- Email: [email protected]
Metamorphic zirconology of continental subduction zones
Accepted Manuscript Metamorphic zirconology of continental subduction zones Ren-Xu Chen, Yong-Fei Zheng PII: DOI: Reference:
S1367-9120(17)30208-0 http://dx.doi.org/10.1016/j.jseaes.2017.04.029 JAES 3065
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
21 January 2017 25 April 2017 27 April 2017
Please cite this article as: Chen, R-X., Zheng, Y-F., Metamorphic zirconology of continental subduction zones, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.04.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Review article
Metamorphic zirconology of continental subduction zones
Ren-Xu Chen*, Yong-Fei Zheng
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
*
Corresponding author. Email: [email protected]
Abstract Zircon is widely used to date geological events and trace geochemical sources in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks of continental subduction zones. However, protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid-state transformation, metasomatic alteration and dissolution reprecipitation; new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks (metamorphic zircon) or peritectic reactions above the wet solidus (peritectic zircon). As a consequence, there are different origins of zircon domains in high-grade metamorphic rocks from collisional orogens. Thus, determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing. We advocate an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotope compositions. Only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology, enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction-zone conditions. The metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in HP to UHP metamorphic rocks of collisional orogens. These different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions. While the protolith nature of metamorphosed rocks dictates water availability, the P-T conditions of subduction zones dictate the property of subduction-zone fluids. The fluids of different properties may be produced at different positions of subducting and exhuming crustal slices, and they may physically and chemically mix with each other in continental subduction channels. Such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element Zr and other fluid-mobile incompatible trace elements from the subducted crust to the mantle wedge. Therefore, the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes.
Keywords: Zircon; regional metamorphism; subduction zone; dehydration reaction; peritectic reaction; fluid action
1. Introduction Zircon is a common accessory mineral in crustal rocks. Because of its high physiochemical stability and refractory nature, zircon is widely used to date geological events and trace geochemical sources (e.g., Hermann et al., 2001, 2013; Valley, 2003; Zheng et al., 2004, 2006a; Watson and Harrison, 2005; Kemp et al., 2006; Harley and Kelly, 2007a; McClelland and Lapen, 2013; Kohn et al., 2015; Taylor et al., 2016). This is because zircon is rich in U and Th but poor in Pb and has the high closure temperature of Pb diffusion (Zheng and Fu, 1998; Cherniak and Watson, 2003). Thus, zircon U-Pb dating has been one of the most commonly used and effective methods in geochronological studies (e.g., Wu and Zheng, 2004; Harley et al., 2007; Rubatto and Hermann, 2007a). As a phase enriched in Hf relative to radioactive Lu, zircon retains a strong fingerprint of the isotopic feature of crustal sources from which it crystallized. This provides robust evidence for growth and reworking of crustal rocks in the Earth’s history (e.g., Kemp et al., 2006; Zheng et al., 2006a, 2007a; Scherer et al., 2007). Zircon may contain significant amounts of temperature- or process-sensitive trace elements such as rare earth elements (REE) and Y as well as high field strength elements (HFSE), which can provide compelling evidence for conditions of zircon growth. This is important to reconstruction of of magmatic and metamorphic processes and to tracing the origin of host rocks (e.g., Harley et al., 2007; Rubatto and Hermann, 2007a). Although mineral O isotopes have a high sensitivity to low-temperature surface processes, zircon has the high stability in preserving its O isotope composition. Thus, zircon O isotope studies have been an effective means to discriminate the role of low-temperature versus high-temperature processes in its host rocks (e.g., Valley, 2003; Chen et al., 2011). Because of these unique properties, zircon is widely used in geological dating and geochemical tracing in various processes such as magma crystallization, partial melting, metamorphism, fluid action and hydrothermal mineralization, leading to the emergence of a new research branch named as zirconology. Zircon can grow or recrystallize under various conditions during hydrothermal, metamorphic, anatectic to magmatic processes (e.g., Rubatto, 2002; Wu and Zheng, 2004; Hoskin, 2005; Harley and Kelly, 2007b; Schaltegger, 2007). Geochemical information recorded in zircon can be correctly interpreted only when zircon growth conditions can be clearly linked to the evolutionary history of host rocks in subduction zones. High-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in collisional orogens generally experience multistages of evolution (Rumble et al., 2003; Zheng et al., 2003a), and metamorphic dehydration and partial melting of crustal rocks at subduction-zone conditions may produce different types of fluids such as aqueous solutions, hydrous melts and supercritical fluids (Zheng et al., 2011a; Zheng and Hermann, 2014). For these reasons, there are different origins of zircon in HP to UHP metamorphic rocks. Relict zircons (protolith zircons of either magmatic or detrital origins) may suffer variable extents of metamorphic recrystallization
through structural modification and chemical alteration in response to changes in P-T conditions and fluid accessibility; new zircon may grow during metamorphic dehydration and partial melting at subduction-zone conditions. As a consequence, the orogenic metamorphic rocks may contain not only recrystallized zircons of different types but also newly grown zircons of different types. The recrystallization of protolith zircons and the growth of new zircons in crustal rocks may take place under a broad range of P-T conditions during prograde, peak or post-peak greenschist-, amphibolite-, eclogite- and granulite-facies conditions (e.g., Fraser et al., 1997; Wu et al., 2006; Baldwin and Brown, 2008; Liu and Liou, 2011). With the advanced application of zircon studies to dating of geological events and tracing of geochemical sources and processes, the term zirconology has being used increasingly in the literature (e.g., Zheng, 2009; Xia et al., 2009, 2010; Chen et al., 2010, 2011; Nemchin, et al., 2012; Tichomirowa et al., 2012; Li et al., 2013). This involves an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotopes. Such a zirconological study is necessary in order to correctly interpret observations from zircons in crustal and mantle rocks. This is a big step in applying a single mineral to studies of metamorphic geology and geochemistry. For this reason, this paper provides a review on the studies of metamorphic zirconology in UHP metamorphic rocks from continental subduction zones. Although many of examples are taken from the Dabie-Sulu orogenic belt in China, available data from the other typical UHP terranes on Earth are also taken into account. The results indicate that different types of zircons can be discriminated by studying their properties during recrystallization and growth in continental subduction zones. This also provides constraints on the fluid action during subduction-zone processes.
2. Fundamentals for metamorphic zirconology In the present review of metamorphic zirconology, magmatic zircon is referred to as that crystallized from magmatic melts, and relict zircon is referred to as those inherited from crustal protoliths (some are of magmatic origin whereas the other is of detrital origin). On the other hand, metamorphic zircon is referred to as that formed through dehydration reactions below the wet solidus of crustal rocks, and peritectic zircon is referred to as those crystallized through peritectic reactions above the wet solidus of crustal rocks. While magmatic melts have separated from their parental rocks and transported upwards with large extent of evolution by fractional crystallization, anatectic melts are not separated from their parental rocks and thus only experienced the smallest extent of fractional crystallization. Nevertheless, anatectic zircon may grow from anatectic melts in which the local oversaturation of Zr is achieved by fractional crystallization of Zr-poor minerals. Therefore, the physical and chemical properties of subduction-zone fluids are a key to mineralogical
processes that form or rework zircon at continental subduction-zone conditions. Understanding these properties and processes is substantial not only to petrogenetic interpretation of zircons from high-grade metamorphic rocks in collisional orogens but also to tectonic interpretation of their U-Pb ages and geochemical signatures (e.g., Wu and Zheng, 2004; Harley and Kelly, 2007; Rubatto and Hermann. 2007; Zheng, 2009, 2012; Hermann et al., 2013). As defined in petrology of magmatic rocks, a magma is a mixture of crystal and melt, whereas the melt is short of crystalline minerals. Thus, a melt is part of a magma rather than whole magma. The difference between melt and magma is the occurrence and amount of crystalline minerals in the melt. A melt becomes a magma as soon as rock-forming minerals have significantly crystallized from the melt. A felsic melt is produced by partial melting of crustal rocks. Anatexis is referred to as partial melting of lower degrees to result in migmatization, generating anatectic melts that have not left their parental rocks. The anatectic melts were produced through peritectic reactions at temperatures above the wet solidus of crustal rocks. Typically, the anatectic melt is produced by migmatization, and its crystallized product is veinlets in metamorphic rocks, leucosomes in migmatites (metatexite and diatexite) and pegmatite veins in felsic gneisses. Geochemically, the anatectic melt has achieved thermodynamic equilibrium with the peritectic mineral in partitioning of water and incompatible trace elements, but it is not with the relict mineral. On the other hand, magmatism requires partial melting of higher degrees with significant transport and accumulation of anatectic melts. Thus, magmatic melts have escaped from their parental rocks (migmatites) with significant evolution in petrology. The anatectic melt becomes the magmatic melt after its significant evolution with fractional crystallization of rock-forming minerals. Therefore, the magmatic melt has achieved thermodynamic equilibrium with the crystallized minerals in partitioning of water and incompatible trace elements. Zircon grown under different environments can entrap the concurrently grown minerals, fluids and melts as its inclusions, which provide important records of its formation conditions and mechanism (e.g., Hermann et al., 2001; Liu and Liou, 2011; Li et al., 2013). The occurrence of inclusions (mineral, fluid or melt) in zircon provides an opportunity to correlate the growth zones of zircon with metamorphic/anatectic conditions. This is usually realized by identifying the distribution of mineral inclusion assemblage, CL images and trace element composition of zircon or mineral inclusions (e.g., Hermann et al., 2001; Liu and Liou, 2011). However, the relationship between inclusion species and host zircon domains is commonly complicated, limiting its application to zircon genesis. Three mechanisms have been proposed to account for the occurrence of inclusions in zircon (e.g., Gebauer et al., 1997; Liu et al., 2001; Zheng et al., 2011b): (1) entrapping of concurrently grown minerals, fluids or melts during zircon growth; (2) squeezing/crystallizing of inclusions along fractures into the preexisting zircon; (3) transforming of
