granitoid petrogenesis and tectonic implications of the...

Granitoid Petrogenesis and Tectonic Implications of the Late TriassicBaoji Pluton, North Qinling Orogen, China: Zircon U-Pb Ages

and Geochemical and Sr-Nd-Pb-Hf Isotopic Compositions

Ying-Yu Xue,1,2 Wolfgang Siebel,2,* Jian-Feng He,1 He Zhang,1 and Fukun Chen1,*

1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026,People’s Republic of China; 2. Institute of Earth and Environmental Sciences, Albert-Ludwigs-

Universität Freiburg, Albertstraße 23b, 79104 Freiburg, Germany

AB STRACT

The Qinling orogenic belt in central China resulted from the final collision of the North China and South China blocksin the Triassic. Early Mesozoic granitoids are widespread in the Qinling orogen. Their genesis can provide keyconstraints on the tectonic evolution of this orogenic belt. The Late Triassic Baoji pluton, located in the North Qinlingunit, consists of granodiorites, monzogranites, quartz monzodiorites, and K-feldspar granites. U-Pb zircon dating viathe laser ablation ICP-MS technique yields ages of 217.4 5 1.4, 212.3 5 1.8, 212.6 5 1.6, and 195.1 5 2.0 Ma forgranodiorites, monzogranites, quartz monzodiorites, and K-feldspar granites, respectively. The granitoids have high-Kcalc-alkaline, metaluminous to weakly peraluminous composition(s). Silica contents increase andMg, Fe, Ti, Ca, and Pcontents decrease from quartz monzodiorites through granodiorites to monzogranites and K-feldspar granites. Com-bining our findings with evidence from whole-rock Sr-Nd and zircon Hf isotopes, we suggest that the granodioritesformed by magma-mixing processes involving metasomatized lithospheric mantle and Mesoproterozoic crustal ma-terial. The monzogranites record the contribution of an asthenospheric mantle source. The quartz monzodiorites werederived fromametasomatized lithosphericmantle source. Partialmelting of crustal lithologies generated theK-feldspargranites. It is argued that the Baoji pluton formed in a late-collision setting.

Online enhancements: supplemental tables.

Introduction

The Qinling orogenic belt (QOB) records a numberof geological processes, such as rifting, oceanic basinformation and destruction, continental growth andcollision, and intracontinental deformation (e.g., Mat-tauer et al. 1985; Zhang et al. 1995, 1996; Wang et al.2013; Li et al. 2015). Based on present knowledge, thebuildup of the QOB involved at least three tectono-magmatic cycles that took place during the Neopro-terozoic, Paleozoic, and Mesozoic eras. Each periodwas associated with extensive granitic magmatism(e.g., Lu et al. 1996;Wang et al. 2013, 2015;Dong et al.2015b; Zhang et al. 2016).Granitoids of the Mesozoic orogenic cycle are

widely distributed in the QOB and are particularly

abundant along the Shangdan suture (SDS in fig. 1).This magmatism is related to the early Mesozoic col-lision between the North China and South Chinablocks (NCB and SCB, respectively). U-Pb age datahave manifested two distinct periods of magma-tism during the early Mesozoic, one between 250and 234Ma and one between 225 and 200Ma (Donget al. 2011b, 2015a;Wang et al. 2013, 2015;Chen andSantosh 2014).Inmany places, the geochemistry of granitoids has

been employed to infer the tectonic setting. In theQOB, however, there is still considerable contro-versy about the relation between granite composi-tion and regional tectonism. Zhang et al. (2001) havesuggested that the Mesozoic granitoids are mainlysubduction related,while others have advocated thatthey were formed in a collisional stage and can bedivided into syncollisional and postcollisional rocks

Manuscript received December 30, 2016; accepted Septem-ber 5, 2017; electronically published November 27, 2017.

* Author for correspondence; e-mail: [email protected]; [email protected].

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[The Journal of Geology, 2018, volume 126, p. 119–139] q 2017 by The University of Chicago.All rights reserved. 0022-1376/2018/12601-0006$15.00. DOI: 10.1086/694765

(e.g., Lu et al. 1996; Sun et al. 2002; J. Zhang et al.2002b; C. Zhang et al. 2008;Wang et al. 2011). In thisstudy, we address this issue through a combinationof systematic geochemical and geochronological dataon the granitoid rocks of the Baoji pluton. The cur-rently available information is insufficient to deter-mine the origin of the melts and the temporal rela-tions between the various intrusions. Here we presentzircon U-Pb ages and Lu-Hf isotopic, bulk-rock geo-chemical, and Sr-Nd-Pb isotopic compositions for theintrusive members of the Baoji pluton. Combiningthese with data collected from previous studies, weaim to decipher the detailed sequence of emplace-ment ages and the source(s) of melting. Finally, sev-eral tectonic settings are evaluated in the light of thenew results.

Geological Background

TheQOB connects the Dabie high-pressure–ultrahigh-pressure (UHP) terrane in the east and theQilian andKunlun Mountains in the west (fig. 1). The orogenformed by multistage collision of the NCB and theSCB as well as intervening microblocks (e.g., Mengand Zhang 1999, 2000; Zhang et al. 2001; Dong et al.2015b). TheQOB is bounded by theLingbao-Lushan-Wuyang fault in the north and the Mianlue, Bashan,

and Xiangguang faults in the south (fig. 1). Two ophi-olitic mélange suture zones, the early Paleozoic Shang-dan and the Paleozoic–Early TriassicMianlue zones,arewell documented in theQOB (fig. 1). On the basisof these faults and suture zones, the QOB has beensubdivided into four tectonic units from north tosouth: the southern margin of the NCB, the NorthQinling belt (NQB), the South Qinling belt (SQB),and the northern margin of the SCB (NSCB; e.g.,Mattauer et al. 1985; Meng and Zhang 2000; Zhanget al. 2001; Dong et al. 2015b).

The Baoji pluton is one of the largest plutons inthe QOB, covering an area of more than 1500 km2

in Shaanxi Province. It is located between East andWest Qinling (fig. 1) in the NQB and shows an east-west elongation (fig. 2). It is a composite pluton andcan be subdivided into several discrete intrusionsof different composition. The pluton was emplacedinto Ordovician and Carboniferous sedimentary coun-try rocks. According to Yan (1985) and BGMRS (1989),it consists of four different facies types. A granodio-rite with phenocrysts of alkali feldspars (1–3 cm indiameter; fig. 3a, 3b) occurs at the eastern and south-ern margins (fig. 2). A medium- to coarse-grainedmafic microgranular enclave (MME)–bearing monzo-granite (fig. 3c, 3d) and a fine- to medium-grainedmonzogranite with fewMMEs (fig. 3d) both crop out

Figure 1. Simplified geological map of the Qinling orogenic belt (QOB) showing the major tectonometamorphicunits and the location of the QOB within China (inset, upper right). Redrawn after Zhang et al. (2001) and Dong et al.(2011b). N-SCB p northern South China block.

