Major and trace element analysis of garnet crystals from a hornblendeite block and rind on Santa Catalina Island, CA: Insights into metasomatic processes in subduction mélange

The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduction initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite mélange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that reflect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that reflect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting.

Forearc

Transform fault

10.1130/l153.1

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Middle Crustal Rocks from the Southern Mariana Trench - Relationship to Boninite and Tholeiitic Magma

AGUFM (2010)

Julie A. Johnson Rosemary L. Hickey-Vargas P. Fryer

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Links between shallow slab dehydration, mantle wedge serpentinization and arc volcanism-insights from the Izu-Bonin-Marianas