Chemical Geology 439 (2016) 71–82

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Chemical Geology

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Lithium isotope fractionation during incongruent melting: Constraintsfrom post-collisional leucogranite and residual enclaves from BengbuUplift, China

He Sun a, Yongjun Gao b,⁎, Yilin Xiao a,⁎, Hai-ou Gu a, John F. Casey b

a CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, Chinab Department of Earth and Atmospheric Sciences, University of Houston, TX 77204-5007, USA

⁎ Corresponding authors.E-mail addresses: ygao@mail.uh.edu (Y. Gao), ylxiao@

http://dx.doi.org/10.1016/j.chemgeo.2016.06.0040009-2541/Published by Elsevier B.V.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 April 2016Received in revised form 7 June 2016Accepted 7 June 2016Available online 08 June 2016

Lithium (Li) elemental and isotopic compositions of the Jurassic Jingshan leucogranites, including garnet-richmafic enclaves andwall rockWuhe gneisses from the southeast margin of North China Craton (NCC)were inves-tigated to understand the behavior of Li isotopes during post-collisional magmatism. The Jingshan leucograniteshave distinct U-shape REE patterns with Y and REE concentrations significantly lower yet Sr/Y ratios higher thantheir presumed source rocks, i.e., the Dabie-Sulu gneisses. Trace elementmodeling of REE and Sr/Y suggests theseelemental signatures of the Jingshan leucogranites can be consistently explained by a fluid-present crustal incon-gruent partial melting: Bt + Qz + Pl + H2O = Grt + melt, leaving mainly Grt + Bt with minor allanite in theresiduum. Themafic enclaves show identical Sr-Nd isotopic compositionswith their host leucogranites, contrast-ing with the Wuhe gneiss and the exposed regional lower crust. The garnet-rich mafic enclaves are thusinterpreted as entrained residual phases formed by this incongruent partial melting.The Jingshan leucogranites show relatively high δ7Li values (+4.0‰ to +9.0‰) and low Li concentrations (4.7–11.3 ppm) in comparison to published data for worldwide granites. In contrast, the residual enclaves show lowδ7Li values (as low as +0.6‰) and high Li concentrations (as high as 118 ppm). Garnet separated from residualenclaves is characterized by a narrow range of low δ7Li values (−1.5‰ to −0.1‰) with high Li concentrationsfrom 32.9 to 81.7 ppm. By contrast, coexisting quartz shows relatively high δ7Li values (+15.0‰ to +16.6‰)with very low Li concentrations (~1 ppm). Biotite from both leucogranite and residual enclaves shows high Liconcentrations (195–382 ppm) and relatively heavy Li isotope compositions (+3.2‰ to+7.5‰). The Li elemen-tal and isotopic signatures of the residual enclaves can be modeled as a Grt-Bt rich residuum mixed withleucogranite melt in various proportions. This work indicates that the Li isotopic compositions for magmaticrocks that are derived from anatexis of mid to lower crustal gneisses may not be a faithful source indicator ascommonly suggested.

Published by Elsevier B.V.

Keywords:Lithium isotopeLeucograniteNorth China CratonPost-Collisional MagmatismIncongruent melting

1. Introduction

Lithium isotopes are potential tracers of processes in subductionzones, as they are significantly fractionated by low-temperature geo-chemical processes that involve fluid-rock interactions, such as alter-ation of oceanic crust (Chan et al., 1994, 2002) and continentalweathering (Huh et al., 1998, 2001; Pistiner and Henderson, 2003;Rudnick et al., 2004). High-temperaturemagmatic differentiation, how-ever, is thought not to produce significant Li isotope fractionation. Apioneering study of cogenetic Hawaiian lavas by Tomascak et al.(1999) showed that high-temperature (N1000 °C)mineral fractionationand accumulation does not produce significant fractionation of Li

ustc.edu.cn (Y. Xiao).

isotopes. Later studies have further demonstrated no apparent fraction-ation of Li isotopes in basalticmagmas (Chan and Frey, 2003; Jeffcoate etal., 2007), or in more evolved granitic magmas (Bryant et al., 2004a;Teng et al., 2004, 2006b, 2009; Magna et al., 2010) and carbonatitemagmas (500 °C, Halama et al., 2007). However, experimental studieshave observed significant Li isotope fractionation between mineralsand coexisting fluid at magmatic temperatures (500–900 °C, Wunderet al., 2006, 2007, 2009). To date, our knowledge of Li isotope behaviorduring magmatic differentiation, in particular with that related to mid-dle to high temperature conditions (500–900 °C), is still limited and inneed of further exploration.

Triassic continental collision between the Yangtze Craton (YC) andtheNorth China Craton (NCC) formed one of the largest ultra-high pres-sure (UHP) metamorphic terranes worldwide, the Dabie-Sulu orogenicbelt (Li et al., 1993, 2000). Here, we explore the composition of the

72 H. Sun et al. / Chemical Geology 439 (2016) 71–82

Jingshan pluton, which consists mainly of garnet-bearing leucogranite,located on the southeastern margin of the NCC. The Jingshanleucogranites were emplaced at ~162 Ma and contain bothNeoproterozoic and Triassic age inherited zircons (Xu et al., 2005;Yang et al., 2010, Xu et al., 2013, Li et al., 2014). The Triassic inheritedzircon ages are similar to those observed in the Dabie–Sulu UHP meta-morphic rocks (Li et al., 1993, 2000), suggesting their derivation frompartial melting of crustal materials of the YC (Xu et al., 2005; Yang etal., 2010; Xu et al., 2013). Abundant peritectic garnets and garnet-richresidual enclaves are incorporated in the Jingshan leucogranites (Xu etal., 2013). These residual enclaves represent mixtures of restite andleucogranite, providing an ideal case to study the Li isotope fraction-ation during crustal anatexis and magma evolution.

We present here the major and trace element concentrations, Sr-Ndand Li isotope data of leucogranites and mafic enclaves from theJingshan pluton, China. The goals of this study are (1) to provide thefirst information on the Li isotope compositions and concentrations ofthe post-collisional Jingshan leucogranites and garnet-rich residual en-claves, and (2) to investigate origin of these Li isotope signatures inthe leucogranites and the Li isotope fractionation resulting from crustalanatexis.

2. Geological backgrounds and samples

Eastern China is comprised of two major tectonic units: NCC and YC(Fig. 1a). The Bengbu Uplift area is located at the southeasternmargin ofNCC, bounded by the Tan-Lu fault zone on the east and the Dabie oro-genic belt on the south. The Tan-Lu fault was a major sinistral strike-slip fault in northeastern Asia. It has a NNE trend and extends morethan 5000 km into the Bohai Sea at its northernmost end. The dominantrock type of the Bengbu Uplift is the Achaean Wuhe complex, which ismainly composed of supracrustal rocks, such as gneiss, marble, meta-

Fig. 1. (a) Sketchmap of China showingmajor tectonic units and the Bengbu uplift on the southshowing the distribution of Jurassic and Cretaceous granites and the studied Jingshan leucogranfault.

sediments and amphibolite layers. Tectonically, the complex is part ofthe NCC basement (Guo and Li, 2009).

Voluminous Mesozoic granitoids (Fig. 1b) intruded into the WuheComplex in the Late Jurassic (~162 Ma) and in the Early Cretaceousfrom ca. 130 to 112 Ma (Yang et al., 2010). The granitoids represent awide range of lithologies including leucogranite, syenogranite, granodi-orite andmonzogranite (Guo and Li, 2009; Yang et al., 2010). One of theJurassic granitoids, the Jingshan intrusion (N30°51′55″, E118°11ʹ06ʺ), islocated in the western part of the Bengbu area (Fig.1b) and consistsmainly of garnet-bearing leucogranite with medium-coarse granulartexture and a weak gneissosity, which is consistent with a late syn-kinematic evolution. Petrographic characteristics, detailed SHRIMP zir-con U-Pb dating and O isotopic analyses were interpreted to indicatethat the Jingshan leucogranite was derived from partial melting of thesubducted crustal materials of the YC at 162 Ma (Xu et al., 2005; Yanget al., 2010; Wang et al., 2013; Li et al., 2014). Decimeter-sized maficgarnet- and biotite-rich enclaves are preserved in the leucogranite.These enclaves have obscure boundaries and represent mixtures ofleucogranite melt and restite (Xu et al., 2013).

