The igneous history of the Sierra Madre Occidental and its relation to the tectonic evolution of western Mexico

Abstract

Igneous rocks of the Sierra Madre Occidental have been mapped and dated along latitude 24° N between Mazatlán and Durango City and near latitude 28° N in Chihuahua. In both of these regions and probably throughout the Sierra Madre Occidental are found two vast and largely coextensive igneous sequences, both calcalkalic and both including ignimbrites, but otherwise dissimilar. The older sequence, ranging in age from 45 m. y. to at least 100 m. y., contains abundant batholithic as well as volcanic rocks dominantly of intermediate composition. The younger sequence is dominated by rhyolitic ignimbrites erupted from numerous large caldera complexes. Smaller outpourings of basaltic lava accompanied the ash flows, but intermediate rocks are rare. Most of the volcanism was confined tu a brief interval 34 to 27 m. y. ago.