Mineralogy and origin of the dumortierite-bearing pegmatites of Virorco, San Luis, Argentina

Abstract

The Virorco dumortierite-bearing pegmatite dikes, in Sierra de San Luis, Eastern Pampean Ranges of central Argentina, are a group of thin, steeply dipping dikes 2 to 10 cm thick with variable lengths (a few dm to <2 m). The pegmatites are emplaced in partially uralitized norite and gabbro that belong to a larger mafic?ultramafic intrusive belt. The pegmatite dikes are symmetrically zoned, with quartz, albite, oligoclase, tourmaline-supergroup and dumortierite-group minerals, muscovite and kyanite as the major phases; the accessory and trace minerals include beryl, chrysoberyl, garnet, fluorapatite, columbite-(Mn) to tantalite-(Mn), pollucite, gahnite, zircon, uraninite and thorite. Holmquistite was found in the exocontact asemblage. Primary textures of magmatic origin were partially disrupted by partial replacements by later minerals and incipient to strong deformation. The whole-rock chemical composition of the dikes shows SiO2 contents normal for rare-element pegmatites whereas amounts of Al2O3 and B2O3 are very high. The composition features high MgO, FeO, CaO and P2O5 and, for pegmatites, unusually low Na2O and K2O contents. Amounts of trace elements are remarkably high in case of Cs (4.3?94.1 ppm), Ta (130?500) and Be (137?261). The normalized REE contents are low (0.1 to 30 times chondrite), highlighted by a strong negative Eu anomaly. Five textural and compositional types of tourmaline-supergroup minerals were identified in the different pegmatitic zones, ranging from dravite-rich compositions to rossmannite, passing through schorl and Mn-rich elbaite. At least four generations of the dumortierite?holtite minerals are texturally and compositionally represented in these dikes: the earliest dumortierite replaces muscovite and tourmaline, locally together with a second generation that grades into As-poor holtite. The third generation is represented by overgrowths or individual crystals of As-poor and As-rich holtite; it is commonly overgrown by the last generation of dumortierite enriched in As. The chemical evolution of dumortierite-group minerals is characterized by an increase of Ta, Nb and minor As, followed by an extensive enrichment in As(+ Sb + Bi) along with gradual decrease in Ta + Nb. The different mineral assemblages and particularly the compositional trends of tourmaline, dumortierite?holtite and columbite reflect superimposed processes. The initial stage comprises the magmatic crystallization of a highly evolved and boron-rich peraluminous melt. The second stage was a prograde medium-pressure metamorphism, with a fluid-phase-related episode of crystallization. The most likely source of the initial melt is an extraction of residual melt from an almost completely crystallized rare-element parental pegmatite.