22 R-23 R-24 R-27 R^29 R-40 R-44 ' R-47 66.18 66.85 67.00 69.87 70.08 66.37 67.43 69.77 67.24 0.42 0.42 0.42 0.44 0.44 0.41 0.42 0.42 0.38 14.42 14.57 13.85 12.71 12.17 12.78 13.53 11.74 14.40 6.63 7.24 6.83 6.64 7.92 7.58 7.41 7.86 6.71 0.13 0.10 0.13 0.16 0.15 0.14 0.16 0.16 0.18 0.00 0.00 0.00 0.00 0.00 1.51 0.00 0.00 0:00 1.29 0.37 0.93 0.39 0.30 0.93 0.81 0.51 0.73 5.63 5.19 5.68 4.72 4.05 5.32 5.22 4.68 5.10 5.21 5.20 5.10: 5.01 4.84 4.90 4.95 4.81 5.20 0.08 0.06 0.06 0.06 0.06 0.06 0.07 0.05 0.06 0.39 1.48 0.52 1.31 1.88 0.49 1.11 1.59 0.76 102.26 101.92 99.33 101.27 101.19 101.41 103.18 99.77 102.33 55.7 66.4 62.3 80.5 79.2 77.9 59.6 100.0 62.6 6.3 7.6 6.6 10.9 8.5 3.6 5.5 9.5 10.4 76.1 25.2 80.5 33.4 39.1 34.2 92.1 17.8 18.6 53.1 66.0 64.2 81.8 98.1 80.6 73.0 94.4 66.1 7.5 8.6 9.5 9.3 8.3 9.6 9.1 9.4 12.2 8.841 8.737 9.439 7.385 9.318 21.639 10.836 10.526 6.019 792 661 674 521 512 528 710 397 704 579.9 1741.4 521.5 1256.9 1037.7 1203.3 459.4 2232.8 2368.3 6847.4 5665.3 6398.2 3805.8 4714.7 11270. 7452.0 4192.3 4140.0 12.1 3.3 12.2 3.1 4.6 9.5 16.8 1.9 1.8 1463.4 348.0 1007.1 255.7 252.3 1846.3 1052.6 383.7 501.7 10.38 13.27 12.34 21.99 24.34 12.63 15.02 22.46 13.95 30.79 30.73 30.14 29.61 28.60 28.96 29.25 28.42 30.73 45.17 43.92 42.85 34.71 34.27 38.46 42.04 33.61 43.15 - 1.10 - - - - - - 1.04 2.18 - 4.59 4.61 - 5.78 1.88 5.28 -2.45 0.14 1.76 0.64 0.15 1.76 1.49 0.92 0.91 - - - - - 0.56 - - -2.79 0.16 2.00 0.73 0.17 1.27 1.69 1.04 1.04 - - - - - 3.20 - - -3.75 6.29 5.42 6.54 6.99 7.26 5.57 7.62 5.11 1.59 2.93 0.47 0.37 3.20 0.16 2.06 0.53 2.71 0.80 0.80 0.80 0.84 0.84 0.78 0.80 0.80 0.72 0.19 0.14 0.14 0.14 0.14 0.14 0.16 0.12 0.14 - 83 -TABLE X. CONTINUED Comendite R-48 R-51 R-54 R-71 R-76 R-81 R-88 R-l 16 SK>2 69.00 67.02 69.03 68.79 65. 43 66. 34 68.12 71.66 T i 0 2 0.41 0.41 0.37 0.36 0. 40 0. 41 0.39 0.41 AI2O3 12.25 14.07 12.77 13.56 14. 49 16. 15 13.67 11.83 Fe 20 3 7.69 7.00 6.29 6.16 7. 48 5. 32 6.47 6.00 MnO 0. 13 0.15 0.12 0.12 0. 13 0. 10 0.13 0.08 MgO 0.00 0.00 0.00 0.00 0. 00 0. 00 0.00 0.00 CaO 0.35 0.71 0.51 0.49 1. 17 1. 26 0.50 0.25 Na20 5.31 5.48 5.89 5.60 5. 51 5. 05 5.77 5.04 K 20 4.82 5.11 4.96 4.85 5. 31 5. 29 4.90 4.68 0.06 0.06 0.06 0.05 0. 08 0. 08 0.06 0.05 L.O.I. 1.48 0.92 0.45 0.38 1. 15 1. 68 0.49 0.00 ORIGINAL SUM 100.42 100.59 98.94 103.06 98. 36 102. 15 101.26 99.95 Rb 70.1 62.8 . 87.0 81.6 59 .8 6] .3 85.4 81.5 Sr 10.5 3.8 2.7 2.5 10.3 '.4 1.2 3.3 Ba 15.8 30.3 11.4 9.7 6f .5 61 .3 10.3 14.3 Nb 98.3 62.7 77.5 79.8 42 .4 53.7 83.5 90.4 Ni 8.7 9.5 7.5 8.4 c 5.7 i 5.5 9.2 8.3 Rb/Sr 6.676 16.526 32.222 32.640 5.806 8.284 74.430 24.697 K/Rb 572 678 467 507 723 729 481 475 K/Ba 2536.4 1404.6 3566.2 4268.0 659.9 729.4 3987.3 2709.8 K/Sr 3800.9 11134. 