Plio-Pleistocene Western Retroarc Magmatic Evolution and Its Relation With Variable Subduction Settings in the Southern Central Andes (34-38 °S)

Plio-Pleistocene magmatism in the Southern Central Andes (34–38 °S) developed contemporaneously with inferred steepening and destabilization of the subducted Nazca plate associated with a slab tearing at 38°S. Within this setting, arc-related magmatic activity developed in N-S striking belts. Particularly, the studied Varvarco Volcanic Field (VVF) formed in an unusual western retroarc position (70 °W), between the eastern retroarc (69°30’ W) and the present-day arc (68°30’ W). Here we present new field data along with major, minor, and trace elements analyses, Sr-Nd-Pb isotope systematics, and a zircon U/Pb age of ~2.36 Ma to better constrain the linkage between emplacement of the VVF during the Nazca plate steepening and the westward migration of the asthenospheric wedge. The VVF is characterized by basic to intermediate lavas interbedded with pyroclastic deposits, and rhyolitic intrusives. The Basal Lava Flows are calc-alkaline basaltic andesites to andesites, whereas the Upper Lava Flows are less evolved tholeiitic basalts to basaltic andesites. Steepening of the Nazca plate and the onset of the upper plate extension during the VVF emplacement favored the rapid ascent of the latest VVF magma pulses, which underwent low degrees of fractional crystallization. A regional comparison is made to evaluate the impact of the variable tectonic setting on Plio-Pleistocene Andean magmatism. The VVF magmatism shows an arc-like signature, although less pronounced than the present-day volcanic arc. This behavior is also seen when considering all Plio-Pleistocene western retroarc volcanism, whose moderate slab fluids input is associated with their higher distance to the trench relative to the main volcanic arc. Meanwhile, the upwelling of a plume-like structure associated with the Nazca plate tearing has only affected the southern Payenia eastern retroarc volcanoes and the southeastern magmatic units of the western retroarc located just above the tearing.