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in an intermediate zone. Locally, bodies of quartzmonzodiorites (a few hundred square meters in size)are exposed within the monzogranites (fig. 3e). Amedium- to coarse-grained aphyric K-feldspar gran-ite dominates in the center of the intrusion (fig. 3f).Common accessory minerals in these rocks includeapatite, titanite, magnetite, ilmenite, epidote, and zir-con (fig. 4).In an earlier study, Triassic zircon U-Pb ages of

206.4 5 1.7 and 211.3 5 1.3 Ma were obtained forthe fine- to medium-grained monzogranites and themedium- to coarse-grained monzogranites of theBaoji pluton, respectively (Zhang et al. 2006). Usingzircon laser ablation ICP-MS analyses, Liu et al.(2011) obtained slightly different ages for a biotitegranite (216 5 1 Ma) and a monzogranite (212 52 Ma) from the Baoji pluton.

Petrography of Samples

Twenty-one representativewhole-rock sampleswerecollected for chemical analyses from the Baoji plu-

ton.Thesecomprise twogranodiorites, 13monzogran-ites, four aphyric K-feldspar granites, and two quartzmonzodiorites. The granodiorites display medium-to coarse-grained porphyritic texture and are com-posed of quartz (25%–30%), plagioclase (35%–45%),alkali feldspar (15%–20%), biotite (7%–10%), andhornblende (5%), together with zircon and apatite asaccessoryminerals. Phenocrysts in the granodioritesare idiomorphic or hypidiomorphic alkali-feldsparcrystals (1–3 cm in diameter). The granodiorite alsohosts numerous MMEs (fig. 3a, 3b).The texture of the monzogranites changes from

medium-to-coarse grained (six samples) to fine-to-medium grained (seven samples). The medium- tocoarse-grained samples contain MMEs (fig. 3c, 3d)and a small amount of idiomorphic or hypidiomor-phicK-feldspar phenocrysts (1–2 cm in diameter) andare composed mainly of quartz (25%–30%), plagio-clase (20%–30%), alkali feldspar (40%–45%), biotite(5%), and hornblende (3%).Compared to themedium-to coarse-grainedmonzogranites, thefine- tomedium-grained monzogranites (fig. 3d) are aphyric and havelower alkali-feldspar content (35%–40%) and fewer

Figure 2. Geological sketch map showing regional distribution of granitoids from the Baoji pluton (after the 1∶250,000geological map of Baoji; Shaanxi Geological Survey 2004). Sample localities shown as yellow circles. Pt1Q p QinlingGroup; Pt2 pMeso-Neoproterozoic Kuanping Group; PzD p Paleozoic Danfeng Group; C2C p Carboniferous CaoliangyiGroup; OC p Ordovician Caotangou Group; Q p Quaternary Pleistocene strata and Holocene alluvium.

Journal of Geology 121G RAN I T O I D P E T ROG EN E S I S I N TH E B AO J I P L U TON

MMEs. Field observations show that the fine- tomedium-grained monzogranites intruded into themedium- to coarse-grainedmonzogranites (fig. 3d).Onthe other hand, intrusive contacts between mon-zogranites and granodiorites were not exposed.

The quartz monzodiorites show fine-grained gran-ular texture and are mainly composed of plagioclase(∼30%), alkali feldspar (∼15%), biotite (20%–25%),hornblende (10%–15%), and quartz (∼10%;fig. 3e). Inthe field, quartzmonzodiorites are closely associated

Figure 3. Field photographs of the granitoids from theBaoji pluton: a, granodioritewith large pinkK-feldspar phenocrystsand biotite schlieren; b, granodiorite with pink K-feldspar phenocrysts and small mafic microgranular enclaves;c, medium- to coarse-grained biotite monzogranite with a mafic microgranular enclave (same facies type as in d) cut by aleucocratic dike; d,fine- tomedium-grained pink biotite granite (right-hand side) intruded intomedium- to coarse-grainedgray biotite monzogranite; the dashed line marks the contact zone; e, quartz monzodiorite cut by leucocratic dikes;f, weathered granite outcrop of medium- to coarse-grained aphyric K-feldspar granite. A color version of this figure isavailable online.

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Figure 4. Photomicrographs of typical minerals from the Baoji granitoids: a, microline with tartan twinning undercross-polarized light; b, plagioclasewith albite twins under cross-polarized light; c, amphibole, epidote, and biotite fromquartz monzodiorite under plane-polarized light; d, amphibole and titanite from quartz monzodiorite under plane-polarized light; e, quartz, microline, plagioclase, biotite, titanite, and ilmenite from K-feldspar granite under plane-polarized light; f, microline and apatite frommonzogranite under plane-polarized light. A color version of this figure isavailable online.

with the monzogranites, although contacts betweenthe two rock types were nowhere observed.

The aphyric K-feldspar granites are light red andshowmedium- to coarse-grained granitic texture andmassive structure.Theyaremainlycomposedof quartz(20%–30%), plagioclase (25%–30%), alkali feldspar(40%–45%), biotite (5%–15%), andhornblende (3%).Occasionally, K-feldspar granites intruded the grano-diorites (fig. 3d ).

Analytical Techniques

Zircon grains from11 granitoid sampleswere isolatedwith standard heavy-liquid and magnetic mineral-separation techniques, handpicked under a binocu-lar microscope, mounted in epoxy resin, and thenpolished to expose the crystal centers. For initialscreening, all grains were examined by transmitted-and reflected-light microphotography. In order todetermine the internal textures and the most suit-able analytical positions, the zircons were photo-graphed under cathodoluminescence (CL) on a scan-ning electronmicroscope. TheCL image acquisition,U-Pb dating, and trace-element analyses were ac-complished at the Key Laboratory of Crust-MantleMaterials and Environments, University of ScienceandTechnology ofChina (USTC).U-Pb analyseswereaccomplished with an ArF excimer laser ablationsystem (193-nm wavelength), which was connectedto an Agilent 7700E ICP-MS instrument. Standardzircon 91500 was used as an external standard forcalibration of the U-Pb ages. The diameter of thelaser ablation spot was 32 mm,with a laser frequencyof 10 Hz and sometimes 6 Hz where necessary. Eachanalysis incorporates c. 20 s of background acquisi-tion (gas blank) followed by 40 s of data acquisitionfrom the sample and another 20 s of backgroundacquisition. Every four-sample analysis was followedby one analysis of zircon 91500 in order to correctthe time-dependent drift of sensitivity and mass dis-crimination. More details on the analytical proce-dure are given in Liu et al. (2007).

Element concentrations of zircon were calibratedagainst the NIST (National Institute of Standards andTechnology) glasses 610, 612, and 614, and NIST610was measured twice every 10 sample spots. Off-lineevaluation and integration of signal to background,time-drift correction and quantitative calibration fortrace-element analyses and U-Pb dating, as well ascommon-lead correction, were processed with thefree software ICPMSDataCal 9.2 (Liu et al. 2008,2010). Correction factors were used for each sampleto correct both instrumental mass bias and elemen-tal and isotopic fractionation.Concordia diagrams and

weighted mean calculations were done with Isoplot4.15 (Ludwig 2009).More than 20 spot analyses werecarried out on the samples; the results are reportedat the 1j level, and uncertainties and the weightedmean ages are quoted at the 95% confidence level.

