Geochemistry and U–Pb zircon ages of metamorphic volcanic rocks of the Paleoproterozoic Lüliang Complex and constraints on the evolution of the Trans-North China Orogen, North China Craton

Precambrian Research 222–223 (2012) 173–190

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Geochemistry and U–Pb zircon ages of metamorphic volcanic rocks of the Paleoproterozoic Lüliang Complex and constraints on the evolution of the Trans-North China Orogen, North China Craton Shuwen Liu a,∗ , Jian Zhang b , Qiugen Li a , Lifei Zhang a , Wei Wang a , Pengtao Yang a a b

The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, PR China Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong

a r t i c l e

i n f o

Article history: Received 12 March 2011 Received in revised form 15 July 2011 Accepted 19 July 2011 Available online 2 August 2011 Keywords: Paleoproterozoic Lüliang Complex Geochemistry Zircon U–Pb age Active continental margin North China Craton

a b s t r a c t The Paleoproterozoic Lüliang Complex is situated in the central part of the western margin of the TransNorth China Orogen and consists of volcanic rocks, sedimentary rocks and Paleoproterozoic granitoid intrusions. The volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist- to amphibolite-facies. These metamorphosed volcanic rocks are dominated by basalts to basaltic andesites. The parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5% to 30% partial melting of spinel lherzolites to spinel-garnet lherzolites which had been enriched by the subduction melts. Mafic magma experienced fractional crystallization and crustal assimilation. U–Pb zircon dating on two metamorphosed volcanic rocks from the Yejishan and Lüliang groups reveals that they formed at 2210 ± 13 Ma and 2213 ± 47 Ma, respectively, and were metamorphosed at ∼1832 Ma. This suggests that the metamorphosed volcanic rocks in the Yejishan and Lüliang groups formed synchronously in the Paleoproterozoic. These new ages, integrated with recently reported U–Pb zircon ages for the Jiehekou Group and Paleoproterozoic granitoids, suggest that all of the lithological assemblages of the Lüliang Complex formed and were metamorphosed in the Paleoproterozoic, not in the Neoarchean. Petrological, geochronological and geochemical data suggest that the geodynamic evolution of the Paleoproterozoic Lüliang Complex was involved in the development of a magmatic arc system at an active continental margin, generating widespread arc-related magmatism at ∼2.2 Ga. The Lüliang Complex then underwent intense deformation and metamorphism, and was incorporated into the Trans-North China Orogen during the 1.88–1.83 Ga collisional event which was followed by post-collision extension at ∼1.80 Ga. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Trans-North China Orogen (TNCO) has been considered to be a convergent orogenic zone along which two discrete continental blocks, called the Eastern and Western Blocks (EB and WB, Fig. 1), amalgamated to finally form the North China Craton (NCC) at ∼1850 Ma (Zhao et al., 1998, 1999a,b, 2000a,b, 2001a,b, 2002, 2005, 2007, 2008a,b, 2009, 2010a; Guo et al., 2002, 2005; Liu et al., 2002a,b, 2004a,b, 2005, 2009; 2010; C.H. Liu et al., 2006; S.W. Liu et al., 2006, 2011; C. Liu et al., 2011; Wilde et al., 2002, 2004a,b, 2005; Kröner et al., 2005, 2006; Wilde and Zhao, 2005; Lu et al., 2008; J. Wang et al., 2010; Li et al., 2011). The main lines of evidence supporting this tectonic model include: (1) SHRIMP zircon U–Pb geochronological data show largely magmatic ages of

∗ Corresponding author. Tel.: +86 10 62754163; fax: +86 10 62754163. E-mail address: [email protected] (S. Liu). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.07.006

∼2500 Ma and metamorphic ages of ∼1850 Ma within the TNCO (Guan et al., 2002; Wilde et al., 2002; Zhao et al., 2002, 2008a; Guo et al., 2005; Kröner et al., 2005, 2006); (2) the EPMA Th–U–Pb monazite ages also reveal that the main metamorphic event occurred at 1880–1822 Ma in the Lüliang and Wutai Complexes (Liu et al., 2005; S.W. Liu et al. 2006); (3) discovery of the ∼1.85 Ga metamorphosed high pressure granulites in the Hengshan Complex and Chicheng area of the TNCO (Zhai et al., 1993, 2003; Liu, 1996; Zhao et al., 2001a,b; O’Brien and Rotzler, 2003; Zhang et al., 2006); (4) counter-clockwise PT trajectories are recorded within the EB and WB, whereas clockwise PT paths are documented from the TNCO (Liu,1995,1996; Liu et al., 1996, 2000, 2002a,b, 2004a,b; Zhao et al., 1998, 1999a, 2000a,b, 2001a, 2010c; Xiao et al., 2011; Guo et al., 2002); and (5) the collision-related deformation occurred at 1880–1820 Ma (Zhang et al., 2006, 2007, 2009; Trap et al., 2007; Faure et al., 2007; Li et al., 2010; Z.H. Wang et al., 2010; Wang, 2010; Zhao et al., 2010b). Although this tectonic model has been broadly accepted by most people (Wu and Zhong, 1998; Guan et al., 2002;

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Fig. 1. Tectonic subdivision of the North China Craton, modified from Zhao et al. (2005). Abbreviations for metamorphic complexes: CD, Chengde; DF, Dengfeng; HA, Huai’an; LL, Lüliang; NH, Northern Hebei; TH, Taihua; WT, Wutai; XH, Xuanhua; ZH, Zanhuang; ZT, Zhongtiao. A location of Fig. 2 is shown.

Guo et al., 2002, 2005; Liu et al., 2002a,b, 2004a,b; Wilde et al., 2002, 2004a,b, 2005; Zhai et al., 2003; Kröner et al., 2005, 2006; Santosh et al., 2006, 2007a,b; Santosh, 2010; Jiang et al., 2010), some researchers still favor a model that the final collision between the Eastern and Western Blocks occurred at ∼2.5 Ga (Kusky and Li, 2003; Polat et al., 2005, 2006; Kusky et al., 2007). Therefore, it is essential to carry out field-based geological investigations combined with geochemical, metamorphic and geochronological studies in the key areas of the TNCO. The tectonic evolution of the NCC has been further refined following recognition of the Paleoproterozoic Khondalite Belt and Jiao-Liao-Ji Belt within the WB and EB, respectively (Zhao et al., 2003, 2005). The Khondalite Belt is a 1.95–1.92 Ga collisional belt along which the Yinshan Block in the north and collided with the Ordos Block in the south to form the WB (Zhao et al., 2003, 2005, 2009, 2010a,b,c; Wu et al., 2006; Xia et al., 2006a,b, 2008; Yin et al., 2009, 2011; Peng et al., 2010, 2011; Santosh, 2010). Recent studies also reported the ultrahigh-temperature (UHT) granulitefacies rocks which underwent the peak UHT metamorphism at ca. 1.92 Ga (Santosh et al., 2007b; Santosh, 2010; Zhao, 2009; Zhao et al., 2010a,b,c). The Jiao-Liao-Ji Belt is a Paleoproterozoic mobile belt within the EB and its tectonic nature remains controversial, with one school of thought arguing that it formed by the closure of a 2.3–1.9 Ga intracontinental rift (Fig. 1; Li et al., 2004, 2005, 2006; Hao et al., 2004; Luo et al., 2004, 2006, 2008; Li and Zhao, 2007), whereas others suggest that it was a Paleoproterozoic arc-continent collisional belt (Bai, 1993; Faure et al., 2004). Numerous geological, geochronological and geochemical data have revealed that the TNCO experienced complicated Pale-

oproterozoic evolution. On the basis of recent discoveries of ∼2.3 Ga metamorphosed mafic dike swarms, ∼2.1 Ga granitoids, and ∼1.9 Ga mafic lavas within the Wutai, Lüliang and Zhongtiao Complexes, some people proposed a two stage model of an active continent marginal arc (Yu et al., 1997; Geng et al., 2000, 2004; Kröner et al., 2005, 2006; Peng et al., 2005, 2007, 2008; S.W. Liu et al., 2006; Liu et al., 2009, 2010; Faure et al., 2007; Trap et al., 2007; Zhao et al., 2008a). Furthermore, some Paleoproterozoic granitoids have been reported in the Archean Fuping Complex (Nanying granitoid gneisses; Liu et al., 2005) and Hengshan Complex (pink granite gneisses; Kröner et al., 2005, 2006). New SHRIMP zircon U–Pb data imply that the whole Lüliang Complex is considered to be composed exclusively of Paleoproterozoic rocks without any Archean components (Yu et al., 1997; Geng et al., 2000, 2004; Wan et al., 2000, 2006; Faure et al., 2007; Trap et al., 2007; Zhao et al., 2010b, 2008a,b; Liu et al., 2009; Z.H. Wang et al., 2010; Wang, 2010). Of particular significance is that all of the lithological assemblages in the Lüliang Complex, including the metamorphic sedimentary and volcanic rocks and granitoids, have extensively experienced intense deformation and metamorphism during the collision between the EB and WB. Therefore, the Lüliang Complex is the key area for understanding the Paleoproterozoic processes of the TNCO. In this contribution, we present new field-based geological, geochemical, LA-ICP-MS U–Pb zircon dating data for the metamorphosed volcanic rocks of the Paleoproterozoic Lüliang Complex (PLC), which are used to determine the petrogenesis of volcanic rocks and provide important insights into understanding the Paleoproterozoic tectonic evolution of the TNCO and the final assembly of the NCC.

