A hybrid origin for the Qianshan A-type granite, northeast China: Geochemical and Sr–Nd–Hf isotopic evidence

A hybrid origin for the Qianshan A-type granite, northeast China:

Geochemical and Sr–Nd–Hf isotopic evidence

Jin-Hui Yang

*, Fu-Yuan Wu

, Sun-Lin Chung

, Simon A. Wilde

, Mei-Fei Chu

a

State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

b

Department of Geosciences, National Taiwan University, Taipei 106, Taiwan

c

Department of Applied Geology, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia 6845, Australia Received 2 March 2005; accepted 31 October 2005

Available online 23 January 2006

Abstract

Major and trace element, whole rock Sr and Nd isotope and zircon Hf isotope data are reported for a suite of A-type granites and mafic microgranular enclaves from the Early Cretaceous (126 F 2 Ma) Qianshan pluton, Liaodong Peninsula, northeast China, with the aim of investigating the sources and petrogenesis of A-type granites. The Qianshan pluton includes hornblende alkali-feldspar granite, graphic biotite granite and mafic microgranular enclaves. The hornblende alkali-feldspar granites have high SiO2, Fe2O3T/

MgO, K2O + Na2O, Rb, Zr and LREE contents and low Ba and Sr concentrations with strongly negative Eu anomalies. Their high

Rb / Sr (87Rb /86Sr = 16.76–24.15) and initial87Sr /86Sr ratios (0.7215 to 0.7283), negative eNd(t) values ( 14.1 to 16.5) and

zircon eHf(t) values ( 18.9 to 11.5) indicate they were mainly derived from a crustal source, but with involvement of high eNd(t)

and eHf(t) materials. Graphic biotite granites have similar geochemical features and Sr–Nd–Hf isotopic compositions to enclaves,

indicating they were the result of crystal fractionation of evolved mafic magmas, but with involvement of low eNd(t) and eHf(t)

materials. The mafic enclaves have an igneous texture and contain acicular apatite, suggesting quenching of mafic magmas that have co-mingled with the host granites. They have low initial87Sr /86Sr ratios (0.7097–0.7148), negative eNd(t) ( 14.5 to 11.9)

and zircon eHf(t) ( 17.1 to 6.9) values, and are enriched in LILEs and LREEs and depleted in HFSEs. When coupled with the

high MgO (Mg# up to 54), this indicates derivation from an enriched lithospheric mantle source, but contaminated by crustal materials.

Geochemical and Sr-, Nd- and zircon Hf-isotopic compositions rule out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mantle-derived mafic and crustal-derived magmas, coupled with crystal fractionation, is compatible with the data. This example shows that at least some A-type granites formed through a complex process involving mantle- and crustal-derived magma mixing, crystal fractionation and infracrustal melting.

D 2005 Elsevier B.V. All rights reserved.

Keywords: A-type granite; Magma mixing; Sr–Nd isotopes; Mafic microgranular enclaves; Zircon Hf isotopes

1. Introduction

bA-typeQ granites commonly occur in post-orogenic or intraplate tectonic settings (Eby, 1990, 1992; Black and Liegeois, 1993) and provide significant information

0024-4937/$ - see front matterD 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2005.10.002

* Corresponding author. Tel.: +86 10 62007900; fax: +86 10 62010846.

E-mail address: jinhui@mail.igcas.ac.cn (J.-H. Yang).

on post-collisional/intraplate extensional magmatic pro-cesses within the continental lithosphere and their con-tribution to the build-up of the upper continental crust (Turner et al., 1992; Mushkin et al., 2003). However, several petrogenetic schemes have been proposed for their origin: (1) direct fractionation products of mantle-derived alkaline basalts (e.g.,Turner et al., 1992; Litvi-novsky et al., 2002; Mushkin et al., 2003); (2) low degrees of partial melting of F- and/or Cl-enriched dry, lower crustal granulitic residue from which a gran-itoid melt was previously extracted (e.g.,Collins et al., 1982; Clemens et al., 1986; King et al., 1997); (3) low-pressure melting of calc-alkaline rocks at upper crustal levels (Skjerlie and Johnston, 1993; Patin˜o Douce, 1997); and (4) hybridization between anatectic granitic and mantle-derived mafic magmas (Be´dard, 1990; Kerr and Fryer, 1993; Wickham et al., 1996; Mingram et al., 2000). An important and fundamental question is whether such granitoids are exclusively crustally de-rived or whether they require significant involvement of mantle materials; or even whether they can be generat-ed directly by fractionation of basalt.

