Journal of Earth Science  2018, Vol. 29 Issue (5): 1116-1131   PDF    
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Metamorphic P-T-t Path of UHT Granulites from the North Tongbai Orogen, Central China
Hua Xiang1, Zeming Zhang1,2, Liming Zhao3, Zengqiu Zhong3, Hanwen Zhou3    
1. Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;
3. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate-to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature (P-T) conditions. In this study, zircon trace elements and Hf isotopes and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon in the granulites from North Tongbai Orogen, Central China. Combined with previous metamorphic P-T path results, a quantitative integrated anticlockwise P-T-t path was established for Tongbai granulites. These granulites recorded an early low-P heating followed by a dramatic pressure increase. Evidence for the prograde history (M1) is provided by cordierite, orthopyroxene and biotite inclusions in garnet. The prograde metamorphism occurred at around 443±3 Ma, with P-T conditions of ca. 730-820℃ and < 6 kbar. The peak metamorphic (M2) condition is >920℃ and 8.5-10 kbar and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered an amphibolite-facies retrograde metamorphism (M3), represented by the replacement of garnet by biotite and plagioclase, and clinopyroxene by amphibole, with metamorphic condition of ca. 700℃ and ca. 7 kbar. The last retrograde metamorphism (M4) is a greenschist-facies overprint with an age of ca. 404 Ma. It is concluded that the metamorphism of Tongbai granulite lasted for more than 40 Ma, including a stage of more than 20 Ma granulite-facies metamorphism. The prolonged granulite-facies metamorphism resulted from the continuous northward subduction of the Shangdan oceanic crust beneath the North Qinling terrane.
KEY WORDS: granulites    zircon U-Pb dating    P-T-t path    trace element    Lu-Hf isotope    Tongbai Orogen    

0 INTRODUCTION

The P-T-t path of metamorphic rocks, generally controlled by tectonic processes of burial and exhumation of rocks at convergent plate boundaries (e.g., Spear, 1993; England and Thompson, 1984), is a key to identify different tectonic models. Linking radiometric ages to metamorphic P-T conditions is key to constrain P-T-t paths, but also difficult. Zircon is an ideal mineral for dating the high-grade metamorphic rocks (Rubatto and Hermann, 2003; Ayers et al., 2002). Metamorphic zircon can grow in different stages of high-grade metamorphism, resulted in uncertainty to determine its relationship to rock-forming mineral paragenetic sequence. Experimental petrology demonstrates that the REE composition and partitioning between zircon and garnet can be used to correlate of zircon and garnet growth (Rubatto and Hermann, 2007). Garnet is the most widely used mineral in geothermobarometer. Therefore, it is possible to link zircon U-Pb ages to metamorphic P-T conditions. Additionally, zircon Lu-Hf isotopes can be used to trace the source and nature of the material in which the zircon grew (Griffin et al., 2004, 2002).

The Qinling-Tongbai-Dabie Orogen, resulted from convergence between South China Block and North China Block, is well-known for its complex evolution, which is characterized by Paleozoic–Mesozoic long duration (over ca. 300 Ma) and multistage amalgamation (Xiang et al., 2014, 2012; Liu et al., 2013; Wang et al., 2013, 2011; Wu et al., 2009; Zheng et al., 2005; Ratschbacher et al., 2003; Li et al., 2000). The Tongbai Orogen is located in the middle of Qinling-Tongbai-Dabie-Sulu orogenic belt, and contains both Paleozoic ultrahigh temperature (UHT)-high temperature (HT) granulites and associated magmatic rocks (Fig. 1a). The petrology and mineralogy of the studied granulites suggest that they record at least four metamorphic stages, and have an anticlockwise metamorphic P-T path with peak condition of ca. 9 kbar and 850–920 ℃ (Xiang et al., 2014, 2012). The metamorphic ages have been limited between ca. 440 and ca. 400 Ma (Xiang et al., 2014, 2012; Liu et al., 2011a; Wang et al., 2011). However, the precise P-T-t evolution of Tongbai granulites has not been well determined, and the precise age of each metamorphic stage has been debated (Xiang et al., 2014, 2012; Liu et al., 2011a; Wang et al., 2011). In this paper, petrology, U-Pb age, trace elements, and Lu-Hf analysis for zircons and trace elements for garnet were used to date different generation zircons and to link the age to various stages of metamorphism. An Early Paleozoic anticlockwise P-T-t path of the granulites is established. These results will provide a sample of linking U-Pb ages to various metamorphic stages by trace element and Hf isotopes in zircon, and shed light on the Early Paleozoic tectonic evolution of Tongbai Orogen.

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Figure 1. Simplified geological map of the Tongbai Orogen and adjacent areas. (a) Sketch geological map of the Qinling-Dabie-Sulu orogenic belt, showing the studied area. NQ. North Qinling; SQ. South Qinling; NTB. North Tongbai; STB. South Tongbai; WDB. West Dabie; EDB. East Dabie. (b) Sketch geological map of the Tongbai Orogen (modified after Liu X C et al., 2008). 1. Kuangping Group; 2. Erlangping Group; 3. Qinling Group; 4. Xinyang Group; 5. Tongbai complex; 6. Early Paleozoic diorite; 7. Early Paleozoic granite; 8. Cretaceous granite. (c) Simplified geological map of the study area (modified after Zhang et al., 2003).
1 GEOLOGICAL SETTING AND PETROLOGY 1.1 Geological Setting

The Tongbai Orogen marks the junction of the Qinling Orogen and the Dabie-Sulu Orogen (Figs. 1a, 1b). This orogen is divided into two orogenic belts by Songba fault. The north belt is a Paleozoic accretionary orogen, and the south belt is a Permo–Triassic collisional orogen. The North Tongbai belt mainly includes three Early Paleozoic metamorphic units: the Qinling Group, the Erlangping Group and the Kuanping Group from southwest to northeast (Fig. 1c). These metamorphic units are separated from each other by large-scale shear zones (Suo et al., 2001; Zhong et al., 2001, 1999).

