The dynamics of big mantle wedge, magma factory, and metamorphic-metasomatic factory in subduction zones

Abstract

The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H2O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.