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Fig. 2. Geological sketch map of the Paleoproterozoic Lüliang Complex (PLC), Western Shanxi Province (modified from Liu et al., 2009), showing geological distribution of the Jiehekou Group, Yejishan Group, Lüliang Goup, granitoid rocks and the geological relationships among these geological units.

2. Geology of the Paleoproterozoic Lüliang Complex (PLC) The PLC is located at the western margin of the central segment of the TNCO (Figs. 1 and 2) and consists of Paleoproterozoic supracrustal rocks and granitoid intrusions (Geng et al., 2000; Wan et al., 2000, 2006; Liu et al., 2009, 2010; Zhao et al., 2008b, 2010c). Previous studies divided the supracrustal rock sequences of the PLC into four major units (Fig. 2), comprising, from the bottom to top: (1) the Jiehekou Group, (2) the Lüliang Group, (3) the Yiejishan Group, and (4) the Heichashan Group or Lanxian Group as low-grade metamorphic or unmetamorphic covers overlying the three former groups. These supracrustal sequences were intruded by the Chijianling, Guandishan and Luyashan granitoid intrusions (Yu et al., 1997; Geng et al., 2000, 2004; Wan et al., 2000, 2006; Zhao et al., 2008b; Liu et al., 2009, 2010). Recent geochronological studies reveal that these granitoid rocks in the PLC formed at 2.3–1.8 Ga, and all components of the Jiehekou, Yejishan and Lüliang Groups show similar ages of 2.2–2.1 Ga (Geng et al., 2000, 2004; Wan et al., 2000, 2006; Xia et al., 2009).

The Jiehekou Group is located in the western part of the PLC and consists mainly of marbles, graphite marbles, calc-silicate rocks, sillimanite mica schists, graphite-bearing pelitic gneisses/schists, quartzites, fine-grained felsic paragneisses, and metamorphic volcanic rocks – amphibolites and garnet amphibolites. These metamorphic volcanic rocks occur as thick- to thin-layered amphibolites intercalated with fine-grained felsic gneisses, marbles and schists or are outcropped as amphibolite blocks in the granitoid plutonic gneisses (Fig. 3a–c). U–Pb detrital zircon dating results and Hf isotopes of metamorphic sediments in the upper sequence of the Jiehekou Group discovered a young 207 Pb/206 Pb age of ∼2.0 Ga (Geng et al., 2000; Wan et al., 2000, 2006; Xia et al., 2006a,b, 2009), indicating that these sediments were most likely derived from the Paleoproterozoic volcanic arc materials and Neoarchean continent materials. Combined with ∼1.85 Ga metamorphic ages (S.W. Liu et al., 2006), these ages indicate that the metamorphosed volcanic rocks in the Jiehekou Group were formed in the late Paleoproterozoic. Our recent field investigations have discovered diverse granitoid intrusions including granodiorites, monzogranites,

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Fig. 3. Photographs of metamorphic volcanites and related rock assemblages in the PLC, with photos A–C showing occurrence features of the metamorphic volcanites of the Jiehekou Group; D–F showing pillow lava, a synchronous basaltic subvolcanic dyke that emplaced into the thick-layer lava and Flysch-type sediments over the lavas of the Yejishan Group; G–I showing features of coarse clastic sedimentary rocks overlaying conformably on strata sequence of the Yejishan Group; and J–L showing outcrop features of the metamorphic volcanites in the Lüliang Group and fault contact with thick-layer coarse clastic rocks of the Heichashan/Lanxian Group.

S-type granites and monzodioritic rocks (Jiehekou intrusion) within the previously defined Jiehekou Group. Identification of igneous plutons (e.g. granitoids in the northwest of the PLC; Fig. 2) and metamorphic volcanic rocks within the original Jiehekou Group led us to propose that the Jiehekou Group may have developed at a an active continental margin. The Yejishan Group is distributed on the east of the Jiehekou Group and consists of greenschist-facies metamorphosed basalts, andesites and minor dacites in the lower sequence and flyschtype sediments in the upper sequence (C.H. Liu et al., 2011). In the lower sequence of the Yejishan Group, the metamorphic basalts appear pillow and the amygdaloidal structures and are cemented by marble and siltstone (Fig. 3d). The volcanic rocks in the middle sequence are thick-layered massive basalts overlying on the pil-

low basalts and are locally intruded by similar-aged subvolcanic dikes (Fig. 3e). The upper sequence consists mainly of flysch-type sediments ranging from 2 cm to over 10 cm in thickness (Fig. 3f). These volcanic and sedimentary rocks are overlain unconformably by coarse-grained conglomerates and sandstones which preserve oblique beddings and mud-cracks (Fig. 3g–i). These coarse clastic sediment layers were weakly metamorphosed and were previously thought as the upper formation of the Yejishan Group. However, our new investigations revealed an unconformity between the weakly metamorphosed upper coarse-grained clastic sediment sequence and the greenschist-facies metamorphosed lower metamorphosed volcanic rocks and flysch-type sediments. Therefore, we preclude the upper coarse-grained sediment sequence from the Yejishan Group and regroup it into the Heichashan or Lanxian groups that

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Table 1 Simplified sample descriptions and sampling locations. Sample Jiehekou Group 06ll37 06ll38 06ll40 06ll42-1 06ll42-2 06ll46 06ll48-4 06ll49-2 L027-1 L027-2 L027-3 L027-5 L027-7 Yejishan Group 06ll06-1 06ll06-2 06ll15-1 06ll15-2 06ll16 06ll18 06ll19-1 06ll21-1 06ll21-2 06ll23 Luliang Group 06ll073 06ll074 06ll081-2 06ll72-1 06ll72-2 06ll79-1 06ll79-2

Latitude

Longitude

Lithology

Mineral assemblage

38◦ 29 06 38◦ 28 17 38◦ 27 29 38◦ 28 13 38◦ 28 13 38◦ 30 06 38◦ 23 08 38◦ 22 55 38◦ 28 51 38◦ 28 51 38◦ 28 51 38◦ 40 11 38◦ 40 11

111◦ 20 00 111◦ 20 53 111◦ 20 20 111◦ 20 53 111◦ 20 53 111◦ 18 25 111◦ 17 02 111◦ 17 13 111◦ 25 23 111◦ 25 23 111◦ 25 23 111◦ 31 51 111◦ 31 51

Garnet amphibolite Amphibolite Amphibolite Garnet amphibolite Amphibolite Amphibolite Amphibolite Amphibolite Coarse-grained amphibolite Fine-grained amphibolite Middle-grained amphibolite Garnet amphibolite Garnet amphibolite

Gt + Pl + Hb + Epi Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Gt + Pl + Hb + Epi Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Pl + Hb + Epi + Chl Gt + Pl + Hb + Epi Gt + Pl + Hb + Epi

38◦ 28 56 38◦ 28 56 38◦ 28 39 38◦ 28 39 38◦ 28 11 38◦ 28 59 38◦ 29 14 38◦ 29 11 38◦ 29 11 38◦ 28 31

111◦ 33 52 111◦ 33 52 111◦ 28 39 111◦ 28 39 111◦ 37 32 111◦ 38 06 111◦ 38 45 111◦ 29 04 111◦ 29 04 111◦ 39 16

Green schist Green schist Green schist Metamorphic subvolcanic rock Green schist Green schist Green schist Metamorphic subvolcanic rock Green schist Green schist

Epi + Chl + Pl + Act Epi + Chl + Pl + Act Epi + Chl + Pl + Act Epi + Pl + Act + Cpx Epi + Pl + Act + Cpx Epi + Chl + Pl + Act Epi + Chl + Pl + Act Epi + Chl + Pl + Act Epi + Chl + Pl + Act Epi + Pl + Act

38◦ 08 21 38◦ 06 49 38◦ 04 33 38◦ 08 096 38◦ 08 096 38◦ 05 18 38◦ 05 18

111◦ 32 38 111◦ 33 14 111◦ 33 21 111◦ 21 04 111◦ 21 04 111◦ 31 57 111◦ 31 57

Green schist Green schist Metamorphic subvolcanic rock Metamorphic subvolcanic rock Green schist Green schist Green schist

