REEVALUATING THE SIGNIFICANCE OF BOUNDARIES WITHIN THE RATTLESNAKE MOUNTAIN PLUTON, SAN BERNARDINO MOUNTAINS, CA

Recent studies at Rattlesnake Mountain have resulted in the need to reevaluate the significance of contacts within the Rattlesnake Mountain pluton. MacColl (1964) described a funnel shaped pluton of porphyritic biotite quartz monzonite separated by screens of mafic (diorite and quartz diorite) country rock. We offer an alternate petrogenetic hypothesis based on field relations, whole rock and mineral chemistry, and geochronology. Petrography indicates that rock types range from diorite to granite, with diorite and granodiorite volumetrically predominant. Field evidence of mingling between contemporaneous diorite and granodiorite magmas include mafic enclaves, enclave swarms and schlieren layers in granodiorite and granite, and highly heterogeneous schlieren layering and hybridization in marginal parts of larger mappable diorite bodies. Field evidence is supported by ion microprobe ages of zircons in quartz diorite and granodiorite that yield indistinguishable ages (151±1 and 152±3 Ma). Whole rock analyses display trends representative of a differentiated suite of calc-alkalic igneous rocks. Zircons also record compositional evidence of mixing. Rare earth element (REE) abundances in early-formed regions of zircons from granodiorite are generally enriched relative to early formed regions of zircons from quartz diorite. Trends of REEs from late stage crystallization zones of zircons from both rock types display significant overlap, indicative of chemical convergence of the two systems. Zircon crystallization temperatures (Watson et al., 2006) record thermal variations within the magmatic systems. Hf concentrations increase with decreasing temperatures in most zircon from both quartz diorite and granodiorite. Late stage zircons from granodiorite formed at lower temperatures than earlier formed regions of the same crystals; however, the opposite trend is observed in zircons from quartz diorite, which suggests heating associated with hybridization.