Introduction to the structures and processes of subduction zones

Accepted Manuscript Review Introduction to the structures and processes of subduction zones Yong-Fei Zheng, Zi-Fu Zhao PII: DOI: Reference:

S1367-9120(17)30344-9 http://dx.doi.org/10.1016/j.jseaes.2017.06.034 JAES 3140

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Journal of Asian Earth Sciences

Please cite this article as: Zheng, Y-F., Zhao, Z-F., Introduction to the structures and processes of subduction zones, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.06.034

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Introduction to the structures and processes of subduction zones

Yong-Fei Zheng1,2* and Zi-Fu Zhao1

1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 2. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China

*

Corresponding author, Email: [email protected] 1

Abstract Subduction zones have been the focus of many studies since the advent of plate tectonics in 1960s. Workings within subduction zones beneath volcanic arcs have been of particular interest because they prime the source of arc magmas. The results from magmatic products have been used to decipher the structures and processes of subduction zones. In doing so, many progresses have been made on modern oceanic subduction zones, but less progresses on ancient oceanic subduction zones. On the other hand, continental subduction zones have been studied since findings of coesite in metamorphic rocks of supracrustal origin in 1980s. It turns out that high-pressure to ultrahigh-pressure metamorphic rocks in collisional orogens provide a direct target to investigate the tectonism of subduction zones, whereas oceanic and continental arc volcanic rocks in accretionary orogens provide an indirect target to investigate the geochemistry of subduction zones. Nevertheless, metamorphic dehydration and partial melting at high-pressure to ultrahigh-pressure conditions are tectonically applicable to subduction zone processes at forearc to subarc depths, and crustal metasomatism is the physicochemical mechanism for geochemical transfer from the slab to the mantle in subduction channels. Taken together, these provide us with an excellent opportunity to find how the metamorphic, metasomatic and magmatic products are a function of the structures and processes in both oceanic and continental subduction zones. Because of the change in the thermal structures of subduction zones, different styles of metamorphism, metasomatism and magmatism are produced at convergent plate margins. In addition, juvenile and ancient crustal rocks have often suffered reworking in episodes independent of either accretionary or collisional orogeny, leading to continental rifting metamorphism and thus rifting orogeny for mountain building in intracontinental settings. This brings complexity to distinguish the syn-subduction processes and products from post-subdduction processes and products. Nevertheless, available results indicate that our definition and understanding of subduction zone processes and products can be advanced by the convergence of observations and interpretations from geochemical, geological, geophysical and geodynamic studies of both oceanic and continental subduction zones. Therefore, insights into subduction zones can be provided by intergration of different approaches from different targets in the near future.

Keywords: Metamorphic dehydration; crustal anataxis; crustal metasomatism; interplate magmatism; intraplate magmatism; extreme metamorphism; rifting orogeny

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1. Foreword Subduction zones occur at convergent plate boundaries where Earth’s lithospheric plates return to the deep mantle. In accord with the plate tectonic theory, convergent boundaries are sites where lithospheric plates move together and the consequent compression causes subduction, whereby one lithospheric plate descends beneath the other and crustal uplifting occurs to result in mountain building. Subduction zones are ultimately responsible for spectacular explosive volcanic eruptions, the planet’s greatest earthquakes, and some of our most valuable concentrations of economic ores. Subduction delivers crustal material to the mantle, playing a primary role in geochemical cycling. On a larger scale, subduction helps drive plate tectonics, as plate movement is chiefly driven by the downward pull of oceanic slabs sinking into the mantle. Furthermore, arc magmatism is produced at convergent boundaries, and it evolves over time to form the continental crust. When oceanic lithosphere collides with continental lithosphere, the denser oceanic crust subducts beneath the lighter continental crust. The subducting oceanic crust not only pushes the continental crust upward into mountain chains but also contribute metasomatic agents to the overriding mantle wedge to form the magma source of volcanic arcs. Although continental crust is also subductable, arc volcanism is absent in the overlying continental crust when subducted is continental lithosphere. This difference is generally dictated by the thermal structure of subduction zones, because the abundance of water in the subducting continental crust is similar to that in the subducting oceanic crust at subarc depths of 80-160 km. On the other hand, ultrahigh-pressure (UHP) metamorphic rocks are common in continental subduction zones, but they are absent in oceanic subduction zones. Such a difference is usually ascribed to continuous subduction of the high-density oceanic slab without breakoff for crustal exhumation. Substantially, the structures and processes of subduction zones have exerted a primary control on these differences at convergent plate margins. This article presents an introduction to the above issues, which also serves as the preface to this special issue.

2. The structure of subduction zones Plate convergence is not independent of plate divergence on Earth. While the two types of lithospheric processes are fundamental to plate tectonics, their structures are different from each other in many aspects. Along divergent plate boundaries, Earth's lithosphere is fractured into dozens of rigid plates that move on top of the asthenosphere. Because Earth's diameter

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remains constant, there is no net creation or destruction of the lithospheric plates. Each lithospheric plate is composed of a crustal layer superficial to an outer layer of the mantle. Oceanic crust is primarily composed of mafic igneous rocks such as basalt and gabbro, which are covered by seafloor sediment. In contrast, continental crust is primarily composed of felsic rocks such as granite and gneiss, which are also covered by sedimentary rocks. The oceanic crust is denser than the continental crust and thus more susceptible to subduction at convergent plate boundaries. Subduction zones form dominant tectonic features on Earth and are the site of large changes in both physics and chemistry at lithospheric scales. Their structures are fundamental to the occurrence of orogenic metamorphism and magmatism at active and fossil convergent plate margins. Although the structures of subduction zones may change with time, they are generally classified in terms of their geometric, geological and thermal properties.

2.1 Geometric structure The geometric structure of subduction zones is defined by their dips between two converging plates (Fig. 1), which are dictated primarily by the rate of slab subduction and subordinately by the age of subducting slabs (Li, 2014; Leng and Mao, 2015; Zheng and Chen, 2016). Young slabs tend to subduct in low angles (<30), and higher subduction rates are more associated with lower angles. In contrast, old young slabs tend to subduct in high angles (>40), and lower rates of subduction are more associated with higher angles. Nevertheless, slab dip may change with time during subduction, and it is common that many slabs were subducted in low angles from forearc depths of <60-80 km to the subarc depths but in high angles at postarc depths of >200 km (Zheng et al., 2016). While this case is generally attributed to gravitational sinking of the subducting slabs in response to the reduced rate of plate convergence, buoyancy of a subducting slab may lead to an increase of its dip from a higher value to a lower value.

2.2 Geological structure The geological structure of subduction zones is defined by the nature of convergent plates (Stern, 2002; Zheng and Chen, 2016). Oceanic subduction zones are defined as sites where oceanic slabs are subducted beneath either oceanic or continental plate, corresponding to accretionary orogens (Fig. 2a). Likewise, continental subduction zones are defined as sites where continental slabs are subducted beneath either continental plate or marginal arc terrane, 4

corresponding to collisional orogens (Fig. 2b). Nevertheless, there is a series of differences in lithology between oceanic and continental plates (Fig. 3). The oceanic lithosphere is primarily composed of mafic igneous rocks, residual harzburgite and lherzolite in the upper part, and normal lherzolite in the lower part. In contrast, the continental lithosphere is generally composed of felsic upper crust and mafic lower crust in the upper part, normal lherzolite in the middle part, residual lherzolite and harzburgite in the lower part. These differences should be taken into account when dealing with the products of subduction zone processes. The occurrence of UHP metamorphic rocks in collisional orogens provides petrological evidence for deep subduction of continental crust to the subarc depths. This is seismically confirmed by the presence of a relict continent slab at a depth of ~75 km in Western Alps (Zhao et al., 2015a). A further seismic study finds consistent preservation of the deeply subducted continental slab beneath the Alpine orogen (Zhao et al., 2016), where slab breakoff was geodynamically suggested for exhumation of UHP slices (Davies and van Blanckenburg, 1995) and syncollisional magmatism (van Blanckenburg and Davies, 1995). In contrast, there is no preservation of the deeply subducted continental slab beneath the Himalayan orogen (Zhao et al., 2017a-this issue), lending support to the hypothesis of slab breakoff there. There is also no relict continental slab beneath the Dabie-Sulu orogenic belt (Dong et al., 2008; Luo et al., 2012; He et al., 2014), which may be ascribed to foundering of the orogenic root via asthenospheric erosion in the Early Cretaceous (Gao et al., 2017a). In addition, slab breakoff and lower crust delamination would also contribute to foundering of orogenic roots (Zheng and Chen, 2016, 2017-this issue). As a consequence, continental subduction zones of pre-Cenozoic age cannot preserve their slab vestages at present.

