香川県高松クレーター産ガラスの岩石学的研究

Abstract

Takamatsu crater has been identified and named by Kono et al., (1994), based on the presence of a concentric gravity low in the southern part of Takamatsu Plain, northeast Shikoku, Japan. The crater is 1∼2 km deep and ca. 4 km across, and buried under the Miocene volcanic formation and alluvium deposit. Both impact and volcanic origins have been proposed for the crater, but remained unresolved. We searched for the evidence of impact origin of the Takamatsu crater, such as shock metamorphic features of minerals, variable and extraordinarily high contents of SiO2, FeO, and NiO, together with low Na2O content of fresh glass in the Miocene deposit. The glass fragments in the pyroclastic flow deposit in the southern part of the Takamatsu crater are characterized by xenocrysts of quartz and feldspar and lack of idiomorphic phenocrysts. We did not find evidence for shock metamorphism in the minerals. The glass shows heterogeneous appearence with variable red tints and high vesicularity under the microscope, somewhat similar to the suevite of Ries crater. The glass of the Takamatsu crater, however, shows uniform and rhyolitic chemical composition, which is common for the calc-alkalic volcanics of the Miocene Setouchi volcanic suite. Furthermore, chemical composition of the glass is almost identical to those of welded tuff blocks and bulk matrix of the pyroclastic flow deposit, indicating that the glass fragments are of volcanic origin. The glass and associated rhyolite are dated by K-Ar method at around 14 Ma, which coincides with the age of the Setouchi volcanic activity. Although deep drilling has not been carried out for determining the subsurface structure, present analytical results argued for cauldron origin of the Takamatsu crater.