2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
Massif peridotite is an important component within orogenic belts. The in-depth study of massif peridotites is useful to effectively reveal the complex geological processes of lithospheric evolution, including continental subduction, collision, exhumation and crust-mantle interaction in subduction zones (Zheng et al., 2006a; Zhang et al., 2005; Katayama et al., 2003).
Zircon U-Pb age plays an important role in tracking geological events. The analysis of zircon U-Pb ages, trace elements and Hf-O isotopic compositions, combined with zircon morphology and internal structure, can provide an essential record for crust-mantle chemical interaction. It is believed that zircons in mantle peridotites are formed in metamorphic recrystallization and metasomatism by fluids/melts (Xiong et al., 2014, 2011; Zheng et al., 2014, 2008, 2006a, b; Zhang et al., 2008; Song et al., 2005; Liati et al., 2004), or derived from the asthenosphere or the subducting slab (Xiong et al., 2014, 2011; Zheng et al., 2014, 2006a; Zhao et al., 2008; Liou et al., 2007; Smith and Griffin, 2005; Griffin et al., 2004; Katayama et al., 2003). This study reviews the previous studies on the occurrence, texture, trace element composition, U-Pb age and Hf isotopes of zircons from the global orogenic peridotites, and comprehensively analyzes the origins of these zircons in the orogenic peridotites, aiming to provide an alternative aspect for studying the geodynamic evolution of the lithosphere, especially the mantle-crust interaction in the deep subduction zones.1 MASSIF PERIDOTITES
Orogenic peridotites are massif-scale peridotites exposed in the orogenic belts, different from xenolith peridotites carried by volcanic rocks. Massif peridotites include orogenic peridotites and ophiolitic peridotites, and the former usually comes from continental lithospheric mantle while the latter from oceanic lithosphere. For the orogenic peridotites, garnet-facies type has been observed in numerous orogenic belts, such as the Alpine Orogen, the Dabie-Sulu Orogen, the Russian Sharyzhalgai Orogen (Ota et al., 2004), the Scandian Orogen (Sweden) and the North Qaidam Orogen (Song et al., 2005, 2004). Most of the orogenic peridotites are exposed in the Eurasian continent, as shown in Fig. 1. These peridotites are mostly layered and lenticular with lenses of eclogites, and surrounded by wall rocks of granulites and gneisses. A striking feature of most orogenic peridotites is that their ages consistent with the peak metamorphic ages of the subduction events. However, the exception of such cases are the orogenic peridotites in the Swedish Scandian and Indonesian Sulawesi orogens (Kadarusman and Parkinson, 2000; Helmers et al., 1990). They have slightly younger ages than the peak metamorphism (Table 1). Medaris and Carswell (1990) and Yang et al. (1993) believed that the orogenic peridotites were directly derived from the upper mantle. In the past decades, however, a growing number of studies show that most of the orogenic peridotites are derived from mantle wedge, which experienced HP-UHP metamorphism in the subduction zones (Spengler et al., 2009; Brueckner and Medaris, 2000). Orogenic peridotites are helpful to understand the subduction depth, dynamic mechanism of continental subduction and the interaction between subduction plate and overlying mantle wedge (mantle-crust interaction). For future research in the field of deep continent subduction, orogenic peridotites have become one of the frontiers and hotspots of geosciences.
