NEOGENE PLATEAU MAGMATISM AND RIDGE SUBDUCTION IN THE SOUTHERN PATAGONIAN ANDES

Relatively large volumes (>2000 km³) of Neogene mafic plateau lavas erupted in the backarc region of the southern Patagonian Andes between 46.5 and 50°S due to ridge collision events that occurred along the Andean margin at ~12 and 6 Ma and thus, is perhaps one of the best places on Earth to investigate magmatic processes associated with ridge subduction. Patagonian plateau magmatism is represented by two distinct sequences, an older (12 to 5 Ma), voluminous, tholeiitic main-plateau sequence (48-55% SiO₂; 4-5% Na₂O+K₂O), and a younger (7 to <0.1 Ma) alkaline post-plateau sequence (43-49% SiO₂; 5-8% Na₂O+K₂O). A dynamic slab window petrogenetic model explains distinctive temporal and spatial variations in the chemistry of both main- and post-plateau sequences. In this model, main-plateau lavas represent relatively large-degree partial melts (10-15%) associated with strong asthenospheric flow around the trailing Nazca Plate edge, whereas post-plateau lavas are small-degree melts (1-4%) generated by weak asthenospheric flow through a wide slab window. Most post-plateau lavas can be explained by a simple one-stage melting model of a dominant OIB-like subslab asthenospheric source characterized by ⁸⁷Sr/⁸⁶Sr ~0.7038 ±0.0003 and ¹⁴³Nd/¹⁴⁴Nd ~0.51275 ±0.00002. Main-plateau lavas from the western backarc are also dominated by the OIB-like subslab asthenospheric mantle, but clearly have evidence for arc and crustal components (relatively low Nb/U, Ce/Pb, high Sr/La) derived from interaction with slab melt/fluid-contaminated mantle wedge and continental lithosphere. West to east decreases in melting percentages and arc signatures in main-plateau lavas results from decreasing amounts of "stored" arc components and the decreasing intensity of mantle upwelling as the Nazca Plate edge subducts to greater depths. Some main- and post-plateau lavas have evidence for an EM1-type enriched continental lithospheric mantle component (relatively high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd, low ²⁰⁶Pb/²⁰⁴Pb) that is geographically associated with the Deseado Massif indicating that important regional differences in lithospheric mantle chemistry exist beneath southern Patagonia.