Timing of Paleozoic amalgamation between the North China ......Timing of Paleozoic amalgamation between the North China and South China Blocks: Evidence from detrital zircon U–Pb

Tectonophysics 586 (2013) 173–191

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Timing of Paleozoic amalgamation between the North China and South China Blocks:Evidence from detrital zircon U–Pb ages

Yunpeng Dong a,⁎, Xiaoming Liu a, Franz Neubauer b, Guowei Zhang a, Ni Tao a, Yiguo Zhang a,Xiaoning Zhang a, Wei Li a

a State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi'an 710069, Chinab Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria

⁎ Corresponding author. Tel.: +86 29 88303028; fax:E-mail address: [email protected] (Y. Dong).

0040-1951/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2012.11.018

a b s t r a c t
a r t i c l e i n f o
Article history:Received 23 April 2012Received in revised form 22 October 2012Accepted 19 November 2012Available online 4 December 2012

Keywords:GeochronologyDetrital zircon U–Pb ageFore-arc basinForeland basinQinling orogen

LA-ICP-MS U–Pb ages of detrital zircons from clastics of the undated fore-arc sedimentary unit (FAS) along theShangdan Suture and theMiddle–Upper Devonian Liuling Group in the South Qinling belt are used to establishthe maximum depositional age and provenance of these tectonic units which were deposited on both sides ofthe Shangdan suture zone between the North China Block (NCB) and South China Block (SCB). The new dataand geological evidence show that the FAS was deposited in a fore-arc basin with an exclusive source ofthe clastics in the North Qinling Belt (NQB). The depositional age of FAS is limited by the youngest U–Pb455 Ma-ages of detrital zircons from clastics and the intrusive age of 435±7 Ma of mafic dykes betweenthe Late Ordovician–Early Silurian, while the NCB was still separated from the SCB by the Shangdan Ocean.However, detrital zircon U–Pb ages from six samples of the Middle Devonian Liuling Group indicate sourcesin both NQB and SCB suggesting pre-Middle Devonian collision of NCB and SCB. All results indicate depositionof the FAS in a forearc setting upon an active continental margin during Late Ordovician–Early Silurian, whilethe Middle–Upper Devonian Liuling Group represents a marine foreland basin after closure of the ShangdanOcean. Together with the unconformity between Middle Devonian and pre-Devonian strata, this reveals stillevolving subduction and accretion on the southern side of the NQB during the Ordovician-Early Silurian,and the Early Devonian collision between the NCB and SCB.

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1. Introduction

The amalgamation of the Pangea supercontinent is one of the mostimportant global tectonic processes, and several models for the Pangeareconstruction were suggested based on paleomagnetism and timingof the collision between the main continents. However, there are stillcontroversies on the amalgamation of the East Asian continents, espe-cially the timing of the collision between the North China Block (NCB)and South China Block (SCB) which resulted to the Qinling OrogenicBelt (QOB) (e.g. Ames et al., 1996; Enkin et al., 1992; Hacker et al.,1998; Hsü et al., 1987; Kröner et al., 1993; Li et al., 1993, 1994;Mattauer et al., 1985; Meng and Zhang, 1999; Okay and Sengör, 1993;Sengör, 1985; Wang et al., 1989; Xu et al., 1988; Zhai et al., 1998;Zhang, 1988; Zhao and Coe, 1987). Almost all the reconstructionmodelsfor the Pangea supercontinent suggest a collision between the NCBand SCB after 240 Ma (Collins, 2003; Golonka, 2007; Metcafe, 2009;Scotese, 2004; Stampfli and Borel, 2002; Torsvik et al., 2008; van derMeer et al., 2010), and various models of Late Triassic continent —

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continent collision have been proposed (Hsü et al., 1987; Li et al.,1994; Sengör, 1985; Wang et al., 1989; Yin and Nie, 1993) based onpaleomagnetic data (Enkin et al., 1992; Zhao and Coe, 1987) andultrahigh-pressure metamorphism at ~230 Ma in the easternmostQinling-Dabie belt (e.g., Ames et al., 1996; Hacker et al., 1998; Li et al.,1993; Okay and Sengör, 1993). However, some authors argued an EarlyPaleozoic age of collision (Kröner et al., 1993; Mattauer et al., 1985;Ren et al., 1991; Xu et al., 1988; Zhai et al., 1998), while a Devoniancollision model was suggested according to the Pb isotopic compositionof granitoids (Zhang et al., 1997) and the geochemistry of Devoniansediments (Gao et al., 1995).

Detrital zircon is a commonly found accessory phase in sedimentaryrocks, and its U–Pb geochronology is widely used as a powerful tool forsedimentary provenance studies (Andersen and Griffin, 2004; Fedoet al., 2003; Guan et al., 2002; Luo et al., 2004; McLennan et al., 2001;Moecher and Samson, 2006; Payne et al., 2006). The age pattern ofdetrital zircon may provide valuable information on the evolutionaryhistory of their source region (Bruguier et al., 1997; DeCelles et al.,2000; Fedo et al., 2003; Košler et al., 2002; McLennan et al., 2001;Nutman, 2001; Veevers et al., 2005; Weislogel et al., 2006; Zhanget al., 2006). Moreover, the youngest age of detrital zircon constrainsthe maximum age of deposition (Fedo et al., 2003; Nelson, 2001).

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In this study, samples of conglomerates from the fore-arc basin(FAS) to the south of the Shangdan suture and sandstones from theMiddle Devonian Liuling Group were collected for U–Pb dating ofdetrital zircons (Fig. 1). Our dataset provides new constraints on thedepositional age and nature of the provenance of the sediments,which will shed light on the timing of the convergence between theNCB and SCB, and the Paleozoic tectonic evolutionary processes ofthe QOB, as well as the amalgamation of East Asian continents withinthe Pangea supercontinent.

2. Geological setting

The Qinling orogen is located between the NCB to the north andthe SCB to the south, extending for several thousand kilometers fromthe Dabie Mountains in the east to the Qilian Mountains and Kunlun

Fig. 1. (a) Sketch map showing the tectonic division of the Qinling and the location of the sQinling belt. (c) Simplified geological map showing the sample locations in the eastern Qin

Mountains in the west. It is bounded by the Lingbao–Lushan–Wuyangfault to the north and by the Mianxian–Bashan–Xiangguang fault tothe south (Fig. 1a). Both of the boundary faults are Mesozoic–Cenozoicintracontinental thrusts, along which the Qinling belt was overthrustonto the South NCB and the North SCB, respectively. Two ophiolitic su-tures are well documented, the Shangdan suture (Dong et al., 2011a)and the Mianlue suture (Dong et al., 1999, 2004, 2011b; Li et al., 1996;Meng and Zhang, 1999; Zhang et al., 1995, 2001). The Shangdan sutureis marked by the occurrence of a mount of ophiolitic mélange and theFAS, which was formed by the closure of an Early Paleozoic oceanbetween the NCB and SCB (Dong et al., 2011a, b; Faure et al., 2001;Ratschbacher et al., 2003; Xue et al., 1996; Zhang et al., 1995, 2001).The Qinling belt itself can be divided into the North Qinling and SouthQingling belts by the Shangdan suture (Fig. 1a). These two units origi-nally belonged to the NCB and SCB, respectively. Many investigations

tudy area. (b) Simplified geological map showing the sample locations in the westernling belt.

175Y. Dong et al. / Tectonophysics 586 (2013) 173–191


suggest that the Mianlue suture was formed after the closure of theMianlue Ocean (Dong et al., 1999, 2004, 2011b; Li et al., 1996; Xuet al., 2002; Zhang et al., 1995, 2001). It was subsequently overprintedby the southward overthrust of the Qinling belt along the Mianxian–Bashan–Xiangguang fault during the Late Jurassic–Early Cretaceous(Dong et al., 2008; Zhang et al., 2001).