precursor mineral inclusions into new mineral inclusions during metamorphism. Once inclusions were entrapped into zircon, their composition can hardly change because of the refractoriness of zircon. Elastic models suggest that transformation of a precursor quartz to coesite appears unlikely to happen (e.g., Gillet et al., 1984; Van Dermolen and Van Roermund, 1986). In this regard, the third mechanism seems unlikely. Healed structures of earlier deformed minerals and disturbed structures such as patch zoning and resorption can be found around secondary inclusions by detailed microstructure imaging (e.g., Gebauer et al., 1997; Dubińskaa et al., 2004). In contrast, cracks or healing traces generally cannot be observed from primary mineral inclusions. As such, detailed structure analyses can distinguish between the secondary and primary origins of mineral inclusions, and only the primary inclusions are of petrological meaning in the zircon genesis. Coesite and other eclogite-facies minerals as well as low-P (e.g., feldspar, biotite and quartz) minerals were found in zircon cores from schist and gneiss from the main hole of Chinese Continental Scientific Drilling (CCSD-MH) in the Sulu orogen (Zhang et al., 2006) and jadeite quartzite from Shuanghe in the Dabie orogen (Gao et al., 2015). However, U-Pb ages, CL images and trace element compositions indicate that these zircon cores are of magmatic rather than metamorphic origin. Detailed structure observations reveal that there are cracks and disturbed structure, indicating these inclusions are formed through metasomatic alteration along fractures due to fluid infiltration during UHP metamorphism (Gao et al., 2015). Thus, it is critical to distinguish between the primary and secondary mineral inclusions when interpreting the formation conditions of host zircon. Detailed studies of mineral inclusions have been carried out on zircons from various UHP metamorphic rocks from the Dabie-Sulu orogenic belt (Liu and Liou, 2011, and references therein). The results show that, at the prograde HP eclogite-facies stage during subduction, mineral inclusions in metamorphic zircons are predominated by quartz, garnet, omphacite, phengite, rutile, K-feldspar, dolomite and apatite. At the peak UHP eclogite-facies metamorphic stage, mineral inclusions are coesite, garnet, omphacite, phengite, rutile, jadeite, kyanite, titanite, K-feldspar, aragonite, magnesite and apatite. At the amphibolite-facies retrogression stage during exhumation, mineral inclusions are only composed of low-pressure minerals such as quartz, plagioclase, albite, amphibole, calcite and apatite. The metamorphic assemblage of mineral inclusions is not only related to metamorphic P-T conditions, but also controlled by the composition of host rocks (Liu and Liou, 2011). For HP to UHP eclogite-facies metamorpohic zircons, inclusion mineral assemblages are Coe/Qtz + Grt + Omp + Phe ± Mgs ± Ap for eclogite/amphibolite in granitic orthogneiss, Coe/Qtz + Grt + Omp + Phe + (Mgs+Arg)/Dol + Ap for eclogite in marble, Coe/Qtz + Grt ± Omp/Jd + Phe + Ttn + Ap for paragneiss, Coe/Qtz + Phe + Ap ± Grt ± Jd ± Rt ± Ttn ± Ky ± Kfs for granitic orthogneiss, Coe/Qtz + Grt + Phe + Rt +Ap + Omp/Jd + Ky for quartzite, and
Coe/Qtz + Grt + Omp + Ap + (Mgs + Arg)/Dol for marble. For the late amphibolite-facies metamorphic zircons, inclusion mineral assemblages are Amp+Pl+Ap for eclogite in granitic orthogneiss, Qtz+Cal+Amp for eclogite in marble, Qtz+Ab for paragneiss, orthogneiss and quartzite and Cal for marble. Analyses of these mineral inclusions, in combination with zircon U-Pb dates, can provide important constraints on P-T-t paths of zircon domains and further their host rocks (Hermann et al., 2001; Liu and Liou, 2011). Magmatic zircon generally exhibits high REE abundances and steep REE patterns, positive Ce anomalies and negative Eu anomalies (e.g., Rubatto, 2002; Whitehouse and Platt, 2003). It has been documented in a number of studies that distribution of trace elements in zircon reflects the metamorphic conditions of crustal rocks and that the individual stages of zircon growth are often associated with specific metamorphic conditions (e.g., Schaltegger et al., 1999; Rubatto, 2002; Rubatto and Hermann, 2003; Whitehouse and Platt, 2003; Kelly and Harley, 2005). Available studies suggest that the trace element composition of newly growth zircon is mainly dictated by the following issues. (1) The ability of trace elements to enter its crystal lattice during subduction-zone metamorphism. For example, during eclogite-facies metamorphism, contraction of the zircon lattice favors the substitution of Zr4+ by Hf4+ due to its smaller ionic radius, but hinders the substitution of Zr4+ by elements other than Hf due to their larger ionic radii. This may induce the fractionation of Th to U, Lu to Hf and LREE to HREE to some extent (e.g., Wu and Zheng, 2004; Chen et al., 2010). Wang and Griffin (2004) argued that enrichment of Hf, and depletion of Y, U and Th in metamorphic zircon may be ascribed to different partition coefficients during metamorphism. (2) The composition of coexisting fluid/melt phases (e.g., Rowley et al., 1997; Keay et al., 2001; Rubatto, 2002; Wu et al., 2007; Li et al., 2013), which records the nature of dehydration/peritectic reactions. (3) The concurrent growth or recrystallization of specific minerals in hosting specific trace elements, such as garnet for HREE and Y, rutile for Nb, Ta and Ti, plagioclase for Eu and epidote for LREE, Th and U (e.g., Rubatto, 2002; Whitehouse and Platt, 2003; Wu and Zheng, 2004). For example, the activity of garnet during zircon growth in metamorphic systems, which dictates the garnet effect on zircon REE partition (Rubatto, 2002; Whitehouse and Platt, 2003). If both garnet and zircon occur as the products of metamorphic and peritectic reactions, the zircon is characterized by the flat HREE pattern due to the preferential partition of HREE into the garnet. Similarly, if garnet is a residual phase during these reactions, due to its high HREE, the product zircon would also exhibit depletion in HREE. In contrast, if garnet is involved in these reaction as reactant, the product zircon would be enriched in HREE. (4) The feature of formation enviroment (Rubatto, 2002; Wu and Zheng, 2004). The openness and closure of metamorphic and peritectic systems would affect zircon composition. Growth velocity of grown zircon was also suggested as a controlling factor (Vavra et al., 1999).
Many laboratory experiments and field-based studies have indicated that aqueous solutions can only dissolve fluid-mobile incompatible trace elements such as LILE, U, Sr and Pb and thus they cannot transport considerable amounts of melt-mobile incompatible trace elements such as LREE and Th (e.g., Hermann et al., 2006a; Zheng et al., 2011a). In contrast, hydrous melts can dissolve higher contents of solutes such as SiO2, Al2O3, CaO, NaO, Th, U, LILE and LREE (Zheng and Hermann, 2014, and references therein). As a result, aqueous solutions and hydrous melts can be distinguished by characteristic trace element contents and specially their ratios. For example, because U is more mobile than Th at temperatures below the wet solidus of crustal rocks, aqueous solutions are characterized by low Th contents and thus low Th/U ratios (e.g., Rollinson and Windley, 1980; Rowley et al., 1997). On the other hand, Th becomes mobile at temperatures above the wet solidus due to the breakdown of Th-rich minerals such as allanite and monazite (Hermann, 2002). Thus, the hydrous melts generally exhibit both higher Th and U contents and higher Th/U ratios than the aqueous solutions. It is expected that the newly grown zircon from aqueous solutions and hydrous melts can be distinguished by its contents and ratios of some trace elements, especially Th and U contents and their ratios. The further evolution of hydrous melts into magmatic melts leads to the common observation that the magmatic zircon shows uniformly higher Th/U ratios than both metamorphic and peritectic zircons. The Ti-in-zircon thermometer is widely used to link temperature to time in metamorphic rocks (e.g., Watson et al., 2006; Ferry and Watson, 2007; Tomkins et al., 2007). Due to the limited diffusivity of the 4+ cations within their lattices as constrained by the experimental diffusion study of Cherniak and Watson (2007), the Ti-in-zircon thermometer is particularly relevant to high-T and ultrahigh-T geological processes (e.g., Kelsey and Hand, 2015). The Ti-in-zircon thermometer not only depends on the Ti contents of zircon, but also depend on the activities of SiO2, and TiO2 as well as pressure (e.g., Watson et al., 2006; Ferry and Watson, 2007; Ferris et al., 2008). Although zircon Ti contents can be well measured, the other parameters especially for pressure cannot be well constrained. As shown by studies from the Dabie-Sulu orogenic belt, the Ti contents of zircon domains formed during a metamorphic event vary significantly (quite common in all types of metamorphic rocks), resulting in a very large variation in Ti-in-zircon temperatures (e.g., Zong et al., 2010; Chen et al., 2013b; Xu et al., 2013; Liu et al., 2015). In addition, nearly all metamorphic zircon domains have similar Ti-in-zircon temperatures despite their formation under different metamorphic P-T conditions. In this regard, it is critical to determine the saturation of Ti, Zr and Si in the target rock, pressure and the possible later resetting of Ti in zircon when applying the Ti-in-zircon thermometer (e.g., Ferry and Watson, 2007; Tomkins et al., 2007; Kelsey and Hand, 2015; Kohn, et al., 2015; Taylor et al., 2016). While the saturation can be demonstrated petrologically by the presence of rutile and quartz, the pressure can only be constrained by index
mineral and barometry. Studies that report Ti-in-zircon temperatures without having established the saturating criteria should be viewed with considerable caution. The Lu–Hf isotope system of a rock can be divided into two subsystems: zircon with low Lu/Hf ratios but matrix (mainly other REE-rich minerals such as garnet, epidote, monazite and apatite) with high Lu/ Hf ratios (Amelin et al., 2000; Kinny and Maas, 2003; Zheng et al., 2005b). In a closed system, zircon and matrix evolve in different ways. After a long time, the zircon has a low
176
Hf/177Hf ratio, whereas the matrix has a high
176
Hf/177Hf ratio. Due to the breakdown or
recrystallization of REE-rich minerals, newly grown zircon would show variably elevated 176
Hf/177Hf ratios (Amelin et al., 2000; Zheng et al., 2005b). For zircon formed via the dissolution
reprecipitation of pre-existing zircon in a closed system, on the other hand, it can inherit the rock Hf isotope composition by weighted meaning of Hf element and isotopes between protolith zircon and matrix (Zheng et al., 2005b, 2006a; Flowerdew et al., 2006). In this regard, the more matrices were involved, the higher
176
Hf/177Hf ratios the newly grown zircons have. If the more protolith zircons
were involved, the newly grown zircons have 176Hf/177Hf ratios that are still higher than, but close to, those for the protolith zircon. In either case, the newly grown zircon in a closed system has similar or elevated
176
Hf/177Hf ratios relative to the protolith zircon. In an open system, on the other hand,
the newly grown zircon would have significantly variable and different Lu–Hf isotope compositions from the inherited zircon (Zheng et al., 2005b; Wu et al., 2006a). Therefore, the Hf isotope composition of newly grown zircon can provide insights into its formation mechanism and system nature (open or closed).
3. Geological setting Because the present review mainly focuses on zircons from UHP metamorphic rocks in the Dabie-Sulu orogenic belt, east-central China, it is necessary to outline its geological setting in order to better understand metamorphic zirconology. There are two UHP terranes in this orogenic belt, which were synchronously generated by northward subduction of the South China Block beneath the North China Block in the Triassic (e.g., Li et al., 1999; Zheng et al., 2003a). They are separated into Dabie and Sulu orogens by 500 km of left-lateral strike-slip displacement along the Tan-Lu fault (Fig. 1). The two orogens are composed of several fault-bounded HP and UHP metamorphic units in association with Mesozoic magmatic rocks and sedimentary covers (e.g., Zheng et al., 2005a; Xu et al., 2006). The UHP metamorphism is demonstrated by the occurrence of coesite and microdiamond in metamorphic minerals from eclogite and gneiss in the Dabie-Sulu orogenic belt (e.g., Okay et al., 1989; Xu et al., 1992; Liu and Liou, 2011). Available observations indicate that this collisional orogen contains one of the largest and best-exposed UHP metamorphic terranes on Earth (e.g., Liou et al., 2009; Zheng, 2012).
