Garnet types from the Cazadero area, California

Journal of Petrology
By: , and 



This is the third in a series of papers on glaucophane schists from the Franciscan Formation near Cazadero, California. Previous papers describe three distinct types of glaucophane-bearing Franciscan metamorphic rocks near Cazadero.

The purpose of this study is to investigate the garnets present in metamorphic types III (bedrock schists) and IV (tectonic blocks) as defined by Coleman & Lee (1963). Twenty-four garnet analyses are presented. Sixteen of these are from (aragonite-bearing) type III glaucophane schists, and eight are from type IV glaucophane schists. Type IV rocks include California eclogites.

Type III rocks include metabasalt, metachert, metashale, meta-ironstone, and metacarbonate that were formed under high pressure and relatively low temperature. These rocks contain garnets that display a wide range of composition, but the dominant molecules represented are consistently almandine, spessartine, and grossular.

Type IV rocks are mainly metabasalts that were probably formed under higher temperatures and pressures than type III rocks. There is a distinct difference between garnets from type III rocks and those from type IV (including eclogites); the lattercontain less spessartine and more pyrope, and the dominant molecules are almandine and grossular. The four analyses of garnets from California eclogites have an average pyrope content of about ten molecular per cent, and they extend the range of composition reported for eclogite garnets.

Quantitative spectrographic determinations of minor elements are listed for each of the garnets described. The values determined for some of the minor elements have a wide range and a capricious distribution over a few feet of outcrop area.

As a group, both the garnets from type III rocks and those from type IV are pyralspites with large contents (as much as 35 molecular per cent) of ugrandite. This unusual admixture of the pyralspite and ugrandite garnet series may have resulted in part from the conditions (high pressures and relatively low temperatures) under which the enclosing rocks were recrystallized.

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Publication type Article
Publication Subtype Journal Article
Title Garnet types from the Cazadero area, California
Series title Journal of Petrology
DOI 10.1093/petrology/4.3.460
Volume 4
Issue 3
Language English
Publisher Oxford University Press
Description 33 p.
First page 460
Last page 492
Country United States
State California
Other Geospatial Cazadero area
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