The Kuanping Group is a suit metamorphosed tholeiitic basalts, clastic and carbonate sedimentary rocks, which deposited during the Neoproterozoic (Liu et al., 2013). The Kuanping Group can be divided into the lower unit and the upper unit by lithological assemblage and structure. The Kuanping Group is generally subjected to amphibolite-facies metamorphism with a clockwise P-T path (Liu et al., 2011a). For the peak metamorphic P-T condition, the upper unit of Kuanping Group is 6.6–8.9 kbar and 630–650 ℃, and the lower unit has relatively low temperature and high pressure of 9.3–11.2 kbar and 570–610 ℃ (Liu et al., 2011a). SHRIMP U-Pb zircon dating on a garnet amphibolite yielded two metamorphic ages of 442±6 and 415±5 Ma (Liu et al., 2011a), and an amphibole 40Ar/39Ar ages of 434±2 Ma obtained from an amphibolite (Zhai et al., 1998).

The Erlangping Group is a metamorphosed volcano- sedimentary sequence. It is composed of mafic to felsic metavolcanics, fine-grained clastic metasedimentary rocks and marbles intercalated with cherts (Liu et al., 2013; Sun et al., 2002; Kröner et al., 1993). The protolith of the Erlangping complex are suggested to form in an Early Ordovician (ca. 490–470 Ma) intra-oceanic arc or a backarc basin (Wu and Zheng, 2013; Ratschbacher et al., 2006, 2003; Sun et al., 2002; Meng and Zhang, 2000; Okay et al., 1993). The Erlangping Group suffered greenschist to amphibolite facies metamorphism. The peak P-T condition is 6.3–7.7 kbar and 550–600 ℃ (Liu et al., 2011a). Two metamorphic zircon ages of 440±3 and 394±5 Ma of an amphibolite from the Erlangping Group are interpreted as amphibolite facies metamorphic age and low-grade metamorphic- deformational age respectively (Liu et al., 2011a).

The Qinling Group consists of three suites of rock assemblages (Fig. 1c). The lower suite comprises felsic, mafic and pelitic granulites, and granitic gneisses; the middle suite consists of gneisses interleaved with minor amphibolites and dolomitic marbles; and the upper suite is dominated by marbles interleaved with minor amphibolites. The Qinling Group is the highest grade metamorphic unit in the north Tongbai Orogen. It represents HT metamorphism with an anticlockwise P-T path. Based on different methods, variable peak metamorphic P-T conditions have been estimated, from 5.5–7 kbar and 600–700 ℃ to 8.0–9.5 kbar and 850–920 ℃ (Xiang et al., 2012, 2009; Liu et al., 2011b; Zhai et al., 1998; Kröner et al., 1993). Recently, numerous zircon U-Pb dating results indicate that the granulite-facies metamorphic age is 440–400 Ma (Table 1; Xiang et al., 2014, 2012, 2009; Wu and Zheng, 2013; Liu et al., 2011a; Wang et al., 2011; Zhang et al., 1998; Kröner et al., 1993).

Table 1 Isotopic age of granulites and associated rocks from Qinling Group, Tongbai Orogen

This study deals with granulites from the lower suite of Qinling Group in the Tongbai orogeny. The lower suite of Qinling Group includes felsic granulite, pelitic granulites, mafic granulites, gneisses and leucosome veins (pegmatite). Granulites are exposed as enclaves in gneisses or interbed with gneisses. Numerous sheeted granites and granitic or pegmatite veins intruded these rocks. Four samples dated in this study include two semi-pelitic granulites (Tb-19 and Tb-23), one mafic granulite (Tb-27) and one pegmatite (Q0938).

1.2 Petrology and P-T Path of Tongbai Granulites

The semi-pelitic granulites consist mainly of garnet, orthopyroxene, biotite, plagioclase, K-feldspar, quartz, ilmenite or/and rutile, with or without cordierite. The sample Tb-19 is characterized by high contents of garnet (ca. 30%), orthopyroxene (20%–25%) and lesser biotite (ca. 5%). The garnet porphyroblasts include inclusions of Bt+Crd+Opx+Pl+Qz (Figs. 2a2c), which represent the prograde metamorphic mineral assemblage. The biotite-rich granulite (sample Tb-23) are distinguished from sample Tb-19 by having lower modes of garnet (ca. 5%), orthopyroxene (ca. 15%), and higher modes of biotite (ca. 40%) and K-feldspar (ca. 5%). Some garnet grains have partly been decomposed to biotite and plagioclase (Figs. 2e2f).

