Insights into lithospheric mantle beneath Patagonia

NTAFLOS, TH.; BJERG, E. A.; ALIANI, P. A.

GEOCHIMICA ET COSMOCHIMICA ACTA

Elsevier

Año: 2008 vol. 72 p. 691 - 692

0016-7037

The lithospheric mantle beneath Patagonia (PTG), as can be inferred from studies on mantle xenoliths, is highly inhomogeneous. In northern PTG the majority of mantle xenoliths show evidences of cryptic metasomatism, except those from Comallo that have traces of amphibole. In contrast, in southern PTG, the lithospheric mantle beneath Gobernador Gregores (Gb) is heavily modal metasomatized by abundant amphibole and/or phlogopite. In northern PTG, gt-bearing peridotites occur in Prahuaniyeu (Pr), close to the NW margin of the Somoncura large igneous povince. They are characterized by high T = 1100–1230 °C and low P = 19.5–24 kbar. Zr is enriched in garnets from fertile gt-peridotites and depleted in garnets from residual gt-peridotites. Ti-content in gt shows no variations and appears to be decoupled from the behaviour of Zr. The cpx trace element patterns from gt-peridotites always have negative Zr anomaly but no Ti anomaly. In contrast, worldwide cpx from sp-peridotites generally have negative Ti anomaly in PM-normalized trace element patterns. Therefore the presence or absence of negative Ti anomaly could reveal whether or not the peridotites have experienced melting processes in the gt-peridotite field. ZrDcpx/Dgt ratios of about 1 from a group of Pr gt-peridotites cannot be explained by simple melting processes and require introduction of melts causing cryptic metasomatism responsible for Zr enrichments in cpx. These gt-peridotites have experienced up to 8% partial melting and slight cryptic metasomatism under low P-high T conditions, suggesting an ascending mantle plume. The heavily metasomatized sp-peridotites from Gb have abundant melt pockets that according to mass balance calculations represent breakdown and melting of amphiboles with minor contribution of external melts. Amph, glass and partly cpx are strongly enriched in incompatible elements. Mantle xenoliths from the southern PT localities Tres Lagos and Cerro Redondo, south of Gb, are more depleted in basaltic components and suggest a lithospheric mantle at this region practically unaffected by metasomatic processes. Those differences and, the fact that the crust at Gb is a metamorphic unit of latest Proterozoic age whereas at the other two localities it is younger (Palaeozoic), suggest a boundary between two different micro tectonic plates.