2.3 Thermal structure The thermal structure of subduction zones is defined by their geothermal gradients, which may vary from low to high with time (Leng and Mao, 2015; Li et al., 2015; Zheng et al., 2016). It directly dictates pressure (P)-temperatures (T) conditions under which metamorphic dehydration and partial melting of crustal rocks take place at different depths of subduction zones. Thus, it can be directly linked to the type of metamorphic facies series in orogenic belts (Fig. 4). Low geothermal gradients of <10C/km are responsible for Alpine-type metamorphism, high geothermal gradients of >30C/km are responsible for Buchan-type metamorphism, and intermediate geothermal gradients of 15 to 30C/km are commonly

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responsible for Barrovian-type metamorphism (Zheng and Chen, 2017-this issue). Liu et al. (2017a-this issue) perform a series of numerical models from oceanic subduction to continental collision based on variable thermal structures of converging plates as well as different convergence rates. The results further suggest that the thermal structures of subduction zones control the temperature and fluid activity at the slab-mantle interface in subcontinental subduction channels, which strongly affect the transport and ascent modes of crustal materials.

Anderson et al. (1978, 1980) were the first to consider the thermal structure of subduction zones for slab dehydration, deep earthquake, mantle melting and arc volcanism. In general, crustal rocks dehydrate more water at higher geothermal gradients than lower geothermal gradients. Geodynamic modelling indicates that much larger amounts of water are released at the forearc depths from the subducting crust at warm subduction zones than at cold subduction zones (van Keken et al., 2011). As a consequence, less amounts of water are left in the subducting crust for its dehydration at the subarc depths. For this reason, more abundant arc magmatism and aubarc earthquake only occurs in regions where the subducting slab experienced less amounts of dehydration at the forearc depths (Peacock and Wang, 1999).

3. The processes of subduction zones Metamorphic dehydration and partial melting are the two most fundamental processes in subduction zones. Within subduction zones, descending crustal rocks can make significant contributions to the overriding mantle wedge. As the lithosphere subducts, supracrustal rocks may be scraped off at shallow depths to form the accretionary wedge. With further subduction from the forearc to subarc depths, the crustal rocks underwent high-pressure (HP) to UHP metamorphism at low geothermal gradients. How are crustal rocks detached, processed and exhumed at different depths has been the focus of subduction channel processes. On the other hand, such processes produce metamorphic fluids that occur in the form of aqueous solutions when dehydrating at temperatures below the wet solidus but hydrous melts when melting at temperatures above the wet solidus. These fluids serve as metasomatic agents to react with the mantle wedge peridotite, generating various ultramafic metasomatites whose partial melting gives rise to interplate and intraplate mafic igneous rocks. How are crustal and mantle rocks processed in subduction channels for geochemical transport and eventually arc magmatism has been the focus of subduction factory.

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3.1 Subduction channel and subduction factory 3.1.1 Subduction channel The subduction channel denotes the narrow space between the subducting oceanic slab and the overlying mantle wedge in subduction zones. It was originally envisaged for deformation and metamorphism of crustal rocks at the slab-mantle interface in oceanic subduction zones (Shreve and Cloos, 1986; Cloos and Shreve, 1988a, 1988b). The oceanic slab is commonly assumed to undergo metamorphic dehydration and partial melting during its subduction to the subarc depths. Slab-derived fluids occur in the forms of aqueous solutions and hydrous melts, which would carry fluid-mobile incompatible trace elements (enriched components) into the mantle sources of oceanic and continental arc volcanics (e.g., Ringwood, 1974; Kay, 1980; Tatsumi, 1986; Hawkesworth et al., 1993; Pearce et al., 1995; Spandler and Pirard, 2013). In the oceanic subduction channel, different types of the crustal and mantle materials are involved in the slab-mantle interaction (Bebout and Barton, 2002; Zheng, 2012; Chen et al., 2014; Dai et al., 2014). For example, the mantle wedge of asthenospheric origin occurs above the Mariana-type oceanic subduction channel of western Pacific (Fig. 5a), with involvement of much less seafloor sediment than altered oceanic basalt in the subduction channel. On the other hand, the mantle wedge of lithospheric origin occurs above the Andean-type oceanic subduction channel of eastern Pacific (Fig. 5b), with involvement of much more seafloor sediment than altered oceanic basalt in the subduction channel (von Huene and Scholl, 1991; Stern, 2010). The metasomatic agents of different compositions contribute the enriched geochemical signatures of crustal components to the mantle sources of mafic igneous rocks (Zheng and Hermann, 2014; Zheng et al., 2016).

The subduction channel operates at different depths, and involves different types of physical and chemical interactions (Beaumont et al., 1999; Gerya et al., 2002; Zheng et al., 2013a; Li et al., 2016). Thus, it deals with various processes such as deformation, metamorphism, anatexis and metasomatism, with variable fluxes of fluids at the plate’s interface. The subduction channel is central to the operation and evolution of shallow and deep Earth systems. Subduction channel processes encompass the subduction of crustal materials, the breakdown and transformation of hydrous minerals with increasing depth, dehydration melting at the slab-mantle interface, reaction of the fluids with the mantle wedge, and recycling of the crustal components to the surface in the form of metamorphic and magmatic rocks. Continental and oceanic arc volcanics contain the crustal signatures derived 7

from metamorphic dehydration and partial melting of the subducted crustal materials. These crustal signatures would be in the forms of metasomatic agents to be incorporated into the overlying mantle wedge. They contribute to the geochemistry of mantle sources for oceanic arc basalts (OAB)-like and oceanic island basalts (OIB)-like mafic igneous rocks at convergent plate margins (Zheng et al., 2015), which represent the growth of juvenile crust and thus provide a probe of physical and chemical processes operating at subduction zones. Subducted materials not returned to the surface along the channel are further carried into the deep mantle, where they contribute to the mantle heterogeneity in both chemistry and rheology.

3.1.2 Subduction factory The subduction factory was originally defined to address material fluxes into and out of oceanic subduction zones (Fig. 6), with focus on the origin of arc magmas at convergent plate boundaries (Plank, 2002; Eiler, 2003; Hacker et al., 2003; Tatsumi and Kogiso, 2003, Tatsumi, 2005; Sun et al., 2014). This involves a series of thermal, chemical and mechanical processes that shape the convergent plate boundaries, the deep mantle beyond, and the air and water above subduction zones. Raw materials include seafloor sediment, oceanic igneous rocks (basalt and gabbro), and mantle wedge peridotite, which are fed into the subduction factory at trenches. In the space between the mantle wedge and the subducting slab, subducted rocks are mixed in association with fluid alteration and melt metasomatism. Output products include hydrous melts, aqueous solutions, gaseous volatiles, metalliferous deposits, serpentine diapirs, arc volcanics and HP to UHP metamorphic rocks, which emerge from the factory on the upper plate and its margin. Waste materials occur in the forms of eclogite and peridotite, which sink into the deep mantle. Thus, the material processing in subduction zones looks like a factory, but it is well hidden for view. Nevertheless, subduction geosystems are an analogue to an industrial plant in material processing, though this factory has conducted much of its business in deeply buried locations.

The study of metamorphic petrology indicates that as soon as the raw materials are fed into the subduction factory, they experience metamorphic dehydration at temperatures below the wet solidus of crustal rocks but partial melting at temperatures above the wet solidus (Zheng and Chen, 2016, 2017-this issue). This leads to a series of metamorphic and peritectic reactions with increasing P and T at the slab-mantle interface in subduction channels. While metamorphic solid products are detached from the subducting lithosphere and then exhumed 8

along subduction channels, metamorphic liquid products buoyantly rise into the overlying mantle wedge for crustal metasomatism (Zheng, 2012). This generates ultramafic metasomatites in the lower mantle wedge, whose partial melting gives rise to mafic melts for arc volcanism, whereby the crustal signatures are returned back to the surface (Zheng et al., 2015). The waste products were processed by metamorphic dehydration and melt extraction at the subarc to postarc depths, becoming sterile in lithochemistry and refractory in mineralogy. This subduction factory is ultimately responsible for generating the continental crust, re-enriching the mantle, and redistributing water and carbon dioxide between the crust and mantle. Although the processes that occur inside the factory are hidden from view, we can examine its raw materials and its products. This is usually done by inference from comparison of geochemical fluxes between crustal input and magmatic output, investigation of exhumed HP to UHP metamorphic rocks and orogenic peridotites, and operation of laboratory experiments and thermodynamic calculations that simulate chemical reactions at the forearc, subarc to postarc depths.