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Figure 2. Photomicrographs showing typical textures of Tongbai granulites. (a) Mineral assemblage of Grt+Opx+Pl+Bt+Ilm from sample Tb-19 (PPL); (b) garnet porphyroblast with inclusions of Crd+Opx+Bt in sample Tb-19 (PPL); (c) the textural aspect of the matrix in sample Tb-19, with granoblastic aggregates mainly consisted of orthopyroxene, garnet, plagioclase, biotite (PPL); (d) the rim of ilmenite is replaced by rutile in sample Tb-19 (BSE image); (e) the textural aspects of the sample Tb-23, with porphyroblastic aggregates; (f) the garnet was replaced by fine-grained Bt+Pl (sample Tb-23, CPL); (g) sapphirine+plagioclase+orthopyroxene in sapphirine-bearing granulite; (h) spinel occurs as inclusion in sapphirine from sapphirine-bearing granulite; (i) photomicrographs of two pyroxene granulite; (j) later amphibole occurs in a fine-grained (< 0.5 mm) symplectite as coronas, separating orthopyroxene and plagioclase (sample Tb-27; BSE image); (k) the spatial relationships among clinopyroxene, amphibole, chlorite and epidote from mafic retrogressed granulite (sample Tb-22; PPL). (a)–(f), (j)–(l) after Xiang et al. (2012); (g), (h) after Xiang et al. (2014).

Mafic granulites from Qinling Group include sapphirine- bearing granulites (Figs. 2g2h), garnet-two pyroxene granulite, two pyroxene granulite (Fig. 2i), garnet-amphibole two pyroxene granulites (Figs. 2j2l). The dated sample Tb-27 consists of amphibole, plagioclase, orthopyroxene and ilmenite. Some orthopyroxenes near plagioclase often have reaction rim of fine- grained amphibole (Fig. 2k). The granulites generally have recorded varying degrees of retrograde metamorphism. Some samples suffered intense green schist facie retrograde metamorphism, which consists of clinopyroxene, plagioclase, amphibole, chlorite and epidote (Fig. 2l). The pegmatite consists of K-feldspar (ca. 40%), plagioclase (ca. 10%), quartz (ca. 45%) and mica (ca. 5%).

Detailed study on petrography, mineral chemistry and phase equilibrium modeling for the dated samples were carried out (Xiang et al., 2012). In sample Tb-19, cordierite-bearing assemblage of Opx+Crd+Bt+Pl+Ilm represents the prograde metamorphic assemblage (M1). The result of phase equilibrium calculation shows the cordierite-bearing assemblage only stable at the low-pressure field of 4–6 kbar and 730–820 ℃ (Xiang et al., 2012). The temperature of peak-metamorphic stage (M2) is estimated at an UHT condition of up to 920 ℃ and ca. 9.2 kbar. In sample Tb-23, the post peak retrograde stage (M3) is characterized by garnet consumption to be biotitle+plagioclase with P-T condition of 6–7 kbar and ~700 ℃. This result indicates a near isobaric cooling after the peak metamorphic stage (M2). The rutile coronas around ilmenite in sample Tb-19 also indicated a near-isobaric cooling P-T path. The greenschist facies retrograde (M4) conditions of the retrograde mafic granulites were constrained to ca. 5.8 kbar and ca. 500 ℃ (Xiang et al., 2012).

Liu et al. (2011a) also reported that Tongbai granulites have an anticlockwise metamorphic P-T path. Xiang et al. (2014) reported sapphirine-bearing granulites in the Qinling Group, providing robust evidence for crustal UHT metamorphism. The result of Zr-in-rutile geothermometry indicates that peak temperature is above 890–940 ℃. The calculated pseudo-sections show that the early spinel-bearing assemblage is stable at a low-pressure condition of 6.7–8.9 kbar and 923–950 ℃, and the peak sapphirine- bearing assemblage is stable at relatively high-pressure conditions of 8.4–10.2 kbar and 922–947 ℃ (Xiang et al., 2014). Overall, the Tongbai granulite experienced the UHT (> 930–1 000 ℃) peak- metamorphism. Thus, in North Tongbai Orogen, the Qinling Group shows distinctive HT-UHT metamorphism with an anticlockwise P-T path.

2 ANALYTICAL TECHNIQUES

Zircons were imaged by cathodoluminescence (CL) using a scanning electron microscope (Quanta 400 FEG) at Northwest University, Xi'an, China. Zirconium U-Pb dating and mineral trace element analyses were carried out using an LA-ICPMS (Geolas 193 nm ArF Excimer laser ablation+Agilent 7500a) at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan (GPMR, CUG). In zircon U-Pb dating analyses, spot size was 32 μm and a repetition rate was 5 Hz. In mineral trace elements analyses, spot size was 24 μm and repetition rate was 5 Hz. Zircon standard 91500 was used to calibrate isotopic ratio during analysis. Element contents were calibrated against NIST610 (Liu et al., 2010). Operating conditions and analytical method for the LA-ICPMS instrument are the same as described by (Liu Y S et al., 2010, 2008).

Zircon Lu-Hf isotopes analyses were carried out using a LA-MC-ICP-MS (Neptune Plus, Thermo Fisher Scientific, Germany) at GPMR, CUG. The analyses were implemented with a spot size of 44 μm, a 10 Hz repetition rate. Zircon 91500 was used as the reference standard (Woodhead et al., 2004). Detailed operating conditions and analytical method are the same as description by (Hu et al., 2012).