Mafic microgranular enclaves (MMEs ofDidier and Barbarin, 1991) occur in nearly all granites and have been the focus of many studies (see reviews byVernon, 1984; Didier and Barbarin, 1991; Bonin, 2004). Al-though MMEs may have originated in many different ways and from different sources, there has been a general consensus that textural criteria allow recogni-tion of a class of enclaves that formed by mingling of an externally derived magma with granitic magma while it was still mobile (Vernon, 1984; Vernon et al., 1988); these can appropriately be termed magmatic enclaves (Wiebe et al., 1997). In spite of partial reequi-libration, enclaves commonly have isotopic composi-tions that are distinct from the enclosing granite and support their formation from mantle-derived magmas (Holden et al., 1987; Yang et al., 2004b). However, MMEs commonly occur in I- and S-type granites and are rare in A-type granites (Didier and Barbarin, 1991), with only a few cases reported byBe´dard (1990) and

Wiebe et al. (1997). We have identified that MMEs in the Early Cretaceous A-type complex of the Liaodong Peninsula, northeastern China possess distinct isotopic characteristics that may be used to argue for magma mixing in the origin of certain A-type granites. In this paper we: (1) present new geochemical, Sr and Nd and in situ zircon Hf isotopic data for MMEs and their host A-type granites from the Qianshan pluton, (2) use these tracers to constrain the sources and petrogenesis of the A-type granites, and (3) discuss the role of magma mixing in the overall origin of A-type granites.

2. Geological setting of the Qianshan pluton 2.1. Regional geological setting

East China is composed of the Xing’an–Mongolia (Xing–Meng) Orogenic belt in the north, the North China Craton (NCC) in the centre and the Dabie–Sulu ultrahigh-pressure orogenic belt in the south and east (Fig. 1a,Wang and Mo, 1996).

The Liaodong Peninsula, located in the eastern seg-ment of the NCC (Fig. 1a), consists of Archean to Paleoproterozoic basement rocks overlain by unmeta-morphosed Mesoproterozoic to Paleozoic sediments and Mesozoic to Cenozoic sedimentary and volcanic rocks (Zhao et al., 2001). Early Archean basement rocks, which range in age from 3.85 to 3.2 Ga, have been reported near Anshan, north of the Liaodong Peninsula (Fig. 1,Liu et al., 1992; Song et al., 1996). In the Liaodong Peninsula, deformed late Archean gneisses include diorite, tonalite and granodiorite that were emplaced at about 2500 Ma (Lu et al., 2004). In the Paleoproterozoic, the Liaohe Group was deposited and then metamorphosed during the ~1.9 Ga collisional event in the Liaodong Peninsula (Zhao et al., 2001). Subsequently, the Liaodong Peninsula was covered by a thick sequence of Meso- to Neoproterozoic and Pa-leozoic sediments (Wang and Mo, 1996). Paleozoic diamond-bearing kimberlites (LBGMR, 1986) and Ce-nozoic gabbros and basalts have also been found here. In the Liaodong Peninsula, about 20,000 km2 of intrusive rocks have been identified along with minor volcanic rocks (LBGMR, 1986) (Fig. 1b). According to our recent work, these intrusions can be divided into three groups: (1) Late Triassic (238–212 Ma) syenites, monzogranites and gabbros (Yang et al., 2004a), (2) Jurassic (180–153 Ma) tonalite, diorite, and two-mica granites that have experienced ductile deformation (Wu et al., 2005b), and (3) Early Cretaceous (130–106 Ma) diorite, granodiorite, monzogranite and A-type granite (Yang et al., 2004b; Wu et al., 2005a). Coeval with the formation of Early Cretaceous I- and A-type granitoids was the exhumation of metamorphic core complexes, indicating that these granitoids were emplaced in an intra-plate extensional environment of regional extent (Wu et al., 2005a).

2.2. Geology and petrography of the Qianshan pluton The Qianshan intrusion (Fig. 1c) is one of several A-type plutons located in the Liaodong Peninsula. It extends E–W and was intruded into Archean supracrustal rocks and granites, the Paleoproterozoic Liaohe Group

and Neoproterozoic sandstones. LA-ICP-MS zircon U– Pb dating indicates that the emplacement age of the Qianshan granitoids is 126 F 2 Ma (Wu et al., 2005a). The pluton consists of two main phases that are texturally distinct. The hornblende alkali-feldspar gran-ite (hornblende grangran-ite) is orange to red-brown and coarse-grained, consisting of 60% subhedral alkali feld-spar, 5% plagioclase, 30% anhedral quartz, 1–5% horn-blende and b1% accessory minerals, including zircon and allanite. The graphic biotite granite (biotite granite) is orange to white and medium- to coarse-grained, consisting of 60% subhedral alkali feldspar, 0–5% pla-gioclase, 32% anhedral quartz, 1–5% biotite and b1% accessory minerals, including zircon and apatite. This

granite contains either individual xenocrysts (mantled plagioclases) or small, fine-grained aggregates (e.g., quartz/K-feldspar oikocrysts enclosing numerous small crystals of hornblende and plagioclase).

All granites contain ubiquitous mafic microgranular enclaves. The enclaves appear circular or lenticular in section, with sharp contacts, locally featuring chilled margins or thin biotite-rich reaction rims (Fig. 2). In-teraction (or hybridization) between the mafic enclaves and host felsic magma is evident from the presence of gradational contacts, suggesting that magma mixing/ mingling occurred. Disaggregation of mafic enclaves yields heterogeneous hybrid rocks containing wispy schlieren and clots of fine-grained mafic material that

Fig. 1. (a) Simplified geological map of eastern China, showing major tectonic units; (b) geological map showing distribution of Mesozoic intrusions in the Liaodong Peninsula; and (c) geological map of the Qianshan pluton in the Liaodong Peninsula.

are heterogeneously distributed throughout the granite. The rocks range from dioritic to monzogranite in com-position and are composed of 20–30% hornblende, 30– 40% alkali feldspar, 20–30% plagioclase, 5% biotite and b1% acicular apatite. Plagioclase occurs both as small euhedral laths and as larger mantled crystals, commonly enclosing small rounded hornblende grains. Hornblende is typically subhedral and forms monomi-neralic clots with lobate outlines or small anhedral crystals, interstitial to plagioclase. Both K-feldspar and quartz are interstitial and poikilitic. Some K-feld-spar occurs as megacrysts (Fig. 2b). Apatite is a com-mon accessory and tends to form long prismatic, acicular crystals.