The Shangdan suture is well documented to represent the mostimportant boundary between the NCB and SCB (Dong et al., 2011a,b;Mattauer et al., 1985; Xu et al., 1988; Zhang et al., 1989, 1995, 2001).The southern Qinling belt originally belonged to the SCB before theDevonian, and was separated from the SCB as a micro-continent duringMiddle Devonian to Middle Triassic times because of the opening ofthe Mianlue Ocean (Li et al., 1996; Xu et al., 2002; Zhang et al., 1995,2001). The southernQinling belt (SQB) consists of a pre-Sinian basementwhich is unconformably overlain by Sinian and Phanerozoic sedimen-tary rocks, including a Sinian clastic–carbonate sequence, Cambrian–Ordovician limestones, Silurian shales, Devonian–Carboniferous clasticswith limestone interlayers, and minor Permian and Lower Triassic sand-stoneswhich only exposed in the Jinjiling area (Zhang et al., 2001). Aboveall, detailed mapping revealed that the FAS and the Middle DevonianLiuling Group are developed, from north to south, along the northernmargin of the SQB.

3. Stratigraphic successions of FAS and Liuling Group

3.1. FAS

The FAS unit is bounded by the Shangdan suture zone (SDS) in thenorth and the Maanqiao-Mianyuzui ductile shear zone (MMSZ) in the

Fig. 2. Images showing the conglomerates and meta-sandstones interlayered with marblesmarble within conglomerates; (c and e) conglomerate with marble; and (f) metasandstone

south (Fig. 1b, c) and is mainly composed of metamorphosed clasticrocks and carbonate rocks (Fig. 2). These include mainly highly de-formed garnet–amphibolite, micaschist, biotite–quartzite with somelenticular calcite marbles in the Shangnan-Danfeng area, east Qinling(Fig. 1b), which are interpreted to represent a fore-arc accretionaryprism. In comparison, the Heihe area (Fig. 1c) is characterized by theoccurrence of metamorphosed conglomerate (Fig. 2a–c), sandstone(Fig. 2d), and carbonate (Fig. 2e, f) sequences from bottom to top, rep-resented by the Shaliangzi and Hubaohe sections. In the Shaliangzisection, the apparent thickness of the conglomerates is up to 50 m,and the upper part of the sequence with sandstones is more than200 m thick. The conglomerates mostly expose highly deformed elon-gated granitic pebbles and boulders of sizes ranging from several cen-timeters to about 30 cm (Fig. 2a), which are documented to originatein the NQB (Meng et al., 1994; Zhang et al., 1997). Among the pebbles,the fine-grained matrix recorded greenschist to amphibolite faciesmetamorphism indicated by the occurrence of new grown amphiboleand biotite (Fig. 2a). The Hubaohe section mainly exposes, frombottom to top, conglomerates (Fig. 2b, c), coarse-grained sandstoneswith pebbles (Fig. 2d), and highly deformed sandstones with inter-layers of marbles (Fig. 2e, f). The boulders are mostly granite, diorite,quartzite, gneiss, amphibolite and minor marbles (Fig. 2b–d). Theseconglomerates were previously considered as typical post-orogenicmolasse (Mattauer et al., 1985). Based on the investigation of the con-glomerates outcropping as two intercalations within sandstones andvolcanic rocks, the FASwas argued to represent a fore-arc sedimentaryprism (Meng, 1994; Meng et al., 1994, 1997; Yu and Meng, 1995).However, the depositional age of this unit has been disputed untilthe present study because of the lack of fossils.

of the FAS rocks: (a, b and d) elongated pebbles and boulders of granite, quartzite ands interlayer with marbles.

image of Fig.�2

Table 1Stratigraphy, petrography and zircon characteristics of the sediments in the Southern Qinling.

Strata Lantitude Longatitue Sample Protolith Petrology Youngestage

Age clusters Th/U

FASHubaohesection

33°52.899’

108°06.201'

08XWJ03-2 Sandstone Meta-sandstone, green-color, coarse-grain with clear sedimentary beddings,low-greenschist facies metamorphism

466 Ma 498 Ma 0.30–1.57

Composed mainly of quartz (40–45%), feldspar (20–25%), chlorite (25–35%), biotite (5%)08XWJ02-2 Sandstone Meta-sandstone, multicolor, fine-grain, low-greenschist facies metamorphism 454 Ma 495 Ma 0.21–1.71

Composed mainly of quartz (30–45%), feldspar (20–25%), chlorite (20–25%), epidote (5–10%),calcite (5%), and ilmenite (b5%)

08XWJ02-3 Sandstone Meta-sandstone, brown to gray-color, coarse-grain with clear sedimentary beddings,low-greenschist facies metamorphism

465 Ma 528 Ma 0.35–1.31

Composed mainly of feldspar (40–45%), quartz (30–40%), biotite (5–10%),epidote (5%),and calcite (5%)

08XWJ03-1 Sandstone bearinggravel

Meta-sandstone, green-color, coarse-grain sandstone interlayered with fine-grainconglomerates with clear sedimentary beddings, low-greenschist facies metamorphism

461 Ma 510, 848, 2316 Ma 0.33–1.15

Composed of quartz (35–40%), feldspar (20–25%), chlorite (20–25%), epidote (5%),and gravels (b10%)

Shaliangzisection

33°53.663’

107°59.251’

10SHL-01 Sandstone Meta-sandstone (Biotite quartze schist), gray-color, greenschist facies mtemorphismwith clear sedimentary beddings

455 Ma 466 Ma 0.52–1.17

Composed of quartz (40–45%), feldspar (10–15%), biotite (25–30%), and chlorite (10–15%)10SHL-02 Sandstone bearing

gravelMeta-sandstone (Biotite amphibole schist), dark-color, low-amphiblitic facies mtemorphismwith clear sedimentary beddings and gravels

462 Ma 485 Ma 0.47–1.18

Composed of amphibole (45–50%), feldspar (20–25%), quartz (15%) and biotite (10–15%),10SHL-03 Granite Rounded gravel of granite; 470±3 Ma 0.44–2.18

Composed of feldspar (35–40%), quartz (45–50%), and biotite (10–15%)33°53.757’

107°58.970’

08SHL-1 Mafic dyke mafic dyke intruded into the conglomerates; composed mainly of plagioclase (35-45%),amphibolite (40–50%), and quartz (10–15%)

435±7 Ma 0.38–1.39

Liuling GroupHubaohesection

33°52.108’

108°06.645’

08XWJ01 Sandstone, turbidite Yellow medium-grained sandstone of turbidite with typical Boma sequence;composed mainly of quartz (35–45%), feldspar (40–50%), rock debris (15-20%) and chlorite (b5%)

492 Ma 492, 719, 929,1035, 1106, 1344,1609, 1953,2447 Ma

0.21–2.91

Shaliangzi-Banfangzi

33°45.486’

107°56.985’

09LL-05 Sandstone medium-coarse grained sandstones with medium-thick layered and massive structures; lightcolor interlayered with green color sandstones, composed mainly of quartz (45-55%), feldspar(40–50%), detrital muscovite (b5%) and chlorite (5–10%)

467 Ma 482, 779, 951,1207, 2146,2607 Ma

0.23–2.71

Wuguansection

33°33.237’

110°39.177’

DLL-02 Sandstone Gray-color, medium-grained sandstone bearing thin-layered mudstone. Sandstone composed ofquartz (40-50%), feldspar (35-45%), chlorite (5%) and detrital muscovite (5%)

415 Ma 470, 718, 835,1024, 1670,2541 Ma

0.16–1.57

33°30.870'

110°37.102'

DLL-07 Sandstone Gray-color, fine-grained sandstone, composed of quartz (35–40%), feldspar (45–55%),chlorite (10%) and detrital muscovite (5%)

400 Ma 429, 858, 934,1248, 1783,2502 Ma

0.20–2.15

Jiuliping GroupShanyangsection

33°26.013’

109°37.038’

LL-06 Sandstone Yellow-color, Coarse-grained sandstone with 10 cm thick beddings; consists mainly of quartz(55–65%), feldspar (30–35%), detrital muscovite (5%)

583 Ma 710, 788,885,1021, 1183, 1720,2432 Ma

0.19–1.53

33°26.008’

109°36.826’

LL-07 Sandstone Yellow-color sandstone interlayered with grayish -color siltstone and mudstone. Samplecomposedmainly of quartz (35–50%), feldspar (45–55%), rock debris (5–10%), detrital muscovite (5%)

