Origin of voluminous Mid-Tertiary ignimbrites of the Batopilas region, Chihuahua: implications for the formation of continental crust beneath the Sierra Madre Occidental.

Abstract

These rhyolitic ignimbrites are interlayered with minor amounts of andesitic and dacitic lava flows and it seems probable that the series is comagmatic. It is chemically distinct from older and younger rocks in the region, lacks systematic variation in isotopic composition, and trace-element variation can be modelled by Rayleigh fractionation calculations. An origin by partial melting seems unlikely and the rhyolites appear to be the products of a closed-system fractional crystallization of subcrustal magmas. Data on other rhyolites in the Sierra Madre Occidental show that they are isotopically heterogeneous and some contain a crustal component. Their Sr-isotope ratios and isotopic compositions can be explained by an assimilation-fractional crystallization model involving <25% of crustal component. The extrusion of the upper volcanic sequence and the emplacement of the underlying batholith marked a major crust-forming event in W Mexico.-R.E.S.