3.2 Orogenic metamorphism There are different types of regional metamorphism at convergent plate margins (Zheng and Chen, 2017-this issue). Alpine-type metamorphism is characterized by the formation of blueschist- to eclogite-facies metamorphic rocks at high to ultrahigh pressures, which are the typical products of subduction zone metamorphism. In contrast, Buchan-type metamorphism is recorded by amphibolite- to granulite-facies metamorphic rocks at low to medium pressures, commonly showing the polymorphic transition from andalusite to sillimanite with increasing temperatures. Such rocks can also be produced by Barrovian-type metamorphism at elevated pressures, exhibiting the polymorphic transition from kyanite to sillimanite with rising temperature. Extreme metamorphism at a regional scale is usually referred to as UHP metamorphism at low geothermal gradients on the one hand and ultrahigh-temperature (UHT) metamorphism at high geothermal gradients on the other hand.

3.2.1 Subduction zone metamorphism Subduction zone metamorphism was recognized early by the occurrence of glaucophane-bearing schist and related metamorphic rocks in elongate, narrow zones where they are often associated with rocks of oceanic affinities such as ophiolites and pelagic sediments (e.g., Dewey and Bird, 1970; Ernst, 1971). Such zones are characteristically developed around the margins of Pacific Ocean and also typify early stages of regional 9

metamorphism in the Alpine (Tethyan) orogen. More and more studies indicate that the subduction zone metamorphism is common in accretionary and collisional orogens. It leads to low-grade greenschist-facies metamorphism at first during crustal subduction to shallow levels of <10 km (Fig. 4). This gives rise to accretionary wedges at both accretionary and collisional continental margins (Zheng et al., 2005; Liu et al., 2017b-this issue). Crustal accretion may also occur at blueschist-facies conditions, which is illustrated the Early Mesozoic subduction of the western paleo-Pacific slab beneath the continental margin of the Central Asian Orogenic Belt (Zhou and Li, 2017-this issue). One of the most exciting advances in petrology during the last century has been the findings of UHP index minerals, such as coesite (Chopin, 1984; Smith, 1984) and diamond (Sobolev and Shatsky, 1990; Xu et al., 1992), in metamorphic rocks of collisional orogens. These findings allow petrologists to estimate the lowest pressure of regional metamorphism and thus the minimum depth of continental subduction. Concurrent with the findings there has been an explosion of interest in eclogites, mafic metamorphic rocks in which the maximum pressures for peak metamorphism can be preserved at best. It turns out that blueschist-eclogite facies series unambiguously occur at convergent plate boundaries and their origin must be linked to a tectonic process which engenders low geothermal gradients. This process is the cold subduction of one plate beneath the other plate. Since this tectonic regime was recognized immediately since the findings of UHP index minerals at convergent plate boundaries, it has been viewed as the characteristic feature of modern plate tectonics (Stern, 2002; Chopin, 2003; Liou et al., 2004; Brown, 2007). In this context, UHP metamorphic rocks in continental subduction zones provide us a direct target to investigate petrological and geochemical changes during metamorphic dehydration and partial melting of the subducting crust at the subarc depths, making a link to the origin of arc magmatism above oceanic subduction zones (Zheng and Chen, 2016). Dehydration of crustal rocks is prominent during their subduction (Zheng et al., 2016), with dissolution of fluid-mobile incompatible trace elements into subduction zone fluids (Zheng et al., 2011, 2016; Xiao et al., 2015, 2017-this issue; Huang et al., 2017-this issue). The liberated water at the subarc depths is partly fluxed into the overlying mantle wedge and partly dissolves into the minerals of higher pressures in the deeply subducting crust (Zheng, 2009; Zheng and Hermann, 2014). Water in UHP metamorphic and metasomatic minerals is primarily present in the forms of structural hydroxyl and molecular water in nominally anhydrous minerals such as garnet and clinopyroxene (Gong et al., 2007; 2013; Sheng et al., 2016). Its transport to deeper mantle by subduction is indicated not only by direct sampling of 10

coesite-bearing eclogites (Chen et al., 2007, 2011; Gong et al., 2013) but also by indirect sampling of continental basalts (Xu et al., 2014, 2016; Kovács et al., 2016), xenolith peridotites (Hui et al., 2016) and ringwoodite (Pearson et al., 2014). Analyses of the total water in nominally anhydrous minerals yield the maximum water contents of ~2500 ppm and ~3500 ppm, respectively, in garnet and omphacite at UHP conditions (Chen et al., 2011; Gong et al., 2013). At pressures of the mantle transition zone, 1.4-1.5 wt% H2O may occur in ringwoodite included in a diamond from kimberlite (Pearson et al., 2014). However, this does not mean that the mantle transition zone is ubiquitously of the high water concentrations. Amphibolite-facies overprinting of UHP metamorphic rocks is prominent in collisional orogens. Traditionally, the externally-derived water was assumed for the secondary alteration. This common wisdom was challenged by Zheng et al. (1999, 2003), who found the close similarity in mineral hydrogen and oxygen isotope compositions between UHP eclogites and their adjacent amphibolites. This led to the proposition that the water for the amphibolite-facies overprinting is of internal origin. This proposition has been verified by FTIR and TC/EA-MS analyses of mineral water abundances in UHP eclogites (Chen et al., 2007, 2011; Sheng et al., 2007; Gong et al., 2012). Further studies indicate that UHP metamorphic rocks commonly undergo decompressional dehydration during their early exhumation, releasing water for the amphibolite-facies retrogression (Zheng, 2009; Zheng and Hermann, 2014; Sheng and Gong, 2017-this issue) and even buleschist-facies retrogression (Xiao et al., 2017-this issue). For this reason, subduction zones fluids are internally buffered in their isotope compositions (Zheng et al., 2003, 2009). If the decompressional dehydration takes place at temperatures above the wet solidus of UHP rocks, partial melting may become conspicuous during the exhumation (Zheng et al., 2011; Chen et al., 2017a-this issue; Gao et al., 2017b-this issue). This leads to granulite-facie overprinting of the UHP rocks at first and then amphibolite-facies overprinting during their late exhumation (Zheng and Chen, 2017-this issue). In modern plate subduction zones where low geothermal gradients prevail, crustal rocks undergo metamorphic dehydration at HP to UHP conditions during their subduction to the subarc depths (Stern, 2002; Zheng et al., 2016). At such depths, aqueous solutions are produced and then react with the mantle wedge peridotite in subduction channels to generate serpentinized to chloritized peridotites, which serve as the source of arc magmas (Zheng et al., 2015). The deeply subducted crustal rocks may undergo partial melting at the subarc depths (Zheng et al., 2011; Chen et al., 2017a-this issue), giving rise to hydrous felsic melts that also react with the mantle wedge peridotite to generate mafic-ultramafic metasomatites such as 11

olivine-poor pyroxenites and hornblendites (Zheng, 2012; Chen et al., 2015; Zheng et al., 2015). These altered peridotites and metasomatites also serve as the sources of arc magmas, transferring the geochemical signature of crustal components from the subducting slab through the mantle wedge to the arc volcanics (Chen et al., 2014; Zhao et al., 2013, 2015; Chen and Zhao, 2017-this issue). In this regard, the HP to UHP metamorphic rocks provide proxies for the generation of subduction zone fluids at the forearc to subarc depths (Fig. 7), where the mantle sources of arc volcanics were generated by metasomatic reaction of the fluids with the mantle wedge peridotite at the slab-mantle interface in subduction channels (Zheng and Hermann, 2014; Zheng and Chen, 2016).

