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Emplacement depth and porphyry copper-gold potential of Late Triassic to Early Jurassic granitoids in… Topham, Matthew James Jun 30, 2015

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       EMPLACEMENT DEPTH AND PORPHYRY COPPER-GOLD POTENTIAL OF LATE TRIASSIC TO EARLY JURASSIC GRANITOIDS IN YUKON   by   MATTHEW JAMES TOPHAM  B.A., The University of British Columbia, 2012   A THESIS  SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  BACHELOR OF SCIENCE (HONOURS)   in   THE FACULTY OF SCIENCE       This thesis conforms to the required standard  ………………………………….  Supervisor  THE UNIVERSITY OF BRITISH COLUMBIA  VANCOUVER, BRITISH COLUMBIA   JUNE 2015    © Matthew James Topham 2015      ii Abstract   The objective of this study is to provide additional constraints through Al-in hornblende geothermobarometry on the nature and tectonic setting of Late Triassic to Early Jurassic magmatism and associated intrusion-related mineralization in west-central Yukon and eastern Alaska. Late Triassic to Early Jurassic (220-180 Ma) plutonic rocks in west-central Yukon and eastern Alaska represent a pulse of magmatism that emplaced the subvolcanic magma chambers of a continental magmatic arc that was superimposed on the Yukon-Tanana terrane. The Late Triassic to Early Jurassic Granite Mountain batholith hosts the Minto and Carmacks Copper deposits, the two most significant examples of Late Triassic to Early Jurassic intrusion-related copper-gold mineralization in the area. Predominantly granitic rocks of the Late Triassic to Early Jurassic Aishihik plutonic suite are exposed along the contact between the Yukon-Tanana terrane and the Mesozoic intraoceanic volcanic arc terranes, Stikinia and Quesnellia. The granitic plutons form mountain ranges on the southern border of the Yukon Plateau where it joins the adjacent Whitehorse Trough. This intermontane basin is occupied by mainly clastic sedimentary rocks overlapping the Stikine and Quesnel terranes. The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin. Specifically, Late Pliensbachian (183 Ma) and younger strata of the Tanglefoot Formation of the Early to Middle Jurassic Laberge Group contain granitic plutonic clasts with igneous ages and detrital uranium-lead zircon age peaks corresponding to Late Triassic to Early Jurassic plutons, indicating rapid exhumation during emplacement. Samples were collected from plutons of the Aishihik and Tatchun batholiths for the purpose of Al-in-hornblende geothermobarometry to provide estimates of pressures and temperatures of crystallization. Assuming an average density of continental crust, calculated pressures were converted into emplacement depths. Based on the depths corresponding to crystallization pressures, the Tatchun batholith was emplaced on average about 10 kilometers deeper (21-31 kilometers) than the Aishihik batholith (12-17 kilometers). Age-equivalent intrusions in British Columbia were responsible for generating significant copper-gold porphyry deposits that are currently preserved at or near surface in Stikinia and Quesnellia. By contrast, the Late Triassic to Early Jurassic Aishihik and Tatchun batholiths were emplaced at deeper levels in the crust than is believed to be required for the exsolution of a mineralizing fluid phase to form conventional porphyry deposits. The rapid exhumation of these batholiths accompanying emplacement produced a strong preservation bias against rocks hosting porphyry deposits that form at shallow crustal levels, generally in the upper six kilometers. While the emplacement depths and exhumation rates of the Aishihik and Tatchun batholiths do not make them prospective in terms of conventional porphyry deposits, they may still be prospective hosts for other unconventional, deformed intrusion-related copper-gold deposits such as Minto and Carmacks Copper.       iiiTable of Contents   Abstract                   ii  Table of Contents                 iii  List of Figures                 iv  List of Tables                   v  List of Appendices                  v  Acknowledgements                 vi  1. Introduction                  1                    2. Regional Geology and LTr-EJ Intrusion-related Mineralization                   5  3. Methodology                 15  3.1 Petrography                15 3.2 Geothermobarometry               17  4. Results                  22   4.1 Petrography                22  4.2 Geothermobarometry               38  4.3 Sources of Error                43  5. Discussion                  46   5.1 Emplacement Depths and Exhumation Rates                       46 5.2 Tectono-magmatic Evolution of the Aishihik Plutonic Suite          48  5.3 Implications for LTr-EJ Intrusion-related Mineral Deposits         53  6. Summary                  55                  7. Recommendations                 57  References                  59  Appendices                  65           ivList of Figures   Figure 1: Regional scale geologic map of western Yukon and eastern Alaska showing the geographic distribution of plutonic suites and lithotectonic terranes as well as significant mineral occurrences (Allan, pers. comm., 2015).  Figure 2: District scale geologic map of south-central Yukon showing the geographic distribution of Late Triassic to Early Jurassic plutonic suites and Early to Middle Jurassic Laberge Group sedimentary rocks as well as sample localities and geochronology data (Allan, pers. comm., 2015).  Figure 3: Images illustrating the process of data collection for Al-in hornblende geothermobarometry (after Tafti, 2005).  Figure 4: International Union of Geological Sciences (IUGS) Quartz-Alkali-feldspar-Plagioclase (QAP) ternary diagram classification of representative plutonic rock units from the Aishihik and Tatchun Batholiths (after Streckeisen, 1976, and Tafti, 2005).  Figure 5: Images from a thin section scan of MA14-TB10 illustrating selected mineral phases and textures.  Figure 6: Images from thin section scans of MA14-AB8 and MA14-TB2 corroborating an inferred process of melt crystallization.  Figure 7: Image of unstained slab and photomicrograph in XPL of MA14-AB1.  Figure 8: Image of unstained slab and photomicrograph in XPL of MA14-AB8.  Figure 9: Image of unstained slab and thin section scan in XPL of MA14-AB10.  Figure 10: Image of unstained slab and photomicrograph in PPL of MA14-TB2.   Figure 11: Image of unstained slab and photomicrograph in PPL of MA14-TB4.  Figure 12: Image of unstained slab and photomicrograph in PPL of MA14-TB10.  Figure 13: Classification diagram after Leake et al. (1997) for hornblende grains analyzed from the six samples selected for geothermobarometry. AB = Aishihik batholith, TB = Tatchun batholith.           vList of Tables   Table 1: Calculated QAP-normal modes for granitoids from the Aishihik and Tatchun batholiths corresponding to the Long Lake (192-180 Ma) and Minto (204-195 Ma) plutonic suites, respectively. Table after Tafti (2005).  Table 2: Results of iteration based on Anderson and Smith (1995) pressure at various thermometers for the six samples selected from the Aishihik batholith (AB) and Tatchun batholith (TB). Calculated maximum and minimum temperatures, pressures, and depths of crystallization for each of the three thermometer reactions are presented for each of the six samples selected for geothermobarometry.    List of Appendices*   *Appendices are inlcuded in digital format in the back pocket of this thesis and their contents are described in more detail in the Appendices section.   Appendix 1: Petrographic descriptions of hand samples and polished thin sections.  Appendix 2: Images of samples and point location maps on back-scattered electron (BSE) images.  Appendix 3: Analytical data from electron probe microanalysis (EPMA), scanning electron microscope (SEM) energy-dispersive X-ray spectrometry (EDS), and calculation spreadsheets related to Anderson and Smith’s (1995) Al-in hornblende geothermobarometer.                  viACKNOWLEDGMENTS   The completion of this thesis has been a significant learning experience that was supported by and benefited from the input of many different people. I would first like to thank my supervisor Dr. Murray M. Allan as well as co-supervisors Dr. James K. Mortensen and Dr. Craig J.R. Hart.  I am indebted to Murray for his valuable feedback, guidance, and assistance over the course of the project, including the staining of samples, the acquisition of analytical data, the compiling of maps, and the provision of scientific literature relevant to this thesis.  I first encountered Jim during the field school on Saltspring Island that he led in 2013. Since then he has provided significant guidance and encouraged my interest in geology.  I am grateful to Craig for valuable constructive criticism and advice at key stages over the course of completing this thesis.  I would also like to thank Maurice Colpron of the Yukon Geological Survey for providing feedback as well as maps and other information related to the geology of the Yukon.   I would like to thank Dr. Reza Tafti for forwarding John Lawford Anderson’s Excel spreadsheet for the Al-in hornblende geothermobarometry calculations. Reza completed a master’s thesis at the University of British Columbia in 2005, which included a section on Al-in hornblende geothermobarometry that I have used as a model in compiling this thesis.  I am grateful to Arne Toma for help with Mineral Deposit Research Unit laboratory equipment and also to Sara Jenkins for help with printing the poster for the presentation of this thesis.  I would like to thank Edith Czech and Lan Kato for providing an introduction to the scanning electron microscope and electron probe microanalyzer and for their assistance in the acquisition of analytical data. I would also like to thank Dr. Mati Raudsepp for his oversight and advice during that process.  Thank you to Dr. Robert Lee for providing helpful advice regarding geothermobarometry calculations.  I would like to thank Dr. Mary Lou Bevier for valuable advice and support during past geology courses at the University of British Columbia and over the course of writing this thesis.        viiI am grateful to my program advisor, Dr. Kenneth A. Hickey, for his support and guidance over the past few years, for his highly effective rugby-inspired coaching attitude, as well as for allowing me to use his slide scanner for this thesis.  I would like to thank instructors Dr. James S. Scoates, Dr. Lori A. Kennedy, Dr. James K. Russell, and Dr. Randal A. Mindell for communicating their passion for geology and for challenging students to excel in the quest for knowledge in geological sciences.   I would like to take the opportunity to thank Francis H.M. Jones for instruction in the writing and critical analysis of scientific literature.  I am indebted to Associate Dean of Science for Students Dr. Paul G. Harrison, who granted me permission to pursue the Honours Geological Sciences program as a second undergraduate degree.  I would like to thank past instructors that have encouraged and supported my education, particularly the dedicated and estimable faculty of St. George’s School, Vancouver, British Columbia.  Finally I would like to thank my family for their love and support and especially my parents, James and Nicole.                  1Introduction   Late Triassic to Early Jurassic (220-180 Ma) plutonic rocks in west-central Yukon and eastern Alaska represent a pulse of magmatism that emplaced subvolcanic magma chambers into a continental magmatic arc that was undergoing contractional deformation and thickening (Mortensen et al., 2000). These rocks are divided into the Aishihik and Taylor Mountain plutonic suites and they form a discontinous belt extending northwest from the Carmacks area and into eastern Alaska (Figure 1). Batholiths comprising the Aishihik plutonic suite are exposed along the contact between the Yukon-Tanana terrane and the Early to Middle Mesozoic intraoceanic volcanic island arc assemblages of the Stikine and Quesnel terranes. The Late Triassic to Early Jurassic Granite Mountain Batholith, located approximately two hundred kilometers north-northwest of Whitehorse, Yukon, hosts the Minto and Carmacks Copper deposits, the two main significant examples of age-equivalent intrusion-related mineralization in the area (Allan et al., 2013). These deposits, along with similar prospects, define the north-northwest-trending Carmacks Copper Belt and have been previously interpreted as deformed porphyry systems related to plutons intruded during a relatively narrow time window between approximately 201 and 196 Ma (Mortensen and Tafti, 2004). Tafti (2005) applied the Al-in hornblende geothermobarometer of Anderson and Smith (1995) to estimate the pressure during crystallization of plutons hosting the Minto and Carmacks Copper deposits. Emplacement depths corresponding to crystallization pressures calculated for a total of seven samples were in excess of nine kilometers, which was considered too deep to have permitted the exsolution of a mineralizing fluid phase required to form conventional porphyry deposits. As a result, Tafti (2005) inferred that the porphyry       2systems formed at shallow depths, were tectonically buried, became enveloped by intrusions at depth, and were subsequently rapidly exhumed. Hood (2012) reinterpreted that the Minto deposit does not represent the envelopment of allochthonous mineralized porphyry rafts but instead indicates plutonism and syngenetic mineralization in an active shear zone. This model relies on strain partitioning within plutons whereby mineralizing fluid flow is concentrated along discrete ductile shear zones during emplacement between 202 and 197 Ma at depths of 10 to 20 kilometers, near the brittle-ductile transition. This requires emplacement and subsequent exhumation in a steady state subduction zone environment of a continental magmatic arc undergoing contractional deformation and therefore eliminates the requirement of rapid burial prior to exhumation.         