伊豆弧衝突帯における大陸地殻形成
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概要
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The tectonic setting of arc-arc collision and arc accretion in the Izu-collision zone is similar to that of the Archean orogenic belts (e.g., Taira et al., 1992). Understanding the petrological processes of granite formation in the Izu-collision zone, where geodynamic information is not modified by polyphase deformation and metamorphism, may contribute to an understanding of ancient orogenic belts, especially those related to collisional settings. The Pacific plate began subducting the Philippine Sea plate about 50 million years ago to produce the currently active Izu–Bonin–Mariana (IBM) arc. The collision between the northern IBM arc system and the Honshu arc of the Eurasia plate has been occurring since the middle Miocene (ca. 15 Ma) as a consequence of the northwestward migration of the Philippine Sea plate (e.g., Yamazaki et al., 2010). Neogene granite plutons are widely exhumed by tectonic uplifts associated with arc collision. Seismic imaging suggests that most of the present Izu-Bonin arc crust was created in the Eo-Oligocene (Kodaira et al., 2008; Kodaira et al., 2010). However, remnants of this older crust have not been found in the Izu collision zone. Tamura et al. (2010) integrated new geochemical results with recent geophysical imaging of the arc and concluded that Miocene plutonic rocks in the Izu collision zone are from the Eocene–Oligocene middle crust, which was partially melted, remobilized, and rejuvenated during the collision. Moreover, (1) the mafic arc lower crust is missing at the collision zone (Kitamura et al., 2003) and (2) the aseismic Philippine Sea plate, which is subducted at depths of 130-140 km without evidence of a tear or other gap, has been detected even beneath areas 120 km NW of the collision zone (Nakajima et al., 2009). These lines of evidence suggest that the down-dragged middle crust would partially melt and coalesce in the upper plate, but the mafic (high in iron and magnesium) lower crust would not melt and subduct into the deep mantle, resulting in delamination and separation of the middle crust from the lower crust. Both processes are inevitable at the collision and are necessary to yield continental crust. Thus, it is suggested that collisional orogeny plays an important role in the genesis of continental crust.