The cenozoic rhyolite-andesite association of the chilean andes

The eruption centres of late volcanism in Chile are situated in two separate areas in the northern and southern High Cordillera. In the north, the ignimbrites of the Rhyolite Formation and the rocks of the « Andesite » Formation occur in about equal proportions, and recent activity is meagre. In the south, the rocks of the « Andesite » Formation predominate, and many volcanoes are in a highly explosive phase of activity. Field relationships, petrological and geochemical data show that the rocks of both Formations are closely related to each other. There is evidence that the magmas of the Rhyolite Formation were formed by fusion of sialic material in the upper parts of the crust. The data for the volcanics of the « Andesite » Formation are inconsistent with their derivation by fractional crystallization of a basaltic parent or by direct mantle derivation involving a single stage process. The authors suggest that the « andesitic » magmas are products of a primary andesitic magma originated by partial fusion of material of the lower crust. Assuming that the « andesitic » magmas of the central parts of the Andes are derived from the upper mantle, this would mean — in the light of the Sr⁸⁷/Sr⁸⁶ data — that the upper mantle in the central region of the Andes is essentially more radiogenic than in other orogenic areas; moroever, it should be very similar in its chemical and Sr⁸⁷/Sr⁸⁶ composition to that of the lower crust.