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Petrology of the Salmon Mountain Stock, Klamath

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Petrology of the Salmon Mountain Stock, Klamath PETROLOGY OF THE SALMON MOUNTAIN STOCK, KLAMATH MOUNTAINS, CALIFORNIA by JAMES JOSEPH BOARDMAN, B.S. A THESIS IN GEOSCIENCES Submltted to the Graduate Faculty of Texas Tech Unlverslty In Partlal Fulflllment of the Requlrements for the Degree of MASTER OF SCIENCE Approved Accepted December, 1985 ^- ^ f\/<^, l'^f ACKNOWLEDGMENTS I would like to thank my advisor, Calvin Barnes, for the interest that he showed in this project from its inception to its corapletion. His encouragement, patience and professionalism brought me through this project. Critical comments of the final manuscript by Dr. Stanley Cebull and Dr. Gary Strathearn were very appreciated. Without the interest, energy, and help of Robert Gribble in the field, this particular project would never have started. Tim Horner generously donated his time and skill to do the lettering on my field map. Jesse O'Halloran (l895-198i*) and James O'Halloran {19OI-I983) provided support for the field work of this project. Sigma Xi also provided money toward the completion of this research. Of course, the encouragement and understanding of my family throughout my graduate career is appreciated more than I can say. ii TABLE OF CONTENTS ACKNOWLEDGMENTS ii LIST OF TABLES v LIST OF FIGURES vi I. PURPOSE AND BACKGROUND 1 Purpose 1 Location 2 Study Methods 5 Regional Geology 5 II. GEOLOGY OF THE SALMON MOUNTAIN AREA 13 Previous Work 13 Metasedimentary Rocks of Salmon Mountain 13 Structure 17 Dikes 18 Glaciation 19 Summary 20 The Salmon Mountain Stock 20 III. ROCKS OF THE SALMON MOUNTAIN STOCK 3*+ Petrographic Descriptions 3^ IV. EMPLACEMENT HISTORY h^ Origin ^5 Mode of Emplacement '•6 Percent Crystallinity During Emplacement ^l iii Origin of Schlieren U8 Emplacement of the Rock Units 50 Dikes 51 V. PETROLOGY 52 Paragenesis 52 Chemical Composition 53 Trondhjemitic Trend ..... 55 Comparison to Other Klamath Mountain Plutons 56 Implications to Plutonism in the Klamath Mountains 62 VI. CONCLUSIONS 63 LIST OF REFERENCES 6k APPENDIX 69 IV LIST OF TABLES 1. Modal mineralogy and plagioclase An content of rocks of the Salmon Mountain stock 37 2. Chemical analyses and CIPW norms of coarse-grained rocks of the Salmon Mountain stock 5** LIST OF FIGURES 1. Plutonic belts of the Klamath Mountains 3 2. Subprovinces and major plutons of the Klamath Mountains 6 3. Geologic Map of Salmon Mountain, California in pocket k, Outline of the shape of the Salmon Mountain stock ... 21 5. Schlieren of the Salmon Mountain stock 30 6. Modal plagioclase-quartz-alkali feldspar diagram for rocks of the Salmon Mountain stock 35 7. Plagioclase-quartz-alkali feldspar diagram for plutons of the Klamath Mountains 57 VI CHAPTER I PURPOSE AND BACKGROUND Purpose The purpose of this study is to characterize the petrology, chemistry, and eraplacement history of the Salmon Mountain stock, and to discern its relation to plutonic belts lying to the north and to the south of the study area. The Salmon Mountain stock is located in a zone between calc-alkaline plutons to the north and mildly alkaline plutons to the south (Barnes, personal coraraunication). Knowledge of the petrology and chemistry of this body should indicate which group, if either, the Salmon Mountain stock belongs to. Furthermore, the Salraon Mountain stock is oriented east-west, transverse to the regional north-south orientation of most Klaraath plutons. Studies of the emplacement history of this body may add to our understanding of rifting and terrane accretion in the Klaraath Mountains (e.g. Saleeby et al., 1982). The Klamath Mountains are thought to be the remnants of ancient island arc systems that formed along the western margin of North America (Irwin, 198I, I98U; Saleeby et al., 1982; Wright, 1982). Arcuate belts of granitic to gabbroic plutons in the Klamath Mountains presumably represent magma charabers that fed these volcanic arcs. Plutonic belts were first recognized in the Klamath province by Lanphere et al. (1968) on the basis of sparse K-Ar dates. With the 1 2 acquisition of raore radioraetric age dates on plutons, twelve plutonic belts are now thought to be present (irwin, I98U). The Salmon Mountain stock is located in a zone between the northern end of the Ironside Mountain plutonic belt and the southern end of the Wooley Creek plutonic belt of Irwin (198U) (Figure l). The Ironside Mountain plutonic belt trends northwest, is raid-Middle Jurassic (about I60 to 166 my) in age, and is thought by Irwin (l98it) to have forraed prior to accretion to the Klamath province. The Wooley Creek plutonic belt trends northeast, is late-Middle Jurassic (about 163 ray) in age, and Í8 believed by Irwin (l981t) to be postaccretion in origin. Inforraation gained from the study of the Salmon Mountain stock will show if the zone between these belts is: l) transitional, 2) a member of one of these belts, 3) unrelated to either belt. Postaccretion versus preaccreion relations in this area can then be further examined in light of this information. Location The gabbroic to trondhjemitic Salmon Mountain stock is located at Salmon Mountain, eleven miles east-southeast of Orleans, California, in the Salraon Mountain 7.5 rainute quadrangle. Elevations in the study area range from 1,707 meters at Red Cap Lake to 2,120 meters at the summit of Salmon Mountain. Access is via unpaved road frora Orleans, California, plus 3.5 miles on an iraproved trail. From'^î^lí n ^98^!