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Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan |
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| Authors: | Saito Satoshi; Arima Makoto; Nakajima Takashi; Misawa Keiji; Kimura Jun-Ichi |
| Institution: | 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 |
| Abstract: | The Miocene Kofu Granitic Complex (KGC) occurs in the Izu CollisionZone where the Izu–Bonin–Mariana (IBM) arc has beencolliding with the Honshu arc since the middle Miocene. TheKGC includes rocks ranging in compositions from biotite-bearinggranite (the Shosenkyo and Mizugaki plutons), and hornblende–biotite-bearinggranodiorite, tonalite, quartz-diorite, and granite (the Shiodaira,Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonaliteand trondhjemite (the Ashigawa–Tonogi pluton), indicatingthat it was constructed from multiple intrusions of magma withdifferent bulk chemistry. The Sr-isotopic compositions correctedto sensitive high-resolution ion microprobe (SHRIMP) zirconages (SrI) suggest that the primary magmas of each pluton wereformed by anatexis of mixed lower crustal sources involvingboth juvenile basalt of the IBM arc and Shimanto sedimentaryrocks of the Honshu arc. After the primary magmas had formed,the individual plutons evolved by crystal fractionation processeswithout significant crustal assimilation or additional mantlecontribution. SHRIMP zircon U–Pb ages in the KGC rangefrom 16·8 to 10·6 Ma and overlap the resumptionof magmatic activity in the IBM and Honshu arcs at c. 17 Maand the onset of IBM arc–Honshu arc collision at c. 15Ma. The age of the granite plutons is closely related to theepisodic activity of arc magmatism and distinct granitic magmabatches could be formed by lower crustal anatexis induced byintrusion of underplated mantle-derived arc magmas. Based onpressures determined with the Al-in-hornblende geobarometer,the KGC magmas intruded into the middle crust. Thus, the KGCcould represent an example of the middle-crust layer indicatedthroughout the IBM arc by 6·0–6·5 km/s seismicvelocities. This granitic middle-crust layer acted buoyantlyduring the IBM arc–Honshu arc collision, leading to accretionof 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 |
| Keywords: | : arc-arc collision crustal anatexis granite Izu-Bonin-Mariana (IBM) arc Izu Collision Zone |
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