Abstract

We define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole-rock geochemistry in the batholiths of California. In so doing, we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano-plutonic arcs of active continental margins. Little-studied metaluminous to strongly peraluminous granites containing Fe-rich biotite with log(XMg/XFe) < -0.21 (I-SCR type; strongly contaminated and reduced I-type) typically occur in north-northwest-striking belts within pre-batholithic wall-rock terranes containing graphitic pelites in the western Sierra Nevada and Peninsular Ranges batholiths. The other pluton types (I-WC, I-MC, and I-SC; weakly, moderately, and strongly contaminated, but not reduced, I-types) range in composition from metaluminous to weakly peraluminous and form a general west-to-east progression across the batholiths defined by increasing F/OH in biotite. This correlates with the well-known petrologic sequence from quartz diorites and granodiorites on the west to quartz monzonites and granites to the east. I-WC types also occur in the central-eastern Sierra Nevada batholith, however, primarily in the vicinity of the Independence dike swarm.

F/OH and Mn in biotite and amphibole increase on a regional scale from western I-WC types to eastern I-MC and I-SC types, parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons. In contrast, the belts of western I-SCR granites and eastern I-WC quartz diorites and granodiorites disrupt the regional west-to-east systematics in both mineral and whole-rock geochemistry. Spatial variations in the Al content of amphibole are regional in scale and reflect pressures of pluton crystallization. We conclude that significant, previously unrecognized complexity exists in regional geochemical systematics in the California batholiths.

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