Pegmatites with allanite-(Ce) related tothe orthopyroxene-bearing orthogneiss of the Quebrada del Río Suquía, Sierras Chicas de Córdoba

Abstract

In the Las Bateas area (Sierras Chicas de Córdoba, 31º21´´´S; 64º21'27''O) there is a pyroxene-bearing orthogneiss metamorphosed to granulite facies during the Early Cambrian. Pegmatitic segregations are scattered within the 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 a 0.47], allanite-(Ce), and occasionally microcline with an ordered structural 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_N ratios 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 pyroxene bearing orthogneiss and the pegmatites derive from the same magma. The presence of allanite-(Ce) is a consequence of REE enrichment in the pegmatite melt induced by the delay or suppression ofmonazite crystallization, this itself being partially due to the early precipitation of apatite which takes up P from the melt.