Scholarly article on topic 'Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt'

Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt Academic research paper on "Earth and related environmental sciences"

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{"Hf-in-zircon isotope" / "Zircon xenocrysts" / Igneous / "Alxa block" / CAOB / "North China Craton"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Jianjun Zhang, Tao Wang, Lei Zhang, Ying Tong, Zhaochong Zhang, et al.

Abstract The southern boundary of the central segment of the Central Asian Orogenic Belt (CAOB) with the Precambrian Alxa Block is not well constrained due to poor recognition of deep crust. Statistical analysis of zircon xenocrysts within igneous rocks from the northern Alxa and its adjacent regions was applied to resolve this problem. We compiled new and previously published geochronological zircon age data obtained by SHRIMP, SIMS, TIMS, and LA-ICP-MS for 316 igneous rock samples of which 61 samples contain zircon xenocrysts. New and previously published Hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study. The zircon xenocrysts are mainly contained within Permian rocks from the Yabulai–Nurgong–Honggueryulin (YNH) zone and igneous rocks from the northwestern margin of the North China Craton (WNCC). A few xenocrysts were also found in Permian igneous rocks from the Zongnaishan–Shalazhashan (ZS) zone, Neo-proterozoic and Paleo-proterozoic intrusive rocks in the YNH zone, as well as Devonian to Carboniferous granitoids and volcanic rocks from southern Mongolia. Altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10% discordant. Xenocryst ages within the Permian igneous rocks from the ZS zone are mainly around ca. 350Ma, ca. 600Ma and ca. 1400Ma. The oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the CAOB (∼1.1Ga). By contrast, abundant zircon xenocrysts within Permian igneous rocks from the YNH zone show highly variable age populations at ca. 2.6–2.1Ga, 1.8–1.6Ga, 930–750Ma and 460–300Ma. Zircon xenocrysts from the ZS zone have positive ε Hf(t) values of +6.3 to +13.9, whereas those from the YNH zone display highly variable ε Hf(t) values from −16.1 to +11.6. From the southern CAOB and the ZS zone to the YNH zone, the zircon xenocrysts show a significant shift from juvenile to crustal Hf isotopic compositions, suggesting that the area between the ZS and the YNH zones constitutes the southern boundary of the CAOB in the deep crust within this region. Our study indicates that statistical analysis (or isotopic mapping) of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly-formed orogenic domain and ancient cratons.

Academic research paper on topic "Tracking deep crust by zircon xenocrysts within igneous rocks from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt"

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Journal of Asian Earth Sciences

journal homepage: www.elsevier.com/locate/jseaes

Tracking deep crust by zircon xenocrysts within igneous rocks ■. c

from the northern Alxa, China: Constraints on the southern boundary of the Central Asian Orogenic Belt

Jianjun Zhang a,b, Tao Wangb'*, Lei Zhang b, Ying Tongb, Zhaochong Zhang a, Xingjun Shib, Lei Guob, He Huang b, Qidi Yangb, Wei Huang cb Jianxin Zhaod'b, Ke Yea'b, Jiyao Houa'b

aState Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China b Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China cNO.1 Detachment of Gold Geological Party, Chinese People's Armed Police Force, Mudanjiang 157021, China d NO.912 Geological Surveying Team, Bureau of Geology and Mineral Exploration and Development, Yingtan 335001, China

ARTICLE INFO

ABSTRACT

Article history:

Received 30 December 2014 Received in revised form 7 April 2015 Accepted 15 April 2015 Available online 22 April 2015

Keywords:

Hf-in-zircon isotope Zircon xenocrysts Igneous Alxa block CAOB

North China Craton

The southern boundary of the central segment of the Central Asian Orogenic Belt (CAOB) with the Precambrian Alxa Block is not well constrained due to poor recognition of deep crust. Statistical analysis of zircon xenocrysts within igneous rocks from the northern Alxa and its adjacent regions was applied to resolve this problem. We compiled new and previously published geochronological zircon age data obtained by SHRIMP, SIMS, TIMS, and LA-ICP-MS for 316 igneous rock samples of which 61 samples contain zircon xenocrysts. New and previously published Hf isotopic compositions of these zircon xenocrysts are combined with zircon ages in this study. The zircon xenocrysts are mainly contained within Permian rocks from the Yabulai-Nurgong-Honggueryulin (YNH) zone and igneous rocks from the northwestern margin of the North China Craton (WNCC). A few xenocrysts were also found in Permian igneous rocks from the Zongnaishan-Shalazhashan (ZS) zone, Neo-proterozoic and Paleo-proterozoic intrusive rocks in the YNH zone, as well as Devonian to Carboniferous granitoids and volcanic rocks from southern Mongolia. Altogether we analyzed more than 270 zircon xenocrysts and considered only ages that are less than 10% discordant. Xenocryst ages within the Permian igneous rocks from the ZS zone are mainly around ca. 350 Ma, ca. 600 Ma and ca. 1400 Ma. The oldest age of zircon xenocrysts in this zone is similar to those of zircon xenocrysts from the CAOB (~1.1 Ga). By contrast, abundant zircon xenocrysts within Permian igneous rocks from the YNH zone show highly variable age populations at ca. 2.6-2.1 Ga, 1.81.6 Ga, 930-750 Ma and 460-300 Ma. Zircon xenocrysts from the ZS zone have positive £Hf(t) values of +6.3 to +13.9, whereas those from the YNH zone display highly variable £Hf(t) values from -16.1 to +11.6. From the southern CAOB and the ZS zone to the YNH zone, the zircon xenocrysts show a significant shift from juvenile to crustal Hf isotopic compositions, suggesting that the area between the ZS and the YNH zones constitutes the southern boundary of the CAOB in the deep crust within this region. Our study indicates that statistical analysis (or isotopic mapping) of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly-formed orogenic domain and ancient cratons.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CCBY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4XI/).

1. Introduction

Zircon is a common accessory mineral in many igneous rocks and has a strong resistance to mechanical and chemical breakdown over long periods of time. It can thus survives erosion and meta-morphism that may have modified or destroyed its host rock (Heaman et al., 1990). Therefore, zircon preserves reliable

* Corresponding author.

geochemical records such as the initial isotopic magma composition at the time of crystallization, variable magmatic processes as well as source heterogeneities and/or melt contamination with mantle-derived or crustal material. It is therefore one of the most widely used minerals to date rocks and to track magma sources (e.g., Belousova et al., 2006). During the past decades, chemical and isotopic analysis of zircon has played a key role in investigating magmatic events and igneous petrogenesis (e.g., Hoskin and Schaltegger, 2003; Valley et al., 2003; Belousova et al., 2006; Liu

http://dx.doi.org/10.1016/jjseaes.2015.04.019 1367-9120/© 2015 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

et al., 2009; Castro et al., 2011; Brown, 2013) and studying the growth and/or recycling of continental crust (Wilde et al., 2001; Kemp et al., 2006; Scherer et al., 2007; Condie et al., 2009, 2011; Lissenberg et al., 2009; Siebel et al., 2009; Dhuime et al., 2012; Cawood et al., 2013; Hawkesworth et al., 2013; Buys et al., 2014; Kröner et al., 2014; Wang et al., 2014). Zircons, initially derived from source(s) or country rocks and significantly older than that of host igneous rocks, are referred to 'xenocrystic/inherited zircons', or 'zircon xenocrysts' in the literatures (Johnson, 1989; Miller et al., 2007; Zheng et al., 2011). Zircon xenocrysts have long been considered to be bothersome because they hinder the precise estimation of the emplacement age of the intrusive rocks. In most cases, they are excluded when calculating the weighted mean age without additional detailed analyses. Conversely, a growing number of studies have shown that the xenocrystic/inherited zircons record much valuable information, which can help to reveal crustal features hidden in the deep continental crust (Wright and Wyld, 1986; Evans and Zartman, 1990; Hoskin et al., 2000; Zheng et al., 2006; Liu et al., 2014) and thereby provide additional constraints on understanding crustal evolution (Hanchar and Rudnick, 1995; Siebel et al., 2009; Charlier et al., 2010; Stern et al., 2010). In some cases, the ages of zircon xenocrysts have been utilized to elucidate the nature of unexposed ancient fragments of continental lithosphere (Qiu et al., 2000; Hargrove et al., 2006; Iizuka et al., 2006; Smyth et al., 2007; Zhang et al., 2012a; Gaschnig et al., 2013; Torsvik et al., 2013; Reimink et al., 2014), and/or provide evidence of pre-existing continental crust (Siebel et al., 2009; Gao et al., 2011). In general, detailed studies of zircon xenocrysts, e.g., deep-seated xenocrysts from volcanic rocks such as lamproites (Zheng et al., 2006), kimberlite (Valley et al., 1998; Griffin et al., 2000; Nasdala et al., 2014; Ashchepkov et al., 2014) and mantle-derived magmas such as ultrapotassic rocks (Liu et al.,

2014), xenoliths (Liati et al., 2004; Zheng et al., 2008; Pan et al., 2014; Tang et al., 2014), oceanic gabbros (Pilot et al., 1998) as well as granitic magmas (Smithies et al., 2001; Bea et al., 2007; Demoux et al., 2009b; Buys et al., 2014; Jeon et al., 2014) have extended our knowledge of unexposed parts of the crust through which the host magmas ascended. To better understand the significance of zircon xenocrysts in juvenile igneous rocks, Stern et al. (2010) summarized the distribution of ancient zircons in juvenile crust, and explained the occurrence of compositional dependence of igneous rocks. However, statistical analysis of the xenocryst ages and their Hf isotopic composition derived from igneous rocks has not yet been applied to reveal signatures of the deep crust.

