NdSr isotopic and geochemical characteristics of two contrasting types of calc-alkaline plutons in the Hida belt, Japan

Abstract

Sr and Nd isotopic and chemical compositions have been determined for Mesozoic plutons in the Hida belt (Japan), which occupied a marginal part of the Sino-Korean Craton before the Miocene opening of the Japan Sea. Associated magmatic activity produced calc-alkaline plutons with a wide spectrum of lithology, chemistry and internal structure. Two types of plutons (Types 1 and 2) are classified on the basis of their chemical and structural characteristics. Type- 1 plutons are characterized by a limited variation in initial Sr and Nd isotope values ( 87 Sr 86 Sr = 0.7044−0.7054 ; ϵNd = −0.8 to + 5.5), irrespective of the rock compositions, whereas Type-2 plutons show a wide range of isotope values ( 87 Sr 86 Sr = 0.7055−0.7105 ; ϵNd = −10.3 to + 0.7) which form a linear data array connecting the most primitive gabbro and basement rocks in an ϵ Nd− 87 Sr 86 Sr diagram. The distinction in isotope systematics between the two types of plutons is due to the degreeof interaction between the middle Proterozoic crust and parental mafic to intermediate magmas. Isotopic and elemental signatures of Type- 1 plutons point to a magmatic differentiation by fractional crystallization as the main evolutional process. In contrast, Type- 2 plutons require a mixing process between parental mafic magma and felsic magma derived from middle Proterozoic basement crust. The lowest Sr and highest Nd isotope compositions of the mafic rock units in each pluton are believed to represent the isotopic compositions of the parental magmas, and possibly those of their source regions (subcontinental mantle and/or mafic lower crust). The two contrasting types of plutons reflect the across-arc difference in crust-mantle structures and the change of tectonic framework around the Hida belt during Mesozoic time.