Detrital zircon fingerprints link western North China Craton with East Gondwana during Ordovician

Gondwana Research 40 (2016) 58–76

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Detrital zircon fingerprints link western North China Craton with East Gondwana during Ordovician Zhentao Wang a,⁎, Hongrui Zhou b, Xunlian Wang b, Mianping Zheng a, M. Santosh b,c, Xiuchun Jing b, Jin Zhang d, Yongsheng Zhang a a

MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, CAGS, Beijing 100037, China School of the Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China c Department of Earth Sciences, The University of Adelaide, Adelaide, SA 5005, Australia d Institute of Geology, CAGS, Beijing 100037, China b

a r t i c l e

i n f o

Article history: Received 23 May 2016 Received in revised form 2 August 2016 Accepted 3 August 2016 Available online 16 September 2016 Handling Editor: S.J. Liu Keywords: Detrital zircon geochronology Provenance North China Craton Gondwana Ordovician

a b s t r a c t The tectonic affinity of the North China Craton (NCC) in the early Paleozoic paleogeographic reconstructions of the Gondwana supercontinent remains controversial. Here we present new detrital zircon U–Pb age data from Middle (Sandaokan Formation) to Upper Ordovician (Lashizhong Formation) sandstone deposits from biostratigraphically well–constrained strata in the Zhuozishan Mountain area, western NCC, to explore the link between NCC and Gondwana. The detrital zircon age spectra from the Sandaokan Formation range in age from 1684 to 2905 Ma with peaks of Neoarchean (~2.5 Ga) and Paleoproterozoic (~1.83 and ~1.96 Ga), and display age distribution similar to that of the Precambrian basement of the NCC. The Lashizhong Formation contains zircons ranging in age from 3626 to 488 Ma, with six main age peaks (1000–900 Ma, 2600–2400 Ma, 900–700 Ma, 700–500 Ma, 1300–1100 Ma and 2000–1500 Ma).The grains represent a record of the Grenville and Pan-African orogenic events, in striking contrast to those for the Sandaokan Formation. The age spectra of the Lashizhong Formation are similar to those from crustal blocks associated with eastern Gondwana. Our results suggest that the western part of the NCC might have been linked with the East Gondwana, rather than being an isolated continent block in the paleo–Pacific, as proposed in current models. The detrital zircon age populations relative to the depositional ages, sandstone lithofacies, sedimentary structures, and paleogeography suggest that the Sandaokan Formation was deposited in a passive continental margin setting, whereas the Lashizhong Formation formed in a collisional setting, which suggests an orogenic belt or suture zone located to the west of the NCC. The significant difference in provenances between the two formations within a short interval (ca. 6 Ma) might suggest a newly identified tectonic shift at ca. 458 Ma, followed by the northwestern margin transforming into a possible foreland basin stage between the NCC and the eastern Gondwana. © 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

1. Introduction The paleogeographic configuration of continental blocks around the eastern segment of the late Neoproterozoic supercontinent Gondwana has been addressed in many studies (e.g., Collins and Pisarevsky, 2005; Meert and Lieberman, 2008; Santosh et al., 2009; Nance et al., 2014), but the location of the North China Craton (NCC) and its history of interaction with the Gondwana fragments during the Paleozoic are poorly constrained (Meert, 2014; Han et al., 2015; Myrow et al., 2015). Contrasting models have been proposed by different workers for the paleogeography of the NCC in relation to Gondwana ranging from Gondwana margin affiliation (Li and Powell, 2001; Zhao et al.,

⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Wang).

2007; Goldman et al., 2011; McKenzie et al., 2011a; Metcalfe, 2013; Han et al., 2015; Myrow et al., 2015) to locations that were unconnected to the margin of this supercontinent during early Paleozoic (Santosh et al., 2009; Wilhem et al., 2012; Cocks and Torsvik, 2013; Xu et al., 2013; Burrett et al., 2014; Yu et al., 2015; Cai et al., 2016; Wang et al., 2016). However, recent detrital zircon, paleomagnetic, stratigraphic, and paleobiogeographic evidence suggests that the NCC had a close affinity to the northern East Gondwana margin during the middle Cambrian through Ordovician (Zhao et al., 2007; McKenzie et al., 2011a; Metcalfe, 2013; Hally and Paterson, 2014; Han et al., 2015; Myrow et al., 2015). Based on detrital zircon U–Pb ages with species– level polymerid trilobite biogeography, McKenzie et al. (2011a) suggested that the southern margin of the NCC was tectonically attached to the northern margin of East Gondwana and that the western NCC (the Ordos region) was isolated from Gondwana–derived sediment during the Cambrian. The detrital zircon spectra from upper Proterozoic

http://dx.doi.org/10.1016/j.gr.2016.08.007 1342-937X/© 2016 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

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to late Middle Ordovician (Darriwilian) strata collected from the western NCC (Darby and Gehrels, 2006; Myrow et al., 2015) display prominent ages at ca. 1.8–2.3 Ga with a distinctive profile from that of other regions of Gondwana (Squire et al., 2006; Myrow et al., 2010), seemingly supporting the view that the NCC was isolated from Gondwana during the Cambrian–Ordovician. However, a revised paleogeographic reconstruction by Myrow et al. (2015), based on the timing and duration of the Cambrian–Ordovician unconformity in the western NCC comparable with those in the North Indian regions indicated a potential paleogeographic link of the NCC to the northern Indian margin of the East Gondwana. This model suggested that the western NCC (the Inner Mongolian region) was likely an along–strike continuation of the northern Indian continental margin during the latest Cambrian, which needs to be tested by further studies. The appropriate placement of the NCC in the reconstruction and regional paleogeography of Gondwana remains a challenging topic (Metcalfe, 2013). Whether or not the NCC was juxtaposed at the northern margin of Gondwana during the Ordovician remain unresolved, partly because of the lack of adequate sediment provenance information and paucity of systematic and complete geochronological data on Ordovician strata in the NCC. In this study, we report laser ablation inductively coupled mass spectrometry (LA–ICP–MS) zircon U–Pb ages from the Middle Ordovician Sandaokan Formation and Upper Ordovician Lashizhong Formation in Zhuozishan Mountain area, which is located in the northwestern NCC (Fig. 1). The results, coupled with recently published species level biogeographic analysis of conodonts, are used to constrain the location of the NCC with respect to Gondwana, thus offering insights the tectonic setting of the western margin of the craton during the Ordovician.

