Geol. Mag. 146 (5 ), 2009, pp. 638–651. c© 2009 Cambridge University Press 638doi:10.1017/S0016756808005918 Printed in the United Kingdom

Petrogenesis and tectonic implications of A-type granitesin the Dabie orogenic belt, China: geochronological and

geochemical constraints

LING CHEN*†, CHANG-QIAN MA*†§, ZHEN-BING SHE*†, ROGER MASON*,JIN-YANG ZHANG‡ & CHAO ZHANG*†

*Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China†State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,

Wuhan 430074, China‡Faculty of Mineral Resources, China University of Geosciences, Wuhan 430074, China

(Received 23 July 2008; accepted 2 October 2008; First published online 19 January 2009)

Abstract – The Dabie orogenic belt is characterized by the presence of large volumes of intrusive andvolcanic rocks that formed in Late Mesozoic times. Most of the intrusive bodies are I-type granites butit is still unclear whether there are contemporary A-type granites. Here, we report the first unambiguousdiscovery of A-type granite from Baiyashan in the North Dabie tectonic belt. The crystallization ageof the body has been fixed as 120.4 ± 1.2 Ma using U–Pb analysis of zircons by LA-ICPMS. TheBaiyashan granite is enriched in Si, K, Na, Rb and REE, has elevated FeOtot/(FeOtot + MgO) andGa/Al ratios, and is depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. The REE composition shows highlyfractionated patterns with (La/Yb)N = 6.95–16.68 and Eu∗/Eu = 0.33–0.59. Its crystallization age,field relationships, petrographic and geochemical data show beyond doubt that the Baiyashan graniteis an Early Cretaceous A-type granite. Sr–Nd isotope systematics are characterized by a high ISr of0.708–0.714 and a low εNd of −7.5 to −19.4, with TDM2 = 1.5–2.5 Ga, and these data indicate thatthe magmas were dominantly sourced from partial melting of middle to lower crustal intermediate-felsic igneous rocks and mingling with mafic to intermediate magmas, during rift-related magmatismassociated with subduction of the Palaeo-Pacific Plate beneath Eastern China in Early Cretaceoustimes.

Keywords: A-type granites, petrogenesis, Late Mesozoic, Dabie orogenic belt.

1. Introduction

The Dabie orogenic belt is the largest ultrahigh-pressure (UHP) metamorphic belt in the world (Hacker,Wang & Eide, 1996) and was formed by a Triassiccollision between the North China and Yangtze blocks(Zhai & Cong, 1996; Hacker et al. 2000). Cretaceousintrusive and volcanic rocks comprise 47 % of thesurface exposure of the Dabie Orogen (Ratschbacheret al. 2000) and were emplaced into the Dabie orogenat c. 120–145 Ma (Ma et al. 1998; Jahn et al. 1999;Ma et al. 1999; Chen, Jahn & Wei, 2002; Wang et al.2005b; Wang et al. 2007). It is still uncertain whetherTriassic collision events directly gave rise to theseextensive magmatic rocks. The time interval betweenthe emplacement of the magmatic rocks and collision ofthe North China and Yangtze blocks is c. 100 Ma (Maet al. 2008) greater than that seen in typical orogenicbelts elsewhere; for example, post-collisional granitesin the European Variscan orogenic belt appearedc. 50 Ma after collision (Finger et al. 1997; VanWagoner et al. 2002). Late Mesozoic magmatism inEastern China has recently attracted much attentionbecause of its relationship to Palaeo-Pacific Platesubduction (Zhang et al. 2002; Wu et al. 2005; Huanget al. 2007; Zhao et al. 2007), although the Dabie

§Author for correspondence: cqma@cug.edu.cn

orogen is over 900 km from present-day subductionzones of the Palaeo-Pacific Plate. Thus the tectonicsetting of Late Mesozoic magmatism in the Dabieorogenic belt is still an open question, and the wide-spread granitoids provide insights into this discussion.Previous studies of granitoids of the Dabie orogen havefocused on I-type granites, although A-type granitesalso occur but are as yet undated (Wang, Zhao &Xiong, 2000). Whalen and others argued that althoughA-type granitoids occur in various locations all overthe world, they are all found in extensional settings,and have great significance for the interpretation ofgeodynamic processes (Whalen, Currie & Chappell,1987; Eby, 1992; Bonin, 2007). The A-type granitoidsof the Dabie orogenic belt are representative examplesand deserve further investigation.

In this paper, we present new zircon U–Pb ages, geo-chemical and isotopic data for the Baiyashan aluminousA-type granitic pluton, which was previously thoughtto be an Early Cretaceous I-type granite (Li & Wang,1991). We discuss the petrogenetic and geodynamicimplications of our data.

2. Geological setting and petrography

The geology of the Dabie orogenic belt (Fig. 1b) hasbeen described in many recent publications (Cong &

A-type granites in Dabie orogenic belt, China 639

Figure 1. (a) Sketch map showing the location of the Dabie orogenic belt and distribution of Early Cretaceous A-type granites inSE and Central China with ages. Abbreviations represent the names of the A-type granitoids (see Table 3). (b) Geology of the Dabieorogenic belt, modified from Zhang et al. (2002). NHY – North Huaiyang block; NDB – North Dabie orthogneiss unit; SDB – SouthDabie HP/UHP metamorphic complex; SS – Susong high pressure belt. (c) Simplified geological map of Baiyashan.

