Continental subduction channel processes: Plate interface interaction during continental collision

Abstract

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth's surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of peridotite, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust-and mantle-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism. In continental collision orogens, exposed ultrahigh-pressure (UHP) metamorphic terranes contain the lithotectonic records of Alpine-type subduction [1]. These records highlight metamorphic P-T conditions and mass transfers, indicating subduction of continental crustal rocks to mantle depths of over 100 km. The UHP terranes are characterized by minor occurrences of mafic eclogites and ultramafic peridotites that are hosted by felsic gneisses. Such UHP eclogite-facies parageneses are absent in Pacific-type subduction zones [2]. These UHP rocks are of crustal and mantle protoliths and now occur together with their lower pressure counterparts in continental collision orogens [3]. It is intriguing how the continental crust is subducted to mantle depths for UHP metamorphism, how UHP slices are exhumed from mantle depths to crustal levels, and how metamorphic rocks of different P-T conditions occur together in the same orogens. The property of plate interface in subduction zones is a key to physical and chemical interactions at interplate boundaries. Physical and chemical transfers at convergent plate margins can occur on various spatial and temporal scales, which is fundamental to subduction-zone processes. Like the plate tectonic theory, the concept of subduction channel was originally developed from the tectonics of oceanic subduction zone [4][5][6]. It was primarily focused on the plate interface interaction in ocean...

Geochemistry of continental subduction-zone fluids

“…In contrast, the mantle wedge above continental subduction channels commonly belongs to the part of the subcontinental lithospheric mantle (SCLM), which is cold (e.g., Zheng et al 2013). This difference may be the reason why continental subduction zones generally have lower geotherms than the hot oceanic subduction zones.…”

Two types of the crust-mantle interaction in continental subduction zones

“…The concept of subduction channel was developed based on studies of oceanic subduction zones Shreve, 1988a, 1988b), providing a reference model for the tectonic processes during subduction of oceanic crust from shallow to deep (Beaumont et al, 1999;Gerya et al, 2002;Guillot et al, 2009). In recent years, the model of oceanic subduction channel has been extended to continental subduction zones, which can well explain structural deformation, HP/UHP metamorphism, physical mixing and chemical reaction between crustal and mantle materials, as well as crustal detachment and decoupling and differential exhumation during continental collision (Beaumont et al, 2009;Guillot et al, 2009;Zheng, 2012;Zheng et al, 2013a). A subduction channel is defined as a shear zone between the upper and lower slabs of convergent plate margins Shreve, 1988a, 1988b).…”

“…In contrast, melts generated by partial melting of subducted continental crust generally exhibit arc-like trace element distribution patterns and enriched Sr-Nd-Hf isotope compositions , and the overlying mantle wedge is ancient cratonic SCLM ( Figure 7). There are a series of differences in lithology and geochemistry between the subduted crust and the overlying mantle wedge (Zheng, 2012;Zheng et al, 2013a), which would lead to the differences in lithology and geochemistry of the metasomatic mantle above subduction zones, and eventually a series of differences in postcollisional mafic igneous rocks. Low degree partial melting of crustal rocks at deep subduction zones produces felsic melts at the slab-mantle interface, resulting in the first element geochemical differentiation; the felsic melts are enriched in LILE and LREE, transferring crustal geochemical signatures to mantle metasomatites.…”

Qian'echong low-F porphyry Mo deposits in the Dabie Mountains, central China

“…The suture between these two blocks is located along the Xiaotian-Mozitan fault and the Huwan Formation (Sun et al, 2002a). Interestingly, Triassic magmatism is not well developed in the Dabie Mountains with only minor foliated garnet-bearing granite veins Zheng et al, 2013) and melt inclusions . No hydrothermal mineralization associated with the Triassic granite has been reported.…”