USING LA-ICPMS U-PB ZIRCON AGES AND HF ISOTOPES TO DETERMINE THE TEMPORAL AND GEOCHEMICAL RELATIONS OF VOLCANIC, PORPHYRY, AND PLUTONIC ROCKS IN THE JACKASS LAKES IGNEOUS COMPLEX, SIERRA NEVADA BATHOLITH

The Jackass Lakes pluton (JLP) is a late-Cretaceous composite pluton in the central Sierra Nevada of California that intruded into roughly coeval volcanic and porphyry host rocks of the Merced Peak caldera complex. Plutons that have preserved related volcanic and hypabyssal porphyry materials are rare, making the JLP igneous complex a great place to study the petrologic volcanic-porphyry-plutonic connection to determine if the system is complementary or equivalent in composition. If the volcanic and porphyry rocks are fractionates from the plutonic rocks now composed of crystal cumulates, the three units represent a complementary system. The alternative end-member model is that their composition is the same and thus the three units are compositionally equivalent. In order to examine such petrologic connections, one must first ascertain which volcanic, porphyry, and plutonic rocks are coeval.

The volcanics in the JLP are meta-rhyolites, meta-dacites, and meta-andesites, while the porphyries are leucogranites, and the plutonic units are largely hornblende biotite granodiorites. Using U-Pb zircon dating, Stern et al. (1981) first dated the JLP to 98 Ma. McNulty et al. (1996) confirmed this age using U-Pb zircon geochronology in the northeastern JLP, dated at98.5 ± 0.3 Ma, and a southwestern sample, dated at 97.1 ± 0.7 Ma. The volcanic rocks of the JLPs eastern boundary were dated ca. 132–144 Ma by Fiske and Tobisch (1994). Six volcanic samples obtained from meta-rhyolites and meta-dacites of the Merced Peak caldera sequence from pendants across the JLP were dated with U-Pb zircon SHRIMP methods to 99-97 Ma. While there are limited age dates on the volcanic and plutonic rocks in the JLP, the Post Peak porphyry and related rocks have not yet been dated.

This study provides new LA-ICP-MS U-Pb zircon ages from five samples of various JLP hornblende biotite granodiorite units, two dacitic metatuffs, and two leucogranitic porphyry samples from the NW and N JLP. Additionally, hafnium isotope analysis on the same zircons allow for an examination of the magma source. Together with petrography and XRF whole rock element analysis, they will also provide additional datasets to compare the three same age units with one another to start testing above hypotheses.