Abstract

Near Durango City, older andesites are overlain by approximately 800 m of rhyolitic volcanic rock, mostly ash-flow tuff of Oligocene age. The rhyolite is unconformably overlain by a few tens of meters of late Miocene basalt in the highlands west of Durango City and by Quaternary basalt and gravel in the Guadiana Valley.

By 32 m.y. ago, voluminous rhyolitic ash-flow tuff was being emplaced over a rugged topography developed on the older andesites. Shortly thereafter, sources in the Durango City area became active and produced two major ash-flow sheets. These ash-flow sheets, together with caldera-fill material, constitute the Carpintero Group. Subsidence in their source areas initiated development of the Chupaderos caldera complex. Most ash-flow sheets overlying the Carpintero Group originated from distant sources, but later eruptions from the caldera area produced one major post-Carpintero ash-flow unit with associated collapse of a distinctive inner caldera.

Rhyolitic volcanism ceased in the Durango area by 28 m.y. ago, but continued until 23 m.y. farther west. Chemical analyses of volcanic rocks from a strip mapped in detail across the Sierra Madre Occidental show systematic variations. Silicic volcanic rocks to the east are higher in silicon and potassium and lower in sodium, aluminum, and calcium than rocks to the west. The phenocryst mineralogy of volcanic rocks along the strip reflects these chemical variations.

Mafic rocks of the Durango area were erupted in three distinct episodes. A small amount of subalkaline basalt is interlayered with the Durango volcanic sequence. Later, alkaline lavas of the Metates Formation were erupted during an episode of late Miocene normal faulting. The large basalt field in the Guadiana Valley is inter-layered with Quaternary gravel, and the youngest flows are Holocene.

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