Origin, stratigraphy and evolution of the Oligocene rhyolitic volcanism in the central and southern Mesa Central of Mexico (Villa García, Zacatecas and La Herradura, San Luis Potosí), by means of geochemistry, petrology and geochronology.

Abstract

"The Mesa Central province forms part of the southern Sierra Madre Occidental silicic large igneous province. Oligocene felsic volcanism is common in the center and southern portion of the Mesa Central physiographic province. Generally, rhyolitic dome complexes and ignimbrites occur aligned with regional fault systems, mostly associated with the southern Basin and Range extension, and thus suggesting that faults controlled the felsic magmas that formed these domes, lava flows, and ignimbrites, however not all have been studied in detail. The present work presents data on the volcanic stratigraphy, geochemistry, geochronology, and petrology of the Oligocene felsic volcanism in this area. In the La Herradura area the garnet-bearing rhyolite and a rhyolitic ignimbrite forms together with an andesite 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. Garnet chemistry as well as the chemistry of the host rhyolite implies the crystallization as a primary phase from a lower crustal peraluminous magma. The absence of crustal xenoliths, the idiomorphic shape and the garnet chemistry as well as the geochemistry of the host rhyolite in combination with geothermobarometric analysis and U-Pb zircon single grain age determination on zircon inclusion in garnet (31.70 ± 0.21 Ma and 32.22 ± 0.67 Ma) and zircons derived from whole rock samples (31.52 ± 0.48 Ma and 31.32 ± 0.97) confirms the crystallization of garnet as a primary phase from a lower crustal peraluminous magma. The appearance of primary magmatic almandine-rich garnets is confirmed by rhyolite-MELTS simulations on the crystallization. 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. The Oligocene Villa Garcia Ignimbrite complex in the southern portion of the Mesa Central is built up by a composite volcanic stratigraphy. A rhyolitic lava flow initiates the volcanic activity and two voluminous rhyolitic ignimbrites form, together with an andesitic lava flow the top of the stratigraphy. Both ignimbrites the older Villa Garcia ignimbrite (31.59 ± 0.14 Ma and 31.15 ± 0.12 Ma) and the younger Loreto Ignimbrite (28.97 ± 0.11 Ma and 28.66 ± 0.07 Ma) are densely welded and show lava-like to rheomorphic structures. Geochemical features show that the rhyolitic magmas are derived in an intra-plate setting from a process including partial melting of the lower crust and fractional crystallization during ascent, in case of the ignimbrites also during magma storage and in a shallow magma chamber. The andesitic lava flow is metaluminous, contains olivine and pyroxene, and its trace element patterns suggest that the parental magma was derived from upper mantle partial melting. I propose that the southern Basin and Range extension event favored the direct ascent of these magmas from the source through the brittle, where slight interaction with the country rock took place, to the surface."