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The NCC is one of the oldest cratonic blocks in the world, containing rocks as old as ~3.85 Ga (Liu et al., 1992, 2008; Liou et al., 2014), and a prolonged early Precambrian crustal evolution history (Zhai and Santosh, 2011; Zhao and Zhai, 2013), including major Neoarchean micro–continent amalgamation and craton building (e.g., Santosh et al., 2015a; Yang et al., 2016). The craton is bound by the Qilian Orogenic Belt to the west and southwest (Song et al., 2013; Dong and Santosh, 2016), the southern branch of the Central Asian Orogenic Belt to the north (Xiao et al., 2003), and the Qinling–Dabie and Su–Lu high–pressure metamorphic belts to the south and southeast (Shi et al., 2013).Traditionally, the basement of the NCC has been divided into the Eastern and Western Blocks, separated by the Trans–North China Orogen with the EW–trending Khondalite Belt (or Inner Mongolia Suture Zone, Santosh, 2010) separating the Western Block into the Yinshan Block in the north and the Ordos Block in the south, and the Jiao–Liao–Ji Belt dissecting the Eastern Block into the Longgang Block and the Langrim Block (e.g., Zhao et al., 1998, 2001, 2005; Santosh et al., 2007, 2012; Zhao and Cawood, 2012; Zhao and Zhai, 2013; Zheng et al., 2013; Fig. 1). However, recent studies indicate that the NCC is a collage of several Archean microblocks and that the final cratonization involved protracted Paleoproterozoic subduction–accretion collision events (e.g., Zhai and Santosh, 2011; Santosh et al., 2015b; Yang et al., 2016). Mesoproterozoic to Cenozoic unmetamorphosed strata overlie Archean to Paleoproterozoic metamorphic basement of the NCC. During the late Proterozoic and early Paleozoic, passive margins developed on all sides of the NCC (Sun et al., 1989; Yin and Nie, 1996; Meng et al., 1997), and the Cambrian to Middle Ordovician strata of the NCC are dominated by siliciclastic and carbonate successions (Zhao et al.,

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Fig. 1. Tectonic subdivision of the North China Craton (after Zhao et al., 2005) and the distribution of detrital zircons reported mainly from Ordovician strata.

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2001). An open platform–ramp basin developed on the western NCC in the Early and Middle Ordovician (Chen et al., 2014), in which restricted platforms, open platforms, and ramps developed successively from east to west. Deep–water deposits characterize the southern and western fringes of the NCC, whereas carbonate platform facies deposits dominate within the craton (Wang, 1985; Wang et al., 2015a). During the Middle Ordovician, the majority of the NCC was submerged, except for its peripheral regions (Wang, 1985; Feng et al., 1990; Guo et al., 2012; Y.S. Zhang et al., 2014; Fig. 2). These parts of the NCC, as well as its marginal belts, witnessed orogenic process during the early Katian of the Late Ordovician, resulting in a regional uplift event, indicated by the unconformities between the Ordovician successions and overlying upper Paleozoic strata (Chen et al., 2013). Although the western boundary of the NCC is debated (Dan et al., 2012, 2014; Zhang et al., 2015; Yuan and Yang, 2015a; Dan et al., 2016; B.H. Zhang et al., 2016; J. Zhang et al., 2016), our present study area in the Zhuozishan Mountain is considered to be an integral part of the NCC during the Ordovician (Fig. 3) and has been in focus for many studies related to paleontology (Chen et al., 1984; Zhou et al., 1989; An and Zheng, 1990; Wang and Zhu, 1997; Xi, 1998; Rong et al., 1999; Chen et al., 2013; Wang et al., 2013; Jing et al., 2016a, 2016b), sedimentary petrology and facies (Li, 1994; Gao et al.,

1995; Fei, 2000, 2001; Fei et al., 2004; Jin et al., 2005; Zhou et al., 2013; Zhao et al., 2014; Myrow et al., 2015; Xu et al., 2015), geochemistry (Xi et al., 2004; Wang et al., 2014) and detrital zircon geochronology (Darby and Gehrels, 2006; Myrow et al., 2015) of the Ordovician strata. 3. Sedimentary characteristics and sample description The Ordovician rocks on the NCC are dominated by shallow–water limestone and dolomitic limestone containing abundant shelly fossil faunas and conodonts (Chen et al., 2013). The Upper Ordovician strata are well developed in the southern and western margin of the NCC (Wang et al., 2015b; Jing et al., 2016a, 2016b). The Ordovician strata in the Zhuozishan Mountain area record deposition on the northwestern margin of the NCC (Fig. 3A). Chen et al. (1984) grouped these strata into seven formations, namely the Sandaokan, Zhuozishan, Kelimoli, Wulalike, Lashizhong, Gongwusu and Sheshan formations in an ascending order, most of which are biostratigraphically well–constrained. Recent biostratigraphy and biofacies analyses of the Middle to Late Ordovician conodonts from the Zhuozishan Mountain area (Jing et al., 2016a, 2016b), has confirmed the earlier sub–division of the strata (Fig. 4).

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The Darriwilian (upper Middle Ordovician) Sandaokan Formation unconformably overlies the Furongian (Cambrian) Abuqiehai Formation (Fig. 4; Fig. 5A) and conformably underlies the Zhuozishan Formation (Fig. 4). This Sandaokan Formation is composed dominantly of a series of gray to dark–gray quartzose sandstone (Fig. 6A,C) and dolomitic quartzose sandstone with some intercalations of dolostone and minor terrigenous detritals (Fig. 6B). The rocks were deposited in the shallow water by high–energy flows (Jing et al., 2016b). Myrow et al. (2015) suggested that the formation records an overall transgression from shoreline and nearshore settings to more offshore environments, suggesting that the Zhuozishan Mountain area was near a paleo–land during the deposition of the formation. Paleocurrents measured from fossil orthocone nautiloids from the middle and upper parts of this formation point approximately to the south (Fei et al., 2004), hinting that the clastic rocks came from the paleoland to the north. The Lashizhong Formation is represented by turbidite sandstone, and siltstone (Fig. 5E,F,G; Fig. 6D,E,F). The trace fossil assemblage in this formation is characterized by the widespread occurrence of Paleodictyon, which corresponds to the Nereites ichnofacies representing depositional environment of far–turbidity current in deep sea (Fei, 2001). The presence of a number of bioendoglyphia Zoophycos and a decrease in abundance of Paleodictyon near the top of this formation suggest that during the late period the water depth became shallower (Fei, 2001). The Zhuozishan Formation is mainly bioturbated and bioclastic limestone, microbialites. Recently, the finding of coral reefs in the Zhuozishan Formation in Zhuozishan Mountain area (Zhou et al., 2013; Zhao et al., 2014) indicates a relatively shallow marine environment in the carbonate platform of the NCC. Graptolitic black shale, thin–bedded limestone, and turbidite in the Kelimoli Formation and

black shale in the Wulalike Formation indicate gradually deepening. The Gongwusu Formation is composed of greenish–gray mudstone, thin–bedded limestone, and sandstone. The sedimentary environment of this formation is proposed as a continental slope during the deposition (Li, 1994). In terms of the succession of sedimentary environments of the formations mentioned above, a carbonate platform and platform margin can be inferred in the northwestern margin of the NCC (Zhuozishan Mountain area) during the Middle–Late Ordovician. The onlap of black shale in the upper of the Wulalike Formation and then argillaceous rock intercalated with carbonate and siliciclastic rocks of turbidite deposit mark the end of carbonate platform development. After this, the Zhuozishan Mountain area evolved into a deep–water turbiditic siliciclastic sedimentation stage. The precise boundary between these two stages corresponds to the location of the end of conodont zone Pygodus Anserinus, earliest Sandbian Age (Jing et al., 2016a), approaching ca. 458 Ma (Cooper and Sadler, 2012; Fig. 4). In this study, we determined detrital zircon U–Pb age spectra for six samples collected from the Zhuozishan Mountain area (Figs. 1,3; Supplementary Table 1), among which three samples (L1R2, L2R2, and L4R1) were collected from the Sandaokan Formation, Laoshidandongshan Section (GPS coordinates of the starting point of the section: 39°22′51.8″ N, 106°51′59.7″ E), and another three (G1R1, G7R1, and G8R2) from the Lashizhong Formation, Hatuke Creek Section (GPS coordinates of the starting point of the section: 39°25′32.66″ N, 106°53′4.77″ E). The biostratigraphically constrained depositional ages of the strata sampled in this study range from Middle to Late Ordovician (Fig. 4). Sample L1R2 was collected at a locality close to the Cambrian– Ordovician boundary in the Laoshidandongshan Section (Figs. 3, 4) within the Sandaokan Formation, and is thus thought to represent the

