Th–U–Pb monazite geochronology of the Lüliang and Wutai Complexes: Constraints on the tectonothermal evolution of the Trans-North China Orogen

Precambrian Research 148 (2006) 205–224

Th–U–Pb monazite geochronology of the L¨uliang and Wutai Complexes: Constraints on the tectonothermal evolution of the Trans-North China Orogen Shuwen Liu a,∗ , Guochun Zhao b , Simon A. Wilde c , Guiming Shu a , Min Sun b , Qiugen Li a , Wei Tian a , Jian Zhang b a

c

The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, China, School of Earth and Space Sciences, Peking University, Beijing 100871, China b Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Applied Geology, Curtin University of Technology, P.O. Box U1987, Perth 6845, WA, Australia Received 7 February 2006; received in revised form 13 April 2006; accepted 22 April 2006

Abstract Recent lithological, metamorphic and geochronological studies show that the basement of the North China Craton (NCC) formed by collision of two discrete Archean to Paleoproterozoic blocks (Eastern and Western Blocks) along a Paleoproterozoic orogen, named the Trans-North China Orogen. However, the timing of the collision is controversial and post-collisional exhumation has not been precisely dated. This study applies the electron microprobe Th–U–Pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low- to medium-grade L¨uliang and Wutai Complexes in the Trans-North China Orogen. Electron microprobe Th–U–Pb monazite data for the L¨uliang Complex reveal five ThO2 * /PbO age ranges: (1) 1940–1938 Ma, (2) 1880–1847 Ma, (3) 1795–1755 Ma, (4) 1720–1703 Ma, and (5) ∼1648 Ma. Of these age ranges, the first four have also been recorded in the Wutai Complex, which yields ThO2 * /PbO ages of (1) ∼1930 Ma, (2) 1838–1822 Ma, (3) ∼1793 Ma, and (4) ∼1719 Ma. The oldest age range of 1940–1930 Ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the L¨uliang and Wutai Complexes. The age range of 1882–1822 Ma is in accord with the SHRIMP U–Pb ages of metamorphic zircons and mineral Sm–Nd and 40 Ar/39 Ar ages obtained for other complexes in the orogen, and is interpreted as the age of the major metamorphic event caused by amalgamation between the Eastern and the Western Blocks. The age range of 1795–1755 Ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800–1765 Ma, interpreted as the time of post-orogenic extension. The 1720–1703 Ma and ∼1648 Ma ages are considered to date later multiple stages of hydrothermal alteration, since monazites with such young ages only occur along fractures in older monazite grains and at the rims. These new monazite ThO2 * /PbO ages support the tectonic model for the evolution of the North China Craton that envisages discrete Eastern and Western Blocks colliding along the Trans-North China Orogen at ∼1.85 Ga and then undergoing post-collisional extension in the period 1795–1755 Ma. © 2006 Elsevier B.V. All rights reserved. Keywords: Electron microprobe dating; Th–U–Pb monazite ages; Crystal age mapping; Paleoproterozoic; Trans-North China Orogen; North China Craton

∗

Corresponding author. Tel.: +86 10 62754163; fax: +86 10 62751159. E-mail address: [email protected] (S. Liu).

0301-9268/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.04.003

206

S. Liu et al. / Precambrian Research 148 (2006) 205–224

1. Introduction The most important progress in recent years in the study of the North China Craton (NCC) is the recognition of an orogenic belt, named the Trans-North China Orogen (Zhao et al., 1998), which divides the craton into two discrete units: the Western and Eastern Blocks (Zhao et al., 1998, 1999a, 1999b, 1999c, 2001a, 2001b, 2005; Wilde et al., 1998, 2002). Based on available lithological, structural, metamorphic and geochronological data, Zhao et al. (2001b) suggested that the Trans-North China Orogen represents a collisional orogen along which the Eastern and Western Blocks were amalgamated to form the North China Craton. This tectonic scenario has subsequently been accepted and advanced by many researchers (Wu and Zhong, 1998; Guan et al., 2002; Guo and Zhai, 2001; Guo et al., 2002; Liu et al., 2002a, 2002b, 2004a, 2004b; Wilde et al., 2002, 2004a, 2004b, 2005; Kr¨oner et al., 2002, 2005a, 2005b; Wu and Zhong, 1998; O’Brien and Rotzler, 2003; Zhai and Liu, 2003; Zhai et al., 2003). However, controversy has surrounded the timing of collision between the Eastern and Western Blocks, with one school of thought proposing that the collision occurred at ∼2.5 Ga (Li et al., 2000; Kusky et al., 2001; Li et al., 2002; Kusky and Li, 2003; Polat et al., 2005), whereas others believe that the final amalgamation of the two blocks was completed at ∼1.85 Ga (Zhao, 2001; Guo and Zhai, 2001; Guo et al., 2002; Kr¨oner et al., 2002, 2005a, 2005b; Guan et al., 2002; Zhao et al., 2002; Wilde et al., 2002, 2004a, 2004b, 2005). In the last few years, some important geochronological data have been obtained for the Fuping, Hengshan and Huai’an Complexes, using the SHRIMP U–Pb zircon and mineral Sm–Nd dating techniques. These data show that metamorphism of these complexes occurred in the period 1880–1820 Ma, suggesting that the collision between the Eastern and Western Blocks took place at ∼1.85 Ga (Zhao et al., 2002, 2005; Guan et al., 2002; Wilde et al., 2002, 2005; Kr¨oner et al., 2002, 2005a, 2005b; Guo et al., 2005; Wang et al., 2001; Wang et al., 2003, 2004a, 2004b; Liu et al., 2002a, 2002b). However, these ages were obtained from upper amphibolite to granulite facies rocks from complexes in the central part of the orogen, and few metamorphic ages have been obtained from low- to medium-grade metamorphic rocks. Recently, it has been established that the same tectonothermal imprint is recorded in the central eastern part of the orogen at Zanhuang (Wang et al., 2004a, 2004b) and at Lushan in the southern part of the orogen (Wan et al., 2006) (Fig. 1), alluding to the wide extent of the activity in the Trans-North China Orogen. However, few data can be used to constrain the timing of post-

collisional events, which is crucial in understanding the full tectonothermal history of the orogen. The electron microprobe Th–U–Pb monazite dating technique (including age mapping) has been widely used to date syn- and post-orogenic tectonothermal events, especially for low- to medium-grade metamorphic terrains where the application of SHRIMP U–Pb zircon dating is limited because of the lack of metamorphic zircon (Suzuki and Adachi, 1991, 1998; Montel et al., 1996, 2000; Braun et al., 1998; Cocherie et al., 1998; Cocherie and Albarede, 2001; Cocherie et al., 2005; Williams, 1998; Santosh et al., 2005). The major advantages of using the electron microprobe Th–U–Pb monazite dating technique include rapid analysis with minimal preparation, low cost and excellent spatial resolution (spot size <2 ␮m), which enables Th–U–Pb analysis on a small-scale, and in situ age mapping of individual monazite grains (Zhu et al., 1997; Zhu and O’Nions, 1999; Teufel and Heinrich, 1997; Catlos et al., 2002; Mathieu et al., 2001; Rasmussen et al., 2001). In addition, numerous studies have shown that monazite can easily dissolve and re-precipitate at low grades of metamorphism, but that Pb-diffusion in monazite is insignificant after its formation, which enables the Th–U–Pb dating of monazite to be a powerful tool for obtaining reliable metamorphic ages from low- to medium-grade metamorphic rocks (Zhu et al., 1997; Zhu and O’Nions, 1999; Catlos et al., 2002; Cocherie et al., 2005; Dahl et al., 2005; Goncalves et al., 2005; Seydoux-Guillaume et al., 2005). The electron microprobe chemical Th–U–Pb dating method is limited in its application to calcium-rich rocks, which have low monazite contents, and also to rocks altered strongly by later hydrothermal fluids, since any pre-existing monazite grains are easily altered through dissolution and precipitation. The precision of the electron microprobe method is also usually lower than the SHRIMP U–Pb, zircon dating technique. However, recently published data show that the precision of electron microprobe chemical dating of monazite for Precambrian geological samples can be improved from ca. 10% ten years ago (Montel et al., 1996) to ca. 1–3% of sample age either through the use of the new model of electron microprobes or by improved calculation methods (Cihan et al., 2006; Swain et al., 2005; Mezeme et al., 2005; Cocherie et al., 1998, 2005; Cocherie and Albarede, 2001; Pyle et al., 2005). This dating precision can now effectively distinguish metamorphic and deformation episodes within a single orogenic event, making the chemical Th–U–Pb dating of monazite a powerful tool when applied to medium- to low-grade metamorphism and associated deformation.

S. Liu et al. / Precambrian Research 148 (2006) 205–224

207

Fig. 1. Map of the North China Craton showing its classification into Eastern and Western Blocks and the Trans-North China Orogen (after Zhao et al., 1998, 2001a, 2001b, 2005). The area as of Figs. 2 and 3 are outlined.

In this contribution, we apply the electron microprobe Th–U–Pb monazite dating technique to the L¨uliang and Wutai Complexes in order to determine the timing of syn- and post-collisional events in the Trans-North China Orogen. The reasons for choosing the L¨uliang and Wutai Complexes is because both areas are at low to medium metamorphic grade and are two of the ‘classical’ complexes in the North China Craton where the “L¨uliang” and “Wutai” ‘movements’ (i.e., tectono-thermal events) were originally established. 2. Geological setting 2.1. Trans-North China Orogen The North China Craton refers to the Chinese part of the Sino–Korea Platform, covering most of north China, the southern part of northeast China, Inner Mongolia, Bohai Bay and the northern part of the Yellow Sea. The North China Craton can be divided tectonically into the Eastern and Western Blocks, separated by the Trans-North China Orogen (Fig. 1; Zhao et al., 1998, 1999c, 2001b, 2005). Detailed lithological, geochemical, structural, metamorphic and geochronological differences between the basement rocks of the Eastern and Western Blocks and the Trans-North China Orogen

and their possible tectonic evolution have been summarized by Zhao et al. (2001b) and are not repeated here. The Trans-North China Orogen is a nearly south to northtrending zone, ∼1200 km long and 100–300 km wide, and it includes several basement complexes (Fig. 1). The main lithotectonic features of the Trans-North China Orogen include reworked Archean basement rocks with the addition of Paleoproterozoic juvenile crust (Sun et al., 1992; Wan et al., 2000; Zhao et al., 2000a), linear structural belts defined by strike-slip ductile shear zones, large-scale thrusting and folding, with transcurrent tectonics (Li and Qian, 1991), sheath folds and strong mineral lineations (Wu and Zhong, 1998), local high-pressure granulites and retrograded eclogites (Zhai et al., 1993; Guo et al., 1993, 2002; Guo and Zhai, 2001; Zhao et al., 2001b, 2001a), clockwise metamorphic P–T paths involving near-isothermal decompression (Zhao et al., 2000a, 2000b; Guo et al., 2002), ancient oceanic fragments and ophiolitic m´elange (Li et al., 1990; Bai, 1986; Bai et al., 1992, 1992; Wang et al., 1996; Wu and Zhong, 1998), syn- or post-tectonic granites (Liu et al., 2000, 2005), and post-collisional mafic dyke swarms (Halls et al., 2000; Zhao et al., 2001b; Kr¨oner et al., 2006). Most of these lithotectonic elements are classical indicators of collision tectonics and have been used to support the view that the Trans-North China Orogen is a