15211. 559.3 4268.6 5919.1 34250. 11743. Ba/Sr 1.5 8.0 4.2 3.9 i 3.4 8.3 8.6 4.3 Ca/Sr 238.2 1335.4 1350.0 1400.8 811.8 1216.9 2977.9 541.4 Q 19.61 12.87 18.17 16.27 9 41 12 .14 14.70 25.09 Or 28.48 30.20 29.31 28.66 31 38 31 .26 28.96 27.66 Ab 36.18 43.92 38.07 42.75 44 .97 42 .73 43.03 34.80 An - - - - 5 .73 - -Ne - - - - -Ac 6.22 2.16 5.06 4.09 1 .46 5.10 4.83 Ns 0.39 - 1.40 - - 0.55 fWo 0.56 1.31 0.89 0.88 2 .21 0.87 0.38 DiiEn - - - - - -Ws 0.64 1.48 1.01 1.00 2 .50 0.99 0.43 Mil 8.09 5.46 6.09 5.72 4 .67 4 .62 6.28 6.18 , rFo _ _ — - -0l{„ VFa — — — — m— Mt — 1.75 - 0.45 2 .30 2 . 15 0.07 -Ilm 0.78 0.78 0.70 0.68 0 .76 0 .78 0.74 0.78 Ap 0.14 0.14 0.14 0.12 0 .19 0 .19 0.14 0.12 Cor - - - - 0 .02 - -TABLE XI. STRONTIUM ISOTOPIC DATA. b Sample S I 0 2 a Rb & S r 3 Rb/Sr S r 8 7 / S r 8 6 S r 8 7 / S r 8 6 o K-Ar Date (wt.%) (ppm) (ppm) (m.y.) AP-12 60.76 54.7 267.9 0.204±3% .7032 + .00015 — — AP-15 50.48 24.9 568.2 0.044±3% .7032 + .00015 - -R-25 50.00 17.5 619.1 0.028±3% .7032 + .00015 - -R-108 54.02 32.5 624.7 0.052±3% .7031 + .0003 - -R-97 66.22 78.4 34.4 2.279±3% .7042 + .0003 .70341.0004 8.7 R-23 66.55 62.3 6.6 9.44±15% .7060 + .0003 .70291.0005 7.9C R-29 67.31 77.9 3.6 21.64±15% .7081 + .0003 .7017±.0006 7.2 R-88 68.98 85.4 1.2 74.43±20% .727 + .0023 .70251.011 8.0C a. SiO Sr done by X-ray fluorescence spectrometry by M.L. Bevier. b. S r 8 7 / S r86 measured on mass spectrometer by M.L. Bevier and K.L. Scott. c. These ages are estimated on the basis of stratigraphic position. d. Precision of Rb/Sr based on replicate analyses of Rb and Sr. : Error i n measured S r 8 7 / S r 8 6 ratios i s 1 a. Error i n calculated S r 8 7 / S r 8 6 ratios represents the com-bination of errors i n Rb/Sr, S r 8 7 / S r 8 6 , and dates. -TABLE XII. CALCULATED AND OBSERVED VISCOSITIES (n) . Temperature 950° C 950° C 1200° C 1200° C Reference Water Content (wt. %) 0 1 0 0 Comenditic trachyte (3) 1 .1 X 10 7 1.5 x 106 9.4 x IO 4 1.2 x 10^ This study*5 Comendite (4) 1 .5 x 10 7 2.0 x 10 6 1.2 x 10 5 1.7 x IO1* This study^ P a n t e l l e r i t e - - - 6.3 x IO 4 Scarfe, 1977^ Calc-alkaline rhyolite (10) 7 .8 x 10 8 4.5 x 10 7 3.3 x 106 4.4 x 10 5 Schminke, 1974b Hawaiian t h o l e i i t e - - ' 4 x 10 2 - Shaw et. a l . , 1969C Ne-bearing alkaline basalt - - 4 x 10 2 - Bottinga and W e i l l , 1972° a. Number of analyses averaged for calculating v i s c o s i t i e s . b. Calculated using the method of Shaw (1972). c. Determined experimentally. - 86 -TABLE X I I I . LEAST-SQUARES MASS BALANCE CALCULATION DERIVING