Petrology of suprasubductive mantle xenoliths from Estancia Sol De Mayo (Central Patagonia, Argentina).

Abstract

A new suite of mantle xenoliths from Estancia Sol de Mayo (Patagonia) has been investigated. They have been sampled and entrained by alkaline lavas from the south western corner of the Meseta Lago Buenos Aires. Chondrite-normalized trace element patterns of these lavas resemble that of the OIB, and of the main and post-plateau lavas from the Triple Junction Province. The xenoliths are mainly anhydrous spinel-bearing harzburgites and dunites, with minor lherzolites and one wehrlite. They are characterized by a coarse grained protogranular texture and devoid of modal metasomatic features. They show two texturally different clinopyroxenes. One is protogranular, defined cpx1, while the second is linked to the spinel (cpx2). Also three different orthopyroxenes are recognized: one is represented by large protogranular crystals with exsolution lamellae (opx1); the second by small clean and undeformed grains without exsolution lamellae (opx2) and the last occurs as smaller grains arranged in vein (opx3). On the whole the geochemical features of clinopyroxenes and orthopyroxenes major element compositions highlight three different trends. Two of them are in common between cpx and opx while the third is observed only for the cpx and is situated in between the other two trends. The trace element concentrations normalized to chondrite do not show any difference between cpx 1 and cpx 2 (characterized by prominent to slightly negative Nb, Zr and Ti anomalies and LREE enriched), but discriminate the three groups of orthopyroxenes: one is represented by the opx 3 characterized by a prominent positive Zr anomaly, while the other two show always prominent to slightly negative Ti and Zr anomalies and LREE depleted. The correlation between incompatible trace elements ((La/Yb)N and SrN) and Al2O3 for the cpx highlights the presence of refertilization/metasomatic events affecting the upper mantle beneath Estancia Sol de Mayo, because of the enrichment of the LREE and Sr correlated to a decreasing of the Al2O3 content. A tholeiitic and a transitional/alkaline melt have to be accounted for a refertilization and a metasomatic event, respectively. The comparison with other localities already studied highlights a global Al2O3 enrichment of the upper mantle from Patagonia. Also the pyroxenes from other Patagonian localities plot on the trends defined by the cpx and opx from Estancia Sol de Mayo. Moreover trace elements evidence metasomatic events affecting the mantle beneath some localities. Others are characterized only by partial melting events. Sr-Nd systematic performed for all the investigated localities show a large range of both 87Sr/86Sr and 143Nd/144Nd isotopic ratios that can be explained by a mixing between a depleted mantle and an enriched component (EM II).