Forwhole-rockmajor- and trace-element analyses,rock samples were split into small chips and thenpowdered to less than 200 mm.Major elements weremeasured by X-ray fluorescence at ALS Minerals–ALS Chemex in Guangzhou. Loss on ignition (LOI)was analyzed by gravimetric methods using an elec-tronic balance. Analytical uncertainties of major el-ements were better than 1%. Trace-element con-centrations were determined on an Elan 6100 DRCICP-MS at the Key Laboratory of Crust-Mantle Ma-terials and Environments. About 50 mg of bulk-rockpowder was weighed into a Teflon bomb for anal-yses, and Rh was used as the internal standard so-lution. Precision is better than 5% for most traceelements. More details on the ICP-MS analytical pro-cedures are given in Hou and Wang (2007).

Rb-Sr, Sm-Nd, and Pb isotope analysis on whole-rock samples was performed in the Laboratory forRadiogenic IsotopeGeochemistry,USTC. Parent anddaughter nuclides were isolated from each other bystandard chromatographic separation techniques. Iso-tope composition was measured on a Finnigan MAT-262 spectrometer. Pb was loaded with silica gel onpreconditioned Ta filaments. Sr was loaded with aTa-HF activator on preconditioned Ta filaments. Ndwas loaded as phosphate on preconditioned Re fila-ments. Sr and Nd isotopic ratios were corrected formass fractionation relative to 86Sr/88Sr of 0.1194 and146Nd/144Nd of 0.7219, respectively. Standard solu-tions of NBS987 for Sr and La Jolla for Nd were mea-sured during analyses. Precision of 87Rb/86Sr and147Sm/144Nd ratios is better than 0.5%. Precision ofthe measured Pb isotopic ratios is better than 0.01%.Analytical precisions are stated as 2j standard errors,and details of analytical technique are described inChen et al. (2000, 2007).

Four samples (BJ13-05, BJ13-12, BJ13-14A, andBJ13-22B) were chosen for Hf isotope analyses. In situzircon Lu-Hf isotopic analyses were performed on aNeptune multicollector ICP-MS equipped with aGeolas-193 laser ablation system at the Institute ofGeology and Geophysics, Chinese Academy of Sci-ences, Beijing. Reference materials GJ-1 (176Hf/177Hfof 0.282020) andMUD (176Hf/177Hf of 0.282500) wereselected for external calibration, and the ablation di-ameter was c. 40–60 mm. More details of the instru-ment parameters, analysis procedure, and interfer-ence correction are given in Xu et al. (2004) and Wuet al. (2006).

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Analytical Results

Zircon U-Pb Ages. Eleven samples from four dif-ferent rock types from the Baoji plutonwere selectedfor U-Pb zircon age dating. U-Pb dating results andrepresentative CL images are presented in figures 5and 6, and the isotopic data are given in table S1(tables S1–S4 are available online). In the chondrite-normalized rare earth element (REE) diagram, all

zircon grains show similar positive Ce and negativeEu anomalies and enrichment in heavyREEs (fig. 5l).Samples BJ13-03, BJ13-04, BJ13-10B, BJ13-12, BJ13-

13, and BJ13-17A are from monzogranites of the in-termediate part of the pluton (BJ13-03, BJ13-04, andBJ13-13 are fine grained, and the others are coarsegrained). Zircons from these samples are short to longprismatic, subeuhedral to euhedral, and light yellowto colorless.Crystal lengths range from100 to 250mm,

Figure 5. a–k, U-Pb concordia diagramswithweightedmean 206Pb/238U ages; l, chondrite-normalized rare earth elementdiagram for zircon grains from the Baoji pluton. Kf-granitep K-feldspar granite. A color version of this figure is availableonline.


with aspect ratios from 1∶1 to 1∶4.5. The grainsshow oscillatory zoning in CL images (fig. 6a, 6b).Twenty zircons from sample BJ13-03 were analyzedand yielded Th contents of 52–652 ppm, U contentsof 78–988 ppm, and Th/U ratios between 0.31 and1.38. The U-Pb ages of the 20 analytical spots varyfrom 217 to 206 Ma, giving a weighted mean 206Pb/238U age of 209.5 5 1.7 Ma and an MSWD value of1.5 (fig. 5a). Twenty-four spots from sample BJ13-04

have Th contents of 39–639 ppm and U contents of104–1011 ppm, with Th/U ratios between 0.30 and1.08. Their 206Pb/238U ages range from 224 to 207Ma,giving a weighted mean 206Pb/238U age of 214.9 51.6 Ma (MSWDp 1.3; fig. 5b). Zircons from sampleBJ13-10B have Th contents of 78–681 ppm and Ucontents of 338–1693 ppm, with Th/U ratios from0.13 to 0.76. Twenty-two spots yield 206Pb/238U agesbetween 220 and 202 Ma, with a weighted mean of

Figure 6. Cathodoluminescence images of representative zircon grains chosen for laser ablation ICP-MS U-Pb andLu-Hf analyses. The grains commonly show oscillatory zoning of magmatic origin. Laser ablation spot size was c.44 mm. 206Pb/238U age and initial εHf values are given beside and in white circles, respectively. A color version of thisfigure is available online.

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211.75 2.3Ma (MSWDp 2.9;fig. 5c). Zircon grainsfrom sample BJ13-12 have variably high Th (116–989 ppm) and high U (316–2557 ppm) contents, withTh/U ratios varying from 0.36 to 1.22. Twenty-fourzircon grains of this sample display 206Pb/238U agesbetween 220 and 205Ma, with a weighted mean ageof 212.3 5 1.8 Ma (MSWD p 2.2; fig. 5d). Twenty-one spots from sample BJ13-13 yield relatively lowTh (62–531 ppm) and U (170–1185 ppm) contents,with Th/U ratios of 0.25–1.05. The U-Pb ages varyfrom 223 to 208 Ma, yielding a weighted mean206Pb/238U age of 212.95 1.7Ma and anMSWDvalueof 1.4 (fig. 5e).Nineteen analytical spots from sampleBJ13-17A have high and variable abundances of Th(274–3396 ppm) and U (487–5465 ppm; one spot has11,390 ppm), with Th/U ratios varying from 0.31to 1.38. The U-Pb ages vary from 227 to 202 Ma,yielding a weighted mean 206Pb/238U age of 214.6 53.2 Ma (MSWD p 4.5; fig. 5f).Sample BJ13-14A comes from a granodiorite close

to the margin of the pluton. Zircon grains of thissample are prismatic, transparent, mostly euhedral,and colorless. In CL images, most grains show os-cillatorymagmatic zoning (fig. 6c), a characteristic ofmagmatic origin. They have lengths of 200–250 mm,with aspect ratios from 1∶2 to 1∶3, low Th contentsof 167–562 ppm, and variable U contents of 149–888 ppm, and Th/U ratios range from 0.53 to 1.32.Twenty-eight spots display concordant 206Pb/238U agesof 224–211 Ma, yielding a weighted mean age of217.4 5 1.4 Ma (MSWD p 1.2; fig. 5j).Zircons from three aphyric K-feldspar-bearing gran-

ite samples—BJ13-20, BJ13-21, and BJ13-22B—aremostly subeuhedral to euhedral and colorless. Grains

are 100–200 mmin length, with aspect ratios between1∶2 and 1∶4, and show oscillatory zoning (fig. 6d).Zircon grains from this granite type have high Th(131–2812 ppm) and high U (311–4120 ppm) con-tents, with Th/U ratios varying from 0.16 to 1.58. Aweighted mean age of 194.3 5 4.1 Ma was obtainedfrom 28 analyses on zircon grains from sample BJ13-20 (fig. 5g). Twenty-three grains of sample BJ13-21yield a weighted mean age of 198.1 5 2.0 Ma, withan MSWD of 2.3 (fig. 5h). Sixteen zircon grains ofsample BJ13-22B have 206Pb/238U ages ranging from203 to191Ma, giving aweightedmeanage of 195.152.0 Ma (MSWD p 1.6; fig. 5i).Zircon grains from quartz monzodiorite sample