Epi + Chl + Pl + Act Epi + Pl + Act Epi + Chl + Pl + Act + Cpx Epi + Chl + Pl + Act Epi + Chl + Pl + Act + Cpx Epi + Chl + Pl + Act + Cpx Epi + Chl + Pl + Act

show similar geological features and relationships with the lower sequences (Liu et al., 2010). This interpretation was supported by C.H. Liu et al. (2011) who showed that the coarse clastic rocks of the Yejishan Group contain 2.0–1.8 Ga detrital zircons, indicating that they were deposited in a Paleoproterozoic foreland basin. Applying the traditional U–Pb zircon dating method, Geng et al. (2000) reported a zircon age of 2124 ± 38 Ma for a meta-volcanic rock of the Yejishan Group. These metamorphosed volcanic rocks display intimate geochemical features with a continental margin arc, and closely associated to the flysch-type sediments on the field. We thus interpret that the Yejishan Group may have developed in an active continental margin arc (Liu et al., 2009, 2010). The Lüliang Group is distributed in the central part of the PLC and comprises greenschist- to amphibolite-facies metamorphosed sedimentary rocks with banded iron formation (BIF) in the lower sequence and metamorphosed volcanic rocks (e.g. basalt, andesite and dacite) in the upper part (Yu et al., 1997). Major lithological assemblages include amphibolites, fine-grained felsic gneisses, quartzites, intercalated magnetite quartzites, and tremolite–actinolite marbles. The metamorphosed volcanic rocks in the Lüliang Group occur as thick- to middle-layers with massive to amygdaloidal structures (Fig. 3j–k) and cover 65% of whole metamorphic supracrustal rocks in the Lüliang Group. Most of the metamorphic supracrustal rocks in the Lüliang Group have experienced greenschist- and locally amphibolite-facies of metamorphism and are unconformably overlain by or tectonically contacted with weakly metamorphosed thick layers of the Heichashan/Lanxian Group (Fig. 3l). Three rough ages were reported for the metavolcanic rocks of the Lüliang Group, including a whole rock Sm–Nd isochron age of 2360 ± 95 Ma (Geng et al., 2000), and single-grained zircon U–Pb ages of 2051 ± 68 Ma and 2099 ± 41 Ma (Yu et al., 1997). Geochemical studies of these volcanic rocks suggest a geochemical affinity with the arc vol-

canic rocks (Yu et al., 1997). Our new detailed field investigations reveal that the Lüliang and Yejishan groups show similarity in the sequence and lithological assemblages, indicating that the two groups may represent different parts of one lithological assemblage. This interpretation is also supported by the similar geochronological and geochemical features of the volcanic rocks from the Lüliang and Yejishan Groups (Yu et al., 1997; Geng et al., 2000; Liu et al., 2010). Four igneous plutons occur in the PLC (Fig. 2), including the Chijianling granitoid intrusion, Guandishan granitoid intrusion, Jiehekou diorite intrusion and Luyashan charnockite intrusion. The Chijianling granitoid intrusion is distributed in the northwestern part of the PLC and east side of Fangshan–Lanxian fault and consists mainly of granodioritic, monzogranitic gneisses that are intruded by some small stocks of massive granites. These granites in the Chijianling intrusion was emplaced at 2173–2199 Ma (Geng et al., 2000; Zhao et al., 2008b). The Guandishan granitoid intrusion is composed of different types of granitoid rocks of variable ages, including early granodioritic and monzogranitic gneisses with magmatic crystallization ages of 2180–2174 Ma and 2088–2056 Ma, respectively, the 1906–1832 Ma monzogranitic gneisses, and ∼1800 Ma massive alkali-feldspar granites and monzogranites (Geng et al., 2000; Liu et al., 2005, 2009; Zhao et al., 2008b). The Jiehekou diorite intrusion consists of monzodiorites and monzonites with magmatic crystallization age of ∼1940 Ma (our unpublished data). The Luyashan charnockite intrusion is composed of enderbite, charnockite, rapakivi granite and alkalinefeldspar granite, which show magmatic crystallization ages of 1815–1800 Ma, representing the end of the Paleoproterozoic orogenic event (Geng et al., 2000; S.W. Liu et al., 2006; Zhao et al., 2008a,b). There are four main fault zones in the PLC. The boundary between the Chijianling granitoid gneisses and the Jiehekou and

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Yiejishan Groups is a thrust fault that strikes NE and dips to the NW at ∼35◦ . The second and third faults are also thrust faults which represent the tectonic and lithological boundaries between the Yejishan and Jiehekou Groups to the west and the Chijianling granitoid gneisses to the east (Fig. 2). The fourth fault, named Jinzhouyu fault (Fig. 2), is a tectonic contact boundary between the coarse-grained clastic sedimentary rocks of the Heichashan Group/Lanxian Group and Lüliang Group. The late three faults also show similar orientations striking NNE–NE and dipping to NWW at 25–40◦ (Fig. 2). The deformational features show a NWW to SEE sense of thrust which is considered as a late Paleoproterozoic nappe stacking (Faure et al., 2007; Trap et al., 2007).

3. Analytical methods and procedures A total of thirty representative samples of metamorphic volcanic rocks collected from the PLC were chosen for whole-rock chemical analyses of the major compositions and trace elements. Of these, twelve samples were collected from the Jiehekou Group (three garnet amphibolites and nine amphibolites), eleven samples were collected from the Yejishan Group (three metamorphosed basaltic subvolcanic rocks and eight green schists), and seven samples were collected from the Lüliang Group (two metamorphosed basaltic subvolcanic rocks and five green schists). Sample locations and simplified lithological features are listed in Table 1. Major components of these samples were analyzed using an automatic X-ray fluorescence (XRF) spectrometry, Thermo Arl Advant XP+. Sample powders were made into glass disks after fusion with lithium metaborate. Samples and flux materials were mixed in a 1:10 ratio and fused at 1100 ◦ C in a Pt-Au crucible for 20–40 min. The resultant melt was then poured into a preheated 34 mm diameter pellet, preparing XRF analyses for major components. XRF system was calibrated by international standards of the GSP-2 and JG-2, and a national standard of the GBW02103. The pellet was excited with a rhodium tube operated at 50 kV and 20 mA. The volatile contents (e.g., CO2 and H2 O) were determined by measuring the weight loss after heating the samples at 1050 ◦ C. The precision is 0.5% for major oxides, and detailed analytical procedures are after Liu et al. (2004a,b, 2005). With respect to trace element analyses of these samples, wholerock powders were dissolved as following procedures. Firstly, powders were accurately weighed (25 mg) into Savillex Teflon beakers within high-pressure bomb, a 1:1 mixture of HF–HNO3 was added and heated up for 24 h at 80 ◦ C, and then evaporated. Secondly, 1.5 ml HNO3 , 1.5 ml HF and 0.5 ml HClO4 were added after the evaporating process, and the beakers were capped for digestion within a high-temperature oven at 180 ◦ C for 48 h or longer, until powders were completely digested. Thirdly, the solutions were diluted with 1% HNO3 into 50 ml for determination. Trace elements were analyzed using ELEMENT-I plasma mass spectrometer (Finnigan-MAT Ltd. German) at Research Institute of Uranium Geology (Beijing), and the international standards, GSR-3 and GSR-15, were used for analytical monitoring of quality. All analytical results of these samples in the major oxides and trace elements as well as some parameters used in the text are listed in Table 2. Two samples from the Yejishan Group (sample 06ll021-1) and Lüliang Group (sample 06ll72-1) were processed by crushing, initial heavy liquid and subsequent magnetic separation. The zircon grains were handpicked under a binocular microscope. Zircon grains from each sample were mounted in epoxy and then polished to approximately half of the average zircon grain thickness. The internal texture of the zircon was examined using cathodoluminescence (CL) images at the Peking University. The CL images were obtained on a FEI PHILIPS XL 30 SEFG SEM with 2-min scanning time under conditions of 15 kV and 120 ␮A.

Zircon analyses of U–Th–Pb elements and isotopes were carried out simultaneously on a Perkin Elmer laser-ablation ICP-MS housed at the State Key Laboratory of Continental Dynamics of the Northwest University, Xi’an, China. The instrument configuration consists of a GeoLas 2005 laser-ablation system (including a COMPexPro 102 ArF excimer laser with wavelength of 193 nm and a GeoLas 2005 PLUS package) and an Elan 6100 DRC Q-ICP-MS (Perkin Elmer). A detailed description for the analytic procedures and processes was documented by Yuan et al. (2003, 2004, 2008) and S.W. Liu et al. (2011). In brief, a helium stream was used to effectively transport the ablated particles and to reduce deposition at the ablation site. The laser beam is 30 ␮m in diameter and ablated depth is 20–40 ␮m. Concentrations of U, Th, Pb elements were calibrated using 29 Si as an internal standard and NIST SRM 610 as the external reference standard. Repeated analyses of standards enhanced 10% of precisions for most elements. Both instrumental mass deviation and depth-dependent elemental and isotopic fractionation were corrected using zircon 91500 as the external standard. 207 Pb/206 Pb, 206 Pb/238 U, 207 Pb/235 U and 208 Pb/232 Th ratios and apparent ages were calculated using GLITTER 4.4 (GEMOC, Macquarie University, Sydney, Australia) and plotted using ISOPLOT (rev.3) (Ludwig, 2001). The analyzed zircon U–Th–Pb isotopic results of the two samples are listed in Table 3.

