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Tectonomagmatic evolution of distinct arc terranes in the Blue Mountains Province, Oregon and Idaho

By
C.J. Northrup
C.J. Northrup
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA
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M. Schmitz
M. Schmitz
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA
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G. Kurz
G. Kurz
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA
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K. Tumpane
K. Tumpane
Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA
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Published:
January 01, 2011

Abstract

Recent mapping, U-Pb zircon geochronology, trace-element geochemistry, and tracer isotope geochemistry of plutonic and volcanic rocks in the Wallowa and Olds Ferry terranes of the Blue Mountains Province yield new insights into their tectonic evolution and pre-accretion history. Igneous rocks of the Wallowa arc terrane formed in two magmatic episodes of contrasting duration and geochemical characteristics. Magmatism in the first episode lasted for at least 20 Ma (ca. 268–248 Ma), spanning the Middle Permian to the Early Triassic and was of generally calc-alkaline affinity. Rock units associated with this episode include the Hunsaker Creek and Windy Ridge formations of the Wallowa terrane, as well as potentially equivalent tonalite and diorite plutonic rocks in the Cougar Creek Complex and related basement exposures, which show midcrustal levels of the terrane. The second episode of magmatism in the Wallowa arc was remarkably brief (U-Pb zircon dates range from 229.43 ± 0.08 Ma to 229.13 ± 0.45 Ma) and dominated by mafic to intermediate compositions of tholeiitic affinity. Rock units associated with the second episode may include the Wild Sheep Creek and Doyle Creek formations, as well as ubiquitous dikes and plutons in the Cougar Creek Complex and similar basement exposures. After 229 Ma, the Wallowa arc apparently became dormant.

The record of igneous activity in the Olds Ferry arc contrasts with that of the Wallowa in its age range and the continuity of calc-alkaline magmatism. Radiometric ages and stratigraphic field relationships allow the magmatic history of the Olds Ferry terrane to be divided into at least three cycles separated by brief hiatuses and collectively spanning the late Middle Triassic through the Early Jurassic (ca. 237– 187 Ma). Rock units related to these episodes are divided by unconformities, and they include the Brownlee pluton, lower Huntington Formation, and upper Huntington Formation. Magmatic activity in the Olds Ferry arc may have persisted until at least 174 Ma, based on the presence of volcanic ash horizons in the lower portion of the overlying Weatherby Formation of the Izee basin. All cycles of Olds Ferry magmatism display generally calc-alkaline affinity.

The contrasting magmatic histories of the Wallowa and Olds Ferry arc terranes provide the basis for at least two conclusions. First, these arcs formed as separate tectonic entities, rather than as a single composite arc. Second, progressive closure of the ocean basin between the arcs in the Late Triassic and Early Jurassic was related to continued subduction beneath the Olds Ferry arc, but the Wallowa arc was apparently dormant during much of that interval.

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Contents

GSA Field Guide

Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain, and Terranes of the U.S. Cordillera

Jeffrey Lee
Jeffrey Lee
Department of Geological Sciences 400 E. University Way Central Washington University Ellensburg, Washington 98926 USA
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James P. Evans
James P. Evans
Department of Geology Utah State University 4505 Old Main Hill Logan, Utah 84322-4505 USA
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Geological Society of America
Volume
21
ISBN electronic:
9780813756219
Publication date:
January 01, 2011

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