Ten fresh leucogranite samples free of obvious inclusions were col-lected and analysed to represent the main granitic body end-membercomposition. Seven residual mafic enclaves in the leucogranite werecollected and analysed to investigate Li elemental and isotopic composi-tions of presumed restite (+leucogranite) material. Minerals (garnet,biotite and quartz) separated from residual enclaves and leucograniteswere analysed for Li concentrations and Li isotopes. TwoWuhe gneisseswere also collected in adjacent regions of the intrusion and analysed torepresent the surrounding country-rock.

3. Sample petrography

The Jingshan leucogranites consist of sub-mm- to mm-sized plagio-clase, quartz, K-feldspar, biotite and garnet, alongwith accessory phases

A

B

margin of NCC. (b) Geological map of the Bengbu uplift modified after Yang et al. (2010),ite. NCC, North China Craton; CCO, Central China orogen; YC, Yangtze Craton; TLF, Tan-Lu

73H. Sun et al. / Chemical Geology 439 (2016) 71–82

such as zircon, allanite, epidote and magnetite. The main body of theJingshan leucogranite in thin section appears unaltered with no evi-dence of obvious weathering or hydrothermal alteration (Fig.2A). Gar-net in the leucogranites can be divided into two groups accordingto size and texture within each individual samples. Group I garnets(Grt I) are large (~mm in size), usually showanhedral to subhedral crys-tal shapes and are ubiquitous throughout the samples (Fig.2B). Group IIgarnets (Grt II) are smaller (b200 μm) and subhedral to euhedral; theyoften occur as clusters in the leucogranites (Fig.2C). A previous study(Xu et al., 2013) indicates that the large garnets of Group I are peritecticin origin and the small garnets of Group II are formed by dissolution–recrystallization. The total proportion of garnet averages about 1 vol.%in the leucogranite, but the abundance of garnets varies somewhatfrom sample to sample.

The residual mafic enclaves are mainly composed of quartz, plagio-clase, biotite and mm-sized euhedral to subhedral garnet, along withmuscovite andminor amounts of allanite and epidote (Fig.2D). The gar-net in the residual enclaves contains abundant mineral inclusions ofquartz, allanite, epidote and biotite.

Mineral modal abundance (vol.%) for the studied samples was typi-cally obtained from petrographic thin-section observations. When thinsection was not available, however, a rough modal estimation wasmade based on direct observations of rock hand samples. The represen-tative mineral abundances for leucogranites and residual enclaves arelisted in Supplementary Table. 1.

4. Analytical methods

Unweathered leucogranites, mafic enclaves and gneiss sampleswere broken into small pieces, handpicked to avoid fractures, veins orweathering, and then powdered to 200 mesh for bulk rock analysis.Mineral separations were performed under an Olympus SZX-7 stereobinocular microscope. The hand-picked minerals were first washed byMilli-Q water, and then ground to 200 mesh powder using an agatemortar.

Fig. 2. Transmitted light photomicrographs of leucogranites andmafic enclaves. A)main body lgarnet in the Jingshan leucogranite; C) small (b200 μm), euhedral to subhedral garnet inGrt + Bt + Ms. + Pl + Qz + Aln + Ep. Mineral abbreviations: Grt Garnet, Bt Biotite, Ms. Mus

Major element concentrationswere determined by ICP-OES analysisusing a Li-metaborate fusion technique at the University of Houston(UH). Losses of ignition (LOI)were determined by gravimetricmethods.Trace element analyses were accomplished using a Varian 810 quadru-pole ICP-MS at UH. All reagents (HF, HNO3 and HCl) were double dis-tilled and ultrapure 18.2 MΩ.cm water was utilized. High-pressureacid digestion bombs from Parr Instruments Co. were used to achievecomplete digestion. Analytical uncertainty was determined to begenerally better than 5% 1σ, with RSD monitored by repeated analysisof USGS standards during analytical runs.

LA-ICP-MS analyses of garnets were performed at the CAS Key Labo-ratory of Crust-Mantle Materials and Environments at the University ofScience and Technology of China (USTC). Trace elements of Group I andII garnet from representative samples of leucogranite and garnet in rep-resentative enclaves were analysed using an Agilent 7700e ICP-MScoupled with a GeolasPro ArF (193 nm) excimer laser ablation micro-probe system. Helium carrier gas and a laser spot size of 44 μm wereused during the analysis. Energy density of the laser beamwas approx-imately 10 mJ cm−2 on the sample surface, with a repetition rate of10 Hz. All data acquisition occurred in time-resolved analysis mode.NIST 610 synthetic glass was used for external standardization, andUSGS glass standards BHVO-2G, BCR-2G and BIR-1G were used forquantification (He et al., 2015). The routine precision of this methodwas generally better than 5–10% RSD.

Sr-Nd isotopic ratios were measured on a Finnigan MAT-262 massspectrometer at USTC. Chromatography separation and analytical pro-cedures for Sr-Nd isotope analysis followed those of Chen et al.(2007). Analyses on the standards give mean values of 87Sr/86Sr =0.710249 ± 0.000012 (2σ, n = 38) for NBS 987 and 143Nd/144Nd =0.511869 ± 0.000006 (2σ, n = 25) for La Jolla.

Separation of Li for isotopic composition analysis was achieved by anorganic solvent free two-step liquid chromatography procedure in aclean lab at UH and at USTC following the procedure described by Gaoand Casey (2012). All separationsweremonitoredwith ICP-MS analysisto guarantee both high Li yield (N99.8% recovery) and low Na/Li ratio

eucogranite showing freshminerals without alteration texture; B) big (N1mm), subhedralthe Jingshan leucogranite; D) mafic residual enclave showing mineral assemblage ofcovite, Pl Plagioclase, Kfs K-feldspar, Qz Quartz, Aln Allanite, Ep Epidote.

Table 1Representative elemental and Li isotope compositions of the Jingshan leucogranites, mafic residual enclaves, minerals and wall rocks.

Sample SiO2 (wt.%) MgO (wt.%) Sr (ppm) Y (ppm) Li (ppm) δ7Li (‰) n# 2σ/2SD

Leucogranite06-js-03Ba 74.6 0.09 151 3.5 10.7 +9.0 1 0.306-js-04Ba 74.2 0.11 222 3.0 5.1 +4.0 1 0.306-js-05Ba na na na na 10.4 +5.2 1 0.306-js-06 Aa 74.2 0.14 187 6.4 10.7 +7.3 1 0.306-js-07Ba na na na na 7.9 +5.0 1 0.306-js-08Ba 74.0 0.11 168 3.5 11.3 +7.3 1 0.306-js-09Aa 74.8 0.09 186 2.7 4.7 +5.5 1 0.406-js-10Aa 74.9 0.11 167 2.4 8.4 +4.8 1 0.406-js-11Aa 73.5 0.12 197 3.0 5.2 +5.6 1 0.310-js-20b 10.9 +8.0 3 0.206-js-03 Biotiteb 195 +7.3 3 0.210-js-20 Biotiteb 382 +7.4 3 0.1

Residual enclave + leucogranite10-JS-16A 69.8 0.16 345 9.6 19.6 +7.9 1 0.110-JS-16A replicate +7.2 3 0.210-JS-14B 67.2 0.18 310 5.2 17.6 +3.4 1 0.110-JS-5a 65.2 0.33 174 15.6 55.8 +6.4 1 0.310-JS-5 replicateb 54.5 +6.7 3 0.210-JS-14A 53.6 3.5 414 17.0 118 +3.5 1 0.210-JS-22 63.0 1.5 250 41.2 94.0 +0.8 1 0.110-JS-22 replicateb +0.6 3 0.109-JS-1rb 51.9 +2.1 3 0.015-Js-2rb 43.8 +5.9 3 0.115-Js-2r replicateb 41.2 +5.9 3 0.110-JS-5 Biotite 290 +7.3 1 0.110-JS-5 Biotite replicateb 285 +7.5 3 0.210-JS-14 A Biotiteb 230 +3.2 3 0.110-JS-5 Garnet 36.3 −0.4 1 0.110-JS-5 Garnet replicateb 32.9 −0.1 3 0.110-JS-6 Garnet 81.7 −1.5 1 0.210-JS-10 Garnet 66.1 −0.8 1 0.210-JS-5 Quartz 1.0 +15.5 1 0.110-JS-6 Quartz 0.9 +15.0 1 0.110-JS-10 Quartz 0.8 +16.6 1 0.1

Wall rock gneiss10-JS-27 A na na na na 24.0 +0.0 1 0.110-JS-27B na na na na 22.0 −0.6 1 0.1

International rock standard UMD USTC UHBHVO-2 +4.3‰ +4.4 ± 0.3‰ +4.5 ± 0.2‰GSP-2 −0.8 ± 0.3‰AGV-1 +5.9 ± 0.5‰BCR-1 +2.7‰JP-1 +4.0 ± 0.4‰JB-2 +4.0‰

n#: number of measures; 2σ: internal error for n = 1; 2SD: external precision for n N 1.Sampleswithout annotations are analysed at theUniversity of Houston (UH). International rock standard data sources: UMDdata from Rudnick et al. (2004); UHdata fromGao and Casey(2012). USTC data from this study.

a Data analysed at the University of Maryland (UMD).b Data analysed at the University of Science and Technology of China (USTC).