208

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Fig. 2. Sketch map of the L¨uliang Complex in the Trans-North China Orogen, showing the spatial distribution of the Chijianling and Guandishan Granitoids, and the Jiehekou, L¨uliang and Yiejishan Groups. Stars show the sample locations.

major collisional belt (Zhao et al., 1999c, 2000a, 2001b, 2005). 2.2. L¨uliang Complex The L¨uliang Complex is located in the central segment of the Trans-North China Orogen (Figs. 1 and 2) and consists of Paleoproterozoic supracrustal rocks and granitoid plutons (Geng et al., 2000; Wan et al., 2000). Five major lithological assemblages have been

recognized (Fig. 2): (1) the Archean to Paleoproterozoic Jiehekou Group, (2) the Paleoproterozoic Yiejishan Group, (3) the L¨uliang Group, (4) the Chijianling granitoids, and (5) granitoids of the Guandishan complex. Recent geochronological studies reveal that most lithological units in the L¨uliang Complex formed in the period 2.3–1.8 Ga (Yu and Wang, 1999; Geng et al., 2000, 2003, 2004; Wan et al., 2000), in contrast with the Wutai, Fuping, and Hengshan Complexes where the major lithologies formed in the late Archean (Guan et al., 2002; Guo et

S. Liu et al. / Precambrian Research 148 (2006) 205–224

al., 2002; Liu et al., 2002a, 2002b, 2004a, 2004b, 2005; Wilde et al., 2002, 2004a, 2004b, 2005; Kr¨oner et al., 2002, 2005a, 2005b; Zhao et al., 2001b, 2002). The Jiehekou Group is developed in the western part of the complex and consists mainly of graphite-bearing pelitic gneiss/schist, quartzite, felsic paragneiss, marble, calc-silicate rock and minor amphibolite. It is considered to have developed at a Paleoproterozoic passive continental margin and was metamorphosed to amphibolite facies at 2.03 ± 0.05 Ga (Wan et al., 2000; Geng et al., 2000). The Yejishan Group occurs east of the Jiehekou Group and consists of green schist facies metabasalts and minor meta-rhyolites in the lower part and flyschtype sedimentary sequences in the upper part. Geng et al. (2000) reported a zircon U–Pb age of 2124 ± 38 Ma for a meta-volcanic rock of the Yejishan Group. The Yiejishan Group is interpreted to have formed in a rift zone (Geng et al., 2003). The L¨uliang Group occurs in the central part of the complex and comprises greenschist- to amphibolitefacies meta-sedimentary rocks in the lower part and meta-volcanic rocks (i.e., basalts to rhyolites) in the upper part (Yu et al., 1997). A Sm–Nd whole rock isochron age of 2360 ± 95 Ma (Geng et al., 2000) and zircon U–Pb ages of 2051 ± 68 Ma and 2099 ± 41 Ma (Yu et al., 1997) have been reported for metavolcanic rocks of the group. On the basis of the occurrence of metamorphosed basaltic and rhyolitic rocks, and limited whole-rock geochemical data, Yu et al. (1997) suggested that the L¨uliang Group probably formed in a rift marginal to a continental block. Similarly, Geng et al. (2003) suggested that the metamorphic volcanic rocks of both the L¨uliang and Yejishan Groups developed during Paleoproterozoic rifting. Major granitoid intrusions occur throughout the L¨uliang Complex and comprise the Chijianling granitoids and the Guandishan granitoids (Fig. 2). The Chijianling granitoids consist of gneissic diorite, tonalite, granodiorite, and minor monzogranite, with gneissic diorites and granodiorites yielding SHRIMP U–Pb zircon ages of 2151 ± 12 Ma and 2152 ± 35 Ma, respectively (Geng et al., 2000, 2004); these rocks exhibit geochemical characteristics of calc-alkaline granitoids. The Guandishan granitoids consist predominantly of gneissic and massive granodiorite and garnet-bearing monzogranite (e.g., Huijiazhuang and Shizhuang intrusions), of which a gneissic garnet monzogranite, considered to be syn-collisional, gives a SHRIMP U–Pb zircon age of 1848 ± 32 Ma and the red massive coarse-grained monzogranite, considered to be post-collisional, yields a SHRIMP U–Pb zir-

209

Fig. 3. Geological sketch map of the Wutai Complex in the TransNorth China Orogen. Stars show the sample locations and Sz10-2 is the site of sample in Liu et al. (2004a).

con age of 1805 ± 8 Ma (Fig. 2; Geng et al., 2000, 2004). The L¨uyashan intrusion in the northernmost part of the L¨uliang Complex (Fig. 2) consists of weakly gneissic enderbite, charnockite and massive quartz monzonite, of which the charnockite gives a SHRIMP U–Pb zircon age of 1800 ± 7 Ma and the quartz monzonite an age of 1794 ± 13 Ma (Geng et al., 2000, 2003, 2004). 2.3. Wutai Complex The Wutai Complex consists of late Archean to Palaeoproterozoic granitic plutons and metamorphosed volcanic and sedimentary rocks, traditionally named the Wutai and Hutuo “Groups” in the Chinese literature (Fig. 3). The Wutai “Group” is conventionally subdivided into the Lower, Middle and Upper Wutai Subgroups (Bai, 1986; Tian et al., 1992), although the presence of a true stratigraphy has recently been questioned (Wilde et al., 2005; Kr¨oner et al., 2005a, 2005b). The Lower Wutai is composed mainly of amphibolite facies peridotites, oceanic tholeiites, cherts, banded iron formations, sandstones, siltstones, shales, calc-silicate rocks and minor limestones, of which the peridotites, oceanic tholeiites and cherts are considered to represent relict oceanic crust (Wang et al., 2004a, 2004b; Polat et al., 2005), whereas sandstones, siltstones, shales, calc-silicate rocks and minor limestones are interpreted as continental margin or back-arc basin sediments (Li et al., 1990; Bai et al., 1992; Wu and Zhong, 1998). The Middle Wutai comprises felsic volcanic rocks and tholeiitic basalts, metamorphosed at greenschist facies.

210

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Geochemical data show that both the felsic and tholeiitic volcanics have volcanic-arc affinity (Wang et al., 2004a, 2004b). Structural, geochronological, and geochemical data suggest that the Middle Wutai was formed in a forearc tectonic environment between 2530 Ma and 2515 Ma. The Upper Wutai consists of conglomerates, quartz wackes, siltstones, and minor mafic to felsic volcanic rocks, metamorphosed at sub-greenschist to greenschist facies, which are interpreted as developing in an intra-arc basin and/or a retro-arc foreland basin (Zhao et al., 2001a). Recent SHRIMP U–Pb zircon data indicate that the volcanic and sedimentary rocks of the “Wutai Group” formed at 2515–2530 Ma (Wilde et al., 1998, 2004a, 2004b). Neoarchean granitoids in the Wutai Complex comprise the Chechang-Beitai, Ekou, Lanzishan and Shifo intrusions (Fig. 3) that are strongly deformed with penetrative foliations, and are thus considered to be pretectonic granites (Tian et al., 1992). New SHRIMP U–Pb zircon data indicate that these granitoids formed between 2531 Ma and 2566 Ma (Wilde et al., 1998, 2005). Geochemical and Sm–Nd isotopic data suggest that they were mostly derived from partial melting of juvenile island arc volcanic rocks (Sun et al., 1992; Liu et al., 2004a, 2004b, 2002b; Zhang et al., 2004). The Wutai Complex also contains several Paleoproterozoic granitoid intrusions, including the Dawaliang pluton and the younger phase of the Wangjiahui granite, of which the former yields a SHRIMP U–Pb zircon age of 2176 ± 12 Ma (Wilde et al., 1997), whereas three samples of the latter were dated at 2117 ± 17 Ma, 2116 ± 16 Ma and 2084 ± 20 Ma, respectively (Wilde et al., 2005). The Wutai Complex is unconformably overlain and also structurally interleaved, by the Paleoproterozoic Hutuo Group, which consists of greenschist-facies metamorphic clastic and carbonate rocks, with minor amounts of tholeiitic metabasalts and associated volcanoclastic rocks, of which a felsic volcanoclastic rock yields two SHRIMP U–Pb zircon age groups at 2180 ± 5 Ma and 2087 ± 9 Ma (Wilde et al., 2004a, 2004b), with the latter being interpreted as the time of eruption. 3. Analytical methods and data treatment The internal structures of monazite grains obtained from samples from the L¨uliang and Wutai complexes were observed under an electron microprobe using backscattered electrons (BSE) and representative images were taken. The Th–U–Pb analyses of monazites were carried out using a JEOL JXA-8100 electron microprobe housed in the Key Laboratory of Orogenic Belts

and Crustal Evolution, Peking University. Analytical methods are briefly described below. Electron beam size was adjusted to 1 ␮m, 15.0 kV accelerating voltage and a 1.004E-07 A current was used to obtain the highest peak/background values and spatial resolution. On the basis of previous experimental data and our own experimental results, we selected crystals PETH for U, and PETJ for Th and Pb compositions. Referring to previous experiments (Montel et al., 1996; Suzuki and Adachi, 1991, 1998; Rhede et al., 1996), the X-ray spectral lines used were YLα, ThMα and PbMα, and UMβ. Spectral interferences of YLγ on PbMα were corrected on line by Lα/Lγ of Y in YAG. ThMξ on UMβ correction was made using the correction factor of Geisler and Schleicher (2000). Background correction used a ZAF model in all cases. We used a ThO2 crystal for the Th standard, metal U for U, and a PbCrO4 crystal for Pb. Counting times (peak + background) were projected to 120 s, 180 s, and 270 s for Th, U and Pb, respectively. The detection limit is about 90 ppm for Pb. Systematic errors were estimated as 1.73% for Th, 0.336% for U, 0.27% for Pb and 0.23% for Y by tests on compositional uniformity of each standard. The analytical stability was also tested by a large number of repeat measurements on a WB.T.329 standard monazite with reference age of 1766 Ma. Concentration errors were calculated at the 2␴ level (Montel et al., 1996) and were propagated through the age equation to reduce age errors at the 2␴ level (Williams et al., 1999). Our estimation method of compositional uncertainty of Th–U–Pb is similar to that applied by Montel et al. (1996) and Pyle et al. (2005) who use the standard deviations (%) in X-ray counts for Th–U–Pb analyses as individual 1␴ compositional uncertainties. Then, the counting standard deviation (%) is multiplied by the respective analytical values of Th–U–Pb as an analytical error of that composition. These analytical errors are then propagated through the age equations to obtain the age error for each analytical spot. A chemical Th–U–Pb apparent age of each analytical spot and weighted mean apparent age for every given data set were calculated using the procedures of Montel et al. (1996), and the most probable ages, proportions, and distinct components in a given data set were determined by mixture modeling, assuming a Gaussian age error distribution by the method described in Sambridge and Compston (1994). If multiple models of apparent ages were shown in an analytical grain or sample, every model was treated as a sub-dataset limited by the two bordering troughs of a main age peak on the mixture modeling curve, then this sub-dataset is used to calculate a weighted mean apparent age and an isochron age. The isochron age