3 RESULTS 3.1 Trace Elements of Mineral 3.1.1 Tb-19 (semi-pelitic garnet-rich granulite)

Plagioclase is characterized by enriched large ion lithophile elements and LREE, strong fractionation between LREE and HREE, distinct positive Eu anomalies (Eu/Eu* > 15; Fig. 3a). The plagioclase has high Sr content (up to 1 269 ppm) and Ba content (112 ppm). Rubidium occurs primarily in biotite (347 ppm–353 ppm), and Ba is mostly abundant in biotite (4 433 ppm–5 087 ppm; Table 2). Biotite, cordierite and orthopyroxene have very low REE content, less than chondrite. Garnet shows obvious zoning in trace elements (Table 2, Fig. 3a). Garnet is enriched in HREE with strong fractionation between LREE and HREE. The compositional variations in the zoning, from inner to outer, are an obvious decrease in Y and HREE content and with minimum enrichment of Y and HREE near the rim. Interestingly, Y and HREE increase abruptly at the outer rim (Table 2). In chondrite normalized REE patterns, the core of garnet is characterized by flat HREEs profiles, low LREEs, and a negative Eu anomaly (Fig. 3a). The mantle of garnet is characterized by right slope HREEs profiles, low LREEs, and a negative Eu anomaly.

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Figure 3. Chondrite-normalized REE patterns of minerals in the studied granulites.
Table 2 Trace elements (ppm) of minerals in granulites from Tongbai Orogen
3.1.2 TB-23 (semi-pelitic biotite-rich granulite)

Similarly, plagioclase is characterized by enriched large ion lithophile elements and LREE, strong fractionation between LREE and HREE, distinct positive Eu anomalies (Eu/Eu* > 15; Fig. 3b). Sr content in the plagioclase is ca. 408 ppm–421 ppm. Rubidium occurs primarily in biotite (479 ppm–581 ppm), and Ba is mostly abundant in biotite (3 377 ppm–3 682 ppm; Table 2). Biotite, cordierite and orthopyroxene all have very low REE content, lower than chondrite. The core of large garnet has relatively high Y (mean of 325 ppm) and HREEs abundances. The chondrite normalized REE patterns show low LREEs, a negative Eu anomaly and flat HREE profiles. The rim of garnet has lower Y and HREEs than the core, and is characterized by right slope HREEs profiles (Table 2, Fig. 3a).

3.2 Geochronology 3.2.1 Zircon morphology

Zircons from semi-pelitic garnet rich granulite (sample Tb-19) are colorless, isometric to short prismatic, with lengths of 100–200 μm. In cathodoluminescence (CL) images, three distinct domains can be distinguished, including inherited cores and two generations of metamorphic overgrowth rims. CL images exhibit two types of internal patterns. Type 1 has soccer ball morphology and exhibit core-rim texture with different CL intensities (for example, Fig. 4c). The cores have relatively strongly luminescent with patchy and sector zoning (MZ1), surrounded by relatively low luminescent overgrowth with sector or fir-tree zoning (MZ2, Fig. 4c). Type 2 is prismatic zircon commonly consisting of low luminescent inherited cores with weak magmatic zoning and metamorphic rims (Fig. 3d).

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Figure 4. Representative cathodoluminescence (CL) images of zircons from the studied granulites. The small circles with numbers are LA-ICPMS U-Pb analytical spots with their identification numbers; the large circles with number are LA-MC-ICPMS Lu-Hf analytical spots with their identification numbers. Scale bars represent 50 μm.

Zircons from semi-pelitic biotite-rich granulite (sample Tb-23) are light yellow, isometric to short prismatic in shape, with lengths of 50–150 μm. Similar to sample Tb-19, three distinct domains can be distinguished on CL images, including inherited cores and two generations of metamorphic overgrowth rims (Figs. 4g4l). Some grains exhibit core-mantle-rim textures with different CL intensities and zoning (for example, Fig. 3k). The inherited zircon cores have a low CL response and oscillated zoning, which is typical of igneous zircons (Corfu et al., 2003; Rubatto and Gebauer, 2000; Hanchar and Miller, 1993). The domains of the first-generation metamorphic zircon (MZ1) occur as the rims of inherited cores or as the cores of individual metamorphic zircons. They have relatively low luminescent with sector, fir-tree or planar zoning. The domains of the second-generation metamorphic zircon (MZ2) occur only as overgrowths on first generation metamorphic zircon cores, exhibit bright luminescent with sector, fir-tree, planar or no zoning.

Zircons from mafic granulites (sample Tb-27) are transparent and colorless. The dominant zircon grains have isometric shape, ranging from ca. 20–100 μm in length. CL images show that most of them have section zoning, or weak zoning (Figs. 4m4p), which is typical of metamorphic zircons (Corfu et al., 2003; Hanchar and Miller, 1993). Some zircon grains have unzoned bright rims (Figs. 4m, 4n).

Zircons from pegmatite (sample Q0938) are light brown to colorless and isometric to short prismatic in shape. The lengths range from 200 to 300 μm. The grains exhibit core-rim textures with different CL intensities. The cores have oscillated or sector zoning, which are similar to the igneous zircons. The overgrowth rims of igneous zircon are characterized by low luminescent with weak or no zoning.

3.2.2 Zircon U-Pb age and trace elements 3.2.2.1 Tb-19 (semi-pelitic garnet-rich granulite)

Twenty-two LA-ICP-MS trace elements and U-Pb analyses on zircons from sample Tb-19 are presented in Tables S1 and S2 and shown in Figs. 5 and 6. Two analytical inherited zircon domains show high U abundances (> 800 ppm), and high REE concentrations (Table S2, Fig. 5a), enriched HREE, with negative Eu anomaly (Fig. 5a). The two analytical spots yielded concordant 206Pb/238U ages of 480±6.1 and 561±7.1 Ma.

