Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan

Abstract

The Miocene Kofu Granitic Complex (KGC) occurs in the Izu Collision Zone where the Izu^Bonin^Mariana (IBM) arc has been colliding with the Honshu arc since the middle Miocene.The KGC includes rocks ranging in compositions from biotite-bearing granite (the Shosenkyo and Mizugaki plutons), and hornblende^biotitebearing granodiorite, tonalite, quartz-diorite, and granite (the Shiodaira, Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonalite and trondhjemite (the Ashigawa^Tonogi pluton), indicating that it was constructed from multiple intrusions of magma with different bulk chemistry. The Sr-isotopic compositions corrected to sensitive high-resolution ion microprobe (SHRIMP) zircon ages (SrI) suggest that the primary magmas of each pluton were formed by anatexis of mixed lower crustal sources involving both juvenile basalt of the IBM arc and Shimanto sedimentary rocks of the Honshu arc. After the primary magmas had formed, the individual plutons evolved by crystal fractionation processes without significant crustal assimilation or additional mantle contribution. SHRIMP zircon U^Pb ages in the KGC range from 16 8 to 10 6Ma and overlap the resumption of magmatic activity in the IBMand Honshu arcs at c. 17Ma and the onset of IBM arc^Honshu arc collision at c. 15Ma.The age of the granite plutons is closely related to the episodic activity of arc magmatism and distinct granitic magma batches could be formed by lower crustal anatexis induced by intrusion of underplated mantle-derived arc magmas. Based on pressures determined with the Al-in-hornblende geobarometer, the KGC magmas intruded into the middle crust. Thus, the KGC could represent an example of the middle-crust layer indicated throughout the IBM arc by 6 0^6 5 km/s seismic velocities. This granitic middle-crust layer acted buoyantly during the IBM arc^Honshu arc collision, leading to accretion of buoyant IBMarc middle crust to the Honshu arc.