S. Liu et al. / Precambrian Research 148 (2006) 205–224

for each data set was calculated using the method of Suzuki and Adachi (1991), where non-radiogenic Pb is considered negligible in the age calculation. Decay constants used are λTh232 = 4.9475 × 10−11 /year, λU238 = 1.55125 × 10−10 /year and λU235 = 9.8485 × 10−10 /year. The UO2 concentration is converted into equivalent ThO2 content (i.e., the amount of ThO2 necessary to produce the same amount of PbO) and this value is added to the measured values of ThO2 , which results in an apparent ThO2 * amount, and the data are plotted in a PbO versus ThO2 * diagram to obtain the isochron age (Suzuki and Adachi, 1998; Kato et al., 1999; Tickyj et al., 2004; Biju-Sekhar et al., 2003). The apparent age for each spot and isochron weighted mean apparent age for a data set were calculated using ChemAge software (Geisler and Schleicher, 2000). The isochron age was computed in the York Ib model to force the regression line through the origin of PbO versus ThO2 * space on the basis of non-radiogenic Pb being assumed as zero and also based on no significant Pb loss (Zhu et al., 1997; Zhu and O’Nions, 1999; Teufel and Heinrich, 1997; Catlos et al., 2002; Mathieu et al., 2001; Rasmussen et al., 2001). The calculations for the slope error of the regression line and age error have been documented elsewhere (Geisler and Schleicher, 2000; Suzuki and Adachi, 1991; Kato et al., 1999). 4. Sample characteristics and results In this study, we collected two samples (IL003-2 and L002-4) from the L¨uliang Complex and one sample (S2010-2) from the Wutai Complex for electron microprobe chemical Th–U–Pb dating of monazite. 4.1. Sample IL003-2 Sample IL003-2 is a fine-grained garnet-bearing felsic gneiss from the Jiehekou Group, located 5 km west of Dongshe (Fig. 2). The rock consists of K-feldspar, plagioclase, quartz, biotite and garnet, with accessory monazite, zircon, ilmenite and other opaque minerals. We consider that the protolith of this rock was probably a felsic igneous rock on the basis of its major and assessory mineral associations. This sample was processed by heavy mineral separation involving firstly crushing to 40–60 mesh size, initial heavy liquid separation and then subsequent electromagnetic separation. Monazites were hand-picked and mounted onto double-sided adhesive tape, enclosed in epoxy resin and then polished. The mount was cleaned and carbon-coated prior to analysis. The crystals were then imaged in BSE using an electron microprobe. Dur-

211

ing analysis, the analytical spot sites were carefully marked onto the BSE images. The analytical sites are shown on Fig. 4A and B and Th–U–Pb analyses of grains IL003-2-1 and IL003-2-2 are listed in Table 1. Monazite grain IL003-2-1 exhibits a patchy internal structure and IL003-2-2 displays irregular zones, reflected by contrasting grey patches in the BSE image (Fig. 4A and B). A total of 58 analyses were carried out on monazite grain IL03-2-1. Based on the apparent age distribution on the BSE image, these analyses can be divided into four groups which define domains A, B, C and D, respectively. Of these, domain A comprises four analyses encircled by a white dashed line in Fig. 4A, 31 in domain B (between the black and white dashed lines in Fig. 4A), seven in isolated domains encircled by black solid lines (domain C) and 16 in domain D The analytical spots in domain A yield apparent ages ranging from 1968 Ma to 1923 Ma, with a weighted mean apparent age of 1942 ± 10 Ma (Fig. 4C). Analytical spots in domain B give apparent ages of 1901–1802 Ma, with a weighted mean apparent age of 1851 ± 7 Ma (Fig. 4C). Analytical spots in domain C give apparent ages of 1789–1761 Ma with a weighted mean apparent age of 1774 ± 8 Ma (Fig. 4C), whereas analytical spots made in domain D yield apparent ages of 1747–1663 Ma, with a weighted apparent mean age of 1705 ± 3 Ma (Fig. 4A). Mixture modeling of the total analyses from each domain yields the most likely ages of 1937 ± 23 Ma, 1857 ± 7 Ma, 1778 ± 14 Ma and 1697 ± 9 Ma (Fig. 4C). The data sets define ThO2 * /PbO isochron ages of 1938 ± 10 Ma (MSWD = 0.92) for analytical spots in domain A, 1847 ± 7 Ma (MSWD = 1.38) for analytical spots in domain B, 1771 ± 58 a (MSWD = 0.61) for analytical spots in domain C and 1703 ± 11 Ma (MSWD = 1.56) for analytical spots in domain D (Fig. 4C), which are comparable to the corresponding weighted mean apparent ages of all four domains. Mixture modeling of the total data set in each domain yields the most likely four peaks of ages at 1937 ± 23 Ma, 1857 ± 5 Ma, 1778 ± 14 Ma and 1697 ± 9 Ma (Fig. 4D). A total of 54 analyses were made on monazite grain IL003-2-2, of which eight were made in domain A encircled by a white dashed line in Fig. 4B, 15 analyses were made on domain B between the whiteand black-dashed lines, eight analyses were made in domain C between the black solid lines, and 23 analyses were made in the outer domain D (Fig. 4B). Analytical spots in domain A yield apparent ages ranging from 1963 Ma to 1920 Ma, with a weighted mean apparent age of 1945 ± 6 Ma (Fig. 4E); the spots in domain B yield apparent ages of 1911–1809 Ma, with a weighted

212

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Fig. 4. Electron microprobe Th–U–Pb analytical spots (age mapping) plotted on back-scattered electron images (BSE) of monazite grains IL003-2-1 (A) and IL003-2-2 (B) from sample IL003-2. Note: Monazite grain IL003-2-1 (A) shows age zoning with four age domains and grain IL003-2-2 (B) also shows four age domains that define the age zoning. Isochron ages are shown for grain IL003-2-1 in (C) and the mixture modeling peak ages in (D). Isochron ages are shown for grain IL003-2-2 in (E) and the mixture modeling peak ages in (F).

S. Liu et al. / Precambrian Research 148 (2006) 205–224

213

Table 1 Electronic microprobe analytical Th–U–Pb data (wt.%) of monazites for sample IL003-2 Label

PbO

Monazite grain of IL003-2-1 1.1 0.2973 1.3 0.3599 1.4 0.4106 1.5 0.4038 1.6 0.3899 1.7 0.4059 1.8 0.4064 1.9 0.3996 1.10 0.4140 2.3 0.3515 2.4 0.4260 2.5 0.5511 2.6 0.4210 2.7 0.3552 2.8 0.3902 2.9 0.4054 2.1 0.3671 3.3 0.3776 3.4 0.3926 3.5 0.4224 3.6 0.3928 3.7 0.4464 3.8 0.5142 3.9 0.4732 3.1 0.5082

Err(Pb)

UO2

Err(U)

ThO2

Err(Th)

Total

T(Ma)

Err(T,Ma)

0.0008 0.0010 0.0011 0.0011 0.0011 0.0011 0.0011 0.0011 0.0011 0.0009 0.0012 0.0015 0.0011 0.0010 0.0011 0.0011 0.0010 0.0010 0.0011 0.0011 0.0011 0.0012 0.0014 0.0013 0.0014

0.2879 0.3923 0.4390 0.4316 0.3781 0.4476 0.4485 0.4713 0.4646 0.3720 0.4328 0.5459 0.4774 0.4213 0.4175 0.4032 0.3901 0.3369 0.4286 0.4899 0.4459 0.4545 0.5587 0.5346 0.5719

0.0010 0.0013 0.0015 0.0015 0.0013 0.0015 0.0015 0.0016 0.0016 0.0012 0.0015 0.0018 0.0016 0.0014 0.0014 0.0014 0.0013 0.0011 0.0014 0.0016 0.0015 0.0015 0.0019 0.0018 0.0019

2.6622 3.1368 3.4955 3.3405 3.3721 3.3211 3.1861 3.1356 3.3020 2.8933 3.6372 4.5460 3.5568 2.9248 3.0442 3.4686 2.9718 3.2810 3.1524 3.3325 3.2387 3.8260 4.2221 3.7144 3.8069

0.0458 0.0540 0.0601 0.0575 0.0580 0.0571 0.0548 0.0539 0.0568 0.0498 0.0626 0.0782 0.0612 0.0503 0.0524 0.0579 0.0511 0.0564 0.0542 0.0573 0.0557 0.0658 0.0726 0.0639 0.0655

3.25 3.89 4.35 4.18 4.14 4.17 4.04 4.01 4.18 3.62 4.50 5.64 4.46 3.70 3.85 4.28 3.73 4.00 3.97 4.24 4.08 4.73 5.29 4.72 4.89

1816 1789 1825 1857 1856 1852 1899 1859 1870 1868 1848 1901 1802 1805 1923 1856 1885 1891 1879 1863 1828 1842 1856 1884 1938

21 20 20 20 21 20 20 19 19 20 20 21 19 19 20 20 20 22 20 19 19 20 20 19 20

A6 A8 A9 A10 A11 A12 A17 A18 A19 A20 A21 A22 A25 A26 A27 A28

0.3727 0.3437 0.4399 0.4464 0.4519 0.4040 0.4219 0.3762 0.4008 0.4368 0.4384 0.4782 0.3861 0.3645 0.3566 0.4658

0.0010 0.0009 0.0012 0.0012 0.0012 0.0011 0.0011 0.0010 0.0011 0.0012 0.0012 0.0013 0.0010 0.0010 0.0010 0.0013

0.4198 0.3434 0.5090 0.5023 0.5005 0.4742 0.4390 0.4563 0.3364 0.4237 0.4681 0.5620 0.4343 0.4365 0.4308 0.5546

0.0014 0.0012 0.0017 0.0017 0.0017 0.0016 0.0015 0.0015 0.0011 0.0014 0.0016 0.0019 0.0015 0.0015 0.0014 0.0019

3.2518 3.0357 3.5374 3.5460 3.6059 3.3532 3.5747 3.1062 3.4116 3.4395 3.7187 3.7879 3.1498 3.1020 3.1072 3.7205

0.0563 0.0525 0.0612 0.0613 0.0624 0.0580 0.0618 0.0537 0.0590 0.0595 0.0643 0.0655 0.0545 0.0537 0.0538 0.0644