S1367-9120(17)30208-0 http://dx.doi.org/10.1016/j.jseaes.2017.04.029 JAES 3065
To appear in:
Journal of Asian Earth Sciences
Received Date: Revised Date: Accepted Date:
21 January 2017 25 April 2017 27 April 2017
Please cite this article as: Chen, R-X., Zheng, Y-F., Metamorphic zirconology of continental subduction zones, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.04.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Review article
Metamorphic zirconology of continental subduction zones
Ren-Xu Chen*, Yong-Fei Zheng
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
*
Corresponding author. Email: [email protected]
Abstract Zircon is widely used to date geological events and trace geochemical sources in high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks of continental subduction zones. However, protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid-state transformation, metasomatic alteration and dissolution reprecipitation; new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks (metamorphic zircon) or peritectic reactions above the wet solidus (peritectic zircon). As a consequence, there are different origins of zircon domains in high-grade metamorphic rocks from collisional orogens. Thus, determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing. We advocate an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotope compositions. Only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology, enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction-zone conditions. The metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in HP to UHP metamorphic rocks of collisional orogens. These different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions. While the protolith nature of metamorphosed rocks dictates water availability, the P-T conditions of subduction zones dictate the property of subduction-zone fluids. The fluids of different properties may be produced at different positions of subducting and exhuming crustal slices, and they may physically and chemically mix with each other in continental subduction channels. Such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element Zr and other fluid-mobile incompatible trace elements from the subducted crust to the mantle wedge. Therefore, the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes.
Keywords: Zircon; regional metamorphism; subduction zone; dehydration reaction; peritectic reaction; fluid action
1. Introduction Zircon is a common accessory mineral in crustal rocks. Because of its high physiochemical stability and refractory nature, zircon is widely used to date geological events and trace geochemical sources (e.g., Hermann et al., 2001, 2013; Valley, 2003; Zheng et al., 2004, 2006a; Watson and Harrison, 2005; Kemp et al., 2006; Harley and Kelly, 2007a; McClelland and Lapen, 2013; Kohn et al., 2015; Taylor et al., 2016). This is because zircon is rich in U and Th but poor in Pb and has the high closure temperature of Pb diffusion (Zheng and Fu, 1998; Cherniak and Watson, 2003). Thus, zircon U-Pb dating has been one of the most commonly used and effective methods in geochronological studies (e.g., Wu and Zheng, 2004; Harley et al., 2007; Rubatto and Hermann, 2007a). As a phase enriched in Hf relative to radioactive Lu, zircon retains a strong fingerprint of the isotopic feature of crustal sources from which it crystallized. This provides robust evidence for growth and reworking of crustal rocks in the Earth’s history (e.g., Kemp et al., 2006; Zheng et al., 2006a, 2007a; Scherer et al., 2007). Zircon may contain significant amounts of temperature- or process-sensitive trace elements such as rare earth elements (REE) and Y as well as high field strength elements (HFSE), which can provide compelling evidence for conditions of zircon growth. This is important to reconstruction of of magmatic and metamorphic processes and to tracing the origin of host rocks (e.g., Harley et al., 2007; Rubatto and Hermann, 2007a). Although mineral O isotopes have a high sensitivity to low-temperature surface processes, zircon has the high stability in preserving its O isotope composition. Thus, zircon O isotope studies have been an effective means to discriminate the role of low-temperature versus high-temperature processes in its host rocks (e.g., Valley, 2003; Chen et al., 2011). Because of these unique properties, zircon is widely used in geological dating and geochemical tracing in various processes such as magma crystallization, partial melting, metamorphism, fluid action and hydrothermal mineralization, leading to the emergence of a new research branch named as zirconology. Zircon can grow or recrystallize under various conditions during hydrothermal, metamorphic, anatectic to magmatic processes (e.g., Rubatto, 2002; Wu and Zheng, 2004; Hoskin, 2005; Harley and Kelly, 2007b; Schaltegger, 2007). Geochemical information recorded in zircon can be correctly interpreted only when zircon growth conditions can be clearly linked to the evolutionary history of host rocks in subduction zones. High-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in collisional orogens generally experience multistages of evolution (Rumble et al., 2003; Zheng et al., 2003a), and metamorphic dehydration and partial melting of crustal rocks at subduction-zone conditions may produce different types of fluids such as aqueous solutions, hydrous melts and supercritical fluids (Zheng et al., 2011a; Zheng and Hermann, 2014). For these reasons, there are different origins of zircon in HP to UHP metamorphic rocks. Relict zircons (protolith zircons of either magmatic or detrital origins) may suffer variable extents of metamorphic recrystallization
through structural modification and chemical alteration in response to changes in P-T conditions and fluid accessibility; new zircon may grow during metamorphic dehydration and partial melting at subduction-zone conditions. As a consequence, the orogenic metamorphic rocks may contain not only recrystallized zircons of different types but also newly grown zircons of different types. The recrystallization of protolith zircons and the growth of new zircons in crustal rocks may take place under a broad range of P-T conditions during prograde, peak or post-peak greenschist-, amphibolite-, eclogite- and granulite-facies conditions (e.g., Fraser et al., 1997; Wu et al., 2006; Baldwin and Brown, 2008; Liu and Liou, 2011). With the advanced application of zircon studies to dating of geological events and tracing of geochemical sources and processes, the term zirconology has being used increasingly in the literature (e.g., Zheng, 2009; Xia et al., 2009, 2010; Chen et al., 2010, 2011; Nemchin, et al., 2012; Tichomirowa et al., 2012; Li et al., 2013). This involves an integrated study of zircon mineragraphy (internal structure and external morphology), U-Pb ages, mineral inclusions, trace elements, and Lu-Hf and O isotopes. Such a zirconological study is necessary in order to correctly interpret observations from zircons in crustal and mantle rocks. This is a big step in applying a single mineral to studies of metamorphic geology and geochemistry. For this reason, this paper provides a review on the studies of metamorphic zirconology in UHP metamorphic rocks from continental subduction zones. Although many of examples are taken from the Dabie-Sulu orogenic belt in China, available data from the other typical UHP terranes on Earth are also taken into account. The results indicate that different types of zircons can be discriminated by studying their properties during recrystallization and growth in continental subduction zones. This also provides constraints on the fluid action during subduction-zone processes.
2. Fundamentals for metamorphic zirconology In the present review of metamorphic zirconology, magmatic zircon is referred to as that crystallized from magmatic melts, and relict zircon is referred to as those inherited from crustal protoliths (some are of magmatic origin whereas the other is of detrital origin). On the other hand, metamorphic zircon is referred to as that formed through dehydration reactions below the wet solidus of crustal rocks, and peritectic zircon is referred to as those crystallized through peritectic reactions above the wet solidus of crustal rocks. While magmatic melts have separated from their parental rocks and transported upwards with large extent of evolution by fractional crystallization, anatectic melts are not separated from their parental rocks and thus only experienced the smallest extent of fractional crystallization. Nevertheless, anatectic zircon may grow from anatectic melts in which the local oversaturation of Zr is achieved by fractional crystallization of Zr-poor minerals. Therefore, the physical and chemical properties of subduction-zone fluids are a key to mineralogical
processes that form or rework zircon at continental subduction-zone conditions. Understanding these properties and processes is substantial not only to petrogenetic interpretation of zircons from high-grade metamorphic rocks in collisional orogens but also to tectonic interpretation of their U-Pb ages and geochemical signatures (e.g., Wu and Zheng, 2004; Harley and Kelly, 2007; Rubatto and Hermann. 2007; Zheng, 2009, 2012; Hermann et al., 2013). As defined in petrology of magmatic rocks, a magma is a mixture of crystal and melt, whereas the melt is short of crystalline minerals. Thus, a melt is part of a magma rather than whole magma. The difference between melt and magma is the occurrence and amount of crystalline minerals in the melt. A melt becomes a magma as soon as rock-forming minerals have significantly crystallized from the melt. A felsic melt is produced by partial melting of crustal rocks. Anatexis is referred to as partial melting of lower degrees to result in migmatization, generating anatectic melts that have not left their parental rocks. The anatectic melts were produced through peritectic reactions at temperatures above the wet solidus of crustal rocks. Typically, the anatectic melt is produced by migmatization, and its crystallized product is veinlets in metamorphic rocks, leucosomes in migmatites (metatexite and diatexite) and pegmatite veins in felsic gneisses. Geochemically, the anatectic melt has achieved thermodynamic equilibrium with the peritectic mineral in partitioning of water and incompatible trace elements, but it is not with the relict mineral. On the other hand, magmatism requires partial melting of higher degrees with significant transport and accumulation of anatectic melts. Thus, magmatic melts have escaped from their parental rocks (migmatites) with significant evolution in petrology. The anatectic melt becomes the magmatic melt after its significant evolution with fractional crystallization of rock-forming minerals. Therefore, the magmatic melt has achieved thermodynamic equilibrium with the crystallized minerals in partitioning of water and incompatible trace elements. Zircon grown under different environments can entrap the concurrently grown minerals, fluids and melts as its inclusions, which provide important records of its formation conditions and mechanism (e.g., Hermann et al., 2001; Liu and Liou, 2011; Li et al., 2013). The occurrence of inclusions (mineral, fluid or melt) in zircon provides an opportunity to correlate the growth zones of zircon with metamorphic/anatectic conditions. This is usually realized by identifying the distribution of mineral inclusion assemblage, CL images and trace element composition of zircon or mineral inclusions (e.g., Hermann et al., 2001; Liu and Liou, 2011). However, the relationship between inclusion species and host zircon domains is commonly complicated, limiting its application to zircon genesis. Three mechanisms have been proposed to account for the occurrence of inclusions in zircon (e.g., Gebauer et al., 1997; Liu et al., 2001; Zheng et al., 2011b): (1) entrapping of concurrently grown minerals, fluids or melts during zircon growth; (2) squeezing/crystallizing of inclusions along fractures into the preexisting zircon; (3) transforming of
