Abstract

GARGIULO, M. Florencia; BJERG, Ernesto A  and  MOGESSIE, Aberra. Metasomatism in orthoamphibolites from the Río de Las Tunas mafic-ultramafic belt, Mendoza. Rev. Asoc. Geol. Argent. [online]. 2012, vol.69, n.2, pp.163-178. ISSN 0004-4822.

In this contribution, petro-mineralogical and chemical evidences of metasomatism in orthoamphibolites from the Río de Las Tunas mafic-ultramafic belt in Frontal Cordillera of Mendoza province are provided. These orthoamphibolites integrate the metamorphic basement of the Frontal Cordillera and they belong to the Guarguaráz Metamorphic Complex. These rocks show mineral associations and textural relationships indicating disequilibrium conditions between some of the mineral phases. Hornblendic amphibole crystals show a reaction border with development of clinopyroxene (ferroan dioside); meanwhile the calcicplagioclase is mostly replaced by a fine-grained matrix integrated by the mineral association of garnet-epidotes.l -prehnite-pumpellyite- albite. The amphibole and clinopyroxene crystals are also partially replaced by pumpellyite and/or chlorite, meanwhile the matrix is slightly replaced by muscovite and calcite. Garnet compositions belong to the grossular-andradite series and their average composition is: Alm05Adr53Prp01Sps04Grs36Uv01. Some crystals with Cr2O3 content between 11.69-13.17 wt.% and with a subtle chemical zonation (uvarovitic core: Alm03Adr13Prp00Sps02Grs35Uv47 with more grossularic border: Alm03Adr12 Prp00Sps01Grs44Uv40) occur in relationship with magnetite crystals with up to 20% of the chromite component. Pressure estimations based on the compositional zoning of amphibole crystals gave values between 2.5-4.5 kbar. The observed mineral associations together with the textural and chemical characteristics of these rocks show that this kind of orthoamphibolites is completely different from the classical amphibolites of igneous protoliths outcropping in the study area. These characteristics also allow us to interpret these rocks and classify them as clinopyroxene-rodingites with garnet and clinozoisite. The pressure conditions together with the replacement sequence observed in the mineral associations suggest they are the result of a metasomatic process developed below 4 kbar, during the final stages of the regional metamorphic cycle that affected the Guarguaráz Complex.

Keywords : Rodingite, Río de Las Tunas belt; Guarguaráz Metamorphic Complex; Frontal Cordillera.

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