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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 766

Last Page: 766

Title: Volcanic Rocks of Sierra Pastorias Caldera Area, Chihuahua, Mexico: ABSTRACT

Author(s): Peter K. Megaw

Article Type: Meeting abstract


The Sierra Pastorias lie just south of Chihuahua City, Mexico. The area contains two resurgent caldera systems and one nonresurgent caldera. With minor variations, both resurgent calderas follow Smith and Bailey's model closely. The major caldera is 22 km in diameter and is characterized by a 600-m thick, densely welded, lithic-rich, two-feldspar, intracaldera facies ash-flow tuff core, surrounded by moat-zone sediments and voluminous porphyritic rhyolite flows. A minimum of 200 cu km of tuff was erupted. The smaller resurgent caldera is 10 km in diameter and it ejected at least 60 cu km of ash-flow tuffs similar to those of the major caldera. Both resurgent calderas are domal structures with apical grabens. Each dome is ringed by vertically foliated rhyolite necks which ntruded along the ring-fracture zones. Nested within the major caldera is a nonresurgent caldera which erupted poorly welded, sanidine "moonstone" bearing tuffs.

The composite section has a total thickness of nearly 3,000 m. Rhyolite ash-flow tuff accounts for 80% of this, with fluvial rhyolite, volcaniclastic sediments, and olivine-augite-bearing basalt comprising the remainder. A small volume of rhyolite intrusion is also present.

Chemically, the rocks define a quartz-normative, bimodal suite composed of calc-alkaline rhyolite and basalt. The alkalinity and other chemical characteristics of these rocks fall between those typical of the Sierra Madre Occidental and eastern Chihuahua; the parental magmas were probably generated by subduction-related processes. No ages have been determined.

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