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Igneous rocks of the Sierra Madre Occidental have been studied along two traverses across the range. One is at lat 24°N between Mazatlán and Durango City, where contiguous mapped areas extend across the Sierra; the other is near lat 28°N, where several separate areas west and north of Chihuahua City have been mapped. In these regions the Sierra contains two vast and largely coextensive igneous sequences, both calc-alkalic and both including ignimbrites. The older sequence of rocks, which ranges in age from 45 m.y. to at least 100 m.y., is characterized by abundant batholithic as well as volcanic rocks and is dominantly intermediate in composition. The younger sequence is dominated by rhyodacitic to rhyolitic ignimbrites erupted from large caldera complexes, generally accompanied by rhyolite flows and domes and small outpourings of mafic lavas. Intermediate rocks are rare, and volcanism was largely confined to the interval 34 to 27 m.y. ago, although some activity persisted until 23 m.y. ago.

Mid-Tertiary volcanic rocks are exposed eastward across Chihuahua between the Sierra Madre Occidental and the alkalic volcanic province of Trans-Pecos Texas. Volcanic rocks of this region are chiefly calc-alkalic in chemistry, but their alkalinity is intermediate between that of the Sierra Madre Occidental and of Trans-Pecos Texas. Ages are both similar to and older than those in the Sierra.

The chronology of both igneous sequences of the Sierra Madre Occidental fits some of the known sea-floor–spreading patterns. The older sequence is similar in age and composition to batholithic terranes of the western United States that were emplaced during Cretaceous–early Tertiary convergence along the western margin of the North American plate. The younger sequence was also emplaced during a period of convergence. Volcanism declined 27 m.y. ago at about the time of reorientation of an east Pacific spreading center and early ridge-trench interaction at the nearest continental margin. The gap in magmatism between 45 and 34 m.y. and the abrupt onset of volcanic activity 34 m.y. ago, however, do not match known sea-floor–spreading events. Furthermore, the brief age span and the bimodal calc-alkalic composition of the younger sequence are not typical for an igneous province at a continental margin.

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