3. Analytical methods 3.1. Major and trace elements

After petrographic examination, a total of 29 fresh rock samples were selected, crushed, and powdered in

an agate mill. Major elements were determined by X-ray fluorescence techniques on fused glass beads using RigakuR RIX-2000 spectrometers at the National Tai-wan University. The analytical procedures were the same as those described by Lee et al. (1997) and

Wang et al. (2004), yielding analytical uncertainties better than F 5% (2r) for all major elements. The fused glass beads were powdered and dissolved using an HF/HNO3(1 : 1) mixture in screw-top Teflon beakers

for 12 h at ~100 8C, followed by evaporation to dry-ness, then refluxing in 7 N HNO3for N12 h at ~100 8C

and, finally, diluting the sample solution with 2% HNO3. An internal standard solution of 5 ppb Rh and

Bi was added and the spiked solution was diluted with 2% HNO3to a sample / solution weight ratio of 1 : 2000.

The internal standard solution was used for monitoring the signal shift during inductively coupled plasma-mass spectrometry (ICP-MS) measurements using an Agilent 7500s spectrometer at the National Taiwan University, which shows a good stability range with ~10% varia-tion. The precision was generally better than F 5% (2r) for most trace elements, as shown by the statistics on duplicate analyses of five rock standards 1, AGV-2, GSP-1, JB-1 and JG-1 (Yang et al., 2005).

3.2. Sr and Nd isotopes

Samples for isotopic analysis were dissolved in Tef-lon bombs after being spiked with 84Sr, 87Rb, 150Nd and 147Sm tracers prior to HF + HNO3(with a ratio of

2 : 1) dissolution. Rubidium, Sr, Sm and Nd were sep-arated using conventional ion exchange procedures and measured using a Finnigan MAT 262 multi-collector mass spectrometer at the Institute of Geology and Geo-physics, Chinese Academy of Sciences, China (Yang et al., 2004b). Procedural blanks were b100 pg for Sm and Nd and b500 pg for Rb and Sr.143Nd /144Nd were corrected for mass fractionation by normalization to

146

Nd /144Nd = 0.7219, and 87Sr /86Sr ratios were nor-malized to 86Sr /88Sr = 0.1194. Typical within-run pre-cision (2r) for Sr and Nd was estimated to be F_{0.000015. The measured values for the La Jolla} and BCR-1 Nd standards and the NBS-607 Sr standard were143Nd /144Nd = 0.511853 F 7 (2rn, n = 3)

and 0.512604 F 7 (2rn, n = 3), respectively, and 87Sr / 86

Sr = 1.20042 F 2 (2rn, n = 12) during the period of

data acquisition. 3.3. Zircon Hf isotopes

Cathodoluminescence (CL) images were obtained using a CAMECA SX50 microprobe at the Institute

Fig. 2. Examples (a and b) of mafic microgranular enclaves (MMEs) occurring within A-type granites in the Qianshan pluton. The diameter of the 1 yuan coin is 25 mm.

Table 1

Whole rock compositions of the Qianshan pluton granites and mafic microgranular enclaves

Sample no. JH-07 JH-08 JH-09 JH-12 JH-15-1 JH-17-1 FW01-162 JH-26 JH-27 !JH-28 JH-29

