Paragenetic relationships in an amphibolitic tectonic block in the franciscan terrain, panoche pass, California

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Two tectonic inclusions within the Franciscan mélange of California display a relict epidoteamphibolitic assemblage that has been modified by rather continuous degrees of retrogressive replacement to blueschist facies minerals. The relict assemblage consists of plagioclase +hornblende+epidote+sphene±rutile±calcic pyroxene±garnet; superimposed during later blueschist metamorphism is the asemblage sodic amphibole+lawsonite±sodic pyroxene±pumpellyite±phengitic mica ±albite+sphene±Ca-carbonate. In an attempt to evaluate chemical redistribution accompanying blueschist metamorphism, electron microprobe analyses have been determined for all major minerals from both assemblages.Hornblende commonly exhibits replacement by crossite around grain boundaries and replacement patches of more riebeckitic amphibole adjacent to mafic grains or inclusions; actinolitic amphibole occurs in veins cross-cutting hornblende and in some cases these veins are zoned to include a central core of sodic amphibole. Omphacitic pyroxene occurs in veinlets and replaces calcic pyroxene in some cases; a more aluminous omphacite variety is commonly associated with the breakdown of epidote. Garnet is highly retrogressed to chlorite. In general, relict Ca-Al silicates are in various stages of breakdown, being largely converted to lawsonite and pumpellyite; concurrently Na-Al silicates formed by replacement where early calcic minerals participated in blueschist reactions. Reactants and products may be separated by phases not participating in the reactions. Whole rock compositions point strongly to a closed system with respect to all major cations, and the principal source for Na participating in replacement reactions appears to be from the breakdown of albite. The mechanism for replacement of the older assemblage is primarily attributed to a contemporaneous series of coupled reactions among nearly all the early minerals and a fluid phase.Sea-floor spreading models may explain the tectonic emplacement of the amphibolite from a relatively high temperature-high pressure regime into a comparatively low temperature-high pressure environment leading to the observed mineralogical re-adjustments. © 1973 Oxford University Press.

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Journal of Petrology