Abstract

Three suites of mid-Tertiary volcanic rocks are recognized in the state of Chihuahua in northwestern Mexico: (1) a dominantly silicic calc-alkalic suite that includes moderate-K and high-K. facies, (2) a ferroaugite rhyolite suite, and (3) a mildly alkalic basaltic suite. Subduction-related calc-alkalic volcanism was predominant until ∼28 m.y. ago when it was replaced by basaltic volcanism. The source of the basaltic magmas in western Chihuahua was enriched in lithophile elements relative to that of the calc-alkalic magmas. The ferroaugite rhyolites, which are restricted to eastern Chihuahua, appeared ∼32 m.y. ago. They are genetically unrelated to the calc-alkalic suite and are interpreted to represent a westward expansion of alkalic volcanism from west Texas.

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