precursor mineral inclusions into new mineral inclusions during metamorphism. Once inclusions were entrapped into zircon, their composition can hardly change because of the refractoriness of zircon. Elastic models suggest that transformation of a precursor quartz to coesite appears unlikely to happen (e.g., Gillet et al., 1984; Van Dermolen and Van Roermund, 1986). In this regard, the third mechanism seems unlikely. Healed structures of earlier deformed minerals and disturbed structures such as patch zoning and resorption can be found around secondary inclusions by detailed microstructure imaging (e.g., Gebauer et al., 1997; Dubińskaa et al., 2004). In contrast, cracks or healing traces generally cannot be observed from primary mineral inclusions. As such, detailed structure analyses can distinguish between the secondary and primary origins of mineral inclusions, and only the primary inclusions are of petrological meaning in the zircon genesis. Coesite and other eclogite-facies minerals as well as low-P (e.g., feldspar, biotite and quartz) minerals were found in zircon cores from schist and gneiss from the main hole of Chinese Continental Scientific Drilling (CCSD-MH) in the Sulu orogen (Zhang et al., 2006) and jadeite quartzite from Shuanghe in the Dabie orogen (Gao et al., 2015). However, U-Pb ages, CL images and trace element compositions indicate that these zircon cores are of magmatic rather than metamorphic origin. Detailed structure observations reveal that there are cracks and disturbed structure, indicating these inclusions are formed through metasomatic alteration along fractures due to fluid infiltration during UHP metamorphism (Gao et al., 2015). Thus, it is critical to distinguish between the primary and secondary mineral inclusions when interpreting the formation conditions of host zircon. Detailed studies of mineral inclusions have been carried out on zircons from various UHP metamorphic rocks from the Dabie-Sulu orogenic belt (Liu and Liou, 2011, and references therein). The results show that, at the prograde HP eclogite-facies stage during subduction, mineral inclusions in metamorphic zircons are predominated by quartz, garnet, omphacite, phengite, rutile, K-feldspar, dolomite and apatite. At the peak UHP eclogite-facies metamorphic stage, mineral inclusions are coesite, garnet, omphacite, phengite, rutile, jadeite, kyanite, titanite, K-feldspar, aragonite, magnesite and apatite. At the amphibolite-facies retrogression stage during exhumation, mineral inclusions are only composed of low-pressure minerals such as quartz, plagioclase, albite, amphibole, calcite and apatite. The metamorphic assemblage of mineral inclusions is not only related to metamorphic P-T conditions, but also controlled by the composition of host rocks (Liu and Liou, 2011). For HP to UHP eclogite-facies metamorpohic zircons, inclusion mineral assemblages are Coe/Qtz + Grt + Omp + Phe ± Mgs ± Ap for eclogite/amphibolite in granitic orthogneiss, Coe/Qtz + Grt + Omp + Phe + (Mgs+Arg)/Dol + Ap for eclogite in marble, Coe/Qtz + Grt ± Omp/Jd + Phe + Ttn + Ap for paragneiss, Coe/Qtz + Phe + Ap ± Grt ± Jd ± Rt ± Ttn ± Ky ± Kfs for granitic orthogneiss, Coe/Qtz + Grt + Phe + Rt +Ap + Omp/Jd + Ky for quartzite, and
Coe/Qtz + Grt + Omp + Ap + (Mgs + Arg)/Dol for marble. For the late amphibolite-facies metamorphic zircons, inclusion mineral assemblages are Amp+Pl+Ap for eclogite in granitic orthogneiss, Qtz+Cal+Amp for eclogite in marble, Qtz+Ab for paragneiss, orthogneiss and quartzite and Cal for marble. Analyses of these mineral inclusions, in combination with zircon U-Pb dates, can provide important constraints on P-T-t paths of zircon domains and further their host rocks (Hermann et al., 2001; Liu and Liou, 2011). Magmatic zircon generally exhibits high REE abundances and steep REE patterns, positive Ce anomalies and negative Eu anomalies (e.g., Rubatto, 2002; Whitehouse and Platt, 2003). It has been documented in a number of studies that distribution of trace elements in zircon reflects the metamorphic conditions of crustal rocks and that the individual stages of zircon growth are often associated with specific metamorphic conditions (e.g., Schaltegger et al., 1999; Rubatto, 2002; Rubatto and Hermann, 2003; Whitehouse and Platt, 2003; Kelly and Harley, 2005). Available studies suggest that the trace element composition of newly growth zircon is mainly dictated by the following issues. (1) The ability of trace elements to enter its crystal lattice during subduction-zone metamorphism. For example, during eclogite-facies metamorphism, contraction of the zircon lattice favors the substitution of Zr4+ by Hf4+ due to its smaller ionic radius, but hinders the substitution of Zr4+ by elements other than Hf due to their larger ionic radii. This may induce the fractionation of Th to U, Lu to Hf and LREE to HREE to some extent (e.g., Wu and Zheng, 2004; Chen et al., 2010). Wang and Griffin (2004) argued that enrichment of Hf, and depletion of Y, U and Th in metamorphic zircon may be ascribed to different partition coefficients during metamorphism. (2) The composition of coexisting fluid/melt phases (e.g., Rowley et al., 1997; Keay et al., 2001; Rubatto, 2002; Wu et al., 2007; Li et al., 2013), which records the nature of dehydration/peritectic reactions. (3) The concurrent growth or recrystallization of specific minerals in hosting specific trace elements, such as garnet for HREE and Y, rutile for Nb, Ta and Ti, plagioclase for Eu and epidote for LREE, Th and U (e.g., Rubatto, 2002; Whitehouse and Platt, 2003; Wu and Zheng, 2004). For example, the activity of garnet during zircon growth in metamorphic systems, which dictates the garnet effect on zircon REE partition (Rubatto, 2002; Whitehouse and Platt, 2003). If both garnet and zircon occur as the products of metamorphic and peritectic reactions, the zircon is characterized by the flat HREE pattern due to the preferential partition of HREE into the garnet. Similarly, if garnet is a residual phase during these reactions, due to its high HREE, the product zircon would also exhibit depletion in HREE. In contrast, if garnet is involved in these reaction as reactant, the product zircon would be enriched in HREE. (4) The feature of formation enviroment (Rubatto, 2002; Wu and Zheng, 2004). The openness and closure of metamorphic and peritectic systems would affect zircon composition. Growth velocity of grown zircon was also suggested as a controlling factor (Vavra et al., 1999).
Many laboratory experiments and field-based studies have indicated that aqueous solutions can only dissolve fluid-mobile incompatible trace elements such as LILE, U, Sr and Pb and thus they cannot transport considerable amounts of melt-mobile incompatible trace elements such as LREE and Th (e.g., Hermann et al., 2006a; Zheng et al., 2011a). In contrast, hydrous melts can dissolve higher contents of solutes such as SiO2, Al2O3, CaO, NaO, Th, U, LILE and LREE (Zheng and Hermann, 2014, and references therein). As a result, aqueous solutions and hydrous melts can be distinguished by characteristic trace element contents and specially their ratios. For example, because U is more mobile than Th at temperatures below the wet solidus of crustal rocks, aqueous solutions are characterized by low Th contents and thus low Th/U ratios (e.g., Rollinson and Windley, 1980; Rowley et al., 1997). On the other hand, Th becomes mobile at temperatures above the wet solidus due to the breakdown of Th-rich minerals such as allanite and monazite (Hermann, 2002). Thus, the hydrous melts generally exhibit both higher Th and U contents and higher Th/U ratios than the aqueous solutions. It is expected that the newly grown zircon from aqueous solutions and hydrous melts can be distinguished by its contents and ratios of some trace elements, especially Th and U contents and their ratios. The further evolution of hydrous melts into magmatic melts leads to the common observation that the magmatic zircon shows uniformly higher Th/U ratios than both metamorphic and peritectic zircons. The Ti-in-zircon thermometer is widely used to link temperature to time in metamorphic rocks (e.g., Watson et al., 2006; Ferry and Watson, 2007; Tomkins et al., 2007). Due to the limited diffusivity of the 4+ cations within their lattices as constrained by the experimental diffusion study of Cherniak and Watson (2007), the Ti-in-zircon thermometer is particularly relevant to high-T and ultrahigh-T geological processes (e.g., Kelsey and Hand, 2015). The Ti-in-zircon thermometer not only depends on the Ti contents of zircon, but also depend on the activities of SiO2, and TiO2 as well as pressure (e.g., Watson et al., 2006; Ferry and Watson, 2007; Ferris et al., 2008). Although zircon Ti contents can be well measured, the other parameters especially for pressure cannot be well constrained. As shown by studies from the Dabie-Sulu orogenic belt, the Ti contents of zircon domains formed during a metamorphic event vary significantly (quite common in all types of metamorphic rocks), resulting in a very large variation in Ti-in-zircon temperatures (e.g., Zong et al., 2010; Chen et al., 2013b; Xu et al., 2013; Liu et al., 2015). In addition, nearly all metamorphic zircon domains have similar Ti-in-zircon temperatures despite their formation under different metamorphic P-T conditions. In this regard, it is critical to determine the saturation of Ti, Zr and Si in the target rock, pressure and the possible later resetting of Ti in zircon when applying the Ti-in-zircon thermometer (e.g., Ferry and Watson, 2007; Tomkins et al., 2007; Kelsey and Hand, 2015; Kohn, et al., 2015; Taylor et al., 2016). While the saturation can be demonstrated petrologically by the presence of rutile and quartz, the pressure can only be constrained by index
mineral and barometry. Studies that report Ti-in-zircon temperatures without having established the saturating criteria should be viewed with considerable caution. The Lu–Hf isotope system of a rock can be divided into two subsystems: zircon with low Lu/Hf ratios but matrix (mainly other REE-rich minerals such as garnet, epidote, monazite and apatite) with high Lu/ Hf ratios (Amelin et al., 2000; Kinny and Maas, 2003; Zheng et al., 2005b). In a closed system, zircon and matrix evolve in different ways. After a long time, the zircon has a low
176
Hf/177Hf ratio, whereas the matrix has a high
176
Hf/177Hf ratio. Due to the breakdown or
recrystallization of REE-rich minerals, newly grown zircon would show variably elevated 176
Hf/177Hf ratios (Amelin et al., 2000; Zheng et al., 2005b). For zircon formed via the dissolution
reprecipitation of pre-existing zircon in a closed system, on the other hand, it can inherit the rock Hf isotope composition by weighted meaning of Hf element and isotopes between protolith zircon and matrix (Zheng et al., 2005b, 2006a; Flowerdew et al., 2006). In this regard, the more matrices were involved, the higher
176
Hf/177Hf ratios the newly grown zircons have. If the more protolith zircons
were involved, the newly grown zircons have 176Hf/177Hf ratios that are still higher than, but close to, those for the protolith zircon. In either case, the newly grown zircon in a closed system has similar or elevated
176
Hf/177Hf ratios relative to the protolith zircon. In an open system, on the other hand,
the newly grown zircon would have significantly variable and different Lu–Hf isotope compositions from the inherited zircon (Zheng et al., 2005b; Wu et al., 2006a). Therefore, the Hf isotope composition of newly grown zircon can provide insights into its formation mechanism and system nature (open or closed).
3. Geological setting Because the present review mainly focuses on zircons from UHP metamorphic rocks in the Dabie-Sulu orogenic belt, east-central China, it is necessary to outline its geological setting in order to better understand metamorphic zirconology. There are two UHP terranes in this orogenic belt, which were synchronously generated by northward subduction of the South China Block beneath the North China Block in the Triassic (e.g., Li et al., 1999; Zheng et al., 2003a). They are separated into Dabie and Sulu orogens by 500 km of left-lateral strike-slip displacement along the Tan-Lu fault (Fig. 1). The two orogens are composed of several fault-bounded HP and UHP metamorphic units in association with Mesozoic magmatic rocks and sedimentary covers (e.g., Zheng et al., 2005a; Xu et al., 2006). The UHP metamorphism is demonstrated by the occurrence of coesite and microdiamond in metamorphic minerals from eclogite and gneiss in the Dabie-Sulu orogenic belt (e.g., Okay et al., 1989; Xu et al., 1992; Liu and Liou, 2011). Available observations indicate that this collisional orogen contains one of the largest and best-exposed UHP metamorphic terranes on Earth (e.g., Liou et al., 2009; Zheng, 2012).
