Abstract

The Potrerillos granitic pegmatites, in the Sierras Pampeanas Orientales of San Luis Province, Argentina, consist of mediumsized bodies with rounded, elliptical, or irregular outcrops varying from 10 to 60 m; vertically, they mostly look like thick lenses. Zoning is ill-defined, with a border zone that grades into a coarse-grained intermediate zone. The core zone also is poorly defined, normally containing milky to pink or gray quartz with large crystals of K-feldspar and, locally, pods of primary fluorite. The pegmatite-forming minerals are quartz, microcline and albite, with accessory biotite, muscovite, tourmaline, beryl, allanite, fluorite, fluorapatite, ilmenite, niobian rutile, rutile and pyrite. Rare primary species are monazite-(Ce), U-rich pyrochlore, scheelite, bismuthinite and molybdenite. Subordinate species are hematite, goethite, Mn oxides, clinobisvanite, bismuthite, and opal. The pegmatites are ascribed to the allanite–monazite type, REL–REE subclass, NYF family. Data on melt+fluid inclusion are consistent with crystallization of an aqueous, CO2-bearing melt at 475° < T < 600°C and ~3.0 to 3.7 kbar, that cooled down to a fluid of low to moderate salinity. The pegmatites are hosted in three dominant facies of two plutons of the Las Chacras – Potrerillos batholith, described as biotite porphyritic granite, biotite-bearing equigranular red granite, and muscovite-bearing equigranular red granite. All of them have high-K compositions, and are metaluminous to mildly peraluminous (ASI in the range 1 to 1.07); the least evolved porphyritic biotite granites of the Las Chacras pluton are depleted in SiO2 and enriched in TiO2, Fe2O3(T), CaO and MgO, poor in Nb, Rb, Th, and HREE, and enriched in P, Zr, Sr and Ba and LREE relative to the red equigranular muscovite bearing facies of the Potrerillos pluton. The asymmetric REE patterns likely reflect the control by accessory monazite-(Ce) and allanite-(Ce) in the porphyritic facies, contrasting with relative HREE enrichment in the muscovite-bearing equigranular granite. Both granite types display Eu/Eu* values <1, indicative of feldspar fractionation in the source; the HREE are strongly depleted, more accentuated in less evolved biotite porphyritic facies, likely reflecting garnet retained in the source. The crystallization-emplacement history of the suite spanned from the Upper Devonian to the Lower Carboniferous in a within-plate regime crossed by deep shear zones. The granite facies that host the pegmatites are redefined as A-type granites. The A-type magmas probably originated by partial melting of a depleted garnet-bearing lower crust, of likely granodiorite or tonalite composition in the minimum melt field of Opx + Cpx + Grt ± Qtz, triggered by mantle-derived heat but previously fertilized by alkali metasomatism produced by mantle degassing.

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