TESTING THE RELAMINATION HYPOTHESIS IN EXHUMED CORDILLERAN SHALLOW SUBDUCTION COMPLEXES: A CALL TO ARMS

Continental arc magmatism is widely regarded to reflect melting of the mantle wedge by addition of fluids produced through metamorphism of trench sediments and oceanic crust in moderately to steeply dipping subduction zones. In contrast, shallow subduction is commonly invoked to explain the termination and/or migration of arc magmatism. However, the tectonics community is beginning to recognize that subduction accretion assemblages may be transported into the magmatic source regions of active arcs during phases of slab flattening that precede shallow subduction events. Subsequent partial melting of fertile underplated material may trigger brief episodes of high-flux magmatism prior to arc shutoff. This effort focuses on the spatial and temporal association of slab shallowing and magmatic flare-up in the Sierra Nevada batholith of California, where devolatilization and/or partial melting of underplated subduction assemblages apparently altered the isotopic composition of, and triggered a magmatic flare-up in, the superjacent Sierra Nevada batholith. I make the case that “relamination” of subducted material to the base of the arc crust drove voluminous magmatism in the southern Sierra Nevada batholith. Based on results from the southern Sierra Nevada batholith, a combined field, geochronologic, and isotopic approach can successfully be used to evaluate the influence of relamination in locations where plutonic roots and subduction accretion assemblages are exposed in close proximity. The North American Cordillera features several locations where this is the case, including: the Central Gneiss Complex (British Columbia), the North Cascades crystalline core (Washington), the Coolwater culmination (Idaho), the Klamath Mountains (Condrey Mountain window; northern California), and the Pelona-Orocopia-Rand schist belt (southern California). Investigation of and comparisons between these areas will lead to an improved understanding of how supracrustal rocks are emplaced into continental arc source regimes and how the input of these materials influences magma productivity and composition.