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sampling Sedimentary location association

Sandaokan Formation

Furongian, Cambrian

L.4.R.1 L.2.R.2

Legend

Histiodella bellburnensis

Pygodus anitae Pygodus serra Pygodus anserinus

calcirudite

bioturbated limestone

Pygodus anserinus

Pygodus serra

marlstone

micrite

Eoplacognathus suecicus

dolomite

Dzikodus tablepointensis

dolosiltite

wormkalk ?

black shale H. bellburnensis

Histiodella kristinae

Laoshidan section

Zhuozishan Formation

Histiodella kristinae

Kelimoli Formation

ca. 457.90Ma

Histiodella cf. holodentata

Wulalike Formation

Yangtzeplacognathus foliaceus

G.1.R.1

Zone

breccia

Eoplacognathus suecicus Pygodus lunnensis

G.8.R.2 G.7.R.1 Lashizhong Formation

Selected conodont species

Dzikodus tablepointensis

Gongwusu Formation

ending of conodont zone Pygodus anserinus

Young Farm Nanshan section

lithological column

Dw2

Darriwilian

Wolonggang section

Hatuke Creek section

Sa1 Dw3

Sandbian

Upper Ordovician Middle Ordovician

Formation

carbonate deposition on the cratonic margin

Ordovician strata Stage Series Stage -slice Section

deep water turbiditic siliciclastic sedimentation

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calcareous mudstone

argillaceous siltstone

calcareous fine sandstone

ca. 463.75Ma H. cf. holodentata

sandstone

? L.1.R.2

Abuqiehai Formation

fine-grained conglomerate

Fig. 4. Measured sections including conodont biostratigraphy through the Middle and Upper Ordovician Sandaokan, Zhuozishan, Kelimoli, Wulalike, Lashizhong, and Gongwusu formations, and the location of samples for detrital zircon analysis. The conodont biozones are from Jing et al. (2016a, 2016b), and the time of conodont zones are after Cooper and Sadler (2012).

oldest Ordovician sample of this study. Sample G1R1 was taken from the Lashizhong Formation, which is close to the location marking the top of conodont biozone Pygodus anserinus, and represents the earliest terrigenous clastic strata after carbonate deposition on the cratonic margin in the Zhuozishan Mountain area (Fig. 4). The detrital minerals in the samples mainly include quartz, feldspar, dolomite and minor biotite. Samples L1R2 and L4R1 are quartzose sandstone with medium– grained quartz (N90 vol.%), L2R2 is sandy dolarenite with medium–

grained quartz (20–30 vol.%), whereas samples G1R1, G7R1 and G8R2 are fine sandstone to siltstone with more lithic grains (ca.10 vol.%). 4. Analytical methods Zircon separation was performed at the Laboratory of the Institute of Regional Geology and Mineral Resources Survey of Hebei Province. Zircon crystals were extracted from crushed samples by using combined

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Ordovician (Sandaokan Formation)

Cambrian(Abuqiehai Formation)

A Sandaokan Formation Sample L.2.R.2

Sandaokan Formation Sample L.1.R.2

C

B Sandaokan Formation Sample L.4.R.1

Lashizhong Formation Sample G.1.R.1

D Lashizhong Formation Sample G.7.R.1

E Lashizhong Formation Sample G.8.R.2

F

G

Fig. 5. Outcrop photos of (A) the Ordovician Sandaokan Formation, Cambrian Abuqiehai Formation, and the Cambrian–Ordovician boundary. (B) ~ (D) Sampling horizon of L.1.R.2, L.2.R.2, and L.4.R.1, respectively, in the Sandaokan Formation. (E) ~ (G) Sampling horizon of G.1.R.1, G.7.R.1, and G.8.R.2, respectively, in the Lashizhong Formation.

methods of magnetic and heavy liquid separation before final hand picking under a binocular microscope. More than 1000 grains were randomly selected from each sample, mounted in epoxy resin, and polished to about half their original thickness. The samples were prepared for U–Pb dating after photographing them under reflected and transmitted light. The cathodoluminescence (CL) imaging was

performed at Beijing GeoAnalysis Co., Ltd. to investigate the origin and structure of the zircons, and to select target sites for U–Pb dating. The CL images of representative zircon grains from each sample are presented in Fig. 6. Measurements of U, Th, and Pb were conducted using laser– ablation multiplecollector–inductively coupled plasma–mass spectrometry (LA–MC–ICP MS) at the Geologic Lab Center, China University of

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Fig. 6. Thin–section photomicrographs of sandstone samples (A)–L.1.R.2, (B)–L.2.R.2 and (C)–L.4.R.1 of the Sandaokan Formation, and (D)–G.1.R.1, (E)–G.7.R.1, and (F)–G.8.R.2 sandstones of the LashizhongFormation. Qtz–Quartz; Felds–Feldspar; Bi–biotite; Ch–chert; Do–dolomite.

Geosciences (Beijing).Instrumental performance and detailed analytical procedures are described by Song et al. (2010) (Supplementary analytical techniques). 5. Results 5.1. Zircon morphology Zircon grains are predominantly colorless to paleyellow, and most of them are rounded to subrounded with clear oscillatory zoning or platy structures. The grain size varies from 60 to 200 μm. CL intensities and zoning patterns of the zircons are highly variable, and core–rim

structures of some zircons under CL have been observed (Fig. 7). Th/U ratios of zircons lie in the range 0.016–9.482 (0.214–2.200 of L1R2, 0.207–2.228 of L2R2, 0.016–2.804 of L4R1, 0.070–9.482 of G1R1, 0.068–2.215 of G7R1, and 0.029–2.897 of G8R2) but are greater than 0.10 in most cases with REE patterns also typical of igneous zircons (Rubatto, 2002; L. Zhang et al., 2014; Supplementary Table 1; Figs. 8,9). 5.2. Detrital zircon U–Pb ages One hundred to 147 zircon grains without inclusions and cracks from each sample were randomly chosen and dated. The

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Fig. 7. CL images of representative zircons showing main types of internal structures within zircon grains. Circles show location of U–Pb age data. Ages are based on 207Pb/206Pb ratios for zircons older than 1.0 Ga and 206Pb/238U ratios for zircons younger than 1.0 Ga.

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Th/U ratio

L1R2 L2R2 L4R1

1

G1R1 G7R1 G8R2

0.1

0.01 0

500

1000

1500

2000

2500

3000

3500

4000

Age (Ma) Fig. 8. U–Pb ages vs. Th/U ratios of detrital zircons from each formation. Note that the majority of the analyzed spot ages have Th/U ratios greater than 0.1.