4. Analytical results 4.1. Geochemical characteristics 4.1.1. Major element compositions Geochemical data and calculated parameters of all analyzed samples are listed in Table 2. On the TAS classification diagram, all twelve samples from the Jiehekou Group plot in the basalt field (Le Maitre, 2002; Fig. 4a). Five of seven samples from the Lüliang Group plot in the basalt field, except samples 06ll72-2 and 06ll07-2, of which sample 06ll72-2 shows lower SiO2 and plots in the left area of the basalt field, whereas sample 06ll07-2 shows higher alkaline concentrations and plots in the trachy basalt field (Fig. 4a). Samples from the Yejishan Group display a wide range of chemical compositions (Fig. 4a). Of these, four samples (06ll06-1, 06ll06-2, 06ll19-2 and 06ll23) plot in the basalt field; three samples (06ll21-2, 06ll15-1 and 06ll15-2) plot in the basaltic andesite field; and the other four samples (06ll19-1, 06ll18, 06ll21-1 and 06ll21-3) show higher alkali concentrations and plot in the trachy basalt and basaltic trachy-andesite fields (Fig. 4a). On the FeOt/MgO versus SiO2 diagram (Myashiro, 1974; Fig. 4b), all thirty samples plot in the tholeiite field. Samples from the Lüliang Group basically display lower FeOt/MgO ratios, whereas samples from the Jiehekou and Yejishan Groups show medium and higher FeOt/MgO ratios, respectively (Fig. 4b). Samples from the Yejishan Group have higher TiO2 (>1.3%) and lower Mg# (MgO/(MgO + FeOt) molar ratio), Al2 O3 /TiO2 and CaO/TiO2 ratios (Fig. 4c and d). In contrast, all other samples from the Lüliang and Jiehekou groups display lower TiO2 concentrations and higher Al2 O3 /TiO2 and CaO/TiO2 ratios (usually TiO2 < 1.3%, Table 2, Fig. 4c and d).

4.1.2. Rare earth elements (REEs) The samples from the Jiehekou and Lüliang groups display lower total REEs ranging from 36.14 ppm to 99.34 ppm and 27.99 ppm to 70.52 ppm, respectively (Table 2). REE patterns are flattened or slightly right-declined with lower (La/Yb)n ratios (1.03–5.12) and slightly positive Eu anomalies (Table 2, Figs. 5a and b). In contrast, samples from the Yejishan Group show distinctly higher total REEs (163–275 ppm) and (La/Yb)n ratios (usually >6), and slightly negative Eu anomalies (Table 2, Fig. 5c). All thirty samples from the PLC

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Table 2 Analytical data of whole-rock major and trace elements and related parameters. Sample

06ll037

06ll038

06ll040

06ll042-1

06ll042-2

Jiehekou Group SiO2 (wt%) TiO2 Al2 O3 Fe2 O3 MgO MnO CaO Na2 O K2 O P2 O5 LOI Total FeOt Al2 O3 /TiO2 CaO/Na2 O Mg#

49.82 0.96 14.43 14.78 5.86 0.21 10.2 1.99 0.55 0.13 0.94 99.87 13.3 15.03 5.13 43.99

50.18 1.22 13.27 15.22 6.1 0.22 9.41 2.7 0.74 0.12 0.73 99.91 13.7 10.88 3.49 44.25

49.74 0.91 14.44 13.25 7.32 0.19 10.85 2.04 0.23 0.08 0.85 99.90 11.92 15.87 5.32 52.26

46.42 1.21 13.52 16.27 9.72 0.21 8.97 2.19 0.33 0.09 0.95 99.88 14.64 11.17 4.1 54.2

49.4 1.24 14.65 13.62 6.92 0.18 9.67 2.92 0.34 0.1 0.82 99.86 12.26 11.81 3.31 50.15

5.65 14.3 1.94 10.9 3.35 1.22 4.04 0.75 4.72 1.06 3.14 0.49 2.89 0.42 55 1.01 1.32 48 152 48 67 45 108 19.1 20 197 30 90 4.68 68 2.12 0.16 0.54 0.15 29.09

6.22 15.7 2.12 11.0 3.16 1.00 4.15 0.77 4.61 1.04 3.11 0.47 2.90 0.42 57 0.85 1.45 44 92 52 43 14.5 99 18.3 18.2 133 31 105 4.79 86 2.26 0.23 0.71 0.40 20.83

3.50 9.78 1.25 6.72 2.13 0.72 2.77 0.52 3.12 0.75 2.24 0.34 2.03 0.30 36 0.91 1.17 35 118 48 80 73 82 16.2 8.88 167 20 67 3.44 38 1.55 0.16 0.34 0.11 21.79

3.88 10.3 1.60 8.70 2.52 0.86 3.21 0.59 3.63 0.82 2.36 0.33 2.18 0.33 41 0.92 1.20 36 111 76 198 29 123 15.9 6.27 157 22 66 3.80 52 1.89 0.12 0.30 0.10 31.90

3.76 11.5 1.54 8.46 2.88 0.97 3.44 0.64 4.17 0.94 2.82 0.42 2.46 0.37 44 0.94 1.03 35 197 52 90 8.02 98 17.4 8.47 403 27 96 4.69 62 2.07 0.18 0.29 0.10 25.88

La (ppm) Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu TREEs ␦Eu (La/Yb)n Sc (ppm) Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Ba Hf Ta Th U Nb/Ta yz17-5

yz17-7

Jiehekou Group 50.24 49.63 1.07 1.04 13.55 14.28 15.08 14.87 6.58 6.15 0.34 0.33 10.48 10.21 2.05 2.11 0.44 0.37 0.11 0.1 0.38 1.11 100.29 100.23 13.57 13.38 13.03 13.35 5.11 4.84 46.36 45.03 8.18 8.43 17.0 17.7 2.28 2.53 11.6 11.1

06ll006-1

06ll006-2

06ll015-1

06ll015-2

Yejishan Group 50.29 1.8 14.71 15.16 3.99 0.18 7.27 2.8 2.13 0.33 1.16 99.82 13.64 8.17 2.6 34.27 31 73 7.90 36

50.52 1.79 14.65 14.93 4.1 0.18 7.66 2.67 1.82 0.32 1.18 99.82 13.43 8.18 2.87 35.24 25 62 6.76 34

52.56 1.47 14.59 12.96 4.39 0.18 7.83 2.45 1.99 0.26 1.14 99.82 11.66 9.93 3.2 40.16 33 75 9.18 43

52.71 2.03 13.14 15.15 3.36 0.16 7.98 1.51 1.42 0.36 2 99.82 13.63 6.47 5.28 30.53 43 86 11.0 48

06ll046

06ll048-4

06ll049-2

yz17-2

yz17-3

47.44 0.62 14.56 12.74 9.43 0.18 10.46 2.56 1.05 0.1 0.74 99.88 11.46 23.48 4.09 59.46

47.68 0.54 11.24 12.6 12.02 0.16 12.21 0.59 1.06 0.13 1.58 99.81 11.34 20.81 20.69 65.39

50.99 1.08 14.93 12.35 5.79 0.2 9.25 3.11 1.15 0.16 0.87 99.88 11.11 13.82 2.97 48.16

49.73 0.92 14.08 13.68 6.8 0.34 9.93 2.16 0.76 0.1 1.85 100.35 12.31 15.3 4.6 49.61

49.55 1 13.66 14.7 6.6 0.31 9.72 2.59 0.59 0.11 1.44 100.27 13.23 13.66 3.75 47.07

5.57 13.8 1.73 8.98 2.48 0.87 3.01 0.59 3.40 0.79 2.41 0.37 2.13 0.33 46 0.98 1.77 36 128 45 63 27 73 14.8 14.1 207 22 75 3.19 125 1.77 0.08 0.40 0.26 40.41

15.6 36.4 3.79 17.8 4.46 2.14 5.33 0.91 5.06 1.10 3.31 0.42 2.54 0.37 99 1.34 4.17 18 1093 47 345 10.8 95 15.9 44 133 24 46 2.80 71 1.81 0.15 2.64 1.18 19.18

11.7 28.0 3.43 16.1 3.76 1.31 4.46 0.72 4.26 0.92 2.74 0.38 2.47 0.38 81 0.98 3.19 41 133 45 66 5.65 118 22 18.0 265 26 101 4.72 69 2.61 0.14 1.04 0.54 33.50

7.05 15.3 2.08 8.94 2.44 0.88 2.98 0.56 3.25 0.75 2.25 0.33 2.12 0.45 49 1.00 2.25 48 * * * * * * 35 238 20 30 2.78 176 1.95 0.16 1.00 0.26 17.38

7.39 16.9 2.34 11.0 2.85 1.02 3.75 0.69 3.97 0.83 2.48 0.36 2.38 0.49 56 0.95 2.10 26 * * * * * * 31 352 23 34 3.10 212 3.60 0.18 1.60 0.16 17.22

06ll016 49.99 1.82 15.08 14.39 4.23 0.15 7.61 2.44 2.07 0.38 1.61 99.77 12.95 8.29 3.12 36.8 35 78 9.15 40

06ll018

06ll019-1

51.32 1.62 15.61 14.14 4.13 0.16 5.26 3.36 2.77 0.27 1.11 99.75 12.72 9.64 1.57 36.66 31 65 8.28 40

49.59 1.65 15.83 13.59 5.11 0.14 5.7 4.26 2.45 0.25 1.21 99.78 12.23 9.59 1.34 42.69 37 79 9.24 43

06ll021-1 53.74 2.08 13.25 14.76 2.53 0.16 6.4 2.89 2.88 0.4 0.71 99.8 13.28 6.37 2.21 25.35 45 92 11.3 52

