Petrogenetic study on granitoids based on Sr,Nd and Pb isotopes.

本文根据岩石学、岩石化学和Sr、Nd同位素研究了日本西南部侏罗纪-中新世花岗岩类的岩石成因。戴森，山阴地区，日本西南部。斜长角闪岩的Rb-Sr全岩-矿物等时线年龄为185 Ma，英云闪长岩的全岩-矿物等时线年龄较年轻。英云闪长岩的年龄大约在180 Ma左右，但在白垩纪花岗岩类侵入作用下已完全或部分重结晶。（2）日本西南部内带白垩纪-古近纪花岗岩类具有不同的Sr、Nd初始同位素比值。山阴带的花岗岩类通常具有低Sr和高Nd同位素比值，而山阳带和濑户内带的花岗岩类具有高Sr和低Nd同位素比值。Ryoke带中的基性岩的年龄为古生代，具有与San-yo和Ryoke带相对应的高Sr和低Nd同位素比值。古陆块位于现今陆块带的南部边缘，它是一个微陆块，沿着。白垩纪时，该微大陆的大部分可能被俯冲到中国区之下，而俯冲的微大陆可能是山阳带和良轭带白垩纪花岗岩类的主要岩浆源。3）外带Asizuri岩体的岩浆应来自上地幔、下地壳和中地壳。

On the basis of petrography, petrochemistry and Sr and Nd isotopes, Jurassic to Miocene granitoids in southwest Japan are petrogenetically examined.1)Rb-Sr isotope ages were determined for a amphibolite and two gneissose tonalites which were newly found in the western skirts of Mt. Daisen, San-in area, southwest Japan. The amphibolite shows a Rb-Sr whole rock-mineral isochron age of 185 Ma, and the tonalites have younger whole rock-mineral isochron ages. The tonalites must have had ages around 180 Ma, but would have been completely or partially recrystallized by intrusion of Cretaceous granitoids. Then these rocks should be correlated to those in the Hida Metamorphic Zone in central Japan.2) Cretaceous to Paleogene granitoids in the Inner Zone of Southwest Japan have variable initial Sr and Nd isotope ratios. Granitoids in San-in Zone have usually low Sr and high Nd isotope ratios, whereas those in San-yo and Setouchi Zones have higher Sr and lower Nd isotope ratios. Basic rocks in the Ryoke Zone are Paleozoic in age, and have high Sr and low Nd isotope rations corresponding to those in San-yo and Ryoke Zones. It has been suggested that the Paleo-Ryoke continent, a micro continent, was situated along the southern margin of the present Ryoke Zone. The greatest part of the micro-continent may have been underthrusted beneath the Chugoku District during the Cretaceous, and the underthrusted micro-continent would have been the main magma source for the Cretaceous granitoids in San-yo and Ryoke Zones.3) The magmas for the Asizuri Pluton in the Outer Zone must have been yielded from upper mantle, lower and middle crust.