4.04 3.72 4.49 4.49 4.56 4.23 4.44 3.94 4.15 4.30 4.63 4.83 3.97 3.90 3.89 4.74

1771 1817 1844 1873 1878 1802 1844 1789 1945 1968 1829 1851 1844 1765 1735 1835

19 21 19 20 20 19 20 19 23 21 20 19 19 19 19 19

1.2 2.1 2.2 3.1 3.2

0.4436 0.6021 0.5690 0.5857 0.5547

0.0012 0.0016 0.0015 0.0016 0.0015

0.3559 0.5434 0.5259 0.5248 0.4931

0.0012 0.0018 0.0018 0.0018 0.0017

4.5782 5.7964 5.5066 5.6133 5.5124

0.0787 0.0997 0.0947 0.0965 0.0948

5.38 6.94 6.60 6.72 6.56

1721 1761 1745 1769 1729

22 21 21 21 21

A1 A2 A3 A4 A5 A7 A13 A14 A15

0.4272 0.3972 0.3695 0.5277 0.3927 0.5625 0.3968 0.4544 0.5560

0.0012 0.0011 0.0010 0.0014 0.0011 0.0015 0.0011 0.0012 0.0015

0.2176 0.1980 0.2035 0.4505 0.2041 0.5540 0.2187 0.2290 0.4899

0.0007 0.0007 0.0007 0.0015 0.0007 0.0019 0.0007 0.0008 0.0016

4.9646 4.4949 4.2407 5.5036 4.6527 5.5314 4.5739 5.6018 5.7514

0.0859 0.0778 0.0734 0.0952 0.0805 0.0957 0.0791 0.0969 0.0996

5.61 5.09 4.81 6.48 5.25 6.65 5.19 6.29 6.80

1694 1735 1693 1686 1663 1700 1687 1617 1685

24 25 24 21 24 20 24 23 21

214

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Table 1 (Continued ) Label A16 A23 A24

PbO

Err(Pb)

UO2

Err(U)

ThO2

Err(Th)

Total

T(Ma)

Err(T,Ma)

0.5504 0.5597 0.5655

0.0015 0.0015 0.0015

0.5007 0.5273 0.5105

0.0017 0.0018 0.0017

5.6206 5.5106 5.5055

0.0972 0.0953 0.0952

6.67 6.60 6.58

1688 1717 1747

21 21 21

0.0013 0.0011 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0008 0.0017 0.0021 0.0018 0.0011 0.0012 0.0012 0.0012 0.0012 0.0012 0.0011 0.0015 0.0015 0.0016 0.0015 0.0012 0.0012 0.0012 0.0012 0.0012 0.0011

0.4961 0.4805 0.4195 0.4176 0.4167 0.4293 0.4272 0.3939 0.3482 0.403 0.5429 0.663 0.657 0.486 0.4662 0.5155 0.5212 0.5072 0.4982 0.4639 0.5472 0.548 0.5209 0.5954 0.4711 0.5026 0.5224 0.5280 0.4954 0.4749

0.0017 0.0016 0.0014 0.0014 0.0014 0.0014 0.0014 0.0013 0.0012 0.0014 0.0018 0.0022 0.0022 0.0016 0.0016 0.0017 0.0018 0.0017 0.0017 0.0016 0.0018 0.0018 0.0018 0.002 0.0016 0.0017 0.0018 0.0018 0.0017 0.0016

4.3172 3.6528 2.0207 2.1654 2.037 2.0048 2.0027 2.0024 2.1245 2.1893 6.0367 7.1194 6.5743 3.7067 3.6636 3.4022 3.3697 3.3529 3.3219 3.5995 5.4402 5.4154 5.9944 5.3205 3.8177 3.5482 3.3827 3.3731 3.3137 3.6069

0.0743 0.0628 0.0348 0.0372 0.035 0.0345 0.0344 0.0344 0.0365 0.0377 0.1038 0.1225 0.1131 0.0638 0.063 0.0585 0.058 0.0577 0.0571 0.0619 0.0936 0.0931 0.1031 0.0915 0.0657 0.061 0.0582 0.058 0.057 0.062

5.28 4.54 2.75 2.89 2.77 2.74 2.74 2.69 2.77 2.90 7.20 8.54 7.92 4.62 4.56 4.36 4.33 4.3 4.26 4.48 6.55 6.53 7.12 6.48 4.72 4.49 4.34 4.35 4.24 4.50

1747 1713 1946 1876 1960 1934 1942 1940 1963 1903 1760 1809 1736 1766 1803 1893 1893 1920 1906 1785 1732 1737 1736 1723 1770 1862 1865 1902 1908 1783

20 19 17 17 17 17 17 18 19 18 21 22 20 19 20 19 19 19 19 19 20 20 21 20 20 19 19 19 19 19

0.0012 0.0012 0.0014 0.0009 0.0016 0.0016 0.0012 0.0012 0.0011 0.0012 0.0012 0.0015 0.0015 0.0014 0.0009 0.0011 0.001 0.0014 0.0013 0.0009 0.0008 0.0011 0.0014 0.0014

0.2168 0.2423 0.5063 0.4081 0.2978 0.5943 0.5015 0.5097 0.5299 0.5099 0.4952 0.5184 0.5129 0.4575 0.4216 0.4879 0.4643 0.4915 0.4643 0.418 0.3622 0.2552 0.502 0.4406

0.0007 0.0008 0.0017 0.0014 0.0010 0.002 0.0017 0.0017 0.0018 0.0017 0.0017 0.0017 0.0017 0.0015 0.0014 0.0016 0.0016 0.0017 0.0016 0.0014 0.0012 0.0009 0.0017 0.0015

5.4158 5.4452 4.9469 2.1652 6.6907 5.3111 3.9054 3.3842 3.3625 3.3378 3.3301 5.314 5.0404 4.9743 2.6009 3.5804 3.4697 5.0991 4.7271 2.3994 2.4558 4.6235 5.0176 5.1069

0.0937 0.0942 0.0856 0.0375 0.1157 0.0919 0.0676 0.0585 0.0582 0.0577 0.0576 0.0919 0.0872 0.0861 0.045 0.0619 0.06 0.0882 0.0818 0.0415 0.0425 0.0800 0.0868 0.0883

6.07 6.14 5.96 2.89 7.56 6.5 4.84 4.33 4.31 4.28 4.26 6.37 6.1 5.94 3.36 4.49 4.32 6.11 5.69 3.13 3.13 5.3 6.04 6.06

1617 1641 1703 1955 1690 1797 1726 1889 1778 1869 1911 1713 1787 1731 1829 1782 1699 1705 1753 1822 1859 1719 1726 1726

23 23 20 18 24 20 19 19 18 19 20 21 21 21 18 19 18 20 21 18 19 23 20 21

Monazite grain of IL003-2-2 4.1 0.4704 4.2 0.4061 4.3 0.3085 4.4 0.3071 4.5 0.3115 4.6 0.3081 4.7 0.3087 4.8 0.2976 4.9 0.2972 4.1 0.3096 5.1 0.6201 5.2 0.7605 5.3 0.6838 5.4 0.4257 5.5 0.4264 5.6 0.4438 5.7 0.4428 5.8 0.444 5.9 0.435 5.1 0.4161 6.1 0.5654 6.2 0.5654 6.3 0.6015 6.4 0.5665 6.5 0.4311 6.6 0.4436 6.7 0.437 6.8 0.4477 6.9 0.4341 6.1 0.4192 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 B24

0.4382 0.4539 0.5071 0.3188 0.5756 0.5923 0.4344 0.4394 0.4148 0.4305 0.436 0.5412 0.5439 0.5047 0.3346 0.4206 0.3845 0.5149 0.4946 0.3159 0.3109 0.4187 0.5188 0.5085

S. Liu et al. / Precambrian Research 148 (2006) 205–224

215

Fig. 5. Electron microprobe Th–U–Pb analytical sites (age mapping) plotted on back-scattered electron image (BSE) of a monazite grain from sample L002-4 (A). It displays patchy zoning with a single dark band: the youngest apparent ages are located along cracks and the oldest apparent ages are located in part of the dark grey domain. Isochron ages are shown in (B) and the mixture modeling peak ages in (C).

mean apparent age of 1883 ± 9 Ma (Fig. 4E); the spots in domain C yield apparent ages of 1787–1760 Ma, with a weighted mean apparent age of 1797 ± 6 Ma (Fig. 4E); whereas analytical spots on the outer domain D yield apparent ages between 1747 Ma and 1617 Ma, with a weighted mean age of 1723 ± 14 Ma (Fig. 4E). Mixture modeling of the total data set yields the most likely four peaks of ages at 1943 ± 8 Ma, 1887 ± 6 Ma, 1789 ± 7 Ma and 1716 ± 5 Ma (Fig. 4F). The data sets for all four domains (A–D) define ThO2 * /PbO isochron ages of 1940 ± 7 Ma (MSWD = 0.77), 1880 ± 9 Ma (MSWD = 1.33), 1795 ± 8 Ma (MSWD = 1.0) and 1720 ± 9 Ma (MSWD = 1.56), respectively (Fig. 4E). 4.2. Sample L002-4 Sample L002-4 was collected from an unmetamorphosed, fine-grained granite vein that intrudes the Jiehekou Group in the southern outcrop area of the L¨uliang Complex (Fig. 2) and is composed of K-feldspar, quartz and minor biotite, with accessory zircon, monazite, ilmenite and xenotime. The sample was prepared as a highly polished thin section which was examined under a microscope to identify and mark monazites in readiness for analysis after carbon coating. Forty-three analytical spots were made on a single monazite grain that displays a darker zone on a grey

background, traversed by bright cracks in BSE (Fig. 5A). Of these, three analytical spots (1.1, 2.3 and 3.3 in Table 2) enclosed by a black dashed line in Fig. 5A, display an apparent age range from 1906 Ma to 1857 Ma, with a weighted mean age of 1871 ± 25 Ma (Fig. 5A and B). Thirty two analytical spots from the grey domain yield an apparent age range from 1782 Ma to 1701 Ma, with a weighted mean age of 1759 ± 7 Ma (Fig. 5B), and eight analytical spots made along or near cracks show an apparent age range of 1675–1625 Ma, with a weighted mean age of 1649 ± 8 Ma; significantly younger than other analytical spots obtained from the grain. Mixture modeling produces three peak ages of 1877 ± 9 Ma, 1763 ± 4 Ma and 1676 ± 5 Ma (Fig. 5C). As shown in Fig. 5A, the data sets yield ThO2 * /PbO isochron ages of 1867 ± 22 Ma (MSWD = 1.54) for the three oldest analytical spots, 1755 ± 6 Ma (MSWD = 1.36) for the thirty two analytical spots, and 1648 ± 8 Ma (MSWD = 0.94) for the eight youngest spots (Fig. 5B). 4.3. Sample S2010-2 Sample S2010-2 is a kyanite–garnet schist collected from the Jingangku ‘formation’ of the Lower Wutai Subgroup, south of Shahe. It consists of kyanite, garnet, biotite, plagioclase, K-feldspar and quartz, with accessory zircon, monazite and ilmenite. The electron

216

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Table 2 Electronic microprobe analytical Th–U–Pb data (wt.%) of monazites for sample L002-4 Label

PbO

Err (Pb)

UO2

Err (U)

ThO2

Err (Th)

Total

T (Ma)

Err (T,Ma)

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 5.1

0.7511 0.4831 0.4901 0.4487 0.5736 0.6600 0.6548 0.6845 0.6789 0.6257 0.7021 0.7576 0.7402 0.5032 0.5401 0.5447 0.6285 0.5838 0.7468 0.6755 0.7690 0.7247 0.8167 0.6374 0.4327 0.4587 0.6350 0.6614 0.6901 0.6555 0.7019 0.7502 0.6193 0.6550 0.5276 0.7517 0.4119 0.4329 0.5484 0.7430 0.7505 0.5992 0.7647

