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Zircon SIMS U–Pb geochronology of the Lushan terrane: dating metamorphism of the southwestern terminal of the Palaeoproterozoic Trans-North China Orogen

Published online by Cambridge University Press:  22 October 2014

JUN-SHENG LU ,
GUO-DONG WANG ,
HAO WANG ,
HONG-XU CHEN ,
TAO PENG  and
CHUN-MING WU
JUN-SHENG LU
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, Shanxi 710069, China
GUO-DONG WANG
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
HAO WANG
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
HONG-XU CHEN
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
TAO PENG
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China
CHUN-MING WU*
Affiliation:
College of Earth Science, University of Chinese Academy of Sciences, P.O. Box 4588, Beijing 100049, China State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, Shanxi 710069, China
*
†Author for correspondence: [email protected]
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Abstract

High-resolution SIMS U–Pb dating of metamorphic zircons of the TTG gneisses, gneissic granitoid and amphibolites of the Lushan terrane, Taihua metamorphic complex, suggests that the metamorphism had taken place at least as early as ~1.96–1.86 Ga. These new dates, along with reference data, demonstrate that the southern and middle terranes of the Trans-North China Orogen had been involved in the continent–continent collision between the Western Block and the Eastern Block of the North China Craton. This orogenic process started as early as 1.96 Ga and lasted as late as 1.80 Ga.

Keywords

SIMS U–Pb datingzirconLushan terraneTaihua metamorphic complexthe Trans-North China Orogen
Type
Rapid Communication
Information
Geological Magazine , Volume 152 , Issue 2 , March 2015 , pp. 367 - 377
Copyright
Copyright © Cambridge University Press 2014 

1. Introduction

The continent–continent collision process is the subsequent process after subduction, which is generally characterized by clockwise P–T paths, including an early prograde process and later isothermal decompression (ITD) process following peak metamorphism, recorded in metamorphic rocks, especially the Precambrian metamorphic terranes. Thus, clockwise P–T paths characterized by ITD segments are considered to be one of the important indicators in recognizing plate tectonics, especially in the Earth's early history (e.g. Zhao, Reference Zhao2007). After reviewing metamorphic P–T paths preserved in the different metamorphic terranes and combining lithological, structural, geochemical and geochronological data, Zhao et al. (Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Cawood, Wilde, Sun and Lu2000a , Reference Zhao, Cawood, Wilde and Lu2001, Reference Zhao, Wilde, Cawood and Lu2005, Reference Zhao, Cawood, Li, Wilde, Sun, Zhang, He and Yin2012) recognized three orogenic belts in the North China Craton (NCC), i.e. the Trans-North China Orogen (TNCO), the Jiao-Liao-Ji Belt and the Khondalite Belt (Fig. 1a), quite similar to present-day subduction zones.

Figure 1. (a) Geological sketch map of the North China Craton (after Zhao et al. Reference Zhao, Sun, Wilde and Li2005); (b) the Lushan terrane in the Taihua metamorphic complex within the Trans-North China Orogen (modified after Xu et al. Reference Xu, Griffin, Ma, O’Reilly, He and Zhang2009); and (c) simplified geological map of the Lushan terrane (modified after Xu et al. Reference Xu, Griffin, Ma, O’Reilly, He and Zhang2009). Sample locations are shown. Pt2ry – Ruyang Group; Pt2xn – Xiong’er Group; Pt3 – Late Proterozoic; ϵ – Cambrian; Q – Quaternary; KB – Khondalite Belt; WB – Western Block; TNCO – Trans-North China Orogen; EB – Eastern Block; JLJB – Jiao-Liao-Ji Belt.

