Contrasting evolution of calc‐alkalic volcanic and plutonic rocks of western Chihuahua, Mexico

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Abstract
Two sequences of continental arc igneous rocks occur in the Batopilas area of the Sierra Madre Occidental of northwestern Mexico. The older includes late Cretaceous to (?) early Cenozoic granitoids, and the younger is a mid‐Tertiary andesite to rhyolite series that is represented largely by voluminous ignimbrites. The latter sequence contains rocks varying from 54 to 78 wt.% SiO2 whereas the granitoids have a more restricted range in SiO2, 57–68 wt %. The major element chemistry of the two sequences is generally similar; however, the older granitoids have lower Zr/Nb, and higher Nb/Y and K/Rb compared to volcanic rocks. In addition, the granitoids have generally smaller Eu anomalies, lower concentrations of REE, and higher (Ce/Yb)N (4.6–7.2 versus 3.8–4.0) than the volcanic rocks. The intermediate rocks of the two sequences, quartz diorites and andesites, had similar petrogenetic histories, and both may have originated by fractionation of mantle‐derived basaltic magmas. In contrast, the more silicic rocks, granodiorites on one hand and dacites and rhyolites on the other, evolved very differently. The granodiorites may have originated from a crustal source leaving a residue of amphibolite or garnet granulite, whereas the rhyolites were the product of protracted fractional crystallization of mantle‐derived melts.