3.2.2 Rifting zone metamorphism Rifting zone metamorphism is a new term in the family of regional metamorphism (Zheng and Chen, 2017-this issue). It corresponds to extensional metamorphism in the literature, denoting the processes of metamorphic reaction and recrystallization that take place at crustal depths with temperatures above the wet solidus of crustal rocks. Its common products are high-temperature (HT) to UHT granulite-facies metamorphic rocks. Although these rocks are generally exposed at fossil convergent plate margins, it does not means that they are the product of subduction-zone metamorphism at high geothermal gradients. Instead, they were mainly generated by asthenospheric heating of the thinned orogens at crustal pressures, which is attributable to superimposition of active continental rifting on preexisting orogens (Zheng and Chen, 2017-this issue). Although UHT metamorphism was recognized very early by the occurrence of sapphirine and quartz in granulite (Dallwitz, 1968; Morse and Talley, 197l; Grew, 1980), it has been puzzled for a long time how the ultrahigh temperatures of >900C can be obtained at crustal pressures below the aragonite/calcite transition (Harley, 1989, 1998; Pattison et al., 2003; Brown, 2007; Kelsey and Hand, 2015). A number of studies have indicated that such temperatures cannot be provided by internal heat sources at the crust-mantle transition zone (Clark et al., 2011). This led to the conventional wisdom that UHT metamorphic rocks were anomalous and UHT conditions were not of general significance to tectonic models of large-scale lithospheric development and evolution (e.g., Bohlen, 1991). It also leads to ambiguous definition of its geodynamic mechanism (e.g., Santosh et al., 2012). For these reasons, it is only in the past three decades that the UHT metamorphism has become more generally accepted as a widespread and important style of regional tectonism that demands 12

explanation. Nevertheless, the asthenospheric mantle is a potential heat source beneath the orogenic lithosphere, and its upwelling is proposed as a geodynamic mechanism for the rifting zone metamorphism in intracontinental orogens (Zheng and Chen, 2017-this issue). If the deeply subducting continental slab undergoes breakoff at the subarc depths, UHP metamorphic rocks are immediately heated by the asthenospheric mantle at the slab window (Fig. 8). This results in superimposition of the UHP rocks by UHT metamorphism (Zheng and Chen, 2017-this issue), which is indicated by the occurrence of coeval UHP and UHT metamorphic rocks in collisional orogens (Kotková et al., 2011; Liu et al., 2015a; Zheng and Chen, 2016). Otherwise thickened orogens are produced by accretionary or collisional orogeny and maintained at convergent plate margins. At a later time, they may be thinned via the mechanism of either gravitational delamination or asthenospheric erosion (Zheng and Chen, 2016, 2017-this issue). As soon as the thickened orogens are thinned to a thickness less than that for normal continental lithosphere, the lithosphere-asthenosphere boundary are elevated to shallower depths. As a consequence, high heat flow is available from the upwelling asthenosphereic mantle, which heats the thinned orogens for active rifting to cause not only dehydration but also anataxis of the lower continental crust (Zheng and Chen, 2017-this issue). This gives rise to both aqueous solutions and hydrous felsic melts, which metasomatize the overlying crustal rocks for hydration melting on the one hand and form felsic leucosomes or even granitic rocks on the other hand (Zheng and Chen, 2016; Chen et al., 2017a-this issue; Zhao et al., 2017-this issue). Residual rocks are granulites that are short of water and melt-mobile incompatible elements (Zheng and Chen, 2017-this issue). It is possible that the granulite-facies

dehydration at

greater

depths

yields

aqueous

solutions

for

the

amphibolite-facies hydration at shallower depths, resulting in the relationship of source and sink in thinned orogens (Zheng and Chen, 2016, 2017-this issue).

3.2.3 Garnet and zircon Traditionally, garnet in high-grade metamorphic rocks is thought to grow via mineralogical reaction at subsoldus conditions. However, more and more studies indicate that garnet may grow not only through metamorphic reaction at subsolidus conditions but also through peritectic reactions at supersolidus conditions (Xia and Zhou, 2017-this issue). In either case, the breakdown of hydrous minerals is necessary for garnet growth. Mineral inclusions in garnet provide a direct record of its growing media, with metamorphic 13

inclusions for metamorphic garnet but anatectic inclusions for peritectic garnet (Xia and Zhou, 2017-this issue; Gao et al., 2017b-this issue). Because of the metamorphic overprinting, garnet cores of metamorphic origin are often surrounded by garnet mantle/rim of peritectic origin. Nevertheless, it is also found that garnet cores of peritectic origin are surrounded by garnet mantle/rim of metamorphic origin. The former is related to superimposition of subsolidus metamorphism by supersolidus metamorphism, whereas the latter is related to superimposition of supersolidus metamorphism by subsolidus metamorphism. Zircon is common in crustal rocks of non-juvenile origin. It can grow from magmatic melts subsequent to fractional crystallization of Zr-poor minerals during magma evolution. Magmatic zircon may suffers different types of metamorphic recrystallization at subduction zones via the mechanisms of solid-stage transformation, metasomatic alteration and dissolution reprecipitation. The type and extent of metamorphic recrystallization are primarily dictated by the accessibility and physicochemical property of subduction zone fluids (Zheng, 2009; Xia et al., 2010; Chen and Zheng, 2017-this issue). On the other hand, zircon can grow not only via metamorphic reaction at subsolidus conditions but also peritectic reactions at supersolidus conditions. In either case, the breakdown of hydrous minerals is necessary for zircon growth during metamorphism. Mineral inclusions in zircon also provide a direct record of its growing media, with metamorphic inclusions for metamorphic zircon but anatectic inclusions for peritectic zircon (Chen et al., 2017a-this issue). Chen and Zheng (2017-this issue) present a review of major contributions that zircon studies have made in terms of understanding key questions involving metamorphic dehydration and partial melting in continental subduction zones. These include the conditions of crustal reworking at subduction zones, the modification of protolith zircons, the growth of metamorphic and peritectic zircons and the effect of fluid properties on zircon growth and modification. Therefore, zircon can be used to sample a broad region of metamorphic rocks in collisional orogens, allowing investigation of a vast area with only minimal sampling. As such, zircon arguably forms the best archive of metamorphic, anatectic and magmatic records in subduction zones.

3.3 Subduction zone metasomatism Studies of metasomatism can be dated back to Goldschmidt (1922), who defined it as a process of chemical alteration which involves enrichment of the rock by new substances brought in from the outside. This process has been widely acknowledged in studies of mantle xenoliths and alkaline igneous rocks in both continental and oceanic settings (e.g., Frey and

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Green, 1974; Bailey, 1987; O’Reilly and Griffin, 2013). As such, mantle metasomatism is referred to as chemical reactions in the mantle (Bailey, 1982). Introduction of metasomatic agents to mantle rocks affects their geochemistry and petrology and depresses their solidi. As a consequence, the enrichment of highly incompatible elements in mantle rocks has been one of the driving forces for their melting at reduced temperatures. Because xenolith peridotites were regarded as lithospheric mantle fragments, metasomatic agents were assumed to originate from the underlying asthenospheric mantle (e.g., Roden and Murthy, 1985; Kelemen et al., 1992, 1998). This kind of mantle metasomatism is usually viewed as the lithosphere-asthenosphere interaction in igneous petrogenesis. Inside subduction channels, however, materials are composed of not only crustal rocks offscrapped from the subducting slab but also mantle rocks eroded by subduction from the mantle wedge base (Zheng and Chen, 2016). They undergo metamorphic dehydration and partial melting at different depths (Fig. 7), producing aqueous solutions and hydrous melts that are variably enriched in fluid-mobile incompatible trace elements (Zheng and Hermann, 2014). These subduction zone fluids tend to rise and infiltrate into the overlying mantle wedge for chemical reactions (Bebout, 2013). In this regard, subduction zone metasomatism builds a bridge between subduction zone metamorphism and magmatism. Metasomatic reactions in subduction zones are of particular interest to the geochemical transfer from the slab to the mantle because they bring enriched signatures into the magma sources of mafic igneous rocks. There are principally two types of metasomatism in subduction zones: one is crustal metasomatism, and the other is mantle metasomatism (Zheng, 2012). The crustal metasomatism is caused by reaction of the mantle wedge peridotite with fluids originated from subducting crustal rocks. While the aqueous solutions react with the mantle wedge peridotite to produce metasomatized rocks such as serpentinized to chloritized peridotites, the hydrous felsic melts react with the peridotite to generate metasomatic rocks such as pyroxenite and hornblendite. These metasomatites are still of ultramafic composition in general, but they are variably enriched in fluid-mobile incompatible trace elements and their pertinent radiogenic isotopes (Zheng et al., 2015, 2016). On the other hand, the metasomatites in the mantle wedge base may be eroded by further subduction and entrained into greater depths in subduction channels. They may undergo dehydration melting to produce hydrous mafic melts, which react with the overlying mantle wedge peridotite, converting pyroxene to olivine. This is referred to as mantle metasomatism (Zheng, 2012). If the subducting slab is decoupled with the mantle wedge, the asthenospheric mantle may laterally flow into the space between the rollback slab and the mantle wedge. In this case, the 15

