New evidence of slab window magmatism from southernmost South America: the Late Miocene mafic volcanics form the Estancia Glencross Area (~ 52º 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.