西南日本黒瀬川テレーンの三滝火成岩類, シルル・デボン系凝灰岩, および, 花崗岩礫に含まれるジルコンのU-Pb年代(古領家帯と黒瀬川帯の構成要素と改変過程) U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)

黒瀬川地帯の三滝火成岩類, シルル-デボン系酸性凝灰岩, 土居層と嘉義尾層の花崗岩礫について, U-Pb法によるジルコンの年代測定を行った.三滝火成岩類については439〜441Ma, また, シルル-デボン系については362〜414Maの年代が得られた.花崗岩礫については263Maおよび250Ma(土居層), 203Ma(嘉義尾層)の年代が得られた.これらは, 各々の地層の堆積年代に近いことから, 礫の供給源となった花崗岩は貫入定置後, 急速に上昇・削剥・供給されたとみられる.また, これらの礫の年代は三滝火成岩類のそれより明らかに若いので, 供給源を三滝火成岩類に求めることはできない.供給源は, 南中国あるいはインドシナ/東マレーシア大陸地塊, および, それらが衝突・合体して形成されたアジア大陸の大陸縁辺域に位置していた火成弧と見なされる.一方三滝火成岩類は, 上記の大陸地塊がゴンドワナ大陸から分裂・移動する以前の結晶質基盤岩類と考えられる.

Zircon U-Pb dating was carried out on three Mitaki igneous rocks, one silicic tuff of the Siluro-Devonian sequence, and three granitic boulders from the Kurosegawa Terrane in an effort to provide a framework of the plutonic-sedimentary history in the terrane. The Mitaki igneous rocks yield ages of 441.9±4.5Ma, 441.5±4.4Ma and 439.7±10Ma. The silicic tuff is dated between 362.7±1.7Ma and 414.1±0.6Ma. Two granitic boulders from the Upper Permian Doi Formation are dated at 263.2±4Ma and 250.8±9.1Ma. A granitic boulder from the Middle Jurassic Kagio Formation gives an age of 203.5±15Ma.The results have led us to the conclusion that zircon U-Pb ages tend to indicate the intrusion of the Mitaki igneous rocks in the late Ordovician time; older than the age given by fossil records of the Siluro-Devonian sequence that overlies the Mitaki igneous rocks. The inherited component of a silicic tuff collected from the Siluro-Devonian sequence indicates that either the magmatic source was quite different from that both the Mitaki igneous rocks and the Permian granites represented in the conglomerates, or that old detrital zircon was incorporated into the tuff during sedimentation. The ages of the granitic boulders fall within the range of sedimentary ages for the conglomerates. This indicates rapid uplift and erosion soon after emplacement of the boulder protoliths, and rapid transport and deposition of conglomerate. The boulders might have been derived from the magmatic arc located in the South China or Indochina/East Malaya continental blocks in which the Mitaki igeneous rocks were involved as a crystalline basement.