Petrological and Geochemical Study of Spilites and Associated Rocks from St. John, U.S. Virgin Islands

Abstract

A continuous section of extrusive and intrusive rocks has been recovered through a drill core of about 2430 ft that was taken at St. John, Virgin Islands. This study is confined to the upper 1180 ft and comprises a cumulative total of 950 ft of altered mafic extrusive rocks (spilite). The spilite is intruded by two amphibole porphyries, one albite-diabase, and oneandesine-diabase. This spilite is further subdivided, according to its appearance, into three zones: a patchy spilite, a non-patchy spilite, and a brecciated spilite. The patchy spilite comprises the upper 650 ft of the core and consists of dark gray patches (2 cm average diameter) containing plagioclase, clinopyroxene, chlorite, and an abundant dark mesostasis. These patches are set into a green matrix rich in chlorite, epi-dote, quartz, and calcite, but low in plagioclase relative to the dark patches. The occurrence of the dark patches and their disappearance with depth (non-patchy spilite) is an igneous feature related to the cooling history of the parental magma of the spilite. The upper part of the flow (patchy spilite) solidified faster than the lower portion (non-patchy spilite), giving rise to centers of crystallization (dark patches) set into a glassy groundmass (green matrix). The effect of autometasomatism as a primary process for the origin of the spilites is still questionable or perhaps masked by a post-consolidation process. It was found that pumpellyite and prehnite disappeared below 700 ft in the core with a concomitant increase of actinolite. Some mobile compounds, mainly K 2 O, also decreased continuously with depth throughout the different lithological units encountered in the core. These mineralogical and chemical variations suggest that the parental magma of the spilites from St. John was altered due to a burial metamorphism of the prehnite-pumpellyite facies grading into the green-schist facies. While the preserved textural features of the spilite show a close correlation with basaltic rocks, the chemistry of the spilite may approach the composition of an amphibolite.