3Figure 1: Regional scale geologic map of western Yukon and eastern Alaska showing the geographic distribution of plutonic suites and lithotectonic terranes as well as significant mineral occurrences. Within the Late Triassic to Early Jurassic Aishihik plutonic suite, AB = Aishihik batholith and TB = Tatchun batholith. The Late Triassic to Early Jurassic Taylor Mountain plutonic suite is mainly exposed in eastern Alaska, approximately 150 kilometers east of Dawson, Yukon. The inset map shows the location of the regional map area within the northern Cordilleran orogen as well as the arcuate trend of the primarily Cretaceous age Tintina Gold Province. (Allan, pers. comm., 2014).    The objective of this thesis is to provide additional constraints on the nature and tectonic setting of Late Triassic to Early Jurassic magmatism and associated intrusion-related mineralization in west-central Yukon and eastern Alaska. Specifically, the Al-in hornblende geothermobarometer of Anderson and Smith (1995) was applied to granitic phases of the Late Triassic to Early Jurassic Aishihik and Tatchun batholiths (Figure 2) located within one hundred kilometers of the Carmacks Copper Belt to provide estimates of emplacement depths and to evaluate their porphyry copper-gold potential on the basis of these depths. Where available, emplacement depths were compared with radiometric ages from plutons as well as Early to Middle Jurassic stratigraphy in the adjacent Whitehorse Trough in order to constrain exhumation rates through correlation with the first appearance of granitic plutonic clasts. This research comprises part of the industry-funded Yukon-Alaska Metallogeny Project, sponsored by the University of British Columbia’s (UBC) Mineral Deposit Research Unit (MDRU) in partnership with the Yukon Geological Survey (YGS), Kaminak Gold Corp., and Sumac Mines Ltd. This study also contributes to a comprehensive characterization study of Late Triassic to Early Jurassic plutonic suites in Yukon by Maurice Colpron and Patrick Sack of the YGS.       4       5Figure 2: District scale geologic map showing the geographic distribution of the two main groups of units: 1) the Late Triassic to Early Jurassic plutonic suites (orange, beige, gold, pink, red) of the Yukon forming the northwest-trending chevron-shaped shoulders flanking the Whitehorse Trough, and 2) Early to Middle Jurassic Laberge Group sedimentary rocks occupying the Whitehorse Trough (pale green). Sample locations are marked with gold triangles and labelled with series MA14-AB for Aishihik batholith or MA14-TB for Tatchun batholith. Uranium-lead, potassium-argon, and argon-argon geochronology data are included as well. The smaller map shows the location of the district scale map area within the northern Cordilleran orogen and the approximate distribution of the Stikine and Quesnel volcanic island arcs (green) and the Yukon-Tanana terrane (tan). (Allan, pers. comm., 2015).   Regional Geology and Late Triassic to Early Jurassic Intrusion-related Mineralization  The Late Triassic to Early Jurassic batholiths that form the subject of this thesis were intruded along the contact between the Mesozoic arc assemblages of Stikinia and Quesnellia and the Yukon-Tanana terrane prior to and at the beginning of the Cordilleran Orogeny, which occurred in the Early to Middle Jurassic. Here follows a brief summary of the tectonic, magmatic, and structural evolution of the region and Late Triassic to Early Jurassic intrusion-related mineralization. During the Late Devonian to Early Mississippian (365-342 Ma), east-dipping subduction of oceanic crust under the western margin of Laurentia was building a continental magmatic arc (Allan et al., 2013). During this time the Simpson Range plutonic suite was intruded into the Proterzoic to Paleozoic rifted passive margin sequence of rocks collectively referred to as the Snowcap Assemblage. These primarily rifted passive margin miogeoclinal sedimentary rocks were deposited during the break-up of the supercontinent Rhodinia, which initiated at approximately 750 Ma during the Late Proterozoic, with another episode of rifting       6superimposed during the Cambrian Period at approximately 530 Ma (Price and Sears, 2000). The Finlayson Assemblage represents the volcanic rocks erupted during the emplacement of the Simpson Range plutonic suite. Back-arc spreading led to the opening of the Slide Mountain Ocean as the continental arc rifted away from the western margin of ancestral North America. During the Late Permian (269-253 Ma), west-dipping subduction of the oceanic crust that had been produced through continued back-arc extension and rifting was responsible for the building of the Klondike Arc on top of the Devono-Mississippian arc. Between 253 and 250 Ma, the Klondike Arc collided with and over-rode the Laurentian margin in a terrane accretion event, the Klondike Orogeny (Beranek and Mortensen, 2011). This collision emplaced small volumes of peraluminous felsic magmas and produced the penetrative, primarily shallowly-dipping ductile shear fabric as well as greenschist to amphibolite facies metamorphism of rocks comprising the Yukon-Tanana terrane (Berman et al., 2007). Late Triassic to Early Jurassic granitic rocks (220-180 Ma) represent the subvolcanic magma chambers of a west-facing continental magmatic arc superimposed on the Yukon-Tanana terrane. While Late Triassic to Early Jurassic plutonic suites are compositionally diverse, they are generally metaluminous and demonstrate continental arc geochemical signatures (Mortensen at al., 2000). Late Triassic to Early Jurassic west-dipping thrust faults near Dawson, Yukon represent evidence of the contractional deformation produced during this phase of arc-building (Allan et al., 2013). Emplacement of continental arc magmas occurred during the lead up to and through the Cordilleran Orogeny, which initiated in the Early Jurassic and continued through the Middle Jurassic. As a result, Early Jurassic magmas emplaced during the Cordilleran Orogeny represent stitching plutons while Triassic magmas       7preceding the onset of collision express ordinary continental arc magmatism. According to the oroclinal enclosure hypothesis (Mihalynuk et al., 1994), the Stikine terrane folded counter-clockwise around the Cache Creek terrane during the Cordilleran Orogeny to amalgamate the composite Stikinia-Yukon-Tanana-Quesnellia terrane. In this model, the Yukon-Tanana terrane forms the hinge zone connecting the roughly linear Stikine and Quesnel intraoceanic volcanic island arc terranes. Plutons of the Late Triassic to Early Jurassic Aishihik Plutonic Suite were emplaced within the Yukon-Tanana terrane immediately north of the synorogenic ‘piggyback’ basin that developed at the center of the apex of the hinge zone of the oroclinal bend (Colpron, pers. comm., 2015). This sedimentary basin is referred to as the Whitehorse Trough, which was uplifted as Stikinia and Quesnellia were sutured together (Colpron, pers. comm., 2015). Oroclinal closure was completed in the mid-Jurassic and corresponded to declining continental arc magmatism, thrusting, uplift, and erosion (Allan et al., 2013).  An Early to Middle Jurassic sedimentary overlap assemblage unconformably overlying the Stikine and Quesnel terranes, the Laberge Group, occupies the Whitehorse Trough in south-central Yukon, an intermontane basin that formed as a forearc basin that was uplifted during the Pliensbachian (191-183 Ma) during the Cordilleran Orogeny. The Early to Middle Jurassic Laberge group is divided into two distinct lateral facies (Colpron, pers. comm., 2015). In the south, the Richthofen Formation consists of more distal facies deep marine turbidites and mass flow conglomerates. In the north, the Tanglefoot Formation consists of more proximal facies shallow marine to fluvial sandstones and conglomerates as well as mudstones and minor limestones. Interbedded       8horizons of Nordenskiold tuff, with ages between 188 and 186 Ma occur in both the Tanglefoot and Richthofen Formations (Colpron and Friedman, 2008). The northern part of the Whitehorse Trough is bounded by Late Triassic to Early Jurassic highlands that are underlain by batholiths of the Aishihik plutonic suite. These highlands occur in a northwest-trending chevron pattern flanking the Whitehorse Trough and they are interpreted to be the source regions for the sediments comprising the Early to Middle Jurassic Tanglefoot Formation (Colpron, pers. comm., 2015). Paleoflow indicators in the Laberge Group indicate southwesterly transport directions at the eastern edge and northern apex of the Whitehorse Trough as well as easterly to northeasterly directions on the western side of the trough (Colpron, pers. comm., 2015). As a result, current directions indicate a converging pattern of paleoflow down from the highlands adjoining the Whitehorse Trough, whose northwest-trending axis bisects the oroclinal hinge zone. The stratigraphy of the Laberge Group is interpreted to record the uplift and erosion of Late Triassic to Early Jurassic plutons that were emplaced during the Cordilleran Orogeny (Colpron, pers. comm., 2015).  Specifically, Late Pliensbachian (183 Ma) and younger conglomerates of the Tanglefoot Formation contain approximately 60 to 80 percent plutonic clasts derived from the adjacent Late Triassic to Early Jurassic plutons (Colpron, pers. comm., 2015).  Zircon age dates from the Minto plutonic suite, representing the Tatchun batholith, are in the range of 204 to 195 Ma while ages from the Long Lake Plutonic Suite, representing the Aishihik batholith, are in the range of 192 to 180 Ma (Colpron, pers. comm., 2015).  Potassium-argon hornblende and biotite cooling ages of 190 ± 8 Ma and 164 ± 6 Ma as well as three biotite mica cooling ages ranging from 160 ± 7 to 166 ± 6 were obtained for the Aishihik batholith (Johnston et al., 1996).       9These cooling ages serve to further constrain the process of the unroofing of these batholiths in the Cordilleran west-facing continental arc environment through the mid-Jurassic period. Mira Geoscience was commissioned by the YGS in 2014 carry out a study to reconcile geologic mapping, geophysical property data, and seismic data with newly-acquired magnetic and gravity data as a three-dimensional geometric model of the subsurface geology. The study confirmed the tectonic interpretation of Johnston and Canil (2007) that the regional tectonic fabric consists of steeply east-dipping sheets that are at least 10 kilometers thick on average. The Aishihik batholith is a west-tapering, east-dipping tectonic sheet that is interpreted to be a syntectonic tabular intrusion (Johnston and Canil, 2007). A weakly to moderately developed magmatic foliation is aligned with the east-dipping orientation of the tabular Aishihik batholith and parallel to its margins (Johnston and Canil, 2007). Johnston and Canil (2007) propose that westward crustal growth in southwest Yukon occurred through a process of synmagmatic subcretion. In this model, the Aishihik batholith was intruded during the Early Jurassic along a southwest-verging crustal scale thrust fault intervening between two supracrustal assemblages: the Lewes River Group of Stikinia underlying the Whitehorse Trough and the Aishihik Metamorphic Assemblage. The process of subcretion is proposed in order to explain the east-dipping imbricated configuration of crustal-scale tabular tectonic elements through successive additions of these elements to the western margin of North America. Previous workers also identified the presence of magmatic epidote as an indication that plutons of the Aishihik batholith were emplaced at depths in excess of twenty kilometers (Johnston et al., 1996; Dusel-Bacon et al., 2009). Similar to the Aishihik batholith, the slightly older Tatchun batholith of the Minto plutonic suite (204-      10195 Ma) is bound by approximately southwest-verging thrust faults, located between Late Devonian to Early Mississippian metasedimentary and metavolcanic rocks of the Snowcap and Finlayson assemblages of the Yukon-Tanana terrane to the northeast and the Late Triassic Lewes River Group of the Stikine terrane to the southwest (Buffett et al., 2006). However, the Tatchun batholith is different in that it intrudes the Semenof block, which consists of Devonian to Permian volcanic and sedimentary rocks of the Boswell assemblage (Buffett et al., 2006; Simard and Devine, 2003). The Mira Geoscience report (2014) models the Tatchun batholith as sitting within the Boswell assemblage dipping neither to the east nor the west. The tectonic setting of the Tatchun batholith broadly mirrors Johnston and Canil’s (2007) interpretation of the Aishihik batholith as a syntectonic intrusion emplaced into a continental magmatic arc between lithotectonic terranes that were in the process of amalgamating in the lead up to and during the Cordilleran Orogeny in the Early to Middle Jurassic. More specifically, the syntectonic emplacement of the Tatchun batholith on the east side of the Whitehorse Trough is the same as the syntectonic emplacement of the Aishihik batholith on the west side of the trough except that the Tatchun batholith intrudes the Semenof block, probably representing a relatively minor volcanic island arc which was caught between the colliding Stikine and Yukon-Tanana terranes. Combining the igneous ages of plutons of the Aishihik and Tatchun batholiths with the timing of the first appearance of granitic plutonic clasts and detrital uranium-lead zircon peaks within the adjacent Whitehorse Trough indicates relatively rapid exhumation of mesozonal to catazonal Late Triassic to Early Jurassic granitoids in Yukon, during an interval of as little as five to ten million       11years. Mica cooling ages provide evidence for continued although much less rapid exhumation through the mid-Jurassic (Allan et al., 2013).  