Paragenetic Relationships in an Amphibolitic Tectonic Block in the Franciscan Terrain, Panoche Pass, California |
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| Authors: | HERMES O DON |
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| Institution: | Department of Geology, University of Rhode Island Kingston, Rhode Island 02881 |
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| Abstract: | Two tectonic inclusions within the Franciscan mélangeof California display a relict epidoteamphibolitic assemblagethat has been modified by rather continuous degrees of retrogressivereplacement to blueschist facies minerals. The relict assemblageconsists of plagioclase +hornblende+epidote+sphene±rutile±calcicpyroxene±garnet; superimposed during later blueschistmetamorphism is the asemblage sodic amphibole+lawsonite±sodicpyroxene±pumpellyite±phengitic mica ±albite+sphene±Ca-carbonate.In an attempt to evaluate chemical redistribution accompanyingblueschist metamorphism, electron microprobe analyses have beendetermined for all major minerals from both assemblages. Hornblende commonly exhibits replacement by crossite aroundgrain boundaries and replacement patches of more riebeckiticamphibole adjacent to mafic grains or inclusions; actinoliticamphibole occurs in veins cross-cutting hornblende and in somecases these veins are zoned to include a central core of sodicamphibole. Omphacitic pyroxene occurs in veinlets and replacescalcic pyroxene in some cases; a more aluminous omphacite varietyis commonly associated with the breakdown of epidote. Garnetis highly retrogressed to chlorite. In general, relict Ca-Alsilicates are in various stages of breakdown, being largelyconverted to lawsonite and pumpellyite; concurrently Na-Al silicatesformed by replacement where early calcic minerals participatedin blueschist reactions. Reactants and products may be separatedby phases not participating in the reactions. Whole rock compositionspoint strongly to a closed system with respect to all majorcations, and the principal source for Na participating in replacementreactions appears to be from the breakdown of albite. The mechanismfor replacement of the older assemblage is primarily attributedto a contemporaneous series of coupled reactions among nearlyall the early minerals and a fluid phase. Sea-floor spreading models may explain the tectonic emplacementof the amphibolite from a relatively high temperature-high pressureregime into a comparatively low temperature-high pressure environmentleading to the observed mineralogical re-adjustments. |
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