METASOMATISM IN SUBCONTINENTAL MANTLE BENEATH NORTHERN PATAGONIA (RIO-NEGRO PROVINCE), ARGENTINA - EVIDENCE FROM SILICA-RICH MELT INCLUSIONS

In an attempt to investigate metasomatic processes in the subcontinent al upper mantle beneath Northern Patagonia, melt and fluid inclusions trapped in spinel Iherzolite and harzburgite xenoliths have been studi ed. The xenoliths contain three types of genetically related inclusion s hosted by olivine, orthopyroxene, clinopyroxene and spinel: silicate glass inclusions, multiphase silicate melt inclusions and CO2 fluid i nclusions. CO2 densities of early fluid inclusions (0.93-1.02 g/cm(3)) and homogenization temperatures of melt inclusions (1220 degrees C) i ndicate that they were trapped at upper-mantle depths. Silicate melt i nclusions occur as isolated inclusions as trails along fractures and d ecorating lamellae deformation. They are composed of glass, or of glas s with ''daughter'' crystals of clinopyroxene, amphibole, apatite and ilmenite. Glass inclusions are characterized by high contents of silic a, alumina and alkali elements. The nature and chemistry of the daught er minerals indicate that melt inclusions can be considered as trapped metasomatic melts. Moreover, glass inclusions show chemical Variation s from high silica (68 wt%) melts trapped as isolated inclusions in ol ivine Fo(91) towards less siliceous (60 wt%) melts trapped as secondar y inclusions in olivine Fo(89-90), orthopyroxene and spinel. This chem ical evolution cannot be reconciled with magmatic processes, like frac tional crystallization or different degrees of partial melting. The ex istence of two stages in their evolution, could reflect the heterogeno us nature of the mantle source of the melts and the presence of a deep -seated fluid phase during melting.