In the last two decades, metamorphic investigations of the metamorphic terranes (e.g. Huai’an, Hengshan, Wutai, Fuping, Lüliang and Zanhuang terranes) within the northern and middle sections of the TNCO have confirmed that these terranes all record clockwise P–T paths including retrograde ITD segments (Zhao, Cawood & Lu, Reference Zhao, Cawood and Lu1999; Zhao et al. Reference Zhao, Wilde, Cawood and Lu2000b , Reference Zhao, Cawood, Wilde and Lu2001, Reference Zhao, Yin, Guo, Sun, Li, Li, Wu and Liu2010; Guo, O’Brien & Zhai, Reference Guo, O’Brien and Zhai2002; Xiao et al. Reference Xiao, Jiang, Wang, Wan, Wang and Wu2011a ,Reference Xiao, Wu, Zhao, Guo and Ren b ). Geochronological studies suggest that the metamorphism of all these terranes occurred at ~1.85 Ga (Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002, Reference Zhao, Wilde, Sun, Guo, Kröner, Li, Li and Wu2008a ,Reference Zhao, Wilde, Sun, Li, Li and Zhang b ; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Liu et al. Reference Liu, Zhao, Wilde, Shu, Sun, Li, Tian and Zhang2006; Xiao et al. Reference Xiao, Wang, Wang, Jiang, Diwu and Wu2013). In recent years, geologists have been aware of the importance of the metamorphic terranes in the southern section of the TNCO, especially the Taihua metamorphic complex (Wan et al. Reference Wan, Wilde, Liu, Yang, Song and Yin2006; Diwu et al. Reference Diwu, Sun, Ling, Liu and Wang2007, Reference Diwu, Sun, Ling and Wang2010; Liu et al. Reference Liu, Wilde, Wan, Wang, Valley, Kita, Dong, Xie, Yang, Zhang and Gao2009; Xu et al. Reference Xu, Griffin, Ma, O’Reilly, He and Zhang2009; Huang et al. Reference Huang, Niu, Xu, Yang and Zhong2010, Reference Huang, Zhang, Wang, Zhang, Dong, Liu and Wan2012; Jiang et al. Reference Jiang, Wang, Xiao, Diwu, Lu and Wu2011; Shi et al. Reference Shi, Yu, Xu, Tang, Qiu and Chen2011; Wang et al. Reference Wang, Wang, Chen, Lu, Xiao and Wu2012, Reference Wang, Lu, Wang, Chen, Xiao, Diwu, Ji and Wu2013; Huang, Wilde & Zhong, Reference Huang, Wilde and Zhong2013; Yu et al. Reference Yu, Liu, Li, Chen and Dai2013; Lu et al. Reference Lu, Wang, Wang, Chen and Wu2013). Our previous work (Lu et al. Reference Lu, Wang, Wang, Chen and Wu2013) suggested that the Taihua metamorphic complex had been involved in the subduction and collision between the Western and Eastern blocks of the NCC and experienced later uplift processes during late Palaeoproterozoic time. However, our previous work focused solely on the gneissic amphibolites. In this contribution, we report detailed secondary ion mass spectroscopy (SIMS) dating of metamorphic zircons separated from the tonalite–trondhjemite–granodiorite (TTG) gneisses, gneissic granitoids and amphibolite boudins with weak or no gneissosity enclosed in the TTG gneisses, to further constrain the Palaeoproterozoic tectonometamorphic evolution of the Taihua metamorphic complex.

2. Regional setting

The Taihua metamorphic complex is located on the southwestern margin of the TNCO, which is comprised of five discrete terranes (Huashan, Xiaoshan, Luoning, Lushan and Wugang) scattered across the Shanxi and Henan provinces, north China (Fig. 1b). The Taihua metamorphic complex consists mainly of TTG gneisses, amphibolites, metapelitic gneisses, marbles and quartzites; experienced upper-amphibolite-facies metamorphism and intense deformation; and records clockwise P–T paths including ITD segments (Lu et al. Reference Lu, Wang, Wang, Chen and Wu2013).