The Dabie-Sulu UHP metamorphic rocks are mainly composed of granitic orthogneiss, with minor proportions of other rock types such as paragneiss, eclogite, peridotite, marble and quartzite. In terms of the differences in metamorphic P-T conditions, eclogite-bearing slices are subdivided into three UHP zones: low-T/UHP, mid-T/UHP and high-T/UHP zones (e.g., Zheng et al., 2005a; Xu et al., 2006; Liu and Li, 2008). The maximum pressure estimates lie in the diamond stability field of >120 km, and the maximum temperatures vary from 730 to 850°C depending on P-T paths of different UHP slices during continental collision. All the three UHP zones underwent amphibolite-facies retrogression during exhumation, and the high-T/UHP zone also underwent widespread migmatization and magmatism at the postcollisional stage (e.g., Wu et al., 2007; Zhao and Zheng, 2009; Chen et al., 2015). The high-T/UHP zone is characterized by overprinting of HP granulite-facies metamorphism over the UHP fabrics during the early exhumation. The maximum temperatures are obtained at the stage of early exhumation, named as "hot" exhumation (Zhao et al., 2007a; Zheng et al., 2011a). Quartz veins and leucosomes within UHP rocks are widely found in the Dabie-Sulu orogenic belt, indicating the presence of fluid flow and partial melting during continental collision (e.g., Zheng et al., 2007b, 2011a; Sheng et al., 2012; Chen et al., 2013a, 2013b; Li et al., 2014, 2016a). The crustal rocks in the Dabie-Sulu orogenic belt underwent UHP eclogite-facies metamorphism at 225-240 Ma, quartz eclogite-facies metamorphism at 215-225 Ma and amphibolite-facies metamorphism at 215-205 Ma (e.g., Zheng et al., 2009). The protoliths of most UHP metaigneous rocks are of middle Neoproterozoic age (mainly at 740 to 780 Ma), whereas some of UHP metasedimentary rocks have Archean and Paleoproterozoic protoliths (Zheng, 2008). Rock-forming minerals in the UHP metaigneous rocks exhibit negative to low δ18O values (e.g., Zheng et al., 2003a, 2009; Chen et al., 2007; Tang et al., 2008a, 2008b). The U-Pb dating of low to negative δ18O zircons demonstrates that the protoliths of metaigneous rocks were strongly altered by unusually
18
O-depleted surface water at high temperatures in the middle Neoproterozoic (e.g.,
Rumble et al., 2002; Zheng et al., 2004, 2009; Chen et al., 2011; He et al., 2016).
4. Newly grown zircons during subduction-zone metamorphism 4.1 Metamorphic zircon The metamorphic zircon is generally produced through subsolidus dehydration reactions between silicate and accessory minerals with the presence of aqueous solutions in the products. Previous studies often ascribe the origin of metamorphic zircon to direct precipitation from metamorphic fluids (e.g., Rubatto and Hermann, 2003; Dubinska et al., 2004; Zheng et al., 2007b). However, the metamorphic fluids produced at subsolidus conditions are highly undersaturated with
HFSE and thus not able to precipitate zircon (e.g., Zheng and Hermann, 2014, and references therein). For this reason, we prefer to interpret the metamorphic zircon as the product of mineral reactions during metamorphic dehydration below the wet solidus of crustal rocks. Metamorphic zircons in UHP metamorphic rocks commonly occur either as small anhedral grains or as overgrowths around relict cores of protolith zircon (Fig. 2). These domains exhibit no zoning, cloudy zoning and weak zoning in CL images (e.g., Zheng et al., 2004, 2005b, 2006a; Wu et al., 2006a; Xia et al., 2009; Chen et al., 2010; Li et al., 2013) and concordant U-Pb metamorphic ages. Metamorphic zircons in UHP metamorphic rocks from the Dabie-Sulu orogenic belt generally exhibit lower REE, Th and HFSE contents but higher Hf contents than the protolith zircons of magmatic origin (Chen et al., 2010; Liu and Liou, 2011). However, REE patterns for the metamorphic zircons are variable (Fig. 3). Eclogite-facies metamorphic zircons exhibit relative lack of negative Eu anomalies due to the breakdown of plagioclase under eclogite-facies conditions. Such zircons in eclogites are generally characterized by flat HREE patterns with low (Lu/Gd)N ratios because of the concurrent growth of metamorphic garnet during eclogitization. In contrast, the majority of metamorphic zircons in eclogite-facies felsic gneisses show steep HREE patterns with high (Lu/Gd)N ratios (Fig. 3d-f), leaving only the minority of metamorphic zircons in these gneisses to exhibit the flat REE patterns similar to those in mafic eclogites (Fig. 3b). This may reflect the garnet effect in the metamorphic system of felsic rocks. While the flat HREE patterns indicate the concurrent growth of metamorphic garnet during dehydration reaction in the felsic rocks, the steep HREE patterns suggest the relative lack of garnet growth during the eclogite-facies metamorphism of felsic rocks (Fig. 3b, d-f). Therefore, the geochemical composition of metamorphic zircons is related not only to the metamorphic P-T condition but also to the composition of host rocks In principle, amphibolite-facies metamorphic zircons would exhibit steep REE patterns and marked negative Eu anomalies. This is indeed observed in some UHP metamorphic rocks from several UHP terranes (e.g., Hermann et al., 2001). However, metamorphic zircons in amphibolite-facies retrograde eclogites from the Dabie-Sulu orogenic belt often exhibit flat REE patterns similar to those in eclogites (Fig. 3d and Liu and Liou, 2011). Mineral inclusions in these zircon domains do indicate their growth under amphibolite-facies conditions. In this regard, the trace element composition of metamorphic zircons may suggest their growth in association with recrystallization of garnet but the limited crystallization of plagioclase. On the other hand, amphiboles formed under amphibolite-facies conditions would inherit the trace element composition from their precursor garnet/omphacite (Sassi et al., 2000), implying the limited release of HREE from these precursors.
Metamorphic zircons also occur as large euhedral crystals in quartz veins within UHP eclogites (Fig. 2), recording the local focus of aqueous solutions during continental collision (Zheng et al., 2007b; Wu et al., 2009; Chen et al., 2012; Sheng et al., 2012, 2013). Although aqueous solutions have the great capacity to dissolve and transport silica and fluid-mobile incompatible trace elements such as LILE, they have very limited capacity to dissolve and transport REE and HFSE (Zheng and Hermann, 2014). In this regard, the occurrence of metamorphic zircons in the quartz veins does not mean the chemical transport of element Zr by the aqueous solutions. Instead, it suggests the growth of metamorphic zircons via locally focused dehydration reactions at decompressional conditions. The occurrence of fluid inclusions in these zircons (Fig. 2f) confirms their origin from dehydration reaction under subsolidus conditions. As such, the metamorphic zircons were carried by the aqueous solutions from their growth sites into the veining sites. This requires the generation of abundant aqueous solutions through the subsolidus dehydration reactions, allowing for sufficient growth of metamorphic zircons as the large euhedral crystals. If only very minor amounts of the aqueous solutions were produced by metamorphic dehydration, the newly grown zircon would occur as small anhedral grains as commonly observed in UHP eclogites. Despite the big difference in zircon morphology and thus fluid abundances, there are no significant differences in trace element and Lu-Hf isotope compositions between the two kinds of the metamorphic zircons in the UHP metamorphic rocks (e.g., Chen et al., 2012a; Sheng et al., 2012, 2013). This indicates the origin of aqueous solutions from the metamorphic dehydration of UHP rocks themselves in both cases. Most zircons exhibit flat REE patterns, indicating their growth in association with the concurrent growth of metamorphic garnet under eclogite-facies conditions. This is confirmed by the mineral inclusions of Grt and Omp in these zircon domains (Fig. 2f). Rare zircons exhibit steep REE patterns with negative Eu anomalies, indicating their growth away from the garnet growth (Fig. 3c). Relict cores of the protolith zircons are common in the quartz veins (Fig. 2g; Chen et al., 2012a; Sheng et al., 2012, 2013), suggesting the physical transport of protolith zircons by the metamorphic fluids. This explains the occurrence of both old relict and newly grown zircons in metamorphic veins and orogenic peridotites. In view of the difference in element abundances between metamorphic zircons and protolith magmatic zircons, the metamorphic zircons are expected to show the difference in some element ratios from the protolith magmatic zircons. The metamorphic zircons generally have lower Th/U ratios (generally <0.1), higher Eu/Eu* ratios, higher Hf/Y ratios and lower (Lu/Gd) N ratios than the protolith magmatic zircons (Figs. 3 and 4). The metamorphic zircons exhibit low and elevated
176
176
Lu/177Hf ratios
Hf/177Hf ratios (Fig. 5a). They may have O isotope compositions similar to, or
different from, the protolith magmatic zircons depending on the O isotope composition of metamorphic minerals and thus the protolith nature of host rocks. For example, metamorphic
zircons in UHP eclogites from Qinglongshan in the Sulu orogen exhibit low or negative δ18O values (Fig. 5b), because the protoliths of UHP metabasites there underwent meteoric hydrothermal alteration in the middle Neoproterozoic (Zheng et al., 2003; Chen et al., 2011). Metamorphic zircons may exhibit higher 18O values than normal mantle values of 5.3±0.3‰ (Valley et al., 1998) if they are produced by metamorphism from supracrustal rocks, otherwise they show lower 18O values than normal values if they are generated by metamorphism from intracrustal rocks that underwent low 18O fluid alteration. Two major episodes of metamorphic zircons have been identified during continental subduction-zone metamorphism in the Dabie-Sulu orogenic belt: one at the prograde HP-UHP transition stage and the other at the retrograde UHP-HP transition stage (e.g., Zheng et al., 2005b; Wu et al., 2006; Zheng, 2009; Xia et al., 2013). This is indicated by the two episodes of zircon growth at c. 240 Ma and c. 225 Ma, respectively, in the UHP eclogites (Zheng et al., 2009). During the prograde HP-UHP metamorphism, aqueous solutions were produced by the breakdown of hydrous HP minerals such as paragonite, amphibole, lawsonite and zoisite. The breakdown of paragonite may be common during the transformation of blueschist to eclogite (Li et al., 2004), and the breakdown of amphibole may be common during the transformation of amphibolite to eclogite (Liu and Ye, 2004). During the breakdown of amphibole, ilmenite was also involved. Ilmenite has been considered as a significant reservoir of Zr in mafic lithology, and its breakdown is especially important to zircon growth (Bingen et al., 2004). Thus, these dehydration reactions may release SiO2 and probably ZrO2 and thus lead to the growth of metamorphic zircons. In addition, zircon is a major reservoir of Zr in metamorphic rocks (e.g., Bea et al., 2006). The Lu-Hf isotope composition of metamorphic zircons suggests that the protolith zircon was also involved in the zircon growth. During exhumation, the breakdown of hydrous UHP minerals such as lawsonite, phengite and zoisite can provide considerable amounts of aqueous solutions for veining, metasomatism and even anatexis (Li et al., 2004; Zheng et al., 2007b). Furthermore, decompression exsolution of structural hydroxyl and molecular water from nominally anhydrous UHP minerals such as garnet, omphacite and rutile is also significant in UHP metamorphic rocks (Zheng et al., 1999, 2003a; Chen et al., 2007b; Zheng, 2009), providing an important source of retrograde fluids for hydration of the UHP rocks during exhumation. The Zr solubility in OH-rich fluids an be enhan e even at o er te
eratures Dubińska et a ., 2004), so that the aqueous solutions generated by local sinking of the
liberated hydroxyls are highly alkaline and thus oxidized (Zheng et al., 2007b). It is an efficient agent to dissolve and transport Zr and Si from relevant minerals for the growth of metamorphic zircons (Zheng, 2009). In addition, rutile is an important reservoir of Zr in eclogites (Sassi et al., 2000; Zack et al., 2002). Rutile may release a lot of Zr due to retrograde recrystallization of
eclogites under HP eclogite-facies conditions.
