The Cedars Ultramafic Mass, Sonoma County, California

By M. Clark Blake, Jr., Edgar H. Bailey, and Carl M. Wentworth

Abstract

The Cedars ultramafic mass is a mantle fragment that consists of partially serpentinized spinel harzburgite and dunite. Compositional layering and a chromite lineation define a penetrative metamorphic foliation that almost certainly formed in the upper mantle. Although detailed petrofabric and mineral chemistry are presently lacking, it seems reasonable that the Cedars peridotite represents a slice of mantle tectonite that once formed the base of the Coast Range ophiolite, and not an abyssal peridotite tectonically emplaced within the Franciscan accretionary prism.

First posted August 6, 2012

Revised April 29, 2016

This report is available only on the Web.

For additional information:
Contact Information, Geology, Minerals, Energy, and Geophysics Science Center—Menlo Park
U.S. Geological Survey
345 Middlefield Road, MS-973
Menlo Park, CA 94025-3591
http://geomaps.wr.usgs.gov/

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Suggested citation:

Blake, M.C., Jr., Bailey, E.H., and Wentworth, C.M., 2016, The Cedars untramafic mass, Sonoma County, California (ver. 1.2, April 2016): U.S. Geological Survey Open-File Report, 2012–1164, 13 p., 1 plate. (Available at http://pubs.usgs.gov/of/2012/1164/.)

Contents

Abstract

Introduction

Regional Geologic Framework

Description of the Peridotite

Peridotite Chemistry

Plate Tectonic Setting

Conclusions

Acknowledgments

References Cited