Chemical composition of glass inclusions of volcanic rocks from the Bonin Arc

The chemical composition of glass inclusions in phenocrystic plagioclase and pyroxene from Sites 792 and 793, drilled during Ocean Drilling Program Leg 126 in the Bonin Arc, is examined. Immiscible liquid, which is preserved as glass inclusions with unmixed textures in plagioclase, is observed in a high-magnesian andesite, which suggests an important role of liquid immiscibility in the fractionation of high-magnesian andesite. In other andesitic rocks (SiO2 = 57-60 wt%), such unmixed textures of glass inclusions in calcic plagioclase with a similar percentage of An (around 80%) is not found. The degree of fractionation and mixing of liquid are inferred from the glass composition in pyroxene.

Chemical and mineral composition of rocks and sediments from the Amirante Arc

New geological and geophysical data on the Amirante Arc, which locates to the south of the Seychelles Islands, are presented. These data were obtained by Pacific Oceanological Institute during the 33-rd cruise of R/V Professor Bogorov in 1990. The Amirante Arc represents a seamount chain, which has submeridional strike and total length about 400 km. To the west of the Amirante Arc there are a deep sea trench and a back-arc basin, i.e. this area is characterized by structural elements associated with the subduction zone of Western Pacific type. According to our data the Amirante Arc is composed by tholeiites of ocean plateau type. This facts are evidences that the Amirante Arc differs from typical Pacific island arcs. This gives an opportunity to distinguish a special type of oceanic structures, i.e. non-volcanic (amagmatic) ridges. The Amirante Ridge has been probably formed as a result of oceanic crust heaping due to horizontal displacements of its blocks in the process of spreding ridge formation in the Indian Ocean during Cretaceous-Paleogene.