Wang, 1996; Ma et al. 1998; Ma et al. 2000; Mason& Sang, 2007; Xu, Ma & Ye, 2007). From south tonorth, the eastern part of the Dabie orogenic beltcomprises: (1) the Su Song (SS) high pressure belt,(2) the South Dabie HP/UHP metamorphic complex,(3) the North Dabie orthogneiss unit and (4) the NorthHuaiyang greenschist-facies meta-sedimentary unit.All four tectonic units were intruded by large volumesof Cretaceous magmatic rocks. Compared with othermetamorphic belts, the North Dabie metamorphic corecomplex was exhumed from deep levels and modifiedby intensive intrusion of Early Cretaceous granitoids.It therefore presents an ideal opportunity for studyinggranitoids (Ma et al. 1998; Huang et al. 2007; Xu,Ma & Ye, 2007). This study focuses on the Baiyashangranitic intrusive body in the North Dabie complex asa typical representative example (Fig. 1).

The Baiyashan granitic body covers an area of22 km2 in the Baiyashan–Taergang area (31◦05′ N,115◦05′ E), and is intruded into the North Dabieorthogneiss unit near the Shang-Ma (Shangcheng-

Macheng) Fault (Fig. 1b). It has the form of a stock,triangular in plan, trending NNE, consistent with theregional tectonic strike. Neogene sediments coverthe intrusion to the west. Near its western margin,the granite has an imposed gneissose foliation parallelto the regional tectonic strike, and away from thewestern part, magma has often been emplaced alongthe gneissose foliation of the metamorphic countryrocks. The Baiyashan granitic body is composed ofmedium-grained biotite granite and porphyritic biotitegranite, with the former dominating. Observations atoutcrop show porphyritic biotite granite intruded bymedium-grained biotite granite. The porphyritic biotitegranite contains ∼ 10 mm phenocrysts of microclineperthite, plagioclase antiperthite (An26), quartz andbiotite (40–50 %) in a groundmass of subhedralK-feldspar, quartz, plagioclase (An25), biotite, apatite,sphene and Fe–Ti oxides. Plagioclase displays normalzoning; biotite is green or green-brown and sometimesaltered to chlorite. Grain boundaries are irregular, andmyrmekitic intergrowths occur in the groundmass. The

640 L. CHEN AND OTHERS

medium-grained biotite granite also contains quartz(25–30 %), plagioclase (An25) (15–20 %), alkali-feldspar (40–45 %), biotite (10–15 %) and accessoryminerals (< 1 %), including zircon, sphene and Fe–Tioxides, and is similar to the porphyritic biotite graniteexcept that the crystals are uniform in size, ranging upto about 10 mm. Both textural types of granite almostalways display deformation features such as strainshadowing in quartz, and cracks and deformed twinlamellae in feldspars. Where deformation is strongest,granite has been metamorphosed into mylonitecontaining feldspar augen in a blastomylonite matrixwith ribbon microstructure (Mason & Sang, 2007).

Mafic microgranular enclaves (MMEs) occursparsely in the porphyritic biotite granite and rarely inthe medium-grained biotite granite. They are ellipticalto lens-shaped, flattened parallel to foliation planesof the granite, have igneous textures and range fromquartz-diorite to monzonite in composition, typicallyconsisting of 20–25 % biotite, 25–40 % K-feldspar, 15–25 % plagioclase, 15–20 % quartz, 5 % hornblende andaccessory minerals (< 1 %) including zircon, spheneand Fe–Ti oxides. Fine-grained, igneous-textured en-claves from I- and A-type granites are usually thoughtto represent remnant mafic end-members of a mafic li-quid component added to intermediate or felsic magmachambers (Bonin, 2004; Perugini, Poli & Bonin, 2004)or alternatively, hybrids of mafic and felsic components(Yang et al. 2006). The enclaves in the Baiyashanhave igneous textures with acicular apatite and allotri-omorphic overgrowths of plagioclase round euhedralcores, typical features of magmatic enclaves (Sparks &Marshall, 1986). Acicular apatite has been attributedto rapid cooling caused by mingling of small volumesof hot basalt with cool granitic melt (D’Lemos, 1996;Yang et al. 2006, 2008). ‘Dents de cheval’ K-feldsparmegacrysts growing across the boundaries of enclavesand biotite-rich basic fronts in the outer layers showthat the enclaves were still liquid when the megacrystswere growing in the granite magmas.

A few microdolerite dykes up to 0.3 m widetypically strike N30◦ and dip 70◦ NE parallel to theregional tectonic strike. They have grain sizes lessthan 0.2 mm, chilled margins and columnar joints andwere intruded during a separate magmatic event afterthe granite body had cooled.

Although Li & Wang (1991) considered the Baiy-ashan intrusion to be an I-type granitic body and repor-ted a crystallization age of 105 Ma obtained by zirconU–Pb isotope dilution, this age has been questioned(Zhao, Yang & Shen, 2002) and may represent a mixedage rather than the actual time of crystallization. Ournew results revise their conclusions.

3. Analytical methods

3.a. Zircon U–Pb dating

Rock samples were crushed in a steel jaw crusher,and shaking bed and heavy liquid techniques wereused to concentrate heavy minerals. Zircon crystals

were separated from heavy minerals by hand-pickingunder a binocular microscope. Sample zircons weremounted in epoxy, and the resin discs were polisheduntil less than half the zircon grains were exposed.Before in situ U–Pb isotope analyses, the morphologyand internal structure of these zircons were studied bybinocular microscope and cathodoluminescence (CL)imaging, which were used as guides to find appropriatespots for the isotope analyses. Representative zirconcrystals were prepared for the CL investigations andin situ isotope analyses by mounting in epoxy resinand polishing down to expose the grain centres. CLimages were obtained using a JEOL JXA-8900RLmicroprobe at the Chinese Academy of GeologicalSciences (CAGS), Beijing at 15 kV. U–Pb dating wasperformed using a laser ablation inductively-coupledplasma-mass spectrometer (LA-ICPMS) at the StateKey Laboratory of Geological Processes and MineralResources (GPMR), China University of Geosciences,Wuhan. The method of LA-ICPMS zircon U–Pb agedating is described by Yuan et al. (2004). Data werereduced using the in-house Glitter online software,which provides for selection of stable intervals in eachtime-resolved analysis. Subsequent data processingwas carried out using the ISOPLOT program (Ludwig,1997), and measured 204Pb was applied for commonlead correction (Andersen, 2002). Errors are quotedat the 1σ level for individual analyses, whereas theerror for weighted mean ages is quoted at the 2σ (95 %confidence) level.

