Abstract:

We analyzed basalt samples from 4 sites at 49.6°E and 50.5°E along the Southwest Indian Ridge, which were collected with television-video guided grab  (TVG ) or dredge  (D ) during 2 hydrothermal expeditions of Chinese vessel “Da Yang Yi Hao” in 2008 and in 2010. These samples are fragments of basaltic pillow lavas and sheet flows. These samples are mainly porphyritic with fine-grained, and euhedral to anhedral phenocrysts of plagioclase, olivine and pyroxene. The matrix contains microcrystalline plagioclase, olivine and pyroxene, in which tabular plagioclase microphenocrysts occur with a radical texture. Whole rock major and rare earth  elements data of these samples show that ratios of K/Ti range from 0.071 to 0.091, and La/Sm from 0.52 to 0.70, suggesting that they are normal mid-ocean ridge basalt (N-MORB). Plagioclase compositions range from An_76-78 to An₆₆, while those of olivine range from Fo₈₆ to Fo₈₄. In comparison of earlier study on basalt samples along SWIR (49°E -70°E), samples in this study are featured with much more depletion in light rare earth elements, Na₈ and K/Ti, but higher Fe₈. These data indicate that mantle melting in the study area occur in higher degree and at deeper depth. Furthermore, petrologic and mineralogic features of basalts at 49.6°E and 50.5°E suggest that magma activity and source in the study area are complex.

Key words: mid-ocean ridge basalt, petrology, mineralogy, element geochemistry, Southwest Indian Ridge