Origin of voluminous mid-tertiary ignimbrites of the batopilas region, Chihuahua: implications for the formation of continental crust beneath the Sierra Madre Occidental.

The voluminous rhyolitic ignimbrites of the Batopilas region of the northern Sierra Madre Occidental are interlayered with minor amounts of andesitic and dacitic lava flows. Several lines of evidence indicate that this andesite -   dacite - rhyolite series is comagmatic. For example, the series is chemically distinct from older and younger igneous rocks found in the same geographical region; it lacks systematic variations in isotopic compositions; and trace element variations within the series can be modeled by Rayleigh fractionation calculations. Variations in Sr concentrations are inconsistent with a partial melting origin for the series, and the voluminous rhyolites of the Batopilas region appear to be the product of essentially closed-system fractional crystallization of magmas from a subcrustal source. Additional published data on mid-Tertiary rhyolites from four other areas of the Sierra Madre Occidental establish that these rocks are isotopically heterogeneous compared to those from Batopilas and that some of them clearly contain a crustal component. The subcrustal source region beneath the Sierra Madre Occidental is believed to have 87 Sr/ 86 Sr between about 0.7042 and 0.7050, and € Nd near 0. Most analyzed rhyolites have initial 87 Sr/ 86 Sr < 0.7070 and their isotopic compositions can be explained by an assimilation - fractional crystallization model that involves less than 25% of a crustal component. The extrusion of the Upper Volcanic sequence and the emplacement of the underlying batholith marked a major crust forming event in western Mexico.