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Description
The Santiago Peak Volcanics (SPV) consist of a belt of weakly metamorphosed volcanic and volcaniclastic rocks exposed discontinuously along the western margin of the Peninsular Ranges batholith (PRB). New mapping, petrography and whole rock geochemistry in the Mission Gorge - San Diego area is presented and discussed in light of previous studies to evaluate geographic differences in the petrology and geochemistry of the SPV. The SPV in the Mission Gorge - San Diego area, the focus of this study, contain very low grade metamorphic mineral assemblages characteristic of thermal and hydrothermal metamorphism. There is no penetrative metamorphic fabric indicating regional metamorphism. Based upon silica content the dominant rock type are andesites (andesitic breccias) with decreasing amounts of basaltic andesite, dacite, and rhyolites. There are no fundamental differences in major-element chemistry between new data reported here and that of previous studies. The combined data set demonstrates that the SPV ranges from basalt to rhyolite in composition and is dominated by andesite. Basalts are rare. The SPV is a strongly subalkaline suite. They have both tholeiitic and calc-alkaline affinities based on a variety of major and trace element criteria. The low titanium content indicates a depletion of the high field strength elements. Variable LIL enrichment are indicative of low to medium-K volcanic suites. Despite the overall coherence of the geochemical data the SPV exhibits both transverse and longitudinal geochemical variations. Three magmatic groups are tentatively recognized. One group is a low silica high magnesium group primarily located within the Santa Ana Mountains and Camp Pendelton areas. This group consists of basaltic andesite and andesite dominate. A second a more diffuse mid-range silica and magnesium group that is located primarily within the Mission Gorge and San Diego area, but does overlap into the Santa Ana Mountain and Camp Pendelton areas. This second group has more andesite, dacite, rhyodacite, and rhyolite present. The third group consists of high-silica low-magnesium volcanic rocks that are primarily rhyodacite to rhyolites and are identifiable throughout the SPV. The SPV are similar in term of whole rock geochemistry to plutonic rocks of the western zone of the PRB. It is suggested that these rocks represent volcanic and plutonic components of the same late Mesozoic Andean-type continental margin magmatic arc.