0.0020 0.0013 0.0013 0.0012 0.0015 0.0018 0.0018 0.0018 0.0018 0.0017 0.0019 0.0020 0.0020 0.0014 0.0015 0.0015 0.0017 0.0016 0.0020 0.0018 0.0021 0.0020 0.0022 0.0017 0.0012 0.0012 0.0017 0.0018 0.0019 0.0018 0.0019 0.0020 0.0017 0.0018 0.0014 0.0020 0.0011 0.0012 0.0015 0.0020 0.0020 0.0016 0.0021

0.8944 0.2800 0.2978 0.2705 0.3638 0.6174 0.6627 0.6819 0.6867 0.6393 0.7095 0.7121 0.8066 0.2992 0.3229 0.3852 0.5513 0.4506 0.8303 0.6903 0.7518 0.7196 1.0287 0.5745 0.2623 0.2783 0.6192 0.6276 0.6572 0.6391 0.7808 0.9556 0.5980 0.6495 0.3124 0.4490 0.2736 0.2871 0.4923 0.9814 1.0088 0.4531 1.0869

0.0030 0.0009 0.0010 0.0009 0.0012 0.0021 0.0022 0.0023 0.0023 0.0021 0.0024 0.0024 0.0027 0.0010 0.0011 0.0013 0.0019 0.0015 0.0028 0.0023 0.0025 0.0024 0.0035 0.0019 0.0009 0.0009 0.0021 0.0021 0.0022 0.0021 0.0026 0.0032 0.0020 0.0022 0.0010 0.0015 0.0009 0.0010 0.0017 0.0033 0.0034 0.0015 0.0037

5.8942 5.1508 5.6030 4.9638 6.0566 6.3340 6.2742 6.3236 6.4094 5.8008 6.3943 7.0596 5.8045 5.4769 5.8731 5.6365 5.9150 6.3866 6.5061 6.4667 7.1186 7.0298 6.1772 6.0169 4.7667 5.2442 5.8000 6.3215 6.2527 6.2612 6.0632 6.0409 6.0048 6.0918 5.9396 8.9467 4.7986 4.9391 5.7972 5.8449 6.0880 6.7942 5.8303

0.1020 0.0891 0.0969 0.0859 0.1048 0.1096 0.1085 0.1094 0.1109 0.1004 0.1106 0.1221 0.1004 0.0948 0.1016 0.0975 0.1023 0.1105 0.1126 0.1119 0.1232 0.1216 0.1069 0.1041 0.0825 0.0907 0.1003 0.1094 0.1082 0.1083 0.1049 0.1045 0.1039 0.1054 0.1028 0.1548 0.0830 0.0854 0.1003 0.1011 0.1053 0.1175 0.1009

7.54 5.91 6.39 5.68 6.99 7.61 7.59 7.69 7.77 7.07 7.81 8.53 7.35 6.28 6.74 6.57 7.09 7.42 8.08 7.83 8.64 8.47 8.02 7.23 5.46 5.98 7.05 7.61 7.6 7.56 7.55 7.75 7.22 7.4 6.78 10.15 5.48 5.66 6.84 7.57 7.85 7.85 7.68

1853 1780 1674 1720 1768 1750 1718 1766 1735 1751 1776 1782 1906 1745 1745 1761 1800 1662 1779 1714 1773 1712 1857 1785 1724 1675 1789 1748 1810 1737 1788 1787 1724 1761 1701 1626 1627 1654 1652 1788 1748 1625 1772

19 24 23 24 24 21 20 20 20 20 20 21 20 24 24 23 22 22 19 20 21 20 18 21 24 23 21 21 21 20 19 18 21 20 23 23 22 22 21 18 17 21 17

microprobe Th–U–Pb analyses were performed on a thin section prepared in the same manner as for sample L002-4. Forty analyses (Table 3) were positioned on two monazite grains (S2010-2-1 and S2010-2-2) enclosed within biotite crystals. The two grains do not exhibit clear internal structures in BSE images, though they show weak zoning of apparent ages (Fig. 6A and B). Twenty analyses on grain S2010-2-1 can be grouped into two data sets, of which the older one (10 spots enclosed by a

black dashed line in Fig. 6A) displays apparent ages ranging from 1901 Ma to 1792 Ma (Fig. 6A), with a weighted mean age of 1827 ± 16 Ma (Fig. 6C), and the younger group (10 spots) shows apparent ages ranging from 1759 Ma to 1685 Ma, with a weighted mean age of 1722 ± 16 Ma (Fig. 6A and C). Mixture modeling of the twenty analyses produces two probable ages: 1823 ± 6 Ma and 1715 ± 6 Ma (Fig. 6D), which are comparable to the weighted mean ages. As shown in Fig. 6C, the two data sets also define ThO2 * /PbO

S. Liu et al. / Precambrian Research 148 (2006) 205–224

217

Fig. 6. Electron microprobe Th–U–Pb analytical sites (age mapping) plotted on back-scattered electron images (BSE) of monazite grains S2010-2-1 (A) and S2010-2-2 (B) from sample S2010-2; age domains are marked by dark and light dashed lines and a dark solid line. Isochron ages are shown for grain S2010-2-1 in (C) and the mixture modeling peak ages in (D). Isochron ages are shown for grain S2010-2-2 in (E) and the mixture modeling peak ages in (F).

218

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Table 3 Electronic microprobe analytical Th–U–Pb data (wt.%) of monazites for sample S2010-2 Label

PbO

Err(Pb)

UO2

Err(U)

ThO2

Err(Th)

Total

T(Ma)

Err(T,Ma)

Monazite grain of S2010-2-1 1.1 0.3075 0.0008 1.2 0.3708 0.0010 1.3 0.3802 0.0010 1.4 0.3142 0.0008 1.5 0.3064 0.0008 1.6 0.2687 0.0007 1.7 0.2599 0.0007 1.8 0.4073 0.0011 1.9 0.3853 0.0010 1.10 0.3564 0.0010 1.11 0.3950 0.0011 1.12 0.4042 0.0011 1.13 0.4125 0.0011 1.14 0.3243 0.0009 1.15 0.2593 0.0007 1.16 0.3151 0.0009 1.17 0.3643 0.0010 1.18 0.4278 0.0012 1.19 0.4056 0.0011 1.20 0.2677 0.0007

0.2943 0.4153 0.4364 0.3129 0.3315 0.2705 0.2668 0.4556 0.4347 0.4476 0.4327 0.4671 0.4554 0.3404 0.2515 0.3159 0.3793 0.4657 0.4637 0.2291

0.0010 0.0014 0.0015 0.0011 0.0011 0.0009 0.0009 0.0015 0.0015 0.0015 0.0015 0.0016 0.0015 0.0011 0.0008 0.0011 0.0013 0.0016 0.0016 0.0008

3.0827 3.4456 3.4194 3.0729 2.7701 2.5895 2.5081 3.4316 3.3956 3.1983 3.2871 3.3566 3.4738 2.7389 2.3423 2.8403 3.2042 3.3711 3.3736 2.7128

0.0533 0.0596 0.0592 0.0532 0.0479 0.0448 0.0434 0.0594 0.0587 0.0553 0.0569 0.0581 0.0601 0.0474 0.0405 0.0491 0.0554 0.0583 0.0584 0.0469

4.48 5.15 5.17 4.46 4.81 4.18 3.80 5.24 5.11 4.92 4.95 5.09 5.19 4.14 3.70 4.26 4.74 5.14 5.14 4.26

1689 1705 1728 1701 1752 1713 1703 1812 1759 1685 1837 1810 1820 1843 1805 1792 1801 1901 1814 1720

20 19 19 20 20 20 20 20 19 18 20 19 20 20 21 21 20 20 19 21

Monazite grain of S2010-2-2 2.1 0.3397 0.0009 2.2 0.3704 0.0010 2.3 0.3756 0.0010 2.4 0.3258 0.0009 2.5 0.3112 0.0008 2.6 0.2739 0.0007 2.7 0.3587 0.0010 2.8 0.4016 0.0011 2.9 0.2831 0.0008 2.10 0.2909 0.0008 2.11 0.3077 0.0008 2.12 0.3161 0.0009 2.13 0.3280 0.0009 2.14 0.2921 0.0008 2.15 0.2741 0.0007 2.16 0.3119 0.0008 2.17 0.3031 0.0008 2.18 0.2797 0.0008 2.19 0.2803 0.0008 2.20 0.3062 0.0008

0.3545 0.5399 0.5432 0.4121 0.3114 0.2966 0.4858 0.6073 0.3602 0.2881 0.3072 0.3726 0.3769 0.3864 0.2924 0.3720 0.3461 0.2664 0.2924 0.3263

0.0012 0.0018 0.0018 0.0014 0.0010 0.0010 0.0016 0.0020 0.0012 0.0010 0.0010 0.0013 0.0013 0.0013 0.0010 0.0013 0.0012 0.0009 0.0010 0.0011

2.6841 2.3281 2.3304 2.4999 2.5993 2.3973 2.6102 2.3574 2.1561 2.5764 2.6990 2.5622 2.6485 2.2753 2.1749 2.5214 2.4018 2.4466 2.4303 2.6498

0.0464 0.0403 0.0403 0.0432 0.0450 0.0415 0.0452 0.0408 0.0373 0.0446 0.0467 0.0443 0.0458 0.0394 0.0376 0.0436 0.0416 0.0423 0.0420 0.0458

4.71 4.91 4.63 4.56 4.78 4.00 4.97 4.80 4.07 4.27 4.46 4.49 4.51 4.20 3.81 4.38 4.28 4.08 4.13 4.48

1922 1932 1949 1840 1881 1787 1848 1962 1844 1818 1825 1825 1844 1797 1905 1821 1868 1851 1817 1808

20 16 16 18 21 20 17 16 18 21 21 19 19 18 20 19 19 21 20 20

isochron ages of 1822 ± 14 Ma (MSWD = 1.60) and 1719 ± 14 Ma (MSWD = 1.68), respectively. The second grain of monazite (S2010-2-2) exhibits three different age groups, separated by a white dashed line and a black solid line in the BSE image (Fig. 6B). The older age group comprises five analytical spots that are located within the white dashed line, whereas the youngest age group consists of two analytical spots encircled by a black solid line; the majority of analytical spots on the BSE image (Fig. 6B) give intermediate ages. The five analytical spots in the older age

group give an apparent age range of 1962–1905 Ma, with a weighted mean age of 1938 ± 12 Ma (Fig. 6E). The majority of analytical spots yield an apparent age range of 1881–1808 Ma, with a weighted mean age of 1838 ± 6 Ma, whereas the two spots in the youngest age group give apparent ages of 1797 Ma and 1787 Ma, with a mean age of 1793 ± 6 Ma (Fig. 6B and E). Mixture modeling for all twenty analyses in this grain yields two peaks, with the most probable ages of 1937 ± 8 Ma and 1831 ± 5 Ma (Fig. 6F). As shown in Fig. 6E, five analyses in the old age group define a ThO2 * /PbO isochron age

S. Liu et al. / Precambrian Research 148 (2006) 205–224

219

Table 4 Summary of the apparent ages, mixture modeling peak ages and isochron ages of monazites from the L¨uliang and Wutai Complexes Sample

Monazite

Weighted apparent age (1␴,Ma)

Mixture modeling peak age (1␴,Ma)

Isochron age (2␴,Ma)

