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Book Chapter

The Laramie anorthosite complex and its contact metamorphic aureole

By
B. Ronald Frost
B. Ronald Frost
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
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Robert L. Bauer
Robert L. Bauer
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
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James S. Scoates
James S. Scoates
Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T1Z4, Canada
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Jeffrey S. Ingram
Jeffrey S. Ingram
Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA
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Published:
January 01, 2013

Abstract

The Laramie anorthosite complex (LAC) is the one of the best-studied and most accessible anorthosite complexes in the world. Its components, which range in age from ca. 1432 to1436 Ma, are exposed over ~800 km2 in the southern Laramie range of southeastern Wyoming. Although the eastern margin of the LAC has been truncated by Laramide faulting, its western and northern margins expose local areas of contact with the adjacent Archean gneiss and supracrustal rocks that were reworked by Paleoproterozoic deformation and metamorphism. Three major compositional units are recognized in the LAC: (1) anorthositic rocks, which occur as three major domal structures, the Poe Mountain anorthosite, the Chugwater anorthosite, and the Snow Creek anorthosite; (2) leucogabbroic rocks, which occur as the Strong Creek Intrusion and as smaller intrusions into the anorthositic rocks; and (3) monzonitic rocks, which form three major intrusions—the Red Mountain, Sybille, and Maloin Ranch, rimming the anorthositic rocks.

The first day of the field trip examines the anorthositic layered zone and leucogabbroic layered zones within the Poe Mountain anorthosite, which is the oldest and northernmost of the LAC anorthositic rocks. The stops illustrate the processes involved in the formation of anorthositic magma chambers at middle to upper crustal levels, and the magmatic structures that form in Proterozoic anorthosites and mafic layered intrusions where plagioclase is a major constituent. The first half of the second day of the trip examines the Sybille monzosyenite intrusion that rims the western margin of the Poe Mountain anorthosite and is in contact with the Archean granitic gneiss and supracrustal rocks to the west and northwest. The stops illustrate mineralogical variations in the Sybille monzosyenite intrusion, a pendant of ferrodiorite and anorthosite, and the role of fluids in stabilizing the mafic mineral assemblages in these rocks. The latter half of the second day will be a walking traverse across the contact aureole on the western margin of the LAC where temperatures in excess of 900 °C were reached.

The last day of the trip examines contact metamorphic zonation and local structural relationships in the Archean country rocks adjacent to the Red Mountain pluton (RMP). The stops illustrate the contact effects along the western margin of the RMP where the country rocks contain interlayers of the RMP. The intrusion of the main RMP body reoriented pre-intrusion deformation features in the country rocks, deformed early intrusive veins of the RMP, and produced contact metamorphic zones that overprint assemblages from previous periods of Paleoproterozoic metamorphism. Thermometry and reactions relationships in the RMP thermal aureole indicate temperatures as high as 800 °C near the contact and within interlayered regions to ca. 600 °C 1.5 km from the contact.

The trip stops include easily accessible outcrops along Wyoming Highway 34, and less accessible stops on a private ranch property.

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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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