180

S. Liu et al. / Precambrian Research 222–223 (2012) 173–190

Table 2 (Continued) yz17-5 2.88 1.01 3.69 0.68 3.79 0.83 2.70 0.40 2.38 0.50 58 0.95 2.39 53 * * * * * * 12.3 201 24 46 3.04 386 3.72 0.19 1.73 0.19 16.00 06ll021-2 Yejishan Group 51.99 1.42 15.11 12.6 4.3 0.14 7.97 2.65 2.05 0.25 1.29 99.77 11.34 10.64 3.01 40.33 33 69 7.92 37 6.57 1.93 7.31 1.05 6.06 1.19 3.43 0.46 2.81 0.39 179 0.85 8.01 18 46 40 37 22 110 22 83 412

yz17-7

06ll006-1

06ll006-2

06ll015-1

06ll015-2

06ll016

3.06 1.03 3.50 0.67 3.76 0.77 2.19 0.35 2.26 0.43 58 0.96 2.45 23 * * * * * * 5.14 181 18.4 50 3.63 188 1.67 0.21 0.57 0.18 17.29

7.35 1.87 7.73 1.20 6.47 1.35 4.13 0.60 3.44 0.50 182 0.75 6.00 25 27 48 24 69 128 23 83 287 38 259 12.0 569 5.52 0.51 5.26 1.39 23.58

7.31 1.90 7.97 1.21 6.63 1.43 4.26 0.60 3.47 0.52 163 0.76 4.95 18 29 48 24 72 125 23 90 424 38 291 15.3 675 5.06 0.72 3.58 1.14 21.20

7.50 2.23 7.41 1.36 6.89 1.51 3.96 0.56 3.48 0.49 195 0.90 6.41 22 42 51 53 53 114 24 80 385 36 97 11.2 642 2.37 2.21 5.56 1.22 5.06

8.01 2.34 8.49 1.38 7.93 1.62 4.53 0.65 4.04 0.59 228 0.86 7.19 29 13 39 25 68 129 23 49 335 40 153 12.1 636 3.98 0.61 7.18 2.66 19.80

7.45 2.12 7.53 1.16 6.19 1.23 3.56 0.53 3.27 0.48 196 0.86 7.22 20 27 47 29 40 124 22 74 443 37 271 15.4 720 5.74 0.60 5.84 1.40 25.74

06ll021-3

06ll023

06ll072-1

06ll072-2

06ll073

06ll074

50.07 1.72 15.18 14.67 4.49 0.17 7.7 2.66 1.89 0.3 0.98 99.83 13.2 8.83 2.89 37.75 29 62 7.82 36 7.10 1.96 6.99 1.10 6.25 1.28 3.79 0.51 3.21 0.50 166 0.84 6.03 21 24 42 35 50 119 20 64 273

Lüliang Group 44.3 0.71 14.2 8.95 7.82 0.14 9.02 2.15 0.95 0.09 11.52 99.85 8.05 20 4.2 63.39 5.32 11.8 1.66 8.28 1.86 0.70 2.16 0.36 2.09 0.42 1.20 0.16 1.09 0.17 37 1.07 3.29 22 268 40 123 32 51 14.9 26 233

48.98 1.34 19.32 11.42 5.38 0.14 3.55 4.29 2.21 0.11 3.11 99.85 10.28 14.42 0.83 48.26 7.13 18.6 2.39 12.6 3.15 0.96 3.43 0.58 2.87 0.61 1.69 0.22 1.45 0.18 56 0.89 3.33 22 91 51 68 46 81 21 71 302

51.1 0.9 15.14 9.73 6.95 0.16 8.98 2.61 0.48 0.12 3.69 99.86 8.76 16.82 3.44 58.58 11.4 25 3.30 15.2 3.21 0.95 3.36 0.50 3.04 0.59 1.63 0.25 1.50 0.20 71 0.88 5.12 23 207 38 91 42 97 17.3 10.7 356

46.49 0.5 19.7 8.34 7.49 0.15 9.04 2.53 1.15 0.05 4.42 99.86 7.5 39.4 3.57 64.03 3.54 8.67 1.12 5.88 1.55 0.63 1.91 0.28 1.75 0.39 1.07 0.16 0.92 0.14 28 1.12 2.60 18 266 37 65 11.4 80 15.5 45 406

55.88 1.75 13.95 12.02 2.61 0.13 5.69 2.58 3.84 0.32 0.99 99.76 10.82 7.97 2.21 30.07 50 108 13.42 58 10.29 2.58 10.45 1.52 8.51 1.80 4.75 0.71 4.47 0.67 275 0.75 7.58 17 23 31 28 40 121 23 140 267

06ll018 7.32 2.13 7.43 1.07 6.89 1.42 3.86 0.57 3.52 0.49 179 0.88 6.00 23 45 54 62 21 133 22 98 376 38 204 13.0 1340 4.69 0.47 5.96 1.88 27.34 06ll079-1 48.5 1.12 16.22 11.99 6.98 0.16 9.9 2.27 0.22 0.12 2.38 99.86 10.79 14.48 4.36 53.55 7.17 16.3 2.28 11.7 2.65 1.01 3.08 0.51 3.30 0.67 1.86 0.28 1.55 0.20 53 1.07 3.12 28 147 50 107 67 76 19.0 3.07 377

06ll019-1 7.50 2.43 7.67 1.32 7.07 1.46 4.03 0.60 3.51 0.55 205 0.97 7.20 26 63 51 56 80 129 28 107 371 41 215 12.1 1185 5.54 0.58 7.06 1.35 20.83 06ll079-2 49.26 0.85 14.67 10.16 7.03 0.15 10.04 2.58 0.16 0.09 4.88 99.87 9.14 17.26 3.89 57.82 7.92 17.7 2.30 12.1 2.68 0.84 2.78 0.50 3.07 0.63 1.63 0.25 1.61 0.23 54 0.93 3.33 25 194 46 111 47 66 14.1 1.72 272

06ll021-1 9.02 2.30 9.49 1.46 8.22 1.65 4.58 0.70 4.52 0.63 243 0.75 6.69 21 18 36 24 38 139 24 110 300 45 301 16.4 911 6.52 0.75 9.24 2.17 21.86 06ll081-2 48.23 0.68 15.58 10.6 8.08 0.17 8.87 1.34 3.06 0.08 3.09 99.78 9.54 22.91 6.62 60.15 5.54 12.8 1.88 8.87 1.95 1.23 2.52 0.42 2.83 0.67 1.67 0.27 1.73 0.24 43 1.69 2.16 29 225 47 110 33 92 15.7 91 274

S. Liu et al. / Precambrian Research 222–223 (2012) 173–190

181

Table 2 (Continued) 06ll021-2

06ll021-3

06ll023

06ll072-1

06ll072-2

06ll073

06ll074

06ll079-1

06ll079-2

06ll081-2

32 151 11.2 596 3.32 0.53 6.35 1.51 21.00

46 298 17.7 1186 7.15 0.83 10.85 2.61 21.31

29 181 9.23 506 5.77 0.63 5.93 1.50 14.74

10.8 42 1.91 289 1.22 0.12 0.48 0.11 16.59

16.7 99 4.67 234 2.14 0.15 0.54 0.14 31.79

16.4 71 5.21 175 1.75 0.23 1.15 0.30 22.56

7.77 27 1.18 197 1.04 0.04 0.61 0.16 27.51

18.3 27 3.06 49 0.81 0.13 0.58 0.17 23.04

15.0 32 2.54 52 1.07 0.13 0.94 0.24 19.55

16.4 19 2.03 961 0.51 0.11 0.56 0.12 17.99

Notes: wt %, major and minor element oxides in weight percent; ppm, trace elements in parts per million, *not analysis; LOI, lost on ignition; ␦Eu, chondrite-normalized Eu/(Sm × Gd)1/2 .

appear positive correlations between the total REEs and (La/Yb)n values (Fig. 5d). 4.1.3. Other trace elements On the primitive mantle normalized multi-element spider diagrams, samples from the Jiehekou Group show depletion of Th, Nb, Ta, P and Ti but enrichment of Rb, K, Sr and LREEs (Fig. 6a). Samples from the Yejishan Group display manifestly enrichment of LILEs, LREEs and Zr, Hf but depletion of Nb, Ta, Sr, P and Ti (Fig. 6b). Samples from the Lüliang Group exhibit variable concentration of Ba, Rb and K, and depletion of Nb, Ta, P and enrichment of LREEs, Sr and other HFSEs (Fig. 6c). In general, all the samples display uniform depletion of Nb, Ta and P and mostly Ti, which reveals their affinities with arc magmatism (Yu et al., 1997; Liu et al., 2010). On the Zr/Ti versus Nb/Y plot (Pearce, 1996), all samples of the Lüliang

Group, most samples of the Jiehekou Group and four samples of the Yiejishan Group plot basalt field, and one sample of the Jiehekou Group and nine samples of the Yejishan Group plot into the andesite or basaltic andesite fields (Fig. 6d). 4.2. LA-ICP-MS U–Pb zircon geochronology Sample 06ll21-1 is a metamorphosed subvolcanic rock with basaltic composition, which intrudes the thick-layered massive metabasalt of the Yejishan Group (Fig. 3e). Similarly, sample 06ll72-1 also is a metamorphosed subvolcanic rock with basaltic composition, which was emplaced into the synchronous eruptive lava of the Lüliang Group. Zircons separated from the two samples were dated using the LA-ICP-MS U–Th–Pb isotopic method (Table 3).