74 H. Sun et al. / Chemical Geology 439 (2016) 71–82

(b0.5). The final concentration of Li in the solutions utilized forMC-ICP-MS analyses was targeted to be about 50–100 ppb to ensure the bestprecision and accuracy. The total procedural blank determined forboth the column procedure alone and the combined sample digestionand column procedure were less than ~0.03 ng of Li. Compared withthe ~200–5000 ng Li used for our analysis, the blank correction is notsignificant at the uncertainty levels achieved. We report resultsas δ7Li = ((7Li/6Li)sample / (7Li/6Li)standard − 1)) × 1000 relative to theL-SVEC Li-isotope standard (Flesch et al.,1973).

The lithium isotopic compositions were analysed at University ofMaryland (UMD), at UH and at USTC. At UMD the lithium isotopic com-positions were analysed on a Nu Plasma MC-ICP-MS following the pro-cedures described by Rudnick et al. (2004) with a precision of ≤1‰. AtUH, the lithium isotopic compositions were analysed on a Nu Plasma IIMC-ICP-MS following the procedure described by Gao and Casey(2012) with a precision better than 0.5‰. At USTC, the lithium isotopic

compositions were analysed on a Neptune PlusMC-ICP-MS on wet plas-mamode using X skimmer cone and Jet sample cone. Samples were in-troduced through a low-flow PFA nebulizer (~50 μL/min) coupled witha quartz spray chamber. The two Li isotopes (7Li and 6Li)weremeasuredsimultaneously in two opposing Faraday cups. Each sample analysiswasbracketed before and after by 100 ppb L-SVEC standard. For a solutionwith 100 ppb Li and solution uptake rate of 50 μL/min, the typical inten-sity of 7Li is about 8 V. The in-run precision on 7Li/6Li measurements is≤0.2‰ for one block of 60 ratios. The external precision, based onlong-term analysis of in-house standards (Li-QCUSTC = +8.8 ± 0.2‰(2SD, n = 161)) is similar to the UH analyses, i.e., ≤0.5‰. For interna-tional rock standards, repeat analysis at USTC yielded +4.4 ± 0.3‰(2SD, n = 8) for BHVO-2, −0.8 ± 0.3‰ (2SD, n = 29) for GSP-2, and+5.9 ± 0.5‰ (2SD, n = 9) for AGV-1, which are within uncertaintyof previously published results (e.g., Rudnick et al., 2004; Gao andCasey, 2012; Lin et al., 2016).

-4

-2

0

2

4

6

8

10

0 100 200 300 400 500 600 700

Teng et al., 2009 Romer et al., 2014 Magna et al., 2010

Teng et al., 2004 Bryant et al., 2004 The Jingshan leucogranite

Teng et al., 2006

-4

-2

0

2

4

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1 10 100 1000

Li (ppm)

δ7L

i (‰

)

δ7 Li (

‰)

Li (ppm)

Fig. 3. Plot of Li concentrations vs. isotopic compositions for the Jingshan leucogranite andpublished worldwide post-Archean A-, I- and S-type granites. For a better view, a smalldiagram with log coordinate was shown in the top-right corner. Compare with thepublished worldwide granite data, the Jingshan leucogranites, which show relativelylow Li concentrations and high δ7Li value, plot on the top-left corner. Worldwide granitedata sources: I- and S-type granite from Southeastern Australia (Teng et al., 2004), I-, S-type granite and 4 leucogranites from New England Batholith (Bryant et al., 2004a); S-type granite from Black Hill, pegmatite is not included (Teng et al., 2006b); A-typegranite from Northeast China and North China Craton (Teng et al., 2009); I-, S-, A-typegranite from Western Carpathians, pegmatite, diorite and tonalitie are not included(Magna et al., 2010); late-orogenic granite from Germany, one granite (Gt-BbB3Burgberg) with anomalously high δ7Li value (14.7‰), which may affect by wall rock-derived fluids, is excluded (Romer et al., 2014). Statistical mean Li concentrationsand isotopic compositions for worldwide granites (n = 137) are [Li] = 66.8 ppm,δ7Li = +1.4‰. Statistical median values are [Li] = 34.5 ppm, δ7Li = +1.2‰.

75H. Sun et al. / Chemical Geology 439 (2016) 71–82

5. Results

Selected major-trace element concentrations and Li isotope compo-sitions of the leucogranites, mafic residual enclaves, minerals and sur-rounding Wuhe gneisses are presented in Table 1. Leucogranitesamples have relatively low concentrations of Li (4.7–11.3 ppm,mean = 8.5 ppm) and high δ7Li values (+4.0‰ to +9.0‰, mean =+6.1‰) compared to published worldwide granite values (n = 137,mean values [Li] = 66.8 ppm, δ7Li = +1.4‰; median values [Li] =34.5 ppm, δ7Li = +1.2‰) (Fig. 3, compilation from Bryant et al.,2004a; Teng et al., 2004, 2006b, 2009; Magna et al., 2010; Romer etal., 2014; granites that experience notably Li diffusion and/or fluid-rock interactions are not included, such as those analysed by Marks etal., 2007). In contrast to the leucogranite, the mafic enclaves have highLi concentrations (19.6–118 ppm) and widely variable δ7Li values(+0.6‰ to +7.9‰). The most mafic enclave samples that are least hy-bridized by the leucogranite and have the lowest SiO2 and highest MgOcontent are samples 10-JS-14A and 10-JS-22. Notably, they have thehighest Li content (118 and 94 ppm) and the lowest δ7Li values(+3.5‰ and +0.6‰), respectively. Two Wuhe gneisses are essentiallyindistinguishable from one another, with Li concentrations (22.0 and24.0 ppm) that are higher than the Jingshan leucogranites by factorsof 2–5. The δ7Li values (−0.3‰ and 0.0‰) are significantly lower thanthe leucogranites.

Garnet from residual enclaves has low δ7Li values (−1.5 to−0.1‰)with high Li concentrations (36.3–81.7 ppm). Biotite from residual en-claves has higher δ7Li values (+3.2 and +7.5‰) and higher Li concen-trations (230 and 285 ppm) than coexisting garnet. Quartz fromresidual enclaves is characterized by high δ7Li values (+15.0 to+16.6‰) and very low Li concentrations (≤1 ppm). Two samples of bi-otite from leucogranite show heavy Li isotope signatures (+7.3 and+7.4‰) with high Li concentrations (195 and 382 ppm).

Trace element concentrations of the garnets from representativesamples of the Jingshan leucogranites and residual enclaves are present-ed in Tables 2. Lithium concentrations in the garnets are generally high

(up to 132 ppm) relatively to the whole rocks and vary widely(Tables 2). All the analysed garnets, including Grt I, Grt II and garnetsfrom mafic enclaves, have similar Li concentration zonations, withhigher Li concentrations in the cores than in the rims (Fig. 4). Thecores of small garnets (Grt II) exhibit much higher Li concentrations(94–132 ppm) than the cores of larger garnets (Grt I) exhibiting Li con-centrations of ~50 ppm, aswell as the cores of garnets from the residualenclaves (22–51 ppm). Garnet rims generally show similar Li concen-trations (~20 ppm). Grt I and garnets from mafic enclaves have similarREE patterns (Fig. 5a). Group II garnets, however, have similar REE pat-terns with Grt I in general, but their negative Eu anomalies are morepronounced and they exhibit higher LREE content than Grt I (Fig. 5a).It is noteworthy that the Y concentrations have zoning patterns similarto Li in garnets and Li and Y are positively correlated within the garnetsanalysed (Fig. 5b).

