TESTING MODELS FOR HIGH-FLUX TRONDHJEMITIC MELTING BETWEEN 98-88 MA ALONG A 2500 KM SEGMENT OF THE CALIFORNIA-BAJA MARGIN

Several models have been invoked to explain the tectonic processes that triggered high flux emplacement (ca. 100 km3/km/m.y.) of high Al, Na, & Sr/Y magma into the California batholith between 98-88 Ma (n > 200 zircon U-Pb ages). The plutonic belt extends for ca. 2500 km along the continental margin and includes both the Sierra Nevada batholith/N.W. Mojave Desert (Sierra Crest suite, 40-35°N) and Peninsular Ranges batholith (La Posta suite, 34-24°N). Model A has primarily been applied to the Sierra Crest suite and holds that thermal relaxation driven by crustal thickening triggered high-flux melting of cratonal rocks that were underthrust beneath the batholith from the east during retroarc shortening. Model B has alternatively been developed primarily for the La Posta suite. It attributes the high-flux magmatism to eastward transfer of forearc and subduction complex sedimentary & mafic rocks by subduction erosion from the forearc to a position beneath the batholith near the region of heightened corner flow within the lithospheric mantle. Models A & B are dominated by melting of crustal materials. Model C is a two-step process that requires massive mantle melting to produce a thick basaltic underplate that is subsequently re-melted to produce trondhjemitic plutons. Three segments of the combined Sierran-Peninsular Ranges batholith belt can be defined on the basis of isotopic and trace element data: (1) plutons from 30-24°N have Sr_i = 0.704-0.705, δ¹⁸O=7.0-8.5‰, and mean Sr/Y=83±47; (2) plutons from 31-38°N have Sr_i=0.705-0.709, δ¹⁸O=9-12‰, and mean Sr/Y = 45±21; (3) plutons from 39-40°N have ⁸⁷Sr/⁸⁶Sr_i =0.704-0.705, δ¹⁸O=7.0-8.5‰, and mean Sr/Y = 65±15. Model B provides the simplest interpretation of the high flux Sierra Crest-La Posta plutonic belt. Plutons within the northernmost (NW Nevada) and southern (central & southern Baja) segments have Sr/Y consistent with a deep source region and are most readily explained by melting a mixture of primitive mantle and altered oceanic crust/trench fill. The central segment (northern PRB to central SNB) can also be explained by model B if significant contamination by continental lithosphere is invoked (possibly via processes described in model A). Model C is disfavored by the massive scale of mantle melting required and its failure to predict the observed Sr-O isotopic array.