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Figure 5. Chondrite-normalized REE patterns for different zircon domains in the granulites. (a) Sample Tb-19; (b) sample Tb-23; (c) sample Tb-27; (d) sample Q0938.
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Figure 6. Concordia diagrams of LA-ICP-MS zircon U-Pb dating of the granulites. (a) Concordia diagrams for sample Tb-19; (b) histograms of zircon 206Pb/238U age for sample Tb-19; (c) concordia diagrams for sample Tb-23; (d) histograms of zircon 206Pb/238U age for sample Tb-23; (e) concordia diagrams for sample Tb-27; (f) concordia diagrams for sample Q0938.

Twelve analytical spots on the MZ1 domains reveal low Th concentrations (26 ppm–151 ppm) and Th/U ratios (0.03–0.67). The REE patterns show a nearly flat HREE profiles with a negative Eu anomaly (Fig. 5a). Some prismatic zircon grains with low CL response and high U abundances (785 ppm–956 ppm), have similar REE compositions and U-Pb ages with the MZ1 domains. Twelve analyses on the MZ1 domains are concordant, yield clustered 206Pb/238U ages of 435±5.3 to 449±5.7 Ma, with a weighted mean of 443±3 Ma (MSWD=0.51) (Fig. 6a). Eight analytical spots on the MZ2 domains show moderate U concentrations (254 ppm–815 ppm) and high Th concentrations (most between 118 ppm to 284 ppm), with high Th/U ratios of 0.26–0.93). They have similar REE pattern to the MZ1, also showing nearly flat HREE profiles, and a negative Eu anomaly (Fig. 5a). The 206Pb/238U ages range from 415 to 425 Ma, with a weighted mean of 419±4 Ma (MSWD=0.44).

3.2.2.2 Tb-23 (semi-pelitic biotite-rich granulite)

Nine analyses on inherited cores zircon domains show a large variation of U abundances (54 ppm–1 500 ppm) and Th abundances (42 ppm–837 ppm), resulting in high Th/U ratios (0.43– 0.89). These zircon domains are characterized by strong HREE enrichment and a negative Eu anomaly (Fig. 5b). Nine analytical spots on the magmatic core or inherited zircon domains yield 206Pb/238U ages with larger variation (from 454 to 1 327 Ma).

Ten spots on the MZ1 domains show moderate U concentrations (399 ppm to 623 ppm) and Th concentrations (most between 47 ppm to 75 ppm), resulting in low Th/U ratios (0.11–0.18). They have a negative Eu anomaly and depleted HREEs (Fig. 5b). Ten analyses on MZ1 domains are all concordant (Fig. 6c). The 206Pb/238U ages range from 428.8 to 436.0 Ma with a weighted mean of 432±4 Ma (MSWD=0.13).

Five analytical spots on MZ2 rims have moderate U (348 ppm–862 ppm) and low Th (22.5 ppm–80.0 ppm), resulting in low Th/U ratios (0.03–0.18). The chondrite normalized REE patterns show negative Eu anomalies and various HREE concentrations (Fig. 5b). The MZ2 domains have distinctly higher HREEs than MZ1. This indicates that the MZ2 domains were formed when garnet begins to breakdown. Five analyses on MZ2 domains are all concordant or nearly concordant, yielding clustered 206Pb/238U ages of 410.0 to 423.7 Ma with a weighted mean of 419±7 Ma (MSWD=0.40) (Fig. 6d).

3.2.2.3 Tb-27 (mafic granulite)

Thirteen analyses on the metamorphic domains, with relatively low CL response, give moderate U concentrations (most between 32 ppm to 164 ppm), low Th concentrations (most between 8.9 ppm to 53.4 ppm), and high Th/U ratios (0.28–0.46). The REE patterns show enriched HREE with no Eu anomaly (Fig. 5c). The concordant ages range from 410 to 427 Ma, with a weighted mean of 419±3 Ma (MSWD=1.02) (Fig. 6e).

The other eleven analyses on outer rims or metamorphic zircons with strong luminescence are characterized by low U and Th abundances. The trace elements are similar to the metamorphic domains except for the high common Pb. A precise discordant line is constituted, with a lower intercept age of 401±7 Ma (MSWD=0.45) (Fig. 6e).

3.2.2.4 Q0938 (pegmatite)

Ten analyses on the magmatic domains, with relatively low CL response, give relatively low U abundances (most between 291 ppm to 968 ppm), moderate Th abundances (most between 132 ppm to 405 ppm) and high Th/U ratios (0.42–0.72). The chondrite normalized REE patterns are characterized by HREE enrichment, clearly negative Eu anomaly (Eu/Eu*=0.19–0.33), and positive Ce anomaly (Fig. 5d). The concordant ages range from 417 to 424 Ma, with a weighted mean of 421±2.0 Ma (MSWD=0.78) (Fig. 6f). This age is interpreted as the crystallization age of the leucosome veins. The other nine analyses on rims are characterized by high U (551 ppm–2 790 ppm) and Th abundances (134 ppm–411 ppm), resulting in relatively low Th/U ratios (0.05–0.51). The trace elements are similar to the magmatic domains. Except spot 08 yields a younger age, the eight others are nearly concordant, giving clustered 206Pb/238U ages of 400–409 Ma with a weighted mean of 404±2.6 Ma (2σ, MSWD=1.3; Fig. 6f). This age should represent the age of later hydrothermal event.

