Abstract

LOCATI, Francisco; COLOMBO, Fernando  and  BALDO, Edgardo G. A.. Pegmatites with allanite-(Ce) related tothe orthopyroxene-bearing orthogneiss of the Quebrada del Río Suquía, SierrasChicas de Córdoba. Rev. Asoc. Geol. Argent. [online]. 2009, vol.64, n.4, pp.646-656. ISSN 0004-4822.

In the Las Bateas area (Sierras Chicas de Córdoba, 31º21´0´´S;64º21´27´´W) there is a pyroxene-bearing orthogneiss metamorphosed to granulitefacies during the Early Cambrian. Pegmatitic segregations are scattered withinthe orthogneiss as lens-shaped bodies up to 150 cm long and 70 cm thick,showing gradational contacts with the host rock. They are composed of quartz,plagioclase (An₃₃-An₄₀), biotite belonging to theannite-phlogopite series [Mg/(Fe_total+Mg) de 0.48 a0.47], allanite-(Ce), and occasionally microcline with an orderedstructural state. Allanite-(Ce) occurs as euhedral black crystals up to 16 cmlong bounded by the forms {001} and {100}. It is metamict due to its Th contentand shows optical and chemical zoning. La_N/Yb_N and La_N/Gd_Nratios show a core-to-rim decrease (34-19 and 3.1-2.1 respectively), while Thincreases towards the rim (1.66-2.59% ThO₂). Dominant substitutionmechanisms are Ca²⁺ + Fe³⁺ ETR³⁺ + Fe²⁺, Si⁴⁺+ M²⁺ Al³⁺ + Fe³⁺ and probably 2ETR³⁺ Ca²⁺+ Th⁴⁺ (where REE stands for rare-earth elements and M²⁺is a divalent cation). The pyroxenebearing orthogneiss and the pegmatitesderive from the same magma. The presence of allanite-(Ce) is a consequence ofREE enrichment in the pegmatite melt induced by the delay or suppression ofmonazite crystallization, this itself being partially due to the earlyprecipitation of apatite which takes up P from the melt.

Keywords : Orthogneiss; Pegmatite; Allanite-(Ce); Rare-earth elements; Granulite-facies metamorphism.

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