VARIATIONS IN CALDERA DEVELOPMENT IN THE TERTIARY VOLCANIC FIELD

Calderas, 4–30 km in diameter, are the major igneous centers of Trans-Pecos Texas. Twelve calderas occur in two north-northwest-trending belts. Eight, including two in adjacent Chihuahua, occur in a western alkali-calcic belt. Four are in an eastern alkalic belt. Additional calderas are probably associated with two major ash-flow sheets in the eastern belt. Sources exist for all major tuffs of Trans-Pecos, other than these two, so additional calderas are unlikely. All calderas were formed between 38 and 28 Ma ago. The two calderas in Chihuahua were formed 30 and 28 Ma ago and show significant differences in composition and eruptive style from the earlier calderas. Calderas are underlain by individual magma chambers that were active for no more than about 1 Ma. The caldera cycle in Texas is highly variable. Minor volcanism commonly occurred before ash-flow tuff eruption and caldera collapse. Tumescence occurred in at most two calderas. Ash-flow eruption and collapse were simultaneous in all calderas, as shown by thick, intracaldera tuffs that are interbedded with breccias shed from caldera walls. Collapse occurred along discrete fracture systems in most of the western calderas but along hinge zones in the eastern calderas and at least part of the two young calderas in Chihuahua. Sedimentation within calderas was minor. Preresurgent volcanism ranged from minor to extensive and was best developed in the western calderas, all of which show some form of resurgence. No eastern calderas were resurgent. Resurgence is expressed in many ways in western calderas, but only a few fit the classic model. Ring-fracture volcanism was irregularly developed. Hydrothermal alteration and ore deposition were abundant and spatially and genetically related to calderas in the western alkali-calcic belt. Alteration and ore deposition were minor to nonexistent in the eastern alkalic belt.

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