Sr-Nd isotope compositions for the Jingshan leucogranites, residualenclaves and two Wuhe gneisses are reported in Table 3 and plottedin Fig. 7. Initial isotopic ratios are calculated back to 160 Ma. The gar-net-biotite- rich enclaves have initial Sr (0.70746–0.70829) and Nd(−13.4 to −18.3) isotopic compositions similar to the leucogranites((87Sr/86Sr)i = 0.70791–0.70876, εNd160 = −13.7 to −15.2), but ap-parently different from the Wuhe gneisses ((87Sr/86Sr)i = 0.70708,0.70712, εNd160 = −10.7, −11.4).

6. Discussion

In broader terms of Li concentrations and isotopic compositions, theJingshan leucogranites have low Li concentrations (4.7–11.3 ppm) andconsiderably variable but generally higher δ7Li values (+4.0 to+9.0‰) when compared to worldwide granites, which have medianLi concentrations ~34.5 ppm and δ7Li ~+1.2‰ (Fig. 3, Bryant et al.,2004a; Teng et al., 2004, 2006b, 2009; Magna et al., 2010; Romer etal., 2014). This suggests that the Jingshan leucogranites may have un-usual formation processes. Factors that may control the Li isotopic com-positions of the Jingshan leucogranites are considered in the followingsections.

6.1. Wall rock assimilation and contamination

As shown in the above section, the wall rock Wuhe gneiss has rela-tively higher Li concentrations and much lower δ7Li values comparedto values for leucogranites. Therefore, any assimilation or contamina-tion of such rocks will increase the Li concentrations and lower theδ7Li values in the Jingshan granitic melt. If the Jingshan leucograniteswere notably affected by wall rock contamination, a negative correla-tion of Li concentrations and δ7Li should be observed in the δ7Li-[Li] di-agram of the leucogranites. However, as shown in Fig. 6, there is nosystematic correlation between the δ7Li and Li concentrations for theJingshan leucogranites (Fig. 6b). Apparently, assimilation or contamina-tion of wall rocks could not account for the low-Li concentrations, high-δ7Li signatures of the Jingshan leucogranites.

6.2. Role of diffusion

As Li diffuses rapidly in both silicate minerals and silicate melts(Richter et al., 2003; Coogan et al., 2005; Teng et al., 2006a; Dohmenet al., 2010), and generally 6Li diffuses ~3% faster than 7Li (Richter etal., 2003), diffusion has extensively been considered as a mechanismfor Li isotopic fractionation. A diffusive loss of Li from the granite meltto the restitemay explain the observed low-[Li] and high-δ7Li character-istics of the Jingshan leucogranites, and the high-[Li] and low-δ7Li char-acteristics of the residual enclaves. However, diffusion is unlikely to bethemajor cause of high-δ7Li of the Jingshan leucogranites for three rea-sons. First, the Li concentrations in the leucogranites (~8.3 ppm) aremuch lower than that in the residual enclaves (up to 118 ppm). A re-verse diffusion of Li from the granite melt to the restite against such

Table 2Representative trace element concentrations of the garnets in the Jingshan leucogranite and residual enclaves.

Sample Grt I-1 Grt I-2 Grt II-1 Grt II-2

rim—————– core—————– rim rim—————————————- core—————————————– rim core rim core rim

Li 28.4 47.9 45.5 42.8 25.1 18.4 24.1 44.6 44.6 56.4 34.0 46.7 39.3 23.0 25.5 132 28.2 114 20.1Sr 0.90 1.3 0.34 0.25 0.19 0.14 0.22 0.21 0.34 0.63 0.11 0.46 0.18 0.07 0.02 2.1 3.3 0.65 0.05Y 963 2603 2549 2194 743 431 607 1379 1644 2722 776 2931 2024 659 592 5990 499 5901 410La 0.00 0.00 0.01 0.01 0.02 0.00 0.01 0.00 0.00 0.03 0.00 0.04 0.00 0.00 0.00 0.36 0.07 0.04 0.06Ce 0.08 0.10 0.08 0.06 0.00 0.00 0.01 0.02 0.03 0.06 0.12 0.05 0.11 0.02 0.00 1.7 2.1 0.49 1.2Pr 0.11 0.05 0.11 0.06 0.00 0.01 0.02 0.00 0.01 0.02 0.03 0.12 0.05 0.02 0.03 0.67 1.7 0.40 0.99Nd 1.5 1.9 1.7 2.0 0.08 0.08 0.14 0.23 0.67 0.81 0.38 1.2 1.2 0.21 0.56 9.1 30.5 8.5 18.2Sm 8.4 9.0 9.3 7.9 0.08 0.84 1.1 3.1 2.8 4.3 3.0 4.2 7.6 2.2 1.5 42.6 46.4 37.1 40.7Eu 2.5 2.4 2.9 2.6 0.42 0.42 0.85 0.68 1.2 1.6 1.5 1.4 1.8 1.4 0.71 2.8 2.7 1.9 2.1Gd 58.0 75.4 52.3 54.5 4.9 7.2 9.7 23.7 27.7 38.8 19.0 46.1 49.7 15.1 12.9 208 70.5 203 63.0Tb 25.8 35.1 19.4 21.0 3.3 2.6 4.4 8.6 11.7 19.1 7.1 20.1 17.9 5.6 4.4 71.1 13.2 72.4 11.0Dy 339 482 208 215 58.5 31.1 55.0 102 142 250 68.7 242 198 46.9 51.0 803 78.3 716 59.7Ho 137 198 65.6 61.8 27.4 10.6 18.3 32.4 46.3 102 21.2 86.1 50.4 15.9 11.0 202 13.9 196 9.8Er 623 908 252 197 125 46.6 67.2 130 180 421 72.5 365 156 52.1 32.3 552 39.2 647 23.5Tm 130 196 48.5 33.3 26.3 9.0 13.5 23.2 34.8 82.1 12.2 72.4 30.1 5.9 6.6 103 6.4 110 3.7Yb 1184 1781 410 246 227 75.3 113 221 293 713 104 618 233 44 45.6 909 50.2 869 26.0Lu 215 326 68.6 39.1 40.0 13.7 21.4 38.0 49.9 124 17.7 107 33.9 9.1 7.8 124 7.2 133 3.7

Table 2 (continued)

Sample Grt II-3 Grt II-4 Grt in restite-1 Grt in restite-2 BHVO-2 BIR-1 BCR-2

core rim Core to rim Core to rim Core to rim Ref. value This study He et al. (2015) Ref. value This study He et al. (2015) Ref. value This study He et al. (2015)

