Paleogene arc-related volcanism in the Southern Central Andes and North Patagonia (39°-41°S).

Abstract

The influence of tectonic processes in evolution of magmatic suitesevaluated through their geochemical signature has always been a subject of debate.Late Paleocene arc volcanism in the Southern Central Andes, particularly in North Patagonia, can be used to infer a direct relationship between magmatic episodes and tectonic changes along the Andean margin. Eocene arc-related volcanism 11 (*44 Ma) in the North Patagonian Andes shows evidence for limited influence of 12 the subducting slab on the composition of arc magmas and they exhibit an alkaline tendency. By Oligocene times (*29 Ma), arc volcanic sequences in the Auca Pan 14 depocenter show predominantly arc-like geochemical signatures and have been 15 derived from a calc-alkaline source. However, a comparison with younger arc 16 sequences (<28 Ma) in the region suggests that the magmatic source turned more 17 tholeiitic in composition with a remarkable increase in the influence of slab-derived 18 fluids, as seen in volcanic rocks from Cura Mallín and Abanico retro and intra-arc 19 basins. It is proposed that the marked geochemical variations between these mag 20 matic periods are related to the tectonic changes associated with the breakup of the 21 Farallon plate at *28?26 Ma. The geochemical data from Eocene and Oligocene