In Lushan County, Henan Province, the southwestern Taihua metamorphic complex is uncomformably covered by unmetamorphosed volcanic strata of the Xiong’er Group dated at ~1.78 Ga (He et al. Reference He, Zhao, Sun and Xia2009), and the northeastern Taihua metamorphic complex is in fault contact with the Mesoproterozoic Ruyang Group and other Neoproterozoic and Palaeozoic covers. The Taihua metamorphic complex exposed in the Lushan terrane was subdivided into the Lower and Upper Taihua subgroups separated by the interstitial characteristic marble stratum exposed approximately along the Dangze River. On the northeastern side of the Dangze River is the Lower Taihua Subgroup, which is mainly composed of TTG gneisses and patchy amphibolites within the TTG gneisses. The TTG gneisses formed at 2.75–2.84 Ga (Kröner et al. Reference Kröner, Compston, Zhang, Guo and Todt1988; Diwu et al. Reference Diwu, Sun, Ling, Liu and Wang2007; Liu et al. Reference Liu, Wilde, Wan, Wang, Valley, Kita, Dong, Xie, Yang, Zhang and Gao2009) and the protolith of the amphibolites formed at ~2.73 Ga and underwent a peak metamorphic event at ~1.95 Ga (Lu et al. Reference Lu, Wang, Wang, Chen and Wu2013). On the southeastern side of the Dangze River is the Upper Taihua Subgroup, which mainly consists of metapelitic gneisses, gneissic granites and marbles. SHRIMP U–Pb dating of zircons (Wan et al. Reference Wan, Wilde, Liu, Yang, Song and Yin2006) determined that the Upper Taihua Subgroup was formed during Palaeoproterozoic time (between 2.26 and 2.14 Ga) and experienced metamorphism at ~1.85 Ga.

3. Sample selection and analytical methods

In this work, six representative metamorphic rock samples were selected for SIMS U–Pb dating of zircons (Fig. 1c).

Samples L10 and L56 are fine-grained garnet-bearing amphibolites showing weak or no gneissosity, occurring as short dykes within the TTG gneisses. Residual ophitic textures and intrusive relationships with the country TTG rocks are locally preserved. The amphibolites mainly consist of hornblende (35–40%) + plagioclase (20–15%) + quartz (20%) + garnet (15%) and clinopyroxene (5%) (Fig. 2a, b).

Figure 2. Photomicrographs of amphibolites (a, b), TTG gneisses (c, d, e) and gneissic granitoid (f). Symbols for minerals are after Whitney & Evans (Reference Whitney and Evans2010).

Samples L51, L54 and L55 are grey coarse-grained TTG gneisses, and are mainly composed of plagioclase (35–53%) + quartz (20–35%) + hornblende (15–20%) and biotite (2–15%) (Fig. 2c–e). Garnets are locally observed in sample L55.

Sample L50 is a gneissic granitoid collected from Limeigou Village and is mainly composed of quartz (50%) + perthite (35%) + plagioclase (8%) + K-feldspar (5%) and biotite (2%) (Fig. 2f).

Zircons for SIMS U–Pb geochronological studies were collected by conventional magnetic and density separation techniques. Zircon grains, together with the zircon standards Plešovice and Qinghu, were mounted in epoxy mounts which were then polished to section the crystals in half for analysis. Prior to the SIMS analysis, micrographs of all zircons were taken using transmitted and reflected light as well as cathodoluminescence (CL) to reveal their internal structures. Measurements of U, Th and Pb isotopes were conducted using a Cameca IMS 1280 large-radius SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. The analytical beam was about 20 × 30 μm in size. Analytical procedures, conditions and data processing procedures are the same as those described by Li et al. (Reference Li, Liu, Li, Guo and Chamberlain2009). Data reduction was carried out using the Isoplot/Ex v. 3.75 program (Ludwig, Reference Ludwig2012). The SIMS zircon U–Pb dating results are listed in Table S1 in the online Supplementary Material available at http://journals.cambridge.org/geo. The uncertainties in Table S1 and on the cathodoluminescence (CL) diagrams for individual analyses are quoted at the 1σ level, whereas the errors on weighted mean ages given in Figure 4 and in the text are quoted at the 95% confidence level.

In order to monitor the external uncertainties of the SIMS U–Pb measurements calibrated against the Plešovice standard, the Qinghu zircon standard was additionally analysed as an unknown together with the unknown zircons that were mounted in mounts A2206, A2420 and A2421 during the course of this study. A total of 27 measurements were conducted on the Qinghu zircon (Table S1 in the online Supplementary Material available at http://journals.cambridge.org/geo), and the Concordia ages (Ludwig, Reference Ludwig2012) of 159.5 ± 2.5 Ma, 160.8 ± 0.8 Ma and 159.0 ± 0.75 Ma were obtained for the Qinghu zircon in mounts A2206, A2420 and A2421, respectively. These three measured ages define a mean of 159.77 ± 0.91 Ma, which is identical to the recommended value of 159.5 ± 0.7 Ma (Li et al. Reference Li, Liu, Li, Guo and Chamberlain2009), within error.

