Abstract

The basaltic rocks of the fragmented East Taiwan Ophiolite have been subjected to “ocean-floor” zeolite facies metamorphism. Depending on the bulk composition and mode of occurrence, various mineral assemblages occur: thomsonite + analcime + chabazite; pumpellyite + chlorite + laumontite in veins of the pillow cores; and pumpellyite + chlorite + K-feldspar; pumpellyite + laumontite + thomsonite; and prehnite (about 5 weight percent Fe203) + hematite in veins of the pillow matrices. Plagioclase phenocrysts were replaced by albite + pumpellyite + Ca-zeolites, pumpellyite + K-feldspar, analcime + chabazite + thomsonite, or by K-feldspar alone, the olivine phenocrysts by brown chlorite + serpentine ± pumpellyite, but the pyroxenes are well preserved. Except for local palagonitization along fractures and rims, the pillowed glassy rims are perfectly fresh. The variations in mineral association among pillow cores, rims, and matrices are believed to have resulted from local variations in compositions of the circulating fluid, in the extent of recrystallization, and in temperature. The occurrence of potash feldspar suggests that the basaltic rocks have been subjected to local metasomatic exchange with a hydrothermal solution under zeolite facies conditions.

Pumpellyites characteristically contain up to 25 weight percent total Fe as FeO, higher than most reported pumpellyite (except julgoldite). Calculation of their structural formulas indicates that they may contain iron dominantly in the ferric state. Substitution of Fe3+ for Al in this phase evidently enlarges the pumpellyite P-T stability field relative to the zeolite facies assemblages under oxidizing conditions. Pumpellyite probably crystallized directly from palagonitized basaltic glass. The basaltic rocks of the East Taiwan Ophiolite were hydrothermally metamorphosed at T = 150-250°C and depths of 0.6 to 1.6 km, probably at some distance away from the ridge.

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