MSWD

Note

L¨uliang Complex IL03-2 garnet felsic gneiss

IL03-2-1

1942 ± 10

1937 ± 23

1938 ± 10

0.92

This study

1851 ± 7 1774 ± 8 1705 ± 3

1857 ± 7 1778 ± 14 1697 ± 9

1847 ± 7 1771 ± 8 1703 ± 11

1.38 0.61 1.56

This study This study This study

0.77 1.33 1.00 1.56

This study This study This study This study

IL02-4 granitic vein

Lower Wutai Complex S2010-2 garnet kyanite biotite gneiss

6 9 6 11

1871 ± 25 1759 ± 7 1649 ± 8

1877 ± 9 1763 ± 4 1676 ± 5

1867 ± 22 1755 ± 6 1648 ± 8

1.54 1.36 0.94

This study This study This study

S2010-2-1

1827 ± 16

1823 ± 6

1822 ± 14

1.60

This study

1722 ± 16

1715 ± 6

1719 ± 14

1.68

This study

1938 ± 12 1826 ± 6 1793 ± 6 (two spots)

1937 ± 8 1831 ± 5

1930 ± 11 1833 ± 6

1.21 1.07

This study This study

1922 ± 24

0.78

Liu et al. (2004a)

1847 ± 62

0.31

Liu et al. (2004a)

of 1930 ± 11 Ma (MSWD = 1.21), whereas the majority of analytical spots define a ThO2 * /PbO isochron age of 1833 ± 8 Ma (MSWD = 1.07). These two isochron ages are consistent with the ThO2 * /PbO isochron ages (1922 ± 24 Ma and 1847 ± 62 Ma) previously obtained from two monazite grains in a sample collected from the Lower Wutai Subgroup at the Ekou Iron Mine (Liu et al., 2004a) (Fig. 3). 5. Discussion and conclusions The Th–U–Pb results from five monazite grains collected from two samples of the L¨uliang Complex and one from the Wutai Complex are summarized in Table 4. Detailed studies based on age mapping, internal structures, peak age mixture modeling and isochron ages, combined with previous data for monazites from the Lower Wutai Complex (Liu et al., 2004a), reveal five age groups: at 1945–1922 Ma, 1887–1822 Ma, 1798–1755 Ma, 1723–1703 Ma and ∼1648 Ma (Table 4; Figs. 4 and 5). BSE images (Figs. 4A, B and 5A) show that the oldest apparent ages and ThO2 * /PbO isochron ages (1943–1922 Ma) are preserved in domain A of the monazite grains although they are not at the core but near

8 6 7 5

1940 1880 1795 1720

± ± ± ±

IL02-4

SZ10-2

1943 1887 1789 1716

± ± ± ±

1945 1883 1797 1723

S2010-2-2

SZ10-2 garnet mica schist

± ± ± ±

IL03-2-2

7 9 8 9

one of the margins, suggesting that growth was not uniform, whereas the younger ages of 1723–1703 Ma are recorded in the outer domains (Figs. 4–6). The youngest ages of all at ∼1648 Ma are only present along cracks in monazite grain L002-4 from the L¨uliang Complex. One interpretation of the oldest group of ages between 1943 Ma and 1922 Ma is that they record crystallization of igneous monazite. Recent SHRIMP U–Pb zircon geochronology has revealed the widespread presence of 2031–1900 Ma igneous rocks in the L¨uliang Complex (Geng et al., 2004) and other complexes in the TransNorth China Orogen (Guan et al., 2002; Zhao et al., 2002; Wilde et al., 2004a, 2004b; Kr¨oner et al., 2005a, 2005b; Peng et al., 2005). Unfortunately, the ages of the Jiehekou Group in the L¨uliang Complex and the lowermost sequence of the Wutai Complex (the Jingangku ‘formation’) are not precisely known. Detrital zircon U–Pb ages of 2.03 ± 0.05 Ga have previously been interpreted as dating metamorphism of the Jiehekou Group (Wan et al., 2000; Geng et al., 2000), whilst conventional U–Pb zircon data for the associated L¨uliang Group define igneous ages of 2051 ± 68 Ma and 2099 ± 41 Ma (Yu et al., 1997). Furthermore, for the Wutai Complex sample, it is impossible that the weighted mean

220

S. Liu et al. / Precambrian Research 148 (2006) 205–224

age of 1925 ± 12 Ma determined from monazite grain S2010-2-1 represents an igneous age. The host-rock is a kyanite-bearing metasediment from the structurally lowermost part of the sequence, overlain by a volcanosedimentary sequence with felsic units dated at 2515 to 2530 (Wilde et al., 2004a, 2004b) and intruded by diorite plutons dated at ∼2510 Ma (Wilde, unpublished data). Any igneous monazite in this rock would be detrital in origin and thus Archean in age. The similarity between the oldest monazite ages in both complexes points to a common origin and it most likely reflects a major thermal pulse associated with the widespread emplacement of mafic dykes at ∼1915 Ma (Peng et al., 2005; Kr¨oner et al., 2006). Importantly, there is no evidence of Archean monazite in the Wutai sample as might be expected if a major tectonothermal event affected these rocks in the late Archean, as suggested by some authors (Kusky et al., 2001; Kusky and Li, 2003). Likewise, published SHRIMP U–Pb zircon and mineral (e.g., garnet and pyroxene) Sm–Nd data show no evidence that metamorphic events occurred before ∼1890 Ma in the Trans-North China Orogen (Guo and Zhai, 2001; Guan et al., 2002; Zhao et al., 2002; Guo et al., 2005; Kr¨oner et al., 2005a, 2005b). The age range of 1887–1822 Ma recorded in monazites from the L¨uliang and Wutai Complexes (Table 4) is consistent with the age of the regional metamorphic event previously dated by SHRIMP U–Pb and mineral Sm–Nd and 40 Ar/39 Ar techniques (Guo et al., 1993; Wang et al., 2001; Zhao et al., 2002, 2005; Wang et al., 2003; Guo et al., 2005; Kr¨oner et al., 2005a, 2005b). Therefore, we interpret monazite ages of 1887–1822 Ma as the time of metamorphism of the L¨uliang and Wutai Complexes. Zhao et al. (2002) carried out detailed SHRIMP U–Pb zircon studies on the Fuping Complex, which is located southeast of the Wutai Complex, and found the presence of only one phase of metamorphic zircon in both the late Archean Fuping TTG gneisses and the Paleoproterozoic Nanying granitic gneisses. These metamorphic zircons occur as either overgrowth rims surrounding older magmatic zircon cores or as new discrete zircon grains. Both yield similarly concordant 207 Pb/206 Pb ages in the range 1870–1800 Ma (Zhao et al., 2002). Wang et al. (2003, 2004a, 2004b) recognized four phases of deformation in the Zanhuang Complex, about 50 km south of the Fuping Complex, and applying mineral 40 Ar/39 Ar dating techniques, they defined the timing of D1 , D2 and D3 events as 1870 Ma, 1870–1826 Ma and 1826–1793 Ma, respectively. In the Wutai Complex, Wang et al. (2001) obtained a hornblende-garnet Sm–Nd isochron age of 1851 ± 9 Ma from a garnet amphibolite, interpreted as the approx-

imate age of peak metamorphism of the complex. North of the Wutai Complex in the Hengshan Complex, Kr¨oner et al. (2005a, 2005b) obtained metamorphic zircon ages of 1881 ± 8 Ma, 1881 ± 0.4 Ma, 1867 ± 23 Ma, 1859.7 ± 0.5 Ma, 1850 ± 3 Ma and 1848 ± 5 Ma from the Hengshan granitic gneisses and high-pressure mafic granulite. North of the Hengshan Complex in the Huai’an Complex, Guo et al. (1993) obtained a garnet–clinopyroxene–orthopyroxene Sm–Nd isochron age of 1824 ± 18 Ma and a U–Pb zircon age of 1833 ± 23 Ma, interpreted as the age of the highpressure metamorphic event. More recently, applying the SHRIMP U–Pb zircon dating technique, Guo et al. (2005) obtained an age of 1817 ± 12 Ma for metamorphic zircons from high-pressure granulites in this complex. All these ages suggest that a major metamorphic event occurred between 1880 Ma and 1800 Ma. This conclusion is further supported by our new chemical Th–U–Pb monazite ages obtained from the L¨uliang and Wutai Complexes. The age range of 1798–1755 Ma is consistent with the time (1780–1750 Ma) of widespread emplacement of mafic dyke swarms in the Trans-North China Orogen. Most of these dykes are unmetamorphosed and undeformed, with chilled contacts (Wang et al., 2003, 2004a, 2004b; Peng et al., 2005; Halls et al., 2000). These mafic dyke swarms are considered to have been emplaced during a post-collisional extensional event occurring in the Trans-North China Orogen shortly after the ∼1.8 Ga collisional event (Zhao et al., 2001a; Wang et al., 2004a, 2004b). Domains in monazite grains with ages of 1723–1703 Ma occur in both the L¨uliang and Wutai Complexes (samples S2010-2, IL003-2). Monazite patches growing along cracks in older monazite from a granite vein (L002-4) in the L¨uliang Complex give a ThO2 * /PbO isochron age of ∼1648 Ma, indicating multiple stages of hydrothermal alteration (Zhu et al., 1997, 1998) at some time after the main metamorphic event resulting from amalgamation of the Western and the Eastern Blocks. In summary, our new monazite U–Th–Pb ages reveal that certain tectonothermal events identified in the medium- to low-grade Wutai and L¨uliang Complexes in the central-southern segment of the Trans-North China Orogen were coeval with events recognized in the highgrade Henshan, Fuping, Huai’an and Xuanhua Complexes to the north and in the Zanhuang (Wang et al., 2004a, 2004b) and Lushan (Wan et al., 2006) complexes to the south. Thus, all metamorphic complexes examined so far in the Trans-North China Orogen, regardless of their location or metamorphic grade, record a

S. Liu et al. / Precambrian Research 148 (2006) 205–224

major metamorphic event at 1890–1800 Ma. There is also strong evidence for a post-collisional extensional event at 1800–1755 Ma, represented by the emplacement of voluminous mafic dike swarms and the granitic vein in this study. Based on these new monazite age data and recent SHRIMP U–Pb zircon and mineral Sm–Nd and 40 Ar/39 Ar ages, we further refine the model as follows. At 1900–1820 Ma, the ocean between the Eastern and Western Blocks was totally consumed by subduction, and its closure led to continent-arc-continent collision, accompanied by regional metamorphism in the TransNorth China Orogen, as recorded by 1890–1822 Ma ages in monazites of the L¨uliang and Wutai Complexes. Subsequently (<1800 Ma), the thickened crust of the orogen underwent exhumation associated with the widespread emplacement of 1789–1750 Ma mafic dyke swarms and granitic veins, probably as a result of post-orogenic collapse, slab break-off or post-orogenic extension, or some combination of these processes. Acknowledgments We wish to thank H.W. Day for assistance in establishing the analytical technique of electron microprobe chemical Th–U–Pb dating of monazite at Peking University and Dr. Y.M. Pan for providing the analytical standards. We also wish to thank the three anonymous reviewers and Dr. A. Polat for their comments which greatly assisted in improving the manuscript. The National Natural Science Foundation of China (Grant Nos. 40420120135 and 40472096) and Hong Kong RGC (7055/03P, 7048/03P and 7058/04P) are thanked for their financial support. References Bai, J., 1986. The Precambrian crustal evolution of the Wutaishan area. In: Bai, J. (Ed.), The Early Precambrian Geology of Wutaishan. Tianjin Science and Technology Press, Tianjin, pp. 376–383 (in Chinese). Bai, J., Wang, R.Z., Guo, J.J., 1992. The Major Geological Events of Early Precambrian and Their Dating in Wutaishan Region. Geological Publishing House, pp. 1–52, (in Chinese). Biju-Sekhar, S., Yokoyama, K., Pandit, M.K., 2003. Late Paleoproterozoic magmatism in Delhi Fold Belt, NW India and its implication: evidence from EPMA chemical ages of zircons. J. Asian Earth Sci. 22, 189–207. Braun, I., Montel, J.M., Nicollet, C., 1998. Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India. Chem. Geol. 146, 65–85. Catlos, E.J., Gilley, L.D., Harrison, T.M., 2002. Interpretation of monazite ages obtained via in situ analysis. Chem. Geol. 188, 193– 215. Cocherie, A., Mezeme, E.B., Legendre, O., Fanning, C.M., Faure, M., Rossi, P., 2005. Electron-microprobe dating as a tool for determin-