Late Triassic to Early Jurassic plutonic rocks in Yukon and eastern Alaska are age-equivalent to intrusions in northern British Columbia emplaced within the Stikine and Quesnel terranes. In British Columbia, the presence of numerous porphyry copper-gold deposits exposed at or near surface, which typically formed at crustal depths between 1 and 6 kilometers (Kesler and Wilkinson, 2008), indicates that only a few vertical kilometers of material has been removed through erosion. Furthermore, these deposits are preserved at or near the present erosional surfrace. By contrast, plutons in west-central Yukon and eastern Alaska representing the subvolcanic granitic magma chambers that underlie porphyry systems are currently exposed at surface. Although Late Triassic to Early Jurassic granitoids in Yukon formed during the same time as porphyry copper-gold deposits in British Columbia, the granitoids in Yukon were emplaced at an inappropriate tectonic environment for the preservation of overlying conventional shallow crustal level porphyry systems. Examples of coeval porphyry deposits preserved at surface in Stikinia within northwest British Columbia include the Kerr-Sulphurets-Mitchell (KSM) deposit, currently one of the largest undeveloped gold deposits in the world, as well as the adjacent exceptionally high-grade Brucejack Lake gold deposit. To explain this preservation bias, exhumation rates must have been significantly higher in the Yukon-Tanana terrane bordering on the Whitehorse Trough than in Stikinia and Quesnellia, the more southerly limbs of the northern Cordilleran orocline. Differential rates of exhumation are probably due to a combination of several factors. First, the heat       12flux and thus the intensity of magmatism in the continental arc may have been higher in the north, contributing to accelerated exhumation of the continental magmatic arc (Hyndman et al., 2005). Second, the intensity of magmatism and rates of deformation are in turn due the velocities of relative plate motion vectors of the North American and Pacific plates during the Cordilleran terrane accretion orogeny (Engebretson, 1985). The pulse of Late Triassic to Early Jurassic continental and volcanic island arc magmatism was probably due to temporarily enhanced mid-Atlantic seafloor spreading and thus accelerated westward migration of the North American plate and subduction along on its western margin. However, local factors are required to distinguish between the higher exhumation rates in the Yukon-Tanana terrane as opposed to the Stikine and Quesnel terranes. One such explanation may be that the Yukon-Tanana terrane is older and thicker and could accommodate less shortening relative to uplift than the thinner Stikine and Quesnel intraoceanic volcanic island arc terranes. Under the oroclinal enclosure hypothesis (Mihalynuk et al., 1994), the duration of collision decreases with distance to the south and away from the hinge zone of the orocline during the Early to mid-Jurassic period of the Cordilleran terrane accretion orogeny. This means that the hinge zone is undergoing contractional deformation during the entire orogeny while the more southerly reaches of the the Stikine and Quesnel arcs are sutured progressively later as one travels further south.   The Minto and Carmacks Copper deposits are the two main significant examples of Late Triassic to Early Jurassic intrusion-related copper-gold mineralization in Yukon and are interpreted to be deformed porphyry systems (Mortensen and Tafti, 2004) or       13intrusion-hosted shear zone deposits (Hood, 2012). Tafti (2005) applied the Al-in hornblende geothermobarometer to samples of the Granite Mountain batholith, which hosts the Minto and Carmacks Copper deposits. Intrusive phases of the Granite Mountain batholith are weakly peraluminous, calc-alkalic, and demonstrate continental magmatic arc isotopic and geochemical signatures (Tafti, 2005). Late-stage potassic alteration at the Minto and Carmacks Copper deposits is dated via argon-argon radiometric dating at 182-183 Ma while the host intrusions are dated at approximately 197-198 Ma. Tafti (2005) developed a deposit model for these rafts or tabular bodies of deformed copper-gold mineralized porphyry embedded in barren granitic rocks of the Granite Mountain batholith. According to the model, the porphyries formed at shallow depths less than approximately six kilometers and were subsequently tectonically deformed and buried to at least nine kilometers in depth, engulfed by younger plutons, and then rapidly exhumed and enriched by supergene hydrothermal processes. Hood’s model (2012) contrasts with Tafti’s model (2005) by suggesting that mineralization at Minto was syngenetic to plutonism and deformation in a mesozonal active shear zone environment. Hood’s model relies on strain partitioning within plutons whereby mineralizing fluid flow is concentrated along ductile shear zones during emplacement between 202 and 197 Ma at depths of 10 to 20 kilometers, near the brittle-ductile transition. This requires emplacement and subsequent exhumation in a steady state subduction zone of a continental magmatic arc undergoing contractional deformation and thus eliminates the additional requirement of shallow crustal level porphyry deposit formation and burial prior to emplacement within granitic host plutons and exhumation.        14Following the emplacement of the Aishihik and Taylor Mountain Plutonic Suites, a hiatus of continental arc magmatism lasted in the region from approximately 180 to 115 Ma (Allan et al., 2013). However, mica cooling ages for the Aishihik batholith (Johnston et al., 1996) and the Tatchun batholith (Tempelman-Kluit, 1984) as well as Early to Middle Jurassic stratigraphy in the Whitehorse Trough indicate that uplift and exhumation persisted at a declining rate into the Middle Jurassic (Allan et al., 2013). Continental arc magmatism that accompanied reinvigorated east-dipping subduction resumed at approximately 115 Ma during the mid-Cretaceous, corresponding to the emplacement of the Whitehorse plutonic suite as well as scattered felsic intrusive and equivalent volcanic rocks (Mortensen et al., 2000; Hart et al., 2004). Volcanism originating in the back-arc spreading region produced several large caldera complexes in eastern Alaska that may have been the sources of widespread synchronous felsic tuff deposits near the Sixtymile and Indian rivers in central Yukon (Allan et al., 2013). To the northwest of the Aishihik Plutonic Suite in the Cretaceous back-arc region, low-angle normal faults separate an upper tectonic plate of relatively shallow crustal level Yukon-Tanana rocks from ductilely-deformed relatively deep crustal level metasedimentary rocks of parautochthonous North America (Allan et al., 2013). The exhumation of lower plate rocks along these normal faults is interpreted to represent Cretaceous back-arc extension. The same tectonic regime gave rise to Cretaceous age and younger orogen-parallel dextral strike-slip faults throughout British Columbia, Yukon, and Alaska including the Denali, Teslin, and Tintina faults (Figure 1). Magmatism migrated inboard at approximately 98 Ma and lasted until 90 Ma, while arc and subduction-related inboard magmatism continued intermittently throughout the remainder of the Cretaceous period       15and the Cenozoic era (Allan et al., 2013). The landscape of the Yukon Plateau in west-central Yukon has not been glaciated since the Pliocene (5.3 - 2.6 Ma) and is therefore mature (Allan et al., 2013). It is therefore composed of relatively high and broad rolling green hills where outcrop consists mainly of grey hilltop tors and castle koppies, collectively described as castellated topography.  Methodology  3.1 Petrography  Twenty-six samples of granitoids representing different intrusive units of the Aishihik and Tatchun batholiths were collected by Dr. Murray Allan of MDRU and members of the Yukon Geological Survey in 2014. Twenty of these samples were selected for this thesis and described in hand sample and then in polished thin section. Sample locations are marked on Figure 2. A total of twenty polished thin sections were prepared by Vancouver Petrographics and examined through transmitted light microscopy. Polished thin sections of intrusive rocks were analyzed using a Nikon Eclipse E600 POL polarizing microscope equipped with a Canon EOS Rebel T2i digital camera to record photomicrographs. In preparation of polished thin sections, minor grain plucking produced up to approximately five percent apparent void space in some samples. Hydrous mafic minerals such as hornblende and biotite were preferentially eroded and the traces of cleavage planes in biotite mica grains were variably curved. In general, thin sections were oriented so that foliation is perpendicular to the long axis of       16the section. Detailed mineralogical and textural descriptions including visually-estimated modal abundances and rock names were recorded for each thin section and compared with hand sample descriptions. Hand sample and thin section descriptions are included in the digital appendix. Approximately 1 cm thick slabs were cut from all 20 hand samples and were stained using amaranth red to highlight plagioclase and sodium cobaltnitrite for potassium feldspar. The staining process was carried out by Dr. Murray Allan as follows. Rock slabs were etched in a shallow bath of 48 percent hydrofluoric acid for 60 seconds. Next, slabs were stained in a shallow bath containing a 7 weight percent grams solute per solvent aqueous solution of amaranth red for 5 to 10 seconds, then rinsed thoroughly with water. Subsequently the rock slabs were air-dried and then stained in a shallow bath containing a 20 weight percent solute per solvent aqueous solution of sodium cobaltnitrite for 60 seconds, then air-dried and sprayed with acrylic to protect friable etched surfaces. Amaranth red is sensitive to the calcium content of feldspar: anorthitic plagioclase stains deep crimson while more albitic plagioclase stains pink and pure albite will not stain. Sodium cobaltnitrite is sensitive to potassium feldspar, which it stains yellow. Six of the polished thin sections satisfied the textural and mineralogical criteria required to apply the Al-in hornblende geothermobarometer of Anderson and Smith (1995). The selected thin sections were subsequently mapped using the scanning electron microscope (SEM) and then selected point locations on adjacent hornblende-plagioclase pairs were analyzed using the electron probe microanalyzer (EPMA) as per the Al-in hornblende geothermobarometry technique that is outlined in more detail in the next section.          173.2 Geothermobarometry  Anderson and Smith’s (1995) Al-in hornblende geothermobarometer was used in this study. Point compositional analyses (approximately 5 micrometer spot size) from the rims of adjacent hornblende and plagioclase grains were determined by electron probe microanalysis. The geothermobarometer is a temperature-dependent correlation between the total aluminum content of hornblende and the pressure of crystallization. Anderson and Smith’s geothermobarometer accounts for the the effect of temperature by combining Holland and Blundy’s geothermometers (1990, 1994) with the geobarometer first formulated by Hammarstrom and Zen (1986) and then calibrated by Schmidt (1992), Hollister et al. (1987), and others. Hammarstrom and Zen’s Al-in hornblende geobarometer was empirically derived by comparing the total aluminum content in hornblende of calc-alkalic granitic rocks with the pressures determined using contact metamorphic geobarometry based on equilibrium mineral reactions. Six thin sections were selected for geothermobarometry on the basis of textural and mineralogical criteria. Specifically, suitable thin sections contain a nine phase equilibrium assemblage consisting of: 1) quartz; 2) plagioclase; 3) orthoclase; 4) hornblende; 5) biotite; 6) magnetite; 7) titanite; 8) melt; 9) water. According to Hammarstrom and Zen (1986), “such a mineral assemblage should adequately buffer the system so that variation of the bulk composition should be largely expressed by modal proportions of minerals rather than by the compositions of individual phases.” In addition, hornblende and plagioclase rims in contact at grain boundaries must be unaltered by secondary mineral reactions. The following conditions for the application of the Al-in-hornblende geothermobarometer       18were also specified. The method is applicable only when the pressure is greater than two kilobars. In hornblende, the ratio of ferric iron, Fe3+, to total iron should be greater than one quarter and the ratio of total iron to the sum of total iron and magnesium should be between 0.40 and 0.65 (Hollister et al., 1987). Only rim compositions of hornblende should be used so as to sample the last melt in the rock before it is completely crystallized. The ratio of ferric iron to total iron provides a measure of oxygen fugacity, which indicates the relative degree of oxidation or reduction of a system in terms of the availability of iron in its oxidized or reduced valence states, respectively. Low oxygen fugacities of reduced systems will result in overestimates of pressures of crystallization. High ratios of total iron to the sum of total iron and magnesium are likewise correlated with low oxygen fugacity and produce overestimates of pressures of crystallization.  Finally, the mole fraction of albite for plagioclase grains should be between Ab65 and Ab75, andesine to oligoclase (Hollister et al., 1987). Prior to electron probe microanalysis, polished thin sections were also coated with a carbon film to prevent charge from building up on the surface of the slides. Back-scattered electron (BSE) images were acquired using the scanning electron microscope (SEM) and used to map suitable grain boundary locations for analysis (Figure 3). Two to eight point locations for analysis per hornblende-plagioclase pair were selected from each of six to eleven mineral pairs per thin section. In addition to rim analyses, a point was placed in the cores of hornblende grains for comparison to rim compositions, which provided an indication of any changes in melt composition as crystallization proceeded. Point locations were analyzed using a 5 micron diameter electron beam and were not spaced closer than 50 microns apart. The geothermobarometer of Anderson and Smith (1995) is given as follows:       19 P (± 0.6 kbar) = 4.76 Altot - 3.01 - {(T - 675)/85} x {0.530 Altot + 0.005294(T - 675)}  where T is the temperature in degrees Celsius and Altot is the total aluminum content of hornblende assuming a 13-cation chemical formula. Temperature is an important variable to account for as it may alter the calculated pressure of crystallization by up to two kilobars per 100 degrees Celsius depending on total aluminum content. Calculations were performed using an Excel spreadsheet prepared by J.L. Anderson that was obtained from Reza Tafti (pers. comm., 2014). EMPA data that was entered into the calculation spreadsheet included measurements of the following oxides and anions in weight percent for hornblende: SiO2, TiO2, Al2O3, FeO, MgO, MnO, CaO, Na2O, K2O, F, and Cl. For plagioclase, the spreadsheet accepted values of mole fractions of plagioclase solid solution minerals albite, NaAlSi3O8, and anorthite, CaAl2Si2O8. These values were tabulated separately beforehand by calculating the number of atoms of Na and Ca given an eight oxygen cation formula and the analytical data in weight percent of: SiO2, Al2O3, Fe2O3, MgO, MnO, CaO, Na2O, and K2O. Total iron is given as FeO for hornblende and Fe2O3 for plagioclase. In order to calculate temperature and pressure of crystallization for each hornblende-plagioclase pair, the spreadsheet iteratively applied the formula of Anderson and Smith (1995) by first calculating an initial pressure using Schmidt’s (1992) formula and temperature using three versions of Holland and Blundy’s (1990, 1994) geothermometers based on equilibrium mineral reactions involving hornblende. The spreadsheet yields three sets of pressure and temperature estimates based on three different thermometer reactions: HB 1, HB 2, and BH, referring to different reactions       20calibrated by Holland and Blundy (1990, 1994). Although results from three of Holland and Blundy’s thermometers are given in Table 2, results from the HB 2 reaction: edenite + albite = richterite + anorthite, are preferred because quartz is not required for this exchange reaction but is required for the other two. The other two thermometer reactions are separate calibrations of the reaction edenite + 4quartz = tremolite + albite (Holland and Blundy, 1990, 1994). Pressure and temperature estimates are given in Table 2 of the results section for each hornblende-plagioclase pair. Point compositional data were averaged for each grain before the average was entered into the calculation spreadsheet.   Figure 3: Images illustrating the process of data collection for Al-in hornblende geothermobarometry: (a) mapped polished thin sections; (b) photomicrograph showing one of the locations that was selected for microprobe analysis; (c) back-scattered electron (BSE) image of same spot; (d) photomicrograph showing the location at higher magnification in XPL. (Figure after Tafti, 2005).        21With regard to the selection of samples for geothermobarometry, petrography of the polished thin sections was focused on identifying the presence of the nine co-precipitating magmatic phases of the required equilibrium assemblage in a relatively fresh or unaltered condition. Melt and water, the eighth and ninth phases, are assumed to have been present during crystallization. Specifically, hornblende grains needed to be anhedral and to have grain boundaries interlocking with the other mineral phases. Hornblende grains should not be euhedral, which would indicate that they are xenocrysts that crystallized in a different melt and then were introduced through magma mixing or melt injection. Point locations for electron probe microanalysis were only placed where adjacent rims of hornblende and plagioclase grains were unaltered. Mapping grain locations with the scanning electron microscrope permitted the identification of feldspar grains as plagioclase or potassium feldspar, which was sometimes ambiguous in thin section, through the collection of energy-dispersive X-ray spectra (EDS) with peaks corresponding to relative elemental abundances. EPMA data including analytical statistics as well as EDS spectra are included in the digital appendix.  Electron probe microanalyses of plagioclase and hornblende were performed at the Electron Microbeam/X-ray Diffraction Facility in the Department of Earth, Ocean, and Atmospheric Sciences at UBC on a fully automated CAMECA SX-50 instrument operating in the wavelength-dispersion mode with the following operating conditions: excitation voltage, 15 kV; beam current, 20 nA; peak count time, 20 seconds (fluorine and chlorine 40 seconds); background count time; 10 seconds (F and Cl 20 seconds); spot diameter, 5 µm. Data reduction was completed with the 'PAP' φ(ρZ) method (Pouchou       22and Pichoir, 1985). For the elements considered in plagioclase, the following standards, X-ray lines, and crystals were used: albite, NaKα, TAP; anorthite, AlKα, TAP; diopside, MgKα, TAP; anorthite, SiKα, TAP; orthoclase, KKα, PET; anorthite, CaKα, PET; synthetic rhodonite, MnKα, LIF; synthetic fayalite, FeKα, LIF. For the elements considered in hornblende, the following standards, X-ray lines, and crystals were used: synthetic phlogopite, FKα, TAP; albite, NaKα, TAP; kyanite, AlKα, TAP; diopside, MgKα, TAP; diopside, SiKα, TAP; scapolite, ClKα, PET; orthoclase, KKα, PET; diopside, CaKα, PET; rutile, TiKα, PET; synthetic magnesiochromite, CrKα, LIF; synthetic rhodonite, MnKα, LIF; synthetic fayalite, FeKα, LIF.   Results  4.1 Petrography  In accordance with the above information on the petrology of the samples as briefly described in the regional geology section, the twenty samples are almost exclusively granitic and metaluminous and contain biotite mica as an essential mineral as well as hornblende as a ubiquitous accessory mineral. Thirteen out of the twenty samples contain the required nine phase equilibrium assemblage of quartz, plagioclase, orthoclase, hornblende, biotite, titanite, and magnetite, with melt and water as assumed eighth and ninth phases. The eight samples of the Aishihik batholith of the Long Lake plutonic suite (192-180 Ma) include granodiorite, K-feldspar megacrystic granodiorite, granite, aplitic       23granite, and quartz diorite. The twelve samples of the Tatchun batholith of the Minto plutonic suite (204-195 Ma) include K-feldspar megacrystic quartz diorite, quartz diorite, pegmatitic diorite, K-feldspar megacrystic quartz monzonite, quartz monzonite, granodiorite, monzodiorite, aplitic granite, and one highly altered mafic plutonic rock consisting of 70 percent combined hornblende, biotite, and epidote. Figure 4 is a Quartz-Alkali-felspar-Plagioclase (QAP) ternary plot of representative plutonic rock units from the Aishihik and Tatchun batholiths. QAP-normal modal abundances for samples plotted in Figure 4 are presented in Table 1. Interestingly, the six samples selected for Al-in hornblende geothermobarometry consist of two compositions. The three samples from the Aishihik batholith include two granodiotites and a quartz diorite while the three samples from the Tatchun batholith are all quartz diorites. Almost all 20 samples were saussuritized or sericite-altered to some degree. Sericite alteration was frequently concentrated in the cores of concentrically zoned plagioclase grains, where the composition is more calcic and anorthitic. Most samples were relatively fresh with less than or equal to approximately 5 to 10 percent replacement, while several were highly altered with up to 80 percent replacement. The presence of magnetite in almost all of the twenty samples indicates that the source magmas of the Aishihik and Tatchun batholiths were magnetite-series, oxidized orogenic granitic magmas (Winter, 2001).         24 Figure 4: International Union of Geological Sciences (IUGS) Quartz-Alkali-feldspar-Plagioclase (QAP) ternary diagram classification of representative plutonic rock units from the Aishihik and Tatchun Batholiths (after Streckeisen, 1976, and Tafti, 2005).         25Table 1: Calculated QAP-normal modes for granitoid rocks from the Aishihik and Tatchun batholiths corresponding to the Long Lake (192-180 Ma) and Minto (204-195 Ma) plutonic suites, respectively. Table after Tafti (2005).   The majority of samples from both the Aishihik and Tatchun batholiths exhibit a weakly developed foliation defined by the alignment of hornblende and biotite mica as well as lens-shaped aggregates of quartz grains. Under the microscope, these lens-shaped quartz aggregates that appear as single quartz grains that are up to several millimeters in diameter in hand sample are shown to consist of numerous smaller quartz grains. This phenomenon is probably due to dynamic grain size reduction in response to the differential stress that produced the foliation. Texturally, the majority of samples were hypidiomorphic to allotriomorphic, consisting primarily of subhedral to anhedral grains. The majority of samples were also inequigranular although medium to coarse-grained on average. Several samples were K-feldspar megacrystic while of four samples of dykes, one was uniformly very coarse-grained or pegmatitic and three were uniformly very fine-grained or aplitic.   Epidote is present in seventeen of the twenty samples and in all six of the samples selected for Al-in hornblende geothermobarometry. In every sample except for MA14-TB6, the epidote occurs as a secondary mineral isocrystallographically replacing hornblende or biotite. In MA14-TB6, magmatic epidote occurs individually as fresh, polygonal, frequently columnar and elongate euhedral grains in association with but not replacing hornblende. Secondary epidote also occurs in MA14-TB6, replacing biotite mica grains. Samples MA14-AB4, -AB5, -AB7, and -AB8 demonstrate infrequent granophyric textures indicating simultaneous precipitation of intergrown quartz and K-      26feldspar. Samples MA14-AB2 and MA14-TB13 demonstrate infrequent micrographic textures consisting of wedge-shaped quartz inclusions in K-feldspar grains, also indicating co-precipitation of quartz and K-feldspar. Samples MA14-AB4, -TB2, -TB3, -TB5, -TB7, -TB10, -TB11, -TB12, and -TB13 contain examples of perthitic potassium feldspar (KAlSi3O8) grains that constitute evidence for the exsolution of lamellae of albite (NaAlSi3O8) during cooling of the high temperature solid solution, (K,Na)AlSi3O8. Hornblende grains were generally anhedral and lacked reaction rims, which indicates that they crystallized directly from the parent granitic magma in equilibrium with the other mineral phases present (Winter, 2001). Hornblende crystals in samples MA14-AB1 and -AB8 from the Aishihik batholith were generally subhedral and displayed more frequent cross-sections that approached rhombohedral shapes while MA14-AB10 and all three samples from the Tatchun batholith were anhedral. K-feldspar grain shape ranges from relatively euhedral to anhedral, and it is typically more euhedral than plagioclase, which is predominantly subhedral to anhedral although a few exceptions of euhedral plagioclase grains were observed. Quartz grains are anhedral and generally smaller than plagioclase and K-feldspar grains. Titanite is generally euhedral and frequently occurs as inclusions within or associated with hornblende. Magnetite occurs generally as subhedral and rounded to blocky, equant black grains that are frequently associated with or included within hornblende grains. Hornblende content for the six samples selected for electron probe microanalysis varies from 5 to 25 volume percent by modal abundance with an average of 10 to 15 percent. Biotite is generally subhedral when occurring individually as primary magmatic grains and anhedral when replacing hornblende as a result of deuteric alteration (Figure 5-a). However, this distinction was sometimes ambiguous as       27individually occurring biotite was also anhedral as well as subhedral, which would either indicate that it was anhedral and primary or had replaced anhedral hornblende.   Figure 5: Images from thin section scan of MA14-TB10 showing: (a) individually occurring subhedral to euhedral magmatic biotite grains (bt-m) as well as secondary biotite grains (bt-s) replacing hornblende (hb); (b) intergrown magmatic texture with anhedral hornblende (hb) displaying curvilinear grain boundaries shared with plagioclase (clear in PPL).    Intergrown, magmatic textures of hornblende grains indicate that they are not xenocrysts but crystallized late from the melt in equilibrium with the generally more euhedral phases that were precipitated earlier in the crystallization history of the granitic magmas. MA14-TB10-C is an example of the magmatic character of hornblende grains (Figure 5-b), demonstrating curvilinear, not angular boundaries with quartz and feldspar grains, indicating magmatic co-precipitation. Samples MA14-AB1 and -AB8 contain subhedral hornblende grains that are generally more rhombohedral in cross-section. Since these grains do not have reaction rims, the hornblende grains crystallized earlier from the melt relative to plagioclase and K-feldspar than in the other four samples, where hornblende displays anhedral, curvilinear grain boundaries. An inferred process of melt crystallization may be described according to the textural relationships observed in the       28six samples selected for electron probe microanalysis. Magnetite and other high-temperature opaque oxide minerals precipitated first, followed by titanite, and then biotite. Next, hornblende crystals began to nucleate and grow, often incorporating magnetite and titanite as inclusions. Potassium feldspar grains precipitated separately from growing aggregates of mafic minerals and assumed euhedral to anhedral crystal shapes depending on the timing of subsequent crystallization of plagioclase and then finally quartz. However, the order of crystallization of hornblende, potassium feldspar, plagioclase, and quartz seems to have been quite flexible with examples of anhedral, irregular grain boundaries shared between all four phases. Granophyric and micrographic intergrowths of potassium feldspar and quartz in particular demonstrates simultaneous precipitation of these minerals. Plagioclase grains were infrequently oriented so as to show concentrically zoned growth patterns indicating regularly alternating minor variation in the chemical composition of the melt leading to the precipitation of either more albitic or anorthitic layers on the surfaces of growing crystals. However, as evidenced by the localization of intense sericite alteration in the cores, the overall composition of the plagioclase grains began as more calcic and gradually became more sodic as fractional crystallization proceeded (Figure 6-a). In the case of MA14-TB2, a set of nested inclusions that supports the inferred crystallization history was observed as follows. A rounded to blocky, equant magnetite grain approximately 0.1 millimeters in diameter was included at the center of an approximately 0.2 mm diameter angular, euhedral titanite grain, which was in turn included within an approximately 1 millimeter diameter subhedral to anhedral hornblende host crystal or oikocryst (Figure 6-b). The remainder of this section is devoted to descriptions of the minerals identified in each of       29the six samples selected for electron probe microanalysis and Al-in hornblende geothermobarometry.    