476 Ma 520, 774, 958,1085, 1727,2525 Ma

0.21–2.31

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3.2. Middle to Upper Devonian Liuling Group

In the northern SQB, the Middle to Upper Devonian Liuling Groupis dominant, and is bounded by the Shanyang-Fengzhen fault to thesouth and the Maanqiao-Mianyuzui ductile shear zone to the north(Fig. 1). The Liuling Group is stratigraphically characterized by theoccurrence of the several thousand meters thick flysch sequence,which mainly contains gray–white and purple sandstone, siltstoneand mudstone. The lower part of the group mainly comprises gray–white sandstoneswithwater-wash structures,which gradually changesupward to siltstone and mudstone. From the lower to the upper parts,the Liuling Group comprises the Niuerchuan, Chigou and QingshiyaFormations of Middle Devonian age, as well as the Xiadonggou andTongyusi Formations of Late Devonian age (Zhang et al., 2001). Thelower Niuerchuan Formation is represented by layered cross-beddedfine- tomedium-grained feldspathic quartz sandstone and quartz sand-stone with minor siltstone and mudstone. The middle portion of theformation comprises storm deposit forming massive sandstone andhummocky cross-bedded sandstone, siltstone, and mudstone (Meng,1994; Zhang et al., 2001). The massive sandstone, cross-bedded sand-stone, sandstone with parallel-stratification and olistostromic sand-stone are developed in the upper part of the Niuerchuan Formation.The Chigou Formation is composed mainly of sandstones with mas-sive, cross-bedded parallel-stratified and olistostromic structures.The Qingshiya and Xiadonggou Formations have a similar composi-tion of parallel- and cross-bedded siltstone/fine-grained sandstone,pelitic sandstone, graded-bedding lamellar siltstone, massive siltsandstone and mudstone. The Tongyusi Formation is the continental-shelf sedimentary system mainly formed under the influence of thetides (Meng, 1994). Cross- and parallel-bedding, fine-medium-grainedfeldspathic quartz sandstone and quartz sandstone can be observed.

In the southern SQB, the Upper Devonian Jiuliping Group is sepa-rated from the Liuling Group by the Shanyang-Fengzhen fault tothe north, and bounded by the Ankang fault to the south (Fig. 1).This Group conformably overlays the continuous Lower- to MiddleDevonian limestones. The Jiuliping Group is stratigraphically charac-terized by the occurrence of deep-water turbidite flysch sequenceswith a thickness of 3000–4000 m. The lower part of the group mainlycontains arkose–quartz sandstone with interlayers of siltstone andmudstone, while the upper part is mainly composed of siltstone andmudstone with interlayers of arkose and marl. This feature shows atypical deepening sedimentary sequence (Meng, 1994; Zhang et al.,2001).

4. Sample description

4.1. FAS

The samples of the FAS are mostly selected from the Hubaohe(08XWJ03-2, 08XWJ02-2, 08XWJ02-3 and 08XWJ03-1) and Shaliangzi(10SHL-01, 10SHL-02, 10SHL-03 and 08SHL-1) sections, inwhichmeta-morphosed conglomerates, coarse-grained sandstones with pebbles,and coarse-grained and fine-grained sandstones are exposed frombottom to top. Their protoliths are mostly lithic sandstone, lithic arkoseand feldspathic greywacke. In the Hubaohe section, mainly conglom-erates and coarse-grained sandstones metamorphosed within lowergreenschist-facies crop out, whereas the Shaliangzi section is character-ized by the occurrence of lower amphibolite-facies metamorphosedconglomerates and coarse-grained sandstones. The sampling positions,petrography and ages are summarized in Table 1.

4.1.1. Meta-sandstone from FAS (samples 08XWJ03-2, 08XWJ02-2,08XWJ02-3, 08XWJ03-1, 10SHL-01, 10SHL-02)

Sample 08XWJ03-2 is collected from the uppermost part of theHubaohe section, which consists mainly of metamorphosed fine-grained sandstones (Fig. 3a). The rock experienced lower greenschist

facies metamorphism. However, the bedding is still well preserved. Itis composed mainly of quartz (40–45%), feldspar (20–25%), chlorite(25–35%), and minor biotite (5%). More than 200 grains of detritalzircon were concentrated from this rock for U–Pb isotopic composi-tion analysis.

Sample 08XWJ02-2 is a metamorphosed multicolored coarse-grained sandstone (Fig. 3b) from the middle part of the Hubaohesection. It is composed mainly of quartz (30–45%), feldspar (20–25%),chlorite (20–25%), epidote (5–10%), and ilmenite (5%). More than 300grains of detrital zircon were picked from this rock for U–Pb isotopiccomposition analysis.

Sample 08XWJ02-3 is a metamorphosed coarse-grained brown togray sandstone collected from the middle part of the Hubaohe section(Fig. 3c). It is composed mainly of feldspar (40–45%), quartz (30–40%),biotite (5–10%), epidote (5%) and calcite (5%). More than 500 grains ofdetrital zircon were concentrated from this sample.

Sample 08XWJ03-1 is a metamorphosed coarse-grained sandstonewith pebbles, which was picked from the lower part of the Hubaohesequence (Fig. 3d). It is composed mainly of quartz (35–40%), feldspar(20–25%), chlorite (20–25%), epidote (5%) and pebbles (10%). Themetasandstone also exhibits a green color due to chlorite, elongatedpebbles and extensive foliation related to the ductile shear zone in thenorthern part of the Hubaohe section. At least 500 grains of detritalzircon were concentrated from this rock for U–Pb age dating.

Sample 10SHL-01 is a gray quartz–micaschist and was collectedfrom the upper sequence of the Shaliangzi section. The protolith isa coarse-grained sandstone with a clear residual graded bedding(Fig. 3e) and minor pebbles, which was overprinted by greenschist-facies metamorphism. It is composed mainly of quartz (40–45%),biotite (25–30%), feldspar (10–15%), and chlorite (10–15%), and smallpebbles (5%). At least 200 grains of detrital zircon were picked fromthis rock for U–Pb age dating.

Sample 10SHL-02 is a dark-colored biotite–amphibole–schist fromthe lower part of the Shaliangzi section. The protolith is coarse-grained sandstone with clearly recognizable residual bedding andboulders and is overprinted by lower amphibolite-facies metamor-phism (Fig. 3f). It consists mainly of amphibole (45–50%), feldspar(20–25%), quartz (15–20%), biotite (10–15%), and fine-grained peb-bles (10%). More than 200 grains of detrital zircon were picked forU–Pb isotopic analysis.

4.1.2. Granitic boulder from the FAS (sample 10SHL-03)Sample 10SHL-03 is a rounded granitic boulder from the lower-

most part of the FAS in the Shaliangzi section (Fig. 3g). The gravel iselongated parallel to the foliation of the matrix. However, it stillshows a granitic texture. It consists of feldspar (35–40%), quartz(45–50%), and biotite (10–15%). More than 1000 grains of magmaticzircon were concentrated from this granite for U–Pb age dating to con-strain the maximum depositional age of the conglomerate.

4.1.3. Mafic dyke intruded into FAS (sample 08SHL-1)Sample 08SHL-1 is selected from a diabase dyke, which intruded

into the conglomerate of the Shaliangzi section (Fig. 3h). Both thedykes and the conglomerates evolved into lower amphibolite faciesmetamorphism. Although the structures of gneiss and granite of theboulders are still well preserved, the matrix was metamorphosedand transformed into amphibolite and mica-schist. Moreover, thestructure of the dyke, which intruded into the conglomerates, isstill recognizable. The metamorphosed dyke sample is composedmainly of plagioclase (35–45%), amphibolite (40–50%), and quartz(10–15%). More than 300 grains of zircon were concentrated fromthis sample for U–Pb age dating, which can be used to constrain thecrystallization age of the intrusion and the minimum depositionalage of the conglomerates.

Fig. 3. Images showing the petrographic associations of the sampling FAS rocks; (a) 08XWJ03-2, (b) 08XWJ02-2, (c) 08XWJ02-3, (d) 08XWJ03-1, (e) 10SHL-01, (f) 10SHL-02,(g) 10SHL-03 of granitic boulder with conglomerate, and (h) 08SHL-1 of mafic dyke intruding into the conglomerate.



