Geochemistry and petrography of the Santiago Peak volcanics, Santa Margarita and Santa Ana Mountains, Southern California

Thesis, (M.S.) Geological SciencesScale of accompanying map on Plate I: 1:24,000Title of accompanying map on Plate I, folded in pocket: Outcrop map of the Case Spring area of Camp PendletonGorzolla???s base map is taken from the northwestern part of the of the Margarita Peak, CA 7.5-minute Quadrangle. Margarita Peak 7.5-minute Quadrangle, contour interval 20 feet, 1968, photorevised 1982. Map edited and published by the Geological Survey. Control by USGS, NOS/NOAA, and USCE. Topography by photogrammetric methods from aerial photographs taken 1946. Field checked 1949. Revised from aerial photographs taken 1967. Field checked 1968. 1927 North American datum. 10,000-foot grid based on California coordinate system, zone 6. 1000-meter Universal Transverse Mercator grid ticks, zone 11. Contour interval 20 feet. Datum is mean sea levelBibliography: pages 124-128The Santiago Peak Volcanics represent an island arc system which existed adjacent to the North American continent during Jurassic to Early (?) Cretaceous time. They are typically composed of mildly metamorphosed volcanic, volcaniclastic and sedimentary rocks that crop out in a N-S trending belt from San Diego to Riverside, California. The objectives of this thesis were to outline and define the geochemical character of the Santiago Peak\ud Volcanics, deduce the tectonic environment of this ancient\ud island-arc system from the stratigraphic, geochemical and\ud petrographic relationships, and to constrain the polarity and orientation of the paleo- subduction zone. XRF analyses of major and selected trace elements for the basaltic and andesitic flows show that the ancient\ud island arc can be divided into a calc-alkaline series and a tholeiitic series. The calc-alkaline rocks are exposed\ud in the northern portion of the volcanic belt and the tholeiitic rocks are found in the south. Geochemical plots, such as FeO*/MgO-SiO2, FeO*/MgO-FeO*, AFM and Zr-Ti-Sr diagrams, were used to distinguish between the two\ud suites of rocks. These trends are similar to those in both the Japanese and Aleutian arc systems. The Japanese arc exhibits an across-arc change from tholeiitic rocks nearest the trench to calc-alkaline rocks toward the\ud continent whereas the Aleutian arc shows a trend from tholeiitic to calc-alkaline nature along strike of the arc. Whether the variation in chemistry is an across-arc\ud or an along-arc trend, the conclusion is the same. The\ud subduction zone was located parallel to the rocks of tholeiitic character and must have dipped to the northeast under the present-day exposures of the Santiago Peak Volcanics