INVESTIGATING ENCLAVE - MAGMA INTERACTIONS IN THE GRANODIORITE OF GRIZZLY CREEK, WEST-CENTRAL SIERRA NEVADA, CALIFORNIA

Mafic magmatic enclaves can provide a window into magmatic processes operating within plutons, and recent studies have documented evidence of fractional crystallization, magma mixing, and magma mingling between enclaves and their surrounding host magmas. This study analyzed enclave-host sample pairs within the 104.2 ± 1.2 Ma Granodiorite of Grizzly Creek to determine whether distinct enclave populations resulted from varying degrees of magmatic interaction with the host.

The porphyritic Kgc contains equigranular plagioclase, quartz, hornblende, and biotite with mm-cm sized plagioclase phenocrysts. Preliminary observations of enclave hand samples identified two populations based on matrix grain size, phenocryst content, and border characteristics. Enclaves classified into Group 1 are characterized by a coarser matrix, fewer phenocrysts, and diffuse boundaries while Group 2 enclaves exhibit a fine-grained matrix, more phenocrysts, and a subset have sharp contacts. Both enclave populations were found across all field locations which suggests a localized process to explain the variations between groups.

Petrography highlighted textural and compositional variations between the enclave populations and between enclave-host pairs. Kgc samples contain larger, oscillatory-zoned, or inclusion-rich plagioclase and alkali feldspar phenocrysts, that are absent from enclave groundmass, but present in some enclave phenocrysts (1-6% abundance). Enclaves are primarily plagioclase (55-60%) and biotite rich (35-40%) and contain varying amounts of hornblende (4-9%) and quartz (2-7%).

Preliminary interpretations of the samples suggest that Group 1 enclaves appear to have experienced minimal mineral exchange with the surrounding host rock compared to enclaves in Group 2 which have similar zoning patterns in large feldspar phenocrysts, comparable to Kgc. However, this does not preclude enclave-magma interactions of either group below the emplacement level.

By utilizing petrographic observations, cathodoluminescence imaging, and field data, this study aims to further define the relationships between populations. These results can provide insight into the evolution of magma chambers at the emplacement level and the role of mafic magmas in plutonic systems.