4.2 Peritectic zircon Partial melting of UHP metamorphic rocks is common not only in the Dabie-Sulu orogenic belt but also elsewhere in the other collisional orogens (e.g., Zheng et al., 2011a; Gao et al., 2012; Liu et al., 2012; Chen et al., 2013a, 2013b; Gordon et al., 2013; Li et al., 2013, 2014, 2016a; Gilotti et al., 2014). It occurs at varying scales from thin sections to outcrops, forming different types of migmatites. Anatectic melts were produced through peritectic reactions above the wet solidus of crustal rocks. Alkalic to granitic dykes and intrusions of synexhumation age are highly evolved products of the anatectic melts (Yang et al., 2005; Liu et al., 2009c; Zhao et al., 2012). Peritectic zircons primarily occur in migmatitic rocks and pegmatites (Wallis et al., 2005; Liu et al., 2009a, 2010, 2012; Zong et al., 2010; Zeng et al., 2011; Chen et al., 2013a, 2013b; Xu et al., 2013), whereas magmatic zircons typically occur in synexhumation magmatic intrusions (Zhao et al., 2012). Because of the low Zr solubility in anatectic melts, zircon is rarely crystallized from the anatectic melts unless the anatectic melts have experienced given degrees of evolution with fractional crystallization of Zr-poor minerals from the melts. Like magmatic zircons, anatectic zircons can only be crystallized from the anatectic melts in which the local Zr oversaturation was achieved during their evolution. While peritectic zircons would generally grow above the wet solidus with increasing temperature, the anatectic zircons would only grow along a temperature decrease path with temperartures on the wet solidus (Fig. 6). In this regard, peritectic zircons would generally occur in migmatitic rocks and pegmatites, whereas anatectic zircons can only occur in the leucosome and pegmatite which have underwent large extent of fractional crystallization along the temperature decrease path.
18
O-depleted surface water at high temperatures in the middle Neoproterozoic (e.g.,
Rumble et al., 2002; Zheng et al., 2004, 2009; Chen et al., 2011; He et al., 2016).
4. Newly grown zircons during subduction-zone metamorphism 4.1 Metamorphic zircon The metamorphic zircon is generally produced through subsolidus dehydration reactions between silicate and accessory minerals with the presence of aqueous solutions in the products. Previous studies often ascribe the origin of metamorphic zircon to direct precipitation from metamorphic fluids (e.g., Rubatto and Hermann, 2003; Dubinska et al., 2004; Zheng et al., 2007b). However, the metamorphic fluids produced at subsolidus conditions are highly undersaturated with
HFSE and thus not able to precipitate zircon (e.g., Zheng and Hermann, 2014, and references therein). For this reason, we prefer to interpret the metamorphic zircon as the product of mineral reactions during metamorphic dehydration below the wet solidus of crustal rocks. Metamorphic zircons in UHP metamorphic rocks commonly occur either as small anhedral grains or as overgrowths around relict cores of protolith zircon (Fig. 2). These domains exhibit no zoning, cloudy zoning and weak zoning in CL images (e.g., Zheng et al., 2004, 2005b, 2006a; Wu et al., 2006a; Xia et al., 2009; Chen et al., 2010; Li et al., 2013) and concordant U-Pb metamorphic ages. Metamorphic zircons in UHP metamorphic rocks from the Dabie-Sulu orogenic belt generally exhibit lower REE, Th and HFSE contents but higher Hf contents than the protolith zircons of magmatic origin (Chen et al., 2010; Liu and Liou, 2011). However, REE patterns for the metamorphic zircons are variable (Fig. 3). Eclogite-facies metamorphic zircons exhibit relative lack of negative Eu anomalies due to the breakdown of plagioclase under eclogite-facies conditions. Such zircons in eclogites are generally characterized by flat HREE patterns with low (Lu/Gd)N ratios because of the concurrent growth of metamorphic garnet during eclogitization. In contrast, the majority of metamorphic zircons in eclogite-facies felsic gneisses show steep HREE patterns with high (Lu/Gd)N ratios (Fig. 3d-f), leaving only the minority of metamorphic zircons in these gneisses to exhibit the flat REE patterns similar to those in mafic eclogites (Fig. 3b). This may reflect the garnet effect in the metamorphic system of felsic rocks. While the flat HREE patterns indicate the concurrent growth of metamorphic garnet during dehydration reaction in the felsic rocks, the steep HREE patterns suggest the relative lack of garnet growth during the eclogite-facies metamorphism of felsic rocks (Fig. 3b, d-f). Therefore, the geochemical composition of metamorphic zircons is related not only to the metamorphic P-T condition but also to the composition of host rocks
Metamorphic zircons also occur as large euhedral crystals in quartz veins within UHP eclogites (Fig. 2), recording the local focus of aqueous solutions during continental collision (Zheng et al., 2007b; Wu et al., 2009; Chen et al., 2012; Sheng et al., 2012, 2013). Although aqueous solutions have the great capacity to dissolve and transport silica and fluid-mobile incompatible trace elements such as LILE, they have very limited capacity to dissolve and transport REE and HFSE (Zheng and Hermann, 2014). In this regard, the occurrence of metamorphic zircons in the quartz veins does not mean the chemical transport of element Zr by the aqueous solutions. Instead, it suggests the growth of metamorphic zircons via locally focused dehydration reactions at decompressional conditions. The occurrence of fluid inclusions in these zircons (Fig. 2f) confirms their origin from dehydration reaction under subsolidus conditions. As such, the metamorphic zircons were carried by the aqueous solutions from their growth sites into the veining sites. This requires the generation of abundant aqueous solutions through the subsolidus dehydration reactions, allowing for sufficient growth of metamorphic zircons as the large euhedral crystals. If only very minor amounts of the aqueous solutions were produced by metamorphic dehydration, the newly grown zircon would occur as small anhedral grains as commonly observed in UHP eclogites. Despite the big difference in zircon morphology and thus fluid abundances, there are no significant differences in trace element and Lu-Hf isotope compositions between the two kinds of the metamorphic zircons in the UHP metamorphic rocks (e.g., Chen et al., 2012a; Sheng et al., 2012, 2013). This indicates the origin of aqueous solutions from the metamorphic dehydration of UHP rocks themselves in both cases. Most zircons exhibit flat REE patterns, indicating their growth in association with the concurrent growth of metamorphic garnet under eclogite-facies conditions. This is confirmed by the mineral inclusions of Grt and Omp in these zircon domains (Fig. 2f). Rare zircons exhibit steep REE patterns with negative Eu anomalies, indicating their growth away from the garnet growth (Fig. 3c). Relict cores of the protolith zircons are common in the quartz veins (Fig. 2g; Chen et al., 2012a; Sheng et al., 2012, 2013), suggesting the physical transport of protolith zircons by the metamorphic fluids. This explains the occurrence of both old relict and newly grown zircons in metamorphic veins and orogenic peridotites. In view of the difference in element abundances between metamorphic zircons and protolith magmatic zircons, the metamorphic zircons are expected to show the difference in some element ratios from the protolith magmatic zircons. The metamorphic zircons generally have lower Th/U ratios (generally <0.1), higher Eu/Eu* ratios, higher Hf/Y ratios and lower (Lu/Gd) N ratios than the protolith magmatic zircons (Figs. 3 and 4). The metamorphic zircons exhibit low and elevated
176
176
Lu/177Hf ratios
Hf/177Hf ratios (Fig. 5a). They may have O isotope compositions similar to, or
different from, the protolith magmatic zircons depending on the O isotope composition of metamorphic minerals and thus the protolith nature of host rocks. For example, metamorphic
zircons in UHP eclogites from Qinglongshan in the Sulu orogen exhibit low or negative δ18O values (Fig. 5b), because the protoliths of UHP metabasites there underwent meteoric hydrothermal alteration in the middle Neoproterozoic (Zheng et al., 2003; Chen et al., 2011). Metamorphic zircons may exhibit higher 18O values than normal mantle values of 5.3±0.3‰ (Valley et al., 1998) if they are produced by metamorphism from supracrustal rocks, otherwise they show lower 18O values than normal values if they are generated by metamorphism from intracrustal rocks that underwent low 18O fluid alteration.
eratures Dubińska et a ., 2004), so that the aqueous solutions generated by local sinking of the
liberated hydroxyls are highly alkaline and thus oxidized (Zheng et al., 2007b). It is an efficient agent to dissolve and transport Zr and Si from relevant minerals for the growth of metamorphic zircons (Zheng, 2009). In addition, rutile is an important reservoir of Zr in eclogites (Sassi et al., 2000; Zack et al., 2002). Rutile may release a lot of Zr due to retrograde recrystallization of
eclogites under HP eclogite-facies conditions.
4.2 Peritectic zircon Partial melting of UHP metamorphic rocks is common not only in the Dabie-Sulu orogenic belt but also elsewhere in the other collisional orogens (e.g., Zheng et al., 2011a; Gao et al., 2012; Liu et al., 2012; Chen et al., 2013a, 2013b; Gordon et al., 2013; Li et al., 2013, 2014, 2016a; Gilotti et al., 2014). It occurs at varying scales from thin sections to outcrops, forming different types of migmatites. Anatectic melts were produced through peritectic reactions above the wet solidus of crustal rocks. Alkalic to granitic dykes and intrusions of synexhumation age are highly evolved products of the anatectic melts (Yang et al., 2005; Liu et al., 2009c; Zhao et al., 2012). Peritectic zircons primarily occur in migmatitic rocks and pegmatites (Wallis et al., 2005; Liu et al., 2009a, 2010, 2012; Zong et al., 2010; Zeng et al., 2011; Chen et al., 2013a, 2013b; Xu et al., 2013), whereas magmatic zircons typically occur in synexhumation magmatic intrusions (Zhao et al., 2012). Because of the low Zr solubility in anatectic melts, zircon is rarely crystallized from the anatectic melts unless the anatectic melts have experienced given degrees of evolution with fractional crystallization of Zr-poor minerals from the melts. Like magmatic zircons, anatectic zircons can only be crystallized from the anatectic melts in which the local Zr oversaturation was achieved during their evolution. While peritectic zircons would generally grow above the wet solidus with increasing temperature, the anatectic zircons would only grow along a temperature decrease path with temperartures on the wet solidus (Fig. 6). In this regard, peritectic zircons would generally occur in migmatitic rocks and pegmatites, whereas anatectic zircons can only occur in the leucosome and pegmatite which have underwent large extent of fractional crystallization along the temperature decrease path.