asthenospheric mantle may undergo decompressional melting to produce MORB-like mafic melts, which react with the overlying metasomatites in the mantle wedge base. This leads to another kind of mantle metsomatism. The crustal metasomatism can be subdivided into modal and crypotic ones according to the occurrence of metasomatic products in the mantle wedge (Zheng and Hermann, 2014). The modal metasomatism is indicated by the presence of new mineral phases such as serpentine, chlorite, amphibole, phlogopite, apatite, carbonate, sulfide, titanite, ilmenite, and zircon, which are absent in the peridotite of primitive and depleted mantle sources. The new mineral phases are mineralogically and geochemically distinguishable from primary peridotite minerals. The cryptic metasomatism is indicated by the absence of such new mineral phases but the enrichment in fluid-mobile incompatible trace elements such as LILE and LREE relative to HFSE and HREE. In either case, the metasomatic products occur as the different types of metsomatites with variable degrees of enrichment in fluid-mobile incompatible trace elements, depending on the thermal structure of subduction zones (Zheng et al., 2016). If subduction proceeds at the low geothermal gradients, crustal dehydration prevails at the subarc depths. This produces the aqueous solutions that are only enriched in highly incompatible water-soluble trace elements (e.g., LILE). If subduction proceeds at the high geothermal gradients, dehydration melting prevails at the subarc depths. This produces the hydrous felsic melts that are enriched in the fluid-mobile incompatible trace elements. The crustal metasomatism of the mantle wedge is evident in peridotites not only in oceanic subduction zones (e.g., Kepezhinskas et al., 1995; Zanetti et al., 1999) but also in continental subduction zones (e.g., Rampone and Morten, 2001; Tumiati et al., 2007; Chen et al., 2015, 2017b). These peridotites provide us with a direct target to investigate the crust-mantle interaction in subduction channels. An important advance in the study of Earth’s geodynamic behaviors is emerging from the linkage between metasomatic reactions in the mantle and the origin of interplate and intraplate mafic igneous rocks (Zheng, 2012; Zhao et al., 2013, 2015; Dai et al., 2014, 2017; Zheng et al., 2015; Chen and Zhao, 2017-this issue; Xu and Zheng, 2017-this issue). For example, metasomatic enrichment of the depleted MORB mantle by subduction zone fluids is a basic mechanism for the ultimate enrichment of fluid-mobile incompatible trace elements and their pertinent radiogenic isotopes in the mafic igneous rocks (Zheng et al., 2015, 2016; Xu and Zheng, 2017-this issue). The metasomatic reaction of subduction zone fluids with the mantle wedge peridotite is a substantial step in transferring the geochemical signatures from the subducted slab to the magmatic products. They also prime the sources of interplate and intraplate mafic magmas. 16

However, the subduction zone metasomatism is a function of multiple variables in geochemistry, geology, geophysics, and geodynamics. Because of the difference in the geological structure of subduction zones, the studies of subduction zone metasomatism must take in account at least three series of mantle and crustal components: (a) mantle wedge peridotite (fertile vs sterile, and enriched vs depleted), (b) cover sediment (terrigenous vs pelagic), and (c) crystalline basement (igneous vs metamorphic). In doing so, more attention should be paid to the composition of crustal fluids, because it is primarily dictated by the P-T conditions at which the subducting crustal rocks undergo dehydration melting at the subarc to postarc depths (Zheng and Hermann, 2014; Zheng et al., 2016).

3.4 Orogenic magmatism Mafic arc volcanics are common above oceanic subduction zones, and their production is usually ascribed to the flux of subduction zone fluids into the mantle wedge. It is generally assumed that mantle melting above the subduction oceanic slab is initiated by water flux in the mantle wedge (e.g., Grove et al., 2012). However, this assumption has been challenged by Zheng et al. (2016), which indicate the presence of a temporal interval between fluid flux into the mantle wedge and mafic arc volcanism (Fig. 9). Thus, the altered peridotites undergo partial melting for mafic magmatism at a later time due to heating by the asthenospheric mantle (Zheng and Chen, 2016; Chen and Zhao, 2017-this issue). Furthermore, the dehydrated UHP rocks may undergo partial melting at the postarc depths, where hydrous felsic melts are produced and also react with the mantle wedge peridotite of asthenospheric origin to generate ultramafic metasomatites such as olivine-poor pyroxenites (Zhang et al., 2015). Such metasomatites may serve as the source of intraplate basalts (Zheng et al., 2015; Xu and Zheng, 2017-this issue). However, these metasomatites did not partially melt immediately upon reaction of the mantle wedge peridotite with the hydrous melts from the subducting slabs (Zheng et al., 2016). Instead, they undergo partial melting at a later time, when they are heated by the asthenospheric mantle.

3.4.1 Interplate magmatism Arc volcanics at convergent plate margins are the typical product of oceanic subduction-zone magmatism (Hawkesworth et al., 1993; Pearce et al., 1995; Stern, 2002; Spandler and Pirard, 2013; Zheng and Chen, 2016). They are commonly categorized into two types of OAB and continental arc andesites (CAA). The formation of their magma sources is 17

temporally and spatially associated with dehydration and anatexis of the subducting oceanic crust at the subarc depths. Subduction zone fluids serve as metasomatic agents to react with the mantle wedge peridotite at the slab-mantle interface in oceanic subduction channels (Zhang et al., 2015; Zhao et al., 2015b; Zheng et al., 2015). Although both aqueous solutions and hydrous melts are involved in the generation of mantle sources for arc volcanics, aqueous solutions play a dominant role in the origin of OAB whereas hydrous felsic melts play a dominant role in the origin of CAA (Chen and Zhao, 2017-this issue; Guo et al., 2017-this issue). As firstly proposed by Chen et al. (2014), the andesitic composition of continental arc volcanics can be explained by much more involvement of the hydrous felsic melts derived from partial melting of subducting terrigenous sediments. Furthermore, the geochemical composition of arc volcanics is primarily dictated by that of metasomatic agents, because the metasomatic agents have much higher abundances of fluid-mobile incompatible trace elements than the mantle wedge peridotite (Xu and Zheng, 2017-this issue). In particular, the relatively depleted Sr-Nd isotope compositions of arc volcanics are inherited from metasomatic agents that are produced by dehydration and anatexis of the subducting oceanic igneous rocks, whereas the relatively enriched Sr-Nd isotope compositions of arc volcanics are inherited from metasomatic agents that are produced by dehydration and anatexis of the subducting terrigenous sediment (Zheng et al., 2015). This paradigm is also applicable to the origin of postcollisional mafic igneous rocks in continental subduction zones (Zhao et al., 2013, 2015). Therefore, the geochemical composition of metasomatic agents is determined by the nature of subducted crustal rocks and the depth of their formation in both oceanic and continental subduction channels.

3.4.2 Intraplate magmatism Many if not all continental basalts show similar trace element signatures to OIB in the primitive mantle-normalized diagram, and moderately depleted Sr-Nd isotope compositions (Allègre, 1982; Farmer, 2014). Such geochemical features are also observed in Cenozoic continental basalts from eastern China (Xu and Zheng, 2017-this issue). Because of their relatively depleted Sr-Nd isotope signatures, they are traditionally interpreted as origination from the depleted MORB mantle (e.g., Flower et al., 1998; Zou et al., 2000) with variable involvement of crustal components in their sources (e.g., Xu et al., 2005; Tang et al., 2006; Liu et al., 2008; Chen et al., 2009). However, these interpretations do not take into account enrichment of melt-mobile incompatible trace elements such as LILE and LREE relative to 18