The definition of tectonic boundaries, particularly those between orogens and cratons, is a major matter of debate, which is concerned with the tectonic evolution of a particular orogenic belt (Zhao et al., 2002; Xiao et al., 2014), and/or the geometry of the nearby craton. The Central Asian Orogenic Belt (CAOB; Jahn, 2004), or the Altaids (Sengor et al., 1993; Wilhem et al., 2012), the world's largest Phanerozoic accretionary orogenic belt, has a complex evolutionary history, represented by multi-stage subduction and juvenile crustal growth (e.g., Jahn et al., 2000; Xiao et al., 2003, 2010a; Yakubchuk, 2004; Han et al., 2011; Kroner et al., 2011; Wilhem et al., 2012; Xu et al., 2013). Despite considerable durative efforts, the southern boundary of the CAOB, particularly the boundary with the Alxa Block, is not well constrained. The Alxa Block, which occupies the major part of Alxa league of Inner Mongolia, is located to the south of the central segment of the CAOB. This block connects the Beishan fold-and-thrust belt in the west with the North China Craton (NCC) in the east (Fig. 1). In the north Alxa, there are two major ophiolite belts: the Enger Us belt in the north and the Qagan Qulu belt in the south (Li, 2006a; Wu and He, 1993; Zhang et al., 2012b). The Enger Us ophiolite belt

Fig. 1. Schematic geological map of the Alxa Block and surroundings showing the distribution of magmatic rocks (modified from Ren et al., 2013). Inset map shows location of the Alxa Block modified after (Sengor et al., 1993; Jahn, 2004; Wilhem et al., 2012). Main tectonic subdivisions were modified from Li (2006b). The boundary of the Alxa Block modified after (Lamb et al., 1999; Zhang et al., 2013a,b; Zhao and Zhai, 2013; Dan et al., 2014a,b).

was previously regarded as a major paleo-plate boundary between the Alxa Block and the Southern CAOB (Wu and He, 1992, 1993; Wang et al., 1998; Dan et al., 2012, 2014a; Gong et al., 2012; Feng et al., 2013; Song et al., 2013; Zheng et al., 2014). However, this assumed suture/tectonic boundary has not been confirmed by robust evidence, due to (1) the difficulty of defining tectonic units due to poor exposure of ophiolites and the allochthonous ophiolites, mélanges; (2) the limited knowledge of the deep crust in this region.

In previous studies, numerous zircon ages and Hf isotopic data were reported for igneous rocks of the Alxa Block and its nearby regions (see Appendix B1 and data sources). Numerous zircon xenocrysts from several tectonic units were identified in many of these studies (Geng and Zhou, 2010, 2012; Li et al., 2010; Wu, 2011; Shi et al., 2012; Wang, 2012; Gong, 2013; Feng et al., 2013; Zhang et al., 2013c; Dan et al., 2014b; Wu et al., 2014; Zheng et al., 2014). These zircon xenocrysts may provide a good opportunity to trace information on the deep continental crust and to constrain the boundary between the CAOB and the Alxa Block.

We present new chronological and Hf isotopic data for zircon xenocrysts within granites from the Alxa Block, and synthesize published zircon xenocrysts data from igneous rocks of the northern Alxa and its surrounding. Statistical analysis and location of the xenocrystic zircons provides strong evidence for the boundary between the Alxa Block and CAOB to be located to the south of Shalazhashan terrane. This study demonstrates that regional analysis and the distribution of ages and compositions of xenocrystic zircons is a powerful tool to trace deep crustal compositions.

2. Geological and geochronological background

The Alxa region is located in the boundary area between the central segment of the southern CAOB and the Alxa Block (Fig. 1). This region is largely covered by Cenozoic sediments and is surrounded by the Badain Jaran and Tengger deserts. The northern Alxa can be geologically subdivided to three units from north to south, namely the southern CAOB, the Zongnaishan-Shalazhasha n (ZS) zone and the Yabulai-Nuoergong-Honggueryulin (YNH) zone (Fig. 1). These are separated by two major Late Paleozoic ophiolite belts, namely (1) the Enger Us ophiolite belt in the north, which is mainly composed of ultramafic and mafic rocks, set in a matrix comprising strongly deformed Carboniferous clastic sediments and tuffs; and (2) the Qagan Qulu ophiolite belt in the south, which is composed of tectonic blocks, including ultramafic assemblages, gabbro and chert occurring in a matrix that includes deformed clastic sediments and limestones (Wu and He, 1992; Zheng et al., 2014).

The southern CAOB includes part of the southern Mongolia, and the area near the southern Mongolia-China border, which is named the Zhusileng-Hangwula zone (Feng et al., 2013). The southern Mongolian domain is dominated by Silurian, Devonian and Carboniferous accretionary complexes and arc-related volcanic and volcanoclastic rocks (Badarch et al., 2002; Rojas-Agramonte et al., 2011; Dolgopolova et al., 2013; Guy et al., 2014; Jian et al., 2014). Many Paleozoic intrusions (mainly 525-277 Ma) are exposed in the southern Dalanzadgad area (Khashgerel et al., 2006; Demoux et al., 2009a; Dolgopolova et al., 2013; Wainwright et al., 2011a,b; Heumann et al., 2012). Most of these are granitoids with positive eNd(t) and eHf(t) values (e.g., Jahn et al., 2000, 2004; Hong et al., 2004; Kovalenko et al., 2004; Helo et al., 2006; Yarmolyuk et al., 2007; Wainwright et al., 2011b; Kroner et al., 2014; Tong et al., 2015; Li et al., in press), indicating that they were derived from juvenile crust. Many Late Paleozoic

intrusions (426-277 Ma) are distributed in the area near the Mongolia-China border (Li, 2006a; Han et al., 2010; Zheng et al., 2013a).

The Zongnaishan-Shalazhashan (ZS) zone mainly consists of the Paleozoic Amushan Formation, Late Paleozoic calc-alkaline batholiths (Liu and Zhang, 2014; Shi et al., 2014a,b) and a few basement rocks. The Amushan Formation consists of neritic volcanic rocks, clastic sediments, and neritic carbonate rocks. The batholiths, characterized by large granitoids plutons associated with gabbros, are of 301-247 Ma in age (Li, 2006a; Lv, 2011; Wu, 2011; Ran et al., 2012; Chen et al., 2013; Liu and Zhang, 2014; Shi et al., 2014a,b; Yang et al., 2014). Previous studies (e.g., Wu and He, 1992; Wang et al., 1994) suggested that the basement rocks of the ZS zone are comparable with these medium- to high-grade metamorphic basements assemblages to the south of the Qagan Qulu ophiolite belt (Fig. 1).

The Yabulai-Nuoergong-Honggueryulin (YNH) zone is extensively covered by Cenozoic sediments. Outcrops in the region predominantly consist of Precambrian basement and widespread Paleozoic igneous rocks and Late Paleozoic strata (Fig. 1). There is a consensus that the Alxa Block belongs to the westernmost part of the NCC since the Late Paleozoic (Li et al., 2012; Song et al., 2013; Dan et al., 2014b; Yuan and Yang, 2015a,b). Although knowledge on the Precambrian history is limited due to the restricted extent of exposed basement, the basement has been interpreted to be mainly composed of pre-Neoproterozoic medium- to high-grade metamorphic assemblages (ca. 2.4-1.7 Ga), such as the Diebusige metamorphic complex, the Bayanwula Shan meta-morphic complex, and low-grade sedimentary sequences (Darby and Gehrels, 2006; Geng et al., 2006; Zhou and Geng, 2009; Dan et al., 2012; Wu et al., 2014), as well as several Neoproterozoic granites (929-904 Ma, Geng and Zhou, 2010; Dan et al., 2014b). The exposed igneous rocks include large granitic intrusions with minor diorite and mafic plutonic rocks (Fig. 1). Most of these are predominantly Permian (~298-262 Ma) in age (Li, 2006a; Chen et al., 2010; Bao et al., 2012; Geng and Zhou, 2012; Shi et al., 2012; Feng et al., 2013; Dan et al., 2014a; Ye et al., submitted for publication). Early Paleozoic strata are lacking, and Late Paleozoic strata are mainly composed of Carboniferous sandstone and Permian felsic volcanic rocks.

The northwestern margin of North China Craton (WNCC) is located near the Alxa Block. It mainly includes the areas around Langshan and Wulate zhongqi of Inner Mongolia (Fig. 2). It is about 100 km to the south of the well-documented Solonker suture zone (Sengor et al., 1993), which represents a major paleo-plate boundary (Li, 2006a; Chen et al., 2000) and has been interpreted as the southernmost boundary of the CAOB (Xiao et al., 2003; Jian et al., 2010; Eizenhofer et al., 2014). The WNCC is characterized by widespread exposures of Paleozoic intrusions, which are similar in age and composition to those from the northern Alxa Block (Luo et al., 2007, 2009, 2010; Peng et al., 2010; Zhang et al., 2011; Zhao et al., 2011; Wang et al., 2012; Wu et al., 2013; Lin et al., 2014; Wang et al., 2015). The basement rocks are predominantly of Paleo-proterozoic ages (~2.5 Ga, Ma et al., 2013).

3. Age and Hf isotopic data of zircon xenocrysts

3.1. New age and Hf isotopic data of zircon xenocrysts

3.1.1. Sample description

Our new zircon xenocryst data were obtained from an aplitic granite (40°13'39.9"N, 105°30'42.0"E), which crosscuts the Permian Yamatu monzogranite in the YNH zone (Fig. 3). The aplitic granite sample is fresh, composed predominantly of quartz (30-35 vol.%), K-feldspar (30-35 vol.%), biotite (15-20 vol.%),

105* 106° 107° 108° 109s

103" 104' 105° 106" 107" 108" 109" 110"

Fig. 2. Schematic map of the Alxa Block and surroundings, showing the spatial distribution of samples containing zircon xenocrysts. Locations of samples, rock type etc. are given in Appendix B1.

plagioclase (5-10 vol.%), muscovite (3-5 vol.%) and minor amphibole (Fig. 3). Zircon, apatite and titanite are common accessory minerals, whereas rare barite and limonite are also present. K-feldspar grains are mostly 200 im in length, whereas biotite is commonly lamellar, subhedral, and mostly coexists with anhedral muscovite. Some biotite grains are included in quartz, or show regular shape contact with quartz, or interstitial texture with quartz and K-feldspar. Some biotite contains apatite and zircon inclusions. Muscovite is subhedral, and contains zircon inclusions. Zircons are mostly irregular in shape, and generally scattered in K-feldspar, quartz or muscovite (Fig. 3). Locally sometimes zircon may also occur in contact with plagioclase and K-feldspar, or is present as subhedral inclusions in biotite.

3.1.2. Results

Sample (10LS54) was selected for zircon U-Pb age and Hf-in-zircon isotope analyses. Zircons are mostly transparent and heterogeneous (Fig. 4), dominantly around ~100 im in size, and occur as euhedral, short prismatic crystals with many being elongated (200-300 im). Zircon aspect ratios range from stubby (1:1 aspect ratio) to elongated (3:1), predominantly between 2:1 and 3:1. Some large zircon crystals contain minor visible inclusions (Fig. 4). CL images reveal that most grains exhibit well-developed concentric, fine-scale oscillatory zoning, suggesting a magmatic origin. Some crystals exhibit weak zoning, are unzoned or display uniform internal textures (Fig. 4, e.g. grains 11, 32 and 42), which may imply a high-temperature crystallization environment (Corfu et al., 2003). A few zircons show high luminosity (Fig. 4, e. g. grains 15, 21 and 34) pointing to some type of discontinuity between core and overgrowths (recrystallized or re-equilibrated area). The

analytical methods of U-Pb dating and Hf isotopic analysis are given in Appendix A.