4.2. Devonian sandstones

The samples of the Middle to Upper Devonian Liuling Group areselected from the Heihe region (samples 08XWJ01 and 09LL-05) in thewestern SQB and the Wuguan region (samples DLL-02 and DLL-07) inthe eastern SQB, while the Upper Devonian Jiuliping Group in the south-ern SQB is represented by the sandstones from the Shanyang region(samples LL-06 and LL-07) (Fig. 1). The sampling positions, petrographyand ages are listed in Table 1.

4.2.1. Sandstone of the Liuling GroupSamples 08XWJ01 and 09LL-05 have been collected from the Liuling

Group in thewestern SQB (Fig. 1b). Sample 08XWJ01 represents a typicalturbidite from the upper part of the Liuling Group. It is characterized by aseries of sedimentary structures in the turbidites, such as a Boumasequence, graded bedding and wash-surface, etc. (Fig. 4a). TheHubaohe section shows turbidites with a predominant lowermost

part of a Bouma sequence, which is characterized by graded beddingstructure within the sandstone beds. There, a single sandstone layeris always about 20 cm thick and exposes, from bottom to top,yellow medium-grained to greenish fine-grained sandstones. Sam-ple 08XWJ01 was selected from the yellow medium-grained sand-stone, which is composed mainly of quartz (35–45%), feldspar(40–50%), rock debris (15–20%) and chlorite (b5%). Sample 09LL-05represents the lower part of the Liuling Group from the Banfangzi sec-tion. It is a medium- to coarse-grained sandstone with medium-thicklayers and massive structures (Fig. 4b). Although the sandstones wereoverprinted by folding and southward thrusting, the sedimentary se-quence and structures are still well preserved. The rock sample is com-posedmainly of quartz (45–55%), feldspar (40–50%), detrital muscovite(b5%), and chlorite (5–10%).

The Liuling Group in the eastern SQB is represented by samplesDLL-02 and DLL-07 collected from north to south along the Wuguansection (Fig. 1c). Sample DLL-02 is thick-layered (10–20 cm) medium-

image of Fig.�3



grained sandstone within a thin-layered mudstone (Fig. 4c), and iscomposed mainly of quartz (40–50%), feldspar (35–45%), chlorite(5%), and detrital muscovite (5%). Sample DLL-07 is a gray-colored,fine- to medium-grained sandstone with a clear sedimentary bedding(Fig. 4d), and consists of quartz (35–40%), feldspar (45–55%), chlorite(10%), and detrital muscovite (5%).

4.2.2. Sandstone of the Jiuliping GroupThe Upper Devonian Jiuliping Group of the southern SQB is

represented by samples LL-06 and LL-07 for upper part and lowerpart of the group in the Shanyang region, respectively (Fig. 1c). Inthe Jinqianhe section, the Jiuliping Group mainly contains turbiditewith well-preserved Bouma sequences, which are characterized bythe occurrence of coarse- to fine-grained sandstones, siltstone andmudstone with blocky-bedding, parallel-bedding, cross-bedding andhorizontal bedding structures, respectively, from bottom to top. Sam-ple LL-06 is a coarse-grained sandstone from the upper sequence ofthe Jiuliping Group (Fig. 4e) and is characterized by yellow-colored,coarse-grained 10–20 cm thick sandstone. It consists mainly of quartz(55–65%), feldspar (30–35%), and detrital muscovite (5%). SampleLL-07 is a turbiditic sandstone collected from the lower part of theJiuliping Group outcropping in the Hujiayuan section. The sample ischaracterized by the typical medium-thick layered sandstones withminor inter-layered black shales and deep channel water glutenites(Fig. 4f). The sample is a medium-grained grayish sandstone andconsists mainly of quartz (35–50%), feldspar (45–55%), rock debris(5–10%), and detrital muscovite (5%).

5. Analytical methods

High spatial resolution U–Th–Pb and REE data of zircons weregenerated using the laser ablation inductively coupled plasma massspectrometer (LA-ICP-MS) at the State Key Laboratory of ContinentalDynamics, Department of Geology, Northwest University, P. R. China.Per sample, about 20 kg of rawmaterial were crushed and powdered,and then washed and dried. Zircons were separated by heavy-liquidand magnetic methods and then purified by hand-picking under abinocular microscope. More than 200 zircon grains were concentrat-ed from each sample, and about 150 grains were randomly selectedand mounted on adhesive tape then enclosed in epoxy resin andpolished to about half of their thickness. After being photographedunder a microscope with reflected and transmitted light, thecathodoluminescence (CL) images of zircons were carried out usingthe Gatan MonoCL 3+ Fluorescence Spectrometer. CL images areused to demonstrate the internal texture of zircons and to select op-timum spot locations for U–Pb dating and trace elements analysis.Zircons were dated in-situ on the laser ablation inductively coupledplasma mass spectrometer (LA-ICP-MS). Trace element (REE, Lu, Hf,Ta, Nb, Th, Ti and P) compositions were simultaneouslyexamined from the same laser ablation spot. The laser-ablation sys-tem used is a GeoLas 200 M equipped with a 193 nm ArF-excimerlaser, and a homogenizing and imaging optical system (MicroLas,Göttingen, Germany). Analyses were performed on the ELAN6100 ICP-MS from Perkin Elmer/SCIEX (Canada) with a dynamic re-action cell (DRC). The laser ablation spot size is approximately40 μm. 207Pb/206Pb, 206Pb/238U, 237Pb/235U and 208Pb/232Th ratioswere calculated using GLITTER 4.0 (Macquarie University), and werecorrected for both instrumental mass bias and depth-dependent ele-mental and isotopic fractionation using Harvard zircon 91500 as theexternal standard. The ages were calculated using ISOPLOT 3(Ludwig, 2003). A detailed analytical procedure of age and trace ele-ment determinations of zircons can be found from Liu et al. (2008).Common Pb corrections were made following the method of Andersen(2002).

6. Result

More than seventy detrital zircons from each sandstone sample ofthe FAS, the Liuling Group and Jiuliping Group were analyzed and arelisted in Tables 2 and 3 (Appendix A), respectively. Most zircon U–Pbanalyses are concordant or near concordant with age concordancewithin 90 to 110%, and are plotted in Figs. 5–8.

6.1. The FAS

6.1.1. Meta-sandstone from the FAS (samples 08XWJ03-2, 08XWJ02-2,08XWJ02-3, 08XWJ03-1, 10SHL-01, 10SHL-02)

Although these meta-sandstones from the FAS were meta-morphosed within greenschist (samples 08XWJ03-2, 08XWJ02-2,08XWJ02-3, 08XWJ03-1 and 10SHL-01) to lower-amphibolite facies(sample 10SHL-02) conditions, all zircon grains are short prismswith length/width ratios ranging from 1:1 to 2:1, and exhibit well-developed crystal faces and well preserved magmatic oscillatoryzoning in cathodoluminescence (CL) images. Consequently, thesezircons are regarded to originally be derived from a magmatic prov-enance, as is also suggested by their high Th/U ratios (>0.2) (Table 2).The Th/U ratios of zircon are routinely used to distinguish their geneticsetting (e.g. Maas et al., 1992). Th/U ratios in magmatic zircons fromvarious rocks mostly range from 0.2 to 1.0, while zircons crystallizedduring metamorphic events exhibit lower Th/U ratios (b0.1) (Kinny etal., 1990; Schiøtte et al., 1988; Williams and Claesson, 1987). Roundedshapes and abrasive rims indicate an erosion-transportation historyand mark them as detrital zircons.

Seventy-five zircon grains were analyzed from the metamor-phosed fine-grained sandstones (sample 08XWJ03-2) have high Th/Uratios (>0.3) (Table 2). Most grains are concordant or nearly concor-dant, yielding a predominant populationwith 206Pb/238U ages between467 Ma and 552 Ma (Fig. 5a), and define a 498 Ma peak age for thesezircons (Fig. 5b).

Seventy-five detrital zircons from the meta-sandstone (sample08XWJ02-2) were analyzed for U–Pb isotopic compositions (Table 2).Sixty-two analyses are concordant or nearly concordant, and yield206Pb/238U ages varying from 455 Ma to 589 Ma (Fig. 5c). All concor-dant ages plotted on the Gaussian probability diagram give a 495 Mapeak age for these zircons (Fig. 5d).