Li 128 21.2 94.3 69.3 26.7 51.3 36.1 21.8 22.2 19.4 11.9 4.8 4.3 4.1 3.2 3.4 2.9 9.0 7.6 8.9Sr 2.17 0.00 0.47 0.13 nd 0.30 0.07 0.02 0.05 nd 0.00 396 361 392 109 98.1 106 340 319 352Y 7468 436 7404 2606 605 1934 434 92.5 104 70.5 7.5 26.0 22.8 24.9 15.6 13.6 14.3 37.0 32.6 36.0La 0.43 0.00 0.02 0.01 0.02 0.01 nd 0.00 0.00 0.00 0.00 15.2 14.4 15.0 0.62 0.56 0.66 24.9 23.6 25.6Ce 2.7 3.8 1.3 0.3 0.6 0.02 0.07 nd 0.01 0.00 0.00 37.5 33.8 37.7 1.9 1.5 1.9 52.9 49.4 54.2Pr 0.73 2.9 1.0 0.4 0.6 0.00 0.04 nd 0.00 0.00 nd 5.4 4.6 5.3 0.37 0.33 0.36 6.7 6.9 7.0Nd 9.7 39.8 18.1 8.5 15.3 0.79 0.59 0.00 0.00 0.17 0.00 24.5 23.0 24.6 2.4 2.6 2.3 28.7 23.3 29.8Sm 33.4 77.9 71.5 39.5 52.3 2.5 1.3 0.02 0.67 0.87 0.00 6.1 5.8 6.1 1.1 1.1 1.1 6.6 6.3 6.7Eu 2.0 1.6 2.9 1.7 2.0 1.5 1.2 0.07 0.33 0.66 nd 2.1 1.9 2.1 0.53 0.49 0.50 2.0 1.6 2.0Gd 195 35.1 361 187 132 19.0 9.0 0.89 4.3 4.8 0.16 6.2 5.8 6.3 1.9 1.7 1.7 6.8 6.4 7.1Tb 77.3 10.8 114.8 52.1 23.7 8.0 3.2 0.41 1.4 1.3 0.09 0.92 0.78 0.94 0.36 0.30 0.34 1.1 1.0 1.1Dy 859 83.3 974 363 103 99.8 31.3 7.6 12.7 8.9 1.1 5.3 4.5 5.4 2.5 2.3 2.5 6.4 5.9 6.4Ho 260 2.7 212 61.8 12.8 40.9 9.6 2.6 2.7 1.6 0.28 1.0 0.8 1.0 0.56 0.53 0.54 1.3 1.2 1.3Er 928 7.8 570 136 28.4 208 38.3 11.1 6.2 4.0 0.87 2.5 2.2 2.5 1.7 1.4 1.6 3.7 3.9 3.8Tm 166 0.4 84. 19.3 4.2 47.7 7.0 2.2 0.77 0.52 0.17 0.33 0.29 0.32 0.25 0.20 0.23 0.54 0.52 0.53Yb 1339 nd 611 128 27.2 489 61.9 15.2 5.8 3.5 1.2 2.0 1.9 2.0 1.7 1.4 1.6 3.4 3.4 3.4Lu 219 0.3 85.6 17.5 4.3 101 10.5 2.0 0.88 0.44 0.19 0.27 0.24 0.29 0.25 0.26 0.25 0.50 0.58 0.49

Trace element in ppm; nd: not detected; The reference values for BHVO-2, BIR-1 and BCR-2 were taken from the preferred values in the GeoReM database. Published values of He et al., 2015 for the same standards were also presented in the table.

76H.Sun

etal./ChemicalG

eology439

(2016)71–82

0

10

20

30

40

50

60

1 2 3 4 5 6 7 8 9 100

10

20

30

40

50

60

1 2 3 4 5

0

20

40

60

80

100

120

140

1 2 3

Core Rim

Li (

ppm

)L

i (pp

m)

Grt II

Core RimRimCore RimRim

Grt I-1 Grt I-2

0

10

20

30

40

50

60

1 2 3

Core Rim

Grt from mafic enclaves

A B

DC

Fig. 4. Li concentration zoning of the garnets. (a) and (b) are type I garnets in the Jingshan leucogranite; (c) type II garnets in the leucogranite; (d) garnets frommafic residual enclaves.

77H. Sun et al. / Chemical Geology 439 (2016) 71–82

great concentration gradients seems implausible. Second, if reverse dif-fusion occurred, the Li concentrations in garnets from the restite shouldexhibit U-shaped profiles across grains. However, all the garnet, includ-ing the garnet in the residual enclaves and in the Jingshan leucogranites,exhibits bell-shaped trace element zonation patterns with Li-rich coresand Li-poor rims (Fig. 4). Such an observation strongly argues againstthe possibility of Li reverse diffusion. Third, on a Li vs. Y plot (Fig. 5b),all garnets show a positive linear trendwithout regard to their differentorigins. Garnet in residual enclaves shows similar REE patternswith thatin leucogranites (Grt I and Grt II), with Grt II having higher REE concen-trations (Fig. 5a), which supports the idea that the garnet in the residualenclaves and Grt I in the leucogranites are of peritectic origin, and Grt IIare formed by dissolution-recrystallization suggested by Xu et al.(2013). If diffusive modification of Li concentration has occurred, thelinear correlation between Li and Y would have been destroyed, as thediffusion rate of Li is considered to be much faster than Y (e.g.,Dohmen et al., 2010; Richter et al., 2014). Thus, the Li (and Y)

Sam

ple/

Cho

ndri

te

A

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu0.01

0.1

1

10

100

1000

10000

Grt I in leucogranite

Grt II in leucogranite Grt I in enclaves

Fig. 5. (a) Chondrite normalized rear earth element (REE) patterns of garnets. Pink lines are typare type-II Grt from leucogranites. (b) Li vs. Y diagram of garnets. Normalization values are fro

comparable zonation and the positive correlation of Li and Y in the gar-nets likely stems from magmatic fractionation, rather than fromdiffusion.

6.3. Role of fractional crystallization

Previous study by Qiu et al. (2011) found that leucosomes in themetapelites have significantly lower Li concentrations but higher δ7Livalues than the metapelites themselves. These authors suggest thatsuch a difference may reflect equilibrium Li isotopic fractionation ac-companying crystallization of the leucosomes from a melt having anisotopic composition similar to the metapelites, as quartz preferentiallytakes 7Li into its two- and four-fold-coordinated interstitial sites duringcrystallization (Teng et al., 2006b). The low-[Li], high-δ7Li Jingshanleucogranites could have resulted from fractional crystallization aswell. If so, a positive correlation of δ7Li with SiO2 content for theJingshan leucogranites should be observed. However, the Jingshan

0

1000

2000

3000

4000

5000

6000

7000

8000

0 50 100 150Li (ppm)

Y (

ppm

)

B

e-I Grt from residual enclaves, yellow lines are type-I Grt from leucogranites and blue linesm (Sun and McDonough, 1989).

Table 3Whole-rock Sr-Nd isotopic compositions of leucogranite, residual enclaves and wall rock Wuhe gneiss from the Bengbu area.

Sample Rb(ppm) Sr(ppm) 87Rb/86Sr 87Sr/86Sr (87Sr/86Sr)i Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd (143Nd/144Nd)t εNd(t)

Jingshan leucogranite09-JS-4 141.1 526.3 0.7762 0.71053 0.70876 1.13 5.46 0.1251 0.51186 0.51173 −13.709-JS-6 141.8 582.7 0.7042 0.70997 0.70837 1.00 4.90 0.1234 0.51186 0.51173 −13.710-JS-19 109.8 451.2 0.7043 0.70993 0.70832 0.99 5.69 0.1052 0.51184 0.51173 −13.710-JS-20 112.2 422.3 0.7691 0.71013 0.70838 0.87 3.79 0.1388 0.51187 0.51172 −13.810-JS-7 91.2 486.8 0.5422 0.70914 0.70791 1.12 3.93 0.1723 0.51184 0.51165 −15.2

Residual enclaves10-JS-16 A 102.3 344.9 0.8582 0.71024 0.70829 0.93 3.63 0.1549 0.51177 0.51160 −16.210-JS-22 328.4 249.7 3.8081 0.71612 0.70746 2.81 7.10 0.2394 0.51180 0.51155 −17.210-JS-5 206.8 174.2 3.4371 0.71593 0.70812 1.66 3.78 0.2664 0.51202 0.51174 −13.410-JS-14 A 223.1 414.0 1.5594 0.71134 0.70779 3.42 20.41 0.1013 0.51160 0.51150 −18.310-JS-14B 92.8 309.7 0.8676 0.70999 0.70802 0.69 2.96 0.1414 0.51181 0.51166 −15.115-JS-2r 258.6 376.9 1.9861 0.71263 0.70811 1.27 6.78 0.1132 0.51185 0.51172 −13.7

Wuhe gneiss10-JS-27 A 88.0 655.4 0.3887 0.70801 0.70712 5.07 28.87 0.1063 0.51196 0.51185 −11.410-JS-27B 61.1 972.0 0.1818 0.70749 0.70708 5.00 28.98 0.1043 0.51199 0.51188 −10.7

Initial isotopic ratios are calculated back to 160 Ma.

78 H. Sun et al. / Chemical Geology 439 (2016) 71–82

leucogranites show no trend in the δ7Li-SiO2 plot (Fig. 6a), indicatingthat equilibrium Li isotopic fractionation during accumulation of quartz(+minor feldspar) seems unlikely.