HREE in continental basalts, which is in contrast to depletion in these elements relative to HREE in normal MORB (Hofmann, 2014). Thus, they cannot explain how the asthenospheric MORB-like trace element character was transformed to the OIB-like trace-element signature in the continental basalts. Traditionally, the enrichment of LILE and LREE relative to HREE in OIB is attributed to their origination from the lower mantle, which is assumed to compensate the depletion of LILE and LREE relative to HREE in the upper mantle due to extraction of continental crust from the primitive mantle (Zindler and Hart, 1986; Hofmann, 1988). For this reason, many studies dealing with the geochemistry of continental flood basalts ascribe their origin to mantle plume magmatism (Farmer, 2014), but few studies link the origin of continental rift basalts to mantle plume magmatism because of their sporadic property. Although the enrichment of radiogenic isotopes in intraplate basalts is ascribed to crustal components, it remains to be resolved how the crustal components were incorporated into the source regions of intraplate basalts. A novel model has been presented for the petrogenesis of continental basalts in order to account for the OIB-like trace element and radiogenic isotope compositions (Zhang et al., 2009; Wang et al., 2011; Xu et al., 2012). This assumes that crustal signatures were transferred in the form of hydrous felsic melts, derived from low-degree partial melting of the subducted oceanic crust, into the mantle source of continental basalts (Fig. 10). Dissolution of rutile in the hydrous felsic melts is used to account for enrichment or no depletion of such high field strength elements (HFSE) as Nb, Ta and Ti in OIB and continental basalts (Ringwood, 1990; Zheng, 2012). As such, a transition lithology of ultramafic metasomatites such as silica-deficient pyroxenites was generated by reaction of the depleted MORB mantle peridotite with the hydrous felsic melts at the postarc depths (Zheng, 2012). Quantitative modeling does demonstrates that the hydrous felsic melts can have much higher abundances of melt-mobile incompatible trace elements than the depleted MORB mantle peridotite, enabling the transfer of enriched geochemical signatures from the crustal components via the metasomatites to the intraplate basalts (Xu et al., 2017; Xu and Zheng, 2017-this issue). It is also emphasized that crustal components in intraplate basalts were incorporated into their mantle sources in the form of metasomatic agents rather than the solid rocks (Zheng et al., 2015). Thus, the geochemical composition of intraplate basalts is primarily dictated by that of metasomatic agents. In this regard, the relatively depleted Sr-Nd isotope compositions of intraplate basalts are inherited from metasomatic agents that are produced by partial melting of the subducting oceanic igneous rocks, whereas the relatively enriched Sr-Nd isotope compositions of intraplate basalts are inherited from metasomatic agents that are produced by 19

partial melting of the subducting terrigenous sediment (Xu and Zheng, 2017-this). Therefore, the nature of subducted crustal rocks is substantial to the geochemical composition of metasomatic agents. This model can be applied not only to the origin of continental mafic igneous rocks that exhibit the radiogenic isotopic signatures of either depletion or enrichment (Zhao et al., 2015), but also to the origin of oceanic intraplate basalts (Zheng et al., 2015).

3.4.3 Postcollisional magmatism Syn-subduction magmatism is common above oceanic subduction zones (Stern, 2002), which are generally developed into accretionary orogens. In contrast, it is absent above continental subduction zones (Zheng and Chen, 2016). Nevertheless, postcollisional magmatic rocks are common in continental subduction zones, which are also referred to as collisional orogens. They are dominated by felsic lithologies, differing from mafic lithologies from syn-subduction magmatism above the oceanic subduction zones (Gill, 1981; Tatsumi and Eggins, 1995; Chen and Zhao, 2017-this issue). Post-subduction reworking is prominent in both accretionary and collisional orogens, giving rise to felsic rocks in postcollisional stages. This is particularly so in collisional orogens like the Qinling-Tongbai and Dabie-Sulu, where granitoid rocks are much more common than either bimodal or intermediate associations (Liu et al., 2015b; Song et al., 2015; Wang et al., 2015; Xu and Zhang, 2017-this issue; Zhao et al., 2015b, 2017b-this issue). Sometimes alkaline magmatism occurs in this stage (Dai et al., 2017), signifying the onset of an extensional setting for very low-degree partial melting of the orogenic lithosphere. However, the geodynamic mechanism for postcollisional magmatism has been controversial though the plate tectonics theory has been developed in the past five decades. Because this theory is originally based on tectonic processes occurring at major plate edges, mid-ocean ridges and active plate margins, it has only given minor consideration of postcollisional settings. This leads to various hypotheses about the origin of orogenic magmatism in postcollisional stages, with little link to operation of plate tectonics. Nevertheless, more and more studies have noticed that intracontinental settings are developed from previous convergent margins (Zheng et al., 2013b, 2015). In particular, subduction zone processes have primed the sources of postcollisional magmas in intraplate settings. Although compressional tectonism is associated with subduction, accretionary and collisional orogenies represent periods of the maximum convergence that are not favorable for ascent of arc magmas. Instead, extensional tectonism is necessary either for exhumation of UHP 20

metamorphic rocks along continental subduction channels or for partial melting of the mantle wedge metasomatites to bring about arc magmatism (Zheng and Chen, 2016). The extensional tectonism is induced by underplating of the asthenospheric mantle along thinned orogens, corresponding to the superimposition of active continental rifting on preexisting orogens (Fig. 11). This active rifting model can account for not only the HT to UHT metamorphism but also voluminous magmatism in the postcollisional stage (Zheng and Chen, 2017-this issue).

4. Implications for orogenic processes Traditionally, orogenic belts are considered as the products of subduction-related tectonism, and generally classified into two types (e.g., Cawood et al., 2009). One is the accretionary orogen, which is generated by subduction of an oceanic slab beneath a continental plate and characterized by the growth of juvenile crust at continental margins. The other is the collisional orogen, that is produced by subduction of a continental slab beneath other continental plate or its newly accreted margin arc terrane and thus characterized by reworking rather than growth of the preexisting crust. In either type, these orogens were developing during plate convergence, with significant thickening of orogenic lithosphere for mountain building. However, more and more studies indicate that many of these orogens underwent remarkable thinning in introcontinental settings after their formation at convergent plate margins. Such thinned orogens show significant reactivation for crustal reworking by lithospheric stretch, and this extensional tectonism considerably postdates the lithospheric contraction during subduction. The reactivation is generally indicated by the voluminous occurrence of both magmatic and migmatitic rocks in felsic composition. If these rocks are so abundant that they have led to mountain building, their production is attributable to the rifting orogeny at fossil convergent plate margins (Zheng and Chen, 2017-this issue). There is also the rifting orogeny at active divergent plate margins, typically giving rise to mid-ocean ridge basalts for the growth of juvenile crust. Although this term looks as if it makes a new type of orogens in continental regions, those reactivated orogens have been noted for a long time and they are often named as intracontinental orogens, intracratonic orogens or extensional orogens in the literature. A common feature of such orogens is that they were developing at a time later than rather than during plate convergence and characterized by reworking of both ancient and juvenile crust.

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The superimposition of active rifting on preexisting orogens is common in intracontinental settings, leading to mountain building in post-subduction stages. This provides a complement to orogenic types that have been enigmatic since the advent of plate tectonics. Zwart (1967, 1969) noticed the metamorphic differences in orogenic belts, proposing three types, which were modified by Pitcher (1979). Further studies indicate that orogenic belts basically develop at convergent plate margins (Zheng et al., 2013, 2015). These have led Zheng and Chen (2017-this issue) to propose the rifting orogen as a new type. In this regard, orogenic belts can be classified into the following three types (Fig. 12): (1) accretionary (Cordillerotype), where regional metamorphism proceeds at low geothermal gradients and at shallow depths, giving rise to low-P to HP metamorphic rocks. There is general lack of not only migmatites but also ophiolite and abyssal sedimentary rocks (black shale, chert). Magmatic rocks are dominated by calc-alkaline basalts and andesites; (2) collisional (Alpinotype), where regional metamorphism proceeds at low geothermal gradients but at greater depths, giving rise to HP to UHP metamorphic rocks that occur in thick zones. Nappe structures are predominant there, and orogens are relatively narrow with large and rapid uplift. There are abundant ophiolites with ultramafic rocks, but there are few contemporaneous migmatites or granites; (3) rifting (Hercynotype), where metamorphism proceeds at high geothermal gradients and at shallow depths, giving rise to HT to UHT metamorphic rocks that occur in thin zones. Nappe structures are rare there, and orogens are very wide with small and slow uplift. There are abundant granites and migmatites, but few ophiolites; ultramafic rocks are virtually absent.