3.1.2.1. U-Pb ages of synmagmatic and xenocrystic zircons. The results of dating of individual zircon points are listed in Table 2. The zircons have highly variable content of Th (24-1591 ppm) and U (41-1462 ppm), with Th/U ratios of 0.4-2.0, which indicates an igneous origin (Hoskin and Ireland, 2000). Unzoned grains have relatively lower U, Th and Pb contents (Fig. 4 and Table 2 for age analysis number of 31, 32 and 42) than those with oscillatory zoning.

Fifty grains were analyzed in this study, of which forty-two analyses are concordant (Fig. 5). The discordance may have resulted from the loss of the radiogenic Pb or mixtures between different zircon growth phases. Forty-two concordant and near-concordant 206Pb/238U ratios define discrete zircon populations ranging from 263 to 2232 Ma. This age distribution spectrum exhibits four major groups (Fig. 5). The largest group (34 spots) comprises ages between 285 and 263 Ma of which 22 analyses yield a concordia age of 280 ± 2 Ma (MSWD = 3.1), and 12 analyses define a concordia age of 269 ± 2 Ma (MSWD = 2.5). We interpret these to reflect the time of the emplacement of the aplitic granite.

The oldest concordant xenocryst age (207Pb/206Pb ages of 2494 ± 18 Ma) was obtained on grain 10LS54-11 (Fig. 5), a simple short prismatic zircon without internal zonation under CL and a Th/U of 1.0. Five other Paleoproterozoic ages of 2071-1884 Ma were obtained on 4 grains (Fig. 4, grains 8, 15, 21 and 33). These grains exhibit oscillatory zoning or core-rim structures under CL and have Th/U ratios of 0.4-1.0. One subhedral grain (10LS54-43) with weak zoning and Th/U of 0.7, has a Meso-proterozoic age of

Fig. 3. Field photos and representative photomicrographs of the aplitic granite from the Yamatu monzogranite in the YNH zone. Previously published geochronological data are from: (1) Li (2006a), (2) Geng and Zhou (2012), (3) Feng et al. (2013), (4) Zhang et al. (2013d), (5) Zhang et al. (in revision). Mineral abbreviations after Whitney and Evans (2010).

1525 ±27 Ma. Another grain (10LS54-9) has 206Pb/238U age of 342 ± 3 Ma (Fig. 4).

3.1.2.2. Hf isotopic compositions. We not only analyzed the Hf isotopic composition of zircon xenocrysts from the above aplitic granite sample (10LS54), but also from zircon xenocrysts found in samples of our previous studies (see Table 1 ). The Hf isotopic data for individual zircon xenocrysts are given in Table 3. The initial Hf isotopic compositions were calculated using the zircon grain spot age.

The xenocrysts of the aplitic granite have variable Hf isotopic compositions with present-day 176Hf/177Hf ratios of 0.2812190.282399 (Table 3). The oldest Paleoproterozoic grain (grain 11, 2494 Ma) has high a eHf(t) value of +12.3 (Fig. 4), and a crustal Hf model age (TDM2) of 2.3 Ga. Five middle to late Paleoproterozoic

grains (2104-1884 Ma) have eHf(t) values between -1.9 and -6.4, with Hf crustal model ages (TDM2) of 2.9-2.6 Ga (Table 3). The Mesoproterozoic (1525 Ma) and Carboniferous grains (342 Ma) have eHf(t) values of +19.7 and -5.8, with TDM2 of 1.1 and 1.6 Ga, respectively (Fig. 8). The discordant zircons with minimum 207Pb/206Pb ages from sample 10LS54 have imprecise eHf(t) values of -2.3 to +21.3 (Table 3).

Hf isotopic compositions of zircon xenocrysts from other Permian granitoids of the YNH zone are from granites of Halinudeng (10LS35), Yabulai Shan (12LS107) and Aolunbulage (10LS32) plutons, and from monzogranite of the Honggureryulin pluton (12LS17). These zircon xenocrysts have variable eHf(t) values (-14.6 to +18.7) with Hf crustal model ages (TDM2) of 1.52.9 Ga (Table 3). Hf isotopic compositions of zircon xenocrysts from the ZS zone are from granites of Wuliji pluton (11LS96 and

Fig. 4. Cathodoluminescence (CL) images of analyzed zircon grains from aplitic granite. Pb ages (207Pb/206Pb > 1000 Ma and 206Pb/238U < 1000 Ma) are shown near the circles.

11LS170) and gabbro from the Baogeqi pluton (11LS106). These zircon xenocrysts are characterized by positive eHf(t) values (+6.3 to +13.9) with two-stage Hf model ages (TDM2) of 1.8-0.5 Ga (Fig. 8).

3.2. Data from the literatures

3.2.1. Age data of zircon xenocrysts

A compilation of 315 published geochronological data from igneous rocks of the northern Alxa and nearby regions was created, in which only high-quality zircon ages obtained by SHRIMP, TIMS,

Circles denote positions of U-Pb age and Hf isotopic analysis. The eH((t) values and U-

SIMS, or LA-ICP-MS are included (Fig. 2; Appendix B1). In Table 1 we summarize the rock samples containing xenocrystic zircons from the southern CAOB, the ZS zone, YNH zone, and the WNCC in our dataset. The locations of these 315 samples are shown in Fig. 2. In addition, the locations of samples containing zircon xeno-crysts are specially marked by red dots.

In this study, U-Pb isotopic data for 270 grains of individual zircon xenocrysts were compiled (Appendix B2) identified in 59 samples. About ~96 percent of these zircon xenocrysts were identified in granitoids and ~4 percent in gabbro, basalt, and felsic volcanic rocks. For the discussion below, some ages were excluded from

207Pb/ 235U

Fig. 5. Concordia diagram for aplitic granite. Inset represents enlarged area of part of the plot.

the discussion below as follows. Sixteen zircons with Th/U < 0.05 were not considered due to their possible metamorphic origin (Rubatto, 2002), and 20 zircons with discordance of more than 10% were excluded as well (Appendix B2). Cathodoluminescence (CL) images of most xenocrystic zircons showing their internal textural features are illustrated in Appendix B3.

In the southern CAOB, the zircon xenocrysts were found within Devonian and Carboniferous granitoids and volcanic rocks (396345 Ma) of the Omnogovi Province of southern Mongolia (Demoux et al., 2009a; Kroner et al., 2010; Wainwright et al., 2011b). These xenocrysts show U-Pb age (207Pb/206Pb > 1000 Ma and 206Pb/238U < 1000 Ma) grouping at around 600-500 Ma and 1200-1000 Ma (Fig. 7a).

In the Zongnaishan-Shalazhashan (ZS) zone, the zircon xeno-crysts occurred in Permian granites and gabbro. In spite of the small number of ages, they yielded three distinct age populations, respectively, peaked at ~350 Ma, ~600 Ma and ~1400 Ma (Fig. 7c).

In the Yabulai-Nuoergong-Honggueryulin (YNH) zone, the zircon xenocrysts were found within Permian igneous rocks (granitoids, gabbros, basalt etc.), Neo-proterozoic igneous rocks (granitic gneiss, granitoids and volcanic rocks), and Paleo-proterozoic rocks (Appendix B2). The zircon xenocrysts within the Permian intrusive rocks show age (207Pb/206Pb > 1000 Ma and 206Pb/238U < 1000 Ma) groupings at around 460300 Ma, 930-750 Ma, 2000-1400 Ma, and further peaks between 2550 and 2200 Ma (Fig. 7c). The xenocrystic zircons within Neo-proterozoic rocks show age groupings mostly at around 2000-948 Ma and ca. 2218 Ma (Fig. 7e), whereas the zircon xenocrysts within Paleo-proterozoic rocks exhibit 207Pb/206Pb age groupings at around 2313-2215 Ma and ca. 2500 Ma (Fig. 7f).

On the northwestern margin of the North China Craton (WNCC), zircon xenocrysts were found within Permian granitoids, Neo-proterozoic meta-volcanic rocks and Paleo-proterozoic rocks (e.g., meta-gabbro, granitoids, gneiss). As illustrated in Fig. 7d,

Table 1

Dataset of rock samples containing zircon xenocrysts.

Units Sample Latitude Longitude Locality Lithology Age (Ma) Error Methods

ZS 11LS170 40°510 52.8" 105°1'19.8" Wuliji Granite 254.3 2.3 LA-ICP-MS

ZS 11LS106 40°550 44.9" 104°13'53.8" Baogeqi Gabbro 264.1 2.8 LA-ICP-MS

ZS 11LS96 Granite 265.5 2.9 LA-ICP-MS

YNH AD113 40°10'58" 104°50'34" Zhulazaga Granite porphyry 280 6 SHRIMP

YNH 09AL101 40°170 52.9" 104°14021.200 Naimumaodao Two-mica Granite 929 6 SIMS

YNH 09AL93 40°210 23.3" 104°5'5.3" Gelintaishan Amphibolitic gneisis 914 8 SIMS

YNH 11S04-7 40°190 31.8" 105°40'24.3" Qinggele Gabbro 262 5 LA-ICP-MS

YNH LS-37 40.610845° 106.305213° Langshan Pillow lava basalt 255246 4.1 LA-ICP-MS

YNH A80 40°300 106°260 Tanyaokou Meta volcanoic rocks 816.9 4.5 SHRIMP

YNH A14-5 40°30' 106°260 Tanyaokou Meta volcanoic rocks 805 5 SHRIMP

YNH AL08054 40°23.652' 104°27.5160 Bijiertai Gneisic dioritie 289 3 LA-ICP-MS

YNH AL0815-3 40°18.406' 104°15.0360 Habuqigai Gneisis 921 7 LA-ICP-MS

YNH AL0815-2 40°18.406' 104°15.0360 Habuqigai Gneisis 926 15 LA-ICP-MS

YNH AL0817-1 40°21.109' 104°03.8040 Keketuolegai Granitic Gneisis 904 7 LA-ICP-MS

YNH LS-11-32 39.4671° 105.0368° Bayanwula Shan Granitic mylonite 359 4.1 LA-ICP-MS

