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Low-grade, M (sub 1) metamorphism of the Douglas Island Volcanics, western metamorphic belt near Juneau, Alaska

Glen R. Himmelberg, David A. Brew and Arthur B. Ford
Low-grade, M (sub 1) metamorphism of the Douglas Island Volcanics, western metamorphic belt near Juneau, Alaska (in Low-grade metamorphism of mafic rocks, Peter Schiffman (editor) and Howard W. Day (editor))
Special Paper - Geological Society of America (1995) 296: 51-66


The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Deformation, metamorphism, and plutonism range from about 120 to 50 Ma. Subgreenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska, record the earliest metamorphic event (M (sub 1) ). The protolith of the M (sub 1) , low-grade metamorphic mineral assemblages is mostly arc-affinity basaltic rocks of the Douglas Island Volcanics. The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low variance assemblages can be attributed to minor differences in pressure (P), temperature (T), or X (sub CO (sub 2) ) . Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 degrees C and 2 to 4.8 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all compatible with metamorphism of the Douglas Island Volcanics at a depth of 7 to 20 km. The low-grade rocks are contiguous with younger (M (sub 5) ), higher grade assemblages that define an inverted metamorphic gradient. The discontinuity in pressure indicated by the M (sub 1) mineral assemblages and M (sub 5) geobarometry (9-11 kbar) suggests juxtaposition of the two metamorphic sequences by vertical uplift along the Coast Range megalineament.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 296
Title: Low-grade, M (sub 1) metamorphism of the Douglas Island Volcanics, western metamorphic belt near Juneau, Alaska
Title: Low-grade metamorphism of mafic rocks
Author(s): Himmelberg, Glen R.Brew, David A.Ford, Arthur B.
Author(s): Schiffman, Petereditor
Author(s): Day, Howard W.editor
Affiliation: U. S. Geological Survey, Columbia, MO, United States
Affiliation: University of California at Davis, Department of Geology, Davis, CA, United States
Pages: 51-66
Published: 1995
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 0-8137-2296-9
References: 55
Accession Number: 1995-032499
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables, geol. sketch maps
N58°15'00" - N58°30'00", W134°40'00" - W134°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 199512
Program Name: USGSOPNon-USGS publications with USGS authors
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