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Masters Thesis

Geochemical and petrogenic study of the Late Cretaceous Mt. Wilson Intrusion and possible comagmatic plutons, San Gabriel Mountains, Southern California

The Mt. Wilson Intrusion is a ~125 km2 Late Cretaceous pluton located north of the San Gabriel fault within the San Gabriel Mountains of the central Transverse Ranges. Subduction of the Farallon plate under the North American plate during Late Cretaceous time resulted in the Mt. Wilson Intrusion, as well as other plutons in the San Gabriel Mountains including the Josephine Mountain Intrusion, the Waterman Mountain Intrusion, and a group of intrusions located within the San Antonio terrane. The Mt. Wilson pluton ranges in composition from diorite to granite, includes magmatic epidote, and is interpreted to have been emplaced at a depth of 22 km in a continental margin arc. Mt. Wilson samples belong to the calc-alkaline magma series, have medium to high K20 concentrations, are relatively alumina rich, and exhibit a smooth transition from metaluminous to peraluminous compositions as silica increases. Samples plotted on major-oxide and trace-element Harker diagrams exhibit broadly smooth variations with respect to Si02. On a REE diagram, samples have overall LREE enrichment, have LaN values averaging -80 times relative to chondrites, and lack a significant Eu-anamoly. On spider diagrams, samples show trends, Nb-Ta trough, conventionally interpreted as resulting from subduction-zone magmatism. Isotopic data from the Mt. Wilson samples show the following ranges: initial 87Sr/86Sr = 0.70546 to 0.70930, 143Nd/144Nd = 0.5 11749 to 0.5 123 02 (ENd = -6.6 to -17.3), 8180qu= 9.0 %0, 206Pb/2�4Pb = 18.758 to 19.703, 207Pb/2�4Pb = 15 .646 to 15.666, and 208Pb/204Pb = 39.038 to 39.162. The Mt. Wilson Intrusion is generally isotopically similar to the underlying 1.4 Ga Mendenhall gneiss. Isotope plots show the Mt. Wilson, Josephine Mountain, and San Antonio Canyon Intrusions all have assimilated crustal material during their petrogenesis. All three intrusions plot in the lower right quadrant of the Sr-Nd covariation plot and above the Northern Hemisphere Reference Line in both 208Pb/204Pb and 207Pbi2�4Pb versus 206Pbi2�4Pb plots. Distinctions occur among rock units and their terrane location on oxygen isotope diagrams; the Mt. Wilson and Josephine Mountain Intrusions, part of the San Gabriel terrane, have 8180 values less than 10 %0, whereas San Antonio canyon samples, part of the San Antonio terrane, have 8180 values that exceed 10 %0. Mineralogical and geochemical data from the the Mt. Wilson Intrusion were compared with those of the coeval Josephine Mountain Intrusion and San Antonio Canyon Intrusions in order to assess whether these bodies are comagmatic. Like the Mt. Wilson Intrusion, the Josephine Mountain Intrusion is calc-alkalic, medium to high K20, and alumina rich. In contrast to the Mount Wilson Intrusion, the Josephine Mountain Intrusion has a distinct bimodal classification of rocks and has distinct spider diagram patterns and Sri and Ndi values. The Late Cretaceous San Antonio Canyon intrusive rocks are also calc-alkalic medium to high K20 , and alumina rich. However, San Antonio Canyon rocks, were emplaced at shallower crustal levels (-15 km) and have different trace-element patterns and 8180 values than Mt. Wilson samples. These differences indicate that although the three units may be coeval, the Mt. Wilson Intrusion is not comagmatic with either the Josephine Mountain Intrusion or the San Antonio Canyon Intrusions. The only two Late Cretaceous igneous units in the San Gabriel Mountains that might be comagmatic are the Mt. Wilson and Waterman Mountain Intrusion, which appear to have been offset from one another along 23 km of right-lateral strike-slip motion along the San Gabriel fault.

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