Magmatic zircons generally show oscillatory zoning, high REE contents and steep HREE patterns, high Th/U (>0.1) and Lu/Hf ratios, positive Ce and negative Eu anomalies. In contrast, peritectic zircons usually exhibit weak zonation (no zoning to cloudy zoning, occasionally oscillatory zoning) (Fig. 7) and low Th/U ratios (generally <0.1), containing multiphase solid inclusions of Qtz±Ab±Kfs±Ap. Compared to the protolith zircons of magmatic origin, the peritectic zircons often exhibits similar or lower REE contents (Fig. 4), variable REE patterns (Fig. 8), lower or similar Th and U contents, higher Hf contents, similar or higher Nb and Ta contents, similar Ce and Eu anomalies, similar or lower Nb/Ta ratios, similar or lower Lu/Hf ratios but higher 176
Hf/177Hf ratios (Figs. 4 and 5). The U-Pb dating indicates that the anatexis of UHP metamorphic
rocks primarily takes place at high-T/HP granulite-facies conditions during the early exhumation.
The partial melting of UHP metamorphic rocks are generally ascribed to the HP granulite-facies overprinting during “hot” exhumation of the deeply subducted continental crust (Zheng et al., 2011a). It is expected that zircon grown under granulite-facies conditions would exhibit negative Eu anomalies and flat REE patterns due to the stability of both plagioclase and garnet (Whitehouse and Platt, 2003). However, peritectic zircons from the North Qaidam orogen in northern Tibet show large variations in mineragraphy and trace element composition (e.g., Chen et al., 2012b; Yu et al., 2014, 2015; Zhang et al., 2015). Peritectic zircons in HP granulites from Dulan in North Qaidam exhibit no zoning, irregular zoning, well-developed sector zoning or oscillatory zoning, variably high Th/U ratios (generally >0.1), weakly negative Eu anomalies and relatively flat REE patterns (Fig. 9b) and contain inclusions of Grt + Qtz + Cpx + Pl + Rt + Ap (Yu et al., 2014). In contrast, peritectic zircons from leocosomes within UHP rocks from Xitieshan and Lüliangshan in North Qaidam exhibit weak zoning or oscillatory zoning, low Th/U ratios <0.1 and flat to steep HREE patterns (Fig. 9b), and contain mineral inclusions of quartz + feldspar + apatite (Chen et al., 2012b; Yu et al., 2015; Zhang et al., 2015). Despite the large difference of trace elements, these peritectic zircons have no large difference in their U-Pb ages, suggesting their growth under similar P-T conditions during continental collision (Zhang et al., 2015). The part of peritectic zircons shows flat HREE patterns and is generally associated with a significant amount of garnet in the leucosome. This indicates the concurrent growth of peritectic garnet and thus the active presence of garnet, which is confirmed by identification of mineral inclusions in these peritectic zircons. The other part of peritectic zircons exhibits steep HREE patterns and occurs in migmatite or leucosome, in which HP/UHP metamorphic zircons also exhibit steep HREE patterns, consistent with the relative lack of garnet in the host rocks. The significant depletion of LREE in peritectic zircons is usually ascribed to the presence of LREE-rich minerals such as allanite, monazite and epidote during zircon growth. However, peritectic zircons from the leucosomes without these minerals also exhibit marked depletion in LREE. In this regard, the composition of peritectic zircons is primarily controlled not only by the composition of reactants but also by that of other peritectic minerals during crustal anatexis. In the Western Gneiss Region of Norway, peritectic zircons with concordant U-Pb ages of 410-406 Ma, coeval with peak or near-peak UHP metamorphism, exhibit low Th/U ratios and flat HREE patterns with lack of negative Eu anomalies. This indicates their growth in association with the concurrent growth of peritectic garnet. The other zircons, including those from crosscutting pegmatite and showing younger U-Pb ages consistent with dates for amphibolite-facies retrogression, exhibit steep HREE patterns and negative Eu anomalies (Gordon et al., 2013). This suggests that their growth is associated with the plagioclase crystallization but with the garnet inertness. The latter observation indicates that peritectic zircons record the transition from UHP
eclogite-facies (garnet-active) to lower P (plagioclase-active) conditions. In this regard, the composition of peritectic zircons is related not only to the metamorphic conditions but also to the activity of coexisting minerals. The composition of peritectic zircons produced at all stages of exhumation changes with the P-T conditions (Fig. 9a), which has also been found in the North-East Greenland (Gilotti et al., 2014).
Hf/177Hf ratios (Figs. 4 and 5). The U-Pb dating indicates that the anatexis of UHP metamorphic
rocks primarily takes place at high-T/HP granulite-facies conditions during the early exhumation.
The partial melting of UHP metamorphic rocks are generally ascribed to the HP granulite-facies overprinting during “hot” exhumation of the deeply subducted continental crust (Zheng et al., 2011a). It is expected that zircon grown under granulite-facies conditions would exhibit negative Eu anomalies and flat REE patterns due to the stability of both plagioclase and garnet (Whitehouse and Platt, 2003). However, peritectic zircons from the North Qaidam orogen in northern Tibet show large variations in mineragraphy and trace element composition (e.g., Chen et al., 2012b; Yu et al., 2014, 2015; Zhang et al., 2015). Peritectic zircons in HP granulites from Dulan in North Qaidam exhibit no zoning, irregular zoning, well-developed sector zoning or oscillatory zoning, variably high Th/U ratios (generally >0.1), weakly negative Eu anomalies and relatively flat REE patterns (Fig. 9b) and contain inclusions of Grt + Qtz + Cpx + Pl + Rt + Ap (Yu et al., 2014). In contrast, peritectic zircons from leocosomes within UHP rocks from Xitieshan and Lüliangshan in North Qaidam exhibit weak zoning or oscillatory zoning, low Th/U ratios <0.1 and flat to steep HREE patterns (Fig. 9b), and contain mineral inclusions of quartz + feldspar + apatite (Chen et al., 2012b; Yu et al., 2015; Zhang et al., 2015). Despite the large difference of trace elements, these peritectic zircons have no large difference in their U-Pb ages, suggesting their growth under similar P-T conditions during continental collision (Zhang et al., 2015). The part of peritectic zircons shows flat HREE patterns and is generally associated with a significant amount of garnet in the leucosome. This indicates the concurrent growth of peritectic garnet and thus the active presence of garnet, which is confirmed by identification of mineral inclusions in these peritectic zircons. The other part of peritectic zircons exhibits steep HREE patterns and occurs in migmatite or leucosome, in which HP/UHP metamorphic zircons also exhibit steep HREE patterns, consistent with the relative lack of garnet in the host rocks. The significant depletion of LREE in peritectic zircons is usually ascribed to the presence of LREE-rich minerals such as allanite, monazite and epidote during zircon growth. However, peritectic zircons from the leucosomes without these minerals also exhibit marked depletion in LREE. In this regard, the composition of peritectic zircons is primarily controlled not only by the composition of reactants but also by that of other peritectic minerals during crustal anatexis. In the Western Gneiss Region of Norway, peritectic zircons with concordant U-Pb ages of 410-406 Ma, coeval with peak or near-peak UHP metamorphism, exhibit low Th/U ratios and flat HREE patterns with lack of negative Eu anomalies. This indicates their growth in association with the concurrent growth of peritectic garnet. The other zircons, including those from crosscutting pegmatite and showing younger U-Pb ages consistent with dates for amphibolite-facies retrogression, exhibit steep HREE patterns and negative Eu anomalies (Gordon et al., 2013). This suggests that their growth is associated with the plagioclase crystallization but with the garnet inertness. The latter observation indicates that peritectic zircons record the transition from UHP
eclogite-facies (garnet-active) to lower P (plagioclase-active) conditions. In this regard, the composition of peritectic zircons is related not only to the metamorphic conditions but also to the activity of coexisting minerals. The composition of peritectic zircons produced at all stages of exhumation changes with the P-T conditions (Fig. 9a), which has also been found in the North-East Greenland (Gilotti et al., 2014).