Rock type Biotite granites Hornblende granites

Major elements (wt.%) SiO2 71.11 71.11 70.84 71.33 71.16 70.49 71.47 70.28 69.67 68.90 69.10 TiO2 0.25 0.25 0.23 0.27 0.27 0.25 0.23 0.32 0.18 0.18 0.18 Al2O3 14.90 14.85 15.34 14.65 15.09 15.05 14.51 14.41 15.32 15.74 15.68 Fe2O3 2.30 2.29 1.99 2.59 2.34 2.21 2.40 2.89 1.98 2.01 2.01 MnO 0.06 0.06 0.05 0.06 0.06 0.06 0.06 0.07 0.05 0.05 0.05 MgO 0.12 0.12 0.09 0.15 0.13 0.13 0.21 0.17 0.01 0.01 0.01 CaO 0.99 0.97 0.93 1.04 1.07 1.03 1.17 1.08 0.69 0.72 0.72 Na2O 4.30 4.12 4.41 4.23 4.24 4.22 4.57 4.14 4.53 4.64 4.67 K2O 5.15 5.39 5.46 5.14 5.50 5.50 5.28 5.04 6.13 6.44 6.33 P2O5 0.05 0.05 0.04 0.05 0.06 0.05 0.05 0.06 0.02 0.02 0.01 Total 99.24 99.20 99.38 99.52 99.92 98.99 99.95 98.45 98.56 98.71 98.75 Trace elements (ppm) Cr 4.79 3.69 6.17 5.38 3.83 4.95 2.43 3.57 3.33 4.49 2.19 Ni 6.62 3.25 4.02 3.15 2.73 3.43 1.82 2.48 4.22 4.83 1.52 Ga 26.8 26.4 27.2 27.0 26.5 26.3 21.6 26.7 25.8 27.2 25.9 Rb 191 199 203 202 211 205 197 202 141 140 137 Sr 88.1 89.8 94.1 91.9 93.9 94.5 82.0 101 20.3 27.5 26.9 Y 40.3 40.8 35.9 43.4 45.3 40.3 35.6 48.3 25.0 26.2 22.1 Zr 3951 3861 345 4035 1029 371 319 2912 5204 1462 537 Nb 27.6 26.7 24.2 29.3 28.5 26.9 26.3 33.4 20.6 19.7 20.1 Cs 2.0 2.1 2.1 2.5 2.6 2.4 2.77 3.0 1.7 1.7 1.6 Ba 403 419 434 422 474 503 419 505 108 138 132 La 73.9 89.7 78.0 98.9 87.1 68.9 78.2 124 91.3 159 95.7 Ce 145 174 150 190 172 136 164 235 173.1 266 181 Pr 15.1 17.7 15.3 19.0 17.8 14.2 16.9 23.2 17.4 28.3 18.2 Nd 55.2 63.4 54.7 67.5 64.9 52.9 62.9 81.7 63.3 98.0 65.7 Sm 9.81 10.7 9.36 11.3 11.2 9.58 10.1 13.0 9.62 12.8 9.49 Eu 0.69 0.72 0.74 0.73 0.74 0.72 0.74 0.73 0.15 0.18 0.17 Gd 8.24 8.90 7.75 9.34 9.23 7.99 9.04 10.67 7.54 9.90 7.40 Tb 1.35 1.40 1.23 1.48 1.49 1.32 1.22 1.64 1.01 1.21 0.96 Dy 7.10 7.25 6.37 7.71 7.81 6.93 6.54 8.42 4.80 5.30 4.41 Ho 1.35 1.38 1.21 1.46 1.46 1.32 1.27 1.60 0.88 0.96 0.81 Er 3.94 3.98 3.51 4.24 4.26 3.79 3.37 4.67 2.58 2.75 2.37 Tm 0.61 0.62 0.54 0.65 0.65 0.57 0.51 0.72 0.40 0.40 0.36 Yb 3.83 3.85 3.41 4.10 4.01 3.60 3.47 4.49 2.63 2.61 2.45 Lu 0.57 0.59 0.51 0.61 0.60 0.53 0.55 0.68 0.44 0.43 0.40 Hf 10.2 9.9 8.4 10.4 9.2 8.8 8.6 11.4 11.6 11.3 11.3 Ta 2.15 2.14 1.97 2.27 2.25 1.99 1.83 2.57 1.17 1.06 1.03 Pb 33.0 32.5 37.1 39.2 33.6 27.7 31.3 33.2 27.4 27.5 62.7 Th 17.8 21.3 18.1 21.4 19.5 16.4 21.4 28.3 16.8 19.1 15.3 U 5.25 5.21 5.08 6.02 3.96 3.26 5.24 6.97 2.86 2.57 2.67 Sample no. JH-31 FW01-164 JH-10 JH-11 JH-13 JH-14 JH-15-2 JH-16 JH-17-2 JH-18 Rock type Hornblende granites Mafic microgranular enclaves

Major elements (wt.%) SiO2 66.75 71.74 57.52 58.89 60.16 55.96 62.60 60.20 61.42 61.69 TiO2 0.33 0.21 1.16 1.02 0.96 1.19 0.59 1.07 0.78 0.63 Al2O3 15.69 14.17 16.34 15.80 16.62 16.95 14.45 16.04 15.78 14.30 Fe2O3 3.63 2.67 8.91 8.97 7.54 8.68 7.45 6.89 6.19 7.04 MnO 0.07 0.05 0.23 0.25 0.18 0.20 0.22 0.17 0.16 0.20 MgO 0.01 0.04 1.79 1.38 1.08 2.71 2.28 1.69 2.75 3.26 CaO 1.07 1.01 3.87 3.71 2.75 4.36 2.78 3.52 4.08 3.43

Table 1 (continued)

Sample no. JH-31 FW01-164 JH-10 JH-11 JH-13 JH-14 JH-15-2 JH-16 JH-17-2 JH-18 Rock type Hornblende granites Mafic microgranular enclaves

