GEOCHEMICAL AND PETROGRAPHIC ANALYSES OF THE BASALTS OF THE DARWIN PLATEAU, INYO COUNTY, CA; EVIDENCE FOR MULTI-DIMENSIONAL VARIATION GOVERNING THE PRODUCTION OF VOLCANIC FIELDS IN THE OWENS VALLEY By Matt Lusk Geological Sciences Department California State Polytechnic University Pomona, CA Senior Thesis Submitted in partial fulfillment of requirements for the B.S. Geology Degree 1 Table of Contents ABSTRACT ………………………………………………………………….1 INTRODUCTION ………………………………………………………….2 Generalized Geology of Southern Inyo Mountains ………………….3 Late Cenozoic Geology of Adjacent Fields ………………………….5 Tectonics ………………………………………………………….6 PETROLOGY ………………………………………………………….9 SAMPLE PREPARATION FOR XRF ………………………………….13 GEOCHEMISTRY ………………………………………………………….14 DISCUSSION ………………………………………………………….18 CONCLUSION ………………………………………………………….26 REFERENCES CITED ………………………………………………….28 APPENDIX A (Basalt Sample Correlation map and Figures) ………….31 APPENDIX B (Hand Sample Description) ………………………………….33 APPENDIX C (Table of Major Oxides) ………………………………….35 APPENDIX D (Table of Trace Elements) ………………………………….37 APPENDIX E (Table of Normative Mineralogy) ………………………….39 2 ABSTRACT The Darwin Plateau lies between the Inyo Mountains to the north, and the Coso Range to the south. A series of basaltic flows and cones were emplaced on the plateau from 8 Ma to 4 Ma. The Coso field (2 Ma-Pres.) lies 50 km to the southwest and the Ricardo volcanics (10-8 Ma) 100 km to the southwest. Sixty samples from basalt flows and cones of the Darwin Plateau were analyzed for major, minor and trace elements. On a Le Bas diagram the basalts show a considerable range in composition from basalt and basaltic andesite, to trachybasalt and basaltic trachyandesite; straddling the alkaline-subalkaline boundary line. Major element and trace ele- ment geochemistry are remarkably consistent within individual flows, but vary non- systematically between adjacent flows. When plotted on a basalt tetrahedron the Darwin basalts again show considerable variation in composition, however the majority are olivine tholeiites. This is in marked contrast to the Ri- cardo volcanics which are quartz-normative tholeiites and the Coso volcanics which are alkali basalts. Petrographic examination of Darwin basalts reveals only small amounts of partially altered olivine, unlike the complete olivine replacement by iddingsite in the Ricardo field and large, unaltered phenocrysts of olivine in the Coso volcanics. These differences may be related to evolutionary trends for the volcanic fields of the southern Owens Valley. Older Ricardo volcanics are quartz normative, Darwin is neither quartz nor nepheline normative, and Coso is distinctly nepheline normative. This can be attributed to variation in the thermal regime represented by differing degrees of partial melt or depth of melt- ing, and/or dissimilarity in water content and oxygen fugacity of the magma. 1 INTRODUCTION The Darwin volcanic field sits atop a horst known as the Darwin Plateau. It is located in the southwest portion of the Great Basin within the Inyo Mountain Range (Fig. 1). The Darwin Plateau sits approximately 1,670 meters above sea level. It displays a relatively moderate to- pography when compared to the steep escarpments of the Panamint Valley to the east and Owens Valley to the west. Local flora is characterized by sage brush, alkali grasses, and sev- eral cacti species that are able to sustain existence in the high desert environment. The Darwin Plateau basalt flows and cones were sampled in June and July of 2006. Volcanic rocks of the Darwin Plateau and adjacent Pinto Peak (Nova basalts) have been ana- lyzed by past researchers (Coleman and Walker, 1990; Schweig, 1989); however a comprehen- sive study of Darwin Plateau has not been undertaken and detailed maps with ages of individual cones and flows does not exist. The goal of this research was to generate a sample population that could possibly delineate indi- vidual flows (based on similar bulk and trace element chemis- try); provide an accurate basis for overall composition of the Dar- win volcanic field; and identify any variation therein. Sample data could then be compared to other volcanic fields within and near to the Owens Valley. Sam- ple localities and a generalized chemical composition map, cor- relating flows, is presented in Appendix A. Figure 1. Location of Darwin Plateau as well as other volcanic fields of the Owens Valley. 