4. Geochronological results and interpretations

4.a. Amphibolites

Zircons separated from the amphibolites (samples L10 and L56) bear close resemblance to one another and are anhedral and round in shape, varying from 40 to 90 μm in length. They are internally homogeneous with weak to medium luminescence in the CL images (Fig. 3a, b), except for a few grains in Sample L10 with weak zoning.

Figure 3. Representative cathodoluminescence (CL) images of zircons separated from amphibolites (a, b), TTG gneisses (c, d, e) and gneissic granitoid (f), Lushan terrane. The numbers refer to the analytical spots and the corresponding data are list in Table S1 in the online Supplementary Material available at http://journal.cambridge.org/geo.

In Sample L10, three age groups have been recognized. The first group is represented by spots 1, 7 and 16. Spots 1 and 7 are distributed near the Concordia line (Fig. 4a) and give a weighted mean 207Pb–206Pb age of 2781 ± 18 Ma (MSWD = 0.000114), possibly indicating the protolith age or possible xenocrystic grains from host TTG rocks. The second group consists of 12 spots (spots 3–6, 8–11, 13, 14, 17 and 18), and their U, Th and Pb contents and Th/U ratios are in the range of 189–295 ppm, 36–99 ppm, 77–122 ppm and 0.19–0.40, respectively. They plot around the Concordia curve (Fig. 4a, b), defining a weighted mean 207Pb–206Pb age of 1927 ± 6.5 Ma (MSWD = 0.73) (Fig. 4b), which is interpreted as possibly the peak metamorphic age. The third group contains three spots (spots 2, 12 and 15), and their U, Th and Pb contents and Th/U ratios are in the range of 207–255 ppm, 56–100 ppm, 105–817 ppm and 0.27–0.39, respectively. They all plot near the Concordia curve, giving a weighted mean 207Pb–206Pb age of 1858 ± 32 Ma (MSWD = 0.5) (Fig. 4b), possibly defining a post-peak metamorphic event.

Figure 4. The U–Pb Concordia diagrams for SIMS U–Pb dating of zircons from the amphibolites (a–c), TTG gneisses (d–g), as well as from the gneissic granitoid (h).

In Sample L56, 20 spots were analysed on 20 grains. Their U, Th and Pb contents and Th/U ratios are in the range of 210–314 ppm, 36–175 ppm, 86–130 ppm and 0.13–0.58, respectively. These spots are distributed on the Concordia line and define a weighted mean 207Pb–206Pb age of 1918 ± 5 Ma (MSWD = 1.2) (Fig. 4c), which is interpreted as possibly recording peak metamorphism.

4.b. TTG gneisses

Zircons extracted from the TTG gneisses (samples L51, L54 and L55) show diverse features. Zircons from samples L51 and L55 are transparent, anhedral and prismatic or stubby in shape varying from 90 to 570 μm in length. Zircons from Sample L54 vary from 80 to 160 μm in length and are obviously smaller than those from samples L51 and L55. They are classified into two different types: the first type are stubby or round in shape and are internally homogeneous with weak luminescence in CL images (Fig. 3d), indicating a possible metamorphic origin. The second type are stubby in shape and show obvious oscillatory zoning in CL images (Fig. 3d), indicating a possible magmatic origin.

Eighteen spot analyses were conducted on 16 zircon grains from Sample L51. The analytical location of spot 14 is on the rim of the zircon (Fig. 3c), plots near the Concordia curve and gives a 207Pb–206Pb age of 1915 ± 14 Ma (Fig. 4d), which is interpreted as possibly representing the metamorphic peak. The other spots show variable Pb loss and yield an upper intercept age of 2897 ± 28 Ma (MSWD = 0.55) (Fig. 4d), which is interpreted to be the possible protolith age of the TTG gneiss. It is noteworthy that, although the lower intercept age 1944 ± 120 Ma (MSWD = 0.55) (Fig. 4d) has a larger error, it is also consistent with the metamorphic age of spot 14.

