室生火砕流堆積物の全岩化学組成 : その給源への手掛り

概要

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The Muro pyroclastic flow deposit (MPFD), distributed in the central part of the Kii peninsula southwest Japan, is composed of rhyolitic welded tuff, representing a single cooling unit. Though the MPFD is a remarkable gigantic igneous unit with the volume of ∼100 km3, its source region is still in dispute. To discuss the problem, bulk rock chemical composition of MPFD were determined by XRF and LA-ICP-MS. The samples used in this study scarcely contain exotic materials. The MPFD welded tuffs indicate rhyolitic compositions with SiO2=74.0-75.5 wt%. They are characterized by LREE enrichments, large Eu negative anomalies (Eu/Eu*=0.30-0.59), and relatively constant chondrite-normalized middle to heavy REE abundances. All the analyzed samples have mole Al2O3/(CaO+Na2O+K2O) ratio higher than 1.1. Garnet is ubiquitously included, and some samples contain cordierite. Some of the rhyolitic rocks in the middle Miocene Setouchi Volcanic Rocks, as found in the lowest horizon of the Nijo Group for example, also have peraluminous bulk rock compositions. However, they are discriminated from MPFD by whole rock chemical compositions, in particular depletion in HREE. Actually, major and trace element compositions of MPFD are very close to those of the middle Miocene S-type granitoids in the Outer Zone of southwest Japan. These facts strongly suggest that MPFD was derived from an S-type felsic igneous body in the Outer Zone, travelling more than 20 km.
2002-11-30