Fig. 4. Petrochemical classifications for the metamorphic volcanic rocks of the PLC and their petrochemical characteristics. (A) A TAS classification diagram (modified from Le Maitre, 2002); (B) a FeOt/MgO versus SiO2 distinguishing the tholeiite and calc-alkaline basalts (after Myashiro, 1974); (C) a TiO2 versus Mg# plot; and (D) a Al2 O3 /TiO2 versus CaO/TiO2 plot. Symbols: square—samples from the Jiehekou Group, diamond—samples from the Yejishan Group and circle—samples from the Lüliang Group. Note sample symbols in other following figures are same as these in this figure except those to be especially explained.

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Fig. 5. Chondrite-normalized REE paterns for samples from the Jiehekou Group (A), from the Yejishan Group (B), from the Lüliang Group (C) and a (La/Yb)n versus TREE plot (D).

On the cathodoluminescence images, most zircon grains from the sample 06ll21-1 display long prismatic shapes (60–110 ␮m in length) and oscillatory zoning structures, indicative of magmatic origin (Corfu et al., 2003; Grant et al., 2009), though few zircon grains do not show any inner structures (Fig. 7). Totally seven analyses were obtained with variable U contents of 23–322 ppm, Th contents of 4.54–246 ppm, radioactive genesis Pb (Pb* in the table) contents of 49–771 ppm, and Th/U values of 0.03–0.77. Of which five analyses are concordant except analyses #4 and #6 which are below the concordant curve (Fig. 8a). The analysis #3 on a structureless zircon grain yields the oldest 207 Pb/206 Pb age of 2516 ± 46 Ma. The second older analysis is #4 which gives an apparent 207 Pb/206 Pb age of 2442 ± 61 Ma, although it shows manifest Pb-loss. Similarly, analysis #6 yields an apparent 207 Pb/206 Pb age of 2307 ± 55 Ma (Fig. 8a), and analyses #3, #4 and #6 define a discordant line with an upper intercept age at 2529 ± 47 Ma (MSWD = 2.1). The youngest age is from analysis #2 on a zircon grain with blurry inner structures and yields an apparent 207 Pb/206 Pb age of 1914 ± 47 Ma with Th/U ratio of 0.12. Three concordant analyses display their Th/U ratios ranging from 0.05 to 0.77 and yield apparent 207 Pb/206 Pb ages of 2180 ± 46 Ma to 2240 ± 61 Ma. The analysis #1 shows low Th/U ratio of 0.05 (Table 3) and gives the youngest apparent 207 Pb/206 Pb age (Table 3), indicating that it has been altered by late thermal events. Three analyses yield a concordant age of 2210 ± 13 Ma (MSWD = 0.01, Fig. 8a inset) with mean weighted 207 Pb/206 Pb age of 2214 ± 56 Ma (MSWD = 0.47, Fig. 8a inset). Based on the Th/U ratios and cathodoluminescence images of these analyzed zircons, we interpret the oldest discordant upper intercept age of 2529 ± 47 Ma as the age of a xenocryst zircon acquired during ascent of the mafic

magma (Corfu et al., 2003; Grant et al., 2009), which implies a possibility to have the Neoarchean crustal materials in the PLC. The concordia age of 2210 ± 13 Ma represents the crystallization age of mafic volcanic rocks which were altered by late thermal events. A total of eleven available U–Th–Pb isotopic analyses on eleven zircon grains have been obtained from the sample 06ll72-1 from the Lüliang Group. These analyzed zircon grains show prismatic shapes (50–80 ␮m in length) and oscillatory zoning inner structures. On the cathodoluminescence images, some zircon grains show blurry inner structures and the others do not show luminescence (Fig. 7). These analyses show variable U contents of 4.19–3832 ppm, Th contents of 3.91–1605 ppm, radioactive genesis Pb (Pb* in Table 3) contents of, 6.53–4469 ppm, and Th/U ratios of 0.11–1.04 (Table 3). On the concordia diagram (Fig. 8b), except analysis #7 that plots obviously away from the concordia curve, other ten analyses show two preferred age groups. One group comprises seven analyses (#2, #3, #4, #5, #9, #10 and #11), five of which (#3, #4, #5, #9 and #10) define an upper intercept age of 2213 ± 47 Ma (MSWD = 0.63) with a weighted mean 207 Pb/206 Pb age of 2156 ± 56 Ma (MSWD = 1.7). The two most concordant analyses #5 and #10 give a concordia age of 2178 ± 16 Ma (MSWD = 3.4) (Fig. 8b). The other group comprises three analyses (#1, #6 and #8), which construct a discordant line trend, and yield a weighted mean 207 Pb/206 Pb age of 1832 ± 56 Ma (MSWD = 1.19). The older age of 2156 ± 56 Ma and the upper intercept age of 2213 ± 47 Ma represent the magmatic crystallization age (Corfu et al., 2003; Grant et al., 2009), whereas the younger age of 1832 ± 56 Ma is consistent with the metamorphic ages of the PLC that were obtained using U–Th–Pb monazites chemical method (S.W. Liu et al., 2006). Therefore, this result further confirms that

S. Liu et al. / Precambrian Research 222–223 (2012) 173–190

183

Fig. 6. Primitive mantle-normalized multi-element spindergrams for samples from the Jiehekou Group (A), from the Yejishan Group (B), from the Lüliang Group (C), primitive mantle values are after Sun and McDonough (1989), and a Zr/Ti versus Nb/Y classification diagram (D) for the metamorphic volcanites of the PLC (after Pearce, 1996). Symbols as in Fig. 4.

the metamorphism of the Lüliang volcanic rocks occurred at the ∼1850 Ma (e.g. Corfu et al., 2003; Grant et al., 2009; S.W. Liu et al., 2011). 5. Discussion 5.1. Chronology of the PLC Recent geological investigations in the PLC were concentrated mainly on the granitoid rocks because these rocks are extensively distributed in the complex and are easy to acquire zircons for U–Pb isotopic dating. SHRIMP U–Pb zircon isotopic data have revealed that the early tonalitic and granodioritic gneisses were emplaced at ∼2174 Ma; the early gneissic monzogranites were emplaced at ∼2056 Ma; the S-type gneissic to massive monzogranites were emplaced at ∼1844 Ma; and the post-collision massive syenogranitic rocks (e.g. Dacaoping syenogranite intrusion and Luyashan charnockite intrusion) were emplaced at ∼1800 Ma, (Geng et al., 2000; Zhao et al., 2008a; Liu et al., 2009). In addition, U–Pb isotopic dating of inherited igneous zircons was considered that the oldest granitoids of the PLC were emplaced at ∼2300 Ma without any older source materials (Geng et al., 2000; Zhao et al., 2008a). However, U–Pb zircon ages in this study reveals the existence of ∼2500 Ma old zircon xenocryst in the metamorphic volcanic rocks of the Yiejishan Group, indicative of existing ∼2500 Ma crustal materials in the PLC. In our interpretation, the old zircon grains are probably detrital zircons that were trapped from the wall rocks during the ascent of mafic magmas. The major population of zircons shows a uniform magmatic crystallization age

of ∼2200 Ma for the metamorphic volcanic rocks. Dating results of the metamorphosed volcanic rock in the Lüliang Group indicate that the magmatic eruption occurred at ∼2200 Ma and metamorphism happened at ∼1830 Ma. This interpretation is also supported by previous single-grained zircon U–Pb ages of 2050 ± 68 Ma and 2100 ± 41 Ma for the volcanic rocks (Yu et al., 1997). These zircon dating results reveal that the Yejishan and Lüliang groups of the PLC were mainly formed during 2200–2000 Ma. Due to the failure of zircon separation, in this study we do not present U–Pb zircon isotopic data for the metamorphosed volcanic rocks of the Jiehekou Group. However, previous U–Pb zircon chronological studies have provided rigorous age constraints on the Jiehekou Group. Geng et al. (2000) and Wan et al. (2000, 2006) obtained a U–Pb zircon age of ∼2030 Ma for the felsic gneisses which are interbedded with the metamorphic volcanic rocks. U–Pb isotopic analyses on detrital zircons obtained from the meta-sedimentary rocks also indicated that the Jiehekou Group was formed between 2.2 Ga and 2.0 Ga and metamorphosed at 1.89–1.83 Ga (S.W. Liu et al., 2006; Xia et al., 2009). So far, Archean geological bodies have not been found in the PLC, though this study has revealed some inherited zircon grains from metamorphic volcanic rocks of the Yejishan Group. We thus suggest that the metamorphosed volcanic and sedimentary sequences of the PLC were formed during late Paleoproterozoic time, which was followed by the emplacement of the younger calc-alkaline magmatic intrusions and regional metamorphism at ∼1850 Ma. The youngest unmetamorphosed granitoid rocks were emplaced at ∼1800 Ma, marking the end of the Paleoproterozoic tectonothermal event (Zhao et al., 2008b).