Usually, the low abundances of trace elements in the metamorphic zircon relative to the magmatic zircon are ascribed to the effect of concurrently grown or recrystallized, specific minerals such as garnet, epidote and rutile. However, the experimental study of Rubatto and Hermann (2007b) for REE partition between zircon, melt and garnet indicates that REE (especially HREE) were preferentially incorporated into zircon rather than garnet when both minerals crystallized from melts at temperatures below 850 °C. Such a temperature effect is of critical importance to petrogenetic interpretation of peritectic, anatectic and magmatic zircons. Thermodynamic equilibrium partition of trace elements between coexisting minerals is also independent of relative proportions between different minerals, implying that even if a large amount of garnet is present during zircon growth or recrystallization in a closed system of anatectic/magmatic melts at the temperatures below 850 °C, the resultant zircon does not become HREE-depleted relative to the coexisting garnet. In this regard, it is necessary to distinguish the magmatic minerals from peritectic minerals in their compositions. Because different minerals may be involved in partial melting via peritectic reactions, the variation of trace element abundances in peritectic zircons may be caused by variable involvement of different reactants in the crustal anatexis. Therefore, it is critical to determine the trace element partition between peritectic zircon and other peritectic minerals at different P-T conditions. Most peritectic, anatectic and magmatic zircons of synexhumation growth exhibit higher 176
Hf/177Hf ratios than the relict cores of protolith zircon (Liu et al., 2009c, 2009d, 2010a; Zhao et
al., 2012; Chen et al., 2013b), suggesting that the anatexis is associated with decomposition or recrystallization of high Lu/Hf minerals in the UHP metamorphic rocks. Although the peritectic, anatectic and magmatic zircons from migmatites, pegmatites and plutonic rocks show significant differences in element contents and ratios, most of them show similar Hf isotopic compositions and thus suggest the similar origins. Therefore, they record the partial melting of UHP metamorphic rock with petrological evolution from the anatectic melt to the magmatic melt during the exhumation of deeply subducted continental crust. Sometimes different generations of peritectic, anatectic and magmatic zircons occur in the same samples (Liu et al., 2009c, 2009d, 2010a, 2012; Chen et al., 2013b). Nevertheless, they often exhibit different Hf and O isotope compositions (Liu et al., 2009d; Chen et al., 2013b), implying that their growth is associated with different origins of
anatectic and magmatic melts. Anatexis of the UHP metamorphic rocks mainly occurs during the exhumation of deeply subducted continental crust (Zheng et al., 2011a), corresponding to the stage of HP granulite-facies overprinting. In fact, the anatexis would have started at the late UHP eclogite-facies stage during the initial exhumation. This has been demonstrated by the following observations: (1) coesite inclusions occur in some domains of the peritectic zircon from the Dabie-Sulu orogenic belt (Chen et al., 2013a, 2013b); (2) leucosomes from the Western Gneiss Region show similar compositions to the experimental melts produced at UHP conditions (Labbrouse et al., 2011); (3) peritectic zircons with concordant U-Pb ages of 410-406 Ma, coeval with peak or near-peak UHP metamorphism, have been found in some leucosomes within UHP rocks in the Western Gneiss Region (Gordon et al., 2013). Furthermore, crustal anataxis in the Kokchetav and Bohemian UHP terranes has temperatures as high as 1000C in the diamond stability field (e.g., Shatsky et al., 1999; Kotkova et al., 2016; Stepanov et al., 2016). The resulted peritectic zircons exhibit sector, fir-tree or oscillatory zoning, high Th/U ratios of >0.1, flat to steep HREE patterns with weakly negative or no Eu anomalies (Fig. 9c). This suggests that the breakdown of Th-rich hydrous minerals is significant whereas the metamorphic garnet is partially activated during the anatexis. For the anataxis during exhumation, with decreasing temperatures, anatectic melts may undergo large extent of fractional crystallization to achieve the local Zr oversaturation. In this regard, anatectic zircons would also grow besides peritectic zircons in felsic veins, especially in the pegmatites which generally have the largest extent of fractional crystallization. This can explain the observed large variation in zircon element compositions and U-Pb ages. However, these anatectic zicons do not show large differences in trace element composition from the peritectic zircons (Figs. 4 and 7). The composition of anatectic zircons are controlled by the composition of evolved melts and concurrent growth minerals. On the other hand, UHP metamorphic rocks may suffer partial melting during their final subduction into the UHP regime, which has been documented in the Dabie-Sulu orogenic belt (Xia et al., 2013; Li et al., 2014, 2016a). Peritectic zircons produced by this stage of crustal anatexis generally exhibit dark luminescent and no zoning in CL images, variable Th/U ratios, flat to steep HREE patterns with variable Eu anomalies (Fig. 9d). Coesite and multiple phase solid inclusions were found in some of these peritectic zircons (Li et al., 2014, 2016a), suggesting at least part of them growth at UHP conditions. Nevertheless, the occurrence of flat to steep HREE patterns in these peritectic zircons suggests discontinuous growth of the peritectic garnet in this period. The UHP metamorphic rocks, especially for those in the high-T/UHP zone from the Dabie-Sulu orogenic belt, also underwent partial melting in the postcollisional stage at pressures below 1.0 GPa, resulting in extensive migmatization and bimodal magmatism (e.g., Zhao and Zheng, 2009; Chen et
al., 2015a). Peritectic zircons of this stage exhibit unzoning, weak zoning and oscillatory zoning, variable Th/U ratios (from <0.1 to >1) and steep HREE patterns (Chen et al., 2015). The steep HREE patterns are consistent with the low-P anatexis, whereas the variable Th/U ratios may be related to the differential involvement of Th- and U- rich minerals such as allanite, epidote and monazite. In summary, peritectic zircons in UHP metamorphic rocks can grow at all stages of continental collision from the final subduction via the peak UHP metamorphism to the initial exhumation, with additional growth at the postcollisional stage. As a consequence, peritectic zircons may exhibit large variations in CL structure from unzoning to oscillatory zoning, in Th/U ratios from <0.1 to >1.0, in REE contents from low to high, in HREE patterns from flat to steep, and in Eu anomalies from no to negative. The variations in the composition of peritectic zircons are primarily related not only to anatectic P-T conditions but also to product mineral species in peritectic reactions. In addition, it is also influenced by the activity of preexisting minerals, either reactant or product. As a consequence, the peritectic zircons may exhibit variable compositions between metamorphic and magmatic zircons. Despite the large variations in element contents and ratios, they generally exhibit higher
176
Hf/177Hf ratios than the inherited magmatic zircons (Fig. 5a), indicating the involvement
of HFSE- and HREE-rich minerals other than zircon in the crustal anatexis.
4.3 Distinction between metamorphic and peritectic zircon Both metamorphic and peritectic zircons show large variations in CL imaging, Th/U ratios, and trace element contents and ratios, depending on P-T conditions and minerals involved in breakdown, recrystallization and growth. This often results in similar geochemical compositions between some metamorphic and peritectic zircons (Fig. 4). Nevertheless, the two types of zircons can be distinguished by a study of zirconology, including CL images for mineragraphy (structure and morphology), U-Pb ages, trace element contents and ratios, stable and radiogenic isotope compositions, and geothermobarometries (Table 1). Lots of important information can be obtained by such a study for zircons from the same samples. Both metamorphic and peritectic zircons exhibit variable CL images. Although most peritectic zircons generally show no zoning in CL images similar to common metamorphic zircons, oscillatory zoning, which is typical for magmatic zircons, often occurs in the peritectic zircons but is rarely observed in metamorphic zircons (e.g., Rubatto and Hermann, 2003). This difference is evident in the peritectic zircons in leucosome and pegamatite relative to the metamorphic zircons in quartz veins (Figs. 2 and 6). Although metamorphic minerals are common in both types of zircons, fluid inclusions only occur in metamorphic zircons (e.g., Wu et al., 2009) whereas melt inclusions
Hf/177Hf ratios than the relict cores of protolith zircon (Liu et al., 2009c, 2009d, 2010a; Zhao et
al., 2012; Chen et al., 2013b), suggesting that the anatexis is associated with decomposition or recrystallization of high Lu/Hf minerals in the UHP metamorphic rocks. Although the peritectic, anatectic and magmatic zircons from migmatites, pegmatites and plutonic rocks show significant differences in element contents and ratios, most of them show similar Hf isotopic compositions and thus suggest the similar origins. Therefore, they record the partial melting of UHP metamorphic rock with petrological evolution from the anatectic melt to the magmatic melt during the exhumation of deeply subducted continental crust. Sometimes different generations of peritectic, anatectic and magmatic zircons occur in the same samples (Liu et al., 2009c, 2009d, 2010a, 2012; Chen et al., 2013b). Nevertheless, they often exhibit different Hf and O isotope compositions (Liu et al., 2009d; Chen et al., 2013b), implying that their growth is associated with different origins of
anatectic and magmatic melts. Anatexis of the UHP metamorphic rocks mainly occurs during the exhumation of deeply subducted continental crust (Zheng et al., 2011a), corresponding to the stage of HP granulite-facies overprinting. In fact, the anatexis would have started at the late UHP eclogite-facies stage during the initial exhumation. This has been demonstrated by the following observations: (1) coesite inclusions occur in some domains of the peritectic zircon from the Dabie-Sulu orogenic belt (Chen et al., 2013a, 2013b); (2) leucosomes from the Western Gneiss Region show similar compositions to the experimental melts produced at UHP conditions (Labbrouse et al., 2011); (3) peritectic zircons with concordant U-Pb ages of 410-406 Ma, coeval with peak or near-peak UHP metamorphism, have been found in some leucosomes within UHP rocks in the Western Gneiss Region (Gordon et al., 2013). Furthermore, crustal anataxis in the Kokchetav and Bohemian UHP terranes has temperatures as high as 1000C in the diamond stability field (e.g., Shatsky et al., 1999; Kotkova et al., 2016; Stepanov et al., 2016). The resulted peritectic zircons exhibit sector, fir-tree or oscillatory zoning, high Th/U ratios of >0.1, flat to steep HREE patterns with weakly negative or no Eu anomalies (Fig. 9c). This suggests that the breakdown of Th-rich hydrous minerals is significant whereas the metamorphic garnet is partially activated during the anatexis. For the anataxis during exhumation, with decreasing temperatures, anatectic melts may undergo large extent of fractional crystallization to achieve the local Zr oversaturation. In this regard, anatectic zircons would also grow besides peritectic zircons in felsic veins, especially in the pegmatites which generally have the largest extent of fractional crystallization. This can explain the observed large variation in zircon element compositions and U-Pb ages. However, these anatectic zicons do not show large differences in trace element composition from the peritectic zircons (Figs. 4 and 7). The composition of anatectic zircons are controlled by the composition of evolved melts and concurrent growth minerals. On the other hand, UHP metamorphic rocks may suffer partial melting during their final subduction into the UHP regime, which has been documented in the Dabie-Sulu orogenic belt (Xia et al., 2013; Li et al., 2014, 2016a). Peritectic zircons produced by this stage of crustal anatexis generally exhibit dark luminescent and no zoning in CL images, variable Th/U ratios, flat to steep HREE patterns with variable Eu anomalies (Fig. 9d). Coesite and multiple phase solid inclusions were found in some of these peritectic zircons (Li et al., 2014, 2016a), suggesting at least part of them growth at UHP conditions. Nevertheless, the occurrence of flat to steep HREE patterns in these peritectic zircons suggests discontinuous growth of the peritectic garnet in this period. The UHP metamorphic rocks, especially for those in the high-T/UHP zone from the Dabie-Sulu orogenic belt, also underwent partial melting in the postcollisional stage at pressures below 1.0 GPa, resulting in extensive migmatization and bimodal magmatism (e.g., Zhao and Zheng, 2009; Chen et
al., 2015a). Peritectic zircons of this stage exhibit unzoning, weak zoning and oscillatory zoning, variable Th/U ratios (from <0.1 to >1) and steep HREE patterns (Chen et al., 2015). The steep HREE patterns are consistent with the low-P anatexis, whereas the variable Th/U ratios may be related to the differential involvement of Th- and U- rich minerals such as allanite, epidote and monazite. In summary, peritectic zircons in UHP metamorphic rocks can grow at all stages of continental collision from the final subduction via the peak UHP metamorphism to the initial exhumation, with additional growth at the postcollisional stage. As a consequence, peritectic zircons may exhibit large variations in CL structure from unzoning to oscillatory zoning, in Th/U ratios from <0.1 to >1.0, in REE contents from low to high, in HREE patterns from flat to steep, and in Eu anomalies from no to negative. The variations in the composition of peritectic zircons are primarily related not only to anatectic P-T conditions but also to product mineral species in peritectic reactions. In addition, it is also influenced by the activity of preexisting minerals, either reactant or product. As a consequence, the peritectic zircons may exhibit variable compositions between metamorphic and magmatic zircons. Despite the large variations in element contents and ratios, they generally exhibit higher
176
Hf/177Hf ratios than the inherited magmatic zircons (Fig. 5a), indicating the involvement
of HFSE- and HREE-rich minerals other than zircon in the crustal anatexis.
4.3 Distinction between metamorphic and peritectic zircon Both metamorphic and peritectic zircons show large variations in CL imaging, Th/U ratios, and trace element contents and ratios, depending on P-T conditions and minerals involved in breakdown, recrystallization and growth. This often results in similar geochemical compositions between some metamorphic and peritectic zircons (Fig. 4). Nevertheless, the two types of zircons can be distinguished by a study of zirconology, including CL images for mineragraphy (structure and morphology), U-Pb ages, trace element contents and ratios, stable and radiogenic isotope compositions, and geothermobarometries (Table 1). Lots of important information can be obtained by such a study for zircons from the same samples.