Petrogenesis of Early Cretaceous adakitic granites from the Kitakami Mountains, Japan

Abstract

Adakitic plutons dominate the Early Cretaceous igneous suite of the Kitakami Mountains of northeast Japan. The zoned plutons typically consist of adakitic granite in their centers (central facies) surrounded by adakitic to non-adakitic granites in their margins (marginal facies). The exposure of the adakitic plutons is divided into two zones: from the eastern part of the north Kitakami belt to the eastern margin of the south Kitakami belt (E zone: Hashikami, Tanohata, Miyako, and Kinkasan plutons), and the inner part of the South Kitakami belt (W zone: Tono, Senmaya, and Hitokabe plutons). The central facies granites are characterized by low Y and high Sr concentrations and fractionated LREE/HREE patterns, characteristics common to Archean TTG and modern adakite. Chemical compositions of the central facies granites can be explained by the “slab melting” model. In addition, from the trace element and isotopic characteristics, the contribution from sediments to slab melting is estimated to be 7 to 20%. The marginal facies granites are characterized by slightly lower Sr/Y ratios, less fractionated REE patterns, and weak negative Eu anomalies. The marginal facies magma is considered to be derived from the reaction of slab melts with mantle peridotite and lower crustal amphibolite. Central facies granites in the W zone may have slightly interacted with mantle wedge peridotite during their ascent, while those in the E zone show little or no evidence for mantle contamination. The difference in the degrees of mantle contamination in the adakitic granites may be related to the crust–mantle structure beneath the accretionary complex (E zone) and the microcontinental block (W zone).