TECTONISM, METASOMATISM AND P-T CONDITIONS OF THE SUBLITHOSPHERIC MANTLE BENEATH PATAGONIA, ARGENTINA

Patagonian alkali lavas (with OIB signature) and cinder cones carrying mantle xenoliths occupy a back arc tectonic environment. A suite of spinel harzburgites, garnet harzburgites, garnet spinel harzburgites, spinel lherzolites, spinel websterites and minor amounts of wehrlites, dunites and pyroxenites, were collected in the region between 40° to 52° S and 67° to 71° W.

Their dominant texture is porphyroclastic followed by equigranular, both tabular and mosaic; protogranular and the transitional types are also present. The different types are randomly distributed in Patagonia and indicate that the sub-lithospheric mantle is mildly to strongly tectonized and recrystallized on both local and regional scale.

In northern Patagonia (between 41° and 44° S), xenoliths depleted in basaltic component dominate. In the south, rather primitive and depleted xenoliths occur together, however, with the majority being depleted ones. The most fertile xenoliths fairly closely approach a primitive UM chemical composition. Bulk rock and mineral analyses indicate complex elemental depletion and variably intensive metasomatic enrichments relative to primitive UM abundances, both modal and cryptic.

Clinopyroxenes from the most primitive samples (Ti/Zr ratio higher than the chondritic ratio), are residues left after partial melting, while the remaining have Ti/Zr ratios lower than the chondritic ratio. There is a positive correlation between LREE-enriched abundance patterns of whole rock and clinopyroxene for all localities.

There is no direct relationship between degree of tectonism and cryptic metasomatism because xenoliths with quite different textures display LREE enrichment in some samples, while others are depleted or only slightly enriched. Modal metasomatism, evidenced by hydrous phases, glass veins and melt pockets responsible for the high contents of incompatible elements in bulk rock analyses, prevails in the samples with equigranular textures.

Mineral equilibria indicate a strongly elevated geotherm beneath Patagonia, similar to the SEA and oceanic geotherms. Therefore, we suggest that the characteristics revealed by the Patagonian samples be most probably related to the presence of quickly rising mantle plume(s) in an extensional tectonic setting.