DECLINING INTERACTIONS BETWEEN MAFIC AND FELSIC MAGMAS DURING DEVELOPMENT OF THE MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Mount Whitney Intrusive Suite (MWIS) is a 1200 km² composite granitic intrusion that was emplaced above an extensional stepover in the Sierra Crest shear zone system between about 83 and 90 Ma. The suite's younger, more felsic members are nested within its older, more mafic ones. The younger members are characterized by more crustal Sr and Nd isotopic compositions and lower abundances of microdioritic enclaves.

Sr_i and ε_Nd are inversely correlated among MWIS samples and suggest that its members are hybrids between subduction-generated hydrous high-alumina basalts and the Proterozoic continental crust that underlies east-central California. Although simple mixing models are unlikely to yield accurate estimates of the fraction of crustal component in a given member because of variations in source compositions and hybridization processes, the isotopic data strongly suggest that both the degree of crust-mantle hybridization and the isotopic diversity of the resulting granitic magmas decreased as the suite developed. Modest volumes of mafic magmas associated with the suite also reached the exposed level where they formed small intrusions and microdioritic enclaves. Enclave abundances decline markedly between successively younger units, falling from about 2% in the oldest member to 0.01% in the youngest.

Together, the isotopic and enclave data suggest that interactions between mafic and felsic magmas occurred at multiple crustal levels in the MWIS system and declined through time. These trends may reflect interception of rising basalts beneath a partially-molten volume of lower crust that grew as the suite developed and led to progressive decoupling of heat and mass transfer. The decline in enclave abundances may also reflect trapping of the basalt magmas that reached the upper crust beneath a growing felsic magma reservoir that was especially well developed at the center of the youngest pluton.