3.b. Major and trace element analysis

Fresh whole-rock samples were crushed in a steelcrusher and powdered to < 200 mesh using an agatemill. Major element oxides were measured using aRegaku 3080E1 XRF spectrometer at the AnalyticalInstitute of the Bureau of Geology and MineralResources, Hubei Province (BGMRHP). Analyticalprocedures are described in detail by Gao et al. (1991,1995). Relative standard derivations for major elementoxides are within 5 %.

Trace and rare earth elements were analysed atthe State Key Laboratory of Geological Processesand Mineral Resources (GPMR), China University ofGeosciences, using an Agilent 7500a ICP-MS. Meth-ods of sample preparation and digestion are describedin Zhang et al. (2002). Analytical precision (relativestandard deviation) estimated from repeated analysesof three standard reference samples G-2, AGV-1and GSR-3 is better than 5 % for rare earth elementsand 5–12 % for other trace elements.

3.c. Rb–Sr and Sm–Nd isotope analysis

Whole rock Sr–Nd isotopic ratios were obtained using aFinnigan Triton thermo-ion mass isotope spectrometerat GPMR, China University of Geosciences, followingthe methods of Zhang et al. (2004). Reported 87Sr/86Srand 143Nd/144Nd ratios were measured by ICP-MS and

A-type granites in Dabie orogenic belt, China 641

Table 1. LA-ICPMS zircon U–Pb data for Baiyashan granite (MC04)

SpotU238

ppmTh232

ppm

206Pb∗

ppm

204Pbc

(%)

207Pb/206Pb ±%

207Pb/235U ±%

206Pb/238U ±%

208Pb/232Th ±%

207Pb/235Uage

206Pb/238Uage

1 117.77 190 8.92 <1.41 0.048 0.0023 0.1251 0.0057 0.0189 0.0002 0.0057 0.0001 120 ± 5 121 ± 12 560.14 428 42.56 <2.11 0.049 0.0021 0.128 0.0054 0.019 0.0002 0.0058 0.0001 122 ± 5 121 ± 13 30.31 53.6 2.25 1.99 0.048 0.0049 0.1238 0.0122 0.0186 0.0004 0.0055 0.0002 119 ± 11 118 ± 34 523.73 253 39.12 <1.50 0.05 0.0011 0.1292 0.0027 0.0187 0.0002 0.0062 0.0001 123 ± 2 119 ± 15 543.29 346 40.77 <1.10 0.05 0.0016 0.1284 0.0039 0.0188 0.0002 0.0053 0.0001 123 ± 4 120 ± 16 4986.6 3292 380.6 <1.94 0.048 0.0009 0.1265 0.0022 0.0191 0.0002 0.0062 8E-05 121 ± 2 122 ± 17 236.94 305 17.89 <1.14 0.048 0.0017 0.1258 0.0043 0.0189 0.0002 0.0057 0.0001 120 ± 4 121 ± 18 1024.6 290 77.17 <1.09 0.049 0.0009 0.1275 0.0022 0.0188 0.0002 0.0064 0.0001 122 ± 2 120.3 ± 0.99 388.97 220 30.73 0.66 0.05 0.0017 0.1267 0.004 0.0184 0.0002 0.0058 5E-05 121 ± 4 117 ± 1

10 1567.4 586 32.86 3.49 0.054 0.0013 0.1423 0.0032 0.0192 0.0002 0.006 6E-05 135 ± 3 123 ± 1

Errors are 1-sigma; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Common Pb corrected using measured 204Pb.

Figure 2. LA-ICPMS zircon U–Pb concordia diagram. Inset:representative CL images of zircons from the sample MC04 ofthe Baiyashan granite; the numbers represent spot numbers.

normalized to 86Sr/88Sr = 0.1194 and 146Nd/144Nd =0.7219, respectively. Total analytical blanks were5 × 10−11 g for Sm and Nd and (2–5) × 10−10 g forRb and Sr.

4. Zircon U–Pb geochronology

A representative sample of medium-grained biotitegranite (MC04, sampling location: 115◦04.606′ E,31◦04.592′ N) was selected for LA-ICPMS dating. Thezircons selected for analysis were prismatic, highlytransparent and free of visible inclusions. They weremostly pale yellow to colourless euhedral crystals,ranging from 120 μm to 350 μm in length, withlength/width ratios of 1:1 to 3:1. Ten analyses wereperformed on ten crystals that were light colouredin cathodoluminescence (CL) images and had highTh/U ratios (0.28 to 1.77), indicative of a magmaticorigin (Xu et al. 2008). Data are listed in Table 1 andshown in a concordia diagram (Fig. 2). The inset inFigure 2 shows that zircons from the Baiyashan plutonexhibit well-developed oscillatory zoning, confirmingtheir magmatic origin, and none contain xenocrysticcores. The ten analyses yield concordant 206Pb/238U

ages ranging from 118 ± 3 Ma to 123 ± 1 Ma, witha weighted mean age of 120.4 ± 1.2 Ma (95 % conf.,MSWD = 2.7).

5. Geochemistry

Ten of the least-altered samples were chosen for major,trace, rare-earth element and Sr–Nd isotopic analyses:seven host rock samples consisting of five medium-grained biotite granites (MC04, MC05, MC06, MC09,MC18-2), two porphyritic biotite granites (MC07,MC18-1), two enclaves (MC19, MC20) from por-phyritic biotite granite and one microdolerite dykesample (MC01-4). Major, trace, rare-earth element andSr–Nd isotope data are given in Table 2.