The orogenic belts can be divided into A-type (Alps type) (Maruyama et al., 1996; Ernst and Liou, 1995) and B-type (circum-Pacific type) (Liou et al., 2004; Ernst, 2001), or continental-type and oceanic-type (Song et al., 2005) according to their rock assemblages and metamorphic deformation characteristics in macro and micro levels. In fact, the B-type or oceanic-type orogenic belts are formed by the ocean-ocean/ ocean-continental subduction. The major rock types are remnants of the oceanic lithosphere and island-arc magmatic rocks. The A-type or continental-type orogenic belts are produced by continental-continental subduction and collision, such as the Dabie-Sulu orogenic belt, the Tethyan-Himalayan orogenic belt and the Alpine orogenic belt. The rock assemblages are main felsic gneisses, with minor garnet pyroxenites and garnet peridotites. An important example is that the North Qaidam Orogen records the continuation from the oceanic subduction to the continental subduction and collision (Song et al., 2005).1.1.1 Oceanic-type orogenic belts
The rock assemblage of peridotites in oceanic-type subduction zone is generally simpler than that of the continental-type subduction zone. Owing to the alteration in the mid-oceanic ridge, back-arc basin and subduction zone, most peridotites show intense serpentinization. Only a few large complexes usually preserved fresh mantle peridotites. In the field structure, the typical section can be found with the mantle peridotite section and overlying cumulus complex, pillow basalt and pelagic sediments (such as siliceous rock) to form a relatively complete ophiolite suite. Typical ophiolites include peridotite massifs in the Yarlung- Zangbo suture zone (e.g., Luobusa-Zedang, Xiong et al., 2016; Bai et al., 2000), Yushigou peridotites in the North Qilian Mountains (Su et al., 1999; Song and Su, 1998) and Songshugou peridotites in the West Qinling Orogen (Yu et al., 2017; Cao et al., 2016). Most of the lithospheric mantle fragments in the oceanic subduction zone are ophiolitic melanges. The cold subduction process with low-temperature and HP metamorphism of the oceanic crust may result in the formation of blueschists and eclogites in the orogenic belts. Mantle peridotites in ocean subduction zone are mainly composed of harzburgites and a small amount of lherzolites and dunite-chromitite lenses. They are formed by partial melting of the asthenosphere and melt-rock interaction in the spreading centers of the ocean and/or forearc and back arc basins.1.1.2 Continental type orogenic belts
Peridotites in continental orogenic belts have been called orogenic root-zone peridotites or Alpine-type peridotites (Evans et al., 1977). According to the petrochemical compositions, peridotites in continental-type orogenic belts can be further divided into Fe-Ti and the Mg-Cr types, or the crust-type (C-type) and the mantle-type (M-type).Fe-Ti type peridotites
The Fe-Ti type peridotites, also named as the C-type peridotites, are derived from ultramafic-mafic cumulates. The peridotites were intruded into the deep continental crust and subducted to the mantle depths along with the continental subduction zone. They were later turned back to the crust after UHP metamorphism (O'Hara et al., 1971). Their major elements constitute a relatively complete ultramafic-mafic magmatic array, with the major elements of garnet pyroxenites and eclogites, and thus have low Cr/(Cr+Al) and Mg/(Mg+Fe) ratios but high TiO2 and FeO contents.Mg-Cr type peridotites
The Mg-Cr type peridotites correspond to the M-type peridotite. They were subjected to UHP metamorphism and exhumed with the felsic and mafic rocks of the continental subducting slab. In the continental-continental collisional orogenic belts, the mantle wedge was usually rim of the ancient cratonic lithospheric mantle. While in the arc-continental collision orogenic belts, the mantle wedge was mainly of the arc lithospheric mantle.
The rock assemblages of continental orogenic belts include felsic gneisses, eclogites, marbles, garnet pyroxenites and garnet peridotites. They are shown as lenticular or tectonic blocks with different sizes. The M-type orogenic peridotite bodies include rocks of garnet lherzolites, garnet harzburgites, dunites and garnet pyroxenites. The Dabie-Sulu UHP metamorphic orogenic belt in East China (Li et al., 2018; Chen Y et al., 2017; Su et al., 2016), the Western Gneiss Region in Norway (Vrijmoed et al., 2013), and the Kokchetav UHP orogenic belt in Kazakhstan (Nakajima, 1998) are some typical orogens of this type.1.2 Formation Mechanism and Tectonic Setting of Orogenic Peridotites
Three main dynamic processes of continental orogenic belts include subduction of oceanic and/or continental crust, exhumation with different rates, and uplift. Initially, the oceanic crust gradually subducts beneath the overlying continental lithosphere. The subduction drives the following continental plate into the mantle depths (about 100 km or more). During this process, the peridotite massifs, including the ultramafic- mafic rocks originally intruded into the deep part of the subduction plate, and those at the bottom of the mantle wedge, experienced a series of UHP metamorphism and transformation during the deep subduction. Later, the gravity of the oceanic lithosphere led to the break-off from the continental slab. The obvious positive buoyancy of the continental crust, once including mantle-wedge-derived components, can result in rapid exhumation of mantle rocks from different sources in the subduction zone (Zheng, 2008). The dynamic process of continental subduction can be obtained from UHP metamorphic minerals, such as coesite and diamond exposed in continental orogenic belts. The later exhumation process can be reflected by the composition and structure of retrograde metamorphic minerals. Therefore, the HP-UHP metamorphic belt is an important sign of the ancient plate convergence and the continental subduction/collision (Song et al., 2007).