In Sample L54, 19 spot analyses were performed on 19 zircon grains. The first group of zircons contains five spots (spot 1, 3, 9, 13 and 16), and their U, Th and Pb contents and Th/U ratios are in the range of 530–1905 ppm, 10–150 ppm, 201–492 ppm and 0.02–0.12, respectively (Table S1 in the online Supplementary Material available at http://journals.cambridge.org/geo), defining an upper intercept age of 1934 ± 7 Ma (MSWD = 2.1) (Fig. 4e, f), which is ascribed to the possible peak metamorphic age. In the second type of zircons, their U, Th and Pb contents and Th/U ratios are in the range of 34–1163 ppm, 20–239 ppm, 26–631 ppm and 0.21–0.87, respectively (Table S1 in the online Supplementary Material available at http://journals.cambridge.org/geo), and yield 207Pb–206Pb ages ranging from 2641 to 2950 Ma. Except for spots 7 and 15, the other spots define an upper intercept age of 2928 ± 14 Ma (MSWD = 1.9) (Fig. 4e), which is coeval with the weighted mean 207Pb–206Pb age of 2928 ± 5.4 Ma (MSWD = 0.68) (Fig. 4e), confining the protolith age of the TTG gneiss.

Twenty-six spots of 19 zircon grains from Sample L55 were analysed. Two rim spots (spots 7 and 16) are plotted near the Concordia curve and their 207Pb–206Pb ages are 1940 ± 15 Ma and 1922 ± 11 Ma, respectively, possibly representing the metamorphic age recorded in the TTG gneiss. The rest of the spots yield 207Pb–206Pb ages ranging from 2602 to 2789 Ma with their U, Th and Pb contents and Th/U ratios in the range of 42–319 ppm, 7–239 ppm, 28–232 ppm and 0.13–0.82, respectively (Table S1 in the online Supplementary Material available at http://journals.cambridge.org/geo). Without spots 5, 9 and 14, the rest of the data define a weighted mean 207Pb–206Pb age of 2778 ± 3.4 Ma (MSWD = 1.8) (Fig. 4g), restricting the protolith age of the TTG gneiss.

4.c. Gneissic granitoid

Zircons separated from the gneissic granitoid (Sample L50) are transparent, prismatic or stubby in shape, with weak zoning in CL images and vary from 70 to 180 μm in length (Fig. 3f). A total of 20 spots on 18 grains were analysed and two age groups have been recognized. The first group is represented by spots 7 and 16. Spot 7 is plotted on the Concordia curve with a 207Pb–206Pb age of 2110 ± 8 Ma, which is interpreted as the protolith age and is consistent with the result of Wan et al. (Reference Wan, Wilde, Liu, Yang, Song and Yin2006). Spot 16 is located above the Concordia curve, which may possibly be influenced by the tiny inclusion minerals within it. The second group of zircons show varying degrees of Pb loss and yield an upper intersect age of 1956 ± 36 Ma (MSWD = 46) (Fig. 4h) in the conventional Concordia diagram, and the modest/weakest Pb loss zircons define a weighted mean 207Pb–206Pb age of 1928 ± 5 Ma (MSWD = 1.0) (Fig. 4h), which is older than those obtained by Wan et al. (Reference Wan, Wilde, Liu, Yang, Song and Yin2006) and is deciphered to represent the peak metamorphic age.

It is suggested that from this new high-resolution SIMS U–Pb dating of the zircons, two periods of protolith ages of the TTG gneisses can be recognized: the first one is ~2.93–2.90 Ga recorded in samples L51 and L54, which is older than previous studies (Kröner et al. Reference Kröner, Compston, Zhang, Guo and Todt1988; Diwu et al. Reference Diwu, Sun, Ling, Liu and Wang2007; Liu et al. Reference Liu, Wilde, Wan, Wang, Valley, Kita, Dong, Xie, Yang, Zhang and Gao2009); the second one is 2.78 Ga recorded in Sample L55, which is in accordance with the results obtained by Diwu et al. (Reference Diwu, Sun, Ling and Wang2010).

