Revista mexicana de ciencias geológicas
versão impressa ISSN 1026-8774
RODRIGUEZ-RIOS, Rodolfo; AGUILLON-ROBLES, Alfredo e LEROY, Jacques L.. Petrologic and geochemical evolution of a topaz-bearing dome complex in the San Luis Potosi volcanic field (Mexico). Rev. mex. cienc. geol [online]. 2007, vol.24, n.3, pp. 328-343. ISSN 1026-8774.
Topaz-bearing rhyolites are widely spread throughout the Mesa Central of Mexico, quite near to the eastern limit of the Sierra Madre Occidental Volcanic Province (Durango, Zacatecas and San Luis Potosí states), whereas in the western part of the USA, they are situated at both sides of the Colorado Plateau and the Rio Grande Rift. These rhyolites are characterized by fluorine enrichments (F >0.2 wt.%) and by the presence of topaz that have crystallized due to post-magmatic processes. In the San Luis Potosí Volcanic Field (SLPVF), topaz-bearing rhyolites (San Miguelito rhyolite) mainly comprise the El Gato (DEG) and Cerro Silva (DCS) domes. These volcanic structures are associated to the extensional tectonics, which developed in the central part of Mexico during the mid-Tertiary. The San Miguelito rhyolite corresponds to a peraluminous, subalkaline, silicic magmatic episode associated to one of the events forming the SLPVF. The San Miguelito rhyolite contains between 25 and 30% of phenocrysts, this being the quantitative proportion of potassium feldspar ~18.7%, quartz ~7.7%, plagioclase ~2.4%, biotite ~1.2% mineral phases. The SLPVF (DEG and DCS) topaz-bearing ryholites present high contents of SiO2, which vary from ~76.8 to ~79.6 wt.% and an alkalies content of ~6.9 to ~8.5 wt.%, as well as strong potassic character. Rhyolitic lava flows also show an enrichment of large ion lithophile elements (LILE), a depletion in elements compatible with feldspars (Ba, Sr, and Eu) and a negative anomaly in Eu. The SLPVF topaz-bearing rhyolites seem to have originated by partial melting of a residual granulitic source from the Precambrian continental crust. In accordance with the model proposed, the presence of contemporary mafic magmas throughout the crust produced small volumes of partial crustal melts, including the breakdown of small amounts of fluorine-rich biotite that gave rise to the high fluorine concentration in these rocks. An extensional tectonic environment allowed these small groups of magma to ascend from the lower crust, without a significant contamination with the contemporary mafic magmas. Subsequent fractional crystallization gave way to the peculiar geochemical features of trace elements.
Palavras-chave : topaz-bearing rhyolites; domes; partial melting; fractional crystallization; crustal melting; granulite; San Luis Potosi volcanic field; Mexico.