Orogenic peridotite is a typical rock in continental subduction zone, which rarely appears in the oceanic subduction zone. Brueckner and Maedaris (2000) summarized garnet peridotites worldwide, and divided them into "crustal-derived" and "mantle-derived" types, according to their chemical characteristics. Based on the tectonic settings, the authors also subdivided the mantle-derived peridotites into three types: (1) UHP metamorphic peridotites in subduction zones, such as the Bohemian and Arami in Alps and the Dabie-Sulu orogenic belt, which entered the continental crust from different depths of mantle wedge. (2) Ultrahigh temperature spinel-facies peridotites which intruded into the crust after cooling from asthenosphere. (3) Garnet peridotites existing in the ancient lithospheric mantle were directly brought into the continental crust. Zhang et al. (2000) and Liou et al.(2009, 2007) suggested that most of the mantle-derived garnet peridotites come from the upper mantle wedge of the subduction zone.
Orogenic peridotites play an important role in explaining the nature and evolution of subduction continental lithospheric mantle, convective mantle, mantle wedge and subduction zone (Medaris, 1999; Brueckner and Medaris, 1998). Although conventional petrological and geochemical techniques can be used to study them (Ye et al., 2009; Song et al., 2004; Zhang et al., 2000), the "fossil" events preserved in peridotites are difficult to find by these conventional methods. Therefore, the application of in-situ micro-analytical techniques is particularly important.2 APPEARANCE OF ZIRCONS IN OROGENIC PERIDOTITES
Orogenic peridotite is one of the common components in the UHP collision orogenic belt (Brueckner and Medaris, 2000; Zhang et al., 2000; Carswell et al., 1983). Fluids released from the subduction slab make the mantle-wedge peridotites tend to experience metasomatism in varying degrees and to achieve crust-mantle interaction between different blocks (Zheng et al., 2016). Therefore, M-type peridotites provide a direct lithologic record from subducted continental lithosphere to overlying mantle wedge (Zheng, 2012).
During the metamorphism of peridotites in orogenic belt, the petrology and geochemistry of the peridotites have been changed obviously due to the addition of melts/fluids, such as the abundances and distribution patterns of trace elements. They exist as minerals due to dominant (obvious) metasomatism, whereas cryptic metasomatism only results in enrichment of elements or isotopes (Zheng et al., 2019a; Xiong et al., 2015; Zheng, 2012). Aqueous minerals have been found in the orogenic peridotites, which provide mineralogical evidence for the metasomatism from crustal-derived fluid. Typical examples include: ① lizardite (liz) as inclusion in olivine (Ol). The cracks of peridotites are filled with antigorite (atg). Since the stable temperature and pressure conditions of atg are higher than those of liz, this structure can be interpreted as fluid flowing through the cracks react with olivine to form atg (Yang and Powell, 2008). ② The early stage of olivine (Ol1) reacts with the silicon-rich melt released by the subduction of the continental crust to form late stage orthopyroxene (Opx2), which wrapped around the Opx1 and Ol1. It is inferred that the Opx2 should inherit the chemical composition of Ol1, but the chemical composition of Opx1 is different. The spinel is encapsulated by garnet, indicating that the protolith may be spinel phase peridotite (Fig. 2a).③ The Ol1 could transform to Ti-clinohumite (Chu) at ultrahigh pressure conditions (Fig. 2b): through the action as Ol+Ti-melt/fluid+ H2O=Chu. The reaction indicates that the Ti-melt/fluid has metasomatized olivines during UHP metamorphism. ④ The appearance of phlogopites (Fig. 2c), apatites and magnesites (Fig. 2d) in peridotites, implies that the peridotites were obviously metasomatized by melt/fluid enriched with K, P, and carbonate. Some orogenic peridotites were also possibly serpentinized prior to UHP metamorphism. It recorded the evolution history of lithosphere from dissociation to collision. The orogenic peridotites entered an open system inevitably exchange chemical and physical material with the crust, resulting in modification of petrological and geochemical compositions.