207 Pb/235U–206Pb/238U concordia diagrams of these samples indicate that most zircon U–Pb ages of the six samples are concordant or nearly concordant (Fig. 10). We used 206Pb/238U and 207Pb/206Pb ages for zircons younger and older than 1.0 Ga (Blank et al., 2003), respectively, for constructing probability density diagrams (Fig. 11). In order to avoid analytical bias owing to lead loss or common lead contamination, the discordant ages (concordance b 90% or N 110%) of these samples

10000 1000

were not used. All ages obtained from the six samples meet the requirements of age distribution statistics (Vermeesch, 2004; Andersen, 2005). The age distribution patterns of zircons from the two formations are markedly different (Fig. 11). The samples of the Sandaokan Formation yield detrital U–Pb zircon ages that range from Mesoarchean (2905 Ma) to late Paleoproterozoic (1684 Ma) (Fig. 11A–D), whereas those from the Lashizhong Formation range from Paleoarchean (3626 Ma) to Cambrian (488 Ma) (Fig. 11E–F). The age distributions of the Lashizhong Formation are more complex (Fig. 11). All the detrital zircons ages of sample L1R2, L2R2 and sample L4R1 are older than 1.68 Ga, and their zircon grains are characterized by two age populations: (1) Neoarchean to early Paleoproterozoic (2.6–2.4 Ga) with a peak at ca. 2.5 Ga (2.469, 2.522, and 2.494 Ga, respectively), and (2) middle to late Paleoproterozoic (2.1–1.7 Ga) with three major age peaks at 1.812 Ga, 1.875 Ga, and 1.965 Ga in L1R2, four peaks at 1.995 Ga, 1.962 Ga, 2.047 Ga, and 1.863 Ga in L2R2, and two peaks at 1.836 Ga, and 1.952 Ga in L4R1. The ca. 2.5 Ga detrital peak is more abundant in sample L2R2 (Fig. 11). Although the detrital zircon ages show a wide range in samples G1R1, G7R1, and G8R2, they possess a similar distribution of age frequencies (Fig. 11). Generally, six main age ranges (1000–900 Ma, 2600–2400 Ma, 900–700 Ma, 700–500 Ma, 1300–1100 Ma and 2000–1500 Ma) could be identified in all samples. Sample G7R1 has more ca. 2.5 Ga detrital zircons than in others. The youngest ages of zircons from the samples are 503 ± 8 Ma (G1R1),

10000

Sandaokan Formation L1R2

1000

100

100

10

10

1

1

0.1

0.1

0.01

0.01

10000

10000 1000

Sandaokan Formation L2R2

1000

100

100

10

10

1

1

0.1

0.1

0.01

0.01

10000

10000 1000

Sandaokan Formation L4R1

1000

100

100

10

10

1

1

0.1

0.1

0.01

Wulalike Formation G1R1

Wulalike Formation G7R1

Wulalike Formation G8R2

0.01 La Ce

Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er

Tm Yb Lu

Fig. 9. Chondrite–normalized REE patterns for the zircons from the Sandaokan and Lashizhong formations. The chondrite–normalization values are from Sun and McDonough (1989).

Z. Wang et al. / Gondwana Research 40 (2016) 58–76

67

D

A 1.0

L1R2

0.6

G1R1

3000

3800

238

238

Pb/

3000

0.6

206

206

2200

0.4

3400

U

2600

0.5

Pb/

U

0.8

1200 0.20

2600

1000

2200

0.4

0.16

1800 1800

800

0.12

0.3

600

1400

0.2

0.08

400

1400 0.04 0.2

0.2

0.6

1.0

1.4

1.8

2.2

0.0

0

4

8

12

207

Pb/

235

16

20

0

20 207

B 0.6

10

U

30

Pb/

235

40

U

E L2R2

G7R1

0.8

238

U

2600

0.5

0.4

3000

0.6

2600

Pb/

2200

206

206

Pb/

238

U

3400

1200 0.20

2200

0.4

1000 0.16

1800 1800

800 0.12

1400

0.3

600

0.2 0.08

400

1400 0.04 0.2

0.6

1.0

1.4

1.8

2.2

0.0

0.2 0

4

8

12

207

Pb/

235

0

16

10

20 207

U

Pb/

30 235

40

U

F

C 1.0

L4R1

0.6

G8R2 3800 3400 238

U

2600

0.5

0.4

3000

0.6

Pb/

2200

206

206

Pb/

238

U

0.8

2600

1200 0.20

1000

2200

0.4

0.16

1800

1800

800 0.12

1400

0.3

600

0.2 0.08

400

1400 0.04 0.2

0.0

0.2 2

4

6

8

10 207

Pb/

12 235

14

16

0

10

207

U

0.6

20

1.0

30

Pb/

235

1.4

1.8

2.2

40

U

Fig. 10. U–Pb concordia diagram of detrital zircon data. (A)–sample L1R2, (B)–sample L2R2, (C)–sample L4R1, (D)–sample G1R1, (E)–sample G7R1, (F)–sample G8R2. Samples L1R2, L2R2 and L4R1 are from the Sandaokan Formation and samples G1R1, G7R1and G8R2 are from the Lashizhong Formation.

488 ± 7 Ma (G7R1), and 508 ± 8 Ma (G8R2), and oldest Archean zircons (N3.5 Ga) are found in all the three samples (Supplementary Table 1). Notably, the age distributions of the three samples from the Sandaokan Formation are significantly different from those from the Lashizhong Formation, suggesting a major provenance change during the Middle to Late Ordovician (ca. 464–458 Ma).

6. Discussion 6.1. Provenance 6.1.1. The Sandaokan Formation In the western NCC, the highlands exerted important influence on regional sedimentary patterns and the nature of the lithofacies during

68

Z. Wang et al. / Gondwana Research 40 (2016) 58–76 35

A

1812 20

E

986

L1R2,N=117

G1R1,N=147 Relative probability

1875

30

25

Number

15

816 20

1965 2469 10

506

15

1182 1296 2146 1614 2514 1724 2800

10

2231 2171 2326

5

1110

560

3238 3588

5

2906 0 18

0

974

B

2522

16

L2R2,N=100

14

Number

1995

802

15

1962

10

580

2047

1152

8

514

10

1863

1636

6

2070 4

F G7R1,N=128 Relative probability

12

2536

20

1715

2197 2333

2706

486

5

2804

3338 3122 3540 2820

2

0

0

20

G

25

C

1836

992

G8R2,N=145 Relative probability

L4R1,N=129 20

Number

2494 808

15

2528

1086

15

644 1174

1952

10

526

10

1718

506 5

2199Ma

0

D

1831

H

80

L1R2+L2R2+L4R1 N=346 2505

986

70

G1R1+G7R1+G8R2 N=420

50

1961

648 602 806 552

30

40 30

20

2330 10

1106 2530 1620 1294 2050

510

20

2199

2804

3536

Relative probability

60

40

Number

3626

0

50

0 1600

3532

5

2696 2775

3124

10 0

1800

2000

2200

2400

2600

2800

3000

3200

Age (Ma)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Age (Ma)

Fig. 11. Histograms and relative probability density diagram of ages with concordance between 90% and 110% for the analyzed samples. (A)–117 analyses of sample L1R2, (B)–100 analyses ofsample L1R2, (C)–129 analyses of sample L4R1, (D)–145 analyses of sample G1R1, (E)–128 analyses of sample G7R1, and (F)–145 analyses of sample G8R2.

Ordovician (Myrow et al., 2015). The Zhuozishan Mountain area was proximal to the ancient Yi–Meng paleohighland (Fig. 2). Lithofacies paleogeography shows that it was an open platform–ramp basin during the Early and Middle Ordovician in the western Ordos Block (Chen et al., 2014).