MUGEARITE FROM HAWAIITE. Parent R-36 Observed Daughter R-99 Calculated Daughter Variable Wt. Fraction SiO„ 48.92 55.00 54.94 Olivine 15.86 TI0 2 2.18 1.71 1.68 Plagioclase 65.37 A1 20 3 17.93 15.71 15.72 Augite .8.27 Fe 0 12.62 11.53 11.40 Magnetite 7.52 MgO 5.19 2.69 2.70 Ilmenite 2.05 CaO 8.13 5.76 5.77 Apatite 0.92 Na20 3.52 4.58 4.57 K 20 1.15 2.60 2.80 P2°5 0.36 0.52 0.44 Er 2= 0.0504 Rb 15.9 31.1 49.9 Sr 649.1 468.7 463.1 Ba 290.0 663.3 705.2 Ni 39.2 10.9 10.2 Mass of New Magma Relative to Old = 29.29 weight percent - 87 -TABLE XIV. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING COMENDITIC TRACHYTE FROM HAWAIITE. Parent R-36 Observed Daughter R-97 Calculated Daughter Variable Wt. Fraction S i 0 2 48.92 68.04 67.88 Olivine 14.84 TI0 2 2.18 0.32 0.28 Plagioclase 63.90 A1 20 3 17.93 14.45 14.45 Augite 9.39 F e2°3 12.62 4.61 4.58 Magnetite 8.97 MgO 5.19 0.00 0.00 Ilmenite 1.74 CaO 8.13 0.95 0.96 Apatite 1.15 Na20 3.52 6.58 6.57 K2° 1.15 4.98 . 5,20 P2°5 0.36 0.07 0.02 Zr2- 0.0645 Rb 15.9 78.4 96.5 Sr 649.1 34.4 203.4 Ba 290.0 781.8 1082.3 Ni 39.2 8.9 3.9 Mass of New Magma Relative to Old = 14.04 weight percent - 88 -TABLE XV. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING COMENDITE FROM HAWAIITE. Parent R-36 Observed Daughter R-48 Calculated Daughter Variable Wt. F r a c t i o n SI0 2 48.92 69.08 68.97 Ol i v i n e 15.10 T i 0 2 2.18 0.41 0.34 Plagioclase 65.22 A 1 2 ° 3 17.93 12.26 12.23 Augite 8.15 F e 2 0 3 12.62 7.70 7.64 Magnetite 8.18 MgO 5.19 0.00 -0.01 Ilmenite 2.04 CaO 8.13 0.35 0.36 Apatite 1.31 Na £0 3.52 5.32 5.46 K 20 1.15 4.83 5.12 P2°5 0.36 0.06 -0.10 Er 2= 0.1515 Rb 15.9 70.1 103.8 Sr 649.1 10.5 173.7 Ba 290.0 15.8 1433.8 Ni 39.2 8.7 3.9 Mass of New Magma Relative to Old = 12.90 weight percent - 89 -TABLE XVI. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING COMENDITIC TRACHYTE FROM MUGEARITE. Parent R-99 Observed Daughter R-97 Calculated Daughter Variable Wt. Fraction S i 0 2 55.00 68.04 68.02 Olivine 10.14 T i 0 2 1.71 0.32 0.31 Plagioclase 57.08 A 12°3 15.71 14.45 14.45 Augite 14.50 F e2°3 11.43 4.61 4.60 Magnetite 15.71 MgO 2.69 0.00 0.00 Ilmenite 0.31 CaO .5.76 0.95 0.96 Apatite 2.26 Na20 4.58 6.58 6.58 K 20 2.60 4.98 5.07 P2°5 0.52 0.07 0.11 Er = 0.0101 Rb 31.1 78.4 64.3 Sr 468.7 34.4 517.9 Ba 663.3 781.8 1204.3 Ni 10.9 8.9 7.9 Mass of New Magma Relative to Old = 48.02 weight percent - 90 -TABLE XVII. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING COMENDITE FROM MUGEARITE. Parent R-99 Observed Daughter R-48 Calculated Daughter Variable Wt. Fraction s i o 2 55.00 69.08 68.89 Olivine 11.89 T i 0 2 1.71 0.41 0.29 Plagioclase 64.60 A 12°3 15.71 12.26 12.17 Augite 7.30 Fe 20 3 11.43 7.70 7.59 Magnetite 11.07 MgO 2.69 0.00 -0.03 Ilmenite 2.01 CaO 5.76 0.35 0.38 Apatite 3.13 Na20 4.58 5.32 5.84 K 20 2.60 4.83 5.14 P2°5 0.52 0.06 -0.22 Er = 0.5286 Rb 31. 