YNH WD6011-TW1 40°22' 28" 104°32'27" Bijiertaiaobao Amphibolite 285.8 1.7 LA-ICP-MS

YNH HT50-3 39°40' 57.2" 105°15'13" Bayanwulashan complex Amphibole plagiogneiss 2290 11 LA-ICP-MS

YNH HT52-2 39°36047.100 105°08'35.5" Bayanwulashan complex Amphibole plagiogneiss 2244 10 LA-ICP-MS

YNH LS035 40°31' 13.1'' 105°44'54.2" Halinudeng Granite 284 1.8 LA-ICP-MS

YNH AB10-19 40°440 400 104°530800 Quangan Qulu ophiolite Gabbro 275 3 SHRIMP

YNH 12LS02 40°220 53.500 105°27025.300 Granite 276 LA-ICP-MS

YNH 11LS08 40°06025.000 105°34052.300 Granite 320 LA-ICP-MS

YNH 11TA1 40°06025.000 105°34052.300 Granite 402 LA-ICP-MS

YNH 10LS54 40°13039.900 105°30'42" Aplite dyke 263.4 1 LA-ICP-MS

YNH 10LS95 40°13039.900 105°30'42" Diabase 287.4 1.8 LA-ICP-MS

YNH 11LS75 Granite 284 LA-ICP-MS

YNH 10LS32 Granite 255290 LA-ICP-MS

YNH 10LS04 Granite 254 LA-ICP-MS

YNH 10LS91 Gabbro 271.1 5.4 LA-ICP-MS

YNH 12LS107 Granite 274.9 1.5 LA-ICP-MS

YNH 12LS110 Granite 415.4 1.7 LA-ICP-MS

YNH 12LS87 Dioritic enclave 271 1 LA-ICP-MS

YNH 12LS80 Monzogranite 271 1 LA-ICP-MS

YNH 13AX01 Granite 270 1 LA-ICP-MS

YNH 13AX21 Granite 269 1 LA-ICP-MS

YNH ZJX-4 Gabbro 272 1 LA-ICP-MS

YNH HZ822-13 40°1603000 106°40'55" Huogeqi Diorite 274 1 LA-ICPMS

WNCC T1 41°55'43" 108°580 Wulatezhongqi Monzogranite 279 3 SHRIMP

WNCC T5 41°55'44" 108°58'24" Wulatezhongqi Monzogranite 266 3 SHRIMP

WNCC 11ND-03 41°07.35150 107°3.87530 Dongshengmiao Monzogranite 259.4 3.3 LA-ICP-MS

Shi et al. (2014a)

Shi et al. (2014a)

Our unpublished data

Li et al. (2010) Dan et al. (2014b) Dan et al. (2014b) Feng et al. (2013)

Zhang et al. (2013c)

Peng et al. (2010) Peng et al. (2010) Geng et al. (2012)

Geng et al. (2010)

Geng et al. (2010)

Geng et al. (2010)

Zhang et al. (2013a)

Wang (2012)

Wu et al. (2014)

Wu et al. (2014)

Shi et al. (2012)

Zheng et al. (2014)

Our unpublished data

Our unpublished data

Our unpublished data

This study

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Our unpublished data

Pi et al. (2010) Wang et al. (2012) Wang et al. (2012) Wu et al. (2013)

(continued on next page)

Table 1 (continued)

Units Sample Latitude Longitude Locality Lithology Age (Ma) Error Methods Data sources

WNCC NOR95- 40°36'52.3" 106°10'13.1" Volcanic rock 804 3.5 LA-ICP- Hu et al. (2014)

WNCC NM0907 40°40'34" 109°30'26" Hademengou Hornblendite -1890 MS SHRIMP Wan et al. (2013)

WNCC NM0803 40°49'21" 110°04'29" Hujigou Amphibolite 1840 10 SHRIMP Wan et al. (2013)

WNCC NM0413 40°48'32" 110°15' 27" Kunduigou Gneiss 2400 10 SHRIMP Wan et al. (2009)

WNCC NM0816 40°48'39" 110°15' 29" Kunduigou Biotite gneiss 2361 18 SHRIMP Dong et al. (2014)

WNCC NM0620 40°49'30" 110°04'19" Kunduigou Biotite gneiss 2361 18 SHRIMP Dong et al. (2014)

WNCC NM0908 40°44'14" 109°48' 19" Kunduigou Biotite gneiss 2467 7 SHRIMP Dong et al. (2014)

WNCC NM1030 40°41'52" 109°43' 15" Kunduigou Biotite gneiss 1954 14 SHRIMP Dong et al. (2014)

WNCC NM0810 40°41'37" 109°3802900 Hademengou Gneiss 2242 8 SHRIMP Dong et al. (2014)

WNCC NM0807 40°41'38" 109°3802800 Hademengou Gneiss 2426 10 SHRIMP Dong et al. (2014)

WNCC NM0809 40°41'38" 109°3802800 Hademengou Gneiss 2446 10 SHRIMP Wan et al. (2009)

WNCC NM06171 40°51'53" 110°17' 03" Dalaohudian Gneiss 1880 10 SHRIMP Dong et al. (2012)

WNCC NM0604 40°45'24" 109°3703500 Maohudong Feldspar quartzite 1840 10 SHRIMP Dong et al. (2012)

WNCC NM0619 40°51'55" 110°04'38" Hujigou Quartzite -1900 SHRIMP Dong et al. (2012)

WNCC NM06211 40°47'01" 110°04'48" Hujigou Gneiss 1960 10 SHRIMP Dong et al. (2012)

WNCC NM0916 40°48'16" 110°12' 25" Maohudong Gneiss 1880 20 SHRIMP Dong et al. (2012)

WNCC 08557-1 40°59'53.8" 109°54' 11.8" Bayn Obo Diorite 445 3 LA-ICPMS Zhang et al. (2014)

Southern AJW6476 43.09° 106.89° Hugo Dummett Granodiorite 365 2 TIMS Wainwright et al.

CAOB (2011a)

Southern AJW3184 43.09° 106.89° Hugo Dummett Granodiorite 367 2 TIMS Wainwright et al.

CAOB (2011a)

Southern Cint! M3883 44°53' 97°480 Devreh Valley Metarhyolite 396.3 3.3 SHRIMP Demoux et al. (2009a)

CAOB Southern AJW3183 43.05° 106.845° Oyu Tolgoi Andesite dike 345 2 SHRIMP Wainwright et al.

CAOB (2011b)

the age distribution of the xenocrysts within Permian intrusive rocks shows age (207Pb/206Pb > 1000 Ma and 206Pb/238U< 1000 Ma) groups at around 410-360 Ma, 2600-1200 Ma. Despite the absence of Neo-proterozoic (1000-600 Ma) ages, the xenocrystic age distribution is comparable to that of the YNH zone (Fig. 7c).

3.2.2. Hf isotopic data of xenocrystic zircons

The Hf isotopic data of 56 zircon grains from igneous rocks of the southern CAOB, the YNH zones, and the WNCC is listed in Table 3. Initial 176Hf/177Hf ratios were obtained on the basis of the corresponding U-Pb ages. One zircon xenocryst (514 Ma) within a ca. 374 Ma granodiorite of the southern CAOB has eHf(t) value of —1.9 with TDM2 of 1610 Ma (Fig. 8), whereas two further zircon xenocrysts have eHf(t) values of +12.6 and +14.5. Published zircon xenocrysts data from the YNH zone are characterized by variable eHf(t) values (—5.5 to +11.1) and corresponding Hf crustal model ages (TDM2) of 2.9 Ga to 1.4 Ga. The zircon xenocrysts from Permian rocks in the WNCC have eHf(t) values from +0.5 to +8.4 and crustal model ages of 2.8-1.5 Ga (Fig. 8).

4. Discussion

4.1. Possible provenance of zircon xenocrysts

The origin of zircon xenocrysts, i.e., magmatic, metamorphic, or detrital must be identified before their zircon U-Pb ages and Hf isotopic compositions can be evaluated. CL images reveals that most xenocrysts are of magmatic origin, and metamorphic cores are fairly rare (Appendix B3). This interpretation is in good agreement with the Th/U ratios that are mostly higher than 0.4 (Appendix B2).

It is unlikely that the zircon xenocrysts from samples of this study and previous studies are due to contamination during sample processing, and should come from the igneous rock itself. Whether zircon xenocrysts were derived from the source region of a magmatic rock or were incorporated into the host magma from surrounding crust during magma ascent is not known. However, it

would appear that there is some difference among the sources of these zircon xenocrysts from the southern CAOB, the ZS zone, the YNH zone and the WNCC.

The oldest zircon xenocrysts in the southern CAOB were found in Devonian and Carboniferous granitoids and volcanic rocks (396345 Ma) in the Omnogovi Province of southern Mongolia, displaying 207Pb/206Pb age of 1104 Ma (Fig. 7a), older than the exposed igneous rocks in the region (Wang et al., 2001; Demoux et al., 2009b). Thus, we consider that these ancient zircon xenocrysts were most probably captured from old igneous suites or basement rocks in the deep crust.

In the Zongnaishan-Shalazhashan (ZS) zone, the zircon xeno-crysts are rarely observed in gabbro and granites, and so far only three zircon xenocrysts were found in three different samples (Fig. 7b). One possible explanation is that only few zircon ages have been reported for intrusions in this belt. However, more than 15 samples were collected for geochronological studies in the ZS zone (Shi et al., 2014a,b; Yang et al., 2014, and our unpublished data), and no xenocrysts were found by examination of the CL images of zircons. Therefore, rare zircon xenocrysts in the ZS zone cannot be attributed to poor sampling, but suggest that old rocks (older than Late Carboniferous) are very limited in the deep crust of this zone (Fig. 7b). Basement rocks of the ZS zone are only exposed in western Zongnaishan and have Rb-Sr isochron ages of 32181982 Ma (Wang et al., 1994). The age of the oldest one of the three zircon xenocrysts found in the ZS zone (1486 Ma) is close to the zircon age for the basement rocks (Shi, 2015), implying that this xenocryst may come from this basement.

Most zircon xenocryst ages of the Yabulai-Nuoergong-Hongg ueryulin (YNH) zone range between 1.8 and 2.6 Ga, with a subordinate age group at 1.7-1.4 Ga (Fig. 7c). Geochronological studies have demonstrated multiple magmatic events in the YNH zone (Geng and Zhou, 2010, 2012; Li et al., 2010; Shi et al., 2012; Wang, 2012; Gong, 2013; Feng et al., 2013; Zhang et al., 2013d; Dan et al., 2014a,b; Wu et al., 2014; Zheng et al., 2014), although magmatic rocks forming at several phases are not exposed or have been destroyed at the present erosion level (Fig. 7c). All these mag-matic events (see Fig. 6c) have left their traces in the crust in the

Table 2

Result of zircon LA-ICP-MS U-Pb dating of aplitic granite of Yamatu pluton in the YNH zone.