Ages of detrital zircon populations are unaffected by fractionation processes within the sedimentary cycle and therefore provide a direct fingerprint for the identification and locations of sources (Cawood and Nemchin, 2000). Ages of samples of the Sandaokan Formation were

Z. Wang et al. / Gondwana Research 40 (2016) 58–76 Main age populations: 2.06, 1.94, and 1.89 Ga Xia et al., 2006b

110°E

Yinshan Block

1.89-1.95 Ga,~2.45 Ga; Liu et al.,2013 2.4Ga-2.7Ga Ma et al.,2013 2.32-1.84 Ga Xia et al.,2006a

1.83Ga-2.97Ga Jiao et al.,2013

2.0Ga-2.75Ga Wang et al.,2015c

2.00-2.56 Ga, 1.83-1.96 Ga; Dong et al.,2012 2.3-2.0 Ga Yin et al.,2009

Khondalite Belt

Datong

Western Block

an

Qianlishan

Postulated detrital flow path 1.90-1.95 Ga, 2.40-2.55 Ga; Dong et al., 2014

2.02-2.45Ga Qiao et al.,2016

80

Late Archean low- grade TTG gneisses and greenstones

Ga Ga

40

.95

.82

2204

Paleoproterozoic khondalites and granulites

A

~1

1812

E

20

L 1R2 , Sandaokan Formation, N=117

1875 Number

Khondalites in the Helanshan -Bayanhulashan -Qianlishan, N=940

Relative probability

Number

~1

B

1900

120

Ordos Block 110°E

~1 .8 ~1 2 Ga .95 Ga ~2 .20 ~2 .33 Ga G ~2 .50 a Ga 160

Late Archean high-grade TTG gneisses and granulites

1.70-2.8 Ga Darby and Gehrels, 2006

2.85-2.53 Ga, 2.2-2.0 Ga Yin et al., 2011

15

1965

2469

10

2231

5

2171

16

12

4

2326

40

2333 1715

2706

2197

2804

0 1836

L4R1, Sandaokan Formation, N=129

2494 15

10

1952

5

2199

2615

6 5 Number

80

1863

2

Relative probability

Number

120

20

Number

Khondalites in theJi’ning area, N=385

8 6

F Relative probability

160

L2R2, Sandaokan Formation, N=100

1995 1962 2047

10

0

C

1888

H

2522

14

2906 0

40°N

1.793-2.502 Ga Cai et al., 2015

1.92-2.23Ga Qiao et al.,2016

2.0-2.1Ga,2.52-2.95Ga Zhou and Geng, 2009

1.89Ga-2.40Ga Santosh et al.,2007

~1 .82 G ~1 .95 a Ga ~2 .2 ~2 0 Ga .33 Ga ~2 .50 Ga

a el

h ns

n

Number

H

n

ha

Relative probability

ya

as

.2 ~2 0 Ga .3 ~2 3 Ga .50 Ga

Ba

l hu

Ji’ning

~2.5Ga, ~2.45Ga, ~1.90Ga;Ma et al.,2015

1.75-2.45 Ga Dan et al.,2012

40°N

GuyangWuchuan Daqingshan-Ulashan

~2

Alxa

th n or ge -N ro ns a O a Tr h i n C

Relative probability

1.76-2.75 Ga Dan et al.,2012

1899

I

Early Ordovician sandstone N=32

1945 4 3 2

2696

2708

2599

2073

Relative probability

1.80-2.46 Ga Wu et al.,2014

69

1

2775

2088

2309

2168

50 0 1600

1998

20

2057

Upper Proterozoic -Cambrian rocks N=171 2655

15

2551 2687 2365 2770 2198

10 5

2000

2400

Age (Ma)

2800

3200

1718 0 1600

J

2000

2400

Age (Ma)

2800

3200

10

2514

1813

8

SBGM-1, Sandaokan Formation, N=53

6 4 2 0 1600

1922 1963

2455

Relative probability

1851

1947

Number

1944

25

G Relative probability

Number

Khondalites in the WulaDaqing shan Mountains, N=1275

Number

250

100

D

2515

Relative probability

2449

200

0

0

0

2361

2085

2000

2400

2800

3200

Age (Ma)

Fig. 12. Tectonic subdivision of the Western Block of the North China Craton (Zhao et al., 2005) and age data of different geological units in the EW–trending Khondalite Belt related to this study (A); Compilation of zircon age distributions for khondalites in the Helanshan–Bayanwulashan–Qianlishan Mountains (B) (Darby and Gehrels, 2006; Yin et al., 2011; Dan et al., 2012; Wu et al., 2014; Qiao et al., 2016), khondalites in the Ji'ning area (C) (Xia et al., 2006b; Santosh et al., 2007;Jiao et al., 2013), khondalites in the Daqingshan–Ulashan Mountains area and gneisses and supracrustal rocks in the Yinshan Block (D) (Jian et al., 2005, 2012; Xia et al., 2006a; Dong et al., 2012; Liu et al., 2013; Ma et al., 2013; Dong et al., 2014; Cai et al., 2015; Ma et al., 2015; D. Wang et al., 2015), sample L1R2 from the Sandaokan Formation (E) (this study), sample L4R1 from the Sandaokan Formation (F) (this study), Upper Proterozoic–Cambrian rocks in the Zhuozishan Mountain (G) (Darby and Gehrels, 2006), sample L2R2 from the Sandaokan Formation (H) (this study), Early Ordovician sandstone(I) (Darby and Gehrels, 2006), and sample SBGM–1 from the Sandaokan Formation (J) (Myrow et al., 2015). Data details are given in Supplementary Table 2.

70

Z. Wang et al. / Gondwana Research 40 (2016) 58–76

previously reported by Darby and Gehrels (2006) and Myrow et al. (2015). Darby and Gehrels (2006) suggested that the source of the zircon grains in the Sandaokan Formation is most likely the exposed parts of the NCC crystalline basement or recycled older sedimentary units. Myrow et al. (2015) suggested the source of detritals as ca. 2.5 Ga trondhjemite–tonalite–granodiorite(TTG) gneisses and supracrustal rocks in the Yinshan and the Khondalite belts. The Khondalite Belt is located in the Western Block of the NCC and is exposed in the Helanshan– Bayanwulashan–Qianlishan Complex, the Daqingshan–Ulashan Complex, and the Ji'ning Complex (Fig. 12A), with different age distributions (Fig. 12B,C,D, Supplementary Table 2). The western (Helanshan– Qianlishan Mountains) and eastern (Ji'ning) belt show main age peaks at ~ 2.2 Ga, 1.90 Ga, and ~ 1.89 Ga (Fig. 12B,C). However, the central belt (Daqingshan–Ulashan Mountains) yields dominant peaks of U–Pb ages at ~2.5Ga, ~1.85 Ma, ~1.95Ga, ~2.30 Ma, and ~2.20 Ga (Fig. 12D). In our study, detrital zircon age spectra from samples L1R2 and L4R1 of the Sandaokan Formation (Fig. 12E,F) yielded a similar age distribution with those of the khondalites in the Wulashan–Daqingshan Mountains area (Fig. 12D), which are characterized by age peaks at ~1.82 Ga,~2.50 Ga, ~1.95 Ga, ~2.33 Ga, and ~2.20 Ga. However, they differ from those of the samples reported by Darby and Gehrels (2006) and Myrow et al. (2015) in some detail. The Late Proterozoic to Cambrian