1 70.1 67.2 Sr 468.7 10.5 453.8 Ba 663.3 15.8 1336.8 Ni 10.9 8.7 8.6 Mass of New Magma Relative to Old = 44.32 weight percent - 91 -TABLE XVIII. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING COMENDITE FROM COMENDITIC TRACHYTE. Parent R-97 Observed Daughter R-48 Calculated Daughter Variable Wt. Fraction SI0 2 68.04 69.08 69.07 Anorthoclase 97.80 T i 0 2 0.32 0.41 0.48 Hedenbergite 2.20 A 12°3 14.45 12.26 11.81 F e 20 3 4.61 7.70 7.36 MgO 0.00 0.00 -0.02 CaO 0.95 0.35 0.57 Na20 6.58 5.32 5.54 K2P. 4.98 4.83 5.10 P2°5 0.07 0.06 0.09 l r 2 = 0.4973 Rb 78.4 70.1 114.9 Sr 34.4 10.5 26.1 Ba 781.8 15.8 375.7 Mass of New Magma Relative to Old = 59.63 weight percent - 92 -XIX. LEAST-SQUARES MASS BALANCE CALCULATION DERIVING ANAHIM PEAK TRACHYTE FROM ANAHIM PEAK HAWAIITE. Parent Observed Daughter Calculated Daughter Variable Wt. Fraction AP-15 AP-12 SiO„ 50.40 61.40 61.39 Olivine 11.17 z TiO„ 2.30 0.64 0.64 Plagioclase 48.01 z A100_ 16.20 17.33 17.29 Augite 28.33 2 3 Fe 0 12.86 6.40 6.40 Magnetite 10.66 Z J MgO 4.32 0.56 0.55 Ilmenite 1.79 CaO 8.02 2.83 2.84 Apatite 0.04 Na20 4.16 6.18 6.50 K2° 1.57 4.24 3.99 P2°5 0.17 0.42 Er 2= 0.41 0.1731 Rb 24.9 54.7 58.9 Sr 568.2 267.9 652.9 Ba 431.9 1050.9 1050.6 Ni 47.0 12.6 15.9 Mass of New Magma Relative to Old = 40.54 weight percent REFERENCES Abbott, M.J., 1967, Aenigmatite from the groundmass of a peralkaline trachyte. Amer. Mineralogist 52, pp. 1895-1901. Abbott,M.J. , 1969, Petrology of the Nandewar volcano, N.S.W., Australia. Contrib. Mineral. Petrol. 20, pp. 115-134. 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Bev ie r , 1976 o S C A L E miles kilometers LEGEND >-cc < z OC LU 3 O Quaternary Alluvium, till Qal Talus, snowfields Late Miocene Trachyte of Anahim Peak Mta Mha Hawaiite of Anahim Peak >-OC < rr LU Mh Mc Mm Mt Hawaiite Unit Comendite Unit Mugearite Unit Comenditic Trachyte Unit SYMBOLS Lithologic contact (defined, approximate, inferred) Contact between flows ( n o t s h o w n in Mm) \ ® Bedding (inclined, vertical, horizontal) BRITISH COLUMBIA INDEX MAP NORTH Mc-Mm 5 0 0 0 SOUTH 8* 6000' ^5000' N 5 W N 31 W N 41 W Hawaiite vent PLATE n. MAP OF SAMPLE LOCALITIES GeoloQic units colored as in Plate I . c SCALE miles 2 kilometers Lithologic contact (defined, approximate, inferred) 2 3 # Sample locality (Prefixes R and AP-not shown on map) Limits of outcrop area 

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