5.a. Major, trace and rare-earth elements

As shown in Table 2, the Baiyashan granites (in-cluding medium-grained biotite granite, porphyriticbiotite granite) are siliceous, with SiO2 rangingfrom 65.9 % to 74.0 %, and enriched in al-kalis (K2O = 4.46–6.29 %, Na2O = 3.50–4.07 % andK2O + Na2O = 8.07–10.25 %). They have low Fe2O3

(0.35–2.49 %), FeO (0.47–1.97 %), MnO (0.02–0.08 %), CaO (0.61–2.52 %), P2O5 (0.05–0.34 %) andMgO (0.24–1.21 %) contents. Mg numbers range from23.3 to 33.9 and FeOtot/MgO ratios vary from 3.48 to5.77 with an average of 4.82, higher than I-type granite(average of 991 samples FeOtot/MgO value is 2.27), S-type granite (average of 578 samples FeOtot/MgO valueis 2.38) and M-type granite (average of 17 samplesFeOtot/MgO value is 2.37) (Whalen, Currie & Chappell,1987). Figure 3a shows that the Baiyashan granites aremetaluminous or weakly peraluminous, and stronglyperaluminous compositions are absent. According tothe classification scheme for granitoids proposed byFrost et al. (2001), the Baiyashan granites are ferroangranites, like other A-type granites worldwide (Fig. 3b).The lack of inherited zircons in granitic igneous rocksmay indicate that most of the zircons in the Baiyashangranites nucleated and crystallized near the liquidus, aconclusion supported by their euhedral form. Zr and Psaturation temperatures were calculated after Watson& Harrison (1983) as 780 ◦C for MC 04, 786 ◦C for

642 L. CHEN AND OTHERS

Figure 3. (a) Molar Al/(K+Na) v. Al/(Ca+Na+K) diagram. (b) Mass % FeOtot/(FeOtot+MgO) v. SiO2. A-type granite field after Frostet al. (2001).

Figure 4. (a) Chondrite-normalized REE patterns, normalizing values from Taylor & McLennan (1985); (b) primitive mantle-normalized element spider diagram, normalizing values from Sun & McDonough (1989).

MC 05, 792 ◦C for MC 06, 813 ◦C for MC 07, 789 ◦Cfor the most felsic component MC09, 806 ◦C for MC18-1, 808 ◦C for B11–1, 821 ◦C for B11–2 and 794 ◦Cfor B1-2.

The Baiyashan granites are characterized by highGa/Al ratios, Ga/Al × 104 ranging from 2.43 to 3.06,and all samples fall in the field of A-type granitesin discrimination diagrams. The Baiyashan granitesare also enriched in rare-earth elements (REE) withtotal concentrations of 238.8–420.3 ppm. The lightrare earth elements (LREE) display a strong negativecorrelation with increasing SiO2, typical of A-typegranites (Rajesh, 2000; Wu et al. 2002). Chondrite-normalized REE patterns show enrichment of LREErelative to heavy rare earth elements (HREE), with(La/Yb)N ratios of 6.95–16.68 and strongly negativeEu anomalies (Eu∗/Eu = 0.33–0.59) (Fig. 4a). In aspider diagram (Fig. 4b), the Baiyashan granites shownegative anomalies of Ba, Sr, P, Eu and Ti, and positiveanomalies of Ce and Y, consistent with the patterns ofA-type granites (Wu et al. 2002), and also suggestinga continental crust affinity. Negative anomalies ofP and Ti probably reflect fractionation of apatiteand ilmenite/sphene, respectively. These geochemicalcharacteristics combined with high zircon saturationtemperatures show beyond doubt that the Baiyashangranites are aluminous A-type granites. The MMEshave higher Fe2O3 (1.32–2.51 %), FeO (2.65–4.80 %)

and Mg numbers (38.9–39.3), and lower FeOtot/MgO(2.76–2.80) than the host Baiyashan granites. However,similar enrichment and depletion patterns shown inthe spider diagram (Fig. 4b) suggest that they belongto the same magmatic suite. The microdolerite dykerock is different: enriched in LREE with (La/Yb)N

ratios of 9.95 and having a weak positive Eu anomaly(Eu∗/Eu = 1.05), indicating that there has been noplagioclase fractional crystallization. In Figure 4b, themicrodolerite displays negative Th, Nb, Ta, Sr, P, Yand Yb anomalies, confirming field and petrographicevidence that it represents magma from a sourceunrelated to the Baiyashan intrusion. Its Zr/Ba ratioof 0.42, higher than 0.2, suggests a depleted mantleorigin (Ormerod et al. 1988).

5.b. Sr and Nd isotopes

Initial 87Sr/86Sr (ISr) and εNd(t) values for the granitesamples were back-calculated for a magma crystalliz-ation age of 120 Ma. Calculated ISr and εNd valuesrange from 0.708 to 0.714 and −7.48 to −19.43,respectively, giving mantle-depletion Nd model ages(TDM2) in the range 1.5–2.5 Ga, indicating that themagma of the Baiyashan granite was derived from anold crustal source. Sr–Nd isotopic data ISr = 0.705 andεNd = 3.71 with TDM2 = 0.61 Ga (t = 120 Ma) for the

A-type

granitesin

Dabie

orogenicbelt,C

hina643

Table 2. Chemical compositions for Baiyashan granitic body; major (wt %) and trace elements (ppm)

Sample no. MC01-4 MC04 MC05 MC06 MC07 MC09 MC18-1 MC18-2 MC19 MC20 B11-1 B11-2 B1-2

