Petrogenesis of Ashigawa and Tonogi granitic intrusions, southern part of the Miocene Kofu Granitic Complex, central Japan: M-type granite in the Izu arc collision zone

Abstract

The Ashigawa and the Tonogi intrusions comprise the southern part of the Miocene Kofu Granitic Complex (KGC), which is the largest pluton exposed in the Izu collision zone. The Ashigawa intrusion mainly consists of tonalite and subordinate amounts of trondhjemite, and the Tonogi intrusion is predominantly composed of tonalite. The rocks in both intrusions are classified as M-type granite. These are characterized by relatively low K₂O (<1.9 wt.%) and Rb (<48 ppm) and LREE enriched chondrite normalized REE patterns with a marked negative Eu anomaly. Compared to the Tanzawa tonalites distributed in the south of the KGC, the Ashigawa and Tonogi granitic rocks have higher LILE, REE and Sr-isotope compositions at a given whole rock SiO₂ content. Although the Ashigawa intrusion has distinctly different petrographical and geochemical characteristics from the Tonogi intrusion, both intrusions exhibit nearly identical Sr-isotopic composition (0.7037-0.7043 at 13Ma) and well-defined variation in whole rock compositions, suggesting that they are comagmatic. The crystal fractionation modeling involving removals of amphibole, plagioclase and magnetite from a tonalitic parental magma composition well represents the major and trace element variation of the Ashigawa and Tonogi granitic rocks. The Rayleigh fractionation modeling for whole rock REE abundance is also in a good agreement with the observed whole rock data. Mass balance modeling suggests that the proposed tonalitic parental magma can be derived by ∼40 % melting of hydrous basalt compositionally similar to the basalt occurring in the rift basin of northern Izu-Bonin arc. Anatexis of hydrous basaltic lower crust of the northern Izu-Bonin arc is a likely process for the formation of Ashigawa-Tonogi parental magma.