INSIGHTS INTO MAGMA RESERVOIR PROCESSES FROM EARLY MESOZOIC EXPLOSIVE VOLCANISM IN CALIFORNIA

The magma mush model relates production of volcanic rocks to crystal-liquid separation, and can be studied realistically by applying a comparison of a system’s extrusive volcanic rocks to its underlying plutonic rocks. The Sierra Nevada is relevant to this topic in that the products of multiple magmatic systems, with ages ranging from Late Triassic to Late Jurassic, are exposed as a result of regional steep tilting of upper crustal rocks. Late Triassic ash-flow tuffs of the Koip sequence in the Saddlebag Lake pendant and coeval rocks in the Mount Morrison pendant are rhyolitic, with 70-75% silica, and overlie granodioritic to granitic plutons of the Triassic Scheelite Intrusive Suite. Middle Jurassic ash-flow tuffs in the Oak Creek pendant are rhyodacitic, with 68-72% silica, and overlie granodioritic to granitic plutons of the Jurassic Palisade Crest Intrusive Suite. Both Triassic and Jurassic ash-flow tuffs have compositional similarities to lower-silica Group 1 or crystal-poor rhyolitic Group 2 tuffs because of gradational compositional zoning and phenocryst texture and abundance. All volcanic rocks have been hydrothermally altered and metamorphosed progressively with time based on metamorphic- and hydrous-mineral abundance, foliation and quartz deformation. Compositional similarity to modern analogues, such as the Andes magmatic arc, reinforces a model of a subduction zone tectonic setting. Triassic tuffs are similar in composition and probably had a similar origin to felsic granodiorite, but Jurassic tuffs are higher in silica than intrusive rocks and may record crystal-liquid separation leading to rhyolite volcanism. A long-term cycle can be tested by classifying and comparing the progression of individual systems to indicate a chronological pattern of eruptions.