BJ13-05 are transparent, mostly subeuhedral to eu-hedral inmorphology, and light yellow. The grains aremore than 180 mm in length, with aspect ratios be-tween 1∶1 and 1∶1.5.Most zircons display oscillatorymagmatic zonings (fig. 6e), and they have relativelyvariable Th (53–2058 ppm) and U (116–1133 ppm)contents, with Th/U ratios ranging from 0.36 to 1.82.Twenty-nine zircon grains of sample BJ13-05 wereanalyzed, and theU-Pbages of 20 analytical spotsvaryfrom 221 to 206 Ma, yielding a weighted mean206Pb/238Uageof 212.651.6Ma (MSWDp2.8;fig. 5k).

Whole-Rock Major- and Trace-Element Composition.Whole-rockmajor- and trace-element data for 21 gran-itoid samples of the Baoji pluton are given in ta-ble S2. The LOI ranges from 0.38 to 1.42 wt%. Inthe R1-versus-R2 diagram, most samples plot in thesyenogranite and monzogranite fields (fig. 7a). Inthe silica-versus-K2O diagram, the samples define ahigh-K calc-alkaline to shoshonitic trend (fig. 8b).Samples from the Baoji granodiorite have high-K calc-

Figure 7. a, R1-versus-R2 classification diagram (de la Roche et al. 1980) showing the compositional range of the Baojigranitoids. Ab p albite; An p anorthite; Or p orthoclase. b, A/CNK-versus-A/NK plot (Maniar and Piccoli 1989)depicting their metaluminous to weakly peraluminous nature. A/CNK p molar Al2O3/(CaO1Na2O1K2O); A/NK pmolar Al2O3/(Na2O1K2O); Kf-granite p K-feldspar granite. A color version of this figure is available online.


alkaline and metaluminous compositions (fig. 7b),with SiO2 contents of 62.3–66.2 wt%, A/CNK (molarAl2O3/(CaO 1 Na2O 1 K2O)) values of 0.86–0.90,MgO contents of 2.1–2.8 wt%, and Mg# (100 # Mg

molar/(Mgmolar1 Fe21molar)) of 56–58.They showhigh total-alkali contents (Na2O 1 K2O of 7.6), withK2O/Na2O ratios of 0.76–0.95, and high Sr contents(522–616 ppm), lowYb contents (1.78–1.92 ppm), and

Figure 8. Harker variation diagrams of some major oxides and trace elements for granitoids from the Baoji pluton.SiO2-versus-K2O diagram ismodified after Peccerillo and Taylor (1976) andMiddlemost (1994). I-type trend is accordingto Chappell (1999), and symbols are the same as in figure 7. A color version of this figure is available online.

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low Y contents (21.1–23.0 ppm), resulting in highSr/Y ratios of 24.7–26.8. In addition, the granodioritesshowa positive Pb anomaly and are enriched in large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs; fig. 9a). Chondrite-normalized REE patterns show enrichment in lightover heavy REEs (fig. 9b), with (La/Sm)N of 3.9–4.2and (La/Yb)N of 16.2–16.7. The granodiorites displaya weak negative Eu anomaly, with Eu=Eu� (EuN=(SmN #GdN)1=2) of 0.62–0.64.Compared to granodiorites, monzogranites have

higher silica and alkali contents: 69.4–74.0 wt%SiO2 and 8.1–8.8 wt% total alkalis (Na2O 1 K2O).K2O/Na2O ratios vary from 0.90 to 1.38; most sam-ples show K2O/Na2O ratios of 11, except sampleBJ13-24, which has a low K2O concentration. Themonzogranites are metaluminous to weakly per-aluminous (A/CNK of 0.98–1.08). They show lowerMgO contents, from 0.20 to 1.12 wt%, and Mg#values varying from 31 to 51. Compared to grano-diorites, the monzogranites have lower Sr contents

(141–469 ppm), low Yb contents (0.26–2.83 ppm),and variable Y contents (3.92–30.4 ppm), resultingin variable Sr/Y ratios of 4.6–120. On the primitivemantle–normalized trace-element diagram, they arecharacterized by enrichment in LILEs, depletion inNb and Ta, and positive Pb and negative Ti and Panomalies (fig. 9a). The chondrite-normalized REEpatterns show enrichment in light REEs over heavyREEs (fig. 9b), with (La/Sm)N of 3.04–6.41 and (La/Yb)Nof 6.59–67 (mean values are 5.09 and 26.60, re-spectively). They shownegative Eu anomalies, withEu=Eu� of 0.33–0.84.The composition of the two quartz monzodiorite

samples differs from that of the other rocks in thatthey have low SiO2 (53.2 and 53.4 wt%), high Na2O1K2O (8.19 and 8.31 wt%), and high K2O/Na2Oratios (5.5 and 9.5). They are strongly metalumi-nous (A/CNK of 0.55 and 0.59) and have high MgO(6.49 and 6.74 wt%) and Mg# (67 and 71). Sr, Y, andYb contents (898 and 1360, 21.7 and 24.7, and 1.52and 1.78 ppm, respectively) are higher than those in

Figure 9. Primitive mantle–normalized trace-element spider diagrams (left) and chondrite-normalized rare earthelement patterns (right) for granitoids from the Baoji pluton. a, b, Data for granodiorites, monzogranites, and quartzmonzodiorites; c, d, Data for K-feldspar granites. Normalization values are from Sun and McDonough (1989) andMcDonough and Sun (1995), and the symbols are the same as infigure 7. A color version of thisfigure is available online.


the other rock types, and Sr/Y ratios are 36 and 63.On amantle-normalized trace-element diagram, theyshow positive Ba, K, Pb, and P and negative Nb andTa anomalies (fig. 9a). Light over heavy REE en-richment andweakly positive Eu anomalies (Eu=Eu�of 1.06 and 1.09) are further characteristics of thequartz monzodiorites (fig. 9b).