Sampling location 115◦04.641′E31◦08.187′N

115◦04.606′E31◦04.592′N

115◦04.693′E31◦05.098′N

115◦04.991′E31◦05.258′N

115◦05.065′E31◦05.483′N

115◦04.123′E31◦05.008′N

115◦03.985′E31◦06.432′N

115◦03.985′E31◦06.432′N

115◦05.720′E31◦04.775′N

115◦05.719′E31◦05.331′N

Rock type Gabbro MBG MBG MBG PBG MBG PBG MBG MBG MBG MBG MBG MBG

SiO2 49.54 73.6 73.65 73.44 70.26 74.01 65.88 72.66 59.56 67.24 73.22 70.12 73.6TiO2 2.43 0.19 0.21 0.23 0.50 0.21 0.79 0.15 1.08 0.68 0.26 0.38 0.2Al2O3 13.8 13.62 13.56 13.55 14.2 13.43 14.78 13.87 15.83 15.18 13.84 15.17 14.05Fe2O3 4.06 0.35 0.48 0.49 1.33 0.49 2.49 0.74 2.51 1.32 0.95 1.04 0.81FeO 6.48 1.07 1.03 1.17 1.63 1.08 1.97 0.47 4.80 2.65 1.03 1.40 0.64MnO 0.14 0.03 0.03 0.03 0.05 0.04 0.08 0.02 0.17 0.08 0.05 0.06 0.04MgO 6.18 0.24 0.27 0.31 0.77 0.28 1.21 0.3 2.56 1.37 0.45 0.46 0.22CaO 6.61 1.12 1.13 1.18 1.94 1.09 2.52 0.61 3.15 3.13 1.33 1.92 1.18Na2O 3.13 3.82 3.53 3.57 3.75 3.6 4.07 3.96 3.9 4.6 3.59 3.62 3.50K2O 2.00 5.10 5.00 5.07 4.46 4.97 4.62 6.29 4.42 2.57 4.48 4.86 4.66P2O5 0.52 0.06 0.05 0.05 0.17 0.05 0.34 0.07 0.58 0.21 0.07 0.14 0.05H2O+ 3.18 0.51 0.64 0.6 0.58 0.49 0.78 0.57 1.07 0.64 0.39 0.47 0.51CO2 1.69 0.12 0.24 0.12 0.1 0.07 0.11 0.06 0.09 0.11 0.08 0.12 0.29LOI 4.02 0.27 0.54 0.39 0.33 0.31 0.56 0.49 0.6 0.32 nd nd ndTotal 99.76 99.71 99.58 99.69 99.64 99.74 99.64 99.77 99.72 99.78 99.74 99.76 99.75Mg no. 52.09 23.6 24.77 25.55 32.69 24.71 33.88 32.01 39.27 38.89 29.84 25.97 22.27A/CNK 0.71 0.98 1.02 1.00 0.98 1.01 0.91 0.96 0.94 0.95 1.05 1.03 1.09Rb 42.3 297 274 279 149 290 133 nd 304 161 239 936 260Sr 701 112 121 132 237 118 499 nd 269 240 139 217 122Ba 446 537 587 657 800 568 1360 nd 476 401 609 287 582Ta 2.44 5.3 2.3 2.13 2.29 3.33 2.89 nd 4.36 3.51 1.9 3.1 2.6Nb 39.4 32.6 20.3 23.1 27.6 24.7 39.1 nd 62.9 43.2 21.8 29.8 22.1Hf 4.95 5.39 5.16 5.36 6.23 5.85 6.99 nd 8.27 7.77 5.5 6.7 5.3Zr 196 161 163 178 241 170 261 nd 305 296 202 249 167Y 24.6 49.7 33.1 38.8 43.7 41.7 38.8 nd 51.7 43.6 27.88 36.09 35.27Ga 20.5 21.9 21.2 21.9 15.7 21.9 19.4 nd 26.3 22.7 nd nd ndLa 24.67 57.2 61.85 64.45 96.58 68.61 91.30 nd 86.50 90.29 54.8 78.17 54.37Ce 51.85 113.57 122.81 123.36 189 138 192.55 nd 190.69 193.52 97.21 138.79 100.07Pr 6.58 11.5 12.4 12.8 19.8 14 20.34 nd 21.71 20.73 12.64 16.31 11.74Nd 29.38 40.2 42.3 43.4 67.9 46.8 72.03 nd 74.96 72.88 46.35 58.62 42.75Sm 7.00 8.03 7.58 7.77 11.3 8.63 12.25 nd 12.68 12.40 8.15 9.25 7.28Eu 2.31 0.82 0.84 0.83 1.49 0.89 2.14 nd 1.69 1.49 0.81 1.26 0.76Gd 6.73 6.9 6.04 6.34 8.8 6.74 9.15 nd 9.82 9.16 6.13 7.08 6Tb 0.98 1.24 0.95 1.01 1.32 1.09 1.37 nd 1.50 1.38 0.94 1.16 1.03Dy 5.40 7.71 5.35 6.07 7.65 6.72 8.06 nd 9.41 8.41 4.97 6.19 5.7Ho 0.91 1.72 1.06 1.21 1.49 1.34 1.58 nd 1.95 1.66 0.96 1.26 1.12Er 2.20 4.61 2.98 3.51 4.18 3.68 4.17 nd 5.50 4.70 2.78 3.69 3.33Tm 0.30 0.75 0.44 0.54 0.57 0.6 0.62 nd 0.91 0.74 0.46 0.61 0.54Yb 1.73 5.56 3.2 3.71 3.86 4.25 4.10 nd 6.32 4.92 2.98 3.87 3.59Lu 0.22 0.87 0.52 0.58 0.55 0.68 0.64 nd 1.00 0.75 0.45 0.58 0.53

644 L. CHEN AND OTHERS

∑R

EE

140.