Traditionally, zircons in peridotites are believed to be contaminated or metasomatized by the crustal materials when they enter the continental crust. Due to the low content and activity of Zr and Si in mantle minerals, zircons can rarely crystallize directly from the original peridotite (Zheng, 2012; Hermann et al., 2006; Palme and OʼNeill, 2003). In recent researches, many scholars have found zircons in the garnet peridotites of the global UHP metamorphic orogenic belts (Fig. 1), including the Dabie-Sulu orogenic belt (Li et al., 2016; Zheng et al., 2014, 2008, 2006a; Zhang Z M et al., 2011; Zhang R Y et al., 2005; Rumble et al., 2002), the North Qaidam (Chen Y et al., 2017; Xiong et al., 2014, 2011; Song et al., 2005), the Alps (Hermann et al., 2006) and the Erzgebirge Mountains (Liati and Gebauer, 2009). Details are shown in Table 1.
The origin of zircon grains with residual cores and newly grown rims in orogenic peridotites is often questioned. The appearance of in-situ zircon grains in thin sections of orogenic peridotites and pyroxenites is important to explain the genesis of zircons in orogenic peridotites (Chen R X et al., 2017; Xiong et al., 2014; Zheng et al., 2014; Zhang et al., 2011). As shown in Fig. 3, these in-situ observations clearly indicate that zircon can be an original mineral of orogenic peridotites, rather than product of artificial contamination. The existence of melt/fluid for the generation of zircon in orogenic peridotites is necessary. The melt/fluid inclusions in peridotite zircons suggests that these inclusions were derived from the host peridotites occurring metasomatism (Liati and Gebauer, 2009; Zhang et al., 2005). Therefore, although zircons are extremely rare in the orogenic peridotites, they provide very important mineralogical evidence for metasomatism and geological events (Zheng et al., 2014, 2008; Katayama et al., 2003; Bea et al., 2001; Gebauer, 1996).3 CHARACTERISTICS OF ZIRCONS IN OROGENIC PERIDOTITES 3.1 Zircon Classification and Significance of Zircon Trace Elements
The trace element abundance and distribution characteristics of zircons can indicate the protoliths and crystallization conditions. The CART tree diagram, constructed by Belousova et al. (2002) using multivariate statistical methods, provides a fast method for associating protoliths. It can identify protolith within a confidence range of 75% or higher. The abundance of trace elements in zircons generally increases from ultramafic rocks to granites. Zircons from kimberlites and carbonatites have flat REE distributions (patterns). The ratio of YbN/SmN ranges from 3 to 30, while the ratio is usually greater than 100 in pegmatites, the ratio of Th/U is generally within 0.1-1. The trace elements of zircons in orogenic peridotites are identified by CART tree diagram, which is helpful for understanding the genesis of zircons. For example, 14 zircons have been reported from the Shenglikou peridotites in the North Qaidam (West China) (Xiong et al., 2011), and 10 of them have been identified as having the same source as kimberlites, and the remaining 4 zircons are similar to syenite. In Xugou, Yangkou and Hujialin of the Sulu orogenic belt (East China) (Zheng et al., 2014), 23 gains belong to basalt-like origin, and 3 gains are similar to diabase. These different characteristics of trace elements in peridotite zircons may indicate the nature of the metasomatism and reveal the complexity of mantle-crust interaction in subduction zones. However, zircon trace elements are variable when crystallized in different P-T-X conditions, it should be cautious to apply Belousova's CART diagram to discriminate zircon sources in orogenic peridotites.
Zircons in orogenic peridotites can be divided into old (mainly the cores of residual magmatic and recrystallized zircons) and newly grown zircons according to U-Pb ages. Old zircons are generally euhedral to subhedral, retaining some of the zircon crystals, with oscillatory zoning or residual zircon cores (Li et al., 2016; Zheng et al., 2014; Liati and Gebauer, 2009; Yang J S et al., 2009). If the metamorphic recrystallization of primitive zircons in UHP metamorphic rocks retained the residual core of magmatic zircons, they almost inherit the characteristics of primitive zircons in composition. However, the U-Pb age is close to or lower than the age of the primitive zircon. The REE distribution (pattern) of magmatic zircons is typical of steep MREE-HREE with high contents of trace elements.