6.4. Role of incongruent melting

As demonstrated by previous studies, Li isotopes are thought not tobe fractionated during partial melting of mafic-ultramafic rocks (to pro-duce basaltic melt) (Tomascak et al., 1999; Chan and Frey, 2003;Jeffcoate et al., 2007) and partial melting (anatexis) of crustal rocks(to produce granitic melt) (e.g., Bryant et al., 2004a; Teng et al., 2009;Qiu et al., 2011; Romer et al., 2014). Hence, the Li isotope signatures ofthese magmatic rocks have been widely used as “probes” to studydeep geochemical processes and their source characteristics. However,although Romer et al. (2014) have argued that the Li and B-isotopiccomposition of partial crustal melts from the Erzgebirge–Vogtland inGermany are similar to their source rocks, they proposed that if the7Li-rich phase (e.g., staurolite) remained stable during partial melting,the melts and the restite may have different Li isotopic compositions.

Recent study of Xu et al. (2013) distinguished three texturally andcompositionally different garnet types in the Jingshan leucogranite.They interpreted the inherited garnets (Grt I) to be of peritectic originin the sources based on their zoning patterns, mineral inclusions, zircondating, major-trace element compositions and oxygen isotope signa-tures. Previous studies of mid-to lower crustal anatectites have sug-gested that peritectic garnet in granite is likely formed by incongruentmelting reactions (Taylor and Stevens, 2010). Additionally, initial Sr-Nd isotopic compositions of the garnet-richmafic enclaves are identicalto the Jingshan leucogranites, which are within the broad range ofDabie-Sulu granitic gneiss and can be clearly distinguished from theWuhe gneiss and exposed regional lower crust (Fig. 7). This supports

2.0

4.0

6.0

8.0

10.0

73.2 73.7 74.2 74.7 75.2

7 Li (

‰)

SiO2 (wt.%)

A

Fig. 6. Plot of δ7Li versus SiO2 and Li concentrations for the Jingshan leucogra

the idea that these garnet-rich enclaves are entrained melting- relatedrestites during magma ascent, rather than xenoliths of wall rocks (Xuet al., 2013). Thus, these observations, together with the residual en-claves in the leucogranites, imply that the parental magma of Jingshanleucogranite should have been generated from an incongruent meltingreaction of Bt + Qz + Pl = Grt +melt (Xu et al., 2013). This incongru-ent melting reaction is in agreement with the mineral compositions ofthe residual enclaves (Grt + Bt + Qz + Pl ± Aln ± Ep ± Ms), andseems to provide a candidate explanation for the generation of thelow-[Li], high-δ7Li signatures in the Jingshan leucogranites.

6.4.1. Melting conditionsIt is well established that, at pressure N 0.5GPa, fluid-absent biotite

dehydration reaction (e.g., Bt + Qz + Pl = Grt + melt) occurs at tem-perature ~760–850 °C (Patiño-Douce and Harris, 1998, Spear et al.,1999; Taylor and Stevens, 2010). Under fluid-present conditions, themelting temperature of the reaction (e.g., Bt + Qz + Pl + H2O =Grt + melt) could be significantly lowered to ~700 °C (Sawyer et al.,2011). The melting temperatures of the Jingshan leucograniteshave been estimated to be 710 ± 18 °C by zirconium saturationgeothermometer and ~700 °C by Ti-in-zircon geothermometer (Xu etal., 2013; Li et al., 2014). The estimated temperatures for the Jingshanleucogranite (700–710 °C) are significantly lower than the fluid-absentmelting temperature, but are similar to the fluid-present melting tem-perature. The melting pressure is hard to quantify. However, accordingto the phengite (Si = 3.58) that is found in the inherited metamorphiczircons from the Jingshan leucogranites, Li et al. (2014) suggests that thesource rocks of the Jingshan leucogranite might have experienced high-pressure metamorphism. The presence of phengite, together with theperitectic garnet that was found in the leucogranite, indicate that themelting pressure should have exceeded 0.5 GPa and in the garnet

2.0

4.0

6.0

8.0

10.0

4.0 6.0 8.0 10.0 12.0

7 Li (

‰)

Li (ppm)

B

nite, note that leucogranite samples show no trend in the two diagrams.

Dabie-Sulu UHP eclogite

(87Sr/86Sr)i

ε Nd(

t)

YC lower crust

NCC lower crust

NCC upper crust

Dabie-Sulu granitic gneiss

0.702 0.704 0.706 0.708 0.710 0.712 0.714 0.716-40

-30

-20

-10

0

10 Leucogranite (Li et al., 2014) Leucogranite (Xu, 2012) Leucogranite (Yang et al., 2010) Leucogranite (this study) Residual enclave (Xu, 2012) Residual encalve (this study) Wall rock gneiss (this study) Tushan amphibolite (Liu et al., 2012)

Fig. 7. (87Sr/86Sr)i vs. εNd(t) (t=160Ma) diagram for the Jingshan leucogranites, residualenclaves, wall rockWuhe gneisses and the Tushan amphibolite. The isotopic compositionof the granites and residual enclaves are shown for the time of granite emplacement, thereference fields and the wall rock gneiss have been calculated for 160 Ma. Previouspublished data for Jingshan leucogranites after Yang et al. (2010), Xu et al. (2013) and Liet al. (2014); for residual enclaves after Xu et al. (2013) and for Tushan amphiboliteafter Liu et al. (2012). The fields for Dabie-Sulu UHP eclogite and Dabie-Sulu graniticgneiss are compile by Xu et al. (2015). The lower and upper crust of NCC and lowercrust of YC are from Jahn et al. (1999).

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

calculated melt melt+1% peritectic garnetaverage Dabie-Sulu gneissaverage Jingshan leucogranite

40% partial melting ofDabie-Sulu gneiss

Sam

ple/

Cho

ndri

te

Fig. 8. Chondrite normalized REE patterns formodeledmelt from partial melting of Dabie-Sulu gneiss. The modeled melt + ~1% peritectic garnet is well matched to the Jingshanleucogranite. Dabie-Sulu gneiss data (grey shaded) are from Li et al. (2000), Bryant et al.(2004b), Zhao et al. (2007) and Tang et al. (2008); Jingshan leucogranite data (brownshaded) are from Yang et al. (2010) and Li et al. (2014); REE concentrations in peritecticgarnets are from Xu et al. (2013) and this study. Chondrite data are from Sun andMcDonough (1989).

79H. Sun et al. / Chemical Geology 439 (2016) 71–82

stability field. Therefore, the estimated melting temperatureand pressure of the Jinghsan leucogranite suggest that their magmamay be produced by a water-present biotite melting reaction, i.e.Bt + Qz + Pl + H2O = Grt + melt.

6.4.2. Trace element modelingThe Jingshan leucogranite enclose sparsemafic garnet-rich enclaves,

which are the residues after variable degrees of magma extraction dur-ing partialmelting (Xu et al., 2013). The residual enclaves were possiblyentrained into the granitic melt during melt accumulation and ascentfrom their source (Stevens et al., 2007; Taylor and Stevens, 2010). Theelemental and O-isotopic compositions of the garnet in the mafic en-claves and the peritectic garnet in the leucogranites can be matched(Xu et al., 2013), suggesting garnet was present during generation ofthe Jingshan granitic magma and left as a residual phase in the source.

Previous studies all suggest that the Jingshan leucogranites were de-rived from partial melting of the deeply subducted Dabie-Sulu gneiss.The consistency between the ages of inherited cores/mantles of zirconsfrom the Bengbu leucogranites and those from the UHPM (Ultra HighPressure Metamorphism) rocks in the Dabie–Sulu orogen present astrong argument that source rock of the Bengbu leucogranites are seg-ments of the subducted YC continental crust beneath the region. How-ever, the Jingshan leucogranites display U-shaped REE patterns withpositive Eu anomalies, which are quite different from the proposedmelting protolith, namely the Dabie-Sulu gneisses (Fig.8, Li et al.,2014). In addition, the Jingshan leucogranites have much lower REEconcentrations than the proposed source rocks, indicating that REEsmay behave as compatible elements during partial melting of theDabie-Sulu gneiss. The observed REE patterns appear opposed to thehy-pothesis that garnet was a main residual phase of the studiedleucogranites, as residual garnet tends to result inHREEs beingmore de-pleted in the melt than those observed. Hence, we conducted REEsmodeling to investigate the formation mechanism of the Jingshanleucogranite compositions.

