4:30 PM - 4:45 PM
[SMP27-17] Structual petrology of ultramafic rocks in the Circum-Izu Massif Serpentine Belt
Keywords:The Circum-Izu Massif Serpentine Belt, Structural petrology, Peridotite, Spinel, Olivine
The Circum-Izu Massif Serpentinite Belt (CIMSB) named by Arai and Ishida (1987) consists of ultramafic blocks within the four geological units surrounding the Izu Peninsula: the Setogawa Belt, the Kobotoke Belt, the Hayama Belt, and the Mineoka Belt. We investigated petrological and structural characteristics of the heavily serpentinized peridotites in all four units. Among the major constituent minerals, we compared chemical compositions of chromian spinels and their mineral inclusions with olivine crystal-fabrics. We found that there are not only two types of spinel grains with/without hydrous mineral inclusions such as aspidrite and amphibole, but their occurrences also have spatial relations with respect to CIMSB. In the Hayama, Kobotoke, and Setogawa belts, both spinel types were present in the same ultramafic samples, whereas, in the Mineoka belt, spinels have no mineral inclusions but plagioclase pseudomorph (i.e. saussurite). Chemical compositions of spinel vary in each unit, but are mostly consistent with those found in the forearc peridotites (Ohara and Ishii, 1998). Although olivine crystal-fabrics vary from AG to A to D type in the studied samples, we found that those for the Setogawa Belt may be compatible with the chemical compositions of the chromian spinel; D-type peridotites not only have higher Cr# and lower Mg# than A-type peridotites, and their spinel grains are also characterized by hydrous mineral inclusions. We argue that the D-type peridotites could be developed associated with slab-derived hydrous melt that reacted to the A-type peridotites. Moreover, as we found certain similarities in the all four sites, the ultramafic rocks of the CIMSB could be petrologically and structurally altered due to the influence of the melt-rock interaction.