Petrology and geochmistry of gabbroic and related rocks from Site 894, Hess Deep

Abstract

The petrogenesis of gabbroic rocks and associated basaltic dikes recovered at Ocean Drilling Program Site 894 has been evaluated using petrography, mineral chemistry, and bulk-rock chemistry as well as Nd isotope analyses. The igneous lithologies recovered at Site 894 include gabbronorite, gabbro, olivine gabbro, olivine gabbronorite, and basalt. The gabbroic rocks and the crosscutting dikes show similar isotope and trace elements signatures, and they are therefore suggested to be cogenetic. The trace element data indicate that the rocks formed from a highly depleted parental magma. Isotopic compositions of Site 894 rocks are within the range of mid-ocean-ridge basalt, and similar to gabbroic rocks recovered from Site 895. The gabbroic rocks show no systematic variation in mineral compositions downhole, but exhibit large variations on a local scale. Plagioclase compositions, for example, vary by up to 25 mole percent An in a single thin section. The rare earth element contents vary by 1 order of magnitude, which is close to the total range seen in the crustal section of many ophiolite complexes. There is, however, no systematic, large-scale variation in bulk-rock compositions downhole. The downhole variations are characterized by abrupt (meter scale) variations in compatible and incompatible trace element contents. There appears to be a decoupling in the behavior of incompatible and compatible trace elements in the gabbroic rocks. Cr shows a well-defined trend when plotted against Ca# whereas TiO2 and Zr do not. The lack of any correlation between TiO2 and Zr with Ca numbers suggests that the abundance of oxides and incompatible element-bearing phases such as zircon and apatite (which is present in the rocks) is unrelated to the composition of the silicate matrix. The local enrichments of incompatible elements in rocks that are relatively rich in compatible elements indicate that the geochemistry of the Site 894 gabbros is not controlled by mineral crystallization and fractionation alone. We suggest that the Site 894 rocks formed along the roof of the magma reservoir where a sequence of mesocumulates and basaltic gabbros developed owing to limited buoyancy-driven migration of interstitial melts. Late magmatic liquids migrated locally into the almost-crystallized matrix. These late magmatic liquids enriched the rocks in incompatible elements but had little influence on the distribution of compatible trace elements and major elements in the rocks. The trace element geochemistry of the rocks may be understood in terms of fractional crystallization and various mixing processes.