(1)(2)(3) (4)

L

(1)-North India Orogen (530-470 Ma)

478

Mid-Upper Ordovician in the Hexi-Corridor Basin n=261

960

(4)-the Wilkes–Albany–Fraser belt (1330–1130 Ma)

1638

K

Southwestern NCC Upper Ordovician Pingliang Formation n=79

976

Dahuangshan Formation southern Alxa Block n=443 2482

529

the North China Craton

D

1949

Western Ordos, NCC Mesoproterozoic-Ordovician n=171

J

1936

I

Detrital zircons in the NCC n=1308

2082

963

Xiangshan Group Mid-Late Cambrian?Early Ordovician n=593

798 530

2597

C

1741

912

Mid-Ordovician Miboshan Formation in the eastern Alxa Block n=159

952 510

3325

2495

2507

596 2530

B

3319 540

2178

Western NCC, Upper Ordovician n=421, this study

986

Xiaoluoshan, Mid-Ordovician Miboshan Formation n=196

1176

O

531

500

974

1500

2000

2500

3000

Age(Ma)

3500

Perth Basin, western Australia Ordovician successions, n=45 2449

1111

4000 4500 0

2503

986

N

Ordovician, Himalaya, northern India, n=773

1166 473

M

Niushoushan, Mid-Ordovician Miboshan Formation n=128

476

2572 2509

Ordovician, South China n=261

962

822

2502 1126

1620

1000

2471 1800

538

1636

1555 0

Qiangtang, Ordovician n=231

941 529

1753

989

565

2530 510

P

2404

1680

G

Pan-African Orogenic Events (650 – 500Ma)

(1)(2)(3) (4)

554 844

1825

A

986

H

2506 Magmatism and metamorphism in the NCC, n=325

Grenville Orogenic Events (1350–900 Ma)

Eastern and southern Alxa Block

2482

451

(3)-the Rayner–Eastern Ghats belt (990–900 Ma)

2500

836 1165

(2)-the Prydz–Darling orogenic belt (500 – 600 Ma)

East Gondwana

Qilian Orogenic Belt (n=814)

848

E

6.1.2. The Lashizhong Formation Detrital zircon spectra for sample G1R1, G7R1, and G8R2 from the Lashizhong Formation are in striking contrast to those of the Sandaokan

(1)(2)(3) (4)

444

QOB

F

strata lack the main peaks at ~ 1.82 Ga and ~ 2.50 Ga (Fig. 12G), and the Early Ordovician (Sandaokan Formation) reported by Darby and Gehrels (2006) with main peaks at ~2.33 Ga and ~2.50 Ga (Fig. 12I). Sample SBGM–1 from the Sandaokan Formation lacks age peaks at ~2.33 Ga and ~2.20 Ga (Fig. 12J). Moreover, the age distribution of zircon grains in the sample L2R2 is more complex with multiple age peaks, among which the minor peak ages between 2.2 Ga and 1.7 Ga do not match. From the above results, it is inferred that, the Late Archean TTG gneisses and granulites in the Yinshan Block provided one of the potential sources (Myrow et al., 2015). Our new data further suggest that the Sandaokan Formation was mainly derived from the khondalites in the Daqingshan–Ulashan Mountains area, and the more proximal Helanshan–Qianlishan– Bayanwulashan Complexes did not provide sediments for this formation. This indicates that the Daqingshan–Ulashan Mountains area remained as a highland above sea water after the earliest Middle Ordovician eustatic fall (Myrow et al., 2015). The Helanshan–Qianlishan–Bayanwulashan Mountains and Ji'ning area could have been buried at that time, and therefore could not provide detritus during the Middle Ordovician.

500

1000

1500

2000

2500

3000

Age(Ma)

3500

4000

4500

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Age(Ma)

Fig. 13. Summary of zircon age distributions of samples from reference regions: the Lashizhong Formation in the Zhuozishan Mountain, northwestern NCC (A) (this study); magmatic and metamorphic zircons in the NCC (B) (Xu et al., 2010a and their Fig. 6F); detrital zircons in the NCC (C) (Xu et al., 2010a and their Fig. 6E); Mesoproterozoic–Cambrian sedimentary rocks from the Zhuozishan Mountain area (D) (Darby and Gehrels, 2006); the Upper Ordovician Pingliang Formation on the southwest margin of the NCC (E) (Yang et al., 2015); the Qilian Orogenic Belt (F) (B.H. Zhang et al., 2016 and their Fig. 12A); the Middle Ordovician sandstones in the Niushoushan Mountain area in the eastern Alxa Block (G) (Zhang et al., 2011); the Middle Ordovician sandstones in the Xiaoluoshan Mountain area in the eastern Alxa Block (H) (Zhang et al., 2011); the Middle Ordovician Miboshan Formation in the eastern Alxa Block (I) (Zhang et al., 2012); the Early Paleozoic Xiangshan Group in the southeastern Alxa Block (J) (Zhang et al., 2015); the Early Paleozoic Dahuangshan Formation in the southern Alxa Block (K) (B.H. Zhang et al., 2016); the Middle–Upper Ordovician strata in the Wuwei City, southern Alxa Block (L) (Xu et al., 2010b); the Ordovician in the southern South China (M) (Xu et al., 2014); the Ordovician in the Himalaya, northern India (N) (Gehrels et al., 2006a, 2006b; Cawood et al., 2007; McQuarrie et al., 2008; Myrow et al., 2010; Long et al., 2011); the Ordovician in the Perth Basin, western Australia (O) (Cawood and Nemchin, 2000); and the Ordovician in Qiangtang (P) (Dong et al., 2011; Zhu et al., 2011). All data have concordance above 90%. N = total number of age data. Ages greater than 1000 Ma calculated using 207Pb/206Pb ratios, and ages less than 1000 Ma calculated from 206Pb/238U ratios. Data details are given in Supplementary Table 3.

