Almandine garnet-bearing rhyolites associated to bimodal volcanism in the Mesa Central of Mexico: Geochemical, petrological and geochronological evolution

Abstract : Garnet-bearing rhyolitic flows and ignimbrites occur together with an andesitic lava flow in the central part of the Mesa Central in the western part of the Mexican state of San Luis Potosí, forming a typical K-rich sub-alkaline intra-plate tectonic setting bimodal series. Electron microprobe analysis indicates that the garnet is homogeneous and primarily almandine with subordinately pyrope, grossular, spessartine, and andradite. The chemistry of the garnets as well as the chemistry of the host rhyolite implies the crystallization of garnet as a primary phase from a lower crustal peraluminous magma. Rhyolite-MELTS simulations on the crystallization confirms this information. Trace element analyses on the rhyolitic ignimbrite and the garnet-bearing rhyolite suggest that the petrogenesis of these acidic rocks involved partial melting of the lower crust and fractional crystallization. The andesitic lava flow is metaluminous and its trace element patterns suggest that the parental magma was derived from upper mantle partial melting and that it was stored at the mantle-crust boundary. We propose that the southern Basin and Range extension event favored the direct ascent of these magmas from the source through the brittle and thin crust (30-33 km), where slight interaction with the country rock took place, to the surface. Usbnd Pb dating on zircons from the rhyolites yielded ages ranging from 32.26 ± 0.92 Ma to 31.52 ± 0.48 Ma, consistent with the 31.25 ± 0.70 Ma whole rock Ksbnd Ar age of the garnet-bearing rhyolite. This information supports the interpretation that the garnets are not xenocrysts but they are crystallized from a rhyolitic magma at lower crustal depths and ascended fast through the brittle crust, so the garnets could be preserved.