A stable-isotope study of the hydrothermal alteration of the East Taiwan Ophiolite

Abstract

The East Taiwan Ophiolite (ETO) occurs as allochthonous fragments in the Lichi Mélange and has been suggested to have formed at an active ridge-ridge transform fault in a marginal basin. The brecciated plutonic sequence has experienced a pre-brecciation ocean-ridge type high-temperature metamorphism as well as a post-brecciation off-axis type low-temperature metamorphism, whereas the extrusive sequence has been subjected only to an off-axis low-temperature hydrothermal alteration. Stable-isotopic studies of the ETO show a slightly decreasing trend in oxygen-isotope compositions from basalt through plagiogranite/gabbro to ultramafic rocks. The isotopic compositions of the ETO basalts indicate low-temperature interaction with seawater under variable water/rock ratios. The amphiboles from the ETO gabbro show low δ 18O- (i.e. +3.1 to + 3.8‰) and high δD- (i.e. −19 to −9‰) values. This is interpreted as having resulted from high-temperature interaction with seawater-derived hydrothermal fluid. The plagioclase in the gabbro is mostly albitic in composition and exhibits very high δ 18O-values (i.e. > + 12.0‰), which may have resulted from low-temperature reaction with 18O-enriched fluid. Such 18O-enriched fluid may have been derived from a high-temperature seawater-plutonic rock interaction system. The antigorite- and lizardite/chrysotile-serpentinites in the ETO formed in oceanic environments under pre- and post-brecciation metamorphic conditions, respectively. The isotopic compositions of the antigorite are compatible with its formation in an oceanic environment. Although the δ 18O-values of the lizardites/chrysotiles are comparable to those of oceanic serpentines, their δD-values, ranging from −91 to −57‰, are too low to be accounted for by reaction with seawater-derived fluid. Post-formation hydrogen-isotope exchange between the lizardite/chrysotile and local meteoric waters during/after on-land emplacement of the serpentinites is postulated as a probable explanation. Calcite veins are ubiquitous in the ETO. Three carbon sources are identified: inorganic seawater carbon, juvenile carbon and organic carbon. In the last case, it is shown that some calcite veins may contain a substantial amount of CO 2 derived from the CH 4 oxidation process. The organic carbon was most likely derived from organics in the surrounding Lichi mudstone. Therefore, the calcite veins with an organic carbon signature are suggested to have formed after the ETO emplacement within the Lichi Mélange.