The K-feldspar granites have a narrow silica range(SiO2 contents of 72.6–73.3 wt%) and Na2O 1 K2O(8.43–8.73 wt%), with high K2O/Na2O ratios, rang-ing from 1.36 to 1.49. They are weakly peralumi-nous,withA/CNKof 0.99–1.10. These samples showlow MgO, from 0.25 to 0.41 wt%, and Mg# variesfrom 31 to 34. They have 158–215 ppm Sr, 1.29–4.08 ppm Yb, and 16.5–36.8 ppm Y, resulting in lowSr/Y ratios, between 4.68 and 13.04. On a mantle-normalized trace-element diagram, they show neg-ativeNb,Ta, P, andTi andpositiveRb, Th, K, La, andNd anomalies (fig. 9c). Chondrite-normalized REEpatterns of the granites are characterized by mod-erate enrichment in light REEs, nearly flat heavyREEs, and negative Eu anomalies (Eu=Eu� of 0.31–0.59; fig. 9d).

Whole-Rock Sr-Nd-Pb Isotopic Composition. Sr, Nd,and Pb isotopic compositions of the granitoid sam-ples are given in table S3. Initial 87Sr/86Sr (ISr) and143Nd/144Nd (INd) ratios and initial εNd values are cal-culated on the basis of the zircon age record. Grano-diorites,monzogranites, and contemporaneous quartzmonzodiorites of the Baoji pluton have almost thesame initial Sr-Nd isotopic compositions, differentfromthoseof theyoungerK-feldspar granites (fig. 10a).

The 87Rb/86Sr ratios of the granodiorites rangefrom 0.48 to 0.61, and the 87Sr/86Sr ratios vary from0.7074 to 0.7077. The ISr ratios vary from 0.7058 to0.7059. The 147Sm/144Nd ratios range from 0.1030 to0.1044, and the 143Nd/144Nd ratios vary between0.512200 and 0.512212. The INd ratios and initial εNd

values range from 0.512052 to 0.512065 and from26.0 to25.7, respectively (fig. 10a). Two-stagemodelages (TDM2) cluster around 1.5 Ga. The samples showthe following initial Pb isotope ratios: 206Pb/204Pb of17.72–17.84, 207Pb/204Pb of 15.54–15.55, and 208Pb/204Pb of 37.91–37.95 (fig. 11).

Monzogranites have higher 87Rb/86Sr ratios (0.67–4.33) and higher 87Sr/86Sr ratios (0.7085–0.7204) thanthe granodiorites, and they have variable ISr ratios(0.7050–0.7083). The 147Sm/144Nd ratios vary from0.0905 to 0.1286, and the 143Nd/144Nd ratios rangefrom 0.511845 to 0.512412. The INd ratios and ini-tial εNd values range from 0.511716 to 0.512236 andfrom212.6 to22.5, respectively (fig. 10a). The TDM2

model ages are between 1.2 and 2.1 Ga. The initialPb isotope composition of the monzogranites is sim-ilar to that of the granodiorites (206Pb/204Pb of 17.50–18.21, 207Pb/204Pb of 15.50–15.63, and 208Pb/204Pb of37.79–38.39; fig. 11).

The 87Rb/86Sr ratios of the two quartz monzodio-rites are 0.22 and 0.54, and the 87Sr/86Sr ratios are0.7084 and 0.7093, while their ISr ratios are almostconstant at ∼0.7077. The quartzmonzodiorites have147Sm/144Nd ratios of 0.0984 and 0.1000 and143Nd/144Nd ratios of 0.511967 and 0.512039. TheINd ratios and initial εNd values are 0.511828 and

Figure 10. a, Initial 87Sr/86Sr ratios versus initial εNd values for the Baoji pluton. Fields for the Qinlin Group, theKuanping Group, the Bikou Group, the Yaolinghe Group, and the Triassic granitoid rocks fromwestern Qinling terrainare based on the data from Zhang et al. (2006), Z. Qin et al. (2015), J. F. Qin et al. (2013), Luo et al. (2012), and referencestherein. b, Initial εHf values versus 206Pb/238U ages of zircons from the Baoji pluton. Data are from this study and Wanget al. (2011), Qin et al. (2009), and Zhang et al. (2006). Kf-granite p K-feldspar granite; MMEs p mafic microgranularenclaves; MORB p mid-ocean ridge basalt; NQB p North Qinling belt.

130 Y . - Y . X U E E T A L .

0.511903 and210.5 and29.0, respectively (fig. 10a).The TDM2 values are 1.8 and 1.9 Ga, and the averageinitial Pb ratios are 17.53 for 206Pb/204Pb, 15.51 for207Pb/204Pb, and 37.99 for 208Pb/204Pb (fig. 11).Compared to the other rock types, two K-feldspar

granite samples show quite different Sr-Nd isotopiccompositions. The 87Rb/86Sr and 87Sr/86Sr ratios are2.94 and 3.22 and 0.7198 and 0.7212, respectively.The ISr ratios are 0.7113 and 0.7122. The 147Sm/144Ndratios range from 0.1015 to 0.1229, and the 143Nd/144Nd ratios vary between 0.511640 and 0.511824.The INd ratios and initial εNd values are low, rangingfrom 0.511509 to 0.511695 and from217.1 to213.5,respectively (fig. 10a). Model ages are old, between2.4 and 2.1 Ga. Initial Pb ratios are 17.39–17.82 for

206Pb/204Pb, 15.48–15.52 for 207Pb/204Pb, and 37.77–38.18 for 208Pb/204Pb (fig. 11).

Zircon Hf Isotopic Composition. Zircon Hf isoto-pic analyses were performed on one sample of eachgranitoid type. Analytical results are given in ta-ble S4, and data are shown in figure 10b. Initial εHf

values (εHf(t)) are calculated back to the average zir-con crystallization age from each sample. Initial εHf

values exhibit a wide range, from weakly positive tostrongly negative.Thirty-seven zircon analyses on granodiorite sam-

ple BJ13-14Agive initial 176Hf/177Hf ratios of 0.282534–0.282668. Most spots have low εHf(t) values, from23.6 to 20.2, and TDM2 values from 1.19 to 1.49 Ga;only one spot has a positive εHf(t) value of 1.1 and amodel age of 0.87 Ga.Initial 176Hf/177Hf ratios of 27 analytical spots on

monzogranite sample BJ13-12 vary from 0.282604to 0.282766. Most analyses show slightly positiveεHf(t) values between 0.1 and 4.4, with TDM1 values of0.69–0.91 Ga, while three spots have negative εHf(t)values of 20.3, 20.7, and 21.3, with TDM2 values of1.27, 1.29, and 1.33 Ga.Thirty analyses were done on zircon grains of

quartz monzodiorite sample BJ13-05. Their initial176Hf/177Hf ratios range from 0.282213 to 0.282376,and initial εHf values are generally lower than thoseof the other granitoids, ranging from 215.1 to 29.3,corresponding to two-stage Hf model ages (TDM2) of1.84–2.20 Ga.Twenty 176Hf/177Hf ratios of K-feldspar granite

sample BJ13-22B vary from 0.282119 to 0.282508.Their εHf(t) values spread from 218.8 to 25.1, corre-sponding to TDM2 values between 1.56 and 2.42 Ga.

