JURASSIC MAGMATISM IN THE EASTERN SIERRA NEVADA AND MOJAVE DESERT SOURCED IN DISTINCTIVE ANCIENT LITHOSPHERE

In the eastern Sierra Nevada between 36.5 and 38°N, broadly coeval mid-Jurassic plutonic and volcaniclastic rocks preserve a record of mafic to silicic arc magmatism during the second pulse of the Mesozoic California arc. The Jurassic arc at this latitude is composed principally of the Palisade Crest Intrusive Suite (PCIS; Bateman, 1992) with ages between 173 and 165 Ma and remnants of the arc volcanic cover, including mafic breccias, flows and silicic ignimbrites in the Oak Creek, Mount Morrison, eastern Ritter Range and Saddlebag Lake pendants with ages between 175 and 164 Ma. The PCIS ranges from mafic granodiorite through porphyritic granodiorite to leucogranite. Ignimbrites erupted during assembly of the underlying PCIS range from dacite to rhyolite in bulk composition, and are petrographically similar to modern arc‐type, monotonous intermediate dacite or phenocryst‐poor, low‐silica rhyolite. Trace elements abundances in dated zircons form parallel arrays when compared to Ti (T-dependence), indicating that Jurassic silicic melts were relatively high T at zircon saturation. Trace element ratios in zircons from both intrusive and extrusive rocks suggest silicic melt compositions were very similar during intrusive and extrusive rock evolution, but had distinctly high melt Th/U, Ce/Yb and Ce/U compared to calculated melts of the earlier Triassic and later Cretaceous magma pulses. Calculated melt compositions suggest that the Jurassic magma pulse tapped a distinct, cratonal lithospheric source not present in the earliest Mesozoic juvenile stage of arc construction. This source is common to Jurassic through Neogene plutonic rocks of the Mojave Desert region, suggesting a long-lived and distinctive geochemical reservoir with a key role in California arc magmatism.