Detrital zircons separated from the coarse-grained sandstone(sample 08XWJ02-3) exhibit rounded shapes with well-developedmagmatic oscillatory zoning and high Th/U ratios (>0.35). Seventyof the seventy-five analyzed data are concordant yielding a predomi-nant population with 206Pb/238U ages varying from 466 Ma to 586 Ma(Fig. 5e), and define a 528 Ma peak age for the zircons from thissample (Fig. 5f).

More than 500detrital zircons grainswere separated from the coarse-grained sandstonewith gravels (sample 08XWJ03-1). Seventy-five grainswere randomly analyzed. Most grains yield a predominant populationof 206Pb/238U ages between 461 Ma and 587 Ma (Fig. 5g), defining a510 Ma peak age (Fig. 5h). In addition, two grains give concordant206Pb/238U ages of 848 Ma and 2316 Ma (Table 2 and Fig. 5g, h).

Zircons separated from a biotite–quartz schist (meta-sandstonesample 10SHL-01) show well-developed crystal faces and magmatic,oscillatory zoning. Thirty zirconswere analyzed, and twenty-five analy-ses are concordant, yielding a predominant population with 206Pb/238Uages varying from 455 Ma to 504 Ma (Fig. 6a), and define a 466 Mapeak age for the zircons from this sample (Fig. 6b).

Zircons separated from thebiotite–amphibole schist (meta-sandstonesample 10SHL-02) exhibit abrasive and rounded shapes, which, togetherwith the blastopsammitic textures and sedimentary beddings in therocks (Fig. 3e), suggest they are detrital zircons. Twenty-seven of thirtyanalyzed grains are concordant, and yield a predominant populationwith 206Pb/238U ages between 461 Ma and 507 Ma (Fig. 6c) defining a485 Ma peak age (Fig. 6d).



6.1.2. Granitic boulder from the FAS (sample 10SHL-03)The zircons from the granitic boulder of the FAS (Fig. 3g) exhibit

well-developed crystal faces with length/width ratios ranging from 1.5:1to 2:1. Light pink to colorless transparent to translucent grains showa well developed magmatic oscillatory zoning in cathodoluminescence(CL) images, typical for magmatic provenance. This is also supportedby their high Th/U ratios (0.44–2.18) (Table 2). Twenty-one out of thirtyanalyses plot on the concordia (Fig. 6e) and twenty analyses define atight cluster giving a weighted mean 206Pb/238U age of 470±3 Ma(MSWD=4.2) (Fig. 6f). With the well defined magmatic oscillatory zon-ing and high Th/U ratios, this age is taken as the formation age of thegranite.

6.1.3. Mafic dyke intruding in the FAS (sample 08SHL-1)For the zircons from the mafic dyke which intruded into the con-

glomerate of the FAS (Fig. 3h), thirty analyses were obtained fromindividual spots in zircons. Twenty-three of the analyzed pointsplot on the concordia (Fig. 6g), however, the remaining seven anal-yses were rejected due to discordant 207Pb/235U and 206Pb/238Uages. Twenty-two concordant analyses yield an average 206Pb/238Uage of 435±7 Ma (MSWD=4.1) (Fig. 6h). Based on the well devel-oped magmatic oscillatory zoning in CL images and their high Th/Uratios (0.38–1.39), the weighted mean 206Pb/238U age of 435±7 Mais interpreted as the intrusive age of the mafic dyke into the FASclastics.

Fig. 4. Images showing the petrographic associations of the samples from the Liuling GroJiuliping Group: (e) LL-06 and (f) LL-07.

6.2. Middle- to Upper Devonian

More than 500 zircon grains were separated from each sandstonesamples of theMiddle Devonian Liuling Group and the Upper DevonianJiulipingGroup along three sections. All the analyzed zircon grains showwell-developed crystal faces andmagmatic oscillatory zoning, aswell ashigh Th/U ratios (>0.2) (Table 3), which indicate their igneous prove-nance. Whereas the abrasive and rounded shapes, together with theclear sedimentary structures in the rocks (Fig. 4) suggest theirerosion-transportation history. Therefore, all the analyzed zirconswere identified as detrital grains.

6.2.1. Liuling Group in the Heihe region (samples 08XWJ01 and09LL-05), western SQB

The detrital zircons from sample 08XWJ01, a turbiditic sandstone ofthe Liuling Group (Fig. 4a) are well rounded with oscillatory zoning.Seventy-two individual zircon grains were analyzed for U–Pb isotopiccompositions, and most zircons have high Th/U ratios (>0.2) exceptfor three with low Th/U ratios (b0.1) (Table 3). Fifty-one analysesgave concordant or nearly concordant ages yielding five major agepopulations at 900 to 1200 Ma, 1300 to 1400 Ma, 1500 to 1700 Ma,1900 to 2000 Ma, and 2200–2600 Ma (Fig. 7a). The concordant agesdefine 206Pb/238U peak ages as 929–1106 Ma, 1344 Ma, 1609 Ma,1953 Ma and 2447 Ma, respectively (Fig. 7b). The most predominantpopulations are from 0.9 to 1.2 Ga, and 2.2 to 2.6 Ga. One grain with

up: (a) sample 08XWJ01, (b) 09LL-05, (c) DLL-02, (d) DLL-07, and samples from the

image of Fig.�4

Fig. 5. U–Pb concordia diagrams, frequency (bars) and probability density distribution (curves) of ages showing the result of the LA-ICP-MS dating of detrital zircons of the FAS fromthe Hubaohe section. Error ellipses represent 2σ uncertainties.



image of Fig.�5

Fig. 6. U–Pb concordia diagrams, frequency (bars) and probability density distribution (curves) of ages showing the result of the LA-ICP-MS dating of detrital zircons of the FAS fromthe Shaliangzi section (a–d). U–Pb concordia diagrams and age histograms showing the dating result of the zircons from a granitic boulder (e, f) and mafic dyke (g, h).



image of Fig.�6

Fig. 7. U–Pb concordia diagrams, frequency (bars) and probability density distribution (curves) of ages showing the result of the LA-ICP-MS dating of detrital zircons of the Middleto Upper Devonian Liuling Group. Error ellipses represent 2σ uncertainties. Paleozoic ages have been enlarged for clarity.



image of Fig.�7

Fig. 8. U–Pb concordia diagrams, frequency (bars) and probability density distribution (curves) of ages showing the result of the LA-ICP-MS dating of detrital zircons of the UpperDevonian Jiuliping Group in the Shanyang area.



an age of 719 Mawas also obtained from this sample. The youngest agefrom this sample is 492 Ma and the oldest age is 3141 Ma.

Seventy-two zircons from sample 09LL-05, a sandstone of theLiuling Group in the Heihe area (Table 3, Fig. 4b), were analyzed,and sixty-five analyses gave concordant or nearly concordant ages(Fig. 7c). The concordant zircons yielded four major age populationsat 480 to 600 Ma, 900 to 1100 Ma, 2000 to 2200 Ma and 2500 to2600 Ma, and 206Pb/238U peak ages at 482 Ma, 951 Ma, 2146 Maand 2607 Ma, respectively (Fig. 7d). The most dominant populationsare from 480 to 600 Ma, and 900 to 1100 Ma. Two grains with agesof 774 Ma and 814 Ma were also obtained from this sample. Theyoungest age of this sample is 467 Ma and the oldest age is 3036 Ma.

6.2.2. Liuling Group in the Wuguan region (samples DLL-02 and DLL-07),eastern SQB

Seventy-two individual detrital zircon grains from the sandstonesample DLL-02 from eastern SQB were randomly analyzed for U–Pbisotopic compositions (Table 3). Fifty-nine zircons are concordant,and yielded two predominant populations of 206Pb/238U ages of415–1100 Ma and 1300–1700 Ma (Fig. 7e). It is noticeable that theyoungest age obtained from this sample is at 415 Ma. In the probabil-ity diagram, the analyses define 206Pb/238U peak ages of 470 Ma,530 Ma, 718 Ma, 835 Ma, 1024 Ma, 1670 Ma, and 2541 Ma (Fig. 7f).These populations are quite similar to those of sample LL-07 fromthe Shanyang area.

