| Text Description: |
In the vicinity of the sampling site, the submerged summit area of the ridge is crested by a number of dome-shaped knolls. Rocks from one of these knolls was sampled at station 70-B29. A seismic reflection profile shows this knoll to be an outcrop of acoustic basement. The recovered rock is chiefly angular, rudely equant-shaped fragments of light-gray hornblende dacite porphyry. Most fragments are 10 to 20 cm across, but some are as large as 50 cm; many have freshly broken surfaces. The blocky dacite appears to represent debris from a large extrusive mass. The hornblende dacite has a porphyritic seriate texture of phenocrysts of hornblende (15 percent), plagioclase (17 percent), and opaque minerals (1 percent) set in a microcrystalline groundmass. Hornblende phenocrysts are typically 0.5 to 1 mm in length and in places occur as glomeroporphyritic clots of as many as 10 individual crystals. Cores of many larger crystals show zones of patchy augite, hypersthene, and plagioclase that reflect an earlier resorbtion. Many hornblende crystals have oxidized or burned reaction rims that consist of granular opaque minerals and augite except where they abut plagioclase phenocrysts; former small hornblende crystals are now entirely replaced by granular aggregates of these two minerals. Plagioclase phenocrysts grade serially in size from 2 mm down to groundmass size (less than 0.1 mm). Phenocryst cores are andesine and show both progressive and oscillatory zoning to oligoclase on the rims. Many larger plagioclase phenocrysts have resorbed cores that now consist of pyroxene, plagioclase, and glass. The felsophyric matrix is composed of euhedral to anhedral crystals, 0.001 to 0.1 mm in length, sodic plagioclase, quartz, alkali feldspar (?), hypersthene, augite, oxyhornblende (?), opaque minerals, and very light brown glass. |