5. Discussion

Zhao et al. (Reference Zhao, Cawood, Li, Wilde, Sun, Zhang, He and Yin2012) summarized the metamorphic chronological data obtained from the different metamorphic terranes within the TNCO, and the dataset suggests that the metamorphism occurred during ~1.89–1.80 Ga based on Sm–Nd isochron dating of metamorphic minerals, electron microprobe analyser (EMPA) chemical dating of metamorphic monazites, 40Ar–39Ar dating of metamorphic hornblendes and a great quantity of single grain evaporation, sensitive high-resolution ion microprobe (SHRIMP) and inductively coupled plasma mass spectrometry (ICP-MS) U–Pb dating of metamorphic zircons. Furthermore, it is noted that the Wutai metamorphic terrane, in the middle segment of the TNCO, experienced metamorphism dated at about 1.91 Ga based on Sm–Nd isochron dating of metamorphic minerals from the garnet-bearing amphibolites, and at about 1.92 Ga via EMPA chemical dating of metamorphic monazites from the metapelites (Liu et al. Reference Liu, Shu, Pan and Dang2004). For the Lüliang metamorphic terrane, in the middle segment of the TNCO, the metamorphism was dated to have occurred at about ~1.90–1.86 Ga via EMPA chemical dating of metamorphic monazites from the metapelites and granitic gneisses (Liu et al. Reference Liu, Zhang, Liu, Li, Lü, Yu, Tian and Feng2007); the Zhongtiao metamorphic terrane, in the southern segment of the TNCO, was dated to have been metamorphosed at about 1.88~1.85 Ga through EMPA chemical dating of metamorphic monazites from the monzogranitic gneisses (Liu et al. Reference Liu, Zhang, Liu, Li, Lü, Yu, Tian and Feng2007).

In recent years, metamorphic geochronological data from the Taihua metamorphic complex have also been reported (Fig. 5), including 1.84 and 1.87 Ga ages from the Upper Taihua Subgroup, Lushan terrane by Wan et al. (Reference Wan, Wilde, Liu, Yang, Song and Yin2006), 1.94 Ga to 1.97 Ga ages from metamorphic zircons through LA-ICP-MS dating in the Luoning terrane (Jiang et al. Reference Jiang, Wang, Xiao, Diwu, Lu and Wu2011), 1.96 Ga to 1.82 Ga ages through LA-ICP-MS and CAMECA SIMS U–Pb dating of metamorphic zircons and a ~1.80 Ga age obtained by 40Ar–39Ar dating of metamorphic hornblendes in the Huashan terrane (Wang et al. Reference Wang, Wang, Chen, Lu, Xiao and Wu2012, Reference Wang, Lu, Wang, Chen, Xiao, Diwu, Ji and Wu2013), 1.94 Ga to 1.86 Ga ages through SHRIMP dating in the Huashan terrane (Huang, Wilde & Zhong, Reference Huang, Wilde and Zhong2013) and 1.88 Ga to 1.87 Ga ages through LA-ICP-MS dating of metamorphic zircons in the Huashan terrane (Yu et al. Reference Yu, Liu, Li, Chen and Dai2013). These data further suggest that the tectonometamorphic event of the TNCO started at least as early as ~1.96 Ga, much earlier than the previously deciphered ~1.85 Ga (Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Sun, Wilde and Li2005, Reference Zhao, Cawood, Li, Wilde, Sun, Zhang, He and Yin2012).

Figure 5. Sketch metamorphic ages of the metamorphic terranes within the Trans-North China Orogen (TNCO) (after Zhao & Zhai, Reference Zhao and Zhai2013). Abbreviations for the metamorphic complexes: CD – Chengde; DF – Dengfeng; FP – Fuping; HA – Huai’an; HS – Hengshan; LL – Lüliang; NH – Northern Hebei; TH – Taihua; WT – Wutai; XH – Xuanhua; ZH – Zanhuang; ZT – Zhongtiao.

Different types of rocks collected from the Upper Taihua Subgroup and the Lower Taihua Subgroup in the Lushan terrane all record an identical metamorphic event varying from 1.96 Ga to 1.86 Ga, which is in accordance with our previous study (Lu et al. Reference Lu, Wang, Wang, Chen and Wu2013). Therefore, it may be confidently concluded that the Taihua metamorphic complex obviously records an older metamorphic process than the northern metamorphic terranes of the TNCO.

