Andean andesites and crustal growth

Abstract

Over the last 200 Ma, the ensialic Andean plate margin has been characterized by calc-alkaline magmatism. The early (Mesozoic), activity was dominantly of basaltic volcanism while the Cainozoic volcanism was of intermediate, calc-alkaline character. The restriction of Recent volcanism to parts of the Andes underlain by thick wedges of asthenospheric mantle, and the Sr and Nd isotopic relations, indicate that the calc-alkaline parental magmas are derived from the asthenospheric mantle. There is no unequivocal geochemical and geophysical evidence that continental crust or sediment has contributed to the mantle source for Andean magmatism. The chemical compositions of the calc-alkaline volcanic rocks of the active volcanic zones are controlled by fractional crystallization, whereas O-Sr isotopic relations reflect crustal interaction of mantle-derived parental magma with the sialic basement of the Andes. The variable extent of fractional crystallization, partial melting, and mixing of crustal contaminant are related to the variable thickness and age of crust in the different volcanic provinces. Calc-alkaline magmatism was largely responsible for post-Mesozoic crustal growth in the Andes and would have depleted the underlying mantle unless balanced by circulation within the asthenospheric mantle wedge. In terms of net growth of the South American continent, it is not certain where the balance lies between growth by magmatic addition and shrinking by erosion.