2726

0.68

268.

3227

5.59

414.

4930

2.03

420.

29nd

424.

6542

3.01

239.

6332

6.84

238.

81δE

u1.

020.

330.

370.

350.

450.

340.

59nd

0.45

0.41

0.46

0.34

0.34

(La/

Yb)

N9.

666.

9513

.06

11.7

416

.68

10.9

115

.04

nd9.

2412

.41

13.6

512

.43

10.2

387

Rb/

86S

r0.

325

7.70

26.

542

6.12

41.

299

7.12

70.

774

nd3.

278

1.94

5nd

5.67

03nd

87S

r/86

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0.70

5362

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6788

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4869

0.72

4233

0.71

2234

0.72

5916

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nd0.

7144

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7141

73nd

0.72

34nd

±2σ

32

22

32

3nd

33

nd1

ndI s

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7140

90.

7140

80.

7141

40.

7100

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7141

70.

7082

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0.70

906

0.71

086

nd0.

7140

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147S

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1467

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1085

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830.

0842

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160.

1028

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5115

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5116

290.

5115

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5117

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microdolerite dyke suggest that the magma had a youngdepleted mantle source.

6. Discussion

6.a. Magma source

The origin of A-type granites is still an active subjectof discussion among petrologists, mainly because somany compositional variants have been found, andthere is no consensus on the origin of A-type granites(Bonin, 2007). There are aluminous A-type granites,in which the sum of Na and K is more or less equalto Al, others are decidedly alkaline, and still others areperaluminous. The Baiyashan granite is an aluminousA-type granite. A number of petrogenetic schemeshave been proposed for the magma sources of A-typegranites (Martin, 2006; Bonin, 2007; Zhang et al. 2007)which mostly fall into two categories, one invokingcrustal, one mantle sources, while a few advocatemixing between crustal and mantle sources (Schmittet al. 2000; Bonin, 2007). Derivation by fractionalcrystallization from alkaline basalts or other mantle-derived magmas and melting of residual granuliticsources have also been proposed (King et al. 1997).

We have used Rb–Sr and Sm–Nd isotopic data toelucidate the source magma of the Baiyashan granitebecause isotopic ratios are useful indicators of granitesource regions. The Sr and Nd isotopic results (ISr =0.708–0.714, εNd = −7.48 to −19.43) indicate acrustal source but do not rule out contributions fromenriched mantle material. Eby (1992) suggested that theY/Nb ratio could distinguish between A-type granitewith mantle (Y/Nb < 1.2) and crustal (Y/Nb > 1.2)origins. Most of the Baiyashan granite samples haveY/Nb > 1.2, implying that they are largely crustal inorigin.

Mineralogical and geochemical features show thatthe Baiyashan A-type granite crystallized at a hightemperature from relatively anhydrous magma, andpossible crustal source rocks are present in the NorthDabie basement and adjacent areas. We have thereforeevaluated various potential crustal sources in theNorth Dabie belt to see if they could make plausiblecontributions to the magma source of the BaiyashanA-type granites. Proposed sources vary from granodi-orite, tonalite and quartz diorite to tholeiites and theirdifferentiated products (Xu, Ma & Ye, 2007).

A great deal of published Sr–Nd isotopic data showsthat εNd(t) and (87Sr/86Sr)t values for the Northern DabieComplex are characterized by more negative εNd(t)values (−8.84 to about −23.56) and restricted narrow(87Sr/86Sr)t variation (∼ 0.707 to ∼ 0.715) comparedwith South Dabie UHP gneiss (Chen & Jahn, 1998;Ma et al. 2000; Wang, Zhao & Xiong, 2000; Zhenget al. 2000; Zhang et al. 2002; Xu, Ma & Ye,2007; Xu, Ye & Ma, 2008). As shown in Figure 5,the Baiyashan A-type granites occupy almost exactlysimilar ranges to the Northern Dabie Complex. TheCretaceous granitoids, irrespective of tectonic unit,

A-type granites in Dabie orogenic belt, China 645

Figure 5. εNd(t) v. (87Sr/86Sr)t diagram for the Cretaceousgranitoids. Each sample calculated at t = 120 Ma, selected meanmagma emplacement age for the Cretaceous granitoids. Fieldsfor the Northern Dabie complex and UHP metamorphic rocksat t = 120 Ma shown for comparison from data by Chen & Jahn(1998), Ma et al. (2000) and Zheng et al. (2000). NDC – NorthDabie complex. Symbols as in Figure 3.

show identical Sr–Nd isotopic compositions, whichagree with the Northern Dabie Complex and differdistinctly from the UHP metamorphic rocks. Ma et al.(1998) and Zhang et al. (2002) have proposed thatthe Early Cretaceous granitoids in Dabie are productsof anatexis of the Dabie Complex, and experimentsalso prove that dehydration and partial melting oftonalite (P ≤ 1.0 Gpa, T ≤ 950 ◦C) can produce meltswith chemical compositions close to Dabie granitoids(Patino-Douce, 1997). King et al. (1997) pointedout that aluminous A-type granites are generallyproduced by high-temperature partial melting of felsicinfracrustal sources. We therefore favour the derivationof Baiyashan A-type granite magmas by direct partialmelting of middle to lower crustal intermediate-felsicigneous rocks, similar to those now occurring in theNorth Dabie Complex.