The newly grown zircons in orogenic peridotites are mostly round and irregular in shape. The U-Pb age is consistent with the age of UHP metamorphism. Cathodoluminescence (CL) images show that the internal structure of zircons is homogeneous without zoning. The composition of trace elements and δ18O are variable. Primitive mantle minerals are imprinted with metasomatic components (Scambelluri et al., 2014, 2006). The trace elements of metasomatic zircons well record the properties of metasomatic agent, the temperature and pressure conditions of metamorphism, the dissolution and preservation conditions of specific minerals during dehydration metasomatism.3.2 Isotopes of Zircons in Orogenic Peridotites
Due to the low ratio of Lu/Hf in zircon (176Lu/177Hf ratio is usually less than 0.002), a few 176Lu decays into 176Hf. Therefore, the 176Hf/177Hf ratio of zircon can represent the 176Hf/177Hf ratio that formed zircon, thus providing important information for the discussion of its genesis (Kinny and Mass, 2003; Knudsen et al., 2001; Patchett et al., 1981). Combined with the U-Pb age of zircons, the accurate initial ratios of Hf isotopes can be used to trace the source of rocks and understand the growth/evolution of lithosphere (Zheng et al., 2005; Amelin et al., 2000; Bodet and Schrer, 2000). Zheng et al. (2006a) analyzed the Hf isotope compositions of zircons in the peridotites of the Sulu orogenic belt (i.e., CCSD-PP1). The εHf values range from -16.3 to -13.8, suggesting that the Sulu peridotites were a fragment of refractory North China Archean mantle and had subjected to metasomatism during the Mesoproterozoic. Hf-O isotope composition of the newly grown zircons in peridotites analyzed by Li et al. (2016), giving that the range of εHf is between -10.7 and -5.8, indicates that the Tengjia peridotites in the Sulu orogenic belt experienced metasomatism by melts/fluids from dehydration of the subducted continental plate during the early stage of exhumation. Both of the zircons were from Sulu orogenic peridotites, but they derived from different source regions. Tang et al. (2014) suggested that decoupled release of zircon Hf and non-zircon Hf from a single crust-derived magma source can lead to Hf-isotope variations. In addition, the Hf isotopic heterogeneity in magmatic zircon also could result in mixing with mantle-derived magmas (Griffin et al., 2000).
Zircon O isotope can provide information on the composition and source of metasomatic fluids. The δ18O values of the continental lithospheric mantle mineral range from 4.8 to 5.5 in olivine, from 5.4 to 5.8 in garnet, from 5.2 to 5.9 in clinopyroxene and from 5.6 to 6.0 in orthopyroxene (Chazot et al., 1997; Mattey et al., 1994). Zircon from Tengjia dunites in the Sulu orogenic belt has a negative value of δ18O, which is caused by the crystallization of UHP igneous protolith in low δ18O magma and the negative δ18O fluid released by the high temperature hydrothermal alteration (Li et al., 2016). The Mg-O isotopes could constrain the source of carbonate metasomatism in Maowu garnet clinopyroxenite from the Dabie orogenic belt (Shen et al., 2018). The Mg isotopic composition of Maowu garnet clinopyroxenite (26Mg from -0.99 to -0.65) is significantly lower than that of mantle (26Mg from -0.25 to 0.07), while the oxygen isotope of Maowu garnet clinopyroxenite is much higher than that of carbonate from mantle. It suggests that the source of metasomatic carbonate is sedimentary carbonate. The Hf-O isotope of zircons in the Shenglikou peridotites in the northern margin of Qaidam recorded the metasomatism from two different source regions. The high-δ18O fluid originated from the deep subduction continental crust and the low-δ18O fluid derived from the dehydration of the subduction oceanic crust before the continental subduction (Chen Y et al., 2017).3.3 Inclusions within Zircons from Orogenic Peridotites
Zircons grown in different conditions can capture minerals, melts/fluids as inclusions. It is difficult to modify the inclusions of zircons due to its stability. Therefore, these inclusions within zircons provide important records of their formation conditions and mechanisms (Li et al., 2013; Liu and Liou, 2011; Hermann et al., 2001). Detailed study on inclusions in zircons from the Dabie-Sulu metamorphic rocks by Liu and Liou (2011), shows that they mainly consist of quartz, garnet, chlorite, rutile, potassium feldspar, dolomite and apatite, produced from subduction zone. During the peak metamorphic stage of eclogite facies, the inclusions in metamorphic zircon mainly include quartz, garnet, chlorite, rutile, K-feldspar, dolomite and apatite. In the retrograde metamorphic stage of amphibolite facies, the inclusions consist of low-pressure minerals such as quartz, plagioclase, albite, amphibole, calcite and apatite. The metamorphic combination of inclusions is not only related to metamorphic temperature and pressure conditions, but also controlled by the composition of host rocks. Combined with the U-Pb age and mineral inclusions in zircons, they can provide further important constraints on the P-T-t conditions of the host rocks (Liu and Liou, 2011; Hermann et al., 2001).
