ORIGIN AND ALTERATION OF TRIASSIC METAVOLCANIC ROCKS IN THE SADDLEBAG LAKE PENDANT, EASTERN SIERRA NEVADA

The Saddlebag Lake pendant, east of Yosemite National Park in the Sierra Nevada, contains lava and ash flows, ranging from andesite to rhyolite, that are intruded and contact metamorphosed by younger granitoids, most of which were emplaced during assembly of Sierra Nevada batholith. The volcanic sources of these deposits have not been characterized in detail, however, this research seeks to determine the magmatic origin of these volcanics, their eruptive style, and their later hydrothermal alteration and deformation. Ultimately this information sheds light on the volcanic systems operating on the western edge of the North America in the Triassic.

Field relations of imbricated pebble conglomerates interbedded with the deposits denote subaqueous eruption. Compressed fiamme indicate that some units were erupted as ash flows. Metamorphism of the pendant to greenschist–amphibolite hornfels facies has resulted in strong foliation and recrystallization of original groundmass to quartz and biotite±muscovite, while maintaining original quartz and plagioclase phenocrysts. Whole rock geochemistry shows that the volcanics range from 54 to 76 wt% SiO₂, and have high total alkalis (9.3 wt% Na₂O and 4.3 wt% K₂O) suggesting alteration. The volcanics are LREE enriched, with La/Lu ratios of 5.8–14.1, Ce concentrations of 41.0-75.1 ppm (n=10), as well as negative europium anomalies in two samples. Oxygen isotope analysis yields magmatic δ¹⁸O values in zircon of 5.9-6.3‰ (n=3) that show a crustal component in the magma source, and also generate a whole-rock magmatic δ¹⁸O of 8.1–8.2‰. Values of δ¹⁸O(quartz) from the same rocks are higher and more variable (11.7-15.0‰; n=4), verifying sub-solidus hydrothermal exchange of the phenocrysts. Metamorphic garnet (7.4‰, n=2) records disequilibrium fractionation with zircon to higher values than predicted by magmatic fractionation (Δ_Zrc-Grt ≈ 0‰), indicating the crystallization of garnet after hydrothermal alteration. Epidote δ¹⁸O values (7.2‰, n=2) are also consistent with their hydrothermal origin suggested by the prevalence of epidote nodules and veins in the deposits.