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Dredge site 70-49B. Approximately 800 kg of blocky fragments of grayish-black vesicular andesite and an equal amount of sedimentary rock, chiefly siliceous mud and cherty diatomicious shale and mudstone were recovered. Although a number of small fragments of greenish-gray altered amygdaloidal lava were retrieved, most of the volcanic rocks are relatively fresh andesite. Vesicles are drawn out or flattened parallel to flow lines delineated by aligned plagioclase microlites. The andesite was presumably erupted as a submarine lava flow, and it either overlies or is interbedded with the siliceous beds. Both lithologic units apparently crop out below the prominent slope break (at a depth near 1000 m) on reflection profile B49. The andesite is porphyritic, with 13 percent augite phenocrysts (0.1 mm to 2 mm), microphenocrysts, and glomerophenocrysts set in a flow-banded hyalo-ophitic-textured matrix of plagioclase, augite, and dark brown glass. Cores of many larger phenocrysts consist of mottled aggregates of augite, opaque minerals, and glass that were produced during an early partial resorption; phenocrysts rims are weakly zoned. Glomerophenocrysts consist of clots of as many as 20 small, euhedral to subhedral grains partly separated by glass. The matrix, too fine grained (0.02 to 0.1 mm) to determine a mode, is composed of subequal amounts of augite, plagioclase, and glass and lesser amounts of opaque minerals, hypersthene, and oxyhornblende. Opaque minerals dust the surface of the matrix augite and occur as equant crystals that formed at an earlier stage. |