Therefore, the orogenic process of the southernmost terminal of the TNCO including subduction, collision and tectonic denudation not only occurred earlier than, but lasted longer than the metamorphic terranes of the northern sections of the TNCO. Meanwhile, it is noticed that the tectonometamorphic event of the Taihua metamorphic complex was almost coeval with the collision of the Jiao-Liao-Ji Belt at ~1.95 Ga (Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Li, Ayers, Xia and Zhang2008; Lu et al. Reference Lu, Wu, Guo, Wilde, Yang, Liu and Zhang2006; Tam et al. Reference Tam, Zhao, Liu, Zhou, Sun and Li2011) and at about ~1.96 Ga of the Khondalite Belt (Yin et al. Reference Yin, Zhao, Sun, Xia, Wei and Leung2009, Reference Yin, Zhao, Guo, Sun, Xia, Zhou and Liu2011; Zhou & Geng, Reference Zhou and Geng2009).

The ubiquitously distributed undeformed, unmetamorphosed Palaeoproterozoic sedimentary rocks (Liu et al. Reference Liu, Zhao, Liu, Sun, Zhang and Yin2012) and mafic dykes (~1.77 Ga) (Wang et al. Reference Wang, Fan, Zhang, Guo, Zhang and Peng2004, Reference Wang, Zhao, Cawood, Fan, Peng and Sun2008; Peng et al. Reference Peng, Zhai, Zhang and Guo2005, Reference Peng, Zhai, Ernst, Guo, Liu and Hu2008, Reference Peng, Liu, Zhai and Guo2012; Peng, Zhai & Guo, Reference Peng, Zhai, Guo, Hanski, Mertanen, Rämö and Vuollo2006) throughout the TNCO post-date the collision events of the orogenic belt.

6. Brief conclusion

  1. (1) Combining available data, it is concluded that the different metamorphic rocks that crop out within the Lushan terrane, including the amphibolites, TTG gneisses and gneissic granitoid, all experienced the Palaeoproterozoic metamorphic event at about 1.96–1.86 Ga.

  2. (2) The possible subduction and collision of the southernmost terminal of the TNCO started obviously earlier than that of the northern terranes of the TNCO.

Acknowledgements

We sincerely thank professors Xian-Hua Li and Qiu-Li Li and Mr Ya-Nan Yang for their help in secondary ion mass spectrometry (SIMS) dating of the zircons. An anonymous referee and the editorial review greatly improved the quality of the original manuscript. This work was financially supported by the National Natural Science Foundation of China (41225007, 40872125) and the State Key Laboratory of Geodynamics, Northwest University (BJ11059).

Supplementary material

To view supplementary material for this article, please visit http://dx.doi.org/10.1017/S0016756814000430.

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Figure 0

Figure 1. (a) Geological sketch map of the North China Craton (after Zhao et al.2005); (b) the Lushan terrane in the Taihua metamorphic complex within the Trans-North China Orogen (modified after Xu et al.2009); and (c) simplified geological map of the Lushan terrane (modified after Xu et al.2009). Sample locations are shown. Pt2ry – Ruyang Group; Pt2xn – Xiong’er Group; Pt3 – Late Proterozoic; ϵ – Cambrian; Q – Quaternary; KB – Khondalite Belt; WB – Western Block; TNCO – Trans-North China Orogen; EB – Eastern Block; JLJB – Jiao-Liao-Ji Belt.

Figure 1

Figure 2. Photomicrographs of amphibolites (a, b), TTG gneisses (c, d, e) and gneissic granitoid (f). Symbols for minerals are after Whitney & Evans (2010).

Figure 2

Figure 3. Representative cathodoluminescence (CL) images of zircons separated from amphibolites (a, b), TTG gneisses (c, d, e) and gneissic granitoid (f), Lushan terrane. The numbers refer to the analytical spots and the corresponding data are list in Table S1 in the online Supplementary Material available at http://journal.cambridge.org/geo.

Figure 3

Figure 4. The U–Pb Concordia diagrams for SIMS U–Pb dating of zircons from the amphibolites (a–c), TTG gneisses (d–g), as well as from the gneissic granitoid (h).

Figure 4

Figure 5. Sketch metamorphic ages of the metamorphic terranes within the Trans-North China Orogen (TNCO) (after Zhao & Zhai, 2013). Abbreviations for the metamorphic complexes: CD – Chengde; DF – Dengfeng; FP – Fuping; HA – Huai’an; HS – Hengshan; LL – Lüliang; NH – Northern Hebei; TH – Taihua; WT – Wutai; XH – Xuanhua; ZH – Zanhuang; ZT – Zhongtiao.

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