Figure 6: Images of thin section scans corroborating the inferred process of melt crystallization: (a) scan of MA14-AB8 showing a large, central concentrically zoned euhedral plagioclase grain with locally intense sericite alteration concentrated in the calcic core and absent from the sodic rim; (b) scan of MA14-TB2 showing nested inclusions of black equant, blocky magnetite (mt) within subhedral to euhedral tan coloured titanite (tit) within subhedral to anhedral green hornblende (hb). Lime yellow epidote (ep) and reddish orange biotite (bt) also occur as secondary minerals isocrystallographically replacing hornblende.    Sample MA14-AB1. is a leucocratic epidote-, apatite-, chlorite-, titanite-, and magnetite-bearing hornblende biotite granodiorite. Texturally, it is phaneritic, holocrystalline, fine- to coarse-grained, inequigranular, weakly foliated, hypidiomorphic to allotriomorphic, and non-vesicular. Quartz grains are translucent, pale grey, anhedral, and generally less than 1 millimeter in diameter, occurring in foliation-parallel slightly elongated lens-shaped clusters, which are 3 millimeters in diameter on average. Undulatory extinction of the clusters on rotating the microscope stage in cross-polarized light demonstrates the development of subgrains or discrete sub-millimeter domains due to dynamic recystallization. Potassium feldspar grains are pale pink, blocky to playing card-shaped, subhedral, 3 to 6 millimeters in diameter on average, frequently displaying       30tartan twinning, pervasive sericite alteration of generally weak but locally varying intensity, and infrequent carlsbad or simple twinning. White to off-white plagioclase grains are blocky and subhedral to anhedral, 1 to 4 millimeters in diameter, displaying pervasive sericite alteration of generally weak but locally varying intensity as well as polysynthetic twinning. Frequently rhombus-shaped, subhedral hornblende grains are pleochroic medium brownish-green to pale moss green, about 0.2 to 1 millimeters in diameter and pervasively altered to biotite, often containing inclusions of titanite and magnetite grains. Biotite grains are anhedral, pleochroic pale yellow to medium forest green to brownish olive green, and occur individually and in elongate aggregates replacing hornblende, with regularly curvilinear traces of {001} cleavage planes, comprising evidence for deformation. Magnetite grains are anhedral to subhedral, blocky and equant opaque grains usually approximately 0.5 millimeters in diameter and associated with titanite as inclusions in pervasively biotite-replaced hornblende. Euhedral titanite grains occur as sub-millimeter scale very high relief, pleochroic pale tan to beige grains that display scalenohedral cross-sections. Pale green tinged yellow anhedral epidote replaces biotite as generally sub-millimeter scale grains. Sub-millimeter scale euhedral to subhedral very high reflief apatite grains are clear in plane-polarized light and medium grey in cross-polarized light.        31 Figure 7: Images of sample MA14-AB1, granodiorite, from the Aishihik batholith: (a) unstained slab; (b) photomicrograph in XPL showing two hornblende grains surrounded by plagioclase.   Sample MA14-AB8. is a leucocratic apatite-, epidote-, chlorite-, titanite-, and magnetite-bearing biotite hornblende granodiorite. It is holocrystalline, predominantly medium-grained and locally potassium feldspar-megacrystic (1 to 3 millimeter diameter grains on average, occasionally up to 1 centimeter in diameter for megacrysts), inequigranular, non-vesicular, phaneritic, weakly foliated, and allotriomorphic to hypidiomorphic with a semi-polygonal texture. Roughly equant, anhedral, transclucent and pale grey quartz grains are approximately 3 to 5 millimeters in diameter on average and demonstrate undulatory extinction. Pale pink, blocky, approximately equant to playing card-shaped subhedral potassium feldspar grains are 5 millimeters in diameter on average and locally up to 1 centimeter in diameter. They are pervasively weakly sericite-altered, preferentially aligned foliation-parallel, and locally display a perthitic texture as well as granophyric intergrowths with quartz. White blocky to elongate subhedral plagioclase grains are approximately 2 to 6 millimeters in diameter and frequently display concentric zonation from calcic, anorthite-rich cores to sodic, albite-rich rims. Pervasive weak sericite alteration of plagioclase is locally intense in the cores of zoned grains.       32Olive green hornblende occurs as predominantly subhedral truncated diamond-shaped grains often altered to biotite and displaying a variably corroded appearance. Reddish-orange to greenish-brown biotite grains occur as primary as well as secondary grains replacing hornblende. Titanite occurs as clusters of sub-millimeter translucent pale tan to amber coloured scalenohedral prismatic grains frequently included with magnetite in hornblende. Magnetite occurs as subhedral blocky equant grains that are approximately 1 millimeter in diameter on average and usually associated with titanite as inclusions in hornblende. Epidote and chlorite occur as minor alteration phases after hornblende and biotite. Sub-millimeter euhedral to subhedral apatite grains are translucent to medium grey and display hexagonal to bar-shaped cross-sections.   Figure 8: Images of sample MA14-AB8, granodiorite, from the Aishihik batholith: (a) unstained slab; (b) photomicrograph in XPL of showing a dark green hornblende grain in contact with plagioclase. Large black dots are ink blotches added prior to electron probe microanalysis to facilitate the location of and navigation between selected plagioclase-hornblende pairs.   Sample MA14-AB10. is a leucocratic apatite-, augite-, epidote-, titanite-, and magnetite-bearing biotite hornblende quartz diorite. It is holocrystalline, medium- to fine-grained, inequigranular, non-vesicular, phaneritic, and hypidiomorphic to       33allotriomorphic. Weakly developed foliation is defined by the preferential alignment of elongate plagioclase laths, lens-shaped quartz aggregates and elongate chains or strings of hornblende and biotite. Quartz occurs as translucent, pale grey lens-shaped clusters approximately 3 millimeters in diameter consisting of anhedral dynamically recrystallized grains that are 0.25 millimeters in diameter average. Extremely pale pink subhedral to anhedral potassium feldspar grains are 2 to 3 millimeters in diameter on average and exhibit pervasive weak sericitization or saussuritization. White tinged pale bluish grey subhedral to anhedral plagioclase grains are 4 millimeters in diameter on average and also display pervasive weak sericite alteration. Forest green coloured hornblende grains, 0.1 to 1.5 millimeters in diameter, are subhedral to anhedral grains that infrequently have truncated rhombus-shaped cross-sections. Elongate anhedral reddish brown coloured biotite grains, 0.1 to 1 millimeters in diameter, with frequently curvilinear cleavage traces are observed to wind along quartz and feldspar grain boundaries parallel to foliation. Subhedral to euhedral pale tan titanite grains up to 2 millimeters in diameter with scalenohedral cross-sections are predominantly associated with magnetite as inclusions in hornblende. Subhedral to anhedral blocky, roughly equant opaque magnetite grains occur individually as well as in aggregates included in hornblende. Euhedral to subhedral translucent to medium grey apatite grains of very high relief occur as elongate bar-shaped and hexagonal grains that are generally less than 0.1 millimeters in diameter. Pale green tinged yellow epidote and dark green chlorite occur as trace alteration products after biotite. A single high relief euhedral primatic grain of dark brown tinged purple augite occurs near the edge of the polished thin section. An       34unidentified isotropic mineral, potentially garnet, is hexagonal in cross-section and intergrown with biotite occupying an intergranular discontinuity.   Figure 9: Images of sample MA14-AB10, quartz diorite, from the Aishihik batholith: (a) unstained slab; (b) thin section scan in XPL of showing a central pale orange hornblende grain in contact with multiple plagioclase grains.   Sample MA14-TB2. is a leucocratic apatite-, epidote-, titanite-, magnetite-, and biotite-bearing hornblende quartz diorite. It is holocrystalline, phaneritic, inequiganular, medium- to fine-grained, non-vesicular, very weakly foliated, and allotriomorphic, with quartz and feldspar grains constituting a semi-polygonal texture. Transclucent pale grey roughly equant subhedral quartz grains are approximately 1 to 2 millimeters in diameter. White tinged pale pink equant subhedral potassium feldspar grains are on average approximately 1 millimeter in diameter and weakly sericite altered. White to off-white plagioclase grains are equant to lath-shaped and on average approximately 0.5 to 2 mm diameter and locally up to 6 millimeters in diameter. They exhibit polysynthetic twinning and minor sericite alteration that preferentially occurs at the cores of concentrically zoned grains. Medium forest green to tan tinged pale yellowish green anhedral hornblende       35grains are approximately 0.2 to 1.5 millimeters in diameter and display infrequent rhombus-shaped cross-sections. Biotite occurs as elongate lath-shaped to acicular anhedral to subhedral light brown tinged orange to moss green grains occurring individually and locally replacing hornblende. Titanite occurs as subhedral, rounded rhombus-shaped very high relief pale tan grains that are approximately 0.1 to 0.2 millimenters in diameter and associated with magnetite grains as inclusions in hornblende. Magnetite occurs as rounded equant anhedral opaque grains less than 0.1 millimeters in diameter that occur as inclusions in hornblende and feldspar grains as well as along quartz and feldspar grain boundaries. Translucent pale grey euhedral to subhedral apatite grains generally less than 0.1 millimeters in diameter are commonly bar-shaped and hexagonal in cross-section and occur as inclusions in feldspar grains. Anhedral pale green tinged yellow epidote occurs as a trace alteration phase replacing biotite.  Figure 10: Images of sample MA14-TB2, quartz diorite, from the Tatchun batholith: (a) unstained slab; (b) photomicrograph in PPL of showing dark, medium, and light green hornblende grains in contact with multiple off-white plagioclase grains.  Sample MA14-TB4. is leucocratic apatite-, titanite-, magnetite-, biotite-, and epidote-bearing hornblende-rich quartz diorite. The texture is holocrystalline, phaneritic,       36inequigranular, non-vesicular, allotriomorphic, and weakly foliated with a semi-polygonal texture defined by quartz and feldspar grains. Quartz occurs as translucent pale grey anhedral grains approximately 0.5 to 1.5 millimeters in diameter that display undulatory extinction. Potassium feldspar occurs as blocky pale pink subhedral to anhedral strongly sericitized or saussuritized grains. White to cream coloured blocky to slightly elongate plagioclase grains are generally anhedral, pervasively sericite altered, and locally display nearly complete replacement by secondary sericite or saussurite alteration minerals. Dark green to brown hornblende displaying characteristic 24 x 56 degree amphibole cleavage angles ocurrs as subhedral grains up to 5 millimeters in diameter with infrequent rhombus-shaped cross-sections. Titanite occurs as very high relief subhedral to anhedral pale tan grains less than 0.1 millimeters in diameter that are frequently intergrown with or included with magnetite within hornblende grains. Anhedral rounded to blocky equant opaque magnetite grains occur primarily as inclusions within hornblende. Pale tan to pale yellowish-green epidote and replaces hornblende as anhedral sheaf-like grains. Clear to medium grey apatite grains are generally less than approximately 0.3 millimeters in diameter and display bar-shaped and hexagonal cross-sections.         37Figure 11: Images of sample MA14-TB4, quartz diorite, from the Tatchun batholith: (a) unstained slab; (b) photomicrograph in PPL showing an anhedral green hornblende grain in contact with clear plagioclase. Large black dots are ink blotches added prior to electron probe microanalysis to facilitate the location of and navigation between selected plagioclase-hornblende pairs.   