Zircon in peridotites is key evidence to indicate the origin of metasomatic melts/fluids. Zircons in the peridotites experienced metasomatism in varying degrees, therefore containing water- bearing minerals such as chlorite, amphibole, and Ti-clinohumite (Zhang Z M et al., 2011; Hermann et al., 2006; Zhang R Y et al., 2005; Katayama et al., 2003). The inclusion assemblages in zircons from orogenic peridotites is generally consistent with mineral assemblages of their host peridotites, implying that the zircon originated in peridotites and crystallized through fluid metasomatism/metamorphism in mantle environment (Fig. 4a). There are two types of inclusions in zircons from orogenic peridotites. One of which is mantle-derived inclusion. For example, in the zircons from the Shenglikou peridotites and pyroxenites in the North Qaidam Orogen, the inclusion assemblages are of garnets, orthopyroxenes and olivines, consistent with the minerals in host peridotites, suggesting that zircons are the product from the mantle-crust interaction (Xiong et al., 2011). The other one is crust-derived inclusion, such as apatites, amphiboles, uranium oxide and feldspar inclusions (as Figs. 4b, 4c) (Chen R X et al., 2017; Zhang Z M et al., 2011; Liati and Gebauer, 2009; Hermann et al., 2006; Zhang R Y et al., 2005). The presence of these non-peridotite mineral inclusions suggests that their formation may be related to the involvement of crustal materials (Hermann et al., 2006). There are also different cases, a large number of carbonate inclusions in the zircons (Figs. 4d-4f) demonstrate that carbonates are formed during metasomatism. Whether these carbonates are derived from mantle or sedimentary carbonates requires Mg-O isotope constraints (Shen et al., 2018). Therefore, combined with the morphological and structural information of the newly grown zircons in the orogenic peridotites, as well as the modification of trace elements and age data, the nature and composition of the metasomatic fluid can be better constrained.4 GENETIC SIGNIFICANCE OF ZIRCONS IN OROGENIC PERIDOTITES
The genesis of zircons in orogenic peridotites is controversial, and the mechanism and source of metasomatic fluids have not been determined. Zheng et al. (2014) suggested that the zircons in the peridotites were related to the metasomatism of deep subducted crustal materials. The zircons were produced by metamorphic dehydration and partial melting at the crust-mantle interface in the subduction channel. Although Zr is rare and with a weak activity in the mantle reservoir, it is the fact that zircon exists in the orogenic peridotites. Therefore, how to break through the predecessorsʼ understanding and scientifically explain the zircon genesis will further promote the study of orogenic peridotites. The formation of zircons in the orogenic peridotites may include the following possibilities.
(1) The strong crystallization ability of zircons. As an incompatible element, Zr preferentially combines with Si, Hf, Th, U elements in the mantle to form zircon under metamorphic conditions. It is important to note that zircon has a stronger ability to combine with other minerals and to crystallize than other mantle minerals (such as garnet, Zheng et al., 2019b).