Average elemental concentrations of the Dabie-Sulu gneisses(Bryant et al., 2004b; Li et al., 2000; Tang et al., 2008; Zhao et al.,2007) are used as starting source composition for the modeling. Resid-ual phases and their relative proportions are inferred from the restitic

garnet-rich enclave (grt + bt + qz + plag ± aln ± ms) found in theJingshan leucogranites. For partition coefficients of mineral/graniticmelt, Dgarnet/melt of Rubatto and Hermann (2007); Dallanite/melt ofMahood and Hildreth (1983); Dplagioclase/melt of Fujimaki et al. (1984)and Dbiotite/melt of Bea et al. (1994) are used for themodeling. Bulk parti-tion coefficients for residual phases composed of 50% Grt + 0.5%Aln + 20% Bt +19.5% Pl + 10% Qz are presented in Table 4. Underthese assumptions, the best-fit for REE abundances in leucograniteswas achieved with 40% partial melting (Fig.8). The LREE concentrationsof the calculated melt agreed well with the Jingshan leucogranites,whereas HREE are much lower than the leucogranites (Fig.8). Giventhe presence of peritectic garnets and later garnets formed by dissolu-tion–recrystallization in the leucogranite (Xu et al., 2013), we considerthat a small fraction of garnet entrained and assimilated into the graniticmelt might be a reasonable explanation for the unexpected elevatedHREE observed in the leucogranites. Calculation results indicate that ap-proximately 1% peritectic garnet addition to the partial melt would fitwell with the average Jingshan leucogranites elevated HREEs and over-all REE patterns (Fig.8). Our modeling and evidence for some entrainedresidual garnet in the leucogranite thus supports the idea that garnetshould be a residual phase during partial melting of the Jingshan sourcerocks.

The Jingshan leucogranites are characterized by high Sr (~460 ppm),low Y (mostly b10 ppm) contents, and thus high Sr/Y ratios. In the Sr/Yvs. Y diagram, theymostly plotted on the adakite area (Fig. 9). However,their source rocks, the Dabie-Sulu gneisses, have high Y (~31 ppm) con-tents and low Sr/Y ratios (Fig. 9). Trace elementmodelingwith the samestarting material and residual phases described for REE modeling is ap-plied to Sr and Y. Modeled melt produced by 40%melting of Dabie-Sulugneiss have Sr ~ 470 ppm and Y ~ 2.5 ppm (Fig. 9 and Table 4), plottedon the upper left corner in Fig. 9. This melt, if mixed with various pro-portions (up to ~1–2%) of peritectic garnet (high Y, low Sr), can repro-duce the Sr\\Y relationships observed in the Jingshan leucogranites.Moreover, the calculated residue, with high Y concentration and verylow Sr/Y ratio, plots in the bottom right corner in Fig. 9. Note that theresidual enclaves all plotted on the mixing line between residue andleucogranite melts, supporting the idea that the residual enclaves aremixtures of restite and granitic melt.

Table 4Modeling parameters, phase proportions of restite (wt.%) and results.

Restite Grt Aln Pl Bt Qz DiBulka Dabie-Sulub

gneissJingshanc

leucogranitePeritecticd

garnetMelt Melt + 1%Grt

P(%) 50 0.5 19.5 20 10 F = 0.4 F = 0.4

La 0.07 2362.0 0.39 0.06 – 11.9 40.6 5.60 – 5.37 5.32Ce 0.31 2063.0 0.25 0.05 – 10.5 77.8 11.9 – 11.6 11.5Pr 1.23 1731.5e 0.22e 0.07 – 9.33 9.54 1.21 – 1.59 1.58Nd 4.50 1400.0 0.19 0.08 – 9.30 36.2 4.48 1.34 6.05 6.00Sm 15.00 756.0 0.14 0.06 – 11.3 6.66 0.94 3.51 0.93 0.95Eu 9.80 100.0 1.11 0.05 – 5.63 1.73 0.46 1.43 0.46 0.47Gd 29.00 167.5e 0.12 0.10 – 15.4 5.71 0.97 15.6 0.59 0.73Tb 32.5e 235.0 0.11e 0.18 – 17.5 0.91 0.17 4.02 0.08 0.12Dy 36.00 123.0 0.11 0.17 – 18.7 5.30 1.06 36.5 0.46 0.78Ho 38.0e 80.0e 0.12e 0.16 – 19.5 1.11 0.25 10.7 0.09 0.19Er 40.00 50.0e 0.12 0.22 – 20.3 2.99 0.78 40.9 0.24 0.60Yb 43.00 24.5 0.13 0.12 – 21.7 3.31 1.06 71.2 0.25 0.89Lu 40.00 22.0 0.14 0.20 – 20.2 0.51 0.19 12.5 0.04 0.15Sr 0.02 0.8 4.40 0.31 – 0.93 456 467 – 475 –Y 39.00 95.0 0.51 0.60 – 20.2 30.9 7.82 – 2.47 –

Qz = Quartz; Pl = Plagioclase; Grt = Garnet; Bt = Biotite; Aln = Allanite.a Bulk partition coefficients are calculated from a residual phase composed of grt+ aln+bt+pl+qtz using DREE

grt/melt of Rubatto and Hermann (2007); DREEaln/melt ofMahood andHildreth

(1983); DREEpl/melt of Fujimaki et al. (1984), DREE

bt/melt of Bea et al. (1994), DSrgrt/melt of Sisson and Bacon (1992), DSr

aln/melt, DSrbt/melt, DY

aln/melt, DYbt/melt and DY

pl/melt of Ewart and Griffin (1994), DSrpl/melt

of Bacon and Druitt (1988) and DYgrt/melt of Rubatto and Hermann (2007).

b Dabie-Sulu gneiss data (grey shaded) are from Li et al. (2000), Bryant et al. (2004), Zhao et al. (2007) and Tang et al. (2008).c Jingshan leucogrenite data (brown shaded) are from Yang et al. (2010) and Li et al. (2014).d REE concentrations in peritectic garnets are from Xu et al. (2013).e REE partition coefficients of mineral/granitic melt that cannot obtained from literatures are calculated by interpolation method.

80 H. Sun et al. / Chemical Geology 439 (2016) 71–82

6.4.3. Li isotopic fractionation during incongruent meltingCompared to the leucogranites, some residual enclaves have unusu-

ally high Li concentration (up to 118 ppm) and relatively light isotopiccomposition, with δ7Li values as low as +0.6‰. Fig. 10 compares theδ7Li values and Li concentrations of the Jingshan leucogranites togetherwith residual enclaves and minerals separated from residual enclavesand leucogranites. Because these mafic enclaves represent mixtures ofrestite and leucogranite, mass balance calculation indicates that therestite end-members should mainly be controlled by garnet and biotite,

Melt+peritectic garnet (~1%)

Start

10%

20%

30%

40%

50%

Jingshan leucograniteResidual enclavesDabie-Sulu gneiss

20% 40%

Residue (calculated)

Partial melting

Adakite field

0

50

100

150

200

250

300

0 10 20 30 40 50

Sr/Y

Y (ppm)

ADR field

Fig. 9. Sr/Y versus Y diagram for the Jingshan leucogranite, residual enclaves, Dabie-Sulu gneiss and modeled melts produced by 10–50 wt. % melting of Dabie-Sulugneiss. Blue line with percentage marks (10%–50%) represent melt from partialmelting of Dabie-Sulu gneiss. The modeled melt (bold red diamond), mixing withvarious proportions (~1%) of peritectic garnet (red-blue gradient arrow) overlapsthe Jingshan leucogranite. Bulk partition coefficients for Sr and Y are calculatedfrom a residual phase composed of grt + aln + bt + pl + qz using DSr

grt/melt ofSisson and Bacon (1992), DSr

aln/melt, DSrbt/melt, DY

aln/melt, DYbt/melt and DY

pl/melt of Ewartand Griffin (1994), DSr

pl/melt of Bacon and Druitt (1988) and DYgrt/melt of Rubatto

and Hermann, (2007). Fields of adakites (yellow region) and classical islandandesite–dacite–rhyolite (ADR) rocks (grey region) are from Defant andDrummond (1990). (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

which have low δ7Li values and high Li concentrations (Fig. 10). Increas-ing hybridization of leucogranitic melt in the enclave will generally in-crease δ7Li values and lower Li content reproducing the enclave trendin Fig. 10. Thus, the Li characteristics of the enclaves and interpretationof the enclaves as residuumof partialmeltingwould be a reasonable ex-planation for the low Li concentrations and relatively high δ7Li of theJingshan leucogranites compared to the worldwide granites (Bryant etal., 2004a; Teng et al., 2004, 2006b, 2009; Magna et al., 2010; Romeret al., 2014).

