Igniting Continental Arcs; A Petrologic Study Of Mafic Rocks And Peridotites From The Coast Ridge Belt, Santa Lucia Mountains (California)

University Of Arizona, Tucson AZ

Investigators

Abstract

Ducea and Chesley EAR-0229470 There is strong evidence that major continental magmatic arcs are being built in episodic "flare-ups" separated by periods of magmatic quiescence, although the mechanisms responsible for the high magmatic fluxes are not resolved. Moreover, flare- up events appears to be fueled by a mafic input, thought to be largely derived from continental lithospheric mantle. This requires the existence of a mantle lithosphere an order of magnitude thicker than allowed by observations in mantle wedges and tectonic models. However, most of this evidence comes from subordinate mafic rocks found in shallow exposures of batholithic terranes. Either the voluminous mafic input to the arc lower crust is not (predominantly) from the lithospheric mantle during high magmatic fluxes, or if it is, we need to revise our thinking regarding the dynamics of subduction beneath continents and the ability of continental lids to episodically feed melt-fertile mass to the mantle wedge. These issues will be addressed in a petrologic study of mafic rocks from a mid-crustal arc exposure in the Big Sur area of the Santa Lucia Mountains, central coastal California. The basement rocks exposed there represent a tilted exposure of a magmatic arc representing paleo-depths ranging from 25 km to 10 km. At about 25 km beneath the arc there is a transition from large intrusions of tonalite to granodiorite composition separated by steeply dipping screens typical of the Sierra Nevada batholith to near horizontal, foliated gneissic rocks with numerous mafic intrusions. Mafic intrusions, now either amphibolites or granulites, are abundant at this level, where they stalled due to buoyancy reasons. Our mapping, petrographic, chemical and isotopic results will be used to resolve crustal vs. sub-crustal, and lithospheric vs. asthenospheric reservoirs involved in continental arc magmatism and provide new constraints for the mechanisms of generation of large batholiths.