(2) Destruction of Zr-bearing minerals and/or precipitation of intergranular melts in UHP metamorphism. The formation and growth of zircons require their host rocks to supply Zr. This element can be obtained from the decomposition of Zr-bearing minerals such as garnet, biotite, amphibole and ilmenite (Zheng, 2012) or from the dissolution of primitive zircon (Ayers et al., 2003; Vavra et al., 1999). As an incompatible element, Zr is usually immobile in aqueous solutions, and can be efficiently transferred via fluid phase in most geological conditions (Harrison and Watson, 1983). It is generally believed that zircon is prone to occur near the reaction zones in which Zr is released. The disintegration and melting of garnets during UHP metamorphism provide conditions for the formation of zircons in peridotites. Degeling et al. (2001) found micro-grain zircons beside cordierite, formed by garnet decomposition in garnet-bearing migmatites from southwestern Norway. In addition, Fraser et al. (1997) and Bingen et al. (2001) reported the growth of in-situ zircons from the decomposition of garnet and ilmenite, respectively.
As an incompatible element, Zr has the ability to transform reactants into local saturation of Zr in the melts and promote the growth of zircon (Harrison et al., 2007; Watson, 1996; Harrison and Watson, 1983). Therefore, zircon can also be grown from the melt in zircon-poor rocks, and their U-Pb ages can constrain the melt crystallization time and correspond to different geological events. The occurrence of garnets in metamorphic reaction also affects the properties of zircon growth (Whitehouse and Platt, 2003; Rubatto, 2002). If both garnets and zircons occur as products of metamorphic reactions, HREE are preferentially assigned to garnets, and zircons reflect smooth HREE characteristics. Similarly, if garnets are the residual phase of the reaction, the zircons will be depleted in HREE due to garnets high HREE content. In the contrary, if garnets are reacted with reactants, the zircons will be enriched by HREE. Therefore, in the process of UHP metamorphism, the disintegration or intergranular melt precipitation of Zr-bearing minerals provide Zr element for the formation of zircons, and the zircon formation under the condition of sub-solidus conditions.
Metasomatism by melts/fluids supply Zr and other elements. Melt/fluid phases play an important role in zircon growth and recrystallization during UHP metamorphism in subduction zones (Zheng Y F, 2009; Wu et al., 2006; Rubatto and Hermann, 2003; Rubatto et al., 1999). The metasomatic zircons are usually attributed to two mechanisms as follows.
(A) Crystallization of mantle-derived metasomatic melts/ fluids (Zheng et al., 2006a; Grieco et al., 2001). For example, the Zhimafang peridotites in the Sulu orogenic belt experienced UHP metamorphism at the Early Mesozoic (223.5±7.5 Ma). The metasomatic agent recorded by zircons has trace elemental nature similar to the kimberlites and carbonatites, showing that these peridotite zircons recorded melts/fluids from asthenosphere mantle in Mesoproterozoic (Zheng et al., 2006a).
(B) Crystallization of metasomatic fluids derived from dehydration of deep subducting crustal plates (Chen R X et al., 2017; Li et al., 2016; Liati and Gebauer, 2009; Hermann et al., 2006). Because of the high Zr content in the crustal rocks, crustal metamorphic zircons (Zheng, 2012) are easily formed in the crust-mantle interface of the subduction channels.
In these two cases, it is emphasized that the source of metasomatic melts/fluids determines the significance of zircons in orogenic peridotites. A large number of studies have shown that metamorphic zircons generally have similar trace element characteristics (Liu and Liou, 2011; Chen et al., 2010). These zircons produced by metamorphic reactions may be homogenized under the similar fluid composition and conditions. On the other hand, the existence of fluids will greatly promote the recrystallization of primary zircons. Melt/fluid metasomatism not only leads to the growth of new zircons, but also results in the metamorphic recrystallization of previous zircons in the mantle wedge rocks. Therefore, studies on zircons in peridotites from orogenic belts can provide important constraints for the melt/fluid processes and crust-mantle interaction in subduction zones.5 GEOCHRONOLOGICAL IMPLICATIONS OF ZIRCONS IN OROGENIC PERIDOTITES
Zircons from orogenic peridotites show a wide range of ages, recording complex crust-mantle interaction and geological evolution history. In different stages of the UHP metamorphism (Chen R X et al., 2017; Li et al., 2016; Liati and Gebauer, 2009; Hermann et al., 2006), zircon U-Pb age, Lu-Hf isotope and O isotope analysis are important methods for comprehensive understanding of geological processes.