Major elements (wt.%) Na2O 4.70 4.44 5.41 5.27 5.39 5.76 5.27 5.95 6.25 5.64 K2O 6.32 5.74 2.26 2.67 3.07 2.16 2.91 2.54 1.45 2.03 P2O5 0.02 0.02 0.43 0.38 0.33 0.40 0.15 0.34 0.18 0.21 Total 98.58 100.09 97.92 98.34 98.09 98.37 98.70 98.39 99.05 98.43 Trace elements (ppm) Cr 2.68 3.00 4.55 3.60 2.88 5.16 65.3 7.09 52.2 88.5 Ni 2.66 3.23 7.44 5.63 3.52 11.1 29.7 8.64 29.0 45.3 Ga 29.4 18.8 30.9 31.9 32.0 25.9 26.7 24.7 24.8 23.1 Rb 138 123 184 198 222 180 191 145 82 146 Sr 25.8 15.9 254 170 336 289 87.1 250 175 102 Y 42.3 27.4 77.3 92.1 55.3 45.4 61.8 42.9 30.8 45.9 Zr 10,270 671 191 334 259 281 260 337 226 255 Nb 28.9 23.6 56.5 62.0 55.9 23.0 30.5 20.3 16.8 22.3 Cs 2.1 1.91 2.6 4.1 3.9 2.2 2.3 2.3 1.0 1.8 Ba 119 45.4 239 277 455 282 270 532 241 249 La 268 127 70.0 60.5 41.5 70.8 53.1 70.8 56.9 58.5 Ce 484 249 180 164 101 153 133 145 118 132 Pr 51.4 22.7 22.8 21.6 12.7 17.0 16.7 15.5 12.8 15.4 Nd 175 82.1 96.0 93.3 54.0 65.5 69.3 58.8 47.3 59.8 Sm 22.5 11.9 19.8 20.2 11.5 11.7 14.3 10.1 8.20 10.8 Eu 0.16 0.16 0.91 0.92 1.04 1.08 0.68 1.27 0.86 0.73 Gd 17.2 10. 8 16.1 16.5 9.74 9.52 11.6 8.39 6.67 8.88 Tb 2.02 1.27 2.72 2.93 1.71 1.52 2.00 1.34 1.05 1.45 Dy 8.59 6.33 14.4 16.0 9.31 7.89 10.8 7.12 5.39 7.71 Ho 1.55 1.12 2.67 3.01 1.79 1.47 2.01 1.37 1.01 1.47 Er 4.53 2.89 7.44 8.60 5.20 4.25 5.69 4.08 2.89 4.22 Tm 0.67 0.40 1.08 1.32 0.81 0.65 0.86 0.63 0.44 0.64 Yb 4.43 2.73 6.47 8.06 4.98 3.95 5.17 3.92 2.64 3.83 Lu 0.73 0.44 0.93 1.18 0.74 0.57 0.75 0.58 0.39 0.55 Hf 22.9 14.7 5.2 8.3 6.5 6.3 6.4 7.5 5.5 6.6 Ta 1.44 1.17 2.58 3.22 2.86 1.43 1.60 1.45 1.19 1.52 Pb 27.9 25.33 33.7 32.7 27.4 26.4 35.5 29.2 15.9 17.3 Th 31.4 16.7 11.8 12.9 12.7 12.5 12.0 14.6 11.3 13.8 U 4.57 3.17 6.82 7.07 6.68 5.02 4.15 12.1 2.84 5.57 Sample no. JH-19 JH-20 JH-21 JH-22 JH-23 JH-24 JH-25 JH-30

Rock type Mafic microgranular enclave Major elements (wt.%) SiO2 61.03 60.39 67.04 59.88 60.24 60.87 62.89 60.04 TiO2 0.96 0.82 0.47 0.83 0.98 0.96 0.46 0.82 Al2O3 17.08 15.79 15.20 15.77 16.30 15.01 16.68 15.20 Fe2O3 6.09 5.38 3.60 4.96 7.29 6.60 5.06 6.42 MnO 0.15 0.10 0.10 0.07 0.20 0.17 0.13 0.15 MgO 1.67 2.81 0.55 2.91 1.31 2.27 0.62 0.91 CaO 3.39 4.16 1.80 4.40 2.99 3.84 2.01 4.09 Na2O 5.67 5.07 4.50 4.61 5.40 5.41 5.24 4.34 K2O 2.15 3.61 5.07 4.34 3.04 2.84 4.78 6.08 P2O5 0.32 0.27 0.15 0.27 0.37 0.26 0.16 0.27 Total 98.50 98.40 98.48 98.03 98.10 98.22 98.03 98.31 Trace elements (ppm) Cr 3.62 64.5 13.5 66.4 3.94 41.0 3.13 7.43 Ni 10.1 37.1 9.46 36.1 3.95 22.9 3.67 6.41 Ga 32.3 22.5 25.4 21.6 28.2 23.7 25.2 22.1

of Geology and Geophysics, Chinese Academy of Sciences in Beijing, in order to identify zircon internal textures and choose potential target sites for Hf analy-ses. The working conditions during the CL imaging were at 15 kV.

In situ zircon Hf isotopic analyses were carried out at the Institute of Geology and Geophysics, Chinese Acad-emy of Sciences using a Neptune MC-ICPMS with an ArF excimer laser ablation system. During analyses, the spot sizes of 32 and 63 Am and a laser repetition rate of 10 Hz with 100 mJ were used. Details of the technique are described by Xu et al. (2004). During analyses, the

176

Hf /177Hf and 176Lu /177Hf ratios of standard zircon (91500) were 0.282294 F 15 (2rn, n = 20) and 0.00031,

similar to the commonly accepted176Hf /177Hf ratio of 0.282284 F 3 (1r) measured using the solution method (Goolaerts et al., 2004; Woodhead et al., 2004).

