THE CHON-AIKE PROVINCE OF PATAGONIA AND RELATED ROCKS IN WEST ANTARCTICA - A SILICIC LARGE IGNEOUS PROVINCE

The field occurrence, age, classification and geochemistry of the Meso zoic volcanic rocks of Patagonia and West Antarctica are reviewed, usi ng published and new information. Dominated by rhyolitic ignimbrites, which form a bimodal association with minor mafic and intermediate lav as, these constitute one of the largest silicic igneous provinces know n, equivalent in size to many mafic LIPs. Diachronism is recognized be tween the Early-Middle Jurassic volcanism of eastern Patagonia (Marifi l and Chon Aike formations) and the Middle Jurassic-earliest Cretaceou s volcanism of the Andean Cordillera (El Quemado, Ibanez and Tobifera formations). This is accompanied by a change in geochemical characteri stics, from relatively high-Zr and -Nb types in the east to subalkalin e are-related rocks in the west, although the predominance of rhyolite s remains a constant factor. All of the associated mafic rocks are wel l fractionated compared to direct mantle derivatives. Petrogenetic mod els favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas, possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidifica tion and remelting. The formation of large amounts of intracrustal sil icic melt acted as a density barrier against the further rise of mafic magmas, which an thus rare in the province. (C) 1998 Elsevier Science B.V.