6.b. Petrogenesis

Many workers have suggested that A-type granites areproduced by combined partial melting and fractionalcrystallization (King et al. 1997; Wei, Zhao &Spicuzza, 2008). The MMEs and their host granitesshow continuous variation in SiO2 from 59.56 % to74.01 %, producing tight linear or slightly curvedtrends on Harker diagrams (Fig. 6a–d), well explainedby fractional crystallization of magma close to thelow SiO2 end of the range. The felsic end-membercould have a composition close to MC04 and the maficmember close to MC19 (Fig. 6e, f). This variationprovides strong support for a genetic link between theenclaves and host granites, supported by field relationsand petrography of the enclaves. We cannot entirelyrule out alternative explanations for the trends, suchas restite unmixing, crystal fractionation of basalticmagma or two-component magma mixing (Arvin,

Dargahi & Babaei, 2004; Yang et al. 2006), but a restitemodel predicts metamorphic xenoliths of metamorphicrocks rather than magmatic enclaves and is thereforeunlikely (Mason, 1996).

The decreasing trends of MgO, FeOtot, TiO2 andP2O5 may be correlated with the crystallization ofaccessory phrases (Fig. 6a–d). Variations of incom-patible trace elements such as Eu, Sr and Ba, inthe major phases K-feldspar, plagioclase and biotiteare useful for estimating the extent of fractionationof these minerals and whether magmatic evolutionwas controlled dominantly by fractional crystallization.Positive correlations between Ba–Sr and Eu–Sr argueagainst a fractional crystallization model, but a positivecorrelation of CaO/Eu with Eu indicates plagioclasefractional crystallization (Zhang et al. 2006). Hallidayet al. (1991) pointed out that high Rb/Sr ratios area possible consequence of fractionation processes, incontrast to the low Rb/Sr ratios of the Baiyashangranitic samples. These conflicting trends may meanthat magma evolution involved limited fractionalcrystallization dominated by fractional crystallizationof plagioclase. There is no evidence of other complexprocesses having controlled the magmatic evolution.The high temperatures required to produce the Baiy-ashan A-type granite may have been initiated by mantleupwelling or basic magma influx into a localized areaof the crust, but there is no direct evidence for the latterat Dabie.

The petrography, geochemistry and isotopic com-positions of the Baiyashan intrusion indicate that thehost granites and their enclaves are the result of com-positional mixing between mafic and intermediate tofelsic magmas combined with plagioclase-dominatedfractionation. The microdolerite dykes represent a later,unrelated intrusion of basic magma derived from themantle.

6.c. Geodynamic implications

It is generally accepted that A-type granites formed incrustal extensional environments in both post-orogenicand anorogenic settings (Collins et al. 1982; Whalen,Currie & Chappell, 1987; Eby, 1992; Frost et al. 2002).As explained above, the Baiyashan A-type graniteintrusion was emplaced in Early Cretaceous times.Here we review geochemical and age data from EarlyCretaceous A-type granites in SE and Central China inorder to determine their tectonic environment (Table 3).

As shown in Figure 1a, Early Cretaceous A-typegranites are also sparsely distributed in South China,but widely distributed in SE China (Wang et al. 2005a).To the west of SE China, Early Cretaceous aluminousA-type granites are said to occur in the Dabie orogenicbelt (Wang, Zhao & Xiong, 2000), although this paperpresents the first fully documented example. EarlyCretaceous A-type granites have also been identifiedfurther west in the East Qinling orogenic belt (ZhangJ. Y., unpub. data; Fig. 1a) and a crystallization age ofone, the Er Langmiao intrusive body, is 118 Ma. All the

646 L. CHEN AND OTHERS

Figure 6. Representative variation diagrams for Baiyashan rock samples. Harker variation diagrams for (a) MgO, (b) TiO2, (c) FeOtot,(d) P2O5. (e) FeOtot/Al2O3 v. Na2O/CaO; (f) MgO/CaO v. Na2O/CaO. Symbols as in Figure 3.

A-type granites appear to have the same geochemicalfeatures, rich in total alkalis and depleted in Fe2O3

and FeO. In spider diagrams they also show depletionin Ba, Sr, P, Eu and Ti. Table 3 lists representativegeochemical data on these granites. Because of theirsimilarity to granites outside the collision zone betweenthe North China and Yangtze blocks, we reckon that theA-type granites in the Dabie orogenic belt were not adirect product of a post-collision environment. A-typemagmatism of similar age is also widely distributedin NE China, and the resultant granites share thesame age, geological and geochemical characteristics(Wang, Zhao & Qiu, 1995; Wu et al. 2002). Thisstage also coincides with large-scale mineralization(Yang, Wu & Wilde, 2003), magmatic underplating(Liu et al. 2004), extension of terrigenous sedimentarybasins (Li, 2000), and uprise of metamorphic corecomplexes (Ratschbacher et al. 2000) in EasternChina. Wu et al. (2005) have pointed out that thestructural framework of Eastern China was dominatedby extensional deformation in Early Cretaceous times.

Thus all the A-type granites in SE China andthe Dabie orogenic belt appear to be controlled bythe same tectonic regime. What was this regime?Previous studies have proposed that Early Cretaceousmagmatism in Eastern China was related either to theearlier Triassic collision between the Yangtze and theNorth China cratons (Gao et al. 2002) or to subductionof the Palaeo-Pacific Plate towards the Asian continent(Chen et al. 2005; Wang et al. 2005a; Wu et al. 2005;Zhao, Maruyama & Omori, 2007). We have eliminatedthe first alternative and therefore conclude that obliquesubduction of the Palaeo-Pacific Plate may have givenrise to the emplacement of A-type granites in Easternand Central China (Ma et al. 2008). This subductionalso caused NE–NNE extensional shear zones andcontinuous fault activity in eastern Asia (Gilder et al.1996; Li et al. 2001; Li & Li, 2007). Some of theshear zones or faults might have cut through to themantle, releasing the basaltic magma that intrudedmicrodolerite dykes into the Baiyashan granites. Thisconclusion is supported by the suggestion that the

A-type

granitesin

Dabie

orogenicbelt,C

hina647

Table 3. Representative geochemical data and age of the A-type granites in Southeastern and Eastern China