The notations of eHf, fLu / Hfand THfare defined as:

eHf(0) = ( (176Hf /177Hf)S/ (176Hf /177Hf)CHUR,0 1) 

10,000

eHf(t) = ((176Hf /177Hf )S (176Lu /177Hf)S(ekt 1)) /

((176Hf/177Hf)CHUR,0 (176Lu/177Hf)CHUR

(ekt 1)) 1)10,000 TDM = 1/k(1+(( 176 Hf /177Hf)S ( 176 Hf /177Hf)DM) / ((176Lu /177Hf)S (176Lu /177Hf)DM)) fLu / Hf = (176Lu /177Hf)S/ (176Lu /177Hf)CHUR 1

where, (176Lu /177Hf)Sand (176Hf /177Hf)Sare the

mea-sured values of samples, (176Lu /177Hf)CHUR= 0.0332

and (176Hf /177Hf)CHUR,0= 0.282772 (Blichert-Toft

and Albarede, 1997); (176Lu /177Hf)DM= 0.0384 and

(176Hf /177Hf)DM= 0.28325 (Griffin et al., 2000), t =

crystallization time of zircon. k = 1.867  10 11 yr 1 (Soderlund et al., 2004) was used in our calculations. 4. Results

4.1. Major and trace element data

All granite samples from the Qianshan pluton are high in silica and alkalis, with SiO2ranging from 66.7

to 71.3 wt.% and total K2O + Na2O varying from 9.2 to

11.0 wt.% (Table 1). They all plot as alkaline rocks on the alkali vs. silica diagram (Fig. 3a). The biotite gran-ites are mostly peraluminous with A / CNK (molar ratio of Al2O3/ [CaO + Na2O + K2O]) and A / NK ratios

rang-ing from 1.01 to 1.03 and 1.10 to 1.18, respectively,

Table 1 (continued)

Sample no. JH-19 JH-20 JH-21 JH-22 JH-23 JH-24 JH-25 JH-30

Rock type Mafic microgranular enclave Trace elements (ppm) Rb 190 118 181 113 199 122 226 134 Sr 477 639 171 835 224 176 151 122 Y 53.0 36.1 51.9 35.7 60.3 43.9 43.2 37.8 Zr 340 212 318 243 189 285 480 379 Nb 35.7 14.9 41.3 11.3 46.9 23.9 32.2 18.9 Cs 2.6 1.0 2.0 0.5 3.8 1.1 4.4 1.2 Ba 606 1115 729 1426 403 552 555 1206 La 69.8 52.1 39.1 50.2 47.3 65.5 54.7 44.5 Ce 153 105 97.1 94.8 118 142 117 102 Pr 17.2 11.3 12.3 9.9 14.9 15.7 13.0 12.2 Nd 66.4 42.7 51.6 37.3 62.9 60.2 48.8 50.9 Sm 11.6 7.28 10.6 6.11 13.0 10.8 8.64 9.91 Eu 1.14 1.20 0.85 1.37 0.80 1.07 0.84 1.17 Gd 9.65 6.08 8.90 5.22 10.6 8.95 7.24 8.51 Tb 1.60 0.97 1.59 0.82 1.85 1.44 1.22 1.38 Dy 8.58 5.38 8.77 4.67 10.1 7.57 6.56 7.08 Ho 1.68 1.11 1.67 1.01 1.91 1.43 1.29 1.30 Er 5.06 3.39 4.80 3.19 5.48 4.06 3.93 3.63 Tm 0.79 0.52 0.72 0.47 0.84 0.61 0.64 0.54 Yb 4.99 3.10 4.44 2.72 5.13 3.75 4.20 3.35 Lu 0.74 0.46 0.65 0.41 0.75 0.55 0.64 0.52 Hf 8.5 5.2 7.2 5.8 4.8 6.7 9.8 7.3 Ta 2.72 1.13 2.13 0.93 2.54 1.76 2.19 1.24 Pb 22.7 23.8 28.7 24.4 17.4 21.6 26.7 27.8 Th 17.0 15.1 10.5 16.7 12.1 14.3 17.9 8.9 U 11.9 73.7 10.4 127 6.98 9.70 7.93 2.28

although one sample (FW01-162) is metaluminous (Fig. 3b). All hornblende granites are metaluminous, with A / CNK and A / NK ratios ranging from 0.92 to 1.0 and 1.05 to 1.09, respectively (Fig. 3b). They have lower total Fe2O3 (2.0–3.6 wt.%), MnO (0.05–0.07

wt.%), CaO (0.7–1.1 wt.%), TiO2 (0.18–0.33 wt.%)

and P2O5 (0.01–0.06 wt.%) contents (Table 1) and

Yb / Hf ratios, but higher Zr, Nb, Ce and Y concentra-tions and (K2O + Na2O) / CaO, Ga / Al2O3, Fe2O3T/

MgO and Rb / Sr ratios than the mafic enclaves (Fig. 4).

