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From ignimbrite to batholith, northeastern San Juan Mountains, Colorado: Bonanza, Cochetopa Park, and North Pass calderas

By
Peter W. Lipman
Peter W. Lipman
U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA
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William C. McIntosh
William C. McIntosh
New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
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Matthew J. Zimmerer
Matthew J. Zimmerer
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Published:
January 01, 2013

Abstract

The Southern Rocky Mountain volcanic field contains widespread andesite and dacitic lavas erupted from central volcanoes; associated with these are ~26 regional ignimbrites (each 150–5000 km3) emplaced from 37 to 23 Ma, source calderas as much as 75 km across, and subvolcanic plutons. Exposed plutons vary in composition and size from small roof-zone exposures of porphyritic andesite and dacite to batholith-scale granitoids. Calderas and plutons are enclosed by one of the largest-amplitude gravity lows in North America. The gravity low, interpreted as defining the extent of a largely concealed low-density silicic batholith complex, encloses the overall area of ignimbrite calderas, most of which lack individual geophysical expression. Initial ignimbrite eruptions from calderas aligned along the Sawatch Range at 37–34 Ma progressed southwestward, culminating in peak eruptions in the San Juan Mountains at 30–27 Ma. This field guide focuses on diverse features of previously little-studied ignimbrites and caldera sources in the northeastern San Juan region, which record critical temporal and compositional transitions in this distinctive eastern Cordilleran example of Andean-type continental-margin volcanism.

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Contents

GSA Field Guide

Classic Concepts and New Directions: Exploring 125 Years of GSA Discoveries in the Rocky Mountain Region

Lon D. Abbott
Lon D. Abbott
Department of Geological Sciences University of Colorado Boulder, Colorado 80305 USA
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Gregory S. Hancock
Gregory S. Hancock
Department of Geology College of William and Mary Williamsburg, Virginia 23187 USA
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Geological Society of America
Volume
33
ISBN electronic:
9780813756332
Publication date:
January 01, 2013

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