O manto litosférico continental na região do Cerro de Los Chenques, Argentina: evidências de heterogeneidade e metassomatismo

Abstract

The Cerro de los Chenques ultramafic xenolith suite from northern Patagonia (44°52'19"S/70°03'57"W), southern South America, consists of continental lithospheric mantle samples. Petrographical, mineralogical and geochemical whole rock data for minor and major elements demonstrate that the Cerro de los Chenques xenoliths include spinel-bearing lherzolite, harzburgite and websterite, with primary mineralogical parageneses formed by olivine, enstatite, diopside and spinel. -Mineral chemistry data suggest that this paragenesis is in equilibrium, as can be observed by the Mg# [Mg/(Mg+Fe total)] of the minerals, lower in olivine and higher in diopside. However, local instabilities occured with the formation of glass and secondary parageneses, constituted by olivine, diopside and spinel. Whole rock geochemical data show depletion on the alkalis and on trace elements in relation to primitive mantle, due to partial melting during two distinct events, one of which involved the peridotites, the other, the websterites. Some samples also present signs of metasomatism involving re-charging of light REE relative to heavy REE. The trace elements of these samples demonstrate slight enrichment of the most incompatible elements in relation to the most compatible, helping the understanding of the metasomatic process.