Chondrite-normalized REE patterns of the horn-blende and biotite granites (Fig. 6a and b) invariably show relative enrichment of light rare earth elements (LREEs), with high (La / Yb)N ratios (14–44) and

significant negative Eu anomalies. In the

PM-normal-ized spidergrams (Fig. 7a and b), all the granites show negative anomalies in Ba, Nb, Ta, Sr, P, Eu and Ti, features characteristic of A-type granites. They are enriched in Rb, U, Th, LREEs, Zr and Hf. Significant features to note are the abundance levels of LREE, Eu, Sr and Ba of the biotite granites which are higher than those of the hornblende gran-ites (Fig. 7).

Mafic enclaves have SiO2 contents ranging from

57.5 to 67.0 wt.% with relatively high Mg-numbers (Mg# = 23.4 to 54.0) (Table 1). They also have higher total Fe2O3, MnO, CaO, TiO2, P2O5, Ba and Sr, but

lower K2O and Zr contents and Rb / Sr and total

Fe2O3/ MgO ratios than the host granites (Table 1,

Figs. 4 and 5). Chondrite-normalized REE patterns for the enclaves are marked by enrichment in the LREEs, variable negative Eu anomalies, and relatively high and variable HREE contents (Fig. 6c). In the primitive mantle-normalized trace element variation diagram (Fig. 7c), most enclave samples show the characteristic negative anomalies in Ba, Nb, Ta, Sr, P, Eu and Ti, whereas the samples with high MgO contents (e.g., JH-22) do not show negative Ba and Sr anomalies (Fig. 7c).

4.2. Sr and Nd isotope data

Rubidium, Sr, Sm and Nd concentrations, 143Nd /

144

Nd and 87Sr /86Sr ratios, and TDM ages for MMEs

and granitoids are listed inTable 2. The initial87Sr /86Sr ratios and eNd(t) values have been calculated at 126 F

2 Ma on the basis of zircon U–Pb dating of rocks from the batholith (Wu et al., 2005a). Depleted mantle model ages (TDM) are reported using the model of DePaolo

(1981). The data are shown in a plot of eNd(t) versus

(87Sr /86Sr)iinFig. 8, and compared on that diagram with

published compositional fields for Late Jurassic and Early Cretaceous granites from the area (Yang et al., 2004b; Wu et al., 2005b).

Two groups of granites can be distinguished on the basis of Sr and Nd isotopic compositions (Fig. 8). The hornblende granite has relatively high87Rb /86Sr (16.8– 24.2) and (87Sr /86Sr)i(0.7258–0.7281) ratios and

high-ly negative eNd(t) values of 15.8 to 16.5. However,

the biotite granite has different Sr and Nd isotopic compositions, with low87Rb /86Sr (6.13–6.43) and ini-tial 87Sr /86Sr (0.7132–0.7135) ratios and negative eNd(t) values of 12.9 to 13.3. The mafic enclaves

have similar Sr and Nd isotopic compositions to the biotite granite, with variable87Rb /86Sr (0.39–6.14) and initial 87Sr /86Sr (0.7097–0.7133) ratios and negative eNd(t) ( 11.9 to 13.6) values.

Fig. 3. Plots of (a) Na2O + K2O vs. SiO2 and (b) A / NK [Al2O3/

(Na2O + K2O)] vs. A / CNK [molar ratio Al2O3/ (CaO + Na2O + K2O)]

for mafic enclaves and host granitoids from the Qianshan pluton in the Liaodong Peninsula, northeastern China. The field of Early Creta-ceous volcanic rocks, MMEs in I-type granites and mafic dykes in (a) is fromYang et al. (2004b).

Fig. 4. Various oxide plots [(a), Fe2O3, (b), Al2O3, (c), TiO2, (d), CaO and (e), P2O5(all expressed in wt.%)], trace element plots [(f), Yb / Hf and (g),

Rb / Sr] and isotopic ratios [(h) (87_{Sr /}86_Sr)

i] vs. SiO2diagrams for enclaves and host rocks from the Qianshan pluton in the Liaodong Peninsula,

4.3. Zircon Hf isotope data

Cathodoluminescence images and in situ Hf isotopic analyses of zircons from the Qianshan granites and mafic enclaves are listed in Supplementary Data file online and shown inFigs. 9 and 10. Zircons from the hornblende granites are fairly homogeneous in CL images (Fig. 9a), but have variable 176Hf /177Hf ratios (0.282161 to 0.282374), with eHf(t) values ranging

from 11.5 to 18.9 (Fig. 10a). Zircons from biotite granites have cores and rims and show strong oscilla-tory zoning (Fig. 9b). They have a large range of

176

Hf /177Hf ratios, from 0.282321 to 0.282524, with eHf(t) values ranging from 6.3 to 16.4 (Fig. 10b).

The cores have higher 176Hf /177Hf ratios than the zircon rims (Fig. 9b). Zircons from MMEs show com-plex patterns in CL images or are characterized by irregular crystal forms indicative of corrosion (Fig. 9c and d). Zircons from enclaves also have variable

176

Hf /177Hf ratios (0.282213 to 0.282513) with eHf(t)

ranging from 6.9 to 17.1 (Fig. 10c). It is evident that the zircons in MMEs have high176Yb /177Hf ratios and eHf(t) values, whereas those in the hornblende

granites have low176Yb /177Hf ratios and eHf(t) values.