29 20 49 27 33 26 34 29 31 33 21 1833 1225 1672 1745 2145 1673 1523 1892 1603 2156 1269 21 18 61 21 22 21 36 23 33 24 22 47 46 125 46 46 49 66 51 73 51 53 0.00594 0.00374 0.00989 0.00556 0.00708 0.00528 0.00658 0.00609 0.00608 0.00708 0.00387 5.07356 3.32837 5.34072 5.64621 7.40960 4.44708 9.42056 5.43810 4.91004 7.66208 3.69059 0.00296 0.00299 0.00972 0.00351 0.00368 0.00300 0.01092 0.00335 0.00532 0.00418 0.00373 0.11188 0.11533 0.13083 0.13175 0.13616 0.10885 0.25654 0.11565 0.12615 0.13995 0.12304 72 403 3.91 32 93 438 45 112 22.7 53 1605 128 647 4.19 190 148 423 97 354 53 483 3832

214 673 6.53 297 310 683 174 615 80 990 4469

0.56 0.62 0.93 0.17 0.63 1.04 0.46 0.32 0.43 0.11 0.42

0.00362 0.00315 0.00460 0.00582 0.00402 0.00475 0.00464 4.54 35 140 10.16 246 7.42 20

06ll21-1 06ll21-1-1 06ll21-1-2 06ll21-1-3 06ll21-1-4 06ll21-1-5 06ll21-1-6 06ll21-1-7 06ll72-1 06ll72-1-1 06ll72-1-2 06ll72-1-3 06ll72-1-4 06ll72-1-5 06ll72-1-6 06ll72-1-7 06ll72-1-8 06ll72-1-9 06ll72-1-10 06ll72-1-11

83 288 223 22.9 322 219 59

179 518 611 49 771 105 53

0.05 0.12 0.63 0.44 0.77 0.03 0.34

0.13624 0.11717 0.16582 0.15871 0.14103 0.14664 0.14035

7.56883 5.52968 10.78898 7.96266 8.09437 6.39150 7.96551

0.12280 0.07833 0.38142 0.13674 0.18221 0.11140 0.36939 0.14346 0.19453 0.20800 0.10161

0.32892 0.20933 0.29612 0.31088 0.39475 0.29639 0.26640 0.34112 0.28236 0.39718 0.21759

1830 1885 2109 2121 2179 1780 3226 1890 2045 2227 2001

1832 1488 1875 1923 2162 1721 2380 1891 1804 2192 1569

2183 1898 2492 2001 2244 1771 2223 21 20 23 31 23 26 27 46 47 46 61 48 55 56 0.00672 0.00566 0.00785 0.00724 0.00694 0.00563 0.00717

5.2. Assessment of element mobility

0.17605 0.13022 0.26379 0.26921 0.20442 0.18781 0.23980

Fig. 7. Zircon cathodoluminescence (CL) images showing the inner structures and analyzed locations.

0.40298 0.34234 0.47198 0.36395 0.41636 0.31619 0.41171

2180 1914 2516 2442 2240 2307 2232

2181 1905 2505 2227 2242 2031 2227

206 Pb/238 U age (Ma)

Error (Ma) 207 Pb/235 U age (Ma)

Error (Ma) 207 Pb/206 Pb age (Ma)

Error Pb/238 U 206

Error Pb/235 U 207

Error Pb/206 Pb 207

Th/U Pb* (ppm) Th (ppm) U (ppm)

Table 3 Analytical zircon elements and U–Pb isotopic data and calculated apparent ages.

31 27 34 34 32 28 33

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Some samples, especially those from the Lüliang Group, show obvious variations of Na2 O and K2 O, and Ba, Rb (Table 2), which may be affected by late metamorphism, alteration or amygdaloids in these samples. Therefore, the element mobility should be assessed before discussing petrogenesis and tectonic backgrounds. According to the criteria for the effects of alteration on volcanic rocks proposed by Polat et al. (2002), the metamorphic rocks of the Jiehekou and Yejishan groups have not undergone significant secondary alteration as inferred from their low LOI of 0.38–2.0% (loss of ignition < 3%) and absence of Ce anomalies. Most samples of metamorphic volcanic rocks in the Lüliang Group show higher LOI values (>3.0%), with the highest up to 11.52 (sample 06ll072-1), which indicates that these samples underwent obviously secondary alterations. Significantly, alkali-element Na and K and Ba, Rb of LILEs display some extent mobility. Therefore, these elements will not be used in discussions on the petrogenesis and tectonic setting in the following text. Zr is one of the alteration-independent indexes of geochemical variations (Pearce et al., 1992; Wang et al., 2011). We use correlation coefficients of Zr versus correlative compositions to test their mobility. The Zr exhibits significant correlations to major compositions of MgO, TiO2 and P2 O5 with correlation coefficients of −0.73, 0.87 and 0.91, respectively, and analogously to HFSEs of La, Sm, Th, Nb, Yb, Y and Hf with correlation coefficients of 0.82, 0.88, 0.80, 0.80, 0.84, 0.94 and 0.90, respectively, indicating that these elements have not been significantly mobilized during late metamorphism and secondary alterations. MnO does not have obvious correlation to the Zr (correlation coefficient = −0.34) but shows a little variation with mean = 0.19 ± 0.06, also suggesting it was not changed during secondary alterations. Although samples 06ll015-1 and 06ll0484 have obvious higher Ta contents (Table 2), other twenty-eight samples of these metamorphic volcanic rocks display a high correlation coefficient (0.92) between Ta and Zr, revealing Ta in most samples was not varied during subsequent alterations. Therefore,

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Fig. 8. Zircon U–Pb isotopic concordia diagrams, (A) for sample 06ll21-1 from the Yejishan Group, (B) for sample 06ll72-1 from the Lüliang Group.

the less mobile elements can be used to discuss the petrogenesis and tectonic backgrounds. 5.3. Petrogenesis of the metamorphic volcanic rocks in the PLC Previous Sm–Nd isotopic studies on the metamorphic volcanics of the Jiehekou Group produced two referrible isochron ages of 2425 ± 237 Ma and 2335 ± 195 Ma, respectively, with the corresponding initial 143 Nd/144 Nd ratios and εNd(t) values of 0.50965 ± 23 and 2.96 ± 1.8 for the first isochron, and 0.50976 ± 20 and 2.96 ± 1.3 for the second isochron (Liu et al., 2001). Although the two isochron ages show big errors, they are largely consistent with the above new U–Pb zircon isotopic ages within an error range. Four samples from the metamorphic volcanic rocks of the Lüliang Group were analyzed using the Sm–Nd isochron dating method and defined an isochron age of 2351 ± 56 Ma with the corresponding initial 143 Nd/144 Nd ratio of 0.50958 ± 5 and εNd(t) of −0.3 ± 0.5 (Geng et al., 2000), which is slightly older than the ∼2.2 Ga zircon age of this study. Previous Sm–Nd isotope data revealed that ␧Nd(t) values vary from +2.96 ± 1.8 to −0.3 ± 0.5, indicating that the parental magma of the metamorphosed volcanic rocks the PLC was derived from the depleted mantle source which might have been enriched by subduction melts. The mafic magma was assimilated

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subsequently by crustal materials when ascending to the surface, leading to the slightly negative ␧Nd(t) values of the samples. On the La/Sm versus La diagram, most of the samples display a high positive slope, indicating that their parental magma was derived from the partial melting of depleted mantle, and only three samples from the Yejishan Group show a low slope (samples 06ll15-2, 06ll19-1 and 06ll021-1), indicating that the parental magma underwent crystallization fractionation (Fig. 9a, Treuil and Joron, 1975). On the Sm/Yb versus Sm diagram, except two samples (06ll048-4 and 06ll490-2) which plot on the garnet-spinel lherzolite (50:50) line (Fig. 9b), all other samples from the Jiehekou Group plot in the array of 20–30% partial melting of the spinel lherzolites (Wang et al., 2007; Caulfield et al., 2008; Aldanmaz et al., 2000; Münker et al., 2004, Münker, 2000), which are consistent with their lowest TREE contents, (La/Yb)n ratios and essential absence of Eu anomalies, suggesting that the mantle source of the parental magma is the spinel lherzolite with less garnet. Compared with the samples from the Jiehekou Group, samples from the Lüliang Group also display similar characteristics on La/Sm versus La diagram (Fig. 9a). On the Sm/Yb versus Sm diagram, most of samples plot over the spinel-garnet lherzolite (50:50) line, indicating that the parental magma was derived from a magmatic source containing more garnets and ∼30% partial melting of the spinel-garnet and garnet lherzolites (Fig. 9b), which is consistent with their higher (La/Yb)n values (Fig. 5d). The sample (06LL081-2) displays a higher positive Eu anomaly (Fig. 5d), which may be connected to concentration of plagioclase during the crystallization process. Samples from the Yejishan Group display an curved trend on the La/Sm versus La diagram (Fig. 9a), indicating that their parental magma was derived from the partial melting of depleted mantle and experienced obviously fractional crystallization (Treuil and Joron, 1975). Based on the Sm/Yb versus Sm diagram, samples from the Yejishan Group exhibit low-degree partial melting of the garnet-spinel lherzolites (50:50) (Fig. 9b, Wang et al., 2007; Caulfield et al., 2008; Aldanmaz et al., 2000), which is consistent with the higher TiO2 concentrations and lower Mg# (Fig. 4c), and also the highest TREE content and (La/Yb)n values (Table 3, Fig. 5d). On the Nb/Th versus Nb/Yb diagram, except several samples from the Jiehekou Group showing higher Nb/Th ratios than that of the primitive and enriched mantle (Fig. 9c), the other samples plot within or below the primitive and enriched mantle fields (Münker, 2000; Münker et al., 2004). All the samples from the Jiehekou Group and most of samples from the Lüliang Group plot along trend A, whereas the samples from the Yejishan Group and the other samples from the Lüliang Group plot along trend B (Fig. 9c), suggesting that the Paleoproterozoic parental mafic magma may be mainly generated from the partial melting of depleted mantle that were enriched by melt of subducted sediments. This interpretation is supported by the Nb/Ta versus Nb diagram, on which all the samples from the Jiehekou, Yejishan and Lüliang groups plot along a nearly vertical trend corresponding with the fractional crystallization and assimilations of lower crustal materials (Fig. 9d, Caulfield et al., 2008). 5.4. Tectonic setting of the Paleoproterozoic volcanic magmatism All the metamorphic volcanic samples of the PLC show the depletion of Nb, Ta and P and partially depletion of Ti (Fig. 6a–c), which display a manifest affinity to the active continental margin setting (Condie, 2005). Geochemical data also support the same interpretation that the mafic parental magma was derived from the lower-degree partial melting of depleted mantle that may have been enriched by the melts of subduction slab (Figs. 4b, 6 and 9b, Pearce, 1982; Aldanmaz et al., 2000; Wang et al., 2007; Caulfield et al., 2008; Münker et al., 2004; Münker, 2000). On the Zr/Y versus