221

ing the closure of Th–U–Pb systems in migmatitic monazites. Am. Miner. 90 (4), 607–618. Cocherie, A., Albarede, F., 2001. An improved U–Th–Pb age calculation for electron microprobe dating of monazite. Geochim. Cosmochim. Acta 65, 4509–4522. Cocherie, A., Legendre, O., Peucat, J.J., 1998. Geochronology of polygenetic monazites constrained by in situ electron microprobe Th–U total lead determination: implications for lead behaviour in monazite. Geochim. Cosmochim. Acta 62, 2475–2497. Cihan, M., Evins, P., Lisowiec, N., Blake, K., 2006. Time constraints on deformation and metamorphism from EPMA dating of monazite in the Proterozoic Robertson River Metamorphics, NE Australia. Precambrian Res. 145, 1–23. Dahl, P.S., Hamilton, M.A., Jercinovic, M.J., Terry, M.P., Williams, M.L., Frei, R., 2005. Comparative isotopic and chemical geochronometry of monazite, with implications for U–Th–Pb dating by electron microprobe: an example from metamorphic rocks of the eastern Wyoming Craton (USA). Am. Miner. 90 (4), 619– 638. Geisler, T., Schleicher, H., 2000. Improved U–Th–total Pb dating of zircons by electron microprobe using a simple new background modeling procedure and Ca as a chemical criterion of fluid-induced U–Th–Pb discordance in zircon. Chem. Geol. 163, 269–285. Geng, Y.S., Yang, C.H., Song, B., Wang, Y.S., 2004. Post-orogenic granites with an age of 1800 Ma in L¨uliang area. North China Craton: constraints from isotopic geochronology and geochemistry. Geol. J. Chin. Univ. 10, 477–487 (in Chinese with English abstract). Geng, Y.S., Wan, Y.S., Yang, C.H., 2003. The Palaeoproterozoic rifttype volcanism in Luliangshan Area, Shanxi Province, and its geological significance. Acta Geoscientica. Sin. 24, 97–104 (in Chinese with English abstract). Geng, Y.S., Wan, Y.S., Shen, Q.H., Li, H.M., Zhang, R.X., 2000. Chronological framework of the early Precambrian important events in the Luliang area, Shanxi Province. Acta Geol. Sin. 74, 216–223 (in Chinese with English abstract). Goncalves, P., Williams, M.L., Jercinovic, M.J., 2005. Electronmicroprobe age mapping of monazite. Am. Miner. 90 (4), 578–585. Guan, H., Sun, M., Wilde, S.A., Zhou, X.H., Zhai, M.G., 2002. SHRIMP U–Pb zircon geochronology of the Fuping Complex: implications for formation and assembly of the North China Craton. Precambrian Res. 113, 1–18. Guo, J.H., Sun, M., Chen, F.K., Zhai, M.G., 2005. Sm–Nd and SHRIMP U–Pb zircon geochronology of high-pressure granulites in the Sanggan area, North China Craton: timing of Paleoproterozoic continental collision. J. Asian Earth Sci. 24, 629–642. Guo, J.H., O’Brien, P.J., Zhai, M.G., 2002. High-pressure granulites in the Sangan area, North China Craton: metamorphic evolution, P–T paths and geotectonic significance. J. Metam. Geol. 20, 741–756. Guo, J.H., Zhai, M.G., 2001. Sm–Nd age dating of high-pressure granulites and amphibolite from Sanggan area, North China Craton. Chin. Sci. Bull. 46, 106–110. Guo, J.H., Zhai, M.G., Zhang, Y.G., Li, Y.G., Yan, Y.H., Zhang, W.H., 1993. Early Precambrian Manjinggou high-pressure granulite melange belt on the south edge of the Huaian complex. North China craton: geological features, petrology and isotopic geochronology. Acta Petrologica Sinica 9, 329–341 (in Chinese with English abstract). Halls, H.C., Li, J.H., Davis, D., Hou, G.T., Zhang, B.X., Qian, X.L., 2000. A precisely dated Proterozoic paleomagnetic pole from the North China Craton and its relevance to paleocontinental reconstruction. Geophys. J. Int. 143, 185–203.

222

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Kato, T., Suzuki, K., Adachi, M., 1999. Computer program for the CHEME age calculation. J. Earth Planet. Sci. Nagoya Univ. 46, 49–56. Kr¨oner, A., Wilde, S.A., O’Brien, P.J., Li, J.H., Passchier, C.W., Walte, N.P., Liu, D.Y., 2005a. Field relationships, geochemistry, zircon ages and evolution of a late Archaean to Palaeoproterozoic lower crustal section in the Hengshan terrain of northern China. Acta Geol. Sin. 79 (5), 532–605. Kr¨oner, A., Wilde, S.A., Li, J.H., Wang, K.Y., Zhao, G.C., 2005b. Age and evolution of a late Archaean to early Palaeozoic upper to lower crustal section in the Wutaishan/Hengshan/Fuping terrain of northern China. J. Asian Earth Sci. 24, 577–595. Kr¨oner, A., Wilde, S.A., Zhao, G.C., O’Brien, P.J., Sun, M., Liu, D.Y., Wan, Y.S., Liu, S.W., Guo, J.H., 2006. Zircon geochronology and metamorphic evolution of mafic dykes in the Hengshan Complex of northern China: evidence for late Palaeoproterozoic extension and subsequent high-pressure metamorphism in the North China Craton. Precambrian Res. 146 (1–2), 45–67. Kr¨oner, A., Zhao, G.C., Wilde, S.A., Zhai, M.G., Passchier, C.W., Sun, M., Guo, J.H., O’Brien, P.J., Walte, N., 2002. A late Archaean to Palaeoproterozoic lower to upper crustal section in the Hengshan–Wutaishan area of North China. In: Penrose Conference Guidebook for Field Trip. Chinese Academy of Sciences, Beijing, China, Beijing, China. Kusky, T.M., Li, J.H., 2003. Paleoproterozoic tectonic evolution of the North China Craton. J. Asian Earth Sci. 22, 383–397. Kusky, T.M., Li, J.H., Tucker, R.D., 2001. The Archean Dongwanzi ophiolite complex, North China Craton: 2.505-billion-year-old oceanic crust and mantle. Science 292, 1142–1145. Li, J.H., Qian, X.L., 1991. A study on the Longquanguan shear zone in the northern part of the Taihang Mountains. Shanxi Geol. 6, 17–29 (in Chinese). Li, J.H., Kusky, T.M., Huang, X.N., 2002. Archean podiform chromitites and mantle tectonites in ophiolitic m´elange, North China Craton: a record of early oceanic mantle processes. GSA Today 12, 4–11. Li, J.H., Kr¨oner, A., Qian, X.L., O’Brien, P., 2000. Tectonic Evolution of an Early Precambrian High-Pressure Granulite Belt in the North China Craton. Acta Geol. Sin. 74, 246–258. Li, S.G., Hart, S.R., Wu, T.S., 1990. Rb–Sr and Sm–Nd isotopic dating of an early Precambrian spilite–keratophyre sequence in the Wutaishan area, North China: preliminary evidence for Nd-isotopic homogenization in the mafic and felsic lavas during low-grade metamorphism. Precambrian Res. 47, 191–203. Liu, S.W., Li, J.H., Pan, Y.M., Zhang, J., Li, Q., 2002a. An Archean continental block in the Taihangshan and Hengshan regions: constraints from geochronology and geochemistry. Prog. Natural Sci. 12, 568–576. Liu, S.W., Pan, Y.M., Li, J.H., Zhang, J., Li, Q.G., 2002b. Geological and isotopic geochemical constraints on the evolution of the Fuping Complex, North China Craton. Precambrian Res. 117, 41–56. Liu, S.W., Pan, Y.M., Xie, Q.L., Zhang, J., Li, Q.G., 2005. Geochemistry of the Paleoproterozoic Nanying granitic gneisses in the Fuping complex: implications for the tectonic evolution of the Central zone, North China Craton. J. Asian Earth Sci. 24, 643–658. Liu, S.W., Liang, H.H., Zhao, G.C., Hua, Y.G., Jian, A.H., 2000. Isotopic chronology and geological events of Precambrian complex in Taihangshan region. Sci. Chin. (Ser. D) 43, 386–393. Liu, S.W., Shu, G.M., Pan, Y.M., Dang, Q.N., 2004a. Electron microprobe dating and metamorphic age of Wutai Group, Wutai Mountains. Geol. J. Chin. Univ. 10, 356–363 (in Chinese with English abstract).