Discussion

Emplacement Age of the Baoji Pluton. Magmatic zir-cons from the Baoji granitoids yield weighted U-Pbages of ∼217 Ma for granodiorites, ∼212Ma for mon-zogranites and quartz monzodiorites, and ∼196 Mafor K-feldspar granites. Previous dating studies onthese granitoids reported U-Pb zircon ages between217 and 210Ma (Lu et al. 1999; Zhang et al. 2006; Liuet al. 2011; Wang et al. 2011). Combining these agedata, we suggest that the Baoji pluton formed as anested diapir during stepwise magma accumulationat 217 Ma (granodiorite), 212 Ma (monzogranite andquartz monzodiorite), and 196 Ma (K-feldspar gran-ite), as shown in figure 12. Thereby, the Baoji plutondisplays a pattern of decreasing ages from themarginto the center (fig. 2): granodiorites occur at the rim ofthe pluton, biotite monzogranites and quartz mon-zodiorites constitute the main body, and K-feldspargranites occur in the center of the pluton.

Figure 11. 207Pb/204Pb versus 206Pb/204Pb (a) and 208Pb/204Pbversus 206Pb/204Pb (b) for granitoids of the Baoji pluton(modified after Qin et al. 2013; Liu et al. 2016). SNCB psouthern margin of the North China block; NQBpNorthQinling belt; NSCB1SQB p northern margin of theSouth China block and South Qinling belt (Zhang et al.2002a).


Petrogenesis of the Baoji Pluton. Genetic Classifi-cation. The granodiorites are low in Si (SiO2 !

67 wt%) and metalumious (fig. 7b). They show themineral assemblage amphibole 1 biotite and lackaluminous mineral phases such as muscovite, gar-net, cordierite, and tourmaline (Barbarin 1999;fig. 4),indicating that they are I-type granitoids (Chappelland White 1992; Chappell 1999; Li et al. 2007).

The majority of monzogranite samples are meta-lumious to weakly peraluminous, with relativelylow A/CNK values (!1.1; fig. 7b) and high total-alkali contents (Na2O 1 K2O 1 8 wt%). Mostly,monzogranites have high SiO2 contents (171 wt%),and some are hornblende free. They display a nega-tive correlation betweenP2O5 andSiO2 and a positivecorrelation between Pb and SiO2 (fig. 8i, 8j). Themonzogranite samples also show increases in Y andTh with increasing Rb, typical of the I-type graniteevolution trend (table S2; Li et al. 2007). All thesefeatures are characteristics of I-type granitoids (Chap-pell and White 1992; Chappell 1999; Li et al. 2007).In addition, most of monzogranites have relativelylow Zr1Nb 1 Ce1 Y (!350 ppm) and 10000Ga/Al(!2.6) but moderate FeOT/MgO (2.1–4.9) and (K2O 1Na2O)/CaO (4.0–8.5), plotting in both the unfrac-tionated and fractionated granitefields (diagramsnotshown). Thus, we propose that the granodiorite andmonzogranite of the Baoji pluton are I-type graniticrocks.

TheK-feldspar granites are evolved in composition,and their major-element compositions are difficult todistinguish from those of fractionated monzogran-ites. They are metalumious to peraluminous, withhigh SiO2 and total-alkali contents (SiO2 1 72 wt%;Na2O1 K2O 1 8 wt%).Most K-feldspar granites have

relatively high Zr 1 Nb 1 Ce 1 Y (242–456 ppm)and 10000Ga/Al (2.4–3.1) but moderate FeOT/MgO(4.4–5.0) and (K2O1Na2O)/CaO (6.5–12.0), plotting inboth the fractionated granite and A-type granitefields (diagrams not shown), whichmeans that the K-feldspar granites have characteristics of both I- and A-type granites.

Magma Mixing and Origin of the MMEs. Theubiquitous mafic enclaves in granitic magmas re-cord interactions of different types of magma (Col-lins et al. 2000). TheMMEs are generally consideredto form bymixing andmingling processes involvingmafic and felsic magma (Qin et al. 2009 and refer-ence therein). According to previous studies, MMEsobserved in QOB early Mesozoic granitoids showtypical igneous texture, with feldspar megacrysts(K-feldspar and plagioclase, compositionally similarto that in host granitoids), and contain acicularapatites. The MMEs have low SiO2 content (mainly!57 wt%), high Nb/Ta ratios, and high Mg# butlower MgO, TiO, and Cr contents, indicating thatthey were probably derived from partial melting ofmantle materials and underwent fractional crystal-lization and crustal assimilation processes duringmagma emplacement. Compared to the host gran-itoids, theMMEs have a similar mineral assemblage,trace-element geochemistry, and initial Sr-Nd isoto-pic composition but a quite lower A/CNK value.Zircons from the MMEs display large variation inεHf values (from211.0 to 8.2), andmostMMEs have anegative zircon εHf(t), with a Neoproterozoic Hfmodel age (Qin et al. 2009, 2010; Wang et al. 2011;Duan et al. 2016). These features suggest that theMMEs are probably derived from an ancient, en-riched lithosphere mantle and underwent magma

Figure 12. Probability of zircon U-Pb ages of the Baoji granitoids, showing three stages of granitoid magmatism. Dataare from Lu et al. (1999), Zhang et al. (2006), Wang et al. (2011), Liu et al. (2011), and this study.

132 Y . - Y . X U E E T A L .

mixing. Therefore, we propose that the presence ofMMEs in the Baoji pluton indicates magma-mixingprocesses.Origin and Source Nature of the Granitoids.

The granodiorites have relatively low Sr/Y and (La/Yb)N ratios (fig. 13a, 13b) and concave chondrite-normalized REE patterns, suggesting that amphibolewas the dominant mineral in source rocks (fig. 9a,9b). The high Sr concentrations and slightly nega-tive Eu anomalies indicate that plagioclase was nota major residual phase. The granodiorites displaylow AMF (Al2O3/(MgO 1 FeOT)) and CMF (CaO/(MgO 1 FeOT)) values of 1.17–1.42 and 0.57–0.58,respectively, implying the derivation of their pre-

cursors mainly from metabasaltic to metatonaliticrocks (fig. 13d). The low ISr ratio (0.7058), negative εNd

(t) (26.0), and negative to slightly positive zircon εHf

(t) (23.6 to 1.1), coupled with two-stage Nd modelages of 1.5 Ga and two-stage Hf model ages of 1.2–1.5 Ga (fig. 10a, 10b), suggest that the magma wasmainly derived from Mesoproterozoic crustal ma-terials. Prouteau et al. (2001) and Gao et al. (2004)argued that the high Mg# in intermediate to felsicmelts are produced from a lithospheric mantle meta-somatized by silica-rich melts or fluids or from an-cient mafic lower continental crust that founderedinto the mantle and subsequently melted and inter-acted with mantle peridotite. The Mg# values (56–

Figure 13. Discrimination diagrams for granitoid rocks from the Baoji pluton. a, b, Sr/Y-versus-Y (a) and chondrite-normalized (La/Yb)N-versus-YbN (b) diagrams for granitoid rocks, compared to adakite and classical island arc/normalarc magmatic rock, after Atherton and Petford (1993) and Petford and Atherton (1996). Chondrite values are from Sunand McDonough (1989). MORBpmid-ocean ridge basalt. c, Mg#-versus-SiO2 diagram, modified after Stern and Kilian(1996). Thefields are shown for partialmelts of crust obtained from experimental studies by dehydrationmelting of low-K basaltic rocks at 8–16 kbar and 10007–10507C (Rapp andWatson 1995) and of moderately hydrous (1.7–2.3 wt%H2O)medium- to high-K basaltic rocks at 7 kbar and 8257–9507C (Sisson et al. 2005). d, Molar Al2O3/(MgO 1 FeOT)-versus-CaO/(MgO1 FeOT) diagram showing source composition for granitoid rocks of the Baoji pluton, modified after Altherret al. (2000). A color version of this figure is available online.


