MINERALOGY AND ORIGIN OF THE DUMORTIERITE-BEARING PEGMATITES OF VIRORCO, SAN LUIS, ARGENTINA
| Authors | |
|---|---|
| Year of publication | 2012 |
| Type | Article in Periodical |
| Magazine / Source | Canadian Mineralogist |
| MU Faculty or unit | |
| Citation | |
| Doi | http://dx.doi.org/10.3749/canmin.50.4.873 |
| Field | Geology and mineralogy |
| Keywords | granitic pegmatite; dumortierite; holtite; chrysoberyl; kyanite; tourmaline; Virorco; Argentina |
| Description | The Virorco dumortierite-bearing pegmatites, in Sierra de San Luis, Argentina, are a group of thin, steeply dipping dikes one to 10 cm thick. The pegmatits are symmetrically zoned, with quartz, albite, oligoclase, tourmaline- 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 assemblage. Primary textures of magmatic origin were partially disrupted by partial replacements by later minerals and incipient to strong deformation. Five textural and compositional types of tourmaline-supergroup minerals were identified in the different pegmatitic zones, ranging from dravite-rich compositions to rossmanite, 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 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. |
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