Abstract:
We present data pertaining to mineral assemblages and composition of the Central Indian Ocean Basin (CIOB) pumices. Eight groups of pumices were identified considering the presence of phenocrysts of plagioclase, clinopyroxene, orthopyroxene, hornblende and biotite together with the occurrence of quartz and glass. Pigeonite, fayalite and ulvospinelare reported for the first time from these pumices. In the eight groups the modal percentage of the constituents are phenocrysts 3 percent to 19 percent (avg 9.6 percent), silicic glass 33 percent to 54 percent (avg 43 percent) and the rest is vesicles. Based on the above factors we have identified the possible sources of the CIOB pumices. The mineral compositions of plagioclase, pyroxenes, and biotite of the CIOB pumices were compared with those of Krakatau and Toba. Most of the plagioclase and pyroxene compositions resemble the Haranggoal Dacite Tuff (HDT) of Toba and Krakatau. Considering the mineral assemblages and compositions there are pumices which do not correlate to any of the above eruptions and are probably from yet unidentified source/s. These sources could either be from nearby terrestrial volcanoes or intraplate seamounts present in the CIOB. In a global context, it is viable that petrological characteristics could be used as initial criteria to determine the source of pumices that occur at abyssal depths in the world ocean.
