pyroxenite–gabbro–diorite suite from xinghe, inner u–pb ......late palaeoproterozoic...
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This article was downloaded by: [b-on: Biblioteca do conhecimento online UP]On: 23 September 2014, At: 23:02Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
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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: msantosh.gr@gmail.com
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
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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)
Plag + K-fs + Opx + Cpx + Bt + Hbl + Mt
7 OY-XH-5k/1 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
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
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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)
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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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