Liu, S.W., Pan, Y.M., Xie, Q.L., Zhang, J., Li, Q.G., 2004b. Archean geodynamics in the Central Zone. North China craton: constraints from geochemistry of two contrasting series of granitoids in the Fuping and Wutaishan complexes. Precambrian Res. 130, 229–249. Mathieu, R., Zetterstrom, L., Cuney, M., Gauthier-Lafaye, F., Hidaka, H., 2001. Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo-Okelobondo and Bangombe natural nuclear reaction zones (Franceville basin Gabon). Chem. Geol. 171, 147–171. Mezeme, E.B., Faure, M., Cocherie, A., 2005. In situ chemical dating of tectonothermal events in the French Variscan Belt. Terra Nova 17 (5), 420–426. Montel, J.M., Kornprobst, J., Vielzeuf, D., 2000. Preservation of old U–Th–Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco). J. Metam. Geol. 18, 335–342. Montel, J.M., Foret, S., Veschambre, M., 1996. Electron microprobe dating of monazite. Chem. Geol. 131, 37–53. O’Brien, P.J., Rotzler, J., 2003. High-pressure granulites: formation, recovery of peak conditions and implications for tectonics. J. Metam. Geol. 21, 3–20. Peng, P., Zhai, M.G., Zhang, H.F., Guo, J.H., 2005. Geochronological constraints on the paleoproterozoic evolution of the North China craton: SHRIMP zircon ages of different types of Mafic dikes. Int. Geol. Rev. 47 (5), 492–508. Polat, A., Kusky, T., Li, J.H., Fryer, B., Kerrich, R., Patrick, K., 2005. Geochemistry of Neoarchean (ca. 2.55–2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt, central orogenic belt, North China Craton: implications for geodynamic setting and continental growth. Geol. Soc. Am. Bull. 117 (11/12), 1387–1399. Pyle, J.M., Spear, F.S., Wark, D.A., Daniel, C.G., Storm, L.C., 2005. Contributions to precision and accuracy of monazite microprobe ages. Am. Miner. 90 (4), 547–577. Rasmussen, B., Fletcher, I.R., McNaughton, N.J., 2001. Dating lowgrade metamorphic events by SHRIMP U–Pb analysis of monazite in shales. Geology 29, 963–966. Rhede, D., Wendt, I., Forster, H.J., 1996. A three-dimensional method for calculating independent chemical U/Pb- and Th/Pb-ages of accessory minerals. Chem. Geol. 130, 247–253. Sambridge, M.S., Compston, W., 1994. Mixture modeling of multicomponent data sets with application to ion-probe zircon ages. Earth Planet Sci. Lett. 128, 373–390. Santosh, M., Tanaka, K., Yokoyama, K., Collins, A.S., 2005. Late Neoproterozoic–Cambrian felsic magmatism along transcrustal shear zones in southern India: U–Pb electron microprobe ages and implications for the amalgamation of the Gondwana supercontinent. Gondwana Res. 8, 31–42. Seydoux-Guillaume, A.M., Paquetta, J.L., Wiedenbeck, M., Montel, J.M., Heinrich, W., 2005. Experimental resetting of the U–Th–Pb systems in the monazite. Chem. Geol. 191, 165–181. Sun, M., Armstrong, R.L., Lambert, R.J., 1992. Petrochemistry and Sr, Pb, and Nd isotopic geochemistry of Early Precambrian rocks, Wutaishan and Taihangshan areas, China. Precambrian Res. 56, 1–31. Swain, G.M., Hand, M., Teasdale, J., 2005. Age constraints on terranescale shear zones in the Gawler Craton, southern Australia. Precambrian Res. 139 (3–4), 164–180. Suzuki, K., Adachi, M., 1991. Precambrian Provenance and Silurian Metamorphism of the Tsubonosaxa Paragneiss in the South Kitakani Terrene, Northeast Japan. Geochem. J. 25, 357–376.

S. Liu et al. / Precambrian Research 148 (2006) 205–224 Suzuki, K., Adachi, M., 1998. Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids. J. Metam. Geol. 16, 23–37. Teufel, S., Heinrich, W., 1997. Partial resetting of the U–Pb isotope system in monazite through hydrothermal experiments: an SEM and U–Pb isotope study. Chem. Geol. 137, 273–281. Tian, Y.Q., Liang, Y.F., Fan, Sikun, Zhu, B.Q., Chen, Y.W., 1992. Geochronology of the Henshan Complex in special reference to Nd isotopic evolution. Geochimica 9, 255–264 (in Chinese with English abstract). Tickyj, H., Hartmann, L.A., Vasconcellos, M.A.Z., Philipp, R.P., Remus, M.V.D., 2004. Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield. J. South Am. Earth Sci. 16, 699–713. Wan, Y.S., Geng, Y.S., Shen, Q.H., Zhang, R., 2000. Khondalite series—geochronology and geochemistry of the Jiehekou Group in L¨uliang area, Shanxi province. Acta Petrol. Sin. 16, 49– 58. Wan, Y.S., Wilde, S.A., Liu, D.Y., Yang, C.X., Song, B., Yin, X.Y., 2006. Further evidence for ∼1.85 Ga metamorphism in the Central Zone of the North China Craton: SHRIMP U–Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province. Gondwana Res. 9, 189–197. Wang, Z.H., Wilde, S.A., Wang, K.Y., Yu, L.J., 2004a. A MORB-arc basalt–adakite association in the 2.5 Ga Wutai greenstone belt: late Archean magmatism and crustal growth in the North China Craton. Precambrian Res. 131, 323–343. Wang, K.Y., Wang, Z., Yu, L., Fan, H., Wilde, S.A., Cawood, P.A., 2001. Evolution of Archaean greenstone belt in the Wutaishan region, North China: constraints from SHRIMP zircon U–Pb and other geochronological and isotope information. In: Cassidy, K.F. (Ed.), Proceedings of the 4th International Archaean Symposium 2001. Extended Abstracts, pp. 104–105. Wang, Y.J., Fan, W.M., Zhang, Y.H., Guo, F., Zhang, H.F., Peng, T.P., 2004b. Geochemical, 40 Ar/39 Ar geochronological and Sr–Nd isotopic constraints on the origin of Paleoproterozoic mafic dikes from the southern Taihang Mountains and implications for the ca. 1800 Ma event of the North China Craton. Precambrian Res. 135, 55–77. Wang, Y.J., Fan, W.M., Zhang, Y., Guo, F., 2003. Structural evolution and Ar-40/Ar-39 dating of the Zanhuang metamorphic domain in the North China Craton: constraints on Paleoproterozoic tectonothermal overprinting. Precambrian Res. 122 (1–4), 159–182. Wilde, S.A., Cawood, P.A., Wang, K.Y., Nemchin, A., 1998. SHRIMP U–Pb zircon dating of granites and gneisses in the Taihangshan–Wutaishan area: implications for the timing of crustal growth in the North China craton. Chin. Sci. Bull. 43, 144– 145. Wilde, S.A., Cawood, P.A., Wang, K.Y., Nemchin, A.A., 2005. Granitoid evolution in the Late Archean Wutai Complex, North China Craton. J. Asian Earth Sci. 24, 597–613. Wilde, S.A., Zhao, G.C., Sun, M., 2002. Development of the North China Craton during the Late Archaean and its amalgamation along a major 1.8 Ga collision zone; including speculations on its position within a global Paleoproterozoic Supercontinent. Gondwana Res. 5, 85–94. Wilde, S.A., Zhao, G.C., Wang, K.Y., Sun, M., 2004a. First precise SHRIMP U–Pb zircon ages for the Hutuo Group, Wutaishan: further evidence for the Palaeoproterozoic amalgamation of the North China Craton. Chin. Sci. Bull. 49, 83–90.

223

Wilde, S.A., Cawood, P.A., Wang, K.Y., Nemchin, A., Zhao, G.C., 2004b. Determining Precambrian crustal evolution in China: a case-study from Wutaishan, Shanxi Province, demonstrating the application of precise SHRIMP U–Pb geochronology. In: Malpas, J., Fletcher, C.J.N., Ali, J.R., Aitchison, J.C. (Eds), Aspects of the Tectonic Evolution of China, Special Publication Geological Society of London 226, pp. 5–25. Williams, M., Jercinovic, M., Terry, M., 1999. Age mapping and dating of monazite on the electron microprobe: deconvoluting multistage tectonic histories. Geology 27, 1023–1026. Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks III, W.C., Ridley, W.I. (Eds.), 1998. Applications of microanalytical techniques to understanding mineralizing processes. Rev. Econom. Geol. 7, 1–35. Wu, C.H., Zhong, C.T., 1998. The Paleoproterozoic SW–NE collision model for the central North China Craton. Prog. Precambrian Res. 21, 28–50 (in Chinese). Yu, J.H., Wang, D.Z., Wang, C.R., Li, H.M., 1997. Ages of L¨uliang Group and its metamorphism in Mt. L¨uliang Region, Shanxi Province: evidence from single grain zircon U–Pb dating. Chin. J. Geochem. 16, 170–177. Yu, J.H., Wang, D.Z., 1999. A Paleoproterozoic A-type Rhyolite. Chin. J. Geochem. 18, 219–228. Zhai, M.G., Liu, W.J., 2003. Palaeoproterozoic tectonic history of the North China Craton: a review. Precambrian Res. 122, 183– 199. Zhai, M.G., Guo, J.H., Li, Y.G., Liu, W.J., Peng, P., Shi, X., 2003. Two linear granite belts in the central-western North China Craton and their implication for Late Neoarchaean–Palaeoproterozoic continental evolution. Precambrian Res. 127, 267–283. Zhai, M.G., Guo, J.H., Yan, Y.H., Li, Y.G., Han, X.L., 1993. Discovery and preliminary research of the Archean high-pressure granulite, North China. Sci. Chin. (Ser. B) 32, 1325–1330. Zhang, J., Liu, S.W., Pan, Y.M., Li, Q.Q., Chu, Z.Y., Yang, B., 2004. Petrogenesis of Neoarchean Wutaishan Granitoids and Its geodynamic significances. J. Peking Univ. (Natural Science Version) 40, 216–227. Zhao, G.C., 2001. Palaeoproterozoic assembly of the North China Craton. Geol. Mag. 138, 87–91. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1998. Thermal evolution of Archean basement rocks from the eastern part of the North China Craton and its bearing on tectonic setting. Int. Geol. Rev. 40, 706–721. Zhao, G.C., Cawood, P.A., Lu, L.Z., 1999a. Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai complex, China. Precambrian Res. 93, 181–199. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1999b. Thermal evolution of two types mafic granulites in the North China Craton: evidence for both mantle plume and collisional tectonics. Geol. Mag. 136, 223–240. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1999c. Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications. Tectonophysics 310, 37–53. Zhao, G.C., Cawood, A., Wilde, S.A., Lu, L.Z., 2000a. Metamorphism of basement rocks in the Central Zone of the North China Craton: implications for Paleoproterozoic tectonic evolution. Precambrian Res. 103, 55–88. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 2000b. Petrology and P–T path of the Fuping mafic granulites: implications for tectonic evolution of the central zone of the North China Craton. J. Metam. Geol. 18, 375–391.

224

S. Liu et al. / Precambrian Research 148 (2006) 205–224

Zhao, G.C., Cawood, P.A., Wilde, S.A., Lu, L.Z., 2001a. High-pressure granulites (retrograded eclogites) from the Hengshan Complex, North China Craton; petrology and tectonic implications. J. Petrol. 42, 1141–1170. Zhao, G.C., Wilde, S.A., Cawood, P.A., Sun, M., 2001b. Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P–T path constraints and tectonic evolution. Precambrian Res. 107, 45–73. Zhao, G.C., Wilde, S.A., Cawood, P.A., Sun, M., 2002. SHRIMP U–Pb zircon ages of the Fuping Complex: implications for Late Archean to Paleoproterozoic accretion and assembly of the North China Craton. Am. J. Sci. 302, 191–226.

Zhao, G., Sun, M., Wilde, S.A., Li, S., 2005. Late Archean to Paleoproterozoic evolution of the North China Craton: key issues revisited. Precambrian Res. 136, 177–202. Zhu, X.K., O’Nions, R.K., 1999. Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain. Earth. Planet. Sci. Lett. 171 (2), 209–220. Zhu, X.K., O’Nions, R.K., Gibb, A.J., 1998. SIMS analysis of U–Pb isotopes in monazite: matrix effects. Chem. Geol. 144, 305–312. Zhu, X.K., Onions, R.K., Belshaw, N.S., 1997. Lewisian crustal history from in situ SIMS mineral chronometry and related metamorphic textures. Chem. Geol. 136, 205–218.