Fifty analyses of the seventy analyzed individual detrital zirconsfrom sample DLL-07 have high Th/U ratios (0.20–2.15) (Table 3).Fifty analyses are concordant or nearly concordant yielding three

predominant populations with 206Pb/238U ages varying from 400 Mato 500 Ma, 700 to 1000 Ma, and 1100 to 1500 Ma (Fig. 7g). All the datadefine 206Pb/238U peak ages of 429 Ma, 858 Ma, 934 Ma, 1248 Ma,1783 Ma, and 2505 Ma (Fig. 7h). The youngest 206Pb/238U age obtainedfrom this sample is at 401 Ma.

6.2.3. Jiuliping Group in the Shanyang region (samples LL-06 and LL-07),central SQB

Seventy-two detrital zircons from the sandstone sample LL-06 ofthe Upper Devonian Jiuliping Group at Shanyang area (Fig. 4c) wereanalyzed for U–Pb isotopic composition (Table 3). Fifty concordantor nearly concordant ages yield four major age populations including700 to 850 Ma, 900 to 1200 Ma, 1700 to 1800 Ma and 2400–2500 Ma(Fig. 8a), and defined 206Pb/238U peak ages as 583 Ma, 710–858 Ma,1021–1183 Ma, 1720 Ma, and 2432 Ma, respectively (Fig. 8b). Themost predominant populations are from 0.7 to 0.9 Ga, and 0.9 to1.2 Ga. The youngest age of this sample is at 583 Ma, and the oldestat 2482 Ma.

The detrital zircons from the sandstone sample LL-07 of the UpperDevonian Jiuliping Group in the Shanyang region (Fig. 4d) showwell abraded and rounded shapes with clear oscillatory zoning, andhigh Th/U ratios (>0.2) in most of the analyzed grains (Table 3).Seventy-two individual zircon grains were analyzed, and fifty-sevenanalyses gave concordant or nearly concordant ages. The most pre-dominant populations were from 470 to 600 Ma, 700 to 800 Ma,and 900 to 1200 Ma (Fig. 8c). In addition, two minor populationsof ~1700 Ma, and 2400–2650 Ma were identified from this sample.All these concordant ages define 206Pb/238U peak ages of 520 Ma,

image of Fig.�8



774 Ma, 958 Ma, 1085 Ma, 1727 Ma, and 2525 Ma (Fig. 8d), as wellas the youngest age of 476 Ma.

7. Discussion

7.1. Timing of deposition of the FAS

The sedimentary deposition must have occurred later than the ageof the youngest detrital zircon if there was no disturbance in the U–Pbisotopic system (e.g. Fedo et al., 2003; Nelson, 2001; Williams, 2001).This method has been successfully applied to the age determinationof sedimentary systems (e.g. Andersen, 2005; Bingen et al., 2001;Griffin et al., 2004; Guan et al., 2002; Liu et al., 2008; Long et al.,2007; Luo et al., 2004; Moecher and Samson, 2006; Payne et al.,2006; Sun et al., 2009; Xia et al., 2006; Yue et al., 2005; Zhang et al.,2011). The FAS was previously interpreted as a Devonian (Mattaueret al., 1985; Meng, 1994; Yu and Meng, 1995) typical post-orogenicmolasse, and consequently the Qinling was regarded as an EarlyPaleozoic, “Caledonic” orogen (Mattauer et al., 1985). However, therewas no reliable geochronological data from this unit until this study.The studied detrital zircons from the four meta-sandstone samples ofthe FAS along the Hubaohe section yield youngest 206Pb/238U ages of467 Ma, 455 Ma, 466 Ma and 461 Ma, respectively (Fig. 5). In compar-ison, the detrital zircons from the two FAS samples of the meta-clasticrocks in the Shaliangzi area give youngest 206Pb/238U ages of 455 Maand 461 Ma, respectively (Fig. 6a–d). Taking the youngest ages fromthese samples into consideration, the deposition of the FAS is reason-ably identified to have taken place not prior to ca. 455 Ma. This conclu-sion is also supported by the formation age of 470±3 Ma (MSWD=4.2) (Fig. 6e, f) of the granitic boulder within the FAS of the Shaliangziarea. As discussed previously, the mafic dyke which intruded intothe conglomerate of the FAS in the Shaliangzi area, gives the intrusive(crystallization) age of the mafic dyke and can be used to constrainthe minimum depositional age of the FAS. The magmatic zirconsfrom the mafic dyke yield an average 206Pb/238U age of 435±7 Ma(MSWD=4.1) (Fig. 6h), which is interpreted as the intrusion ageof the mafic dyke in the FAS. This age limits the FAS depositionalage to prior to ca. 435 Ma. Thus the depositional age of the FAS isconstrained to between ca. 455–435 Ma (Late Ordovician–EarlySilurian).

7.2. Sedimentary provenance of the FAS

The studied detrital zircons from the four meta-sandstone samplesof the FAS along the Hubaohe section cluster into distinct groups with206Pb/238U ages between 467–552 Ma, 455–589 Ma, 466–586 Ma and461–587 Ma, and define peak ages of 498 Ma, 495 Ma, 528 Ma and510 Ma, respectively (Fig. 5). In comparison, the detrital zircons fromtwo FAS samples of the meta-clastic rocks in the Shaliangzi area yield206Pb/238U age groups of 455–504 Ma and 461–507 Ma, and peakages of 466 Ma and 485 Ma, respectively (Fig. 6a–d). These similarage populations and predominant peak ages suggest that the prove-nance of the FAS must be attributed to the NQB instead of the SQB.

Detailed investigations andmapping revealed that the Phanerozoicmagmatism in the SQB are characterized by the occurrence of UpperTriassic granitoids (Dong et al., 2011b, 2012a) and minor basalt andmafic–ultramafic dyke with a formation age of 433±4 Ma in thesouthern edge of the SQB (Zhang et al., 2007), which could not bethe provenance of the population of ca. 455–600 Ma in the FAS.Neoproterozoic volcanics in the SQBwere represented by theWudangGroup at ca.780–755 Ma (Ling et al., 2008, 2010) and the YaolingheGroup at ca. 685 Ma (Ling et al., 2008). No Cambrian–Ordovicianmagmatic rocks were identified in the SQB. Therefore, the largeamount of detrital zircons obtained from the FAS cannot be attributedto the SQB.

Instead, the NQB is characterized by the existence of the large vol-ume of Lower Paleozoic volcanic rocks including the discontinuouslyexposed tectonic mélange in the Shangdan suture and the arc-relatedgabbroic-granitic plutons to the north of the suture. Our previousinvestigations reveal that the ophiolite suite and subduction-relatedvolcanic rocks consist mainly of metamorphosed mafic and ultramaficrocks outcropping, from west to east, at Yuanyangzhen-Wushan at457±3 Ma (Li, 2008), Guanzizhen at 534–471 Ma (Li, 2008; Peiet al., 2005, 2007a; Yang et al., 2006), Tangzang, Yanwan at 523–483 Ma (Chen et al., 2008b; Dong et al., 2011a; Lu et al., 2003),Heihe at 442±7 Ma (Yan et al., 2008) and Danfeng at Cambrian toOrdovician (Cui et al., 1995; Dong et al., 2011a). Further to the east,on the northern side of the Tongbai-Dabie Mountains, the Shangdansuture zone is marked by an ophiolitic mélange zone (Xie et al.,1999; Yan et al., 1989; Zhang, 1985) with island-arc basalts whichwas formed at 472±17 Ma (Liu et al., 2011). The above presentedgeochronological evidence suggests that the spreading and formationof the Shangdan oceanic crust occurred from ca. 534 Ma to ca. 442 Ma,and could be part of the provenance of the FAS.