Sample MA14-TB10. is a leucocratic apatite-, titanite-, magnetite-, epidote-, and biotite-bearing hornblende-rich quartz diorite. It is holocrystalline, non-vesicular, phaneritic, inequigranular, medium- to fine-grained, weakly foliated, and allotriomorphic. Quartz occurs as pale grey translucent equant anhedral grains. Minor potassium feldspar occurs as blocky anhedral grains approximately 1 to 2 millimeters in diameter. White to off-white generally anhedral plagioclase grains display frequent polysynthetic twinning as well as pervasive weak to locally intense sericite alteration. Hornblende occurs as dark forest green to yellowish tan anhedral grains locally replaced by biotite and epidote. Biotite occurs as elongate chestnut brown to reddish orange anhedral grains. Epidote occurs as yellowish lime green anhedral grains. Titanite occurs as subhedral to anhedral very high relief pale tan truncated rhombus-shaped to elongate scalenohedral grains. Magnetite occurs as equant, rounded to blocky or prismatic opaque grains that are generally between 0.1 and 0.4 millimeters in diameter and included in hornblende grains or along quartz and feldspar grain boundaries. Apatite occurs as very high relief subhedral clear to medium grey grains that are less than 0.1 millimeters in diameter and display hexagonal and bar-shaped cross-sections.        38  Figure 12: Images of sample MA14-TB10, quartz diorite, from the Tatchun batholith: (a) unstained slab; (b) photomicrograph in PPL showing anhedral green hornblende grains in contact with clear plagioclase grains.    4.2 Geothermobarometry  This section presents the results of electron probe microanalysis of selected hornblende-plagioclase pairs for each of the six samples analyzed and the temperatures and pressures of crystallization calculated using Anderson and Smith’s (1995) formula as well as calculated depths of emplacement. Table 2 is a summary table presenting the ranges of pressures and temperatures of crystallization as well as ranges of calculated depths of emplacement for each of the six samples according to the three different thermometer reactions. While results of the HB 2 thermometer reaction (Holland and Blundy, 1994) are preferred and are the most consistent, results from the other two thermometers, HB 1 and BH, are included for comparison. The range of pressures calculated using HB 2 for the Aishihik batholith samples is 3.2 ± 0.6 to 4.6 ± 0.6 kilobars while that for the Tatchun batholith samples is 5.8 ± 0.6 to 8.6 ± 0.6 kilobars. The       39corresponding ranges of depths are 11.9 ± 2.1  to 17.2 ± 2.1 kilometers for the Aishihik batholith and 21.4 ± 2.1 to 31.4 ± 5.6 kilometers for the Tatchun batholith. Pressure estimates are consistently approximately three kilobars higher or ten kilometers deeper for the Tatchun batholith relative to the Aishihik batholith. Results using thermometers HB 1 and BH yielded much larger ranges between samples within the batholiths and each thermometer yielded clearly erroneous pressure estimates that were either negative or too low for at least one sample. Both consistently underestimated pressures of crystallization relative to HB 2. These discrepancies and the poorer performance of BH and HB 1 thermometers relative to HB 2 is attributed to the presence of quartz in these thermometer reactions. One explanation could be that textures described in the previous section indicate that quartz was often more anhedral than either plagioclase or hornblende and therefore crystallized later. Ranges of crystallization temperatures were less consistent among samples from each batholith and did not differ significantly between batholiths. The total temperature range using HB 2 is approximately 605 to 785 degrees Celsius. The mole fraction of albite for the plagioclase grains that were analyzed varied from 64 to 67 (andesine) for MA14-AB1, 74 to 77 (oligoclase) for MA14-AB8, 59 to 62 (seven of nine pairs, labradorite to andesine) and 78 to 80 (oligoclase, pairs gave signficantly higher pressure estimates and were excluded from the total range) for MA14-AB10, 80 to 86 (oligoclase) for MA14-TB2 (one grain had a value of 98 and corresponded to a pressure estimate of approximately 1 to 1.5 kilobars higher than other estimates using the same geothermometer reaction and was excluded), 77 to 81 (oligoclase) for MA14-TB4 (increasing the mole fraction albite by 5 consistently increased all pressures for each geothermometer reaction (HB1: 1-1.5 kb, HB2: ~ 0.6, HB: 1-1.5 kb) and decreased       40temperatures by ~ 25 degrees Celsius), and 76 to 80 (oligoclase) for MA14-TB10. Although the mole fraction of albite for plagioclase grains should be between Ab65 and Ab75 as this was the range used for the empirical calibration of Hollister et al. (1987), the Ab ranges of the six samples fall within the range (An10-An90) specified for the thermometer reactions of Holland and Blundy (1990, 1994). In order to convert the calculated pressures of crystallization to inferred depths of emplacement, the formula for lithostatic pressure Pl = ρgh was employed using an average density of continental crust of 2700 kg/m3 until 15 kilometers depth and then 2900 kg/m3 after 15 kilometers depth (Van der Pluijm and Marshak, 2004). In this formula, ‘ρ’ (rho) equals density (kg/m3), ‘g’ is the acceleration due to gravity at the Earth’s surface (9.8 m/s2), and ‘h’ equals depth in meters. This method gave a crustal lithostatic gradient of approximately 3.78 kilometers per kilobar until 15 kilometers depth and then 3.52 kilometers per kilobar at depths greater than 15 kilometers. The increase in density occurs at around 15 kilometers depth at the location of the brittle-ductile transition where brittle upper crustal level rocks begin to deform in a ductile manner as temperature and pressure increase with depth.        41 Table 2: Results of iteration based on Anderson and Smith (1995) pressure at various thermometers for the six samples selected from the Aishihik batholith (AB) and Tatchun batholith (TB). Note: J.L. Anderson prefers HB2 results. Calculated maximum and minimum temperatures, pressures, and depths of crystallization for each of the three thermometer reactions are presented for each of the six samples selected for geothermobarometry.    The composition of hornblende influences the applicability of the Al-in-hornblende geothermobarometer. Figure 13 is a classification plot of all analyzed hornblende grains based on the number of silicon atoms per 13-cation chemical formula  of hornblende and the ratio of magnesium to the sum of magnesium and total iron after Leake et al. (1997). There was no significant variation in the composition of individual hornblende cores relative to rims. This indicates that the melt composition remained stable during the crystallization process. Anderson and Smith (1995) specified that in hornblende, the ratio of ferric to total iron should be greater than one quarter. The ratio of       42ferric to total iron is approximately equal to or greater than 0.25 for the hornblende compositions of all six samples. Specifically, the ratio for MA14-AB1 ranges from 0.32 to 0.38, from 0.30 to 0.33 for -AB8 (one pair had a ratio of 0.24 and a pressure estimate of approximately 1 kilobar higher than the other pairs and a temperature underestimate of 20 to 100 degrees Celsius and was excluded from the total range), from 0.34 to 0.38 for -AB10, from 0.23 to 0.28 for -TB2 (no significant difference in pressure or temperature among pairs), from 0.24 to 0.29 for -TB4, and from 0.23 to 0.26 for -TB10. The above observation for MA14-AB8 agrees with Anderson and Smith’s (1995) statement that relatively low oxygen fugacities correspond to overestimates of pressures of crystallization. Another condition specified by Anderson and Smith (1995) is that the ratio of total iron to the sum of total iron and magnesium should be in the range of 0.40 to 0.65 for hornblende compositions. This is true for all six samples. As a result the ratio of magnesium to the sum of total iron and magnesium for hornblende compositions displayed in Figure 13 is approximately in the range of 0.35 to 0.60. A notable exception in terms of silicon atoms per 13-cation chemical fomula is the hornblende composition of MA14-AB8, centred around 0.68. Hornblende grain compositions from the Tatchun batholith are in the hastingsite field while those from the Aishihik batholith are on the boundary between the edenite and ferro-edenite fields.       43 Figure 13: Classification diagram after Leake et al. (1997) for hornblende grains analyzed from the six samples selected for geothermobarometry. AB = Aishihik batholith, TB = Tatchun batholith. The ellipse indicates the approximate range of hornblende compositions for the Granite Mountain batholith of the Minto plutonic suite, which hosts the Minto and Carmacks Copper deposits (Tafti, 2005).   4.3 Sources of Error  The error of the formula given by Anderson and Smith (1995) is ± 0.6 kilobars or approximately 2.3 kilometers at depths less than 15 kilometers and 2.1 kilometers at greater crustal depths. According to Anderson and Smith, this error incorporates the largest 2σ regression error between the data sets used to derive their formula. It is important to note that this error does not include the effects of temperature or analytical imprecision. To address this issue, Anderson and Smith propose the following additional uncertainties applicable under specific conditions. First, an uncertainty in temperature of       44± 50 degrees Celsius adds an additional 0.8 kilobars of uncertainty for a total uncertainty of 1.4 kilobars or 4.9 kilometers at depths less than 15 kilometers and 5.3 kilometers at greater crust depths. Temperature results using HB 2 for five of the six samples have ranges of less than ± 50 degrees Celsius while MA14-TB2 has a temperature range of almost 150 degrees Celsius (See Table 2). As a result, an additional ± 1.2 kilobars of uncertainty is applied to pressure estimates from MA14-TB2. Furthermore, a one percent analytical error in aluminum content adds an additional 0.1 kilobars of uncertainty. The analytical imprecision of aluminum EPMA data is on average 0.6 percent, representing the relative standard deviation. Furthermore, the natural variation in aluminum content is on average 18.5 percent, much larger than the analytical imprecision. This means that the error introduced through the analytical imprecision in the measurement of the aluminum content of hornblende grains in the six samples selected for Al-in hornblende geothermobarometry is insignificant in comparison with the observed natural variation. The natural variation and analytical imprecision attached to the other analyzed elements that affect the thermometer reactions are reflected in the temperature uncertainty of an additional ± 0.8 kilobars for a ± 50 degree uncertainty.  Another potential source of error in the calculated depths of emplacement involves the values used in the formula for calculating lithostatic pressure, Pl = ρgh. Density, ‘ρ’ (rho), is given as 2700 kg/m3 until the brittle-ductile transition zone at 15 kilometers depth and then 2900 kg/m3 at greater crustal depths (Van der Pluijm and Marshak, 2004). These values are constant average crustal densities for discrete depth intervals and therefore the calculated depth only approximates the true depth, which       45represents the continuously varying density gradient due to vertically varying lithology and fluid composition and distribution that existed in the Yukon-Tanana terrane during the Late Triassic to Early Jurassic. Similarly a value of 9.8 m/s2 is used for the gravitational constant, ‘g’, although this is the acceleration due to gravity at the Earth’s surface. In reality, the acceleration due to gravity varies with distance from center of the Earth. The gravitational constant at 20 kilometers in depth, which is approximately 0.3 percent of the Earth’s radius, 6370 kilometers, would be slightly greater than 9.8 m/s2, producing a slightly shallower lithostatic gradient in terms of kilobars per kilometer. Corresponding depths would be slightly overestimated.  Another potential source of error involves the assumption that the crystallization temperature of plutons is near a wet-granite solidus temperature (Anderson and Smith, 1995). Plutons emplaced at hotter temperatures than wet-granite solidus temperatures generate artificially higher pressures relative to near wet-granite solidus temperatures (Anderson and Smith, 1995). Water-undersaturated solidus temperatures increase the Al content of hornblende thus increasing the calculated pressure. Anderson and Smith (1995) correct the pressure calculation for temperature and oxygen fugacity although they maintain that crystallization of plutons does not necessarily occur near water-saturated solidus temperatures, such as the wet granite solidus temperature of approximately 650 degrees Celsius. This means that one of nine phases in the equilibrium buffer assemblage specified by Schmidt (1992), water, may not have been present, which would affect the calibration of his Al-in hornblende geobarometer.        46Discussion  5.1 Emplacement Depths and Exhumation Rates  Based on the depths of emplacement corresponding to calculated crystallization pressures, the Tatchun batholith was emplaced on average about 10 kilometers deeper (21-31 kilometers) than the Aishihik batholith (12-17 kilometers). The Tatchun batholith of the Minto plutonic suite (204-195 Ma) and the Aishihik batholith of the Long Lake plutonic suite (192-180 Ma) occur in highlands flanking the Whitehorse Trough. The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin (Colpron, pers. comm., 2015). Specifically, Late Pliensbachian (183 Ma) and younger strata of the Tanglefoot Formation of the Early to Middle Jurassic Laberge Group contain granitic plutonic clasts with igneous ages and detrital uranium-lead zircon age peaks corresponding to Late Triassic to Early Jurassic plutons. Combining the igneous age ranges of the Long Lake and Minto plutonic suites and calculated emplacement depths of the Aishihik and Tatchun batholiths with the timing of the first appearance of granitic plutonic clasts within the adjacent Whitehorse Trough indicates relatively rapid exhumation of mesozonal to catazonal Late Triassic to Early Jurassic granitoids in Yukon during an approximate interval of as little as five to ten million years. In particular, the exhumation rates for the Aishihk batholith may have been between 1.2 to 3.4 kilometers per million years or millimeters per year while for the Tatchun batholith, exhumation rates may have been between 2.1 to 6.2 kilometers per million years or millimeters per year. These       47values may be compared to the 5.1 mm/year erosion rate of the St. Elias Mountains in southwestern Yukon which are currently being uplifted due to the collision of Yakutat block in southeastern Alaska since approximately 5-10 Ma (Sheaf et al., 2003).  