3.3 Zircon Lu-Hf Isotopes

In semi-pelitic garnet rich granulite Tb-19, seven analyses on metamorphic zircon domains show low 176Lu/177Hf ratios (0.000 025–0.000 094), and similar 176Hf/177Hf ratios of 0.282 345–0.282 448. The calculated εHf(t) values are -1.7 to -3.1, except one is -5.9, with a weighted mean of -2.5±0.6 (MSWD=1.9) (Table S3, Fig. 7). Two-stage Hf model ages (TDM2) range from 1 533±129 to 1 583±91 Ma, with a weighted mean of 1 570±40 Ma (MSWD=0.26) (Fig. 7). Two analyses on inherited cores show relatively high 176Lu/177Hf ratios (0.000 307– 0.001 077), and low 176Hf/177Hf ratios of 0.282 141–0.282 267. The calculated εHf(t) values is -10.4 to -7.4 (Table S3, Fig. 7). Another two analyses on metamorphic zircon domains, which are similar to the inherited zircon with prismatic in shape, have similar Lu-Hf isotope compositions (176Hf/177Hf ratios are 0.282 267– 0.282 301, and εHf(t) values vary from -8.2 to -7.1).

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Figure 7. Hf isotope evolution for zircons from Tongbai granulites, depleted mantle evolution is calculated with values after Vervoort and Blichert-Toft (1999).

In semi-pelitic Biotite-rich granulite Tb-23, eleven analyses on metamorphic zircon domains reveal low 176Lu/177Hf ratios varying from 0.000 010 to 0.000 784, most < 0.000 110, and homogeneous 176Hf/177Hf ratios of 0.282 442– 0.282 600. The εHf(t) values are -2.2 to 3.1 with a weight average of 1.1±1.1 (MSWD=18). Calculated TDM2 range from 1 206±93 to 1 551±89 Ma, with a weighted mean of 1 342±69 Ma (MSWD=4.7). The inherited cores have relatively high 176Lu/177Hf ratios of 0.001 075–0.002 905, and low 176Hf/177Hf ratios of 0.281 504– 0.281 946. The εHf(t) values are -14.5 to -30.4.

In mafic granulite Tb-27, five analyses spots on metamorphic zircon domains show low 176Lu/177Hf ratios (0.000 053– 0.000 237), and relatively homogeneous 176Hf/177Hf ratios of 0.282 570–0.282 675. The calculated εHf(t) values are2.0–5.7, with an average of 3.7±2.1 (MSWD=18) (Table S3). The calculated TDM2 range from 1 038±88 to 1 274±101 Ma, with an average of 1 168±140 Ma (MSWD=4.5). Five spots on the outer rims or new growth zircons have relatively high 176Lu/177Hf ratios (0.000 244–0.000 442), and low 176Hf/177Hf ratios (0.282 494–0.281 560). The εHf(t) values are -1.2–1.2, with an average of 0.4±1.2 (MSWD=4.9).

In pegmatite Q0938, nine analyses on magmatic zircon domains show varying 176Hf/177Hf ratios of 0.282 356–0.282 488. The calculated εHf(t) values are -5.9 to -0.8 with an average of -2.8±0.95 (Table S3). The calculated TDM2 are 1 455±98 to 1 745±96 Ma, with an average of 1 579±59 Ma (MSWD=3.2). The other eight analyses on the rims or new growth zircons have relatively clustered 176Hf/177Hf ratios of 0.282 376–0.282 445. The εHf(t) values range from -5.2 to -2.8, and the weighted mean is -3.65±0.56 (MSWD=5.6). The calculated TDM2 is 1 564±90 to 1 717±86 Ma, with an average of 1 616±40 Ma (MSWD=1.4).

4 DISCUSSIONS 4.1 The Nature and Age of Protolith

The age of inherited or magmatic zircons in the semipelitic granulites ranges from ca. 480 to 1 327 Ma. This suggested that the protolith of the semi-pelitic granulites was sedimentary rocks with a maximum age of ca. 480 Ma. Previous study shows that the range of detrital zircon ages from metasediments in Qinling Group is from ca. 450 to 2 635 Ma, with a main peak at 440–490 Ma, a subordinate peak at 660–950 Ma (Liu et al., 2011a; Wang et al., 2011). The youngest age peak of ca. 450 Ma is interpreted as the maximum depositional age of their protoliths (Wang et al., 2011). The Early Paleozoic subduction-related magmatic arcs along the North Qinling-Tongbai Orogen is possible source for 440–490 Ma detrital zircon (Jiang et al., 2009; Wang et al., 2009; Sun et al., 2002; Meng and Zhang, 2000, 1999; Kröner et al., 1993).

4.2 Metamorphic Age and P-T-t Path

The Tongbai granulites experienced UHT metamorphism and multistage evolution with an anticlockwise P-T-t path (Xiang et al., 2014, 2012; Liu et al., 2011a). There are four metamorphic U-Pb zircon age populations of ca. 440, 430, 420, and 404 Ma (Table 1). However, linking these ages to P-T conditions or metamorphic stages is the key to establish the P-T-t path, which is an important parameter to recognize the tectonic evolution of Tongbai Orogen.