58) of granodiorites are much higher than those ofexperimental basaltic melts (Mg# ! 45; Rapp andWatson 1995), and they plot above the experimentalmelts derived from thepartialmelting of pure crustalmaterials or basalts and amphibolites, suggesting theinvolvement of mantle material (fig. 13c). Besides,the presence of MMEs in the granodiorites indicatesthat these rocks were probably created by partialmelting ofmafic lower crust,with contributions fromthe uppermost lithospheric mantle (Li et al. 2015 andreferences therein).

The monzogranites have relatively higher Sr/Yand (La/Yb)N ratios (fig. 13a, 13b). Fractionated REEpatterns andmoderate negative Eu and Sr anomalies(fig. 9a, 9b) suggest that the magma was probably gen-erated by an amphibole dehydration reaction, leav-ing a plagioclase-enriched residue (Sisson et al. 2005).Partial melting of hydrous, mafic to intermediatemetamorphic rocks in the crust canproducemedium-to high-K calc-alkaline graniticmagmas (Roberts andClemens 1993). Dehydration partial-melting exper-iments of intermediate to felsic rocks (SiO2 p 59–70wt%, A/CNKp 0.90–1.04) performed undermid-crustal pressures (1.0–3.2 GPa) can generate graniticmelts that have relatively high A/CNK ratios (1.03–2.07) and high SiO2 contents (170%; Skjerlie andJohnston 1993). The Baoji monzogranites are met-aluminous to weakly peraluminous, with high K2Ocontents, Na2O/K2O ! 1, SiO2 concentrations rang-ing from 69.4 to 74.0 wt%, and A/CNK ratios rang-ing from 0.98 to 1.08. In the AMF-versus-CMF dia-gram, they plot in the range ofmetagreywacke partialmelting (fig. 13d). Consequently, the Baoji monzo-granites were likely derived from intermediate tofelsic sources, probably from the partial melting ofmetagreywacke. Compared to the granodiorites, themonzogranites of the Baoji pluton have higher ISr ra-tios (0.7050–0.7083) and lower εNd(t) (212.6 to 22.5),with two-stage Nd model ages of 1.2–2.1 Ga (mostly!2.0 Ga; fig. 9a), suggesting that the magma wasmainlyderived fromMeso- toPaleoproterozoic crustalmaterials.Most zircons displayweakly positive εHf(t)ranging from 0.1 to 4.4, with single-stage Hf modelages of 0.7–0.9 Ga (fig. 10b), indicating more involve-ment of Neoproterozoic juvenile mafic crust or amantle component in the source region. These char-acteristics and the presence of coeval quartz mon-zodiorites and MMEs suggest that the magma wasmainly derived fromMeso- to Paleoproterozoic crus-talmaterials,with the involvement of amantle sourcecomponent.

In the Harker variation diagrams (fig. 8), grano-diorites and monzogranites show linear trends, sug-gesting that they may come from the same magma

source and are related by fractional crystallization.This view is also supported by their overlapping Sr-Nd and zircon Hf isotopic compositions (fig. 10). Inthe 87Sr/86Sr (t)-versus-εNd(t) diagram (fig. 10a), grano-diorites and monzogranites display Sr-Nd isotopecharacteristics similar to those of NeoproterozoicBikou and Yaolinghe metabasalts but distinct fromthose of the Qinling Group, the Kuanping Group,and the North Qinling basement. As shown in fig-ure 11, the Pb isotope compositions of the grano-diorites andmonzogranites mainly plot in the regionof the NSCB and the SQB, close to NeoproterozoicBikou and Yaolinghe metabasalts. In this regard, theintrusions in Baoji probably record the contributionof crustal material of the Yangtze block.

The quartz monzodiorites have a hydrous min-eral association of hornblende and biotite, indicat-ing crystallization fromwater-saturatedmelts. Theyare characterized by high Sr, TiO2, and K2O concen-trations but low Rb/Sr and Sr/Y ratios, suggestingthat theywere not significantly contaminated by thecoeval monzogranite melt. The weakly positive Euanomalies (fig. 9b) indicate plagioclase accumula-tion or derivation from a very primitive melt. Thequartz monzodiorites are enriched in LILEs (partic-ularly Ba and K) and light REEs and depleted inHFSEs (fig. 9a). Th, light REEs, andHFSEs aremainlytransported by the melt, whereas the LILEs are ef-fectivelymoremobile in the presence of afluid phase(e.g., Pearce and Peate 1995; Elliott et al. 1997). Inaddition, the quartz monzodiorites have negativeεNd(t) and εHf(t) values and PaleoproterozoicNdmodelages. All these signatures suggest that the quartzmonzodiorites formed by partial melting of amantlethat had been metasomatized (or enriched) by slab-derived fluids or melts.

TheK-feldspar granites define colinear trendswiththe granodiorites and monzogranites (fig. 8). Theserocks, however, have a Sr-Nd isotopic compositionquite different from that of granodiorites and monzo-granites, suggesting a distinct source. The K-feldspargranites display strongly evolved Sr-Nd isotopic com-position (ISr ratio of 0.7117 and εNd(t) of 213.5 to217.2) and variable εHf(t) values ranging from 218.8to25.1. In the Sr-Nd isotope correlation diagram, theK-feldspar granites granites plot into the field of Tri-assic granitoids fromWest Qinling and show affinityto Qinling Group gneiss (fig. 10a). Furthermore, two-stageNdmodel ages (2.4–2.1Ga) and zircon Hf modelages (2.4–1.6 Ga) of K-feldspar granites are close tothe age of the Qinling Group complex (Zhang et al.2006). The isotopic composition points toward theinvolvement of old crustal material derived mainlyfrom Qinling Group.

