Jadeite-rocks and glaucophane-schists from Angel Island, San Francisco Bay, California

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ID: 301189
1960
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Abstract
The rocks of Angel Island comprise a series of Franciscan (Jurassic) arkosic graywackes with subordinate amounts of conglomerate and radiolarian chert. Almost all the graywackes have been metamorphosed to jadeite-bearing metagraywackes independently of proximity with basic or ultrabasic intrusions. A diabase sill, originally intrusive into the sedimentary series, has also been metamorphosed and its feldspar converted to chlorite, lawsonite, and jadeite. Glaucophane occurs in the altered diabase and is particularly abundant at its margins. Towards the diabase the jadeite-bearing metagraywackes become richer in glaucophane and pass into jadeite-free schistose glaucophane-rocks. Chemical analyses show that the change is accompanied by some desilication and an addition of Fe, Mg, Ca, Ti, and possibly Na. This metasomatism, which may have preceded, or coincided with, the metamorphism, was probably hydrothermal in nature and localized along igneous contacts and shear-planes. Similar, but more extensive developments of glaucophane-schist, also derived from graywacke, occur in association with a dike of serpentinized pyroxenite. Hydrothermal processes directly related to the serpentinization may have played a significant part in the chemical alteration of the graywacke. Movements along serpentinite contacts complicates the relationships, and many large masses of glaucophane-schist and jadeite-bearing metagraywacke occur as tectonic inclusions within the serpentinite. Plagioclase is absent from, and considered unstable in, all the metamorphic assemblages. While jadeite appears to be stable in the arkosic metagraywackes deficient in Fe, Mg, and Ca; its place is taken by acmitic-jadeite and eventually glaucophane in the glaucophane-schists.
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Authors T. W. Bloxam
Journal american journal of science
Year 1960
DOI
10.2475/ajs.258.8.555
URL
https://ajsonline.org/article/58835.pdf
Keywords Keywords not found

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