Analysis Spots Elemental data (ppm) Th/U Isotopic ratios Age (Ma)

Pb Th U 206pb/238u 1 r 207Pb/235U 1 r 207Pb/206Pb 1r 207Pb/206Pb 1r 207Pb/235U 1 r 206Pb/238U 1 r

10LS54-1 9 177 166 1.1 0.04330 0.00041 0.31467 0.03730 0.05271 0.00629 316 271 278 33 273 3

10LS54-2 37 359 702 0.5 0.04253 0.00040 0.32745 0.00792 0.05584 0.00127 446 50 288 7 269 2

10LS54-3 18 202 378 0.5 0.04511 0.00044 0.33170 0.01810 0.05333 0.00285 343 121 291 16 284 3

10LS54-4 60 660 1314 0.5 0.04419 0.00046 0.32724 0.00624 0.05370 0.00084 359 35 287 5 279 3

*10LS54-5 81 349 220 1.6 0.30763 0.00236 6.78236 0.08526 0.15990 0.00176 2455 19 2083 26 1729 13

10LS54-6 54 706 1163 0.6 0.04501 0.00033 0.33324 0.00536 0.05370 0.00081 359 34 292 5 284 2

10LS54-7 41 827 794 1.0 0.04500 0.00036 0.33304 0.00832 0.05367 0.00131 357 55 292 7 284 2

10LS54-8 157 257 406 0.6 0.36218 0.00347 6.51302 0.08292 0.13043 0.00140 2104 19 2048 26 1992 19

10LS54-9 7 105 132 0.8 0.05443 0.00051 0.38915 0.04824 0.05186 0.00645 279 285 334 41 342 3

10LS54-10 5 102 103 1.0 0.04415 0.00046 0.34804 0.04157 0.05717 0.00698 498 269 303 36 279 3

10LS54-11 204 457 448 1.0 0.41380 0.00303 9.33894 0.10923 0.16368 0.00171 2494 18 2372 28 2232 16

10LS54-12 64 1591 1462 1.1 0.04456 0.00042 0.34241 0.00561 0.05573 0.00085 442 34 299 5 281 3

10LS54-13 55 448 1164 0.4 0.04460 0.00034 0.32081 0.00528 0.05217 0.00084 293 37 283 5 281 2

10LS54-14 28 518 581 0.9 0.04506 0.00041 0.32819 0.01198 0.05283 0.00181 321 78 288 11 284 3

10LS54-15 169 189 471 0.4 0.34166 0.00251 6.03129 0.06825 0.12803 0.00136 2071 19 1980 22 1895 14

*10LS54-16 80 512 538 1.0 0.13832 0.00114 2.48739 0.03179 0.13043 0.00142 2104 19 1268 16 835 7

10LS54-17 49 777 932 0.8 0.04351 0.00027 0.31442 0.00577 0.05240 0.00094 303 41 278 5 275 2

*10LS54-18 619 1603 2387 0.7 0.23648 0.00178 4.53276 0.05443 0.13902 0.00145 2215 18 1737 21 1368 10

*10LS54-19 15 792 319 2.5 0.04972 0.00035 0.39487 0.01301 0.05760 0.00186 514 71 338 11 313 2

10LS54-20 37 949 726 1.3 0.04398 0.00030 0.31106 0.00642 0.05129 0.00103 254 46 275 6 277 2

10LS54-21 208 572 562 1.0 0.33310 0.00276 5.29452 0.06566 0.11528 0.00121 1884 19 1868 23 1853 15

10LS54-22 5 77 119 0.6 0.04407 0.00043 0.33744 0.03285 0.05553 0.00551 434 221 295 29 278 3

10LS54-23 35 1133 768 1.5 0.04517 0.00038 0.33005 0.00687 0.05299 0.00107 328 46 290 6 285 2

*10LS54-24 492 3414 1454 2.3 0.30293 0.00194 6.31778 0.07067 0.15126 0.00158 2360 18 2021 23 1706 11

10LS54-25 6 28 133 0.2 0.04453 0.00044 0.33796 0.03069 0.05505 0.00490 414 199 296 27 281 3

10LS54-26 42 983 924 1.1 0.04299 0.00038 0.31495 0.00613 0.05313 0.00096 335 41 278 5 271 2

*10LS54-27 263 444 872 0.5 0.28694 0.00263 6.03230 0.07887 0.15247 0.00159 2374 18 1981 26 1626 15

10LS54-28 43 381 987 0.4 0.04433 0.00034 0.32359 0.00569 0.05294 0.00090 326 39 285 5 280 2

10LS54-29 20 254 437 0.6 0.04343 0.00034 0.36003 0.01043 0.06013 0.00170 608 61 312 9 274 2

*10LS54-30 19 144 360 0.4 0.04324 0.00040 0.50683 0.01625 0.08500 0.00282 1316 64 416 13 273 2

10LS54-31 4 58 89 0.7 0.04245 0.00031 0.303 0.019 0.05174 0.00327 274 145 269 17 268 2

10LS54-32 3 29 72 0.4 0.04184 0.00031 0.298 0.020 0.05166 0.00340 270 151 265 17 264 2

10LS54-33 286 445 1017 0.4 0.26753 0.00285 4.437 0.045 0.12028 0.00074 1960 11 1719 17 1528 16

10LS54-34 79 167 206 0.8 0.33670 0.00203 5.408 0.036 0.11649 0.00074 1903 11 1886 12 1871 11

10LS54-35 21 333 440 0.8 0.04375 0.00028 0.313 0.005 0.05193 0.00080 282 35 277 4 276 2

10LS54-36 13 286 263 1.1 0.04295 0.00027 0.328 0.009 0.05535 0.00142 426 57 288 7 271 2

10LS54-37 18 172 408 0.4 0.04311 0.00032 0.308 0.005 0.05178 0.00077 276 34 272 4 272 2

10LS54-38 50 798 1114 0.7 0.04279 0.00026 0.307 0.003 0.05208 0.00041 289 18 272 2 270 2

10LS54-39 14 175 301 0.6 0.04421 0.00031 0.344 0.007 0.05649 0.00116 472 45 300 6 279 2

10LS54-40 3 46 67 0.7 0.04475 0.00034 0.320 0.021 0.05193 0.00341 282 150 282 19 282 2

10LS54-41 8 125 164 0.8 0.04322 0.00029 0.309 0.009 0.05192 0.00148 282 65 274 8 273 2

10LS54-42 3 36 60 0.6 0.04164 0.00034 0.296 0.030 0.05164 0.00511 269 227 264 26 263 2

10LS54-43 11 30 41 0.7 0.24606 0.00181 3.218 0.047 0.09485 0.00134 1525 27 1461 21 1418 10

10LS54-44 14 488 246 2.0 0.04349 0.00027 0.310 0.007 0.05174 0.00116 274 51 274 6 274 2

10LS54-45 3 52 60 0.9 0.04252 0.00039 0.303 0.027 0.05163 0.00458 269 203 269 24 268 2

10LS54-46 5 56 120 0.5 0.04519 0.00043 0.351 0.014 0.05629 0.00214 464 84 305 12 285 3

10LS54-47 10 79 252 0.3 0.04233 0.00029 0.304 0.006 0.05201 0.00096 286 42 269 5 267 2

10LS54-48 7 61 161 0.4 0.04560 0.00030 0.330 0.011 0.05249 0.00173 307 75 290 10 287 2

*10LS54-49 14 103 289 0.4 0.04644 0.00029 0.359 0.008 0.05607 0.00119 455 47 311 7 293 2

10LS54-50 16 24 379 0.1 0.04474 0.00040 0.327 0.005 0.05306 0.00076 331 32 288 5 282 3

The discordance zircons.

Table 3

Our new obtained and published Hf isotopic data for zircon xenocrysts.

Units Age Spot No. Zircon age Sample age 176Yb/177Hf 176Lu/177Hf 176Hf/177j.