Pluton (abbreviation) Sample no. Rock typeSiO2

(wt%)FeOtot/

(FeOtot+MgO)Nb

(ppm) Ga/Al × 104 Age (Ma) Method Sources

Erlangmiao (ELM) 05DB20-1 Gneissic granite 72.08 0.79 32.4 2.72 Zhang J.Y., unpub. data.05DB21-1 K-feldspar granite 77.28 0.82 36.9 3.93 118 ± 2 LA-ICPMS

Huangshan (HS) DB13-1 Porphyritic mozonitic granite 75.11 0.79 8.35 2.47 132.8 ± 0.8 LA-ICPMS Zhou et al. 2008DB15-1 Mozonitic granite 73.19 0.82 14.2 2.73DB15-2 Mozonitic granite 75 0.86 18.8 2.58

Zushiding (ZSD) DB15-3 Mozonitic granite 76.05 0.82 26 3.33 Zhou et al. 2008DB18-1 Mozonitic granite 73.25 0.81 18.4 2.98 131.9 ± 1.1 LA-ICPMS

Jiaozishan (JZY) DB16-1 Porphyritic mozonitic granite 78.05 0.79 26.6 2.86 120.9 ± 0.8 LA-ICPMS Zhou et al. 2008Yanzigang (YZG) YZ03-5 Mozonitic granite 65.59 0.72 25.1 2.76 134 ± 1 LA-ICPMS Zhou H. S., unpub. Ph.D. thesis, China

Univ. of Geosciences, Wuhan, China,2008

Baiyashan (BYS) MC04 Biotite granite 73.60 0.85 32.6 3.04 120.4 ± 1.2 LA-ICPMS This paperMC07 Porphyritic biotite granite 70.26 0.79 27.6 2.43

Shacun (SHC) BH14 Biotite monzonite 62.73 0.79 32 3.00 119.0 ± 3.2 SHRIMP Xie et al. 2004BH16 Porphyritic mozonitic granite 67.07 0.78 40 3.22

Huashan (HS) 98LZ087-2 Magnesioriebekite granite 125 ± 2 SHRIMP Wang et al. 2005a

Huangmeijian (HMJ) Quartz syenite 125 ± 4 Zircon U-Pb dilution Zheng, Fu & Gong, 1995

Suzhou (SZ) SuG2 Biotite granite 76.23 0.96 103.6 3.60 123–133 Biotite 39Ar–40Ar Chen, Foland & Liu, 1993; Charoy &Raimbault, 1994

Sucun (SC) 99ZJ012 Cavity K-feldspar granite 133.42 ± 0.24 K-feldspar 39Ar–40Ar Wang et al. 2005a

Honggong (HG) 3339 Quartz syenite 66.45 0.96 38 4.29 124.5 ± 0.8 Biotite 39Ar–40Ar Chen, Zhou & Yin, 1991; Lu et al. 20063394 Quartz syenite 63.99 0.93 40 3.06

Xiangshan (XS) 89-36 Dacite porphyry 66.9 0.82 17 134.2 ± 1.9 Zircon U-Pb dilution Fan et al. 2005; Zhang, Liu & Li, 200589-37 Granite porphyry 67.33 0.82 15

Eji’nao (EJN) LY-1 Nepheline sodalite syenite 59.46 0.85 81.6 137 ± 2 SHRIMP Bao, Zhao & Xiong, 2000; Wang et al.2005a

SHRIMP – Sensitive High Resolution Ion MicroProbe.

648 L. CHEN AND OTHERS

eastern part of Shangma fault is a deep ductile shearzone (Wang et al. 2008).

In summary, we speculate that all the Early Creta-ceous A-type granites of SE China are related to riftingof the Asian Plate following subduction of the Palaeo-Pacific Plate beneath Eastern China in Early Cretaceoustimes.

7. Conclusions

(1) Zircon U–Pb isotopic dating of the Baiyashanmedium grained biotite granite yields a crystallizationage of 120.4 ± 1.2 Ma.

(2) Its geochemical characteristics combined withthe presence of alkali-feldspar show that the Baiyashangranite is an aluminous A-type granite.

(3) Trace element and isotopic data show thatthe principal source of magma for the BaiyashanA-type granites is partial melting of middle tolower crustal intermediate-felsic igneous rocks, butcontributions from enriched mantle cannot be ruledout. A combination of magma mixing between maficand intermediate to felsic magmas, partial melting andplagioclase-dominated fractionation seems to be themost viable mechanism to explain the features of theBaiyashan granite.

(4) The close spatial–temporal relationship of A-typegranitoid magmatism with metamorphic core com-plexes like Dabie and extensional terrigenous basinsconfirms that the Early Cretaceous was a period ofextension in Eastern China. Data compiled from EarlyCretaceous A-type granites in SE and Central Chinashow that these granites spread all the way from SouthChina to the Dabie orogenic belt, and even into theeastern Qinling. This magmatism was controlled bysubduction of the Palaeo-Pacific Plate under EasternChina in the Early Cretaceous and the A-type granitesare most likely related to rifting following subduction.

Acknowledgements. This study was financially supportedby the Key International S & T Cooperation Project (no.20071077) of the Ministry of Science and Technology, theNational Nature Science Foundation of China (nos 40521001and 40334037), and the Program for Changjiang Scholars andInnovative Research Teams in Universities (IRT0441). Prof.Yongsheng Liu and Prof. Wenli Ling in GPMR are thankedfor their help in LA-ICPMS zircon U–Pb dating and Sr–Ndisotope analysis, respectively. We are also grateful to Prof.Yusheng Wan in CAGS for his help in zircon CL imaging. Weare indebted to Dr David Pyle and two anonymous reviewersfor their constructive comments and Mrs Jane Holland foreditorial handling.

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