However, these ranges of exhumation rates for the Aishihik and Tatchun batholiths are at best approximate due to the lack of stronger constraints on the ages of the plutons and the absence of identified age-equivalent clasts of the same lithologies.   Tafti (2005) applied Al-in hornblende geothermobarometry to estimate the pressure during crystallization of plutons hosting the Minto and Carmacks Copper deposits. Emplacement depths corresponding to crystallization pressures calculated for a total of seven samples of the Granite Mountain batholith (202-197 Ma) were in excess of nine kilometers. This estimate is significantly shallower than those for the Aishihik (12-17 kilometers) and Tatchun (21-31 kilometers) batholiths. However, Tafti’s estimate reflects the minimum emplacement depth consistently yielded when using any of the three thermometer calibration reactions, while the estimates presented in this thesis reflect only the results of employing the HB 2 thermometer reaction in Anderson and Smith’s (1995) formula. Depths estimates for samples from the Granite Mountain batholith using the preferred thermometer HB 2 are consistently deeper, in the range of five to six kilobars or 18 to 22 kilometers (Tafti, 2005). If the estimates produced by HB 2 more accurately reflect the emplacement depths for sampled plutons of the Granite Mountain batholith, then the emplacement depths of those plutons would be intermediate between estimates of emplacement depths for the Aishihik and Tatchun batholiths that are presented in this thesis. The frequent disagreement between pressures of       48crystallization estimated using thermometer reactions other than HB 2 as presented in the results section above, highlights the uncertainty attached to the applicability of the Al-in hornblende geothermobarometry technique.   5.2 Tectono-magmatic Evolution of Aishihik Plutonic Suite  This section comprises a review of the Late Triassic to Early Jurassic regional geology presented at the beginning of this thesis, which addresses the tectono-magmatic evolution of the Aishihik plutonic suite, within the context of additional constraints provided by results of Al-in hornblende geothermobarometry for plutons of the Aishihik and Tatchun batholiths. Late Triassic to Early Jurassic (220-180 Ma) plutonic rocks in west-central Yukon and eastern Alaska represent a pulse of magmatism that emplaced the subvolcanic magma chambers of a continental magmatic arc that was undergoing contractional deformation and thickening (Mortensen et al., 2000). Predominantly granitic rocks of the Late Triassic to Early Jurassic Aishihik plutonic suite are exposed along the contact between the Yukon-Tanana terrane and the Mesozoic intraoceanic volcanic arc terranes, Stikinia and Quesnellia. The granitic plutons form mountain ranges on the southern border of the Yukon Plateau where it joins the adjacent Whitehorse Trough. This intermontane basin is occupied by mainly clastic sedimentary rocks overlapping the Stikine and Quesnel terranes. The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin (Colpron, pers. comm., 2015). Emplacement of continental arc magmas occurred       49during the lead up to and through the Cordilleran Orogeny, which initiated in the Early Jurassic and continued through the Middle Jurassic. As a result, Early Jurassic magmas emplaced during the Cordilleran Orogeny represent stitching plutons while Triassic magmas preceding the onset of collision express ordinary continental arc magmatism. According to the oroclinal enclosure hypothesis (Mihalynuk et al., 1994), the Stikine terrane folded counter-clockwise around the Cache Creek terrane during the Cordilleran Orogeny to amalgamate the composite Stikinia-Yukon-Tanana-Quesnellia terrane. In this model, the Yukon-Tanana terrane forms the hinge zone connecting the roughly linear Stikine and Quesnel intraoceanic volcanic island arc terranes. Plutons of the Late Triassic to Early Jurassic Aishihik Plutonic Suite were emplaced within the Yukon-Tanana terrane immediately north of the synorogenic ‘piggyback’ basin that developed at the center of the apex of the hinge zone of the oroclinal bend (Colpron, pers. comm., 2015). This sedimentary basin is referred to as the Whitehorse Trough, which was uplifted as Stikinia and Quesnellia were sutured together (Colpron, pers. comm., 2015). Oroclinal closure was completed in the mid-Jurassic and corresponded to declining continental arc magmatism, thrusting, uplift, and erosion (Allan et al., 2013).  An Early to Middle Jurassic sedimentary overlap assemblage unconformably overlying the Stikine and Quesnel terranes, the Laberge Group, occupies the Whitehorse Trough in south-central Yukon, an intermontane basin that formed as a forearc basin that was uplifted during the Pliensbachian (191-183 Ma) during the Cordilleran Orogeny. Specifically, Late Pliensbachian (183 Ma) and younger conglomerates of the Tanglefoot Formation contain approximately 60 to 80 percent plutonic clasts derived from the       50adjacent Late Triassic to Early Jurassic plutons (Colpron, pers. comm., 2015).  The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin (Colpron, pers. comm., 2015). Zircon age dates from the Minto plutonic suite, representing the Tatchun batholith, are in the range of 204 to 195 Ma while ages from the Long Lake Plutonic Suite, representing the Aishihik batholith, are in the range of 192 to 180 Ma (Colpron, pers. comm., 2015).    Johnston and Canil (2007) proposed that westward crustal growth in southwest Yukon occurred through a process of synmagmatic subcretion. In this model, the Aishihik batholith was intruded during the Early Jurassic along a southwest-verging crustal scale thrust fault intervening between two supracrustal assemblages: the Lewes River Group of Stikinia underlying the Whitehorse Trough and the Aishihik Metamorphic Assemblage. The process of subcretion is proposed in order to explain the east-dipping imbricated configuration of crustal-scale tabular tectonic elements through successive additions of these elements to the western margin of North America. Previous workers also identified the presence of magmatic epidote as an indication that plutons of the Aishihik batholith were emplaced at depths in excess of twenty kilometers (Johnston et al., 1996; Dusel-Bacon et al., 2009). Similar to the Aishihik batholith, the slightly older Tatchun batholith of the Minto plutonic suite (204-195 Ma) is bound by approximately southwest-verging thrust faults, located between Late Devonian to Early Mississippian metasedimentary and metavolcanic rocks of the Snowcap and Finlayson assemblages of the Yukon-Tanana terrane to the northeast and the Late Triassic Lewes River Group of the Stikine terrane to the southwest (Buffett et al., 2006). However, the Tatchun batholith       51is different in that it intrudes the Semenof block, which consists of Devonian to Permian volcanic and sedimentary rocks of the Boswell assemblage (Buffett et al., 2006; Simard and Devine, 2003). The Mira Geoscience report (2014) models the Tatchun batholith as sitting within the Boswell assemblage dipping neither to the east nor the west. The tectonic setting of the Tatchun batholith broadly mirrors Johnston and Canil’s (2007) interpretation of the Aishihik batholith as a syntectonic intrusion emplaced into a continental magmatic arc between lithotectonic terranes that were in the process of amalgamating in the lead up to and during the Cordilleran Orogeny in the Early to Middle Jurassic. More specifically, the syntectonic emplacement of the Tatchun batholith on the east side of the Whitehorse Trough is the same as the syntectonic emplacement of the Aishihik batholith on the west side of the trough except that the Tatchun batholith intrudes the Semenof block, probably representing a relatively minor volcanic island arc which was caught between the colliding Stikine and Yukon-tanana terranes. Combining the igneous ages of plutons of the Aishihik and Tatchun batholiths with the timing of the first appearance of granitic plutonic clasts and detrital uranium-lead zircon peaks within the adjacent Whitehorse Trough indicates relatively rapid exhumation of mesozonal to catazonal Late Triassic to Early Jurassic granitoids in Yukon, during an interval of as little as five to ten million years. Following the emplacement of the Aishihik and Taylor Mountain Plutonic Suites, a hiatus of continental arc magmatism lasted in the region from approximately 180 to 115 Ma (Allan et al., 2013). However, mica cooling ages for the Aishihik batholith (Johnston et al., 1996) and the Tatchun batholith (Tempelman-Kluit, 1984) as well as Early to Middle Jurassic stratigraphy in the Whitehorse Trough indicate       52that uplift and exhumation persisted at a declining rate into the Middle Jurassic (Allan et al., 2013).  Plutons corresponding to samples analyzed from the Tachun batholith (21-31 kilometers) were emplaced on average about ten kilometers deeper than those from the Aishihik batholith (12-17 kilometers). Exhumation rates for the Aishihk batholith may have been between 1.2 to 3.4 kilometers per million years or millimeters per year while those for the Tatchun batholith may have been between 2.1 to 6.2 kilometers per million years or millimeters per year.  These estimates of emplacement depths and exhumation rates represent approximate ranges and therefore provide only rough constraints on the Late Triassic to Early Jurassic tectono-magmatic evolution of south-central Yukon. In general, earlier phases of both the Aishihk and Tatchun batholiths have mesozonal to catazonal calculated crystallization pressures while observed younger phases of both batholiths include granitic pegmatites, aplitic dykes, and rocks with miarolitic cavities (Tafti, 2005). Dr. Murray Allan recently located a porphyritic component of the Aishihik batholith that is exposed at surface that he has dated to the Early Jurassic and believes was emplaced at a maximum of 5 kilometers in depth (pers. comm. Allan, 2015). These observations suggest the successive emplacement of plutons at progressively shallower crustal levels during a relatively narrow time window in the Early Jurassic and therefore corroborate the hypothesis of rapid exhumation during emplacement.           535.3 Implications for Intrusion-related Mineral Deposits  Estimates of crystallization pressures and corresponding depths of emplacement of plutons of the Late Triassic to Early Jurassic Aishihik and Tatchun batholiths in south-central Yukon provide rough constraints on the tectono-magmatic evolution of the area and therefore the potential for the formation and preservation of intrusion-related mineral deposits such as copper-gold porphyry systems. The Late Triassic to Early Jurassic Granite Mountain batholith, located within 100 kilometers of the Aishihik and Tatchun batholiths, hosts the Minto and Carmacks Copper deposits, the two most significant examples of Late Triassic to Early Jurassic intrusion-related copper-gold mineralization in the area. Tafti (2005) inferred that the rafts or tabular ore bodies of the Minto and Carmacks Copper deposits represent deformed porphyry systems that formed at shallow crustal depths, were tectonically buried and engulfed by plutons at depths greater than nine kilometers and then were subsequently rapidly exhumed. Hood (2012) reinterpreted that mineralization at Minto was syngenetic to plutonism and deformation in a mesozonal active shear zone environment, near the brittle-ductile transition at 10 to 20 kilometers depth. Strain partitioning within plutons whereby mineralizing fluid flow is concentrated along ductile shear zones during emplacement Hood’s model thus eliminates the requirement of shallow level porphyry system formation and burial prior to emplacement within mesozonal host intrusions and subsequent exhumation. Tafti (2005) interpreted that hosts of mineralization at the Minto and Carmack Copper deposits include early phases of the Granite Mountain batholith as well as metamorphosed supracrustal wall rocks. Mineralization that Tafti identified as hypogene occurs as disseminations and leaf-      54shaped stringers of copper sulphide minerals including chalcopyrite and bornite along with native gold, electrum, and silver telluride minerals hosted in strongly deformed metaplutonic and metasedimentary rocks. Subsequent supergene alteration deposited native copper in narrow veinlets and abundant copper oxide minerals including malachite and azurite along fracture planes in the oxidized zone. Supergene alteration also deposited less abundant copper sulphide minerals at greater depth including covellite and chalcocite at grain discontinuities and fractures (Tafti, 2005).   