Temporal and spatial variations in the distribution and geochemistry of Cenozoic Patagonian basalts in Argentina

KAY, S.M.; RAMOS, V.A.; RAPELA, C.W.

Denver

Congreso; Geological Society of America, Centennial Meeting; 1988

Geological Society of America

The vast Patagonian basalt field extends north-south behind the main Andean chain for some 1600 km (38-52°S). Volcanism, which has occurred throughout the Cenozoic in different places at different times, is linked with minimal extension and represents excess mass loaded on the crust. Several tectonic processes including the near approach and subduction of oceanic ridges along the western margin and periods of major plate reorganization are linked to the distribution of the basalts. In general, important retroarc volcanism has occurred when are volcanism has been minimal. In the Eocene, important basaltic volcanism occurred behind the inactive are between 44-50°S, are volcanism dominated north of 40°, and a transitional region occurred between 40-44°S. In the Oligocene, are magmatism was minimal along the entire margin and basaltic magmatism was absent south of 45°. However, some of most extensive basaltic volcanism in the Patagonian plateau occurred between 40-43° at this time. Late Miocene to Early Pliocene basaltic volcanism was concentrated east of the subducted Chile rise between 47-49°S and scattered behind the are north of about 40°S. Plio-Pleistocene volcanism was volumetrically minor except in the retroarc region north of 39°S. The Patagonian retroarc rocks have non-are to transitional arc geochemical characteristics (i.e, La/Ta < 22) and the available data suggests that incompatible element ratios vary with age and geographic distribution. For instance, La/Ta ratios in Oligocene rocks between 40-43°S decrease from west to east reflecting changes in the mantle source region. Trace element abundances in these rocks are variable but appear to generally increase towards the east. Changes in mantle source regions in time in this same region are shown by a decrease in La/Ta ratios in Eocene to Oligocene mafia rocks coincident with a major reorientation of oceanic plates to the west. Other types of basalts from north of – 45°S, compared to those from the south.