Mg# values of M-type peridotites in the Dabie-Sulu orogenic belt generally range from 92 to 90, and mainly of them are transitional type between refractory and fertile, suggesting that the southeastern margin of the North China Craton was subjected to metasomatic alteration by fluid components released by the subducting Yangtze lithospheric block (Zheng J P, 2009). Zircons of peridotites in the Dabie-Sulu orogenic belt have a ~3.1 Ga ancient age and an Hf model age < 3.2 Ga, which are similar to the Archean peridotite xenoliths oldest ages (3.2 Ga, mainly at 210-240 Ma) of Xinyang volcanic rocks in the southern margin of the North China Craton (Zheng et al., 2006a).
The M-type peridotites in Xugou, Yangkou, Hujialin, Ganyu, and Raobazhai of the Sulu orogenic belt underwent Proterozoic metasomatism. A strong tectonic thermal event of the Early Paleozoic (~470 Ma), was subsequently recorded by the eruption of kimberlites in Mengyin and Fuxian of the North China Craton in Early Paleozoic (Li et al., 2011; Yang J S et al., 2009; Yang Y H et al., 2009; Lu et al., 1998). This event is interpreted as the result of melt/fluid metasomatism from a variety of sources including asthenosphere, lithospheric mantle and subduction continental plate. The U-Pb age of metamorphic zircons in the Dabie-Sulu orogenic belt is mainly distributed in the Triassic UHP metamorphism age. As a word, zircons from peridotites in the Dabie-Sulu orogenic belt record the northward subduction/collision of the Yangtze Block with the North China Craton during the Early Mesozoic (Xia et al., 2013; Zheng J P, 2009; Wu et al., 2006; Zheng et al., 2005). The U-Pb ages of the peridotite zircons range from 205 to 244 Ma, recording the subduction/collision of the Yangtze Block beneath the North China Craton. Most of these U-Pb ages are concentrated at 212-227 Ma, younger than the UHP metamorphic age (225-240 Ma), suggesting that they grew under the metasomatism of the early exhumation stage of subducted continental crust. The Th, U contents and Th/U ratios of these metasomatic zircons vary greatly between 0.01-1, implying that they grow within the fluid released from crustal rocks.
As shown in the above discussion, zircons in the M-type peridotites of the Sulu-Dabie orogenic belt have the same Hf depleted mantle model age (TDM peak value is 1.3 Ga) and Hf crustal model age (Tcrust peak value is 1.75 Ga). The data suggest that the continental lithospheric mantle beneath the southeastern margin of the North China Craton has subjected to an intense Mesoproterozoic metasomatism with agents from asthenosphere, similar to the ~1.3 Ga record of lithosphere beneath the North China Craton (Zheng et al., 2014). Therefore, the U-Pb age, Hf isotopic system and trace element compositions of zircons from peridotites in the Dabie-Sulu UHP metamorphic orogenic belt mainly record that the deep lithosphere (> 3.2 Ga) in the southern margin of the North China Craton and the Proterozoic metasomatism, followed by strong tectonic thermal events in the Early Paleozoic (~470 Ma) and Early Mesozoic (~230 Ma). The northward subduction/collision (Zheng et al., 2014) of the Yangtze Block strongly affected (physically and chemically) the lithospheric mantle of the southeastern margin of the North China Craton. These processes were occurred in the overlying mantle wedge above the subduction continental lithosphere, leading to multiple metasomatism and modification.6 CONCLUSION
Zircon in the orogenic peridotite is a product of metamorphic recrystallization and melt/fluid metasomatism. The original zircons were modified to varying degrees under HP-UHP conditions, depending on their physical, chemical properties (i.e., crystallinity of zircon, existence of cracks in zircon and other Zr-rich mineral crystals) and the availability of metamorphic melts/fluids during the processes of continental subduction and collision. Zircon from orogenic peridotites is an important indicator for revealing material and energy cycling between crust and mantle. It further constrains the evolution of mantle wedge, crust-mantle interaction, melt-rock interaction and chemical geodynamics of continental subduction zones. So far, the data of zircon O and Li isotopes of orogenic peridotites are very few, which should be a direction worthy of attention in the future.ACKNOWLEDGMENTS
We sincerely appreciate the support from the National Natural Science Foundation of China (Nos. 41520104003 and 41873023) and the DREAM project of the MOST (No. 2016YFC0600403). We also thank Profs. Jingsui Yang and Changqian Ma for their invitation. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1220-2.
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