Three dimensional distributions of chemical and isotopic compositions of granitic pluton and its formation process

通过对单个花岗岩体元素组成和Sr同位素比值的三维空间分布特征的分析，探讨了浅部岩浆体的结构及其在冷却凝固过程中的分异过程。在深达1500 m的索里岩体中，岩石中SiO_2wt%、Fe_2O_3、MgO、MnO、K_2O/Na_2O、Ba、Rb、Y、Zn、U、Th等元素含量随深度增加而增加，Al_2O_3、CaO、Na_2O、Sr等元素含量随深度增加而减少。这种成分的垂直分布与岩石的岩相学特征有很好的对应关系，但与有明和奥重山等其他岩体的垂直分布不一致。在水平分布上，从边缘到中心，元素浓度随深度增加的趋势是一致的。岩石Sr同位素比值集中在一个很窄的范围内，表明岩浆来自同位素均质的源区，没有明显的围岩混染。Sori岩体的这些元素含量分布和岩石学特征是由从边缘到内部的冷却和凝固过程中的原位分馏作用解释的。并对希达带的中生代岩体与古生代、前寒武纪岩体进行了对比研究。

In this research, three dimensional distributions of element compositions and Sr isotope ratios of single granitic pluton were analyzed for the purpose of investigating the structure of shallow magma body and its differentiation process during the cooling and solidification. The Sori pluton was selected for this study.For the vertical distributions of element concentrations for bulk rocks, which are measured for boling core samples with 1500m depth in the middle of the pluton, SiO_2 wt%, Fe_2O_3, MgO,MnO,K_2O/Na_2O,Ba, Rb, Y,Zn, U,Th increase, while Al_2O_3, CaO,Na_2O,Sr decrease with increasing depth. This vertical distributions of compositions correlate well with the petrography of the rocks, but not agree with the vertical distributions of other plutons such as Ariake and Okueyama. In horizontal distributions, the same tendency of element concentrations with increasing depth was recognized from the margin toward the core of the pluton. The Sr isotope ratios of the rocks are concentrated in a very narrow range, indicating that the magma was derived from isotopically homogeneous source without marked contamination of wall rocks.These distributions of element contents and petrographical characteristics for the Sori pluton are explained by in situ fractionation during the cooling and solidification from margin to inner part.The other plutons in different tectonic settings, Mesozoic plutons in the Hida belt and Paleozoic and Precambrian plutons, are also studied for comparison of the structure and formation process of the plutons.