The topaz-bearing rhyolite lavas from the Chivinar volcanic complex (24°17?38?? S - 67°25?28?? W, Central Andes, NW Argentina)

PAOLO ORLANDI; ANNA GIONCADA; LUIGINA VEZZOLI; RICARDO H. OMARINI; ROBERTO MAZZUOLI; VANINA LOPEZ DE AZAREVICH; RICARDO SUREDA; MIGUEL AZAREVICH; DIVIDE BIANCHI2; VALERIO ACOCELLA; JOEL RUCH; HERVÉ GUILLOU; PHILIPPE NONNOTTE

TUCUMAN

Simposio; Primer Simposio sobre Petrología Ígnea y Metamórfica asociada. San Miguel de Tucumán; 2012

INSTITUTO SUPERIOR DE CORRELACION GEOLOGICA

The Chivinar volcanic complex is located in the Central Andes, at the boundary between the active volcanic arc and the backarc, in correspondence with the Calama-Olacapato-El Toro strike-slip fault system. The volcanic rocks of Chivinar and of the nearby Guanaquero volcanoes include silicic lavas that are reported to bear topaz (Koukharsky & Munizaga, 1990). The reconstruction of the magmatic processes responsible for the genesis of these peculiar lavas may furnish new elements for the understanding of petrogenetic processes at the arc-backarc boundary. Moreover, the study of these topaz-bearing rhyolites can contribute to the understanding of the formation of economic mineralizations present in the region. For these reasons, geological, stratigraphical, petrographical and mineralogical investigations have been undertaken in the Chivinar and Guanaquero region. In this framework, the results of the study of the Chivinar topaz-bearing rhyolites are presented in this contribution. The Chivinar topaz-bearing silicic lavas have low phenocryst abundance, with phenocrysts of sodic plagioclase and alkali feldspar. The groundmass is nearly holocrystalline and mainly consists of rounded quartz crystals, zoned feldspars (sodic plagioclase and sanidine) and minor magnetite. A Mn-garnet (spessartine) and, sporadically, a calcic amphibole are also present. The accessory minerals phases include zircon (with Th, U and Hf), xenotime (Y, REE), monazite (Ce), and Nb-Ti-Fe minerals. The rocks present a very fine but diffuse vesicularity. Locally, the vesicles occur in clusters of millimetric size, containing an assemblage including topaz, F-mica (montdorite/fluor-phlogopite), quartz and sodic plagioclase, indicating the final phases of crystallization of the silicic magma in the presence of a F-H2O-rich fluid.