Publication Date:
2011-01-13
Description:
In the Sierra de Velasco, northwestern Argentina, undeformed Lower Carboniferous granitoids (350-334 Ma) intrude deformed Lower Ordovician granites and have been emplaced by passive mechanisms, typical of tensional environments. The semi-elliptic, about 300 km2 shallow-emplaced San Blas pluton is 340-330 Ma old, with {varepsilon}Ndt between -1.3 and -1.8 which indicates that, different from the nearby Famatinian-Ordovician granitoids, the San Blas pluton had a relatively brief crustal residence, with an interaction between asthenospheric material and greywackes. The cupola of the pluton was almost totally eroded down during the Upper Carboniferous. The San Blas pluton is a porphyritic granite composed mainly of monzogranite to syenogranite and shows graphic intergrowth and miarolitic cavities up to 5 cm in diameter, filled with quartz. Two different textures are recognized: perthitic microcline megacrysts (30-45 vol%) set in a medium- to coarse-grained groundmass of quartz, microcline and oligoclase, with sericitic alteration. Biotite, muscovite, apatite, zircon, fluorite and opaque minerals are the accessory phases. The other textural variation consists in microcline megacrysts (10%-15 vol%) and a fine-grained groundmass, of quartz, microcline and oligoclase, biotite, apatite, muscovite, zircon and magnetite. The average SiO2 content in this pluton is 74.94%, the ASI=1.1, CaO and MgO are less than 1%, total Fe2O3 and P2O5 contents are low, and K2O〉Na2O. Low Ba, Sr and high Rb contents, coupled with Sn contents (c. 15 ppm), W (c. 380 ppm), Nb, Y, Ta, Th and U confirm this is a special granite. The K/Rb ratio (c. 75) indicates that Rb has been fractionated to the residual melt whereas the Zr/Hf (c. 25) demonstrates that hydrothermal alteration occurred. The Sr/Eu ratio of c. 75 along other geochemical features characterize this pluton as a fertile evolved granite. The chondrite-normalized rare earth element (REE) diagram shows the tetrad effect that allows the subdivision of the lanthanides into four groups. In general, the tetrad effect is recognized in evolved granites and products of hydrothermal alteration such as greisens. The above-mentioned features show that the San Blas granite is fertile, and the absence of ore deposits has been probably caused by erosion of a mineralized cupola during Carboniferous and Cenozoic exhumation. The finding of alluvial cassiterite and wolframite in drainage systems is the first evidence of the fertile character of this granite.
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