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Professional Paper 1776–E

Prepared in cooperation with the National Park Service

Geochronology of Plutonic Rocks and their Tectonic Terranes in Glacier Bay National Park and Preserve, Southeast Alaska

By David A. Brew, Kathleen E. Tellier, Marvin A. Lanphere, Diane C. Nielsen, James G. Smith, and Ronald A. Sonnevil

Thumbnail of and link to report PDF (1.6 MB)Abstract

We have identified six major belts and two nonbelt occurrences of plutonic rocks in Glacier Bay National Park and Preserve and characterized them on the basis of geologic mapping, igneous petrology, geochemistry, and isotopic dating. The six plutonic belts and two other occurrences are, from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous (145.5–99.6 Ma) granodiorite and tonalite of the Muir-Chichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet (65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, quartz diorite, and granite of the Muir-Fairweather felsic-intermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) layered gabbros of the Crillon-La Perouse mafic belt; and (8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz syenite of the Tkope belt. The rocks are further classified into 17 different combination age-compositional units; some younger belts are superimposed on older ones. Almost all these plutonic rocks are related to Cretaceous and Tertiary subduction events.

The six major plutonic belts intrude the three southeast Alaska geographic subregions in Glacier Bay National Park and Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr Inlet Suture Zone (which contains the Border Ranges Fault Zone), and (3) the Central Alexander Archipelago. Each subregion includes rocks assigned to one or more tectonic terranes.

The various plutonic belts intrude different terranes in different subregions. In general, the Early Cretaceous plutons intrude rocks of the Alexander and Wrangellia terranes in the Central Alexander Archipelago subregion, and the Paleogene plutons intrude rocks of the Chugach, Alexander, and Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture Zone, and Central Alexander Archipelago subregions.

First posted March 17, 2014

For additional information, contact:
Alaska Science Center staff
Geological Survey
4210 University Dr.
Anchorage, AK 99508
Alaska Mineral Resources
Alaska Science Center

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Suggested citation:

Brew, D.A., Tellier, K., Lanphere, M.A., Nielsen, D.C., Smith, J.G., and Sonnevil, R.A., 2014, Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska in Dumoulin, J.A., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2008–2009: U.S. Geological Survey Professional Paper 1776-E, 18 p., https://dx.doi.org/10.3133/pp1776E.

ISSN 2330-7102 (online)



Contents

Abstract

Acknowledgments

Introduction

Subregions and Tectonic Terranes

Nonplutonic Rocks of the Subregions

Plutonic Rocks

Composition and Geochronology of Rocks in the Plutonic Belts

Descriptions of the Plutonic Rocks

Discussion

Conclusions

References Cited


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