ZS 11LS96-13 1461 266 0.024365 0.000998 0.282045

ZS 11LSI06-8 357 264 0.007758 0.000353 0.282946

ZS 11 LSI 70-29 607 254 0.027577 0.001101 0.282715

YNH '10LS54-05 2455 263 0.054046 0.001494 0.281219

YNH 10LS54-08 2104 263 0.026753 0.000654 0.281414

YNH 10LS54-09 342 263 0.021024 0.000571 0.282399

YNH 10LS54-11 2494 263 0.02443 0.000674 0.281568

YNH 10LS54-15 2071 263 0.027683 0.000668 0.28138

YNH 10LS54-19 313 263 0.03411 0.00098 0.282302

YNH 10LS54-21 1884 263 0.022802 0.000586 0.281487

YNH '10LS54-24 2360 263 0.051665 0.001403 0.281497

YNH '10LS54-27 2374 263 0.013339 0.000374 0.281407

YNH 10LS54-33 1960 263 0.011907 0.000326 0.281366

YNH 10LS54-34 1903 263 0.021933 0.000636 0.281528

YNH 10LS54-43 1525 263 0.013967 0.000416 0.282379

YNH 10LS35-03 1789 284 0.006871 0.000239 0.281575

YNH 10LS35-08 531 284 0.007575 0.000329 0.282398

YNH 10LS35-14 2333 284 0.009309 0.000365 0.281833

YNH 10LS35-22 1919 284 0.010750 0.000401 0.281480

YNH 10LS35-26 2066 284 0.011650 0.000457 0.281461

YNH 10LS35-29 2103 284 0.011292 0.000409 0.281986

YNH 12LS17-05 2056 287 0.012223 0.000500 0.281323

YNH 12LS17-09 2304 287 0.021136 0.000836 0.281605

YNH 12LS17-14 1310 287 0.014124 0.000586 0.282014

YNH 12LS17-17 1923 287 0.015217 0.000566 0.281521

YNH 12LS17-19 2444 287 0.016818 0.000660 0.281579

YNH 12LS107-05 406 275 0.026827 0.001000 0.282115

YNH LS032-08 1676 255 0.100441 0.002056 0.281763

YNH LS032-20 1499 255 0.046345 0.000774 0.281874

YNH 12LS80-3 333 271 0.035959 0.001406 0.282115

YNH 12LS87-8 329 271 0.049137 0.001569 0.282175

YNH 12LS87-15 352 271 0.022408 0.000835 0.282196

YNH 12LS87-16 1396 271 0.023575 0.000816 0.282085

YNH 12LS87-25 582 271 0.048090 0.001821 0.282058

YNH 09AL101-7 1613 929 0.001833 0.281965

YNH 09AL101-11 1590 929 0.000936 0.281932

YNH 09AL101-12 1827 929 0.001181 0.281736

YNH 09AL93-1 1464 914 0.00093 0.282046

YNH 09AL93-2 1368 914 0.00209 0.282218

YNH 09AL93-3 1243 914 0.000961 0.282115

YNH 09AL93-4 1123 914 0.001714 0.282271

YNH 09AL93-5 1119 914 0.000684 0.282257

YNH 09AL93-7 1188 914 0.000984 0.282277

YNH 09AL93-8 1420 914 0.001469 0.282084

YNH 09AL93-9 1108 914 0.000799 0.282331

YNH 09AL93-10 1844 914 0.0002 0.281462

YNH 09AL93-12 1371 914 0.001548 0.282209

YNH 09AL93-13 1208 914 0.0025 0.282146

YNH 09AL93-16 1344 914 0.00136 0.282256

YNH 09AL93-17 1590 914 0.001189 0.281966

YNH 09AL93-18 1714 914 0.001498 0.281761

YNH 09AL93-19 1504 914 0.000926 0.282071

YNH 09AL93-22 1042 914 0.000386 0.282182

YNH 09AL93-23 1488 914 0.001256 0.282072

YNH 09AL93-24 1195 914 0.000902 0.282036

l76Hf/l77Hfi ohKO) £hK0

TDM2 (Ma) /'lu/hf Data sources

0.000027 0.282017 -25.7 5.8

0.000022 0.282944 6.2 13.9

0.000025 0.282703 -2.0 10.9

0.000038 0.28117 -54.9 -2.3

0.000024 0.28139 -48 -1.9

0.000025 0.2824 -13.2 -5.8

0.000031 0.28154 -42.6 12.3

0.000019 0.28136 -49.2 -3.9

0.00003 0.2823 -16.6 -10.0

0.000017 0.28147 -45.4 -4.2

0.000025 0.28145 -45.1 -5.6

0.000016 0.2814 -48.3 -4.4

0.000023 0.281357 -49.7 -6.4

0.000025 0.281505 -44.0 -2.4

0.000027 0.282368 -13.9 19.7

0.000025 0.281568 -42.3 -2.8

0.000022 0.282395 -13.2 -1.6

0.000018 0.281820 -33.2 18.6

0.000018 0.281466 -45.7 -3.4

0.000019 0.281446 -46.4 -7.8

0.000025 0.281976 -27.8 -0.8

0.000030 0.281304 -51.2 -6.0

0.000030 0.281569 -41.3 9.1

0.000077 0.282000 -26.8 1.8

0.000018 0.281500 -44.2 -2.1

0.000030 0.281549 -42.2 11.6

0.000024 0.282108 -23.2 -14.6

0.000029 0.281698 -35.7 -0.7

0.000030 0.281852 -31.8 0.8

0.000030 0.282108 -23.2 -17.5

0.000031 0.282165 -21.1 -14.2

0.000026 0.282191 -20.4 -12.8

0.000017 0.282063 -24.3 6.0

0.000019 0.282038 -25.2 -13.1

0.000013 5.4

0.000019 4.6

0.000015 2.6

0.000025 6

0.000029 8.9

0.000024 3.5

0.000026 5.9

0.000026 6.1

0.000027 8

0.000027 5.8

0.000028 8.4

0.000027 -5.5

0.000032 9.1

0.000023 2.6

0.000031 10.4

0.000027 5.6

0.000029 0.7

0.000029 1.1

0.000029 1.9

0.000027 7.1

0.000038 -0.3

1804 -0.97 This study

459 -0.99 This study

831 -0.97 This study

3045 -0.96 This study

2745 -0.98 This study

1583 -0.98 This study

2264 -0.98 This study

2828 -0.98 This study

1997 -0.97 This study

2697 -0.98 This study

2531 -0.96 This study

2610 -0.99 This study

2882 -0.99 This study

2611 -0.98 This study

1069 -0.99 This study

2543 -0.99 This study

1493 -0.99 This study

1784 -0.99 This study

2681 -0.99 This study

2654 -0.99 This study

1590 -0.99 This study

2935 -0.98 This study

2292 -0.97 This study

1913 -0.98 This study

2610 -0.98 This study

2264 -0.98 This study

2132 -0.97 This study

2338 -0.94 This study

2116 -0.98 This study

2169 -0.96 This study

2052 -0.95 This study

1991 -0.97 This study

1743 -0.98 This study

2186 -0.95 This study

1993 Dan et al. (2014b)

2022 Dan et al. (2014b)

2334 Dan et al. (2014b)

1842 Dan et al. (2014b)

1581 Dan et al. (2014b)

1827 Dan et al. (2014b)

1583 Dan et al. (2014b)

1568 Dan et al. (2014b)

1495 Dan et al. (2014b)

1817 Dan et al. (2014b)

1411 Dan et al. (2014b)

2856 Dan et al. (2014b)

1566 Dan et al. (2014b)

1858 Dan et al. (2014b)

1465 Dan et al. (2014b)

1961 Dan et al. (2014b)

2370 Dan et al. (2014b)

1760 Dan et al. (2014b)

1772 Dan et al. (2014b)

1788 Dan et al. (2014b)

2032 Dan et al. (2014b)

Units Age Spot No. Zircon age Sample age 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 2r 176Hf/177Hfi £Hf(0) eHf(t) Tdm2 (Ma) fLu/Hf Data sources

YNH 09AL93-25 969 914 0.00028 0.282113 0.000029 -2 1968 Dan et al. (2014b)

YNH 09AL93-26 1176 914 0.001208 0.28221 0.000025 5.3 1664 Dan et al. (2014b)

YNH 09AL93-27 1531 914 0.000618 0.281994 0.000021 5.9 1896 Dan et al. (2014b)

YNH 09AL93-28 1410 914 0.001461 0.282059 0.000031 4.7 1879 Dan et al. (2014b)

YNH 09AL93-29 1244 914 0.000326 0.282244 0.000028 8.7 1498 Dan et al. (2014b)

YNH ZL2-1.16 1176 270 0.001321 0.282087 0.000039 0.282058 -24.2 0.8 1932 Bao et al. (2012)

YNH ZL2-1.28 1633 270 0.001616 0.282072 0.000032 0.282022 -24.8 9.9 1720 Bao et al. (2012)

YNH ZL2-1.54 1492 270 0.000344 0.281909 0.000037 0.281899 -30.5 2.3 2081 Bao et al. (2012)

YNH ZL2-1.55 1550 270 0.000959 0.281997 0.000038 0.281969 -27.4 6.1 1891 Bao et al. (2012)

YNH AL815-2-2 1288 926 0.025787 0.000915 0.282127 0.000021 0.282105 -22.8 5.0 1665 Geng and Zhou (2011)

YNH AL815-2-6 1025 926 0.052218 0.001733 0.282257 0.000042 0.282224 -18.2 3.3 1537 Geng and Zhou (2011)

YNH AL815-2-7 1031 926 0.022574 0.000715 0.282203 0.000029 0.282189 -20.1 2.2 1596 Geng and Zhou (2011)

YNH AL815-2-20 1125 926 0.074915 0.002505 0.282232 0.000039 0.282179 -19.1 4.0 1585 Geng and Zhou (2011)

YNH AL815-2-29 1436 926 0.026195 0.000905 0.282208 0.000023 0.282183 -19.9 11.1 1477 Geng and Zhou (2011)

YNH AL815-2-35 1707 926 0.020396 0.00072 0.281902 0.000021 0.281879 -30.8 6.5 1934 Geng and Zhou (2011)

YNH AL815-2-36 1810 926 0.029028 0.001072 0.281808 0.000028 0.281771 -34.1 5.0 2092 Geng and Zhou (2011)

YNH AL815-2-42 1008 926 0.014143 0.000484 0.282212 0.000032 0.282203 -19.8 2.2 1579 Geng and Zhou (2011)

YNH AL815-2-44 1044 926 0.048197 0.001543 0.282159 0.000040 0.282129 -21.7 0.4 1699 Geng and Zhou (2011)

YNH AL815-2-34 1004 926 0.01647 0.000573 0.282134 0.000031 0.282123 -22.6 -0.73 1723 Geng and Zhou (2011)

YNH AL815-3-12 1212 921 0.107058 0.003503 0.282287 0.000038 0.282207 -17.2 6.9 1507 Geng and Zhou (2011)

YNH AL815-3-23 986 921 0.021559 0.000677 0.282334 0.000030 0.282321 -15.5 5.9 1374 Geng and Zhou (2011)

YNH AL815-3-35 1168 921 0.045002 0.001578 0.282195 0.000033 0.282160 -20.4 4.3 1605 Geng and Zhou (2011)

YNH AL815-3-42 991 921 0.012215 0.000422 0.282313 0.000025 0.282305 -16.2 5.4 1401 Geng and Zhou (2011)

YNH AL815-3-47 970 921 0.033714 0.001144 0.2821 0.000030 0.282079 -23.8 -3.0 1811 Geng and Zhou (2011)

YNH AL817-1-42 1025 904 0.01008 0.000309 0.282232 0.000021 0.282226 -19.1 3.4 1532 Geng and Zhou (2011)

YNH AL820-2-46 1043 913 0.010256 0.000286 0.282226 0.000021 0.282220 -19.3 3.6 1537 Geng and Zhou (2011)

Southern CAOB EGRDC066 406 374 0.027155 0.001060 0.282937 0.000030 0.282929 14.5 475 Dolgopoloba et al. (2013)

Southern CAOB OTD514 514 374 0.02226 0.000756 0.282405 0.000019 0.282398 -1.9 1610 Dolgopoloba et al. (2013)

Southern CAOB EGRDC066 396 374 0.026329 0.000967 0.282890 0.000030 0.282883 12.6 587 Dolgopoloba et al. (2013)

Southern CAOB EGRDC066 406 374 0.027155 0.001060 0.282937 0.000030 0.282929 14.5 475 Dolgopoloba et al. (2013)

WNCC 08406-1-6 2300 432 0.021563 0.000459 0.281258 0.000013 -53.5 -2.8 3044 Zhang et al. (2014)