Garnet is strongly enriched in HREE and Y, then the relationship between zircon and garnet is a major factor affecting the HREE distribution in metamorphic zircon (Wu et al., 2008; Whitehouse and Platt, 2003; Rubatto, 2002). Zircon crystallized with garnet would display a flat or even negative HREE profile (Bingen et al., 2004; Rubatto, 2002; Schaltegger et al., 1999). On the other hand, metamorphic zircon, crystallized during garnet decomposition, would increase HREE concentrations. Moreover, experimental results show that there is remarkable negative correlation of temperature and zircon/garnet HREE partitioning (Fig. 8c) (Rubatto and Hermann, 2007). Therefore, the REE composition of zircon and zircon/garnet HREE partitioning can determine zircon formation condition and allows linking zircon U-Pb ages with metamorphic P-T conditions, which is a fundamental step in defining metamorphic P-T-t path (Rubatto, 2002).

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Figure 8. Chondrite-normalized REE patterns for garnets in granulites from Tongbai. (a) Sample Tb-19; (b) sample Tb-23.
4.2.1 Age of peak metamorphism (M2)

Two semi-pelitic granulites samples (Tb-23 and Tb-19) have similar bulk REE compositions, and the garnets in sample Tb-23 have relatively high HREE than in sample Tb-19. Nevertheless, the MZ1 zircon domains in sample Tb-23 have the lowest HREE and Y, and much lower than any other domains, with the result that the HREE partitioning between MZ1 and the garnet in sample Tb-23 is the lowest (DLuZrn/Grt=0.1, Table 3, Fig. 8c). This indicates the MZ1 zircon domains formed at UHT condition, and the age 432±4 Ma of the MZ1 represents the age of the granulite- facies peak or near-peak metamorphic stage (M2). The LA-ICP- MS zircon U-Pb dating for the sapphirine-bearing mafic granulite yield two group ages of 430 and 411 Ma, which interpreted as the near peak age and the retrograde age, respectively (Xiang et al., 2014). The results of U-Pb dating of zircon from a mafic granulite (sample Tb-25) also show that the peak metamorphic age is 432±4 Ma (Xiang et al., 2012). This interpretation is also supported by the age (432 Ma) of impure marble from the upper unit of Qinling Group. Additionally, zircons from the leucosome within the granulites yield a weight mean age of 428±4 Ma (Wang et al., 2011), which represents the melt crystallization age during the cooling near the peak metamorphic condition. These indicate that the peak metamorphic age of Tongbai granulites is ca. 430 Ma.

Table 3 Arithmetic mean of REE composition in garnet and zircon and partitioning between zircon and garnet
4.2.2 Age of prograde metamorphism (M1)

In sample Tb-19, the mean age of the MZ1 domains is 443±3 Ma (MSWD=0.51). The ca. 443 Ma zircon domains in sample Tb-19 have relatively flat HREE patterns, suggesting that the metamorphic zircons were coexisted with garnets. Nevertheless, the ca. 443 Ma zircons in sample Tb-19 have much higher HREE than the ca. 432 Ma zircons in sample Tb-23, the HREE compositions of garnets are variable in both sample, the HREE partitioning between ca. 443 Ma zircon and the garnet in sample Tb-19 (ca. 1.5) is much higher than that of the ca. 432 Ma zircon and garnet in sample Tb-23 (Table 3, Figs. 8a8c). This suggests that the MZ1 domains from sample Tb-19, with the mean age of 443±3 Ma, were formed during the prograde metamorphic stage (M1) rather than the peak stage. It is consistent with the fact deduced by the prograde metamorphic reaction texture of Crd+Opx=Grt+Qz in sample Tb-19.

4.2.3 Age of retrograde metamorphism (M3)

The MZ2 domains from sample Tb-23 show nearly flat HREE patterns with negative Eu anomalies, variable HREE and markedly higher HREE contents than the MZ1 domains from sample Tb-23. This implies that the MZ2 zircon domains were formed during the retrograde metamorphic stage, because garnet breakdown will release HREEs, and result in an increase of HREE contents in coeval metamorphic zircon (Hermann et al., 2001). This is consistent with the fact that massive garnets were replaced by biotite and plagioclase in sample Tb-23. Clustered 206Pb/238U ages of 410–423 Ma with a weighted mean of 419.0±7.3 Ma (MSWD=0.38) (Fig. 8) are interpreted as the age of the retrograde metamorphic stage (M3). This interpretation is also supported by that the crystallization age of pegmatite vein is 421±2.0 Ma (MSWD=0.78). Furthermore, the age of the MZ2 domains from Tb-19 is consistent with the age of the retrograde metamorphism, so it indicates the retrograde metamorphic (M3) age is 419 Ma.

4.2.4 Age of greenschist facies retrograde metamorphism (M4)

The older zircons in mafic granulite Tb-27 yield an age of 418.7 Ma, which is consistent with the retrograde metamorphic age for the samples Tb-19 and Tb-23. The bright rims of zircons in sample Tb-27 yield age of 400 Ma. In sample Tb-27, the ca. 419 Ma zircons have a relatively high initial 176Hf/177Hf ratio with a mean εHf(t)=3.3±1.9. However, the ca. 400 Ma zircon domains have a high proportion of common Pb and a relatively low initial 176Hf/177Hf ratio with mean εHf(t)=0.4±1.9. The ca. 400 Ma zircons have much lower initial 176Hf/177Hf ratio than the ca. 419 Ma zircons. This indicates that low initial 176Hf/177Hf material was added into the rock at ca. 400 Ma. It is extremely possible that hydrothermal fluids with low176Hf/177Hf ratios were added into the rock, which is manifested by abundant hydrous minerals, such as chlorite, epidote and actinolite, of the greenschist facies retrograde metamorphism. In the pegmatite, the black rim of the zircons yield weighted mean age of 404±2.6 Ma (2σ, MSWD=1.3). Liu et al. (2011a) also obtained three metamorphic ages of ca. 440, 420, and 400 Ma (Table 1). They proposed that the ca. 420 Ma was the post-peak cooling age, the ca. 400 Ma was the age of greenschist facies overprinting (Liu et al., 2011a). Zhai et al. (1998) reported a ca. 404±2 Ma amphibole Ar-Ar age of amphibole granulite. This age represents the time of cooling through the amphibole closure temperature (~500 ℃).