134 Y . - Y . X U E E T A L .

Tectonic Implications. It is well accepted that theQOB was built up through the collision of the NCBand the SCB and intervening microcontinents (e.g.,Zhang et al. 2001; Dong et al. 2011b; Dong andSantosh 2016; Hu et al. 2016). During Paleozoic toearly Mesozoic times, the QOB underwent predom-inantly two episodes of collision: the Paleozoic sub-duction between the NQB and the SQB along theShangdan suture and theTriassic collisional orogenybetween the SQB and the SCB along the Mianluesuture.The Shangdan suture is a tectonic boundary be-

tween the NQB and the SQB and represents theShangdan Ocean between the NCB and the SCB inthe Paleozoic. The geochemical and geochronologi-cal data of the gabbroic rocks, fossils, and meta-morphic rocks, together with a large number of gran-itoids in the NQB, clearly indicate that the ShangdanOcean had evolved in a northward subduction un-derneath the NQB during ∼514–420 Ma (Cui et al.1995; Wang et al. 2009; Dong et al. 2011a; Dong andSantosh 2016 and references therein). Subsequently,the subduction of the Shangdan oceanic lithosphereresulted in the closure of the Shangdan Ocean, fol-lowed by continental subduction of the SQB (Donget al. 2013 and references therein). This can also ex-plainwhy the Baoji granitoids have isotopic signaturessimilar to those of the SQB basement.The timing of thefinal ocean closure and collision

is still in debate (Chen and Santosh 2014 and refer-ences therein). TheUHPmetamorphism in theDabie-Sulu area was dated at 246–225 Ma, which meansthat the continental collision was before 225 Ma, atleast in the eastern part of the orogen (Liu et al. 2004;Liu and Liou 2011 and reference therein). So far, noevidence of Triassic UHP metamorphism in theQinling area has been reported (Dong et al. 2011b;Qin et al. 2013; Wang et al. 2013), and the exactcollision timing inQinling is still not clear.Wanet al.(2005) and Zheng et al. (2007) reported the retrogres-sivemetamorphic ageof 224–215Ma forhigh-pressureto UHP eclogite-facies recrystallization during theexhumation of the subducted Yangtze continentallithosphere to a higher crustal level.Paleomagnetic data suggest that the NCB and the

SCB collided first in the eastern part, while an oce-anic basin remained in the western part that did notcloseuntil themid-Jurassic (Zhao andCoe1987;Zhuet al. 1998). As a result, the so-called scissor or zippercollision model was proposed. Yin and Nie (1993)argued that the NCB and the SCB were assembledprogressively during an east-to-west collision, andChen and Santosh (2014) suggested that there wasa scissor-like closure of the Mianlue Ocean and a

transition from oceanic subduction to continentalcollision in the QOB between 200 and 190 Ma. Onthe basis of geochemical studies of detrital heavyminerals and detrital zircon ages of Early–MiddleTriassic sedimentary rocks, Yang et al. (2012) sug-gested that the northward subduction of theMianlueOcean continued at least until the Triassic in thewestern portion of South Qinling.The emplacement ages of early Mesozoic granit-

oids in the QOB are regarded as representing the timeof postcollisional oceanic-slab break-off, and conse-quently mantle upwelling induced by slab break-offis widely used in interpreting granitoid magmatismin the QOB (e.g., Sun et al. 2002; Qin et al. 2008).However, the earlyMesozoic tectonic setting duringwhich the QOB granitoids were produced is still con-troversial. Recently, Wang et al. (2013) provided anoverview of early Mesozoic magmatism in the QOB.In the QOB, magmatism occurred predominantly inthe SQBbetween 252 and 185Ma.The authors couldidentify two distinct stages of magmatism, one be-tween 250 and 240Ma and another between 225 and185 Ma, with a cluster of ages between 225 and200 Ma. Wang et al. (2013) stated that the first gran-itoid pulse mainly consists of I-type rocks that formedin a continental arc setting related to the subductionof the Mianlue Ocean. In contrast, voluminous mag-matism at 225–185 Ma represents a transition froma syn- to a postcollisional setting in response to thecollision between the NCB and the SCB. Dong et al.(2015b) proposed a further subdivision of the Trias-sic granitoids of theQOB.On the basis of systematicchanges in composition and age, they distinguishedbetween subduction-related (245–238 Ma, identicalto the first group of Wang et al. 2013), syncollisional(237–210Ma), andpostcollisional (210–200Ma)groups.According to this division, there is an extensive over-lap in ages of the Baoji granitoids and the syn- to post-collisional granitoid group of the QOB. In addition,theBaoji granitoidshave 206Pb/238Uagesof 217–194Ma,which are younger than the Triassic UHPmetamor-phic event in the Dabie-Sulu orogenic belt but closeto the retrogressive metamorphism event. Therefore,we conclude that the Baoji pluton formed in a post-collisional setting.As discussed above, all samples from the Baoji plu-

ton show enrichment in light REEs and LILEs butdepletion in HFSEs, which are typical characteris-tics of arc- or subduction-related granitoids. In theSr/Y-versus-Yand (La/Yb)N-versus-YbNdiagrams, theymainly plot in the fields of normal arc or classic is-land arc magmas (fig. 13a, 13b). It has been arguedthat the compositions of the granitoids were largelycontrolled by their source rocks, so the arc signatures


may not represent an arc setting but could have beeninherited from a previous subduction period. Dur-ing continental subduction, crustal materials can berecycled into the mantle in the subduction channel.The crustal compositions could then be introducedinto the overlying subcontinental lithospheric man-tle by dehydration and/or partial melting. Thus, iden-tification of crust-mantle interaction from orogenicmagmatic rocks could confirm the collision process.

In the following, we present a possible formationmodel for the Baoji granitoids: continental crust wastransported tomantle depth by subduction, and thenslab break-off induced asthenospheric upwelling. Thisprocess caused partial melting of the lithosphericmantle as well as partial melting of the lower crust.Such a process might have involved two differentmantle sources, onemore refractory/residual and onemore enriched/metasomatized. The magmas thatbuilt up the Baoji composite pluton are products ofthese different sources and formed bymagmamixingand subsequent fractional-crystallization processes.The first melts emplaced a mixture of the enriched/metasomatized lithosphericmantle andmafic lowercrust, producing the granodiorites. Ongoing partialmelting in the upper mantle and lower crust pro-duced hybrid melts from which the monzograniteswere derived. At the same time, a mantle source me-tasomatized by fluids or melts from the subductedslab produced the geochemically and isotopically en-riched quartz monzodiorites. Subsequently, partialmelting of thickened lower crust produced the K-feldspar granites.

Conclusions

The granitoids of the Baoji pluton formed duringlate and postcollisional processes between theNorthChina and South China blocks. Zircon U-Pb datingindicates that the pluton assembled during threeepisodes over 20 million years: (1) granodioritesformed at ∼217Ma, (2) monzogranites and quartzmon-zodiorites at ∼212 Ma, and (3) K-feldspar granitesat ∼196 Ma. The granodiorites and monzograniteswere generated by partial melting of crustal mate-rials with the involvement of mantle source that wasenriched by slab-derived fluids and melts. Monzo-granites and quartz monzodiorites formed simulta-neously, while the latter were derived from a stronglymetasomatized lithospheric mantle wedge. The K-feldspar granites formed in a postcollisional settingand were derived from partial melting of lower-crustalmetagreywacke.

ACKNOWL EDGMENT S

This studywasfinancially supported by theNationalKey Research and Development Program of the Min-istryof Science andTechnology (grant 2016YFC0600404)and the National Nature Science Foundation ofChina (grants 41372072 and 41090372). We wish tothank editor D. B. Rowley and an anonymous re-viewer, for their constructive comments that helpedto improve our discussion, and H. Nie and H. Zhangfor their careful revision and comments. We thankP. Xiao for help in analysis and Y.Wang and Y. Qi forhelp in fieldwork.

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