Furthermore, it is well documented that the NQB was an island-arc terrane above a subduction zone during Early Paleozoic times(Dong et al., 2007), and is mainly composed of the Paleoproterozoicplutonometamorphic basement of the QinlingGroup andNeoproterozoicand Lower Paleozoic gabbro-granitoid intrusions (Dong et al., 2011a,b;Lu et al., 2003;Wang et al., 2005; Zhang et al., 2004). The Lower Paleozoicsubduction related mafic intrusions with zircon U–Pb ages of ca.514–422 Ma (Dong et al., 2011b and references therein) are represented by,from west to east, the Guanzizhen gabbro (507±3.0 Ma and 499±1.8 Ma) (Pei et al., 2005), Baihua gabbroic–dioritic intrusion (434±1.5 Ma) (Pei et al., 2007b), Houzhenzi gabbro (475±4 Ma) (Liu et al.,2007), Lajimiao gabbro (422±7 Ma) (Liu et al., 2009), and Fushui gabbro(514–490 Ma) (Li et al., 2006; Lu et al., 2003; Su et al., 2004). Further tothe east, this subduction-related mafic magmatic zone can be connectedwith the gabbros to the north of the Tongbai Mountains with SHRIMPU–Pb zircon ages ranging between 470 and 432 Ma (Jiang et al., 2009).

Additionally, many subduction and/or collisional granitoids withzircon U–Pb ages of ca. 500–403 Ma were revealed that intrudedinto the Qinling Group on the northern side of the Shangdan suture(Dong et al., 2011b and references therein). In the western NQB,the Lower Paleozoic granitoids are represented by the plutons with206Pb/238U zircon ages varying from 455 Ma to 414 Ma, includingthe Caochuanpu granite (434±10 Ma), Yanjiadian diorite (441±10 Ma)(Zhang et al., 2006), Wangjiacha diorite (455±1.7 Ma),Tangzang granitoid (455±1.9 Ma)(Chen et al., 2008a), and theHonghuapu diorite (414±1.5 Ma, Xu et al., 2007). In the middlepart of the NQB, the granitic plutons from the Heihe to Zhongnanshanareas yield U–Pb formation ages of 442–401 Ma (Dong et al., 2011b).In the eastern NQB, a large number granitic plutons attributed tothe northward subduction of the Shangdan oceanic crust or collisionalong the Shangdan suture (Chen et al., 1995; Wang et al., 2009)show U–Pb zircon ages ranging from 500 Ma to 400 Ma (Dong etal., 2011b and references therein). For instance, the Piaochi granite(495±6 Ma), Shiziping granite (492±14 Ma) (Wang et al., 2009),Taohuapu diorite (487±1.1 Ma) and trondjemite (488±1.4 Ma)(Xue et al., 1996), Kuanping monzonite (446±19 Ma) (Zhang et al.,2006), Wuguan monzonite (439±10 Ma) and the Shangnan diorite(428±7 Ma) (Lu et al., 2003).

Comparison of the age populations of the FAS with all above agesfrom the plutons in the NQB, and volcanics in the SQB (Fig. 9) indi-cates that the sedimentary provenance of the FAS are mostly erosionfrom an island-arc terrane in the NQB in Early Paleozoic times.

7.3. Sedimentary provenance of the Middle to Upper Devonian in the SQB

The Middle to Upper Devonian strata of the Liuling Group is repre-sented by sandstone 09LL-05 from the Heihe area in the western



SQB, DLL-02 and DLL-07 from the Wuguan section in the eastern SQB.All the detrital zircons from each of the three sandstone samples ofthe Middle Devonian turbidite cluster into three distinct groupswith 206Pb/238U ages between 400 and 600 Ma, 700–850 Ma and900–1100 Ma, respectively (Fig. 7c–h). The youngest age limits thedeposition of the Liuling Group to not earlier than theMiddle Devonian.In view of their clear oscillatory zoning and high Th/U ratios (>0.2),all these zircons are derived from igneous rocks. Detailed mappingrevealed that the Liuling Group, in most regions, conformably overliesthe pre-Devonian strata except forminor outcropswith a disconformityor overlap unconformity (Dong et al., 2011b; Zhang et al., 2001). Thisfact indicates that the Liuling Group is an in-situ deposit ruling out itsorigin as an overthrust sheet. As discussed previously the basement ofthe SQB is represented by the Neoproterozoic Wudang and YaolingheGroups (Zhang et al., 2001), which are characterized by rift-relatedigneous rocks, with zircon U–Pb ages of ca. 780–755 Ma, and ca.685 Ma, respectively (Ling et al., 2008, 2010). These volcanic rockscould be the provenance of the ca. 700–850 Ma zircon populationwith-in the Liuling Group. In addition, large amounts of mafic- to graniticintrusions were investigated at the northwestern SCB, including theintrusions in the Hannan massif with 206Pb/238U zircon ages betweenca.705 and 825 Ma, in the Micangshan massif between ca. 814 and860 Ma (Dong et al., 2012b and references therein), and in theHuangling dome between ca. 744 and 795 Ma (Ling et al., 2006).These intrusions are also an alternative provenance of the zirconswith ages of ca.700–850 Ma.

Although minor basalts and mafic–ultramafic dykes with a forma-tion age of 433±4 Ma were reported from the southern SQB (Zhanget al., 2007), this short mafic–ultramafic magmatism seems unlikelyto be a major source of the zircon population of ca. 600–400 Ma. Par-ticularly the fact that there were no igneous rocks with formationages of ca. 900–1000 Ma in the SQB rules out that they were deriveddirectly from the SQB. In comparison to the large amounts of zirconsfrom the FAS and NQB, as well as the rather active magmatism in theNQB during the Early Paleozoic (Dong et al., 2011a,b), the provenanceof the predominant zircon population with ages from ca. 400 to ca.600 should be attributed to the NQB (Fig. 9). In addition, the populationof ca. 900–1000 Ma can only be derived from the NQBwhich is the onlyregion developing igneous rocks with ages of ca. 900–1000 Ma (Fig. 9).For instance, the Dehe granite (943–964 Ma) (Chen et al., 2004),Niujiaoshan granite (929–959 Ma) (Wang et al., 1998, 2005) and theCaiwa granite (889±10 Ma) (Zhang et al., 2004) in the eastern Qinling,and the Tianshui granitoid in the western Qinling (915–978 Ma) (Liuet al., 2006; Pei et al., 2007c).

In addition, some populations of ca. 1600–1800 Ma, ca. 2000 Ma andca. 2500–2600 Ma are also present in these three samples (Fig. 7c–h).Comparing these populations with the data from the NCB and SCB, itis clear that the provenance of the 1600 to 1800 Ma-population is theNCB, while the ca. 2000 Ma-cluster is attributed to SCB, and the ca.2500–2600 Ma-population derived from both the NCB and SCB (Fig. 9).

The detrital zircons from sample 08XWJ01 of the Upper Devoniansandstone from the upper Liuling Group yield five major age popu-lations, which are ca. 900–1150 Ma, ca. 1300–1400 Ma, ca. 1600 Ma,ca. 2000 Ma, and ca. 2200–2600 Ma (Fig. 7a, b). The most predominantpopulation of ca. 900–1200 Ma was denudated from the NQB, and

Fig. 9. Probability density distribution (curves) of ages for (a) the NCB and NQB, (b) theUpper Ordovician to Lower Silurian FAS, (c) Middle to Upper Devonian Liuling andJiuliping Group, and (d) the SQB and SCB. The data for the FAS and Devonian samples,the SQB and the SCB are single grain zircon analyses, while the data for the NCB andNQB are the formation age of the igneous rocks. The data for the NCB are from Liuet al. (2008 and references therein), while data for the NQB are from literatures(Chen et al., 2004; Dong et al., 2011b and references therein; Liu et al., 2006; Peiet al., 2007c; Wang et al., 1998, 2005; Zhang et al., 2004). The data for the NCB arefrom Liu et al. (2008 and references therein), while data for the SCB are from Liuet al. (2008), Dong et al. (2012b and references therein), and data for SQB are fromLi et al. (2003), Ling et al. (2008, 2010) and Wang et al. (2009).

the data of ca. 1600 and ca. 2000 Mawere derived fromNCB and SCB, re-spectively, whereas the provenance of the ca. 2200–2600 Ma-group canbe attributed to both the NCB and SCB (Fig. 9). It is noticeable that there

image of Fig.�9



is no dominant population of ca. 400–600 Ma in the Upper Devoniansandstone, which is common in the Middle Devonian within the LiulingGroup. This fact likely implies that the vast Lower Paleozoic igneousrocks accumulated on the western NQB had already vanished by LateDevonian, and therefore, the erosion mainly affected the Precambrianbasement of the NQB.

Two samples from the Upper Devonian Jiuliping Group located tothe south of the Liuling Group have detrital zircon age populations ofca. 470–600 Ma, ca. 700–850 Ma, ca. 950–1200 Ma, ca. 1700–1730 Ma,and ca. 2400–2500 Ma (Fig. 8). The major populations of ca. 470–600 Ma and ca. 950–1200 Ma, as well as ca. 1700–1730 Ma indicatethat their provenance was dominated by the NQB and NCB. In compari-son, the population of ca. 700–850 Ma is clearly attributed to the SQB,while the minor ages of ca. 2400–2500 Ma can be from both the SCBand NCB (Fig. 9).

7.4. Tectonic implications

The FAS zone is exposed as intensely deformed thrust slices com-posed of different litho-tectonic units from east to west due to themultiple tectonic movements along the Shangdan suture. Detailedmapping reveals that the FAS consists mainly of low-grade metamor-phosed sandstones with some isolated lenticular conglomerates(exposed, i.e., in the Shaliangzi and Hubaohe areas of the Heihe val-ley), volcaniclastic rocks and limestones. This unit, especially the con-glomerates, was previously considered to be typical Upper Devonianpost-orogenic molasse (Mattauer et al., 1985), or to represent anOrdovician–Carboniferous fore-arc sedimentary wedge (Meng, 1994;Yu and Meng, 1995; Zhang et al., 2001). This conclusion is alsosupported by the geochemical studies of the greywackes in the Tianshuiarea in western Qinling (Xu, 2005) and in the Shangxian area in easternQinling (Song et al., 1995; Zhang et al., 2001).

However, based on our study, all meta-sandstone samples fromthe FAS have relatively uniform detrital zircon age spectra of ca.455–600 Ma, except for two grains with ages of 848 Ma and 2316 Main sample 08XWJ03-1. This homogeneous detrital zircon populationis taken as evidence that sedimentation is related to an arc-relatedmagmatism in theNQB. Considering the following evidence: (1) zirconsshowing euhedral to subhedral shapes, and simple detrital zirconswithan age spectra of ca. 455–600 Ma (Figs. 5, 6a–d), (2) vast boulders ofigneous rocks from the NQB (Fig. 3g), (3) metamorphosed limestoneinterlayered within the conglomerates (Fig. 2c, e) and sandstone(Fig. 2f), (4) sedimentary facies and associations of subaqueous debrisflows and highly concentrated turbidity flows (Meng et al., 1994),(5) typical volcanic-sedimentary sequences and island-arc geochemicalsignature (Song et al., 1995; Xu, 2005; Zhang et al., 2001), we proposedthat the FAS was originally formed as diluvial fans to turbidite fans, anddeposited in a fore-arc basin correlated to the northward subduction ofthe Shangdan oceanic crust beneath the NQB island-arc terrane duringLate Ordovician to Early Silurian (ca.455–435 Ma) (Fig. 10a).

The relatively uniform detrital zircon age spectrum of ca. 455–600 Ma in the FAS without any contributions from the NCB (Fig. 9)may imply that the NQB was separated from the NCB by theErlangping back-arc basin (Dong et al., 2011a); whereas the missingcontribution from the SQB implies that the Shangdan Ocean stillexisted between the NCB and SCB during the deposition of the FASin Late Ordovician to Early Silurian times (Fig. 11a).

The northern SQB is characterized by the Middle- to UpperDevonian turbidite succession named Liuling Group (Zhang et al.,2001). This turbidite succession was previously proposed to have de-posited in a passive-continental marginal basin (Mei et al., 1999),which would mean that the Shangdan Ocean still existed in Middleto Late Devonian times. Zhang et al. (2001) argued that the turbiditesof the Liuling Group were formed in a remnant oceanic basin, whileothers proposed a fore-arc accretion zone along an active-continentalmargin (Yan et al., 2007). As discussed previously, our detrital zircon

age dating of the clastic rocks indicates the origin of the Liuling Groupto have been fromboth theNQB and SQB, and particularly that theprov-enance was dominated by the NQB. This result is consistent with thegeochemical composition of sandstones from the Middle- to UpperDevonian Liuling Group, which revealed that the clastic materialscame from both the NQB and SQB (Gao et al., 1995). Together withthe foreland sedimentary successions (Li et al., 1994), we confirmedthat the Liuling Group might have been formed in a foreland basinafter closure of the ShangdanOcean (Fig. 10b). The previous sedimenta-ry work revealed that a Devonian highland had existed along theShanyang-Fengzhen fault, which separated the Northern Devonianfrom different southern Devonian (Meng et al., 1995). We interpretthis highland as a peripheral bulge in front of the Liuling foreland flex-ural basin (Fig. 10b).

Taking into account the unconformity between themiddleDevonianand prior strata in the SQB, all geological and geochronological datain this work, dating results of the ophiolite and subduction related igne-ous rocks in the NQB (Dong et al., 2011a,b), we propose that the closureof the Shangdan Ocean between the NCB and SCB occurred in EarlyDevonian times (Fig. 11b). However, therewas no full collision betweenthe blocks after the closure of the Shangdan Ocean, as indicated by thecontinuous deposition from Devonian to Lower Triassic successions inthe SQB. These dataset and timing constrain the detail tectonic processbetween theNCB and SCB, aswell as the amalgamation of the East Asiancontinents within the Pangea supercontinent.

Our detrital zircon age dating shows that the provenance of theJiuliping Group was also dominated by both the NQB and SQB, whichis similar to that of the Liuling Group. Taking into account the turbiditicassociations and sedimentary facies, therefore, the Jiuliping Group wasalso deposited in a foreland basin similar to the upper part of the LiulingGroup. This also implies that the peripheral bulge disappeared duringLate Devonian.

In addition, the zircon provenance of the NCB implies that theErlangping back-arc basin, which separates the NQB island-arc fromthe NCB, had already closed by the Middle Devonian.

8. Conclusions

Our new data allow the following major conclusions:

(1) The FAS was deposited in a fore-arc basin at ca. 455–435 Ma,which is related to the northward subduction of the Shangdanoceanic crust in the Late Ordovician to Early Silurian.

(2) The uniform age spectra of ca.455–600 Ma in the FAS is evi-dence that sediments were derived only from the NQB whichindicates that the Shangdan Ocean still existed until the endof the Silurian.

(3) The detrital zircon ages of the Middle to Upper DevonianLiuling and Jiuliping Groups show a provenance from the SQBand NQB, as well as the SCB and NCB, implying that the closureof the Shangdan Ocean between the NCB and SCB occurredprior to Middle Devonian. Together with available geologicaland geochronological data, the closure time of the ShangdanOcean as well as the collision between the NCB and SCB wasproposed to have been Early Devonian.

Acknowledgments

This study was jointly supported by the National Natural ScienceFoundation of China (grants: 41190074, 41225008 and 40972140)and MOST Special Fund (BJ081331) from the State Key Laboratoryof Continental Dynamics, Northwest University. We gratefully ac-knowledge Dr. W. Xiao and Dr. C. Hauzenberger for their constructivecomments.

Fig. 11. Reconstructionmodels for the North China and South China blocks in Pangea during: (a) Late Ordovician to Early Silurian and (b)Middle to Late Devonian intervals (Modifiedfrom Scotese, 2004).

Fig. 10. Models for the tectonic evolution between the NCB and SCB: (a) Late Ordovician to Early Silurian and (b) Middle to Late Devonian intervals.



image of Fig.�11

image of Fig.�10



Appendix A. Supplementary data

Detailed U-Pb data for zircons from the FAS and Middle to UpperDevonian rocks are provided in Tables 2 and 3, respectively, and canbe found in the online version, at http://dx.doi.org/10.1016/j.tecto.2012.11.018.

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Timing of Paleozoic amalgamation between the North China ......Timing of Paleozoic amalgamation between the North China and South China Blocks: Evidence from detrital zircon U–Pb