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Fig. 9. A group diagrams of the petrogenesis discrimination for metamorphic volcanites of the PLC, (A) a La/Sm-La plot (after Treuil and Joron, 1975); (B) a Sm/Yb–Sm plots showing source compositions of the mafic parental magmas and partial melting degrees (after Wang et al., 2007), and the tracks of melting degree after Aldanmaz et al. (2000). Abbreviations: DM—depleted mantle; PM—primitive mantle; MORB—mid-oceanic basalts; (C) a Nb/Th–Nb/Yb plot showing partial melting factors of the depleted mantle where the mafic magmas were generated, arrows A—OIB trend, B—trend for subduction melt and C—trend for subduction fluid (after Münker, 2000), and abbreviations: OIB—oceanic island basalts, EM—enriched mantle, PM—primitive mantle; (D) a Nb/Ta–Nb plot showing the mantle sources were enriched for these metamorphic volcanites of the PLC, a dash line with arrow indicates increasing partial melting degree of the depleted mantle. Abbreviations both DM and PM showing the depleted and primitive mantles, respectively (after Caulfield et al., 2008). Sample symbols are same as those in Fig. 4.

Ti/Y diagram, except one sample (06ll72-2) from the Lüliang Group which plots in the within-plate field, all other samples plot in the plate margin range (Fig. 10a; Pearce and Gale, 1977). This is consistent with the plots of the MnO × 10 − TiO2 − P2 O5 × 10 discrimination diagram, on which most of the volcanic samples from the Lüliang Group and Jiehekou Group plot into island-arc tholeiite and island-arc calc-alkaline basalt fields except one sample into the MORB field, whereas almost all samples of the Yejishan Group into the ocean-island alkali basalt or sea-mount alkali basalt field, with some samples close to the boninite field (Fig. 10b, Mullen, 1983). Moreover, on the Hf/3-Th-Ta and Hf/3-Th-Nb/16 tectonic distinguishing diagrams (Fig. 10c and d; Wood, 1980), these samples mainly plot in the volcanic-arc basalt field. All of the geochemical data mentioned above strongly support that the metamorphic volcanic rocks of the PLC may have developed under a continental margin arc setting, most likely in areas of the pre-arc accretionary wedge (Liu et al., 2010). 5.5. Implications for Paleoproterozoic evolutions of the TNCO The petrogenetic and geochronological data presented in this study, integrated with previous geochronological and petrological data of the granitoid rocks in the PLC (Liu et al., 2009; Zhao et al., 2008b; Geng et al., 2000) suggest that the volcanic and granitoid rocks in the PLC occurred under a continent marginal arc setting at ∼2.2 Ga, as a consequence of the Paleoproterozoic subduction between the EB and WB of the NCC. In addition, recently published SHRIMP U–Pb zircon data certified the absence of Archean rocks in the PLC (Yu et al., 1997; Geng et al., 2000; Wan et al., 2000, 2006;

Zhao et al., 2008a; Liu et al., 2009). These facts indicate that the main magmatic event was controlled by a Paleoproterozoic subduction of an oceanic lithosphere, which was probably related to the closure of a major oceanic basin prior to the final collision between the EB and WB to form the Trans-North China Orogen (Fig. 1). Recent studies in the geochronology and petrogenesis of the Guandishan granitoid intrusions revealed that syn-collision granitoid magmatism occurred at 1.9–1.85 Ga (Geng et al., 2000; S.W. Liu et al., 2006; Liu et al., 2009; Zhao et al., 2008b), with the peak metamorphism at 1.88–1.85 Ga (S.W. Liu et al., 2006). Our new U–Pb zircon data (e.g. sample 06ll72-1) have also revealed that the PLC underwent thermal alteration at ∼1.83 Ga, synchronously with the regional metamorphism. Based on the available geochronological data, it is suggested that the collision between the EB and WB (or Ordos Block) should have occurred at ∼1.85 Ga, corresponding to the regional peak metamorphism at ∼1.85 Ga in the TNCO (Wan et al., 2000, 2006; Guan et al., 2002; Wilde et al., 2002, 2005; Zhao et al., 2002, 2005; Liu et al., 2004a,b; S.W. Liu et al., 2006; Kröner et al., 2005, 2006). After the collision, the TNCO underwent extensively orogenic collapse and extension at ∼1.80 Ga, characterized by the emplacement of post-collision syenogranitoids (e.g. Dacaoping granite intrusion and Luyashan charnockite intrusion) and mafic dike swarms throughout the TNCO (Halls et al., 2000; Wang et al., 2003, 2004; Peng et al., 2005, 2007, 2008; Hou et al., 2006), which marks the final stabilization of the NCC. Petrological, geochronological and geochemical data suggest that the geodynamic evolution of the PLC records an integrated orogeny that began as part of a magmatic arc system at an active continental margin, generating widespread arc-related magma-

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Fig. 10. Discrimination diagrams for metamorphic volcanites of the PLC. (A) A Zr/Y–Ti/Y plot showing fields of within-plate and plate margin basalts (after Pearce and Gale, 1977); (B) a La/Yb–Sc/Ni plot for mafic and andeside rocks (after Bailey, 1981); (C) and (D) Hf/3-Th–Ta and Hf/3-Th–Nb/16 diagrams (after Wood, 1980), the fields are: A, N-type MORB; B, E-type MORB and within-plate tholeiites; C, alkaline within-plate basalts; and D, volcanic-arc basalts. Sample symbols are same as those in Fig. 4.

tism at ∼2.2 Ga. The PLC then underwent intense deformation and metamorphism and was incorporated into the Trans-North China Orogen during the 1.90–1.83 Ga collisional event that was followed by post-collision extension at ∼1.80 Ga. 6. Conclusions The above geological, geochemical and geochronological data lead us to draw the following conclusions: 1. Metamorphosed mafic volcanic rocks are major components of the supracrustal rock sequences of the Paleoproterozoic Lüliang Complex. The parental magma of metamorphosed volcanic rocks in the PLC was mainly derived from the 5% to ∼30% partial melting of subduction—melts enriched spinel lherzolites and spinel-garnet lherzolites. The mafic magma experienced subsequently fractionation crystallization and was assimilated by the continental materials when ascending to the shallow crustal level. 2. New LA-ICP-MS U–Pb zircon analyses on two metamorphosed volcanic rocks from the Yejishan and Lüliang groups reveal magmatic crystallization ages of 2210 ± 13 Ma and 2213 ± 47 Ma, respectively, suggesting that the Yejishan and Lüliang groups were formed synchronously in the Paleoproterozoic. All the lithological assemblages of the Lüliang Complex were formed

and metamorphosed in the Paleoproterozoic, not in the Neoarchean. 3. The geodynamic evolution of the PLC records an integrated orogeny that began as part of a magmatic arc system at an active continental margin, generating widespread arc-related magmatism at ∼2.2 Ga. The PLC then underwent intense deformation and metamorphism and incorporated into the Trans-North China Orogen during the 1.90–1.83 Ga collisional event which was followed by post-collision extension at ∼1.80 Ga. Acknowledgements We wish to thank B. Yang and L. Gu at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University. Dr. Honglin Yuan and Dr. Chunrong Diwu are thanked for their assistance in LAICPMS zircon U–Pb isotopic analyses at the State Key Laboratory of Continental Dynamics, Northwest University. This research was financially supported by the National Natural Science Foundation of China (grants: 40821002, 40872120, 40420120135 and 40472096). References Aldanmaz, E., Pearce, J.A., Thirlwall, M.F., Mitchell, J.G., 2000. Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey. Journal of Volcanology and Geothermal Research 102, 67–95.

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