Z. Wang et al. / Gondwana Research 40 (2016) 58–76

Formation (Fig. 11D,H), which suggest that the provenances for the sedimentary rocks in the northwestern NCC changed significantly in a short time during the Middle to Late Ordovician. Darby and Gehrels (2006) suggested a major change in the provenance of sediments between the Cambrian and Ordovician, which is associated with the Cambrian– Ordovician unconformity in the NCC (Myrow et al., 2015). However, the time of the tectonic transformation event revealed by our study is different from that presented by Darby and Gehrels (2006) and Myrow et al. (2015). The position of the tectonic transformation event presented in this study is close to the central Wulalike Formation, ca. 458 Ma, whereas Myrow et al. (2015) placed this event between the Abuqiehai and Sandaokan formations (latest Cambrianto earliest Ordovician). There is a marked difference between the age spectra ofthe NCC and the Lashizhong Formation (Fig. 13A–E). No Neoproterozoic and Cambrian zircons appear in the NCC, and no age peak appears between 1900 Ma and 1800 Ma inthe age spectrum of the Lashizhong Formation (Fig. 13B,C,D). Zircon grains analyzed from the Upper Ordovician sandstone in the southwestern NCC shows only a narrow peak at 451 Ma (Fig. 13E). Thus, the NCC was excluded as the provenance of the Lashizhong Formation. Moreover, the Proterozoic zircons, especially the Paleoproterozoic and Mesoproterozoic grains are uncommon in the Qilian Orogenic Belt (Fig. 13F), and only one zircon with age between 500 and 400 Ma was identified in the Lashizhong Formation, which excludes the Qilian Orogenic Belt as a probable provenance for the Lashizhong Formation. Recently, many studies have reported detrital zircon ages from the Cambrian–Ordovician strata on the eastern and southern fringe of the Alxa Block, which shows similar age spectra and peaks (ca. 960–990 Ma, ca. 490–510 Ma, ca. 1080–1180 Ma, and ca. 2.5 Ga) (Fig. 13G–L). There is increasing evidence for marked differences between the Alxa Block and the NCC, ranging from the Precambrian geology (Dan et al., 2014) to detrital zircon age distributions during the Paleozoic (Zhang et al., 2011; Dan et al., 2014; Yuan and Yang, 2015b). Thus, the Alxa Block was regarded as an independent block during the early Paleozoic (B.H. Zhang et al., 2016).We therefore do not consider the Alxa Block as the principal provenance for the Lashizhong Formation, although the age spectra of the Lashizhong Formation are very similar to those of the eastern and southern domains of the Alxa Block. Moreover, we place the contact between the Alxa and the NCC along the Western Ordos fault and link it with the Bayanwulashan fault to the north (Fig. 3), consistent with the work of Zhang et al. (2011) and Yuan and Yang (2015a) which shows that the Western Ordos fault runs along the east side of the Xiaoluoshan, Niushoushan, and Helanshan Mountains (Fig. 3).This means that the samples collected by Zhang et al. (2011, 2012) are from the Alxa Block, which can not represent the sediments of the NCC. In addition, the age spectra of the Lashizhong Formation are similar to those of the Ordovician formations in the southern part of South China (Fig. 13M), the Himalayan segment in northern India (Fig. 13N), the Perth Basin in western Australia (Fig. 13O), and the Qiangtang region in China (Fig. 13P). They show similar age spectra and peaks, which suggest similar provenances. Mckenzie et al. (2011b) and Myrow et al. (2010) suggested that the Eastern Ghats–Rayner orogenic belt in the eastern Gondwana was the main provenance of the detritus shed onto the North Indian and the Himalayan regions, and possibly also the provenance for Cambrian strata in South China (Xu et al., 2013). Yuan et al. (2012) argued that the provenances of the Xiangshan Group in the Niushoushan Mountain region in the eastern Alxa Block were linked to the Albany–Fraser orogenic belt and the Yilgarn Craton in the western Australia, and Zhang et al. (2015) compared the age spectra of the Xiangshan Group in the southeastern Alxa Block with those of the western Australia, North India, the Qiangtang Block and South China, and also suggested that the predominant sources of the Xiangshan Group were the eastern Ghats–Rayner and Wilkes–Albany– Fraser orogenic belts in the eastern Gondwana, rather than the NCC,

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the Alxa Block and the Qilian Orogenic Belt. Based on the similar age spectra of the Xiangshan Group and Dahuangshan Formation, B.H. Zhang et al. (2016) proposed that they shared the same provenance. Remarkable similarity in age spectra are also revealed when comparing the Lashizhong Formation (Fig. 13A) and the Xiangshan Group (Fig. 13M) or the Dahuangshan Formation (Fig. 13N). These features suggest that the Neoproterozoic orogenic belts (the Eastern Ghats– Raynerand Wilkes–Albany–Fraser orogenic belts) in the eastern Gondwana may also be the main provenances of the Lashizhong Formation. The cumulative probability spectra of measured crystallization ages for detrital zircon grains relative to the depositional ages of the Lashizhong, Dahuangshan formations and the Xiangshan Group are also compared (Fig. 14), with a view to identify the possible tectonic settings in which a succession is deposited (Cawood et al., 2012). All samples of the Lashizhong Formation lie in the transition zone between the collisional and extensional settings (Fig. 14).Because the youngest 5% of the detrital zircons with crystallization age (CA)– deposition age (DA) younger than 150 Ma constitute only a minor population, an extensional setting can be excluded, and the Lashizhong Formation is indicated to have been deposited in a collisional setting. However, the Sandaokan Formation clearly formed in a passive setting, which is in line with the evidence of sandstone lithofacies, sedimentary structures of the Sandaokan Formation (Myrow et al., 2015; Jing et al., 2016b). This implies a prominent event of provenance change between the two formations, which accompanied a tectonic shift from a passive setting to a collisional one. The three sandstone samples from the Lashizhong Formation represent the early sedimentation of the collisional regime. The transition to collision could have taken place at some point during deposition of the underlying shale of the central Wulalike Formation, which has been well constrained by the top of conodont biozone Pygodus Anserinus, which is ca. 458 Ma (Fig. 4).

1.0

A 0.8

Dahuangshan Fm. Xiangshan Gp. Lashizhong Fm.

0.6

Samples of Lashizhong Fm. (G1R2, G7R1 and G8R1) Sandaokan Fm. Samples of Sandaokan Fm. (L1R2, L2R2 and L4R1)

0.4

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Fig. 14. Cumulative probability curves of measured crystallization ages for detrital zircon grains relative to the depositional age of samples from the Sandaokan, Lashizhong, Dahuangshan formations and Xiangshan Group. The base figure is modified from Cawood et al. (2012). Convergent basin (A, pink), collisional basin (B, light blue), and extensional basin (C, light green).

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6.2. Paleogeographic model

The main age ranges (1000–900 Ma, 700–500 Ma, and 1300–1100 Ma) of detrital zircons from the Lashizhong Formation correspond to the Rayner–Eastern Ghats belt (990–900 Ma), located between East Antarcticaand eastern India; the Prydz–Darling orogenic belt (600–500 Ma) in the interior of Antarctica; and the Wilkes– Albany–Fraser belt (1330–1130 Ma) between Antarctica and the southwestern Australia, respectively. These three Neoproterozoic and Mesoproterozoic orogenic belts originated from the assemblage of the eastern Gondwana based on their ages, its northern part received common siliciclastic sediment from these three orogenic belts (Zhang et al., 2015). It should be noted that ca. 500 Ma detrital zircon age peaks (the youngest age peaks are 506 Ma, 486 Ma, and 506 Ma, respectively) were found in the three samples from the Lashizhong Formation. The abundance of ca. 500 Ma zircon grains was interpreted as a shared Cambrian tectonic event in the East Gondwana continents (McKenzie et al., 2011a; Myrow et al., 2015). Our study provides clear imprints from the northwestern NCC on the final assembly of the Gondwanaat ca. 500 Ma. Combined with the provenance information, we suggest a direct tectonic link between the western NCC and the East Gondwana. As mentioned above, the Lashizhong Formation records tectonic transition to a collisional setting in the early Late Ordovician from a passive margin setting (Fig. 4), which would require that an orogenic belt developed to the west of the western boundary of the NCC, as has been recently been inferred (Dan et al., 2014, 2016). Dan et al. (2016) suggested that this suture zone recorded the amalgamation of the Alxa Block and NCC, and further suggested that the Helanshan Mountain area probably constituted a part of a foreland basin system

