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 K 2 O (<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 SiO 2 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.

Petrogenesis of the Kaikomagatake granitoid pluton in the Izu Collision Zone, central Japan: implications for transformation of juvenile oceanic arc into mature continental crust

“…They are composed of various types of granitoids including tonalite, trondhjemite, granodiorite, monzogranite, and granite (e.g., Takahashi 1989;Sato 1991;Saito et al 2007b). Recent petrogenetic investigations of the Tanzawa plutonic complex and Kofu granitic complex have shed new light on the deep crustal processes beneath the collision zone (e.g., Kawate and Arima 1998;Saito et al 2004Saito et al , 2007a. Kawate and Arima (1998) suggested that the tonalitic Tanzawa plutonic complex (Fig.…”

“…Note the northward convex structure of Median Tectonic Line in central Japan (Kanto Syntaxis). Light solid lines outline seamounts shallower than 2,000 m. b Geological map of the Izu Collision Zone (modified after Takahashi and Saito 1997;Saito et al 2004Saito et al , 2007b. ISTL ItoigawaShizuoka Tectonic Line, BTL Butsuzo Tectonic Line, TATL Tonoki-Aikawa Tectonic Line, KF Kannawa Fault, KO Koma terrane, MI Misaka terrane, TA Tanzawa terrane, IZ Izu terrane.…”

Geochemical diversity of Neogene granitoid plutons in the Izu Collision Zone: implications for transformation of juvenile oceanic arc into mature continental crust

“…SiO 2 versus SrI (at zircon U Pb age) of the rocks of the Tanzawa plutonic complex (Kawate, 1996) (a), the Kofu granitic complex (b), and the Kaikomagatake pluton (c). The compositionalˆeld of Izu Bonin arc basaltic rocks (Taylor and Nesbitt, 1998;Hochstaedter et al, 2001) and Shimanto metasedimentary rocks in the Izu Collision Zone (Saito et al, 2004(Saito et al, , 2007a(Saito et al, , 2007b are also shown for comparison.…”

“…Petrogenesis of the Izu Collision Zone granitoid plutons together with other oceanic arc collision related magmatism collectively suggest that collision has played an important role in transforming juvenile oceanic arc into mature continental crust. Takahashi and Saito, 1997;Saito et al, 2004Saito et al, , 2007aSaito et al, , 2007bSaito et al, , 2012 (Condie, 1993;Gao et al, 1998;Borodin, 1998;Wedepohl, 1995;Rudnick and Gao, 2003) . SiO 2 versus SrI (at zircon U Pb age) of the rocks of the Tanzawa plutonic complex (Kawate, 1996) (a), the Kofu granitic complex (b), and the Kaikomagatake pluton (c).…”

“…齊藤 哲 る組成変化だけでもたらされるのではなく，親マグマの組 成が既に多様であることがわかる( Table 2) (Kawate and Arima, 1998;Saito et al, 2004;Saito et al, 2007b;Saito et al, 2012). Qtz, quartz; Pl, plagioclase; Kfs, K feldspar; Bt, biotite; Hbl, hornblende; Mag, magnetite.…”