NA-RICH PARTIAL MELTS FROM NEWLY UNDERPLATED BASALTIC CRUST - THE CORDILLERA BLANCA BATHOLITH, PERU

The late Miocene Cordillera Blanca Batholith lies directly over thick (50 km) crust, inboard of the older Cretaceous Coastal Batholith. Its peraluminous 'S' type mineralogy and its position suggest recycling of continental crust, which is commonly thought to be an increasingly im portant component in magmas inboard of continental margins. However, t he peraluminous, apparent 'S' type character of the batholith is an ar tefact of deformation and uplift along a major crustal lineament. The batholith is a metaluminous 'I' type and the dominant high-silica rock s (>70%) are Na rich with many of the characteristics of subducted oce anic slab melts. However, the position of the batholith and age of the oceanic crust at the trench during the Miocene Preclude slab melting. Instead, partial melting of newly underplated Miocene crust is propos ed In this dynamic model newly underplated basaltic material is melted to produce high-Na, low HREE, high-Al 'trondhjemitic' type melts with residues of garnet, clinopyroxene and amphibole. Such Na-rich magmas are characteristic of thick Andean crust; they are significantly diffe rent from typical calc-alkaline, tonalite-granodiorite magmas, and the ir presence along the spine of the Andes provokes questions about mode ls of trondhjemite genesis by melting of subducted oceanic crest, as w ell as any generalized, circum-Pacific model involving consistent isot opic or chemical changes inboard from the trench.