Paleozoic reconstructions for the NCC and Gondwana have mostly relied on sparse paleomagnetic and biogeographic data and, as a result, have yielded contrasting paleogeographic models. The NCC has been assigned a variety of locations relative to Gondwana in various early Paleozoic paleogeographic reconstructions, although there is increasing evidence that the NCC had a close affinity to the northern East Gondwana margin during the Middle Cambrian and Ordovician (Zhao et al., 2007; McKenzie et al., 2011a; Metcalfe, 2013; Han et al., 2015; Myrow et al., 2015). McKenzie et al. (2011a) placed the NCC connected to, and sharing a detrital belt with, the northern margin of India during the Cambrian. Han et al. (2015) suggested that the southern NCC likely collided with the northern Australia margin of the East Gondwana at ca. 500 Ma. McKenzie et al. (2011a) proposed that Gondwana–derived strata were shed onto the southern margin of the NCC, but the western NCC (Ordos region) was isolated from such sediment. However, Ordovician vertebrates fossils in the Zhuozishan Formation in the Zhuozishan Mountain area suggest the proximity of the western NCC to the Gondwana in the Ordovician (Wang and Zhu, 1997). Myrow et al. (2015) regarded the timing and duration of the Cambrian–Ordovician unconformity in the western NCC, and compared it with those in the North India regions, as a potential paleogeographic link of the NCC to the northern Indian margin of the eastern Gondwana. They proposed that the western NCC was an along–strike continuation of the northern Indian continental margin. These hypotheses, however, must be tested, particularly using geochronologic data.

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Grenville age provinces Peri-Gondwana terranes Terra-Australis Orogen Infered transport direction of detritus Location of samplings: Gondwana-derived Location of samplings: not Gondwana-derived

Fig. 15. Reconstruction of the eastern Gondwana during the Middle to Late Ordovician. Orthographic projection, outlines of major blocks/cratons and locations are mostly modified after Burrett et al. (2014), and Wang et al. (2016).

Z. Wang et al. / Gondwana Research 40 (2016) 58–76

formed during the collision between the Alxa Block and the NCC. However, B.H. Zhang et al. (2016) proposed that the southern and eastern parts of the Alxa Block also received sediments from the eastern Gondwana during the early Paleozoic. Our new data indicate that the orogenic belt should be placed between the NCC and Gondwana, instead of the Alxa Block (Fig. 15). Recent geological and paleomagnetic studies indicate that the Alxa block was not part of the NCC until the Middle to Late Permian (Yuan and Yang, 2015b; J. Zhang et al., 2016). This means that the Alxa Block may be an isolated continental block close to the NCC instead of a direct connection, and both of them share the same provenance from East Gondwana in the Ordovician (Fig. 15). Paleomagnetic investigations indicate that the NCC was located in the low latitudes, roughly at 18°S (Scotese et al., 1979), ~17°S (reference points at 39.7°N, 118.5°E, Yang et al., 2002), 10.6°–14.4°S (Huang et al., 1999) or 9.2°–14.2°S (reference points at 37.5°N, 110.3°E, Yan and Zhang, 2014) in the Ordovician. Recently, based on the positions of Plume Generation Zones, paleomagnetic data, lithofacies analysis, and biogeographical information, Wang et al. (2016) suggested a revised paleoposition of the NCC during the Middle Ordovician, specifically a paleolatitude of approximately 16.6°–19.1°S (reference points at 35°N, 110°E), which is in the range of the paleolatitude of India, and markedly different from that of Australia (Wang et al., 2016 and their Fig.2) at this time. Burrett et al. (2014), however, suggested that the NCC may have been closer to northern Australia rather than India during the Early– Middle Ordovician, which is based on palaeomagnetic data for Australia presented by Ripperdan and Kirschvink (1992). The paleopositions of global plates were recently presented by Wang et al. (2016) based on latest paleomagnetic database (Li et al., 2014). Moreover, Yang et al. (2004) recalculated the early Paleozoic pole of Australia for the reference point at 15°S, 120°E (northwestern Australia) and showed that northwestern Australia had a paleolatitude ranging from ~ 18°S to 12°N. The mid– value was close to the equatorial region at ca. 460 Ma. In the absolute paleoposition reconstruction of the NCC during the Middle Ordovician by Wang et al. (2016), a potential position next to North India was excluded due to likely overlap with Lhasa and Qiangtang Blocks in space. This contradicts with the absolute paleoposition of these two blocks. Our new data suggest a position next to North India as reliable and reasonable. More importantly, new data from the western NCC are similar to those from North India and the Perth Basin in the western Australia. Thus, the western NCC might have been at proximal position between North India and the western Australia. Yang et al. (1999) and Huang et al. (2000) proposed that the NCC was attached to the paleo–Pacific side of Antarctica during Cambrian, until its detachment in the Early Ordovician and subsequent migration to reside outboard of Siberia. Our new data, however, provide evidence that the western NCC collided with East Gondwana at some point between deposition of the Sandaokan and Lashizhong formations, instead of moving away from East Gondwana (Wang et al., 2016). The Gondwana began to rotate clockwise at ca. 460 Ma (Torsvik and Voo, 2002; Torsvik et al., 2012; Li et al., 2014; Wang et al., 2016), and the NCC rotated counterclockwise at that time (Huang et al., 2000). McKenzie et al. (2011a), Myrow et al. (2015) and Han et al. (2015) suggested that the southern margin of the NCC had a paleogeographic continuity with Gondwana. If this is true, the relative rotational movement with respect to each other may have resulted in the southwestern or western or northwestern NCC being attached to Gondwana. The detrital zircon spectra for the Upper Ordovician units from the southwestern (Fig. 13E) and northern NCC (Wang et al., 2016, unpublished data) is different from those of the East Gondwana, which suggests that only the northwestern margin of NCC was connected with the eastern Gondwana (Fig. 15). 7. Conclusion This study investigates the paleogeographic position of the NCC with respect to the Gondwana supercontinent during the Middle to Late

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Ordovician through detrital zircon geochronology of Middle–Upper Ordovician sandstone units from western NCC. The Middle Ordovician Sandaokan Formation was derived from Archean and Paleoproterozoic sources within the NCC, with ages of detrital zircons ranging from 1.68 Ga to 3.0 Ga. Detrital zircon grains from the Upper Ordovician Lashizhong Formation were mainly derived from the eastern Ghats– Rayner and Wilkes–Albany–Fraser orogenic belts in eastern Gondwana, and show magmatic ages with a wide range from 0.48 Ga to 3.63 Ga that are distinctly different from the detrital zircon data of the Sandaokan Formation. The Alxa Block, Qilian Orogenic Belt, and NCC are excluded as the main sources for the Lashizhong Formation. The marked distinction in detrital zircon age spectra of the two formations suggests a major provenance shift during the Middle (ca. 464 Ma) to Late Ordovician (ca. 458 Ma). The Sandaokan Formation was deposited on a passive continental margin on the northwestern NCC, whereas the Lashizhong Formation formed in a collisional setting, with an orogenic belt situated to the west of the NCC. The provenance of the Lashizhong Formation suggests that northwestern margin attained a possible foreland basin stage between the NCC and the eastern Gondwana. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2016.08.007.

Acknowledgements We thank Ian Metcalfe, Paul Myrow, Qiong-Yan Yang, and Associate Editor Shou-jie Liu for their comments that led to significant improvements in the manuscript. This work was financially supported by the Chinese National Basic Research 973 Program (2011CB403001). We thank Li Su, Hong-yu Zhang, Xiao-hong Huang, Xun Wang, Chunpeng Bian, Li-chao Yang, Yu-ming Bai, Zhi-jun Shen for technical support during the LA–ICP–MS U–Pb dating and data analyses.

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