WNCC YT27 1246 274 0.023701 0.000772 0.282247 0.000014 -18.6 8.4 1510 Pi et al. (2010)

WNCC 11ND-3-4.1 2485 259 0.024340 0.000707 0.281332 0.000012 -50.9 1.1 2775 Wu et al. (2013)

WNCC 11ND-3-2.1 2363 259 0.028608 0.000794 0.281319 0.000008 -51.4 0.5 2807 Wu et al. (2013)

WNCC 11ND-3-10.1 2226 259 0.023258 0.000752 0.281314 0.000011 -51.6 0.4 2814 Wu et al. (2013)

WNCC 11ND-3-13.1 2381 259 0.030203 0.000946 0.281355 0.000008 -50.8 0.8 2790 Wu et al. (2013)

WNCC 11ND-3-18.1 2309 259 0.030398 0.001068 0.281339 0.000009 -51.7 0.7 2793 Wu et al. (2013)

WNCC 08429-1-15 1612 433 0.043818 0.000879 0.282110 0.000019 -23.4 11.6 1579 Zhang et al. (2014)

WNCC 08557-1-5 504 445 0.046075 0.001040 0.282680 0.000018 -3.3 7.5 998 Zhang et al. (2014)

WNCC 08406-1-6 2300 432 0.021563 0.000459 0.281258 0.000013 -53.5 -2.8 3044 Zhang et al. (2014)

Calculation of eHf values of this study is based on the chondritic values of 176Hf/177Hf and 176Lu/177Hf as reported by Blichert-Toft and Albarède (1997). The depleted mantle line in Fig. 8 is defined by (176Lu/177Hf)DM = 0.0384 and (176Hf/177Hf)DM = 0.28325 (Griffin et al., 2000), and the value of (176Lu/177Hf)continental crust = 0.0125 (Chauvel et al., 2014) is used to calculate crustal model ages. * The discordance zircons.

■9 15

Southern CAOB N = 85

100 200 300 400 500 Age of igneous rock (Ma)

-Q £ = 10

100 200 300 400 Age of igneous rock (Ma)

40 35 30 25

£ 20 n

15 10 5 0

500 1000 1500 2000 Age of igneous rock (Ma)

500 1000 1500 2000 2500 Age of igneous rock (Ma)

Fig. 6. Comparison of zircon crystallization age distributions for igneous rocks from the southern CAOB, the ZS zone, YNH zone, and WNCC. For data sources see Appendix B1.

form of relict zircons that may have been supplied to Permian magmatic rocks. Our investigation of the morphologies and internal textures (Pupin, 1980; Vavra, 1994; Corfu et al., 2003) of the zircon xenocrysts from the YNH zone reflects lithological heterogeneity of the sources of these zircon xenocrysts (Peltonen et al., 2003). The existing age data for igneous rocks of the YNH zone indicate that the surface occurrences related to 1500-1000 Ma events have not yet found (Fig. 7c). Therefore, we infer that the most likely source of the abundant ^1500-1000 Ma zircon xenocrysts (Fig. 7b and e) are at deep crustal levels. A few zircon xeno-crysts with dark metamorphic rims surrounding dark cores have ages of 1800-1500 Ma, comparable to ages of previously studied metamorphic rocks from those regions (Geng et al., 2007, 2010; Geng and Zhou, 2011). The oldest rocks in the YNH zone record zircon U-Pb ages up to 2.3 Ga, and 2.2-1.7 Ga meta-igneous rocks are locally exposed within the basement. Zircon xenocrysts with ages up to 2.5 Ga in the plutonic rocks were probably derived from res-tite of the protolith in the deep crust. This suggests that these zircon xenocrysts were not only accidentally entrained from wall rocks during magma ascent through late-stage contamination, but were also derived from a deeper level basement or reflect old multistage older magmatic activities.

Zircon xenocrysts within the Permian rocks of the northwestern margin of North China Craton (WNCC) (Fig. 7c) record crystallization during different phases of magmatic evolution, indicating that the most likely sources of these zircons are older intrusive rocks or metamorphic xenoliths. This is evidenced by previously reported Neo-proterozoic meta-volcanic and Paleo-proterozoic rocks (e.g., meta-gabbros, granitoids and gneiss) in the WNCC (Fig. 6d, for data sources see Appendix B2).

4.2. Tracking of deep crustal composition using zircon xenocrysts

Zircon forms in complex magmatic systems at different levels of the crust. Small melt portions in the source area within the lower crust may already saturate and crystallize zircons (Watson and Harrison, 1983), whereas other melt batches crystallize zircon during ascent through the lower and middle crust, or during further crystallization and cooling. In addition to their ages, the Hf isotopes of zircon xenocrysts may provide significant information on unexposed crustal levels through which the host magma has passed.

Zircon xenocrysts in the southern CAOB mainly have eHf(t) values of+ 12.6 to+ 14.5, similar to the Hf isotopes in synmagmatic zircon of granitoids (Fig. 8), indicating juvenile sources of the CAOB. The ancient zircon xenocrysts (~1.1 Ga) in southern Mongolia area (Appendix B2) and the negative eHf(t) value of —1.9 for a zircon xenocryst (514 Ma) within granodiorite (374 Ma) with aHf crustal model age of 1610 Ma support the previous conclusion of Proterozoic microcontinents in CAOB (Wang et al., 2001; Demoux et al., 2009b).

In the Zongnaishan-Shalazhashan (ZS) zone here are no >1.5 Ga xenocrysts in any rock sample (Fig. 9a). In addition, all xenocrysts are characterized by positive eHf(t) values of +6.3 to +13.9 (Fig. 8), indicating juvenile sources.

Hf isotopic values of zircon xenocrysts within Permian igneous rocks of the Yabulai-Nuoergong-Honggueryulin (YNH) zone mostly fall in the field defined by 1.7 Ga and 3.0 Ga evolutional trends (Fig. 8), indicating highly variable source components. Xenocrysts from Neo-proterozoic rocks have eHf(t) values ranging from —5.5 to +10.4 with Hf crustal model ages of 1.4-2.2 Ga. The occurrence of ancient as well as juvenile

2 0.2 Q_

(a) Zircon xenocrysts within Devonian rocks of the southern CAOB

207Pb/ 235U

1000 1400 Ma

(b) Zircon xenocrysts within Permian rocks of the ZS zone

207Pb/ 235U

000 1400 Ma

' (c) Zircon xenocrysts within

Permian rocks of the YNH zone

6 8 10 12 14

207Pb/ 235U

(e) Zircon xenocryst within Neo-proterozoic rocks of the YNH zone

207Pb/ 235U

(f) Zircon xenocrysts within Paleo-proterozoic rocks of the YNH zone 220

12 14 0 2 4 6 8 10 12 14

207pb/ 235U 207Pb/ 235U

Ellipses represent single spot analyses and 1o standard error

U-Pb ages (207Pb/206Pb >1000 Ma and 206Pb/238U <1000 Ma) for zircon xenocrysts are plotted in inset histograms

Fig. 7. Composite concordia diagrams showing U-Pb isotopic data for zircon xenocrysts and histograms for xenocrystic zircon ages from the southern CAOB, the ZS zone, YNH zone and WNCC. For data sources see Appendix B2.

Ï -10-

This study

Previous studies

Legend

Zircon xenocrysts contained within:

Devonian granodiorite from southern CAOB A Permian granitoids from the |Z \7 Permian gabbro from thelZ

□ Permian granitoids from the

■ Neo-proterozoic rocks from the Y ^ Permian granitoids from WNCC

Zircons within igneous rocks from: f -¿t Devonian granitoids from southern CAOB A Permian intrusive rocks in the Z V Permian mafic rocks in the Z A Precambrian basement from the |Z O Permian felsic rocks in the Y O Permian mafic rocks in the Y

□ Neoproterozoic felsic rocks in the Y

■ Neoproterozoic mafic rocks in the Y O Precambrian basement from the Y o Precambrian rocks from the western Alxa Block

^ O Permian intrusive rocks in the WNCC O TTG from WNCC

1500 2000 Age (Ma)

Fig. 8. Hf isotopic compositions of data and compiled intrusive rocks data. The Hf evolution line is based on 176Lu/177Hf = 0.0125 (Chauvel et al., 2014). The zircon xenocrysts from the southern CAOB are from Dolgopolova et al. (2013). The zircon xenocrysts from the YNH zone are from the literature (Dan et al., 2012,2014b; Geng and Zhou, 2011; Shi et al., 2012) and our unpublished data. The zircon xenocrysts from the WNCC are from (Pi et al., 2010; Wu et al., 2013). Previous Hf isotopic data of intrusive rocks in ZS zone cited after Shi et al. (2014a, 2014b). Hf isotopic data of Precambrian rocks in ZS zone cited after Shi (2015). Hf isotopic data of the Permian rocks from the YNH zone cited after (Shi et al., 2012; Dan et al., 2014a; Zhang et al., in revision). Data of the Neoproterozoic rocks in YNH zone cited after (Geng et al., 2010; Dan et al., 2014b). Hf isotopic data of the Precambrian rocks in YNH zone cited after Dan et al. (2012). Hf isotopic data of the Precambrian magmatic rocks from the western Alxa Block cited after (Gong et al., 2012; Gong, 2013; Zhang et al., 2013b). Hf isotopic data of the Permian rocks from WNCC cited after (Zhang et al., 2011; Wu et al., 2013; Lin et al., 2014). TTG from WNCC cited after Ma et al. (2013).

source features in zircon xenocrysts within the YNH Permian and Neo-proterozoic igneous rocks coincides with zircons from igneous rocks in the YNH zone and the western Alxa Block (Fig. 8). This provides unequivocal evidence for multiple

Precambrian magmatic activities in the deep crust of the YNH zone. Moreover, these zircon xenocrysts have older Hf crustal model ages (TDM2) than synmagmatic zircons (Fig. 9b), indicating ancient crustal rocks in deep crust.

Zuunbayan Fault

Solonker suture zone

Legend

Oldest age of zircon xenocrysts

359-300 Ma 542-359 Ma 1000-582 Ma 1.5-1.0 Ga 1.8-1.5 Ga 2.2-1.8 Ga

SP 2.5-2.2 Ga

Latitude (°N)

Qagan Qulu ophiolite belt Southern Boundary of CAOB ?

