砕屑粒子の化学組成からのメッセージ : 砕屑性ザクロ石の化学組成に基づく後背地解析と東アジアの地殻変動  [in Japanese] Message from chemistry of clastic grains : Provenance analysis based on chemical composition of detrital garnet and tectonic events in East Asia  [in Japanese]

砕屑性ザクロ石の化学組成から, その源岩の種類を推定でき, また化学組成の累帯構造から, 変成岩ならば変成作用のタイプを明らかにできるので, 砕屑性ザクロ石の化学組成を分析することは, 後背地解析において有効な手段である.日本列島に分布するペルム系から白亜系中に産する砕屑性ザクロ石の化学組成を総括すると, 大きく3つのタイプが認められる.それらは, (1)グランダイト, (2)パイロープ成分に富むアルマンディン, (3)その他(多くはスペッサルティン成分に富むアルマンディン)からなる.これらの産出は時代と地域に密接に関係して変化し, 大きく3つの時期, (1)ペルム紀末から三畳紀初期, (2)後期三畳紀からジュラ紀初期, (3)白亜紀中頃に, 地殻変動による後背地の変化があったことが明らかになった.

Chemical analysis of detrital garnets in an available manner in provenance analysis, because we can know the source rocks for detrital garnets based on its chemistry and clarify the metamorphic type in the provenance by chemical zoning pattern.Chemistry of detrital garnets from sandstones of the Permian to Cretaceous systems in Japan is divided into three types, (1) grandite, (2) pyrope-rich almandine and (3) the others (mainly spessartine-rich almandine). Grandite garnets were derived from skarn deposits in the contact metamorphic belt along the Asian continental margin, and pyrope-rich almandine garnets were derived from granulite-facies gneiss in the Asian continental basement. The others, mainly spessartine-rich almandine, are originated from acidic igneous rocks or low-grade metamorphic rocks.The types of garnet chemistry change in relation to age and tectonic setting. The changes of provenance corresponding to tectonic movements are found out in three periods, (1) latest Permian to earliest Triassic, (2) Late Triassic to earliest Jurassic, (3) middle Cretaceous. The first event represents a juxtaposition of the Maizuru and Ultra-Tanba Terranes with the continental margin where the Akiyoshi Terrane and shelf-type sediments were distributed. The second event may correspond to a collision between Sino-Korea Massif and Yangtze Massif. All terranes are suffered from the movement. The third event represents left-lateral movement of the Median Tectonic Line (MTL) in southwest Japan. The effect is recognized in the terranes near MTL in southwest Japan.