PETROLOGIC AND GEOCHEMICAL LINKS IN THE JACKASS LAKES VOLCANIC-PLUTONIC COMPLEX, SIERRA NEVADA BATHOLITH

The 99-97 Ma Jackass Lakes pluton (JLP) within the central Sierra Nevada batholith consists mainly of granodiorite, which intruded into rhyolitic/dacitic volcanic ejecta. Additionally, various fine-grained leucogranite bodies are distributed throughout the JLP. U-Pb zircon geochronology indicates all units are coeval between 99 – 97 Ma. This timing relationship is essential to determine the petrologic connection of the volcanic–plutonic units.

Our hypothesis is that the granodiorites of the JLP are compositionally complementary to the meta-rhyolites/dacites and leucogranites, the latter of which formed through melt-extraction from a JLP magma reservoir, leaving behind granodioritic crystal cumulates. The leucogranites and meta-rhyolites/dacites are compositionally equivalent, and the leucogranites may represent un-erupted melt-rich magma pools.

Ti-in zircon temperatures are <850°C in all units. Zircon hafnium isotopes show εHf values ranging between +5 and -5 across all rock units, indicating a similar source. Whole rock Sr isotope ratios differ only slightly between rock units. Petrographic observations and XRF bulk-rock analyses support a complementary relationship between some granodiorites and leucogranites, further corroborated by crystal accumulation of plagioclase in thin sections. Crystal-melt separation is inferred from granodiorite and leucogranite bulk-rock Ba and Zr vs. SiO₂ plots. At 68 wt.% SiO₂ the two "unmix" from the linear trend, likely reflecting a magma crystallinity conducive to crystal-melt separation at this composition. This magma mush was subsequently intruded by younger granodiorites that did not “unmix.” Volcanic samples exhibit a spread between the granodiorites and leucogranites, suggesting a weak complementary/equivalent relationship with the two. Zircon trace element data are further testing this hypothesis.

Our results support a common magma source for all JLP rocks and a complementary relationship of early granodiorites to leucogranites and some metavolcanics that underwent crystal-melt separation in a ca. 2-myr long active magma mush. However, the leucogranites and metavolcanics largely do not appear to be direct equivalents to one another. We suggest that the metavolcanics relate to an earlier melt extraction event, while the leucogranites pooled and solidified late.