Garnet-bearing xenoliths from Prahuaniyeu N-Patagonia: a LA-ICP-MS study.

NTAFLOS, TH.; BJERG, E. A.

Melbourne Exhibition and Convention Centre, Australia.

Congreso; 16th Annual VM Goldschmidt Conference 2006; 2006

Executive Organising Committee: Chair - Simon Turner, Sponsorship - Janet Hergt, Field Trips - John Foden, Scientific Program Chair - Hugh O'Neil, Scientific Program Subchair - Greg Yaxley

Mantle xenoliths from Prahuaniyeu, northern Patagonia, occur in tuffs, alkali basalts and basanites and comprise fertile and depleted garnet-peridotites, garnet-spinel-peridotites and spinel-peridotites. The garnet bearing peridotites are characterized by high temperature (T= 1100-1230°C) and low pressure (P= 19.5-24 kbar). On the basis of texture and their corresponding trace elements two groups of garnets can be distinguished: group 1 consists of fine grained (up to 1mm) garnets in which no trace element zoning exists and, group 2 which consists of coarse grained (up to 8 mm) garnets with strong trace element zonation. Compared to the core, the rims of group 2 garnets are depleted in REE about factor of 10 and Zr varies from 37 ppm in the rim to 70 ppm in the core. In contrast cpx from the same group have slightly higher Zr than the core. Primitive mantle-normalized trace element abundance patterns for garnets show Zr anomalies that in respect to the neighbor elements range from positive to negative. Garnets form the fertile garnet peridotites have positive Zr anomaly and the depleted ones have pronounced negative Zr anomaly.Ti shows, in both fertile and depleted garnet peridotites, generally weak negative anomaly and appears to be decoupled from the behavior of Zr. The cpx trace element patterns from garnet peridotites always have negative Zr anomaly and no Ti anomaly. In contrast, worldwide cpx from spinel peridotites have generally negative Ti anomaly and therefore the presence or absence of negative Ti anomaly could reveal whether or not the peridotites have experienced melting processes in the garnet peridotite field. The ZrDcpx/Dgt ratio for the majority of the analyzed samples is close to 0.44, similar to the experimental determined partition coefficients. Trace element patterns for these samples can be obtained by simple fractional melting alone. However ratios ZrDcpx/Dgt of about 1 from group 1 cannot be explained by simple melting processes and requires introduction of melts that caused cryptic metasomatism responsible for Zr enrichments in cpx. The garnet peridotites from Prahuaniyeu have experienced up to 8% partial melting and partly cryptic metasomatism in a low pressure, high temperature environment that might point to an ascending mantle plume.