Abstract

DAHLQUIST, Juan A. et al. The El Pilón alkali-feldespar stock (Sierra Brava, La Rioja): an example of tourmaline granite magmatism. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.3, pp.369-382. ISSN 0004-4822.

Tourmaline is an abundant accessory mineral in the El Pilón stock, a body discordantly emplaced in schist, gneiss, and migmatites of the Sierra Brava basement. It is also a common accessory mineral in the metamorphic rocks, lying in foliation planes. The El Pilón stock is alkali-feldspar granite and the magmatic assemblage is Mc - Pl (An < 1 % molar) - Qtz with Bt - Ms - Tur + Opq + Zrn + Ap as primary accessory minerals, and secondary muscovite after biotite and plagioclase. Petrography and mineral chemistry indicate that the tourmaline (schorl) and some muscovite crystallized from the parental magma of the stock. The chemical composition is typical of peraluminous granitic rocks (ASI = 1.23-1.29) with magmatic tourmaline, showing some chemical characteristic similar to A-type granite (high Ga/Al, low CaO/Al₂O₃, etc.). Low concentrations of LIL elements (average in ppm) such as Ba (89), Sr (32) and the high concentrations of Rb (464) and Cs (106) suggest separation from orthoclase-rich feldspar in the source during partial melting of muscovite-rich metapelites, where the referred feldspar are generated by break-down of muscovite, with the production of a small amount of melt. Ba, Sr and Eu (Eu/Eu* = 0.33-0.42) were retained in the source, while Cs and Rb were transferred to the melt. The studied granitic rocks could also represent a highly differentiated facies of a parental peraluminous magma.

Keywords : Sierras Pampeanas; Peraluminous; Tourmaline; Geochemistry; Mineral chemistry.

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