Although granites and migmatites are common in orogenic belts, their relationship to orogenesis has been enigmatic for a long time. If they are associated with contemporaneous HT to UHT metamorphic rocks, they are certainly produced by the rifting orogeny in intracontinental setting (Zheng and Chen, 2017-this issue). This is also the reason why such rocks are often considered as the products of intracontinental orogeny. Now they can be termed as the products of rifting orogeny if their volumes are large enough to result in mountain building. Although very small volumes of magmatic rocks are also common in rifting settings, they cannot be regarded as the products of rifting orogeny because they have not resulted in mountain building. In either case, nevertheless, extensional tectonism is prominent but its scale is variable. This is also illustrated by the occurrence of alkaline igneous rocks in orogenic belts. Whereas the rifting orogeny leads to large-scale magmatism for mountain building, anorogenic settings are independent of orogenic processes. Therefore, 22

it is critical to distinguish the anorogenic associations of magmatic rocks to the magmatic products of rifting orogeny at fossil convergent plate margins. Likewise, it is important to discriminate the syn-subduction processes and products from post-subdduction processes and products. Because high heat flow can be supplied from mantle plume to continental lithosphere, the concept of mantle plume has been utilized to explain the origin of some granites in continental regions (e.g., Li et al., 2003, 2008; Haapala et al., 2007; Shellnutt and Zhou, 2007; Wang et al., 2010). Intraplate basalts, represented by OIB, are usually assumed as the product of mantle plume magmatism. They correspond to large igneous provinces (LIPs) with OIB-like composition in continental regions. In either case, both matter and energy would be supplied from deep mantle to the lithosphere for basaltic magmatism. As such, we must keep in mind that OIB and LIPs are geochemically characterized by enrichment in HFSE in addition to their enrichment in LILE and LREE relative to HREE in the primitive mantle-normalized diagram (Hofmann, 1997; Zheng et al., 2015). The enrichment in HFSE is in contrast to depletion of HFSE in OAB, the typical product of subduction-zone magmatism (Kelemen, 2003). If the mantle plume could have played a primary role in triggering granitic magmatism, it is necessary to have the occurrence of contemporaneous OIB-like LIPs in the same region. Such a LIP is indeed present at Emeishan in South China (Xu et al., 2004), where Permian A-type granites are associated with contemporaneous OIB-like basalts (Shellnutt and Zhou, 2007). Nevertheless, this mantle plume did not develop into lithospheric rupture for seafloor spreading. In contrast, the Tristan mantle plume in the southwestern edge of Africa led to continental rifting that developed into seafloor spreading at about 125–127 Ma, resulting in opening of the Atlantic Ocean by separating South America from Africa (White and McKenzie, 1989; Gladczenko et al., 1997; Gibson et al., 2005). A bimodal LIP even occurs at Damaraland in Africa, where a composite intrusive composed of mafic, silicic and alkaline plutonic and volcanic rocks was emplaced into the Neoproterozoic Damara orogen at 124 to 137 Ma (Haapala et al., 2007). However, such a LIP is absent in the Jiangnan orogen of South China, where the composite granites of middle Neoproterozoic were interpreted as the product of mantle plume magmatism (Li et al., 2003, 2008; Wang et al., 2010). In this regard, the active continental rifting may have served as the geodynamic mechanism for the generation of composite granites in the Jiangnan orogen, and it was superimposed on the accretionary arc-continent collisional orogens (Zheng et al., 2008). This explains the observation that such granites show subduction-related geochemical signatures (Zhao et al., 2008; Wang et al., 2010). 23

On the other hand, subduction of spreading ridges has been suggested to occur beneath continental margins (Palmer, 1968; Delong et al., 1979; Cole and Basu, 1992; Farrar and Dixon, 1993; Thorkelson, 1996). The concept of ridge subduction was utilized to explain the origin of adakites in continental regions (Sun et al., 2010; Tang et al., 2010; Zhang et al., 2010). This was based on the assumption that the mid-ocean ridge is hot and its subduction beneath continental margins can provide not only the eclogite of MORB composition (matter) but also the high heat flow (energy) for adakitic magmatism (Defant and Drummond, 1990; Peacock et al., 1994; Cole and Stewart, 2009). Although partial melting of the eclogite in the garnet stability field has the capacity to produce adakitic melts with high Sr/Y and La/Yb ratios, it is not able to produce adakites with arc-like trace element distribution patterns (e.g., the positive Pb anomaly). This is because the mid-ocean ridge basalt and its underlying gabbro are geochemically characterized by depletion of LILE and LREE relative to HREE (Hofmann, 1988). Available studies indicate that adakitic rocks generally occur at convergent plate margins, corresponding to precedingly accretionary/collisional orogens (e.g., Stern and Kilian, 1996; Xu et al., 2002, 2007; Chung et al., 2003; Wang et al., 2005, 2006, 2007; Huang et al., 2008; Liu et al., 2010; Tang et al., 2010; Zhang et al., 2010; Ling et al., 2011; Castillo, 2012). In this regard, partial melting of the thickened orogenic lower crust is capable of producing the adakitic melts with high Sr/Y and La/Yb ratios. This process may take place prior to the superimposition of active continental rifting on the previous orogens (Zhao et al., 2017-this issue). Therefore, the active continental rift model can account for the generation of adakitic rocks in continental regions.

5. This special issue This special issue has arisen from thematic sessions on subduction zones in three annual meetings, which were sponsored by the Geological Society of America (GSA2015 at Baltimore), the Geochemical Society and the European Association of Geochemistry (Goldschmidt2010 at Yokohama), and the Asia Oceania Geosciences Society (AOGS2016 at Beijing), respectively. These sessions were designed to provide a snapshot of the current research into the structures, processes and products of subduction zones, with particular focus on advances in observational and interpretations of subcontinental subduction zones. There are eighteen papers in this special issue, and they are composed of ten review articles and eight research papers. They deal with the following eight aspects in subduction zones: (a) the thermal and geometric structures of subduction zones, (b) regional

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metamorphism at convergent plate margins, (c) metamorphic dehydration of crustal rocks during their subduction to and exhumation from subarc depths, (d) partial melting of UHP metamorphic rocks at subarc depths; (e) geochemical metasomatism of the mantle wedge in subduction channels, (f) mineralogical records of subduction zone processes, (g) mafic magmatism above oceanic subduction zones, and (h) felsic magmatism in continental subduction zones. This organization provides a simple progression from subduction to orogenesis through the topic. As a consequence, the ordering of the papers shifts focus on the fundamental problems at the forefront of subduction zone studies. The contributions in this special issue address, to varying degrees, ten cutting-edge questions about subduction zones: (1) what governs the thermal structure of subduction zones and how does it influence their evolution and internal processes? (2) what is the geometric structure of continental subduction zones with reference to the known occurrence of UHP metamorphic rocks at collided continental margins? (3) how crustal accretion proceeds at collided and accretionary continental margins? (4) what is the geodynamic mechanism for the extreme metamorphism at convergent plate margins? (5) how do subduction zone processes influence the metamorphic dehydration and partial melting of crustal rocks at different depths? (6) where are hydrous melts produced in subduction zones and what processes control their characteristic compositions? (7) what are the mineralogical records of metamorphic and peritectic reactions in HP to UHP metamorphic rocks? (8) how has the property of subduction zone fluids influenced the composition of interplate and intraplate mafic igneous rocks? (9) how are postcollisional granites produced in continental subduction zones? (10) to what extent can the knowledge of geophysical structure be combined with geochemical inputs and outputs to construct a predictive model for subduction zone magmatism? These ten questions provide the framework to advance the study of subduction zones in the future. This special issue is not intended as a comprehensive volume on the structures and processes of subduction zones but, rather, is a collection of the current research into this important field. Despite the diverse nature of research into subduction zones, a number of far-reaching outcomes have been produced in the past decade. Consequently, a unified definition and understanding of the subduction zone processes that drive interplate and intraplate magmatism is emerging from an integrated study of geochemistry, geology, geophysics and geodynamics. It is our hope that insights into subduction zones can be provided by integration of different approaches from different institutions in the near future.

