Miocene magmatism of Southernmost Tierra del Fuego: evidence of repeated subduction of oceanic spreading ridges in Patagonia.

Abstract

Ridge subduction is known to generate anomalous magmatism before and during the opening of a slab window. High-Sr/Y intermediate magmas and slightly alkaline basalts with transitional arc-to-intraplate (OIB-like) signatures have been reported worldwide in such tectonic settings. Southernmost South America has been affected by the subduction of several spreading ridges from the Late Oligocene to the present. Here we present new geochronological, petrographic, geochemical, and isotopic data to evaluate the petrogenesis of two Early Miocene volcanic units from islands at ≈56°S, formed during subduction of the Nazca–Phoenix spreading ridge. Comparison with igneous suites related to subduction of the Chile Ridge (currently being subducted beneath central Patagonia at ≈46°30′S) reveals notable similarities in timing relative to slab-window formation and in chemical signatures. Furthermore, there is a volcanic arc-gap of around 200 km during the Early Miocene, which coincides with the location of the Nazca–Phoenix slab window. We interpret the chemistry of the high Sr/Y rocks as the result of the melting of a mantle source contaminated by terrigenous material derived from erosion of the accretionary prism associated with young, highly rugose oceanic crust. In contrast, the slightly alkaline magmas reflect lower degrees of partial melting under a different thermal structure of the slab–mantle wedge.