YNH zone (northern Alxa Block)

Zircon xenocrysts contained within:

^ Devonian granitoids from southern CAOB A Permian granitoids from ZS zone ▼ Permian gabbro from ZS zone

■ Permian granitoids from YNH zone

■ Neo-proterozoic rocks from YNH zone Zircons from igneous rocks:

Devonian granitoids from southern CAOB A Permian intrusive rocks in the ZS zone V Permian mafic rocks in the ZS zone A Precambrian rocks from the ZS zone O Permian feisic rocks in the YNH zone O Permian mafic rocks in the YNH zone Neoproterozoic feisic rocks in the YNH zone

■ Neoproterozoic mafic rocks In the YNH zone - O Precambrian basement from the YNH zone

Fig. 9. (a) Cartoon showing the oldest age (207Pb/206Pb > 1000 Ma and 206pb/238u < 1000 Ma) for zircon xenocrysts within igneous rocks from the southern CAOB, the ZS zone and YNH zones and the WNCC. (b) Section across Hf crustal model age maps for zircon xenocrysts and igneous rocks of the southern CAOB and the ZS and YNH zones. Background data sources as in Fig. 8. Data for the southern CAOB form Dolgopolova et al. (2013).

As mentioned above, most previous studies proposed that the ZS and YNH zones share a common basement (e.g., Wu and He, 1992; Wang et al., 1994). If this were the case, the zircon xenocrysts from these two zones should be expected to yield similar age spectra and Hf isotopic signatures. However, the zircon xenocryst ages and Hf isotopic compositions of the two zones differ significantly, as illustrated in Fig. 9. The marked shift from a juvenile source in the ZS zone to ancient sources in the YNH zone probably indicates that, on a regional scale, the deep continental crust in the ZS zone is younger than that of the YNH zone (Fig. 9b). This is in good agreement with the differences in Hf crustal model ages for synmagmatic zircons from the ZS and YNH zones, as shown in the cross-section across of Fig. 9b. In conclusion, we propose that the ZS and the YNH zones have different basements and thus constitute two distinct and separate tectonic units.

Along the northwestern margin of North China Craton (WNCC), the xenocryst Hf isotopic data mostly plot in the field defined by the 2.5 Ga and 3.0 Ga evolutional trends (Fig. 8), indicating ancient source components. Some zircon xenocrysts have ages older than the oldest age of exposed igneous rocks (Fig. 9a), suggesting the presence of old crustal material in the deep crust.

4.3. Constraints on the southern boundary of the CAOB

The Enger Us ophiolite belt (Fig. 1 ) was previously considered to represent a suture between the southern CAOB and the North China Craton and was therefore considered as the southern boundary of the CAOB (Wu and He, 1992, 1993; Wang et al., 1998; Dan et al., 2012, 2014a; Feng et al., 2013). However, this suture was not well constrained due to the poor exposure of the ophiolitic mélanges.

Generally, the CAOB consists of juvenile accretionary zones and ancient (Proterozoic) microcontinents. The ZS zone has zircon xenocrysts with similar ages as zircon xenocrysts from southern Mongolia. Moreover, the oldest zircon xenocryst from the ZS zone (1486 Ma) is consistent with the age of the oldest intrusive rock (~1.4Ga) in this zone (Shi, 2015) and the zircon crystallization age for gneiss (1.5-1.4 Ga) in southern Mongolia (Kozakov et al., 2007; Yarmolyuk et al., 2007; Demoux et al., 2009a). This indicates that the ZS zone is most likely a constituent part of the southern CAOB. This inference is further reinforced by whole-rock Nd iso-topic studies of Phanerozoic granitoids and volcanic rocks (Yarmolyuk et al., 2007; Dolgopolova et al., 2013; Shi et al., 2014a) and similar Hf-in-zircon isotopes for the ZS zone and the southern CAOB (Fig. 9b).

In contrast, the YNH zone has abundant zircon xenocrysts with ages older than 1.5 Ga (Fig. 9a). These old zircon xenocryst populations (1.8-1.6 Ga and 2.6-2.1 Ga) are fairly similar to the ages of Precambrian igneous rocks in the YNH zone (with age groups at 2.0-1.7 Ga and 2.3-2.1 Ga) and the western Alxa Block. In addition, the Hf isotopic data for the zircon xenocrysts is comparable to those of zircons from intrusions in the YNH zone and the western Alxa Block (Fig. 8). All these features indicate an Alxa Block-affinity with the YNH zone.

In summary, the deep crust of the ZS zone is similar to that in the southern CAOB, but distinct from the YNH zone (Alxa Block). Thus we propose that the border between the ZS and YNH zones, rather than the Enger Us ophiolite belt is the most likely terrane boundary and constitutes the southernmost boundary between the southern CAOB and the Alxa Block (Fig. 9b).

From the perspective of research methods, the current study indicates that the regional statistical study of the distribution of ages and isotopic compositions of zircon xenocrysts from igneous rocks is an effective way to trace deep crust compositions and to identify tectonic boundaries between orogenic belts and ancient cratons. This may help to provide new clues to constraint the tectonic boundary of the southern CAOB with the NCC, where continuous exposures of ophiolites are hard to find. Previous speculation that the Enger Us ophiolite belt reflects a major suture zone (Wu and He, 1993; Wang et al., 1998; Dan et al., 2012; Gong et al., 2012; Feng et al., 2013; Song et al., 2013; Dan et al., 2014a; Zheng et al., 2014) and represents the western extension (Zhou et al., 2011) of the Solonker suture seems questionable. From a large-scale perspective, the central part of the Tianshan-Solonker suture zone (see Fig. 1, Xiao et al., 2003, 2010b; Li, 2006a; Shi, 2006; Zhou et al., 2011; Feng et al., 2013; Zheng et al., 2013b), which extends along the northern margins of the Tarim and North China Cratons, is located between the ZS and YNH zones (Fig. 9a). However, since only scarce studies upon zircon xeno-crysts have been performed so far, more multidisciplinary work is needed to evaluate the practicability of such method.

5. Conclusions

Based on the U-Pb age and Hf isotopic composition of zircon xenocrysts from igneous rocks of the northern Alxa and its nearby regions, combined with published data, several conclusions can be drawn:

(1) Abundant zircon xenocrysts within Permian igneous rocks of the Yabulai-Nuoergong-Honggueryulin (YNH) zone difine age populations at ca. 2.6-2.1 Ga, 1.8-1.6 Ga, 930-750 Ma, and 460-300 Ma, and old Hf model ages (TDM2, mainly 3.21.4 Ga); while those from the Zongnaishan-Shalazhashan (ZS) zone show younger age populations (mainly around 1400 Ma, 600 Ma and 350 Ma), and younger model ages of 1.8-0.5 Ga.

(2) The ages and Hf isotope of these zircon xenocrysts indicate that the basements rocks beneath the ZS zone are young and similar to these of the southern CAOB, whereas those beneath the YNH zone are old and similar to the Alxa Block.

(3) The southernmost boundary of the CAOB with the Alxa Block is located along the boundary between the ZS and the YNH zones (the Qagan Qulu ophiolite belt), rather than along the Enger Us ophiolite belt as previously considered.

Acknowledgements

We thank Profs. Xiaoxia Wang and Antonio Castro for their fruitful discussion and research cooperation. Prof. Alfred Kroner is

specially appreciated for the language polishing and fruitful suggestions related to the manuscript. We gratefully acknowledge the valuable comments and constructive suggestions of Prof. Wenjiao Xiao and an anonymous reviewer. This work was financially supported by the Major State Basic Research Program of the P.R. China (grant 2013CB429803), the National Natural Science Foundation of China (NSFC grants 41372077 and 41372006) and China Geological Survey Projects (grant numbers, 1212010611803, 12120113096500 and 12120113094000).

Appendix A. Analytical methods

A.1. Zircon dating

Zircon grains were separated by the conventional magnetic and density separation techniques, and representative zircons were selected by handpicking under a binocular microscope. Transmitted and reflected light microphotographs as well as CL images were obtained for the polished mounts of zircon grains prior to U-Pb and Lu-Hf isotopic analysis in order to reveal their internal structure and and to guide the selection of most suitable analytical spots. The CL images were obtained by using a Quanta 200 FEG scanning electron microscope (SEM) at Peking University.

U-Pb dating of zircons of aplitic granite sample 10LS54 was conducted at the Tianjin Institute of Geology and Mineral Resources, using a Neptune MC-ICP-MS with 9 Faraday cups (Thermo Fisher Scientific, Inc., Waltham, MA, USA). The system is coupled to a (ESI) UP193FX ArF Excimer laser ablation system with 193 nm wavelength and a homogenizing imaging optical system. The set-up delivers a flat top beam onto the sample surface, and the laser wavelength and energy density (13-14J/cm2) allow controlled ablation of highly transparent samples. Spot sizes of 35 im and 50 im were adopted in this study. Helium was used as a carrier gas during laser ablation to enhance transport efficiency of the ablated material. The standard TEM zircons (417 million years, Black et al., 2003) were used to monitor inter-element fractionation, and U, Th, and Pb concentrations were determined based on the standard zircon NIST612 (Pearce et al., 1997). Data processing was carried out using the ICPMSDataCal (Liu et al., 2008) and Isoplot4.15 (Ludwig, 2003), and the 208Pb-based method of common Pb correction was applied (Andersen, 2002).

A.2. Hf-in-zircon isotopic analysis

In situ zircon Hf-in-zircon isotopic analysis was conducted using a Neptune MC-ICP-MS equipped with a new wave UP213 laser-ablation system at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Instrumental conditions and data acquisition were described by Wu et al. (2006). A spot with a beam diameter of either 40 or 55 im depending on the size of ablated domains, was used for the present analyses. Helium was used as carrier gas to transport the ablated sample from the laser-ablation cell to the ICP-MS torch via a mixing chamber mixed with argon. In order to correct for the isobaric interferences of 176Lu and 176Yb on 176Hf, the 176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 ratios were determined (Chu et al., 2002). To correct for instrumental mass bias, by using an exponential law the Yb isotope ratios were normalized to 172Yb/173Yb = 1.35274 (Chu et al., 2002) and Hf isotope ratios to 179Hf/177Hf of 0.7325. The mass bias behavior of Lu was assumed to follow Yb, and the mass bias correction was described by Wu et al. (2006) and Hou et al. (2007). Standard zircon GJ1 (Morel et al., 2008) was used as the reference standard with a weighted mean 176Hf/177Hf ratios of 0.282000 ±0.000019 (2r, n = 11) during the analyses. These are not distinguishable from the weighted mean 176Hf/177Hf ratio of 0.282013 ± 19 (2r) from in-situ analysis by Elhlou et al. (2006).

Appendix B. Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/jjseaes.2015.04. 019.

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