TRACKING MAGMA SOURCE AND PROCESS SINGALS USING TRACE ELEMENTS IN HORNBLENDE IN THE BUCKS LAKE PLUTON, NORTHERN SIERRA NEVADA, CALIFORNIA

The Bucks Lake pluton, emplaced ca.139 Ma (Langenheim et al. 2021) during a magmatic lull, exhibits reverse zonation. The oldest, central zone consists of a two-pyroxene diorite that grades into pyroxene-hornblende diorite, a hornblende diorite transition zone containing mafic magmatic enclaves, and an outer zone of hornblende-biotite-quartz diorite (Hietanen, 1973). This study explores whether the pluton's compositional variation primarily reflects signals from magmatic sources or emplacement-level processes through preliminary petrographic observation and LA-ICP-MS analysis of hornblende in enclave and host between the central and transitional zones

The host rock diorite contains plagioclase (40-60%), hornblende (25-50%), biotite/chlorite (15-30%), pyroxene (10-20%), K-feldspar (5-10%), and quartz (5-10%). Hornblende grains are green or yellow and subangular or rounded, with very finely cleaved and fractured grains appearing next to regularly cleaved grains. Some hornblende crystals display patchy zoning, and others display hornblende-to-biotite replacement and embayment.

In contrast, the enclave contains plagioclase (40%), hornblende (25%), biotite (15%), and pyroxene (10%), with minor quartz and K-feldspar. Millimeter-sized clusters of hornblende are unzoned and regularly cleaved.

Trace element analysis of the hornblendes indicates distinct magmatic sources between units, shown by contrasting Sr, Nb, Ba, La/Lu and Ti against Zr, Si, and Ti indices. Enclave and host trends are distinct in slope and position, precluding a shared fractional crystallization event. Core-to-rim zoning patterns identified two closely related populations in the host diorite, and a third, distinct population in the enclave. REE patterns show pronounced core-rim variations in host, with elevated cores and Eu anomalies while enclave hornblendes show uniformly flat patterns and minimal Eu anomalies.

While magmatic affected both enclave and host samples, initial interpretations suggest that the enclaves represent a less evolved, mafic source unrelated to the host rock. The processes that did occur in the host included fractionation and mixing, involving resorption of core-rim zones. Future work will expand the dataset to the primary units, to evaluate the signals from central to outer zones.