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Juana Norma Rossi, Universidad Nacional de Tucuman (Argentina)
Miguel Baez, UNIVERSIDAD NACIONAL DE TUCUMAN (Argentina)
Miguel A. Basei, Universidad Sao Paulo (Brazil)
Alejandro J. Toselli, UNIVERSIDAD NACIONAL DE TUCUMAN (Argentina)
Alcides Nobrega Sial, NEG-LABISE (Brazil)
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At Sierra de Velasco, northwestern Argentina, undeformed Lower Carboniferous granites (350-334Ma) intrude deformed Lower Ordovician granites, and heve been emplaced by passive mechanisms, typical of tensional environments. The semi-eliptic shallow-emplaced San Blas pluton, about 300 km2, is one of these Carboniferous granites (340 to 328 Ma) and its cupola has been almost totally eroded in the Upper Carboniferous. The finding of alluvial cassiterite and wolframite in drainage systems reported by the Mining and Geological Survey of Argentina has been the first evidence of the fertile character of this granite.
The San Blas pluton is composed of granite porphyry to porphyritic granite with monzogranite to syenogranite compositions that shows graphic intergrowts and miarolitic cavities up to 5cm in diameter filled with quartz. Perthitic microcline phenocrysts constitute 30- 45 % of the rock and the matrix is equigranular, medium to coarse-grained, and constituted by quartz, microcline and oligoclase, with sericitic alteration. Biotite, muscovite, opaques, apatite, zircon and fluorite are the accessory minerals. In the granite porphyry, microcline megacrysts constitute from 10% to 15%, in fine-grained matrix, composed by quartz, microcline and oligoclase, biotite, apatite, muscovite, zircon and magnetite.
The average SiO2 content in this pluton is of 74.94 %, CaO and MgO are less than 1%, total Fe2O3 is low, and K2O>Na2O. P2O5 content is low and same happens to the ASI index = 1.1. Low Ba, Sr and high Rb contents, coupled with Sn contents (∼15 ppm), W (∼380 ppm), Nb, Y, Ta, Th and U confirm this is a specialized granite. The K/Rb ratio (∼75) indicates that Rb has been fractionated to the residual melt whereas the Zr/Hf (∼25) evidences hydrothermal alteration. The Sr/Eu ratio of ∼ 75 along other geochemical features characterize this pluton as a fertile evolved granite.
The REE diagram shows the tetrad effect that allows for the subdivision of lanthanides in four groups according to their distribution patterns, normalized to chondrite, that use La-Nd and Gd-Ho. In general, the tetrad effect is recognized in evolved granites and with hydrothermal alteration (greisenization). The above-mentioned features support a fertile granite and the absence of ore deposits has been probably caused by erosion of a mineralized cupola during Carboniferous exhumation.
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