Metallogenesis of the Totoral LCT rare-element pegmatite district, San Luis, Argentina: A review

Abstract

The Totoral Pegmatite District (TPD) is the southernmost rare-element LCT pegmatite field of the Pampean Pegmatite Province. The TPD produced intermittently in the last 60 years Ta-Nb ore minerals, spodumene, beryl and ceramic raw materials. It is located in the southern part of the Eastern Pampean Range of San Luis province. It was developed during the late stage of accretion of the Famatina terrane to the West Gondwana in the Lower Paleozoic (≈450 Ma). The parental S-type leucogranites and rare-element pegmatites of REL-Li subclass form three groups aligned NNE. The leucogranites were originated by muscovite (±incipient biotite) dehydration melting of preferably metapelites (±metagreywackes) of the Pringles Metamorphic Complex (PMC). The resultant bimodal suite of S-type muscovite-tourmaline and muscovite-biotite leucogranites show major, trace elements, and Pb-Ba ratios compatible with both low-T and higher-T collisional leucogranites. These leucogranites were emplaced after regional metamorphism in the upper part of the mica schists unit of PMC at 640–725 °C and ≈400–500 MPa, and they fractionated to their associated pegmatites as is supported by the spatial association, similar age and fractionation trends of leucogranites and pegmatites. The regional integrated pegmatite zoning shows the sequence: leucogranite, pegmatitic leucogranite, barren-transitional to beryl-type, beryl-columbite-phosphate subtype, albite-spodumene type, complex-type spodumene-subtype and albite type rare-elements pegmatites. This zonation follows a path towards decreasing pressure of emplacement. The crystallization of pegmatites was triggered by the rapid undercooling provoked by the thermal contrast due to the fast forced emplacement in the hosting mica schists and the H 2 O and fluxes content of the melts that produced nucleation delay and high crystal growth rate of the minerals. The Li-bearing pegmatites have genetic links with the higher-T leucogranites. The emplacement of the pegmatites was facilitated by a shear zone, and they show synkinematic ductile-state deformation ascribed to the late stage of the Famatina terrane accretion. Later on, most of them were tectonically affected in brittle state by the diastrophism attributed to the westward accretion of the Cuyania terrane.