Two types of granitic intrusions in the Hida belt, Japan: Sr isotopic and chemical characteristics of the Mesozoic Funatsu granitic rocks

Abstract

Initial 87 Sr 86 Sr ratios, major- and trace-element compositions have been determined for the Late Triassic to Early Jurassic granitic rocks (Funatsu granitic rocks) in the Hida belt, Japan. Six intrusions, which are exposed in a transverse section of the belt, were selected for analyses. Based on the characteristics of initial 87 Sr 86 Sr ratios (Sr I-values) and element concentrations, the above intrusions are classified into two types. One is the intrusion with nearly constant Sr I-values over the variation of rock compositions. The Utsubo (Sr I=0.7053) and Hirose (0.7044) intrusions in this study, and the already reported Funatsu and Shimonomoto intrusions (both 0.7048) belong to this group, and rocks in this group are interpreted to have been formed mainly by fractional crystallization from mafic magma that originated in the lower crust or upper mantle. In contrast, the rocks in the other four intrusions (Hodatsusan, Shokawa, Yatsuo and Nagarehayama) show relatively higher and variable Sr I-values (0.7047–0.7105). Two-stage models, crustal mixing by the parental mafic magma at mid-crustal levels and subsequent differentiation by fractional crystallization, are interpreted to have been the main mechanisms for producing granitic rocks in these intrusions. Two contrasting types of Sr isotopic and chemical compositions recognized in the intrusions of the Funatsu granitic rocks, which are attributed to a difference of the predominant mechanism (crustal mixing or differentiation), correlate well with two geological features observed in the Hida belt. One is the existence or absence of shear zones which contemporaneously formed with granitic emplacement, and the other is the pattern of chemical zonation of the intrusion. Intrusions with a wide range of high Sr I-values are accompanied by a dextral shear zone in one margin and show a reverse- (less normal-) type chemical zonation. On the other hand, intrusions with constant Sr I-values do not relate with shear deformation and have normal chemical zonation. Considering these correlations, it is concluded that the stress state in middle-to upper-crustal levels is one of the important factors controlling the mechanisms of magma formation and way of emplacement.