Petrology and Geochemistry of DSDP Leg 35 Basalts, Bellingshausen Sea

Abstract

Basaltic rocks were recovered from two sites on the abyssal plain of the Bellingshausen Sea during Leg 35 of the Deep Sea Drilling Project. Disoriented basaltic cobbles, some of which have glassy rinds, hyaloclastite fragments, and numerous chips of black glass cored at Site 322 are interpreted to be the upper surface of a brecciated pillow flow. However, high penetration rates (12-13 m/hr) indicate the drilled interval may consist of interbedded lava flows and sediments, and true oceanic basement (layer 2) probably was not reached. Except for an abnormally high K2O content, the mineralogy and chemistry are typical of average mid-ocean ridge tholeiites. Many of the plagioclase microphenocrysts contain irregular to sharply bounded cores of almost pure potassium feldspar, a feature previously reported from Sites 165 (Leg 17) and 192 (Leg 19). This feature probably results from the filling of hollow centers of hopper crystals by potassic-rich solutions produced during alteration. Textural variation, mineralogy, and drilling rates suggest that one and possibly two sills were penetrated at Site 323, and oceanic basement (layer 2) was not reached. The mineralogy and chemistry are also typical of average mid-ocean ridge basalt although the K2O content is abnormally high. The high oxidation index and abnormally low K-Ar age suggest that the apparent alkalinity of these rocks has been increased during alteration. A possible sill with even stronger alkaline tendencies has been cored at Site 170 (Leg 17) in the central Pacific, and the occurrence of sills with alkaline affinities in deep oceanic basins indicates the oceanic crust may be far more "leaky" than was originally assumed.