°"'' "'"' °' *^' '''"""*'^ Mountains. x = study area. nriiiiTiii rLVTIIIC lElTS riSTUCIETICI riEtcciniti IIST of D«Tco mnoHS k Athltnd Aflnon Rldgt Btlln Gulch B«ar Hountiln klr Ullloa Clrlbou Hountlln Cisllc Crigl CrlcKer HcidOM Criggy Ptik Dcidmln Pcik (l>t Fork Engllsh Pcik Forkl of SllWfl Grcybick GC Glcn Crcck GH Golll Hlll Grtnts Pist Horsctnoc lakc IronsiOc Hcuntctn Jlckson>l1lc Lower Coon Hountiln Hulc hsuntlln No name gibbro Plt Itlvcr flustlln Pcik Sllnkird Siddlc Gjlcn Stir î^untaln Sugar Plnc Veta eluff uoolcy Crctk Walktr folnt Unltt Kock Tcllo> Buttc 40'-f Vo> *> -(fe *^ '^ *'^ ^%:'*:%.''^.:\^ •<> % V Study Methods Field studies were performed during the summer of 198î|. Mapping was completed on 1:6,000 and 1:12,000 scale enlargements of the Salmon Mountain 7.5 minute quadrangle, using brunton compass and altimeter methods. Lab studies were done at Texas Tech University during the 198I+-85 academic year. Plagioclase anorthite content was determined using the A-norraal, Michel-Levy and Carlsbad twinning methods on a flat stage petrographic microscope. All raodal analyses are based on at least one thousand points per thin section. Regional Geology Subprovinces The Klamath Mountains form an elongate north-trending geological province located in northwestern California and southwestern Oregon and occupy more than 11,000 square miles between the Cascade province to the east and the Coast Range province to the west (Figure 2). The province contains predominately eugeosynclinal sedimentary, raetasedimentary, and metavolcanic rocks that are intruded by numerous granitic plutons. Rocks of the Klamath Mountains that were involved in the late Jurassic Nevadan orogeny will be briefly discussed in this section. Overlying weakly deformed rocks occurring sparsely throughout the Klamath province, such as the Great Valley sequence at the extrerae southern part of the province, will not be considered. Figure 2. Subprovinces and major plutons of the Klamath Mountains. EK: Eastern Klamath belt, SM: Central Metamorphic belt, TrPz: western Paleozoic and Triassic belt, WJ: western Jurassic belt, CMS: Condrey Mountain Schist, AP: Ashland Pluton, VB: Vesa Bluffs, GP: Grants Pass, GB: Greyback, IMB: Ironside Mountain batholith, EP: English Peak, WCB: Wooley Creek batholith, SL: Slinkard, BMC: Bear Mountain coniplex. After Irwin (1981). study area *''*" Of fflop 8 The Klamath Mountains have been divided into four arcuate, north- trending subprovinces, or lithic belts, separated by thrust faults that dip to the east, so that each belt is thrust over the adjacent belt to the west (irwin, 196O, 1966) (see Figure 2). From east to west, these belts are the eastern Klamath belt, the Central Metamorphic belt, the western Paleozoic and Triassic belt, and the western Jurassic belt. These belts are differentiated on the basis of lithology and structure, and in a general sense become progressively younger from east to west. The rocks range in age frora Ordovician to Jurassic, as determined by radiometric dating (Lanphere et al., I968) and sparse fossil evidence (lirwin, I966; Irwin et al. 1978). The eastern Klamath belt consists of an eastward dipping horaoclinal sequence of eugeosynclinal clastic sediments and volcanic rocks that exhibit variable degrees of low-grade metaraorphisra and have an aggregate thickness of as much as 15,2^0 meters (irwin, I966; Davis, 1966; Hotz, 1971)« These rocks range in age from Ordovician to Jurassic as shown by fossil evidence (irwin, 1978, 1981) and uranium- lead and rubidium-strontium dating methods (Lanphere et al.,1968; Mattinson and Hopson, 1972). The central raetaraorphic belt is separated frora the eastern Klaraath belt on the east by discontinuous elongate bodies of ultramafic rocks. The central metaraorphic belt consists of the raetavolcanic Salraon Hornblende Schist and the overlying raetasediraentary Abraras Mica Schist (Davis et al. , I965). Primary metamorphism of the Salmon Hornblende Schist and the Abrams Mica Schist is believed to have occurred approxiraately 380 Ma (Devonian) (Lanphere et al., I968). 9 The western Paleozoic and Triassic belt is a complex eugeosynclinal terrane coraposed of thinly bedded cherts, argillites, metaserpentinites, mafic volcanic rocks, detrital rocks and coarsely crystalline limestone lenses. These rocks are generally metamorphosed to the lower greenschist facies, but locally attain grades as high as upper amphibolite facies near the Condrey Mountain Schist and pyroxene-hornfels facies near the margins of intrusions (irwin, 1966; Davis, 1966; Barnes, I983, Snoke et al., I981). The age of this most extensive of the four belts is not tightly constrained, but radiolarian fossil evidence suggests a mid-Perraian to Early Jurassic age (Hotz, 1971; Irwin, 1972; Irwin and Galanis, 1976; Irwin et al., 1977; Irwin et al., 1978).
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