MAGMA MINGLING & MIXING: MAFIC ENCLAVES AND DIKES IN THE WOOLEY CREEK BATHOLITH, N. CALIFORNIA

The tilted 159-156 Ma Wooley Creek batholith is a large, tilted intrusion with about 9 km of structural relief that provides a means to explore magma mixing and mingling at different structural levels. The pluton is zoned with structurally lowest gabbro, diorite, quartz diorite, & tonalite and structurally higher tonalite, granodiorite, & granite with sparse gabbro at contacts. Syn-plutonic mafic dikes and mafic magmatic enclaves (mme) occur throughout the batholith but are sparse in the lowest part and in greatest abundance in the central region. Enclave swarms and single mme are common in the upper part. Dikes and disrupted dikes are typically gabbro and diorite, whereas mme vary compositionally between gabbro and granodiorite.

Some enclaves are clearly the result of disruption of mafic dikes in still-molten host magma, thus allowing for chemical and mechanical mixing and dispersion of enclaves into the system. The mafic dikes are predominantly fine-grained and may have sparse plagioclase ± hornblende phenocrysts. The mme vary in shape from angular to ovoid and fusiform and in grain size from fine to medium with variable amounts of plagioclase, hornblende, & biotite ‘phenocrysts’. At least some of these ‘phenocrysts’ are identical to crystals in the host rocks.

For the batholith as a whole, the mme have major & trace element compositions that are broadly intermediate between mafic synplutonic dikes and the enclave’s host rock compositions. In this sense, mme compositions may represent mixing of existing magmas in the pluton with injected mafic (basaltic) magmas. Mixing probably began during disruption and dispersal of magmatic dikes. However, the fact that the groundmass mineral assemblage in the mme mimics that of their host rock type suggests that chemical exchange between host and mme magmas was commonly pervasive and continued to near-solidus conditions.