Granite subduction: Arc subduction, tectonic erosion and sediment subduction Shinji Yamamoto a, ⁎, Hiroki Senshu a,b , Shuji Rino a , Soichi Omori a , Shigenori Maruyama a a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan b Faculty of InformationTechnology and Business, Cyber University, Higashi-Shimbashi 1-9-1, Minato-ku, Tokyo 105-7311, Japan abstract article info Article history: Received 8 August 2008 Received in revised form 14 December 2008 Accepted 16 December 2008 Available online 31 December 2008 Keywords: Continental growth Subduction Intra-oceanic arc Western Pacific region Continental growth has been episodic, reflecting the episodic nature of mantle dynamics as well as surface dynamics of the Earth, the net result of which is exhibited by the present mantle with two huge reservoirs of TTG rocks, one on the surface continents and the other on the D″ layer on the Core-Mantle Boundary (CMB). During the early half of the Earth history, the felsic continental crust on the surface which formed in an intra- oceanic environment has mostly been subducted into the deep mantle, except in the rare case of parallel arc collision. The growth history of continental crust shows that with its simultaneous formation, a considerable amount must have also been subducted. Such ongoing subduction processes can be seen in the western Pacific region, through tectonic erosion, arc subduction, and sediment-trapped subduction. © 2008 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. 1. Introduction Geologists have long believed that the granitic crust (TTG-Tonalite- Trondhjemite-Granodiorite) once formed on the surface must have survived through the whole history of the earth without any subduction because of its buoyant nature. Thermal considerations (Fyfe, 1978; Armstrong, 1981, 1991) for the formation of continental crust suggest that major continental growth must be Hadean to Archean, because of its higher mantle temperature, which brought extensive amounts of melt to fractionate, “finally” TTG magma. Hence, Fyfe (1978) and Armstrong (1981) speculated that presumably more than 100% of the present continental crust must have been formed on the Hadean-Archean Earth. On the other hand, the proposed growth curves of continental crust, while highly variable, show some evidence for only are ca. 20% by the end of Archean (Fig. 1 , Rino et al., 2008). To explain the observed growth curve, the possible extensive subduction of continental crust must have occurred through time (Santosh et al., 2009-this issue), although the physical processes have not been addressed in detail. Moreover, from the recent study of active subduction zone along the Circum-Pacific subduction zone, mainly by seismological methods, tectonic erosion and even sediment subduction have been identified as common processes (e.g. von Huence and Scholl, 1991; Scholl and von Huene, 2005). In this paper, we summarize the geological aspects of the western Pacific region to evaluate the processes of oceanic arc subduction, tectonic erosion and sediment trapped subduction. There- after we extend the observations in the western Pacific to Archean dynamics, and consider whether or not intra-oceanic arcs subduct in the deep mantle, and briefly discuss the fate of subducted continental crusts. 2. Arc subduction, tectonic erosion and sediment subduction The concept of subduction of continental crust has generally not been accepted for several reasons. For example, the geological aspects of the Himalayan orogen suggest that collision and subduction seldom exceeded the Moho depth, particularly as demonstrated by the Himalayan regional metamorphism (England and Thompson, 1984). Nevertheless, the occurrence of ultrahigh- pressure (UHP) metamorphic assemblages in the collisional oro- genic belts suggests that continent subduction might reach depths of 200–300 km (Liou et al., 2002). On the other hand, recent geochemical studies on the hotspot lavas report the trace-element and isotopic data which indicate the presence of a recycled sediment (continental crust) component in the mantle plume (e.g. Jackson et al., 2007). The geochemical evidence clearly show that subducted sediment is entrained in upwelling plumes and returned to the Earth's surface at hotspot. It is thus evident that variable segments of the continental crust may have been subducted into the deep mantle. 2.1. Arc subduction in the western Pacific The Western Pacific region offers important clues on arc subduc- tion (Figs. 2 and 3). The island arcs of Japan, Izu-Ogasawara, Mariana, Kyushu-Palau, Philippine arcs, Malacca arcs, Indonesia, Vanuatu, Solomon, Georgia and others are all intra-oceanic in nature. A number of cases of arc collision are present in the western Pacific. These Gondwana Research 15 (2009) 443–453 ⁎ Corresponding author. E-mail address: syamamot@geo.titech.ac.jp (S. Yamamoto). 1342-937X/$ – see front matter © 2008 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. doi:10.1016/j.gr.2008.12.009 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr