pyroxenite–gabbro–diorite suite from xinghe, inner u–pb ......late palaeoproterozoic...

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Late Palaeoproterozoic post-collisional magmatismin the North China Craton: geochemistry, zirconU–Pb geochronology, and Hf isotope of thepyroxenite–gabbro–diorite suite from Xinghe, InnerMongoliaQiong-Yan Yanga, M. Santosha & Guochen Donga

a School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing,ChinaPublished online: 29 Apr 2014.

To cite this article: Qiong-Yan Yang, M. Santosh & Guochen Dong (2014) Late Palaeoproterozoic post-collisional magmatism inthe North China Craton: geochemistry, zircon U–Pb geochronology, and Hf isotope of the pyroxenite–gabbro–diorite suite fromXinghe, Inner Mongolia, International Geology Review, 56:8, 959-984, DOI: 10.1080/00206814.2014.908421

To link to this article: http://dx.doi.org/10.1080/00206814.2014.908421

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Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry,zircon U–Pb geochronology, and Hf isotope of the pyroxenite–gabbro–diorite suite from Xinghe,

Inner Mongolia

Qiong-Yan Yang, M. Santosh* and Guochen Dong

School of Earth Sciences and Resources, China University of Geosciences Beijing, Beijing, China

(Received 4 February 2014; accepted 22 March 2014)

The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to latePalaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonicframework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North ChinaOrogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical,and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in InnerMongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from thegabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from thesuite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The εHf(t) showsmostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantlyjuvenile sources. The generally low SiO2 and high MgO values, and other trace element features of the Xinghe suite areconsistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustalcontamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within theColumbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This,together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest atectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following thewestward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwellingand heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithosphericmantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.

Keywords: mafic–ultramafic suite; post-collisional magmatism; geochemistry; zircon U–Pb geochronology and Lu–Hfisotopes; North China Craton

Introduction

In continental collision zones, the detachment of oceaniclithosphere from continental lithosphere following oceanclosure leads to slab break-off. Slab break-off resultsfrom extensional deformation of the subducted slabcaused by opposing buoyancy forces induced by thesubduction of dense oceanic lithosphere and buoyantcontinental lithosphere (Huw Davies and vonBlanckenburg 1995). The resultant slab window providesa pathway for asthenospheric upwelling and heat inputleading to magma generation from multiple sourcesincluding the upwelling asthenospheric mantle, litho-spheric mantle enriched by subduction components, andthe overlying lower crust (Zhu et al. 2013). Post-collisionalmagmatism induced by slab break-off thus leads to theformation of a wide variety of ultramafic, mafic, andintermediate magmatic suites, distinct from the bimodalsuites developed within intraplate rift zones (e.g. Coulonet al. 2002; Zhu et al. 2009, 2013). A typical example ofmagma flare-up associated with slab break-off and

asthenospheric upwelling has been reported from Tibet,generating nearly 2000 m-thick volcanic sequences in theLinzou Basin together with a diverse rock suite withheterogeneous composition correlated with astheno-spheric upwelling following slab break-off (Mo et al.2008; Zhu et al. 2013). Post-collisional magmatisminvolving discrete pulses and producing volcano-plutonicassociations with different chemical compositions hasbeen reported from the western Anatolia region byDilek and Altunkaynak (2009). The thermal input andmelt sources for this prolonged magmatism are correlatedwith asthenospheric flow initially through slab break-off,followed by delamination and finally by tectonic exten-sion. The magmatism has been correlated with decom-pressional melting of asthenospheric mantle. Slab break-off magmatism can also lead to magma mixing, as illu-strated in the case of the Kalatongke diorites by Gao andZhou (2013), where the magmatism is correlated with thesubduction and post-collision tectonics in the ChineseAltay. The dioritic suite in this case formed from an

*Corresponding author. Email: [email protected]

International Geology Review, 2014Vol. 56, No. 8, 959–984, http://dx.doi.org/10.1080/00206814.2014.908421

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evolved mantle-derived basaltic magma through the par-tial melting of subduction-modified mantle source.

Whereas most of the examples for magmatism trig-gered by slab break-off have been documented fromPhanerozoic terranes, similar processes also operated inPrecambrian continental collision zones. A typical exam-ple is the Trans-North China Orogen (TNCO), a majorPalaeoproterozoic collision zone in the North ChinaCraton (NCC) (Zhao and Zhai 2013, and referencestherein). Following the collision between the Easternand Western blocks of the NCC along the TNCO at ca.1.85 Ga (or 1.80 Ga; see Lu et al. 2013), a number ofmagmatic suites were emplaced along the northern mar-gin of the craton (Zhang et al. 2007) including A-typegranites, anorthosites and rapakivi granites, and maficdikes (Zhang et al. 2007; Zhao et al. 2009a; Peng et al.2010; Jiang et al. 2011). Late Palaeoproterozoic post-collisional magmatic suites also occur in the central andsouthern segments of the TNCO, which include char-nockites, granites, mafic dikes, and volcanic suites(Li et al. 2001; Geng et al. 2004, 2006; Wang et al.2004, 2010; Peng et al. 2005, 2008; Han et al. 2007;Hou et al. 2008; Liu et al. 2009; Zhao et al. 2009a). Thepost-collisional suite of rocks with ages ranging from ca.1.78–1.68 Ga is considered to extend for more than500 km across the border between the TNCO and theEastern Block along the northern margin of the NCC,marking one of the major magmatic belts. Debates sur-round the origin of the late Palaeoproterozoic magmaticsuites emplaced along the northern margin of the NCCwith some models linking these to the rifting of the NCCin response to the global break-up of the supercontinentColumbia (e.g. Lu et al. 2008). However, recent studiesthat integrate precise geochronology and palaeogeo-graphic models have clearly established that the plume-related rifting of the NCC within the Columbia super-continent and associated magmatic activity occurredmuch later in the late Mesoproterozoic (1.3–1.2 Ga)(Wang et al. 2014).

In this study, we report a pyroxenite–gabbro–dioritesuite from Xinghe in the northern domain of the TNCOand present petrologic, geochemical, zircon U–Pb, andLu–Hf data. Our results link the magma genesis to post-collisional slab break-off and asthenospheric upwelling,with magma generation through partial melting of meta-somatized mantle and subduction-related components.

Geological setting

The NCC (Figure 1), covering an area of over 1.7 millionsquare kilometres in the northeastern part of China andInner Mongolia and extending into Korea as part of theSino-Korean Craton, preserves one of the oldestPrecambrian cores of Asia. The NCC is bordered on thesouth by the Qinling–Dabie Shan Orogen, to the north by

the Central Asian Orogenic Belt, and to the east by theSu–Lu Belt (Kusky and Li 2003; Zhai and Santosh 2011;Zhao and Zhai 2013). The Archaean to Palaeoproterozoicbasement of the NCC has been divided into the Easternand Western blocks (Figure 1), separated by the TNCO(Zhao et al. 2001, 2005; Santosh 2010; Zhai and Santosh2011; Zhao and Zhai 2013). The Western Block is acollage of two discrete crustal segments, the ArchaeanYinshan Block to the north and the dominantlyPalaeoproterozoic Ordos Block to the south, weldedalong the Inner Mongolia Suture Zone (incorporating the‘Khondalite Belt’) at around 1.95–1.90 Ga (Santosh 2010;Zhai and Santosh 2011; Wang et al. 2013; Zhao et al.2013; Zhao and Zhai 2013, and references therein;Santosh et al. 2013). The Jiao–Liao–Ji Belt in the eastrepresents a Palaeoproterozoic rift that closed an internalocean, and this, together with the Inner Mongolia SutureZone and the TNCO, define the three majorPalaeoproterozoic sutures within the NCC (Santosh et al.2012, 2013; Zhao and Zhai 2013). The Western Block is astable craton with a thick subcontinental mantle root, inthe absence of any major earthquakes, and low heat flow,albeit mostly covered by younger sediments (Kusky 2011;Zhao and Zhai 2013). In contrast, the Eastern Block ischaracterized by numerous earthquakes, high heat flow,and an eroded mantle root with a differentially thinnedlithosphere. The Eastern Block of the NCC is consideredto be a classic example of craton destruction associatedwith extensive Mesozoic magmatism and metallogeny inresponse to crust–mantle interaction induced by Pacificplate subduction from the east as well as the tectonicsassociated with plate reorientation (Menzies et al. 1993;Guo et al. 2013; Yang et al. 2014a; Goldfarb and Santosh2014; Yang and Santosh 2014).

The TNCO is a major subduction–collision belt devel-oped during the amalgamation of the Western and Easternblocks and the final cratonization of the NCC at ~1.85–1.80 Ga (Zhao et al. 2005; Santosh 2010; Santosh et al.2012; Lu et al. 2013; Zhao and Zhai 2013). Our study area(Figure 2) is located in the northern part of the NCC,where three different terranes, the Huai’an, Fengzhen,and Yinshan, are juxtaposed from southeast to northwest(Figure 1). The Huai’an terrane corresponds to the ca. 2.5Ga tonalite–trondhjemite–granodiorite (TTG) gneisseswith some Palaeoproterozoic mafic dikes that have under-gone ca. 1.85 Ga high-pressure granulite facies meta-morphism, together with 2.0–1.9 Ga high-potassicgranitoids (Zhai et al. 1992; Liu et al. 2009; Peng et al.2010). The Yinshan terrane is composed of ca. 2.5 Ga lateArchaean TTG gneisses and granulites, together with agranite–greenstone belt (e.g. Zhao et al. 2005) and cov-ered by Palaeoproterozoic sediments (Wan et al. 2009).The Fengzhen terrane is represented by the ‘KhondaliteBelt’, comprising a vast Palaeoproterozoic accretionarysequence of metasediments and metacarbonates

960 Q.-Y. Yang et al.

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metamorphosed to high and ultra-high temperature condi-tions, together with imbricated oceanic fragments (amphi-bolites, metagabbros, banded iron formations), and arc-related continental fragments (charnockites, granitoids)developed during a prolonged subduction–accretion his-tory prior to final collision in late Palaeoproterozoic(Santosh et al. 2012, 2013).

Our present study area in the Xinghe country of theInner Mongolia region in North China is located atthe junction between the E–W-trending Inner MongoliaSuture Zone between the Yinshan and Ordos blocks, andthe TNCO between the Western and Eastern blocks of theNCC (Figures 1 and 2). The geology of this region hasbeen summarized by Santosh et al. (2013) and the majorrock types include charnockites, TTG, khondalites (gran-ulite facies metapelites), and minor banded iron formationsformed within a subduction–accretion–collisional setting.The TTG gneisses locally incorporate lenses and blocks ofgarnet-bearing mafic granulites. The surrounding khonda-lites represent continental shelf sequences incorporatingabundant graphite mineralization such as that in theHuangtuyao mine. Some workers consider that the

protoliths of these rocks formed within an active marginin an arc-related setting (Dan et al. 2012).

Samples for the present study were collected fromtwo locations in the Xinghe county; one near Lujiayingvillage (40° 37′ 56.37″ N; 113° 50′ 42.06″ E; height1452 m) and the other from Xinghuagou village (40°52′ 47.98″ N; 113° 59′ 05.54″ E; height 1240 m)(Figure 2(a) and (b). In the Lujiaying location, a steeproad cutting exposes fresh rocks for more than 1 km onthe hill slopes on both sides. The dominant rock types areTTG gneisses and massive charnockites which constitutethe Neoarchaean basement rocks. The gabbro–dioritesuite of rocks occurs as large bands and lenses of up to60 m thickness intruding the TTG gneisses (Figure 3(a)).They are medium to fine grained, dark, and homogenous.The base of the gabbro unit is lined by 0.5–1 m thickbands of pyroxenites (websterites) (Figure 3(b)). Thewebsterites are medium to coarse grained and dark green-ish, showing cumulate texture. All the rocks have beendeformed and locally faulted, sliced, and imbricated, withrepeating sequences visible for over 500 m along the roadcutting. In Xinghuagou, medium-grained, homogenous,

Figure 1. Generalized tectonic framework of the North China Craton showing the major crustal blocks and intervening suture zones(after Zhao et al. 2005; Santosh 2010). The location of Figure 2(a) is shown by the box. The locations of some of the other majorPaleoproterozoic magmatic suites as discussed in the text are also shown (red circles).

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dark gabbroic rocks, ranging in thickness up to 10 m,occur within granulite facies metapelites (khondalites)(Figure 3(c)). The khondalites are garnet- and sillima-nite-bearing aluminous granulite facies gneisses and atplaces show cordierite and spinel (Figure 3(d)). Ourrecent petrological and zircon U–Pb geochronologicalstudies (Yang et al. 2014b) reveal that these rocks weresubjected to ultra-high temperature metamorphism under

P–T conditions of 6.5–7.5 kbar and 930–1050°C at ca.1.88 Ga. The websterite-gabbro–diorite suite fromXinghe is unmetamorphosed, and resembles those ofthe latest Palaeoproterozoic post-collisional magmaticsuites described from elsewhere along the TNCO suchas the anorthosite–mangerite–charnockite–granite asso-ciation in the Damiao region of Chengde (Zhang et al.2007; our unpublished age data).

Figure 2. (a) Geological map of part of the Inner Mongolia region in the North China Craton, showing the study area and the twolocations described in the text from where samples were collected for this study. The location of Figure 2(b) is shown by the box. (b)Geological map of the Xinghe area, showing the locations of samples for zircon geochronology and Lu–Hf isotope studies (map modifiedfrom Lu et al. 1996, Figure 2-1).


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Sampling and analytical techniques

Sample description

The salient details of all the samples used for whole rockgeochemical and zircon analysis, including the webster-ites, gabbros, and diorite from Xinghe, are listed inTable 1, and a summary of the mineral assemblages inthe representative samples is given below.

The websterite (samples OY-XH-5c, OY-XH-5d, andOY-XH-5e; GPS co-ordinates 40° 37′ 56.37″ N; 113° 50′42.06″ E; height 1452 m) shows sub-equal amounts ofeuhedral and coarse grained (0.5–3 mm) orthopyroxene(Opx) and clinopyroxene (Cpx), with very minor biotite(Figure 4(a) and (b)). Both Opx and Cpx show minoralteration along cleavage traces and grain margins intohornblende and biotite. Biotite also occurs along the inter-stice or the margin of Opx and Cpx crystals.

Samples OY-XH-5h, OY-XH-5i, OY-XH-5j, OY-XH-5k, and OY-XH-5k/1 are from the gabbros in the Xinghe

area, from the same location of the websterites nearLujiaying village. These rocks are composed of bothfemic minerals (43%) and felsic minerals (57%). Thefemic minerals include sub-equal amounts of medium-tocoarse-grained Cpx (8–10%) and Opx (8–10%), togetherwith hornblende (15–18%), minor biotite, and few mag-netite grains. The felsic minerals are mainly plagioclasewith minor K-feldspar. The pyroxene and plagioclasegrains show subhedral morphology set in a typical gab-broic texture (Figure 4(c) and (d)).

The diorite sample OY-XH-3B is from a stream sectionnorth of the above location (40° 52′ 47.98″ N; 113° 59′05.54″ E; height 1240 m) in Xinghuagou village. The majorminerals in this rock are feldspar (up to 60–65%) andhornblende (up to 25–27%), with minor pyroxene (up to4% including Opx and Cpx) (Figure 4(e) and (f)). Thefeldspar is mainly plagioclase (65–70%, together withminor K-feldspar) which occurs as phenocrysts of up to1 mm size and shows slight alteration. The amphiboles are

Figure 3. Field photographs from Lujiaying and Xinghuagou locations discussed in the text. (a) Gabbro emplaced within TTG gneissesand showing displacement through later faulting (location: Lujiaying) (b) Contact between websterite and gabbro (location: Lujiaying).(c) Dioritic rocks exposed along a stream section (location: Xinghuagou). (d) Ultra-high temperature granulites with cordierite and spinelfrom Hongsigou.


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also partly altered. The accessory minerals are magnetite(1–2%), quartz (2%), and very minor biotite (<1%).

Sample preparation and imaging

Polished thin sections were prepared for petrographicstudy at Peking University and the petrographic studywas done at the China University of Geosciences,Beijing. Zircons grains were separated using standardprocedures for U–Pb dating and Hf analyses at theYu’neng Geological and Mineral Separation SurveyCentre, Langfang city, Hebei Province, China. The cath-odoluminescence (CL) imaging was carried out at theBeijing Geoanalysis Centre. Individual grains weremounted along with the standard TEMORA 1, with a206Pb/238U age of 417 Ma (Black et al. 2003), onto dou-ble-sided adhesive tape and enclosed in epoxy resin discs.The discs were polished to a certain depth and gold coatedfor CL imaging and U–Pb isotope analysis. Zircon mor-phology and internal structure were examined using aJSM-6510 Scanning Electron Microscope (SEM, JEOL,Tokyo, Japan) equipped with a backscatter probe andChroma CL probe. The zircon grains were also examinedunder transmitted and reflected light images using a pet-rological microscope. The CL images of representativezircons are shown in Figures 5 and 6.

Zircon U–Pb and Hf isotopic analysis

Zircon U–Pb analysis was performed by laser ablationinductively coupled plasma spectrometry (LA-ICP-MS) atthe National Key Laboratory of Continental Dynamics of

Northwest University (Xi’an) (samples OY-XH-5h and OY-XH-5i), Peking University (Beijing) (sample OY-XH-3B),and the Tianjin Institute of Geology and Mineral Resources(Tianjin) (sample OY-XH-5j). In situ zircon Hf isotopicanalyses were performed at the National Key Laboratoryof Continental Dynamics of Northwest University (samplesOY-XH-5h and OY-XH-5i) and the Tianjin Institute ofGeology and Mineral Resources (samples OY-XH-3B andOY-XH-5j). The analyses were conducted on the samespots or in adjacent domains where U–Pb dating wasdone. At the Tianjin Institute of Geology and MineralResources, zircon U–Pb dating and in situ Hf isotopicanalyses were conducted using a Neptune MC-ICP-MS(Thermo Fisher Scientific, Waltham, MA, USA) equippedwith 193 nm Geolas Q Plus ArF exciplex at the laserablation, with spot sizes of 35 and 50 μm, respectively.Zircon GJ-1 was used as an external standard for U–Pbdating and in situ zircon Hf isotopic analyses. Common Pbcorrections were made using the method of Andersen(2002). Data were processed using the GLITTER andISOPLOT X (Ludwig 2003) programs. Errors on individualanalyses by LA-ICP-MS are quoted at the 95% (1σ) con-fidence level. Details of the technique are described by Liet al. (2009) and Geng et al. (2011).

Zircon U–Pb analysis at Peking University and at theNational Key Laboratory of Continental Dynamics ofNorthwest University followed the analytical proceduresreported in Yuan et al. (2004). In the LA-ICP-MS method,the laser spot diameter and frequency were 30 μm and 10Hz, respectively. Zircon 91500 was employed as a stan-dard and the standard silicate glass NIST was used tooptimize the instrument. Raw data were processed using

Table 1. Details of samples analysed for whole rock geochemical analysis and zircon U–Pb and Hf isotopes.

No. Sample No. Rock type Locality Mineralogy

1 OY-XH-5c Websterite Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Opx + Cpx + Bt

2 OY-XH-5d Websterite Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Opx + Cpx + Bt

3 OY-XH-5e Websterite Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Opx + Cpx + Bt

4 OY-XH-3B Diorite Xinghuagou village (GPS co-ordinates 40° 52′47.98″ N; 113° 59′ 05.54″ E; height 1240 m)

Plag + K-fs + Opx + Cpx + Bt + Hbl + Sph + Qtz

5 OY-XH-5j Gabbro Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Plag + K-fs + Opx + Cpx + Bt + Hbl + Mt

6 OY-XH-5k Gabbro Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)


7 OY-XH-5k/1 Gabbro Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)


8 OY-XH-5h Gabbro Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Plag + K-fs + Bt + Hbl+ Opx + Cpx + Qtz + Mt

9 OY-XH-5i Gabbro Lujiaying village (GPS co-ordinates 40° 37′56.37″ N; 113° 50′ 42.06″ E; height 1452 m)

Plag + K-fs + Bt + Hbl+ Opx + Cpx+ Qtz + Mt

Note: Mineral abbreviations: Opx, orthopyroxene; Cpx, clinopyroxene; Bt, biotite; Hbl, hornblende; K-fs, K-feldspar; Plag, plagioclase; Qtz, quartz; Mt,magnetite; Sph, sphene.


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the GLITTER program to calculate isotopic ratios andages of 207Pb/206Pb, 206Pb/238U, and 207Pb/235U(Table 2). Data were corrected for common lead, accord-ing to the method of Andersen (2002), and calculated the

ages by ISOPLOT 4.15 software (Yuan et al. 2004). Theanalytical procedures of in situ Hf isotope followed thosedescribed by Yuan et al. (2008). An energy density of15–20 J/cm2 and a spot size of 45 μm were used. The

Figure 4. Photomicrographs from (a) and (b) websterite (OY-XH-5c), (c) and (d) gabbro (OY-XH-5h), and (e) and (f) diorite (OY-XH-3B). (a), (c), and (e) in parallel nicols; (b), (d), and (f) in crossed nicols. Mineral abbreviations: Opx, orthopyroxene; Cpx, clinopyroxene;Hbl, hornblende; Plag, plagioclase; Mt, magnetite.


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flattest, most stable portions of the signal were selected foranalysis. Isobaric interference of 176Lu on 176Hf wasadjusted by measuring the intensity of the interference-free 175Lu isotope and using a recommended 176Lu/175Luratio of 0.02669 (DeBievre and Taylor 1993) to calculate176Lu/177Hf ratios. Adjustment for the isobaric interferenceof 176Yb on 176Hf was performed in ‘real time’ as advo-cated by Woodhead et al. (2004), which involved measur-ing the interference-free 172Yb and 173Yb during theanalysis, calculating the mean βYb value from 172Yb, and173Yb and using the recommended 176Yb/172Yb ratio of0.5886 (Chu et al. 2002). Zircon 91500 was used as thereference standard with a recommended 176Hf/177Hf ratioof 0.282306 ± 10 (Woodhead et al. 2004). All the Lu–Hfisotope analysis results are reported with an error of 1σ.The decay constant of 176Lu of 1.865 × 10−11 year−1 wasadopted (Scherer et al. 2001). Initial 176Hf/177Hf ratios εHf(t) were calculated with reference to the chondritic reser-voir (CHUR) of (Blichert-Toft and Albarede 1997) at thetime of zircon growth from the magma. Single-stage Hf

model age (TDM) was calculated with respect to thedepleted mantle with present-day 176Hf/177Hf = 0.28325and 176Lu/177Hf = 0.0384 (Griffin et al. 2000). Crustalmodel age (TDM

C) was calculated with respect to theaverage continental crust with a 176Lu/177Hf ratio of0.015 (Griffin et al. 2002).

Whole rock major and trace element analysis

Major elements were analysed on glass discs fused withLi-borate flux by X-ray fluorescence spectrometry (XRF)at China University of Geosciences, Beijing, following themethod of Harvey (1989). Trace elements (including rareearth element (REE)) were analysed using a FinniganMAT Element 2, high resolution ICP-MS at the StateKey Laboratory of Geological Processes and MineralResources (GPMR), following procedures described byLiang et al. (2000). The Chinese national standard GSR-1 (Xie et al. 1989) was used to monitor analyses.

Figure 5. Cathodoluminescence (CL) images of representative zircon grains from diorite sample OY-XH-3B and gabbro sample OY-XH-5j. The values written against each grain represent the spot age in Ma (top) and the εHf(t) calculated for the 207Pb/206Pb mean age(bottom).


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Results

Zircon U–Pb geochronology

The analytical results of zircons from four samples fromXinghe are presented in Table 2 and representative zirconCL images are shown in Figures 5 and 6.

Zircons in the diorite sample OY-XH-3B are prismaticand euhedral to subhedral with grain sizes ranging from150 × 200 to 400 × 200 μm and length/width ratios of 2:1to 4:1. The zircon grains are colourless or light brown withno obvious inclusions and some of the grains displaytypical core-rim texture in CL images (Figure 5), suggest-ing dissolution of the grain margins and recrystallizationduring the late magmatic stage. However, the rims are toothin for age dating. The cores are weakly zoned orunzoned and dark luminescent with medium brightnessand many of the cores show prismatic morphology, sug-gesting their magmatic origin. The Th contents range from31 to 287, and U contents from 13 to 323 ppm, with highTh/U values from 0.36 to 3.06. Twenty-six zircon grainsfrom the diorite sample were analysed. All the analysedspots yield concordant 207Pb/206Pb ages with a weightedmean age of 1767 ± 13 Ma (n = 26, MSWD = 1.6; Figure

7(a) and (b)). Although typical oscillatory zoning is notwell defined in CL images, comparable to the case forzircons from gabbroic rocks crystallizing under high tem-perature, the high Th/U ratios (0.36–3.06) of these zirconsare indicative of magmatic origin. Thus, the weightedmean age of ca. 1767 Ma is interpreted to represent thetiming of magma emplacement of the gabbro.

Twenty-eight zircons were analysed from gabbro sam-ple OY-XH-5j for U–Pb dating. Most of the zircons arecolourless and some grains are light brown. The grainsrange from 80 × 50 to 200 × 100 μm with aspect ratios of1.5:1 to 2.5:1. They are mostly stubby to long prismatic inshape with some grains occurring as small ellipsoidalgrains. Most zircon grains in the sample are subhedral asshown in CL images (Figure 5) and exhibit well-devel-oped oscillatory zoning, suggesting magmatic crystalliza-tion (Vavra et al. 1999; Hoskin and Ireland 2000; Hoskinand Schaltegger 2003). U–Pb analyses were conducted on28 spots (Table 2), and the results show Th contents of10–201 ppm and U contents of 99–308 ppm. The Th/Uratios are in the range 0.48–0.83 (except one spot, with aTh/U ratio of 0.06), suggesting magmatic origin. The 28analyses yield concordant 207Pb/206Pb ages, ranging from

Figure 6. Cathodoluminescence (CL) images of representative zircon grains from gabbro samples OY-XH-5h and OY-XH-5i. Thevalues against each grain represent the spot age in Ma (top) and the εHf(t) calculated for the 207Pb/206Pb mean age (bottom).


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Table

2.ZirconU–P

bagedata

ofgabb

roic

anddioriticrocksfrom

Xingh

e,North

China

Craton.

Sam

ple

Pb

Th

U207Pb/

206Pb

207Pb/

235U

206Pb/

238U

207Pb/

206Pb

207Pb/

235U

206Pb/

238U

spot

(ppm

)(ppm

)(ppm

)Th/U

ratio

1σratio

1σratio

1σMa

1σMa

1σMa

1σDiscordance

OY-X

H-3B-01

210

9012

91.44

0.10

500.00

144.51

620.05

900.31

220.00

3417

1524

1734

1117

5217

2OY-X

H-3B-02

211

5113

22.59

0.10

490.00

144.58

390.05

890.31

730.00

3417

1324

1746

1117

7617

4OY-X

H-3B-03

282

108

171

1.58

0.10

930.00

154.80

290.06

390.31

900.00

3517

8825

1785

1117

8517

0OY-X

H-3B-04

381

156

235

1.50

0.10

710.00

154.64

850.06

230.31

510.00

3417

5125

1758

1117

6617

1OY-X

H-3B-05

138

3486

2.51

0.10

550.00

154.61

240.06

250.31

740.00

3517

2425

1752

1117

7717

3OY-X

H-3B-06

3037

130.36

0.10

960.00

264.83

870.1149

0.32

050.00

4917

9343

1792

2017

9224

0OY-X

H-3B-07

495

250

302

1.21

0.10

750.00

144.63

790.05

870.31

320.00

3317

5823

1756

1117

5716

0OY-X

H-3B-09

241

7015

02.16

0.10

620.00

144.65

400.05

850.31

830.00

3417

3523

1759

1117

8116

3OY-X

H-3B-12

154

5196

1.89

0.10

800.00

174.64

430.07

130.31

230.00

3617

6628

1757

1317

5218

1OY-X

H-3B-13

194

7512

11.62

0.10

750.00

154.63

560.06

180.31

300.00

3417

5825

1756

1117

5517

0OY-X

H-3B-14

4531

250.80

0.10

800.00

214.65

170.08

740.31

290.00

4117

6534

1759

1617

5520

1OY-X

H-3B-16

196

6512

11.84

0.10

880.00

184.75

240.07

580.31

720.00

3817

8029

1777

1317

7619

0OY-X

H-3B-20

455

198

277

1.40

0.1103

0.00

134.84

320.05

620.31

890.00

3318

0422

1792

1017

8416

1OY-X

H-3B-21

228

4915

03.06

0.10

670.00

164.52

900.06

460.30

830.00

3417

4326

1736

1217

3217

1OY-X

H-3B-23

444

158

273

1.73

0.1106

0.00

164.88

320.06

690.32

060.00

3518

0925

1799

1217

9317

1OY-X

H-3B-24

239

8814

81.69

0.10

640.00

164.64

580.06

730.31

720.00

3617

3827

1758

1217

7617

2OY-X

H-3B-25

509

287

309

1.08

0.10

970.00

144.71

680.05

770.31

210.00

3217

9523

1770

1017

5116

3OY-X

H-3B-27

305

134

186

1.39

0.10

890.00

144.76

730.06

080.31

780.00

3417

8124

1779

1117

7916

0OY-X

H-3B-28

198

6312

01.91

0.1109

0.00

164.95

090.06

810.32

420.00

3618

1425

1811

1218

1017

0OY-X

H-3B-29

303

126

182

1.45

0.1107

0.00

164.92

770.06

790.32

320.00

3518

1125

1807

1218

0517

0OY-X

H-3B-30

207

5513

42.42

0.10

690.00

154.57

840.06

130.31

090.00

3317

4825

1745

1117

4516

0OY-X

H-3B-31

353

117

221

1.88

0.10

870.00

174.72

450.07

160.31

550.00

3617

7828

1772

1317

6818

1OY-X

H-3B-32

539

248

323

1.30

0.1105

0.00

144.91

280.05

920.32

290.00

3318

0722

1804

1018

0416

0OY-X

H-3B-33

257

6216

82.69

0.10

680.00

164.56

200.06

520.31

020.00

3417

4526

1742

1217

4217

0OY-X

H-3B-34

314

128

193

1.51

0.10

940.00

174.80

400.07

110.31

880.00

3617

9027

1786

1217

8418

0OY-X

H-3B-35

149

5095

1.92

0.10

640.00

174.52

070.06

870.30

860.00

3517

3828

1735

1317

3417

0OY-X

H-5h-11

1959

1010

771

1.31

0.10

720.00

224.75

180.07

260.32

170.00

4617

5237

1776

1317

9822

3OY-X

H-5h-12

876

326

387

0.84

0.10

740.00

244.69

830.08

320.31

740.00

4717

5540

1767

1517

7723

1OY-X

H-5h-13

1086

396

505

0.78

0.10

600.00

224.62

950.07

410.31

680.00

4517

3238

1755

1317

7422

2OY-X

H-5h-14

1006

332

500

0.66

0.10

470.00

224.52

010.07

280.31

330.00

4517

0938

1735

1317

5722

3OY-X

H-5h-15

803

290

389

0.74

0.10

620.00

244.71

250.08

390.32

190.00

4717

3640

1770

1517

9923

4OY-X

H-5h-16

1049

417

529

0.79

0.10

480.00

234.63

830.07

760.32

110.00

4617

1139

1756

1417

9523

5OY-X

H-5h-17

421

134

223

0.60

0.10

530.00

244.65

530.08

500.32

090.00

4717

1941

1759

1517

9423

4OY-X

H-5h-18

578

217

305

0.71

0.10

480.00

234.61

440.08

050.31

950.00

4717

1140

1752

1517

8723

4OY-X

H-5h-19

508

161

270

0.60

0.10

710.00

244.73

780.08

480.32

110.00

4717

5041

1774

1517

9523

3OY-X

H-5h-20

602

206

324

0.63

0.10

860.00

244.80

190.08

240.32

080.00

4717

7640

1785

1417

9423

1OY-X

H-5h-21

339

9318

80.50

0.10

970.00

264.91

520.09

320.32

510.00

4917

9442

1805

1618

1524

1OY-X

H-5h-22

566

151

328

0.46

0.10

640.00

234.67

880.07

520.31

910.00

4617

3938

1764

1317

8522

3OY-X

H-5h-23

278

8315

90.52

0.10

880.00

254.79

470.08

770.31

980.00

4717

7941

1784

1517

8923

1OY-X

H-5h-24

846

359

464

0.77

0.10

790.00

224.92

650.07

310.33

130.00

4617

6436

1807

1318

4522

5OY-X

H-5h-25

752

327

424

0.77

0.10

970.00

224.92

240.07

360.32

570.00

4617

9437

1806

1318

1722

1OY-X

H-5h-26

434

174

247

0.70

0.10

850.00

244.81

600.08

250.32

220.00

4717

7440

1788

1418

0023

2OY-X

H-5h-27

247

6915

10.46

0.10

700.00

254.72

630.08

660.32

060.00

4717

4941

1772

1517

9323

3

(Con

tinued)


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P] a

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02 2

3 Se

ptem

ber

2014

Table

2.(Con

tinued).

Sam

ple

Pb

Th

U207Pb/

206Pb

207Pb/

235U

206Pb/

238U

207Pb/

206Pb

207Pb/

235U

206Pb/

238U

spot

(ppm

)(ppm

)(ppm

)Th/U

ratio

1σratio

1σratio

1σMa

1σMa

1σMa

1σDiscordance

OY-X

H-5h-28

240

6714

10.48

0.10

740.00

254.75

690.08

850.32

130.00

4817

5642

1777

1617

9623

2OY-X

H-5h-29

312

8219

30.42

0.10

790.00

244.79

380.08

450.32

230.00

4717

6540

1784

1518

0123

2OY-X

H-5h-30

330

9519

30.49

0.10

760.00

244.79

580.08

440.32

330.00

4717

6040

1784

1518

0623

3OY-X

H-5h-31

288

9017

30.52

0.1102

0.00

244.89

380.08

150.32

230.00

4618

0239

1801

1418

0123

0OY-X

H-5h-32

487

219

292

0.75

0.10

790.00

224.78

050.07

370.32

150.00

4517

6437

1782

1317

9722

2OY-X

H-5i-06

506

7229

90.24

0.10

640.00

224.84

430.07

360.33

010.00

4617

3937

1793

1318

3922

6OY-X

H-5i-12

420

9125

50.36

0.10

720.00

224.82

630.07

560.32

650.00

4617

5338

1790

1318

2122

4OY-X

H-5i-16

270

7116

70.43

0.10

650.00

244.83

840.08

810.32

960.00

4917

4041

1792

1518

3624

6OY-X

H-5i-17

330

104

205

0.51

0.10

750.00

254.83

710.08

960.32

650.00

4817

5742

1791

1618

2124

4OY-X

H-5i-18

435

6927

60.25

0.10

670.00

244.83

780.08

570.32

880.00

4817

4441

1792

1518

3323

5OY-X

H-5i-19

271

7516

30.46

0.10

710.00

244.87

390.08

440.33

010.00

4817

5140

1798

1518

3923

5OY-X

H-5i-21

449

104

280

0.37

0.10

670.00

234.86

130.08

060.33

050.00

4717

4439

1796

1418

4123

6OY-X

H-5i-22

254

7815

60.50

0.10

770.00

254.90

130.08

920.33

000.00

4917

6141

1803

1518

3924

4OY-X

H-5i-23

359

101

219

0.46

0.10

660.00

234.85

470.07

920.33

030.00

4717

4239

1794

1418

4023

6OY-X

H-5i-24

406

103

246

0.42

0.10

750.00

234.89

670.07

830.33

030.00

4717

5838

1802

1318

4023

5OY-X

H-5i-25

365

9322

40.42

0.10

670.00

234.85

090.08

050.32

990.00

4717

4339

1794

1418

3823

5OY-X

H-5i-26

509

160

309

0.52

0.10

760.00

234.89

530.07

640.32

990.00

4717

6038

1801

1318

3823

4OY-X

H-5i-27

442

8227

30.30

0.10

950.00

234.96

400.07

810.32

880.00

4717

9138

1813

1318

3323

2OY-X

H-5i-28

305

9418

50.51

0.10

950.00

244.95

930.08

430.32

850.00

4817

9140

1812

1418

3123

2OY-X

H-5i-29

463

111

282

0.40

0.1106

0.00

235.02

640.07

850.32

970.00

4718

0938

1824

1318

3723

2OY-X

H-5i-30

375

6223

60.26

0.10

660.00

234.84

020.07

920.32

950.00

4717

4139

1792

1418

3623

5OY-X

H-5i-31

8122

500.44

0.10

910.00

324.95

310.12

900.32

930.00

5617

8453

1811

2218

3527

3OY-X

H-5i-32

401

110

247

0.45

0.1108

0.00

245.01

090.08

120.32

810.00

4718

1238

1821

1418

2923

1OY-X

H-5i-33

163

1510

10.15

0.10

880.00

274.95

000.09

960.33

000.00

5017

7944

1811

1718

3924

3OY-X

H-5i-34

356

8221

70.38

0.10

990.00

245.00

000.08

310.33

000.00

4717

9839

1819

1418

3823

2OY-X

H-5i-35

303

7318

30.40

0.10

950.00

254.98

270.08

840.33

010.00

4817

9141

1816

1518

3923

3OY-X

H-5i-36

167

3610

00.35

0.1121

0.00

275.10

140.09

950.33

000.00

5018

3443

1836

1718

3824

0OY-X

H-5i-37

434

6726

90.25

0.1121

0.00

245.10

890.08

280.33

040.00

4718

3438

1838

1418

4123

0OY-X

H-5i-38

547

171

333

0.51

0.1102

0.00

244.99

960.08

120.32

910.00

4718

0239

1819

1418

3423

2OY-X

H-5i-39

397

9024

70.36

0.1119

0.00

245.07

140.08

240.32

860.00

4718

3139

1831

1418

3123

0OY-X

H-5i-40

297

8217

70.46

0.1126

0.00

265.13

100.09

080.33

030.00

4818

4341

1841

1518

4023

0OY-X

H-5i-41

123

1275

0.17

0.1141

0.00

315.20

730.12

130.33

080.00

5318

6648

1854

2018

4226

1OY-X

H-5i-42

351

7021

30.33

0.1113

0.00

255.08

870.08

870.33

140.00

4818

2140

1834

1518

4523

1OY-X

H-5i-43

118

2773

0.37

0.1139

0.00

315.17

810.1188

0.32

960.00

5318

6248

1849

2018

3726

1OY-X

H-5j-6

7217

020

80.82

0.10

900.00

074.74

780.03

250.31

590.00

1817

8312

1776

1217

7010

1OY-X

H-5j-9

6610

319

30.53

0.10

930.00

074.87

140.03

690.32

330.00

2017

8712

1797

1418

0611

1OY-X

H-5j-10

4579

130

0.61

0.10

860.00

074.83

180.03

500.32

280.00

1917

7613

1790

1318

0311

2OY-X

H-5j-12

6213

117

30.75

0.10

870.00

074.82

810.03

500.32

200.00

1917

7812

1790

1318

0011

1OY-X

H-5j-13

7116

219

50.83

0.10

980.00

084.89

850.03

620.32

360.00

1917

9613

1802

1318

0711

1OY-X

H-5j-14

5610

415

90.66

0.1106

0.00

084.90

720.03

670.32

190.00

2018

0913

1803

1417

9911

1OY-X

H-5j-15

6112

516

90.74

0.10

970.00

084.88

530.03

570.32

310.00

1917

9413

1800

1318

0511

1OY-X

H-5j-16

57114

162

0.70

0.10

930.00

074.86

590.03

440.32

300.00

1917

8712

1796

1318

0411

1

(Con

tinued)


Dow

nloa

ded

by [

b-on

: Bib

liote

ca d

o co

nhec

imen

to o

nlin

e U

P] a

t 23:

02 2

3 Se

ptem

ber

2014

Table2.

(Con

tinued).

Sam

ple

Pb

Th

U207Pb/

206Pb

207Pb/

235U

206Pb/

238U

207Pb/

206Pb

207Pb/

235U

206Pb/

238U

spot

(ppm

)(ppm

)(ppm

)Th/U

ratio

1σratio

1σratio

1σMa

1σMa

1σMa

1σDiscordance

OY-X

H-5j-17

4676

132

0.57

0.10

880.00

084.84

910.03

540.32

320.00

1917

7913

1793

1318

0610

1OY-X

H-5j-18

5189

144

0.61

0.10

910.00

084.85

250.03

750.32

260.00

1917

8413

1794

1418

0211

1OY-X

H-5j-19

4810

153

0.06

0.10

900.00

084.84

490.03

590.32

240.00

1917

8313

1793

1318

0111

1OY-X

H-5j-20

5498

152

0.64

0.10

950.00

084.87

310.03

530.32

260.00

1917

9213

1798

1318

0311

1OY-X

H-5j-22

3559

103

0.57

0.10

840.00

084.78

010.03

430.31

990.00

1717

7213

1781

1317

8910

1OY-X

H-5j-23

4882

141

0.58

0.10

840.00

074.8115

0.03

270.32

190.00

1817

7312

1787

1217

9910

1OY-X

H-5j-24

5810

817

00.63

0.10

920.00

074.80

960.03

180.31

940.00

1717

8612

1787

1217

8710

0OY-X

H-5j-25

5612

716

20.78

0.1104

0.00

074.82

880.03

190.31

720.00

1718

0612

1790

1217

7610

2OY-X

H-5j-26

6114

217

40.81

0.1106

0.00

074.89

730.03

410.32

110.00

1818

1012

1802

1317

9510

1OY-X

H-5j-27

3453

990.53

0.10

830.00

144.79

070.05

970.32

080.00

1817

7123

1783

2217

9410

1OY-X

H-5j-28

5498

159

0.62

0.1100

0.00

074.82

590.03

240.31

830.00

1717

9912

1789

1217

8110

1OY-X

H-5j-29

4785

137

0.62

0.10

880.00

094.82

810.03

910.32

180.00

1817

7914

1790

1417

9910

1OY-X

H-5j-30

6614

518

90.77

0.10

780.00

074.78

450.03

140.32

190.00

1717

6312

1782

1217

9910

2OY-X

H-5j-31

6813

119

60.67

0.10

980.00

094.84

280.04

100.31

990.00

1717

9615

1792

1517

8910

0OY-X

H-5j-32

5590

158

0.57

0.10

880.00

114.87

440.05

090.32

480.00

1817

8018

1798

1918

1310

2OY-X

H-5j-33

5480

163

0.49

0.10

800.00

084.69

190.03

440.31

520.00

1717

6513

1766

1317

669

0OY-X

H-5j-34

4466

128

0.51

0.10

960.00

094.92

900.03

980.32

620.00

1817

9314

1807

1518

2010

1OY-X

H-5j-35

6110

317

30.60

0.10

960.00

084.93

980.03

440.32

690.00

1717

9313

1809

1318

2310

2OY-X

H-5j-38

111

201

308

0.65

0.10

860.00

074.92

910.03

210.32

920.00

1717

7612

1807

1218

3410

3OY-X

H-5j-39

5576

160

0.48

0.10

910.00

084.90

140.03

400.32

570.00

1717

8513

1802

1318

1810

2


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1810 to 1763 Ma, with a weighted mean age of1786.1 ± 4.8 Ma (n = 28, MSWD = 0.91; Figure 7(c)and (d)). We interpret this age to represent the magmaemplacement time for Xinghe gabbro.

Another two gabbro samples (OY-XH-5h and OY-XH-5i) from the Xinghe area were also analysed for U–Pb.Most of the zircon grains from the sample OY-XH-5h arecolourless and some are light brownish. The grains rangefrom 80–200 × 50–120 μm in size with aspect ratios of 3:1to 2:1. They are mostly stubby to long prismatic in shapewith some grains occurring as small ellipsoidal grains.Most zircon grains in the sample are subhedral, and inCL images (Figure 6), they display core-rim texture. Thecores are weakly zoned or unzoned and very dark lumi-nescent with medium brightness and many of the coresshow prismatic morphology, suggesting magmatic origin.The rims are unzoned and weakly luminescent, and sug-gest late recrystallization by fluids. The unzoned zirconrims have Th contents of 67–217 ppm and U contents of141–305 ppm. The Th/U ratios show a range of 0.42–0.71(Table 2). The eight analyses are relatively concordant(discordance ranging from 1% to 4%) and show 207Pb/206Pb ages of 1794–1711Ma. Analytical data from thezircon cores show 207Pb/206Pb ages of 1802–1709 Ma,broadly overlapping with the rim ages. Their Th contentsshow a range of 90–1010 ppm and U contents show a

range of 173–771 ppm. The Th/U ratios are in the range0.46–1.31 (Table 2). The above features, together withtheir oscillatory zoned CL images, indicate that the zirconscrystallized from magma (Vavra et al. 1999; Hoskin andIreland 2000; Hoskin and Schaltegger 2003). The 22 spotscombining the rims and cores of zircons from this sampleyield a 207Pb/206Pb weighted mean age of 1754 ± 16 Ma(MSWD = 0.48, n = 22, Figure 8(a) and (b)).

In the gabbro sample OY-XH-5i, most of the zircongrains are colourless and range from 50–160 × 30–80 μmin size with aspect ratios of about 2.5:1 to 1:1. They aremostly subhedral, and in CL images (Figure 6), they dis-play medium brightness and long prismatic morphology,suggesting magmatic origin. Some grains carry unzonedrims that are weakly luminescent, but are too narrow foranalysis. A total of twenty-nine spots were analysed fromthe core domains and the results show Th contents of 12–171 ppm and U contents of 50–351 ppm. The Th/U ratiosare in the range 0.15–0.56 and the 207Pb/206Pb ages rangefrom 1866 to 1651 Ma (Table 2). The 29 spots form acoherent group on the concordia yielding a 207Pb/206Pbweighted mean age of 1783 ± 15 Ma (MSWD = 0.91,n = 29, Figure 8(c) and (d)). In summary, the weightedmean ages in the narrow range of 1.78–1.77 Ga obtainedfrom zircons in the two samples are considered to repre-sent the timing of emplacement of the rocks.

Figure 7. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the diorite sample OY-XH-3B.Zircon U–Pb concordia plots (c) and age data histograms with probability curves (d) for the gabbro sample OY-XH-5j.


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Zircon Lu–Hf isotopes

The analytical results of Lu–Hf isotopic ratios are listed inTable 3. The data show that the 176Lu/177Hf ratios are lessthan 0.002, indicating the absence of any major enrich-ment of radiogenic Hf after the formation of the zircons.Therefore, the initial 176Hf/177Hf ratios can be used toevaluate the source characteristics of these zircons (Wuet al. 2007). The fLu/Hf values display a tight rangebetween –1.00 and –0.95, which are obviously lowerthan the fLu/Hf values of mafic crust (–0.34, Amelin et al.2000) and sialic crust (–0.72, Vervoort et al. 1996). InTable 3, TDM represents the depleted mantle age to eval-uate the time of source material extraction from thedepleted mantle, and TDM

C represents the residence timeof the source material in the crust (Blichert-Toft andAlbarede 1997). A total of 28 zircon grains were analysed,and the results show εHf(t) values, computed using the207Pb/206Pb mean age as defined from the concordiaplots of each sample, ranging from –4.2 to 5.8(Figures 7, 8 and Table 2). The slightly negative valuessuggest a reworked lower crustal source and the positivevalues suggest the input of juvenile components. The εHf(0)values (see Table 3) are in the range – 43.8 to –32.8. Thismight suggest that the source material of the magmas had along residence time in the mantle. Hf model ages (tDM and

tDMC) are within the range 2350 to 1948 Ma and 2710 to

2069Ma, respectively. We briefly discuss below the Hf dataon zircons from the individual samples.

OY-XH-3B (diorite)

Fifteen zircons in the diorite sample OY-XH-3B wereanalysed for in situ Lu–Hf isotopic composition, and theresults show lower (176Hf/177Hf)i (initial ratio) tightly ran-ging from 0.281562 to 0.281649 (Table 3). The εHf(t)values also show a relatively narrow range from –3.5 to–0.4, with the Hf depleted model ages (TDM) of zirconsranging from 2314 to 2198 Ma and the Hf crustal modelages (TDM

C) of 2653 to 2462 Ma against their 207Pb/206Pbmean age from the concordia (Figure 9). The values fallclose to the chondrite for the corresponding time, suggest-ing the reworking of Neoarchaean and Palaeoproterozoicjuvenile components.

OY-XH-5h (gabbro)

Only three zircons in sample OY-XH-5h were analysed forin situ Lu–Hf isotopic composition, and the results show(176Hf/177Hf)i (initial ratio) tightly ranging from 0.281754to 0.281831 (Table 3). The εHf(t) values define a clear

Figure 8. Zircon U–Pb concordia plots (a) and age data histograms with probability curves (b) for the gabbro sample OY-XH-5h.Zircon U–Pb concordia plots (c) and age data histograms with probability curves (d) for the gabbro sample OY-XH-5i.


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positive range between 3.1 and 5.8, with the Hf depletedmodel ages (tDM) of zircons ranging from 2052 to 1948Ma and the Hf crustal model ages (tDM

C) of 2238–2069Ma with respect to their 207Pb/206Pb mean age derivedfrom concordia (Figure 9). The data fall close to thedepleted mantle at that time, suggesting that the parentalmagma was likely derived from Palaeoproterozoic juve-nile sources.

OY-XH-5i (gabbro)

Five zircons in sample OY-XH-5i were analysed for in situLu–Hf isotopic composition, and the results show lower(176Hf/177Hf)i (initial ratio) tightly ranging from 0.281531to 0.281783 (Table 3). The εHf(t) values range from –4.2.to 4.7, and the Hf depleted model ages (tDM) of zirconsrange from 2350 to 2013 Ma. The Hf crustal model ages(tDM

C) show 2710–2156 Ma against the 207Pb/206Pb meanage (Figure 9). The data suggest magma derivation frommixed sources of both Neoarchaean and Palaeoproterozoicjuvenile and reworked components.

OY-XH-5j (gabbro)

Five zircons in sample OY-XH-5j were analysed for in situLu–Hf isotopic composition, and the results show lower(176Hf/177Hf)i (initial ratio) ranging from 0.281732 to0.281790 (Table 3). The εHf(t) values show a tight rangeof positive values between 3.0 and 5.1, with the Hfdepleted model ages (tDM) of zircons ranging from 2082to 2003 Ma and the Hf crustal model ages (tDM

C) of 2267to 2138 Ma against their 207Pb/206Pb mean age (Figure 9).The data clearly fall in the region between the depletedmantle and chondrite, suggesting that their parental

Table 3. LA-MC-ICP-MS Hf isotope data of zircons from Xinghe, North China Craton.

No.Age(Ma)

176Yb/177Hf

176Lu/177Hf

176Hf/177Hf 1s

176Hf/177Hfi

eHf(0)

eHf(t)

TDM

(Ma)TDM

C

(Ma) fLu/Hf

OY.XH.3B.1 1767 0.0222 0.0006 0.2816 1.8E-05 0.28158 −41.5 −2.9 2292 2615 −0.98OY.XH.3B.2 1767 0.019 0.0005 0.2816 1.9E-05 0.28158 −41.6 −2.9 2291 2616 −0.99OY.XH.3B.3 1767 0.0276 0.0007 0.28164 1.9E-05 0.28162 −40.0 −1.5 2240 2530 −0.98OY.XH.3B.9 1767 0.0141 0.0004 0.28163 1.8E-05 0.28162 −40.4 −1.5 2238 2533 −0.99OY.XH.3B.12 1767 0.041 0.001 0.28162 0.00002 0.28159 −40.7 −2.6 2287 2597 −0.97OY.XH.3B.13 1767 0.0149 0.0004 0.28162 1.7E-05 0.28161 −40.8 −1.9 2254 2558 −0.99OY.XH.3B.20 1767 0.0567 0.0015 0.28165 1.9E-05 0.2816 −39.6 −2.1 2274 2567 −0.95OY.XH.3B.21 1767 0.0173 0.0005 0.28158 1.7E-05 0.28156 −42.3 −3.5 2314 2653 −0.99OY.XH.3B.23 1767 0.0248 0.0006 0.28167 1.8E-05 0.28165 −39.0 −0.4 2198 2463 −0.98OY.XH.3B.24 1767 0.0206 0.0006 0.28161 1.5E-05 0.28159 −41.0 −2.3 2272 2582 −0.98OY.XH.3B.27 1767 0.0392 0.0012 0.28163 1.6E-05 0.28159 −40.4 −2.5 2285 2591 −0.96OY.XH.3B.30 1767 0.0172 0.0005 0.28161 1.3E-05 0.28159 −41.1 −2.4 2272 2584 −0.98OY.XH.3B.32 1767 0.0188 0.0005 0.28163 1.6E-05 0.28162 −40.3 −1.6 2241 2534 −0.98OY.XH.3B.33 1767 0.0288 0.0007 0.28167 1.8E-05 0.28165 −38.9 −0.4 2199 2462 −0.98OY.XH.3B.35 1767 0.0207 0.0005 0.2816 1.8E-05 0.28158 −41.6 −2.9 2293 2619 −0.98OY-XH-5h-11 1754 0.009 0.0004 0.28177 8E-06 0.28175 −35.6 3.1 2052 2238 −0.99OY-XH-5h-12 1754 0.0098 0.0004 0.28184 8E-06 0.28183 −32.8 5.8 1948 2069 −0.99OY-XH-5h-13 1754 0.0146 0.0006 0.2818 0.00001 0.28178 −34.4 4.0 2019 2182 −0.98OY-XH-5i-26 1783 0.0103 0.0004 0.2818 9E-06 0.28178 −34.5 4.7 2013 2156 −0.99OY-XH-5i-35 1783 0.0044 0.0002 0.28171 1.1E-05 0.2817 −37.7 1.7 2125 2342 −0.99OY-XH-5i-40 1783 0.0093 0.0004 0.28176 8E-06 0.28175 −35.7 3.6 2056 2227 −0.99OY-XH-5i-41 1783 0.0025 0.0001 0.28153 0.00001 0.28153 −43.8 −4.2 2350 2710 −1.00OY-XH-5i-43 1783 0.0038 0.0002 0.28156 8E-06 0.28156 −42.8 −3.3 2315 2653 −1.00OY.XH.5J.14 1786 0.0113 0.0004 0.28179 2.1E-05 0.28178 −34.6 4.7 2015 2158 −0.99OY.XH.5J.16 1786 0.0105 0.0003 0.28174 1.9E-05 0.28173 −36.4 3.0 2082 2267 −0.99OY.XH.5J.17 1786 0.0116 0.0004 0.2818 1.7E-05 0.28179 −34.3 5.1 2003 2138 −0.99OY.XH.5J.19 1786 0.0085 0.0003 0.28179 1.7E-05 0.28178 −34.9 4.6 2022 2169 −0.99OY.XH.5J.28 1786 0.0105 0.0003 0.28176 1.7E-05 0.28175 −35.9 3.5 2064 2238 −0.99

Figure 9. εHf(t) versus207Pb/206Pb mean age diagram of zircons

from the diorite and gabbros.


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magma was likely derived from Palaeoproterozoic juve-nile mantle sources.

Geochemistry

The results of major and trace (including rare earth) ele-ment analyses of nine samples including three websterites,five gabbros, and one diorite are listed in the Table 4.Their loss on ignition (LOI) values, ranging from 0.04 to3.83, imply that these rocks have been variably altered,consistent with the petrographic observation (Figure 4).

Websterites

The websterite samples show relatively low SiO2 contentranging from 42.33 to 46.18 wt.% (Figure 10(a)), highMgO content from 11.06 to 17.33 wt.%, and Al2O3 from16.11 to 18.56 wt.%. The rocks also display high K2Ocontent (2.22–3.69 wt.%), consistent with the presence ofsecondary biotite in these rocks (Figure 10(b)). The TiO2,Na2O, and Fe2O3

t contents are 0.02–0.03 wt.%, 1.18–2.18 wt.%, and 6.07–7.82 wt.%, respectively. The rockshows moderately high Mg# values (Mg# = 100 ×MgOmolar/(MgOmolar + FeOmolar)) from 78.86 to 82.79.

The websterites display low and variable total REEconcentrations from 2.58 to 18.98 ppm with a light rareearth element (LREE) content of 1.28–16.07 ppm andheavy rare earth element (HREE) of 1.30–3.35 ppm. Onthe chondrite-normalized REE diagram (Figure 11(a)), theLREEs are only slightly enriched relative to the HREEs,with LaN/YbN ratios ranging from 2.03 to 12.39. TheHREEs display a relatively flat distribution pattern. TheEu anomaly is obviously positive, with Eu/Eu* rangingfrom 1.66 to 2.74, suggesting a lack of any major fractiona-tion of plagioclase during magma ascent and emplacement,and hinting at a magma source from juvenile mantle. Theprimitive mantle normalized spider diagram (Figure 11(b))shows that the websterites are relatively enriched in largeion lithophile elements (LILEs) (Rb, Sr) relative to the highfield strength elements (HFSEs) (Nb, Ta, Ti), although Baand Th are depleted. The elements K, U, La, Pb, Eu, Yb,and Lu are enriched, in contrast to Ce, Pr, Zr, and Y.Notably, Nb, Ti, and Pr show depletion.

Gabbros

The five gabbro samples from near Lujiaying exhibit atight range of SiO2 contents from 49.99 to 51.92 wt.% andlower MgO contents from 4.10 to 6.68 wt.% relative to thewebsterites. Other major element contents are as follows:0.82–2.04 wt.% TiO2, 10.54–18.72 wt.% Fe2O3

t, 12.04–14.86 wt.% Al2O3, 2.62–3.96 wt.% Na2O, and 0.43–0.90 wt.% K2O. Their Mg# values range from 30.99 to55.32, and the rocks show high Na2O/K2O ratios in therange of 3.08 to 9.12. They classify as typical gabbros in

the SiO2 versus Na2O + K2O diagram (Figure 10(a)). Allthe samples plot between the field of medium-K calc-alkaline series and low K-tholeiite series in the K2O–SiO2 diagram (Figure 10(b)),

The rocks display variable total REE concentrationsfrom 83.35 to 157.75 ppm with LREE content of 43.31–83.24 ppm and HREE of 35.64–85.82 ppm. The chon-drite-normalized REE diagram (Figure 11(c)) of the gab-bros shows nearly flat HREE patterns, very slightenrichment in LREE with LaN/YbN ratios of 1.23 to3.08, and slight positive Eu anomalies with Eu/Eu* valuesof 0.93 to 1.18, comparable to those of E-MORB andmarginal basin basalts (Sun and McDonough 1989). Theprimitive mantle normalized spider diagram (Figure 11(d))is characterized by minor enrichment of LILE relative toHFSE with obvious enrichment in Pb and Ba. Somesamples show negative anomalies for Nb, Ti, and Zrresembling those of E-MORB.

Diorite

The diorite sample (OY-XH-3B) from Xinghuagou to thenortheast of the above location displays a SiO2 content of53.28 wt.% and high MgO content of 11.33 wt.%. Othermajor element contents are as follows: 0.84 wt.% TiO2,9.83 wt.% Fe2O3

t, 14.85 wt.% Al2O3, 2.02 wt.% Na2O,and 0.90 wt.% K2O. The Mg# value is 70.25, and the rockshows moderate Na2O/K2O ratios in the range of 2.25. Therock classifies as gabbroic diorite in the SiO2 versusNa2O + K2O diagram (Figure 10(a)). As shown in theK2O–SiO2 diagram (Figure 10(b)), the plot falls in thefield of medium K calc-alkaline series. The rock showstotal REE concentration of 161.63 ppm with LREE contentof 137.68 ppm and HREE of 23.96 ppm. On the chondrite-normalized REE diagram (Figure 11(e)), the LREEs areenriched relative to the HREEs, with a LaN/YbN ratio of15.60, and the HREEs display a relatively flat distributionpattern with the an Eu/Eu* value of 1.01, similar to those ofE-MORB and marginal basin basalts (Sun and McDonough1989). The primitive mantle normalized spider diagram(Figure 11(f)) shows that the gabbro is characterized byminor enrichment of LILE (Ba) relative to HFSE (Nb, Ta,Ti), resembling those of E-MORB.

Discussion

Late Palaeoproterozoic magmatism in the NCC

Our new zircon LA-ICP-MS U–Pb age data on threegabbros and one diorite from Xinghe yield emplacementages of 1786.1 ± 4.8, 1764 ± 16, 1783 ± 15, and 1767 ± 13Ma, respectively. The data reveal that these magmaticrocks were emplaced within the short time intervalof 1786–1754 Ma during the late Palaeoproterozoic(Figures 7 and 9–10). Contemporaneous intrusions have


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Table 4. Whole rock geochemical data of websterites, gabbros, and diorites from Xinghe, North China Craton.

Websterite Gabbro Diorite

Element. OY-XH-5c OY-XH-5d OY-XH-5e OY-XH-5h OY-XH-5i OY-XH-5j OY-XH-5k OY-XH-5k/1 OY-XH-3B

SiO2 46.18 42.33 45.19 49.99 50.03 51.29 51.68 51.92 53.28TiO2 0.03 0.02 0.02 1.95 2.13 0.97 1.08 0.82 0.84Al2O3 18.56 16.42 16.11 12.19 12.04 13.62 14.59 14.86 14.85TFe2O3 6.07 7.38 7.82 18.32 18.72 12.98 10.54 11.06 9.83MnO 0.16 0.12 0.12 0.32 0.30 0.23 0.16 0.19 0.13MgO 11.06 17.33 15.52 4.14 4.10 6.17 6.33 6.68 11.33CaO 9.71 9.90 7.50 7.84 7.99 10.19 6.46 10.61 6.13Na2O 1.18 2.18 1.84 2.72 2.77 2.62 3.96 2.86 2.02K2O 3.69 2.30 2.22 0.68 0.90 0.61 0.43 0.81 0.90P2O5 0.04 <0.03 0.02 0.24 0.25 0.12 0.11 0.10 0.24LOI 3.36 1.03 3.71 0.79 1.10 0.29 3.83 0.04 0.91Total 100.04 99.00 100.06 99.16 100.33 99.08 99.16 99.94 100.46Fe/Fe+Mg 0.35 0.30 0.34 0.82 0.82 0.68 0.62 0.62 0.46Na+K–Ca −4.84 −5.42 −3.44 −4.45 −4.33 −6.97 −2.07 −6.94 −3.21A/CNK 0.79 0.68 0.85 0.63 0.60 0.58 0.78 0.60 0.96A/NK 3.13 2.70 2.97 2.34 2.18 2.74 2.09 2.66 3.46Na+K 4.87 4.48 4.06 3.39 3.67 3.23 4.39 3.67 2.92Na/K 0.32 0.94 0.83 4.02 3.08 4.33 9.12 3.54 2.25Mg# 78.86 82.79 80.26 31.62 30.99 49.32 55.15 55.32 70.25Li 12.30 24.20 36.40 11.40 12.60 7.45 28.30 9.47 14.26Be 0.19 0.07 0.32 0.93 1.19 0.67 0.79 0.68 1.25Sc 38.10 19.80 19.40 46.00 50.50 55.00 52.00 47.50 22.70V 95.10 39.00 41.20 414.00 444.00 360.00 446.00 302.00 127.99Cr 947.00 1167.00 1183.00 33.00 32.70 75.30 63.60 154.00 658.85Co 52.10 85.00 73.00 52.70 55.70 55.80 80.20 54.70 38.09Ni 242.00 654.00 573.00 31.20 29.30 52.50 69.50 61.10 136.74Cu 9.55 4.10 2.15 64.30 68.80 50.90 536.00 48.70 39.80Zn 65.30 61.10 129.00 104.00 105.00 112.00 78.10 92.90 147.85Ga 9.08 6.49 11.60 20.30 22.40 17.00 18.80 17.70 20.06Rb 68.80 42.80 71.00 7.89 16.30 4.05 7.00 11.90 26.02Sr 67.30 57.30 155.00 244.00 292.00 217.00 265.00 355.00 536.79Y 2.11 0.81 1.66 45.30 52.50 28.40 25.00 22.30 13.72Zr 2.94 0.65 5.48 93.60 116.00 60.10 55.60 33.50 218.95Nb 0.20 0.05 1.50 8.03 9.28 3.66 3.20 3.51 7.15Cs 0.05 1.00 1.19 0.02 0.02 0.01 0.04 0.10 0.35Ba 566.00 85.10 252.00 312.00 428.00 226.00 156.00 356.00 495.97La 0.87 0.49 3.97 15.20 10.70 8.97 6.87 10.00 30.19Ce 1.49 0.43 7.55 33.60 25.50 22.80 19.20 21.90 62.62Pr 0.15 0.05 0.86 4.62 3.77 3.37 2.35 2.86 7.63Nd 0.58 0.21 2.98 22.00 19.30 16.10 10.90 12.60 30.71Sm 0.14 0.05 0.41 5.88 6.10 4.36 3.04 3.09 5.04Eu 0.09 0.05 0.31 1.94 2.11 1.26 0.95 1.18 1.48Gd 0.18 0.06 0.41 5.69 6.09 3.80 2.84 2.94 3.65Tb 0.04 0.02 0.05 1.25 1.41 0.80 0.64 0.59 0.45Dy 0.30 0.13 0.25 8.46 9.80 5.16 4.31 3.74 2.48Ho 0.07 0.03 0.05 1.79 2.07 1.08 0.93 0.79 0.48Er 0.24 0.09 0.18 4.98 5.76 2.99 2.58 2.22 1.36Tm 0.05 0.02 0.03 0.85 0.99 0.50 0.45 0.38 0.21Yb 0.31 0.13 0.23 5.38 6.26 3.16 2.85 2.33 1.39Lu 0.05 0.02 0.04 0.81 0.95 0.48 0.45 0.36 0.22Hf 0.11 0.03 0.25 3.45 4.10 2.12 2.04 1.53 5.09Ta 0.02 0.01 0.20 0.58 0.64 0.23 0.19 0.21 0.24Pb 1.47 1.94 2.49 2.06 2.71 3.13 1.96 4.42 2.84Th 0.33 0.14 0.13 1.06 1.46 0.37 0.05 0.11 0.70U 0.17 0.09 0.14 0.27 0.42 0.08 0.03 0.04 0.14Sr/Y 31.90 71.18 93.37 5.39 5.56 7.64 10.60 15.92 39.13Nb/Zr 0.07 0.08 0.27 0.09 0.08 0.06 0.06 0.10 0.03K/Rb 444.31 446.02 259.27 710.02 457.06 1238.90 514.26 563.21 286.24

(Continued )


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been reported along the northern margin of the NCC(Zhang et al. 2007), and include the Wenquan A-typegranites (1697 Ma, Jiang et al. 2011), the Lanying–Changsaoying–Gubeikou alkaline granitoid and anortho-site intrusions (1753–1692 Ma, Zhang et al. 2007), maficdikes (1731 Ma, Peng et al. 2012), the Shachang rapakivigranites from Miyun (1710–1680 Ma, Yang et al. 2005),the ca. 1742–1726Ma Damiao anorthosite complex (Zhaoet al. 2009b), and other intrusive suites (1696–1721, Wanget al. 2013). The Xinghe rocks are older than those of theDahongyu high potassc volcanic rocks in eastern Beijing(TIMS zircon U–Pb age = 1625 ± 6 Ma, Lu and Li 1991),which are considered to have formed in an anorogeniccontinental rift environment (e.g. Yu et al. 1993; Luet al. 2002). Late Palaeoproterozoic post-collisional intru-sions also occur in the central and southern segments of theTNCO, including the Luyashan charnockites and theYunzhongshan and Dacaoping granites of ca. 1.80 Ga,

mafic dikes of 1.78–1.76 Ga, and volcanic rocks of theXiong’er belt dated as 1.78–1.75 Ga (Li et al. 2001; Genget al. 2004, 2006; Wang et al. 2004, 2010; Peng et al.2005, 2008; Han et al. 2007; Hou et al. 2008; Liu et al.2009; Zhao et al. 2009a). The Xinghe suite might formpart of a large late Palaeoproterozoic magmatic belt withinthe TNCO. This magmatic belt is notably younger than theporphyritic monzogranites and alkaline syenites (ca. 1.86–1.84 Ga) within the Jiao–Liao–Ji Belt (Cai et al. 2002; Luet al. 2004; Li and Zhao 2007).

The geochemical features of the Xinghe pyroxenite–gabbro–diorite suite of rocks are different from typicalbimodal rocks formed in anorogenic intra-continent riftsettings. Also, recent studies have clearly established thatthe plume-related rifting of the NCC and magmatic activ-ity occurred much later at around 1.27–1. 21 Ga (Wanget al. 2014), generating mafic dikes, flood basalts, andlayered intrusions. A Mesoproterozoic hot spot track has

Table 4. (Continued).

Websterite Gabbro Diorite

Element. OY-XH-5c OY-XH-5d OY-XH-5e OY-XH-5h OY-XH-5i OY-XH-5j OY-XH-5k OY-XH-5k/1 OY-XH-3B

Rb/Nb 349.24 839.22 47.33 0.98 1.76 1.11 2.19 3.39 3.64Rb/Ta 3276.19 4280.00 358.59 13.65 25.31 17.69 37.43 57.21 106.73La/Ta 41.57 49.30 20.05 26.30 16.61 39.17 36.74 48.08 123.85La/Yb 2.83 3.71 17.26 2.83 1.71 2.84 2.41 4.29 21.74Th/Ta 15.71 13.70 0.67 1.83 2.27 1.62 0.27 0.52 2.87Th/Yb 1.07 1.03 0.57 0.20 0.23 0.12 0.02 0.05 0.50Nb/Yb 0.64 0.38 6.52 1.49 1.48 1.16 1.12 1.51 5.15Th/.Ce 0.221 0.322 0.017 0.032 0.057 0.016 0.003 0.005 0.011LREE 3.32 1.28 16.07 83.24 67.48 56.86 43.31 51.63 137.68HREE 3.35 1.30 2.91 74.51 85.82 46.36 40.04 35.64 23.96REE 6.67 2.58 18.98 157.75 153.30 103.22 83.35 87.27 161.63LaN/YbN 2.03 2.66 12.39 2.03 1.23 2.04 1.73 3.08 15.60Eu/Eu* 1.66 2.74 2.26 1.01 1.05 0.93 0.97 1.18 1.01

Note: LOI, loss on ignition.

Figure 10. (a) SiO2 versus Na2O + K2O diagram, showing the compositional plots of websterites, gabbros, and diorite.The compositional fields are after Middlemost (1994). (b) SiO2 versus K2O diagram showing the plots of gabbros and diorite. Thecompositional fields are after Rollinson (1993).


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also been established with the NCC located betweenLaurentia and Baltica in the Columbia supercontinentassembly at this time (Wang et al. 2014). However, theXinghe intrusions are much older than these rift-relatedmagmatic suites, and are closely comparable with thoseemplaced within post collisional/post-orogenic extensionaltectonic setting. Although the different rock types from theXinghe suite display variations in major element composi-tion, their similarity in trace element and REE geochem-istry, as well as U–Pb ages, suggest that they are relatedthrough fractional crystallization of a common parentalmagma. The generally low SiO2 and high MgO valuesof the rocks suggest magma derivation from a mantlesource without any significant crustal contamination.Plots in Figure 10(b) suggest a transitional compositionalfrom tholeiitic to calc-alkaline for the magma. On thechondrite-normalized REE diagrams (Figure 11(a)), thewebsterites display marked positive Eu anomalies. Thegabbros also show a relatively flat REE pattern, but inthe absence of any Eu anomaly (Figure 11(c)), suggesting

no significant plagioclase fraction, consistent with themarkedly positive Eu anomaly displayed by the webster-ites. On the primitive mantle-normalized diagrams, bothwebsterites and gabbros display strong positive Pb anoma-lies (Figure 11(b) and (d)), whereas there is only slightenrichment in the case of diorite (Figure 11(f)).

The similar Lu–Hf isotopic compositions of zirconsfrom the Xinghe gabbros and diorite (Table 3) also suggestthat the rocks may have a common source (Figure 9).Furthermore, the Xinghe gabbros and diorite exhibitbroadly similar model ages (TDM and TDM

C) and initial176Hf/177Hf ratios and εHf(t) values, indicating that theywere derived from a common parental magma or camefrom a similar source. Although some of the εHf(t) valuesare negative, all the data are higher than –4.2, with manyzircons possessing positive values, suggesting fractiona-tion from a mafic parental magma derived from litho-spheric mantle. The zircon TDM and TDM

C ages andεHf(t) values of the zircons clearly indicate that reworked(mostly juvenile) Neoarchaean and Palaeoproterozoic

Figure 11. Chondrite-normalized REE patterns for the websterites (a), gabbros (c), and diorite (e). Chondrite normalization values areafter Sun and McDonough (1989). Primitive mantle-normalized spider diagrams for the websterites (b), gabbros (d), and diorite (f).Primitive mantle values are after Sun and McDonough (1989).


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components were involved. The source heterogeneity, aswe note in the present case, might suggest a subduction-modified lithospheric mantle.

Nature of the magma source

Most of the rocks from the Xinghe suite show low SiO2

(42.33–53.28 wt.%), high MgO contents (4.10–14.33 wt.%), and high Mg# values (30.99–82.79) (Table 4). In theFeOt/MgO versus SiO2 diagram (Figure 12(a)), the gab-bros plot in the tholeiitic field, whereas the diorite showscalc-alkaline affinity. In the A/CNK (Al2O3/(CaO + Na2O+ K2O)) versus A/NK (Al2O3/(Na2O + K2O)) diagram(Figure 12(b)), the gabbros and diorite show metalumi-nous nature. In the FeOt–Na2O + K2O–MgO diagram(Figure 12(c)), the rocks show predominantly tholeiiticaffinity. In Figure 13, we examine the tectonic affinityand magma source characteristics of the studied rocks.The Al2O3 versus TiO2 plots (Figure 13(a)) show a transi-tion from arc to within-plate affinity for the gabbros anddiorite, suggesting a heterogeneous source composition.The Zr versus Zr/Y plots (Figure 13(b)) also bring outthe same feature, with the gabbros falling in the fields of

both island arc and mid-ocean-ridge basalt fields, similarto magmas generated from subduction zone components,suggesting the involvement of Neoarchaean andPalaeoproterozoic subducted oceanic crust beneath theTNCO. The Nb/Yb versus Th/Yb relationship shown inFigure 13(c) indicates that most rocks possess E-MORBaffinity, and all the plots generally fall within or near themantle array, indicating little crustal contamination. In theTh/Ta versus La/Yb diagram (Figure 13(d)), the samplesplot around the primordial mantle (PM), EMII, and assim-ilation and fractional crystallization (AFC) fields, but farfrom the field of depleted mantle, suggesting a mixedorigin from both PM and EMII mantle sources. The vari-able plots from tholeiitic through arc to calc-alkaline fieldsdisplayed in the geochemical discrimination diagramsmight be the reflection of a heterogeneous source compo-sition, probably related to a subduction-modified sub-lithospheric mantle. A prolonged subduction–accretionprocess has been traced in the NCC from the early tolate Palaeoproterozoic, associated with the amalgamationof the Yinshan and Ordos blocks within the unifiedWestern Block, followed by the assembly of the Westernand Eastern blocks (Zhao et al. 1999, 2005; Liu et al.

Figure 12. (a) Na2O + K2O versus SiO2 (after Miyashiro 1974) plots of the Xinghe suite. (b) A/CNK (Al2O3/(CaO + Na2O + K2O))versus A/NK (Al2O3/(Na2O + K2O)) plots of gabbros and diorite. The fields are after Maniar and Piccoli (1989). (c) Na2O + K2O – FeOt– MgO diagram (after Irvine and Baragar 1971) showing the plots of the Xinghe rocks.


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2004, 2005, 2012; Santosh 2010; Wang et al. 2011, 2012,2013; Santosh et al. 2012). These subduction processesmight have enriched the subcontinental mantle litho-sphere. The low Th/Ce ratios (0.003–0.322) and chondritenormalized Hf/Sm and Ta/La ratios (0.70–1.45 and 0.14–1.01, respectively) for the Xinghe rocks suggest that themantle source was probably metasomatized by slab fluidswith minor melts derived from either subducted oceanicslabs or pelagic sediments (Hawkesworth et al. 1997).

Implication for the tectonic evolution of the northernNCC

The Palaeoproterozoic world witnessed the assembly ofEarth’s first coherent supercontinent, Columbia (Rogersand Santosh 2002, 2009; Zhao et al. 2002; Meert 2012;Nance et al. 2014). The NCC was an integral part ofColumbia (Zhao et al. 2002, 2009a), with the culminationof the subduction–accretion–collision process among themajor crustal blocks in the NCC coinciding with the finalassembly of this supercontinent in the latePalaeoproterozoic (Zhao et al. 2009a; Santosh 2010).

The early Precambrian evolution of the NCC, whichinvolved the amalgamation of microcontinents into

continental blocks, was followed by a prolonged rifting–subduction–accretion collision cycle until the final crato-nization in the late Proterozoic (Zhao et al. 2001; Santosh2010; Zhai and Santosh 2011). Zhao et al. (2005) sug-gested that the basement of the NCC was assembledthrough two discrete Palaeoproterozoic collisional events.The first one occurred at ca. 1.95 Ga, when the Yinshanand Ordos blocks amalgamated along the Khondalite Belt(within the Inner Mongolia Suture Zone) to form theWestern Block, and the second event occurred when theWestern Block collided with the Eastern Block along theTNCO at ca. 1.85 Ga. In contrast, Kusky and Li (2003)proposed that the Western and Eastern blocks collidedalong the Central Orogenic belt to form the coherent base-ment of the NCC at ca. 2.5 Ga, although recent studiesreveal that the prolonged subduction–accretion–collisionhistory from early to late Palaeoproterozoic was responsi-ble for the final assembly of all the crustal blocks definingthe cratonic architecture of the NCC, coinciding with theglobal amalgamation of the supercontinent Columbia(Santosh 2010; Santosh et al. 2012, 2013). The magmaticevents post-dating the final 1.85 Ga collision in the NCChave been explained through two models. The first oneproposes rifting related to mantle plumes linked to the

Figure 13. (a) TiO2 versus Al2O3 diagram (after Muller et al. 1992) showing the plots of gabbros and diorite from Xinghe. (b) Zr/Yversus Zr diagram from Pearce and Norry (1979) showing the plots of the Xinghe gabbros. (c) Nb/Yb versus Th/Yb (after Pearce 2008)showing the plots of Xinghe suite. (d) Th/Ta versus La/Yb diagram for the Xinghe suite. The fields shown are after Condie (2001).Abbreviations: CA, calc-alkaline; TH, tholeiitic; SHO, shoshonite; PM, primordial mantle; DM, depleted mantle; EM, enriched mantle;AFC, assimilation and fractional crystallization; UC, upper crust; HIMU, high μ (U + Th)/Pb reservoir.


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break-up of the Columbia supercontinent (Zhai and Liu2003; Zhao et al. 2004; Peng et al. 2005). The second oneenvisages post-collisional extension subsequent to theassembly of the Western and Eastern blocks (e.g. Zhaoet al. 2001; Wang et al. 2004). Our new data for the latePalaeoproterozoic Xinghe rocks show geochemical fea-tures which are distinct from typical intracontinentalplume-generated or rift-related anorogenic bimodal mag-matic rocks. Accordingly, we envisage the formation ofthese rocks in a post-collisional extension setting asso-ciated with slab break-off following the westward subduc-tion of the Eastern Block and its final collision with theWestern Block at ca. 1.85 Ga (Santosh 2010). The result-ing asthenospheric upwelling and heat input might havetriggered the generation of the source magmas for theXinghe suite as well as the other late Palaeoproterozoicpost-collisional intrusives in the TNCO. The magmasources might have involved metasomatized mantle andsubduction-related components from the prolonged sub-duction-accretion history along the TNCO. The tightlyclustered ages of 1.78–1.75 Ga that we obtained fromthe Xinghe suite are remarkably close to the final collisionevent at 1.85–1.80 Ga, further supporting our inference onmagmatism being related to the post-collisional exten-sional phase. The heat input through asthenosphericupwelling following slab break-off generated a range ofmagmatic suites with isotopic signatures attesting to juve-nile and mixed juvenile and reworked components ofNeoarchaean/Palaeoproterozoic age. However, alternatemodels of magma generation including asthenosphericupwelling through delamination remain to be evaluatedby more detailed studies.

Conclusion

Our study on the websterite–gabbro–diorite suite ofXinghe from the Inner Mongolia region of the NCCleads to the following conclusions.

(1) The field relations of the magmatic suite suggestemplacement of the rocks immediately followingthe final collisional event between the Eastern andWestern blocks of the NCC.

(2) Zircon grains from the gabbros and diorite displayinternal structures and Th/U values consistent withcrystallization from magma. Zircon LA-ICP-MSU–Pb age data from the gabbros and diorite definea tight 207Pb/206Pb age range of 1786 to 1754 Ma,clearly suggesting that the magmas were emplacedimmediately after the major collisional event at1850 Ma.

(3) The geochemical features of the Xinghe suite sug-gest a common magmatic source of transitionalcharacter from low K tholeiitic to calc-alkalinewith fractional crystallization as the major

controlling factor. These rock associations andgeochemical features are distinct from the bimodalmagmatic suites generated through plume-relatedcontinental rifting during late Mesoproterozoic inthe NCC.

(4) The εHf(t) values of zircons from the gabbros anddiorite show mostly positive values (up to 5.8),with the lowest value at –4.2, suggesting a com-mon source of dominantly juvenile components.The zircon TDM and TDM

C ages and εHf(t) valuessuggest that these components also involvedNeoarchaean and Palaeoproterozoic juvenile mate-rial, probably underplated during the prolongedsubduction–accretion history of the Western andEastern blocks prior to their final collision. Thus,our tectonic model envisages post-collisional slabbreak-off, following the westward subduction ofthe Eastern Block and its collision with theWestern Block. The magma generation from sub-duction-modified sub-lithospheric mantle sourcesis correlated with asthenospheric upwelling.

AcknowledgementsWe thank Professor Robert Stern, Editor, and three anonymousreferees for their detailed and critical comments, which greatlyhelped in improving our paper. This study forms part of thedoctoral research of Q.Y. Yang at the China University ofGeosciences, Beijing.

FundingFunding for this study was provided through the Talent Awardproject to M. Santosh through the 1000 Talents Plan of theChinese Government.

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pyroxenite–gabbro–diorite suite from xinghe, inner u–pb ......late palaeoproterozoic...

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