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

The Estancia Glencross Area (EGA) volcanic rocks form a series of five isolated buttes located at the southern end (similar to 52 degreesS) of the discontinuous belt of Cenozoic basaltic lava formations occurring in the extra-Andean Patagonia. EGA volcanics are subalkaline basalts and basaltic andesites 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 compositions 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 their age and location are consistent with a model of slab window opening beneath 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 ascent through the mantle lithosphere. The melt/harzbugite reaction, favoured by a slow melt ascent rate, as well as the low magma production at EGA, are likely related to the dominantly compressive stress regime operating in this area during Late Miocene. (C) 2001 Elsevier Science B.V. All rights reserved.