Samuele Agostini

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

Abstract The Estancia Glencross Area (EGA) volcanic rocks form a series of five isolated buttes located at the southern end (∼52°S) 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 Pali 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 OIB-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 lherzolite source followed by the reaction of these melts with harzburgite country rocks during their ascent through the mantle lithosphere. The melt/harzburgite 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.

Tertiary high-Mg volcanic rocks from Western Anatolia and their geodynamic significance for the evolution of the Aegean area

Abstract Scattered Late Miocene high-Mg basaltic andesites to dacites can be found in Western Anatolia. These rocks display Mg#>65, high CaO/Al2O3 ratio, low alkalies and TiO2 contents. Trace element distribution shows typical orogenic signature with higher values of Fluid Mobile Elements and lower values of HREE and HFSE with respect to Early Miocene Western Anatolia calc-alkaline rocks. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios virtually overlap the values of the less evolved calc-alkaline rocks. The variations observed in this association have been attributed to an FC process combined with interaction with crustal material. A thermal anomaly affecting a depleted mantle source has been invoked for the genesis of these products. Such an anomaly was produced by the ascent of deep sub-slab mantle, which replaced the underthrust lithosphere, already thinned and stretched by extensional process.