2 Generalized Geology of Southern Inyo Mountains Devonian, Pennsylvanian, and Permian marine sedimentary and metasedimentary rocks make up a large volume of the basement rocks within and adjacent to the study area (Fig. 2). These rocks are intensely folded; the folds best exposed in the areas of Keeler and Rainbow Canyon. The Paleozoic rocks have been intruded by post-kinematic Mesozoic granites (Taylor, 2002). Devonian rocks consist of the Lost Burro formation which is comprised of light-gray dolomite, (prominently striped with nearly black limestone and dolomite), interbedded light-and dark-gray dolomite, quartzite, and sandy or cherty dolomite. The Stewart Valley Formation is comprised of dark-gray limestone, cherty limestone, sandstone, quartzite, and conglomerate (Death Valley Sheet, California Division of Mines and Geology, 1974). Pennsylvanian units are exposed to the north and south of Darwin Plateau. These in- clude the Bird Spring Formation, Keeler Canyon Formation, Lee Flat Limestone, and Rest Spring Shale. The Bird Spring Formation consists of pebbly sandstone, coarsely bioclastic limestone, sandy limestone, limestone, dolomite, and chert nodules. Keeler Canyon Formation contains bluish-gray limestone, shaly limestone, black siliceous shale, pink fissile shale, and limestone breccia. Lee Flat Limestone is comprised of white to light-gray marble, light-brown dolomite marble, dark-gray limestone, and chert lenses. The Rest Spring Shale consists of ovine-gray to olive-brown argillaceous shale and siltstone (Death Valley Sheet, California Divi- sion of Mines and Geology, 1974). Permian marine rocks contain sections of the Owens Valley Formation; gray, brown, red, and yellow conglomerate, quartzite, sandstone, siltstone, shale, limestone, and limestone breccia. Triassic marine sedimentary and metasedimentary rocks make up a small part of the southern Inyo Mountains; they are comprised of limestone and shale (Death Valley Sheet, Cali- fornia Division of Mines and Geology, 1974). To the northwest of the Darwin Plateau, a section of the Inyo Mountains has been deemed the Inyo Mountains Volcanic Arc Complex by Dunne and others (1998). These arc- flank rocks consist of marine and non-marine units interspersed with periods of volcanism that lie unconformable atop the Paleozoic metasedimentary strata. The complex was intruded by the Jurassic French Springs Formation; U-Pb dating gives an age of 148 Ma for the French Springs 3 Figure 2. Stratigraphic column of the Darwin Plateau geology. 4 Granite (Dunne et. al, 1998). The Volcanic Complex is comprised mostly of epiclastic and py- roclastic rocks; ranging from rhyolite tuff to basalt as well as sandstones, mudstones, conglom- erates, and fluvial/debris flows. The Paleozoic/Mesozoic basement rocks are overlain unconformably by the two domi- nant rock types exposed on the plateau; Miocene-Pliocene volcanics (basaltic rocks age 4-8 Ma) and alluvial/colluvium fanglomerates and stream deposits (Schweig, 1989; this study). Late Cenozoic Geology of Adjacent Fields The Coso volcanic field is located approximately 50 km south-southwest of the Darwin Plateau. Two major periods of volcanism have occurred; the first approximately 4 to 2.5 Ma and the second 1.1 to 0.04 Ma. Basalt, rhyodacite, dacite, andesite, and rhyolite were erupted with rhyolite being the most voluminous rock type (Groves, 1996). Rhyolite flows and domes are all high-silica rhyolite; xenocrysts of basalt and mafic inclusions are present however rare. Slightly alkalic basalts were erupted during the same time as the rhyolite, however the basalts were always derived from vents peripheral to the rhyolitic fields. Coso, today, continues to be associated with a high heat flux and hydrothermal activity (Bacon, 1982; Groves, 1996). Big Pine volcanic field lies approximately 80 km north-northwest of Darwin Plateau, stretching from Independence to Big Pine, CA, a distance of about 20 km. The field lies along the flank of a graben, proximal to oblique range-front faults. The field ranges in age from 1.2 Ma to 500 ka and shows a trend of decreasing age to the northwest (Bierman et al., 1991). The rocks of the field vary from subalkaline basalt and basaltic andesite to alkaline basalts, trachybasalts, and basaltic trachyandesite. One rhyolite dome/flow is present within the Big Pine field (Varnell, 2006). The Ricardo volcanics are located approximately 100 km south-southwest of the Darwin Plateau. These are the oldest Cenozoic volcanics in the region at 10.1 to 8.2 Ma. They consist of approximately twenty-three individual flows ranging from basalt to andesite as well as two notable rhyolite flows. The volcanics are commonly interbedded with fluvial/lacustrine sedi- ments. Basaltic rocks are dominantly tholeiitic and believed to be derived from a shallow, lithospheric source (Anderson, 2005). 5 The volcanic rocks of Pinto Peak (Nova basalts) are located on the eastern side of the Panamint Valley, adjacent to the Panamint and Hunter Mountains. They are Miocene-Pliocene in age, range from basalt to rhyolite and are characterized by high total alkalis. It has been speculated,