It is well established that, in addition to temperature, the differencesin coordination among coexisting phases are the first-order effects thatcontrol equilibrium isotopic fractionation (Oi et al. 1989). For Li iso-topes, the lighter isotope preferentially occupies the more highly coor-dinated site (Sartbaeva et al., 2004; Teng et al., 2006b; Wunder et al.,2006). For the phases involved in the incongruent melting reaction(Bt + Qz+ Pl + H2O= Grt +melt), Li enters the six-fold coordinatedsites in biotite (Sartbaeva et al., 2004; Teng et al., 2006b) and in feld-spars (Giletti and Shanahan, 1997), and it occupies two- and four-foldcoordinated interstitial sites in quartz (Sartbaeva et al., 2004; Teng etal., 2006b). The coordinate number of Li in water is generally between3–6 and it may increase with the increasing of fluid density (Jahn andWunder, 2009). For the reaction product, Li may substitute for Mg orundergo coupled substitution with Y (or REE) ions at the 8-fold coordi-nated site in garnet (Halama et al., 2011; Cahalan et al., 2014), and it oc-cupies the 4-fold coordinated site in silicate melt (Soltay andHenderson, 2005). The δ7Li values of minerals from residual enclavesshow that garnet (−1.5 to−0.1‰) do exhibit much lighter Li isotopiccompositions than coexisting biotite (+3.2 to +7.5‰) and quartz(+15.0 to+16.6‰). Thus, during the incongruentmelting that formedthe Jingshan leucogranite, garnets would preferentially take 6Li into its8-fold-coordinated sites and leave isotopically heavy Li in the meltphase. In addition, the breakdown of biotite (usually enriched in Li)in the source gneiss would release Li that could be incorporated by thegarnet, resulting in high concentrations of Li in garnet. This is consistentwith the observed unusually high Li concentrations in garnetsand residual enclaves. Therefore, we conclude that the Jingshanleucogranite originated from an incongruent melting reaction, i.e.Bt + Qz + Pl + H2O = Grt + melt, of Dabie/Sulu gneisses of thelower collided YC plate leaving a high Li concentration, but a light Li iso-topic garnet-rich residuum in the source.

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0 50 100 150 200 250 300 350 400

GraniteResidual enclave+granitegarnetquartzbiotite

90%Bt

Bt

Grt

Qz

50%Bt

80%Bt

10%

30%

30%

Li (ppm)

δ7L

i (‰

)

60%

70%

Fig. 10. Plot of δ7Li versus Li concentrations for the Jingshan leucogranite, residualenclaves, and minerals that separated from residual enclaves and leucogranites. TheJingshan leucogranites have low Li concentrations (4.7 ppm–11.3 ppm) with high δ7Livalues (+4.0‰ to +9.0‰), while the residual enclaves have high Li concentrations(17.6 ppm–118 ppm) with low δ7Li values (+0.6‰ to +7.9‰), imply that the restite ofthe Jingshan leucogranite should have Li concentrations and relatively low δ7Li values.The residual enclaves can be modeled by a Grt-Bt rich restite (along grey dashed line)mixed with leucogranite melt at various proportions (green dashed lines, percentagenumbers represent the proportions of granitic melt). The high [Li], low δ7Li garnet-richresiduum was left in the source, offers a reasonable explanation for the high Li isotopesignature in the Jingshan leucogranite. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

81H. Sun et al. / Chemical Geology 439 (2016) 71–82

6.5. Implications for crustal anatexis

Crustal anatexis can give rise to granitic melts and is a fundamentalprocess in the geochemical differentiation of the continental crust.In the crust, the dominant mechanism of anatectic melting is the incon-gruent dehydration melting reaction of hydrous silicates; amongwhich, the muscovite dehydration reaction (e.g., Qz + Ms. + Pl =Als + Kfs+melt ± Grt ± Tur) at temperature ~710–790 °C and the bi-otite dehydration reaction (e.g., Qz+ Bt+ Pl=Grt+melt) at temper-ature ~760–850 °C are the most commonly inferred incongruentmelting reactions during anatectic melting (Patiño Douce and Harris,1998, Spear et al., 1999; Taylor and Stevens, 2010). These incongruentmelting reactions usually leave a less hydrous restitic assemblage as re-siduum in the source. For the biotite dehydration reaction, cordierite isthe stable peritectic phase at lower pressures (b0.5 GPa), whereas atpressures above 0.5 GPa, peritectic garnet is stable (Searle, 2013).

To date, it is generally accepted that, when contamination or assim-ilation of foreign material is insignificant, the stable isotope ratios ofmelts are valid tracers that reflect the isotopic compositions of thesource rocks. Indeed, with respect to Li isotopes, isotopic fractionationduring magmatic process is insignificant for the basaltic magmas, forinstance fresh MORB exhibits uniform Li signatures that generallyreflect its mantle sources (e.g., Tomascak et al., 2008). However, the ob-servation that residual enclaves and peritectic garnet have contrasting[Li] and δ7Li compositions relative to the host Jingshan leucogranites in-dicates that significant Li isotope fractionation may have occurred dur-ing incongruent melting. Thus, for crustal anatexis, we interpret thatthe crustal sources together with the peritectic phases are governingthe Li isotopic composition of anatectic melt. Melting will typicallybegin with preferential breakdown of hydrous phases (e.g., Muscovite,Biotite), producing an initial melt that coexists with the peritecticphases (e.g., Grt, Crd). As Li in garnet (in 8-fold coordination) is differ-ently coordinated than in felsicmelt (4-fold coordination), theperitecticgarnet will preferentially take up 6Li from the coexisting melt, leaving7Li relatively enriched in the melt. We interpret our results to indicatethat incongruent dehydration melting at relatively high pressures (i.e.,in garnet stability field) has the potential to produce high δ7Li anatecticmelts by leaving an isotopically light garnet-rich residuum in their

source. Thus, care must be taken when using Li isotopic signatures incrustal magmatic rocks to infer the composition of their source, espe-cially for leucogranites that originate from incongruent anatectic melt-ing of a source with residual garnet.

7. Conclusion

The Jingshan leucogranites are characterized by relatively low Liconcentrations and high δ7Li isotopic compositions and can be distin-guished from the average worldwide granite. Garnet-rich mafic en-claves represent mixtures of the restite and leucogranite. They havehigh Li concentrations and low δ7Li values indicating that the residualphase of the Jingshan leucogranite should have [Li] N 100 ppm andδ7Li b +0.8‰. Trace elements (REE and Sr/Y) and Li isotope modelingcalculations indicate that the Li isotopic and elemental signaturesof Jingshan leucogranites can be explained as a result of deepercrustal anatexis of a protolith similar to the Dabie-Sulu gneisses,possibly via the fluid-present incongruent melting reaction:Bt + Qz + Pl + H2O = Grt + melt. This would leave an isotopicallylight, but Li-rich garnet in the residuum. The observed Li isotopic frac-tionation between restite and granitic melt in this study implies thatLi isotopic composition of the anatectic melt may be different fromtheir source.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2016.06.004.

Acknowledgment

Funding for this work was provided by the National Natural ScienceFoundation of China (NSFC 41273037), the National Basic Research Pro-gram of China (2015CB856102) and the Fundamental Research Pro-grams for the Central Universities (WK3410000003). Profs. ShuguangLi, Fangzhen Teng and Dr. Lijuan Xu are thanked for assistance duringfield work. We thank Huimin Yu, Hongqiong Wan and Ping Xiao forhelps during Li and Sr-Nd isotopic analysis. Profs. Tomas Magna, PaulTomascak and Roberta Rudnick are sincerely thanked for providingcareful and constructive reviews that improved the manuscript. Weacknowledge Prof. Klaus Mezger for his exceptional editing work.

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