Evidence in Cerro Pampa volcanic rocks for slab-melting prior to ridge-trench collision in southern South America

Abstract

Late Miocene hornblende-bearing andesitic to dacitic "adakite' flows at the small Cerro Pampa center in Patagonia have some of the clearest slab-melt geochemical signatures yet seen in a Phanerozoic center on continental crust. These magmas formed in response to melting of the hot, thin slab that was subducting beneath South America prior to the collision of the Chile rise. They were generated by ~3-5% partial melting of eclogite facies N-MORB oceanic crust. Low FeO/MgO ratios and high Cr and Ni concentrations indicate some interaction with mantle peridotite. Low εNd, high 207Pb/204Pb ratios, and high Ba, Cs, U, and Th concentrations indicate some upper crustal contamination. -from Authors