Late Triassic to Early Jurassic plutonic rocks in Yukon and eastern Alaska are age-equivalent to intrusions in northern British Columbia emplaced within the Stikine and Quesnel terranes. In British Columbia, the presence of numerous porphyry copper-gold deposits exposed at or near surface, which typically formed at crustal depths between 1 and 6 kilometers (Kesler and Wilkinson, 2008), indicates that only a few vertical kilometers of material has been removed through erosion. Furthermore, these deposits are preserved at or near the present erosional surfrace. By contrast, plutons in west-central Yukon and eastern Alaska representing the subvolcanic granitic magma chambers that underlie porphyry systems are currently exposed at surface. Although Late Triassic to Early Jurassic granitoids in Yukon formed during the same time as porphyry copper-gold deposits in British Columbia, the granitoids in Yukon were emplaced at an inappropriate tectonic environment for the preservation of overlying conventional shallow crustal level porphyry systems. Geochemical analyses obtained by Dr. John B. Chapman of the Geological Survey of Canada for samples from unmineralized plutons from across west-central Yukon indicate strong potential for having generated higher       55level porphyry systems although high uplift rates may have eroded these systems away to expose the mid-crustal residue of porphyry-related magmatism (Chapman, 2014). The Al-in hornblende geothermobarometry results presented in this thesis support the interpretation that Late Triassic to Early Jurassic granitic rocks in Yukon are not generally prospective for conventional porphyry deposits. Specifically, the Aishihik batholith was emplaced at depths between approximately 12 and 17 kilometers while the Tatchun batholith was emplaced at depths between approximately 21 and 31 kilometers. By contrast, conventional porphyry deposits form at much shallower crustal levels, generally in the upper six kilometers (Kesler and Wilkinson, 2008). The Late Triassic to Early Jurassic Aishihik and Tatchun batholiths were emplaced at deeper levels in the crust than is believed to be required for the exsolution of a mineralizing fluid phase to form conventional porphyry deposits. However, porphyry deposits were probably emplaced at shallower levels related to porphyrytic stocks emanating from mesozonal batholiths.    Summary  The objective of this study is to provide additional constraints through Al-in hornblende geothermobarometry on the nature and tectonic setting of Late Triassic to Early Jurassic magmatism and associated intrusion-related mineralization in west-central Yukon and eastern Alaska. Late Triassic to Early Jurassic (220-180 Ma) plutonic rocks in west-central Yukon and eastern Alaska represent a pulse of magmatism that emplaced the       56subvolcanic magma chambers of a continental magmatic arc that was superimposed on the Yukon-Tanana terrane. The Late Triassic to Early Jurassic Granite Mountain batholith hosts the Minto and Carmacks Copper deposits, the two most significant examples of Late Triassic to Early Jurassic intrusion-related copper-gold mineralization in the area. Predominantly granitic rocks of the Late Triassic to Early Jurassic Aishihik plutonic suite are exposed along the contact between the Yukon-Tanana terrane and the Mesozoic intraoceanic volcanic arc terranes, Stikinia and Quesnellia. The granitic plutons form mountain ranges on the southern border of the Yukon Plateau where it joins the adjacent Whitehorse Trough. This intermontane basin is occupied by mainly clastic sedimentary rocks overlapping the Stikine and Quesnel terranes. The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin. Specifically, Late Pliensbachian (183 Ma) and younger strata of the Tanglefoot Formation of the Early to Middle Jurassic Laberge Group contain granitic plutonic clasts with igneous ages and detrital uranium-lead zircon age peaks corresponding to Late Triassic to Early Jurassic plutons, indicating rapid exhumation during emplacement. Samples were collected from plutons of the Aishihik and Tatchun batholiths for the purpose of Al-in-hornblende geothermobarometry to provide estimates of pressures and temperatures of crystallization. Assuming an average density of continental crust, calculated pressures were converted into emplacement depths. Based on the depths corresponding to crystallization pressures, the Tatchun batholith was emplaced on average about 10 kilometers deeper (21-31 kilometers) than the Aishihik batholith (12-17 kilometers). Age-equivalent intrusions in British Columbia were responsible for generating significant copper-gold porphyry deposits that are currently preserved at or       57near surface in Stikinia and Quesnellia. By contrast, the Late Triassic to Early Jurassic Aishihik and Tatchun batholiths were emplaced at deeper levels in the crust than is believed to be required for the exsolution of a mineralizing fluid phase to form conventional porphyry deposits. The rapid exhumation of these batholiths accompanying emplacement produced a strong preservation bias against rocks hosting porphyry deposits that form at shallow crustal levels, generally in the upper six kilometers. While the emplacement depths and exhumation rates of the Aishihik and Tatchun batholiths do not make them prospective in terms of conventional porphyry deposits, they may still be prospective hosts for other unconventional, deformed intrusion-related copper-gold deposits such as Minto and Carmacks Copper.   Recommendations  In order to provide stronger constraints on the emplacement depths and exhumation rates of Late Triassic to Early Jurassic plutonic rocks in west-central Yukon, future work could focus on accounting for limitations of calibration ranges of the thermometer reactions used in Al-in hornblende geothermobarometry as well as more closely establishing the time differential between emplacement and erosion of plutons. More precisely accounting for limitations of geothermobarometry calculations using different thermometer reactions would significantly reduce the uncertainty attached to estimated emplacement depths. Establishing a more precise time differential between the generation of erosional clasts and the emplacement of parent plutons would require       58stronger constraints on the timing of the first appearance of plutonic clasts in the Tanglefoot Formation as well as identifying and more precisely dating the specific plutonic sources in the highlands adjacent to the Whitehorse Trough intermontane basin. 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Colpron, M. and Friedman, R.M., 2008, U-Pb zircon ages for the Nordenskiold Formation (Laberge Group) and Cretaceous intrusive rocks, Whitehorse trough, Yukon in Emond, D.S., ed., Blackburn, L.R., ed., Hill, R.P., e.d., and Weston, L.H., ed., Yukon exploration and geology 2007: Yukon Geological Survey, p. 139-151.  Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Relative plate motions between oceanic and continental plates in the Pacific basin: Geological Society of America, Special Paper n. 206, 59 p.  Hammarstrom, J.M. and Zen, E., 1986, Aluminum in hornblende: An empirical igneous geobarometer: American Mineralogist, v. 71, p. 1297-1313.  Hart, C.J.R., Goldfarb, R.J., Lewis, L.L., and Mair, J.L., 2004, The northern Cordilleran mid-Cretaceous plutonic province: Ilmenite/magnetite-series granitoids and intrusion-related mineralisation: Resource Geology, v. 54, p. 253-280.        61Holland, T.J.B. and Blundy, J.D., 1994, Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry: Contributions to Mineralogy and Petrology, v. 116, p. 433-447.  Holland, T.J.B. and Blundy, J.D., 1990, Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer: Contributions to Mineralogy and Petrology, v. 104, p. 208-224.  Hood, S.B., 2012, Mid-crustal Cu-Au mineralization during episodic pluton emplacement, hydrothermal fluid flow, and ductile deformation at the Minto deposit, YT, Canada: M.Sc. thesis, Vancouver, Canada, The University of British Columbia, 220 p.  Hyndman, R.D., Currie, C.A., and Mazzotti, S.P., 2005, Subduction zone backarcs, mobile belts, and orogenic heat: GSA Today, v. 15, n. 2, p. 4-10.  Johnston, S.T., Mortensen, J.K., and Erdmer, P., 1996, Igneous and metaigneous age constraints for the Aishihik metamorphic suite, southwest Yukon: Canadian Journal of Earth Sciences, v. 33, p. 1543-1555.        62Johnston, S.T. and Canil, D., 2007, Crustal architecture of SW Yukon, northern Cordillera: Implications for crustal growth in a convergent margin orogen: Tectonics, v. 26, p. 1-18.  Kesler, S.E., and Wilkinson, B.H., 2008, Earth’s copper resources estimated from tectonic diffusion of porphyry copper deposits: Geology, v. 36, n. 3, p. 255-258.  Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V .G., Linthout, K., Laird, J., Mandarino, J.A., Maresch, W.V., Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W., and Youzhi, G., 1997, Nomenclature of amphiboles: Report of the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names: Canadian Mineralogist, v. 35, p. 219-246.  Mihalynuk, M.G., Nelson, J.A., and Diakow, L.J., 1994, Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera: Tectonics, v. 13, p. 575-595.  Mira Geoscience, 2014, Geologically-constrained inversion of magnetic and gravity data over parts of the Yukon-Tanana terrane and Whitehorse trough: Yukon Geological Survey Miscellaneous Report MR-10, Mira Geoscience Project No. 3984, 81 p.       63 Mortensen, J.K., 1992, Pre-mid-Mesozoic tectonic evolution of the Yukon-Tanana terrane, Yukon and Alaska: Tectonics, v. 11, n. 4, p. 836-853.  Mortensen, J.K., Emon, K., Johnston, S.T., and Hart, C.J.R., 2000, Age, geochemistry, paleotectonic setting and metallogeny of Late Triassic-Early Jurassic intrusions in the Yukon and eastern Alaska: a preliminary report, in Emond, D.S. and Weston, L.H., eds., Yukon exploration and geology 1999: Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 139–144.  Pouchou, J.L. and Pichoir, F., 1985, PAP φ(ρZ) procedure for improved quantitative microanalysis: Microbeam Analysis, v. 20, p. 104-106.  Price, R.A. and Sears, J.W., 2002, Chapter 5: A preliminary palinspastic map the Mesoproterozoic Belt-Purcell Supergroup, Canada and USA: Implications for the textonic setting and structural evolution of the Purcell anticlinorium and the Sullivan depoit, in Lydon, J.W., ed., Hoy, T., ed., Slack, J.F., ed., and Knapp, M.E., ed., The geological environment of the Sullivan deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication n. 1, p. 61-81.        64Schmidt, M.W., 1992, Amphibole composition in tonalite as a function of pressure: An experimental calibration of the Al-in hornblende barometer: Contributions to Mineralogy and Petrology, v. 110, p. 304-310.  Sheaf, M.A., Serpa, L., and Pavlis, T.L., 2003, Exhumation rates in the St. Elias Mountains, Alaska: Tectonophysics, v. 367, p. 1-11.  Simard, R.L., and Devine, F., 2003, Preliminary geology of the southern Semenof Hills, Yukon (105E/1,7,8) in Emond, D.S., ed., and Lewis, L.,L., ed., Yukon exploration and geology: Exploration and Geological Services Division, Yukon Region, Indian and Northern Affairs Canada, p. 213-222.  Streckeisen, A., 1976, To each plutonic rock its proper name: Earth Science Reviews, v. 12, p. 1-33.  Tafti, R., and Mortensen, J.K., 2004, Early Jurassic porphyry-copper (-gold) deposits at Minto and Williams Creek, Carmacks copper belt, western Yukon, in Emond, D.S., and Lewis, L.L., eds., Yukon exploration and geology 2003: Yukon Geological Survey, p. 289–303.  Tafti, R., 2005, Nature and origin of the Early Jurassic copper (-gold) deposits at Minto and Williams Creek, Carmacks Copper Belt, western Yukon: Examples of       65deformed porphyry deposits: M.Sc. thesis, Vancouver, Canada, The University of British Columbia, 213 p.  Tempelman-Kluit, 1984, Geology, Laberge (105E) and Carmacks (105I), Yukon Territory: Geological Survey of Canada, Open File 1101, 1:250 000 scale.  Van der Pluijm, B.A., and Marshak, S., 2004, Earth structure: New York, W.W. Norton and Company, 656 p.  Winter, J.D., 2001, An introduction to igneous and metamorphic petrology: Upper Saddle River, New Jersey, Prentice-Hall, 697 p.   Appendices*  *The following three appendices are included in digital format in the back pocket of this thesis with the following file descriptions and navigation instructions.  Appendix 1: Petrographic descriptions of hand samples and polished thin sections.  This appendix contains two pdf documents of 20 pages each with approximately one page for each of the descriptions for each hand sample and corresponding polished thin section.        66Appendix 2: Images of samples and point location maps on back-scattered electron (BSE) images.  This appendix contains the following sets of images for all 20 samples from the Aishihik and Tatchun batholiths: 1) 20 JPEG images showing juxtaposed stained and unstained slabs; 2) 20 JPEG images of polished thin section scans in plane-polarized light (PPL); 3) 20 JPEG images of polished thin section scans in cross-polarized light (XPL). This appendix also contains the following sets of images for the six samples selected for Al-in hornblende geothermobarometry: 1) 54 JPEG images of photomicrographs of polished thin sections for all six samples except MA14-AB10; 2) 31 TIF images of point location maps on BSE images for each hornblende-plagioclase pair location on each of the six samples; 3) 6 polished thin section scan images in PPL with inset rectangles showing the locations of analyzed hornblende-plagioclase pairs.  Appendix 3: Analytical data from electron probe microanalysis (EPMA), scanning electron microscope (SEM) energy-dispersive X-ray spectrometry (EDS), and calculation spreadsheets related to Anderson and Smith’s (1995) Al-in hornblende geothermobarometer.  This appendix contains: 1) .xls files of hornblende and plagioclase EPMA data with analytical statistics; 2) .xls files of J.L. Anderson’s calculation spreadsheet for each of the six samples selected for Al-in hornblende geothermobarometry; 3) .xls files related to the preparation of hornblende and plagioclase analytical data prior to entry in calculation spreadsheets; 4) .doc files of SEM energy-dispersive X-ray spectra.   Note on navigation: All three digital appendices, labelled ‘Digital Appendix 1’ and so on, are secondary folders within the primary ‘Digital Appendices’ folder. Within the three secondary folders, the contents as outlined above are organized in appropriately titled tertiary folders. 

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