Abstract
Abstract Textural, chemical, and areal relations of jadeitic pyroxene‐bearing, quartzose metagreywackes near Pacheco Pass, east‐central California Coast Ranges, indicate that: Na‐clinopyroxene is a newly generated mineral rather than clastic material; pumpellyite is a minor or nonexistent neoblastic phase in associated albite‐bearing metagreywackes, hence growth of jadeitic pyroxene must have resulted from a reaction of the sort chlorite + Na‐plagioclase + iron oxide = jadeitic pyroxene + glaucophane + lawsonite + quartz ± H2O; and, because of sluggish reaction rates, sodic plagioclase persisted in a metastable condition into the higher pressure, nearly end‐member jadeitic pyroxene + quartz P‐T field. Irregular zoning within single jadeitic pyroxene prisms and variable clino‐pyroxene/albite modes unrelated to Na‐clinopyroxene compositions are consequences of kinetics rather than reflecting equilibrium recrystallisation within a P‐T transition zone; high‐pressure overstepping of the stability boundary can ...
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