Slab window-related magmatism from southernmost South America: the Late Miocene mafic volcanics from the Estancia Glencross area (similar to 52 degrees S, Argentina-Chile)

The Estancia Glencross Area (EGA) volcanic rocks form a series of five isol ated buttes located at the southern end (similar to 52 degreesS) of the dis continuous belt of Cenozoic basaltic lava formations occurring in the extra -Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andes ites erupted at 8.0-8.5 Ma in a region closely behind the Andean Cordillera . EGA volcanism predated by about 4-5 my the onset of the volcanism in;the nearby pall Aike Volcanic Field, which produced highly primitive, alkaline lavas. Incompatible trace-element distributions and Sr-Nd isotope compositi ons of EGA rocks are those typical of within-plate GIB-type basalts and are indicative of minimal interaction of sub-lithospheric magmas with enriched reservoirs. The geochemical characteristics of EGA volcanics, as well as t heir age and location are consistent with a model of slab window opening be neath this region. The high silica content and the garnet signature of the estimated EGA primary magma are explained by a two-stage process involving the initial production of melts from a garnet Iherzolite source followed by the reaction of these melts with harzburgite country rocks during their as cent through the mantle lithosphere. The melt/harzbugite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, ar e likely related to the dominantly compressive stress regime operating in t his area during Late Miocene. (C) 2001 Elsevier Science B.V. All rights res erved.