As discussed previously, the Tongbai granulites record a low-P heating followed by a pressure increase within the sillimanite field, and a near-isobaric cooling P-T path (Fig. 9; Xiang et al., 2014, 2012). The prograde metamorphism with P-T condition of < 6 kbar and 750–830 ℃ occurred at around 443 Ma. The peak metamorphic (M2) condition is about 9 kbar and > 920 ℃, and the peak metamorphic age is ca. 432 Ma. At around 419 Ma, the granulites suffered a strong amphibolite facies retrograde metamorphism (M3) under conditions of ca. 700 ℃ at ca. 7 kbar. During the amphibolite facies retrograde metamorphism (M3), the average cooling rate of about 20 ℃/Ma can be obtained, indicating a fast cooling stage. The last retrograde metamorphism (M4) is represented as a greenschist-facies overprint with an age of ca. 400 Ma and P-T condition of ca. 5.8 kbar at 500 ℃. Therefore, the metamorphism of Tongbai granulite would have lasted for more than 40 Ma, including a period of more than 20 Ma with granulite-facies metamorphism.

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Figure 9. The metamorphic P-T-t path for Tongbai UHT granulites.
4.3 Geological Implication

A number of magmatic rocks with island arc characteristics occur in the North Qinling-Tongbai Orogen with ages of ca. 400– 490 Ma (Wu and Zheng, 2013; Dong et al., 2011; Wang et al., 2011; Ratschbacher et al., 2006, 2003; Hacker et al., 2004; Xue et al., 1996). However, coeval metamorphism and magmatism are absent in the South Qinling-Tongbai terrane. Thus, it was generally proposed that the northward subduction of the Shangdan Ocean result in voluminous arc magmatism during the period of ca. 490–400 Ma (Wu and Zheng, 2013; Wu et al., 2013). A major detrital zircon age cluster of metasedimentary rocks of Qinling Group in Tongbai Orogen is ca. 450–490 Ma (Liu et al., 2011a; Wang et al., 2011). This indicates that the Early Paleozoic arc magmatic rocks are probably the major source of the sediments, which are most likely in fore-arc, and this sediment rocks immediately were involved in a prolonged granulite-facies metamorphism (Xiang et al., 2012). A possible tectonic evolution is that consumption of Shangdan Ocean Basin by northward subduction will bring a sea-floor-spreading ridge toward a deep-sea trench. After 445 Ma, the ocean ridge intersects the subduction zone, producing anomalous thermal, triggered UHT metamorphism and voluminous magmatism (Xiang et al., 2014, 2012).

5 CONCLUSIONS

The ca. 432 Ma zircons from semi-pelitic granulite in the North Tongbai Orogen, show strong HREE depletion and the lowest DLuZrn/Grt≈0.1, are suggested formation at UHT condition during peak or near-peak metamorphic stage (M2). The ca. 443 and 419 Ma zircons from semi-pelitic granulite, show flat HREE patterns and relative high DLuZrn/Grt, are suggested formation at prograde stage (M1) and retrograde stage (M3) respectively. The ca. 400 Ma zircons have much lower initial 176Hf/177Hf ratio than the ca. 419 Ma zircons. It should be the result of the greenschist facies retrograde metamorphism with the addition of low 176Hf/177Hf ratios hydrothermal fluids.

The Tongbai granulites have an anticlockwise P-T-t path evolution. The prograde metamorphism (M1) occurred at < 6 kbar and ca. 800 ℃ at 443±3.1 Ma. The peak metamorphic (M2) conditions are > 920 ℃ at a pressure around 8.5–10 kbar, and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered amphibolite-facies retrograde metamorphism (M3) with P-T conditions of ca. 7 kbar and ca. 700 ℃. Finally, the greenschist-facies retrograde metamorphism (M4) under P-T condition of ca. 5.8 kbar and ca. 500 ℃ occurred at ca. 400 Ma. This indicates that the metamorphism of Tongbai granulite would have lasted more than 40 Ma. The HT-UHT metamorphism in the North Tongbai Orogen may be results of the continuous northward subduction of the Shangdan oceanic crust and spreading ridge beneath the North Qinling terrane.

ACKNOWLEDGMENTS

This paper is dedicated to Prof. Zhendong You on the occasion of his 90th Birthday. We are grateful to three anonymous reviewers for critical and constructive reviews of the manuscript. This work was supported by the National Natural Science Foundation of China (No. 41302040) and the China Geological Survey (No. DD20160201). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0855-8.

Electronic Supplementary Materials: Supplementary materials (Tables S1–S3) are available in the online version of this article at https://doi.org/10.1007/s12583-018-0855-8.


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