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Journal of Petrology Advance Access originally published online on July 27, 2007
Journal of Petrology 2007 48(9):1761-1791; doi:10.1093/petrology/egm037
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

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

Satoshi Saito1,2, Makoto Arima1,*, Takashi Nakajima3, Keiji Misawa4 and Jun-Ichi Kimura5

1Geological Institute, Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai, Hodogaya-Ku, Yokohama 240-8501, Japan
2Department of Geology, University of Maryland, College Park, MD 20742-4211, USA
3Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan
4National Institute of Polar Research, 1-9-10 Kaga, Itabashi 173-8515, Japan
5Department of Geoscience, Shimane University, Matsue 690-8504, Japan

RECEIVED SEPTEMBER 9, 2006; ACCEPTED JUNE 25, 2007


  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–biotite-bearing 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 10·6 Ma and overlap the resumption of magmatic activity in the IBM and Honshu arcs at c. 17 Ma and the onset of IBM arc–Honshu arc collision at c. 15 Ma. 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 IBM arc middle crust to the Honshu arc.

KEY WORDS: arc–arc collision; crustal anatexis; granite; Izu–Bonin–Mariana (IBM) arc; Izu Collision Zone

*Corresponding author. Telephone: +81-45-339-3354. Fax: +81-45-339-3264. E-mail: arima{at}server2.edhs.ynu.ac.jp


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