6. Epilogue 25

Subduction zones are is a rich topic, spanning from marine geology to igneous petrology, phase equilibrium to seismic tomography, and geochemical kinetics to chemical geodynamics. Accordingly, diverse and multidisciplinary observations and measurements are required to understand the subduction zones for various variables. Geophysical studies provide valuable information about the thermal and compositional structure of present subduction zones. Numerical geodynamic models provide valuable information about how subduction zones could have evolved over time. Nevertheless, the structures and processes of fossil subduction zones can only be studied by geological and geochemical investigation of their direct and indirect products. Integration of these results can provide both qualitative and quantitative constraints on the composition and evolution of subduction zone fluids, the extent and scale of crust-mantle interaction in subduction channels, and a time-integrated perspective about the physicochemical changes in the subduction factory through geologic history. While oceanic subduction zones have been studying comprehensively since the advent of plate tectonics in 1960s, attention to continental subduction zones has being paid only since the discovery of coesite in metamorphic rocks of supracrustal origin in 1980s. Much advances have been made on the structures, processes and products of subduction zones in the past three decades. However, there are still a lot of major questions that remain to be answered in the study of subduction zones. These are primarily related to the geodynamics of subduction zones, including aspects such as the structure and evolution of subduction zones, the temperature and volatile distribution in the descending slab and overlying mantle wedge, metamorphic dehydration and partial melting of the subducting crust, the role of accessory minerals in dictating the composition of subduction zone fluids, and the role of metasomatic agents in the formation of mantle sources for mafic magmatism. In order to understand the structures and processes of subduction zones, it is necessary to have an integrated study of geochemistry, geology, geophysics and geodynamics. In doing so, the following eight aspects are to be taken into account: (1) the composition of sediments in the trench and crystalline rocks in oceanic and continental crust; (2) the melting P-T conditions of HP to UHP metamorphic rocks and the composition of their melting products; (3) the laboratory experiments and thermodynamic calculations of crustal and mantle metasomatism in subduction zones; (4) the melting P-T conditions of metasomatized and metasomatic mantle rocks (i.e. serpentinized and chloritized peridotites, and pyroxenites and horblendites) and the composition of their melting products; (5) the qualitative and quantitative models of element transfer at the slab-mantle interface in subduction channels; (6) the concentration and dilution of specific elements in subduction zone fluids; (7) the change 26

in the geometric and thermal structures of subduction zones with time, and (8) the geodynamic modeling of subduction zone processes at different geothermal gradients. However, the challenges to our understanding are wide and deep, and the stakes are high – subduction has built the continents, stirred the mantle, and developed our hydrosphere for human existence. In addition, rifting orogens involve the reactivation of former subduction zones that had developed into accretionary/collisional orogens. Thus, it is important to answer how subduction zone inheritance can be identified and how it controls the architecture of rifting orogens. Therefore, the study of subduction zones should be performed through coordinated, international programs involving hundreds of scientists, with efforts crossing modern and fossil subduction zones. We wish that the contributions in this special issue can stimulate in-depth studies of subduction zones, with a better link between ancient oceanic and continental subduction zones in order to advance the plate tectonics theory to continental dynamics.

Acknowledgments This special issue was initiated by Prof. J.G. Liou, who has made tremendous efforts in promoting the study of continental subduction zones in China and elsewhere in the world. We thank his continuous encouragement in this aspect. We also thank the following individuals who have devoted much time and effort in careful reviews of manuscripts submitted for this special issue: O. Bartoli, Lin Chen, Ren-Xu Chen, Yi-Xiang Chen, Hao Cheng, Li-Qun Dai, S. Ferrero, S.M. Gordan, Jinghui Guo, Chuansong He, Wei Leng, Hongyan Li, Xi Liu, Xiaochun Liu, Huaiwei Ni, A. Perchuk, S.M.M. Straub, Daoyuan Sun, Qiang Wang, Chunjing Wei, Yuan-Bao Wu, Qun-Ke Xia, Wenjiao Xiao, Haijin Xu, Wenliang Xu, Zheng Xu, Xiaozhi Yang, Lingsen Zeng, Haijiang Zhang, and Yong Zheng. We thank journal Editor in Chief, Meifu Zhou, for his efforts in keeping the standards when accepting the manuscripts for the journal. Preparation of this special issue has been supported by grants from the Chinese Ministry of Science and Technology (2009CB825000) and the Natural Science Foundation of China (41590620).

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Figure captions Figure 1. Schematic diagram illustrating the geometric structure of subduction zones, whose thermal structure is a function of several variables (revised after Zheng et al., 2016).

Figure 2. Schematic diagrams illustrating orogenic structures at convergent plate margins (revised after Zheng et al., 2015). (a) Accretionary orogeny at oceanic subduction zone, (b) collisional orogeny at continental subduction zone.

Figure 3. Schematic diagrams illustrating the lithological difference between oceanic and continental lithospheres (abstracted with revision from Zheng and Chen, 2016).

Figure 4. Metamorphic facies series at different geothermal gradients (revised after Zheng and Chen, 2017-this issue). Bands delineating fields indicate phase boundaries according to the bulk composition of metamorphic lithologies. Red curve denotes the granite wet solidus. Green lines denote the geothermal gradients at 5C/km, 10C/km and 30C/km, respectively. Metamorphic facies abbreviations: LG = low grade; GS = greenschist facies; BS = blueschist facies; EC = eclogite facies; UHP = ultrahigh-pressure facies; GP = garnet pyroxenite facies; HGR = high-P granulite facies; GR = granulite facies; PA = plagioclase amphibolite facies; UHT = ultrahigh-temperature facies.

Figure 5. Two types of oceanic subduction zones in circum-Pacific regions. While Mariana-type is indicated by the occurrence of oceanic arc basalts due to thrusting of oceanic lithosphere under oceanic lithosphere in high angles, Andean-type is indicated by the occurrence of continental arc andesites due to thrusting of the continental lithosphere over oceanic lithosphere in low angles. Figure 6. Role of the subduction factory in generating interplate and intraplate basalts (modified after Tatsumi, 2005). Raw materials, such as seafloor sediment, oceanic igneous rocks and mantle wedge peridotite, are fed into the factory and are manufactured into HP to UHP metamorphic rocks on the one hand and volcanic arc magmas on the other hand. The waste materials processed in this factory, such as refractory eclogite and peridotite after extraction of aqueous solutions and hydrous melts, are transported and

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stored in the deep mantle, and recycled as crustal components in the mantle sources of intraplate basalts.

Figure 7. A schematic cartoon illustrating the production of subduction-zone fluids by metamorphic dehydration and partial melting crust at forearc to subarc depths, corresponding to regional metamorphism at high-pressure to ultrahigh-pressure conditoons (revised after Zheng and Chen, 2016).

Figure 8. A schematic diagram illustrating the formation of HT to UHT granulites by breakoff of the deeply subducting continental slab at a subarc depth (modified after Santosh et al., 2012). The model suggests upwelling of the asthenospheric mantle through the slab window onto the lower curst of a collisional orogen during continental subduction, leading to the superimposition of UHT metamorphism on UHP metamorphic assemblages (Zheng and Chen, 2017-this issue).

Figure 9. A schematic diagram illustrating hysteresis of the mantle melting for arc magmatism relative to fluid flux into the mantle wedge in oceanic subduction zones (revised after Zheng et al., 2016). This is primarily caused by the difference in temperature between the mantle wedge base and the subducting slab surface. As soon as the mantle wedge temperature is elevated by asthenospheric heating to achieve its wet solidus, the arc magmatism results at a later time. Dashed lines denote the geothermal gradients at 5°C/km, 10C/km, 20C/km and 30°C/km, respectively, in subduction zones. Abbreviations: GS, greenschist facies; BS, blueschist facies; AM, amphibolite facies; EC, eclogite facies; GP, garnet pyroxenite; LH, lherzolite.

Figure 10. A schematic diagram illustrating the generation of mantle sources for continental basalts through metasomatic reaction of the depleted MORB mantle wedge peridotite with hydrous felsic melts derived from partial melting of the subducting oceanic crust at postarc depths of >200 km (abstracted with revision from Zheng, 2012).

Figure 11. Schematic diagram illustrating the tectonic transformation from lithospheric thickening by accretionary/collisional orogeny to asthenospheric heating of the thinned lithosphere by rifting (revised after Zheng and Chen, 2017-this issue). The crust is light gray and the mantle lithosphere is light green. 44

Figure 12. Classification of orogenic types into three interrelated end-members: accretionary, collisional and rifting (modified after Cawood et al., 2009). As highlighted by Zheng and Chen (2017-this issue), accretionary orogens are generated by subduction of oceanic crust beneath continental margins (Cordillerotype), collisional orogens are produced by subduction of one continental lithosphere beneath the other (Alpinotype), and rifting orogens are induced by underplating of the asthenospheric mantle on thinned lithosphere along preexisting orogens (Hercynotype).

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Highlights 

Subduction zones are different in their geometric, geological and thermal structures;

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Subduction zones processes include metamorphic dehydration, partial melting and crustal metasomatism;

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Subduction zone fluids serve as metasomatic agents for crust-mantle interaction in subduction channes;

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The composition of metasomatic agents is primarily dictated by the thermal structure of subduction zones;

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Plate tectonics develops from interplate compression by subduction to intraplate extension by rifting.

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