Analysis of consolidation processes of granitic magma based on partition of rare earth elements between accessory minerals

点击翻译按钮获取中文摘要

The consolidation processes of granitic magmas were investigated on the basis of the XRF and EPMA analyzes and CHIME geochronology. The main results are as follows :(1) The intra-grain distribution of rare earth elements in accessory minerals is closely related to the sequence of their crystallization. The successive appearance and disappearance of accessory minerals are explained in terms of deviation from cotectic saturation levels owing to kinetic crystallization from a supersaturated magma.(2) The Hikami, Okuhinotsuchi and Hirasawa granites in the South Kitakami terrane were dated through the CHIME method. The CHIME ages, 350-250 Ma, do not support a current view that these granites are pre-Silurian basement. Rather, the CHIME ages require the detailed stratigraphical re-examination of the South Kitakami terrane.(3) The Mitsuhashi granite in the Ryoke belt is a zoned pluton. Structural, petrochemical and geochronological analyzes revealed that the zoned pluton formed ca.80 Ma ago by in situ fractionation of a dioritic parent melt.(4) The composition of hydrothermal titanite in granite is controlled by both substitutions (Al, Fe^<3+>)FTi_<-1>0_<-1> and (Al, Fe^<3+>)(OH)TI_<-1>0_<-1>. Weathered biotite in granite contains microdomains of K-vermiculite and interstratification of K-vermiculite and biotite, and contains much lower iron than the parent biotite. Oxidation of Fe^<2+> to Fe^<3+> in the octahedral sheet causes the iron loss to maintain local electrostatic balance.(5) Paleozoic and Mesozoic sediments which form walls of granite bodies contain detrital monazite and zircon grains of middle Precambrian (2000-1400 Ma) as well as Paleozoic (430-250 Ma) ages.