Zircons in the biotite granites have intermediate

176

Yb /177Hf ratios and eHf(t) values between zircons

in MMEs and those in the hornblende granites.

5. Discussion

5.1. Genetic type: highly fractionated I-type or A-type? Since the introduction of the terms I- and S-type granite by Chappell and White (1974), granitic rocks have commonly been divided into I-, S-, M- and A-types according to the nature of their protolith ( Pitch-er, 1982, 1993). However, distinction between differ-ent types is not always straightforward. This is particularly true in the case of A-type and highly frac-tionated I-type granites (White and Chappell, 1983; Chappell and Stephens, 1988; Landenberger and Col-lins, 1996; King et al., 1997, 2001; Wu et al., 2003a). Several attempts have been made to discriminate A-types from the others (e.g.,Collins et al., 1982; Whalen et al., 1987; Sylvester, 1989; Eby, 1990, 1992). In general, A-type granites are comparatively enriched in high field strength elements (HFSEs), such as Zr, Nb, Y, REE and Ga.

Fig. 5. (a) Na2O + K2O, (b) Zr and (c) Nb vs. 10,000 Ga / Al and (d) (Na2O + K2O) / CaO vs. (Zr + Nb + Ce + Y) discrimination diagrams ofWhalen et al. (1987), showing the A-type nature of the Qianshan granites. I, S & M: unfractionated M-, I- and S-type granites; FG: fractionated felsic granites.

The Qianshan granites in NE China have most characteristics of A-type granites. They have high K2O + Na2O, Zr, total FeO / MgO and Ga / Al ratios.

In the discrimination diagrams of K2O + Na2O, Nb

and Zr vs. Ga / Al (Fig. 5a and b), they plot in the A-type granite field of Whalen et al. (1987). Further-more, in the (K2O + Na2O) / CaO vs. (Zr + Nb + Y + Ce)

diagram, all of them fall in the field of A-type granites (Fig. 5d). This is also confirmed by the criteria of

Sylvester (1989) using the diagram of (Al2O3+ CaO) /

(FeO* + Na2O + K2O) vs. 100(MgO + FeO* + TiO2) /

SiO2, which is fairly effective in discriminating

alka-line granites from calc-alkaalka-line and strongly peralumi-nous granites (unshown). We therefore conclude that

Fig. 6. Chondrite-normalized REE patterns for (a) hornblende gran-ites, (b) biotite grangran-ites, and (c) enclaves of the Qianshan pluton. The chondrite values are fromSun and McDonough (1989).

Fig. 7. Primitive mantle (PM) normalized trace element patterns, elements arranged in the order of decreasing incompatibility from left to right. (a) Hornblende granites, (b) biotite granites, and (c) enclaves of the Qianshan pluton. The PM values are fromSun and McDonough (1989).

the Qianshan granites described in this study are A-type granites.

5.2. Restite or magma mixing?

The MMEs and host granites show a continuous variation in SiO2from 56 to 71 wt.%, producing tight

linear to slightly curved trends on Harker diagrams (Fig. 4). Although this variation provides a strong argument for a genetic link between the enclaves and host granites, there are several other potential reasons for such trends, including restite unmixing, crystal frac-tionation of a mafic magma or two-component mixing. In the restite unmixing model (e.g.,Chappell et al., 1987; Chappell and White, 1992), MMES are thought to represent residual material (restite) which successive-ly unmixes from the melt during the rise of a crystal mush from its source region. A critical feature of the restite model is linear chemical variations, which are observed in many plutonic suites (e.g.,Chappell et al., 1987; Chappell and White, 1992; Collins, 1998). Al-though these features are observed in some element

variation diagrams (Fig. 4), the large Sr, Nd and Hf isotopic differences between enclaves and host rocks from the Qianshan pluton clearly rule out any strictly cogenetic origin. Restite derived from the source rock of the granitoids should be in isotopic equilibrium with its enclosing melt. It is therefore unlikely that the mafic microgranular enclaves from the Qianshan pluton are restitic in origin. The heterogeneous isotopic composi-tions of this granitoid-mafic enclave association also preclude a simple, common evolution by closed-system fractionation processes.

The enclaves from Qianshan have igneous microtex-tures and contain acicular apatites, which are analogous to other cases from around the world (Eichelberger, 1980; Vernon, 1984; Barbarin, 1988; Didier and Bar-barin, 1991; Bonin, 2004; Yang et al., 2004b). Acicular apatite habits are common in comingled mafic pillows (Vernon et al., 1988) and have been attributed to rapid cooling, the result of mingling of small volumes of hot basalt with a cool granitic melt (Sparks and Marshall, 1986). Furthermore, the presence of megacrysts, such as K-feldspar, in the enclaves implies that the enclaves are

Table 2

Rb–Sr and Sm–Nd isotopic data for Qianshan granites and mafic microgranular enclaves Sample no. Rock type Rb

(ppm) Sr (ppm) 87 Rb / 86 Sr 87 Sr / 86 Sr 2r Sm (ppm) Nd (ppm) 147 Sm / 144 Nd 143 Nd / 144 Nd 2r (87Sr / 86 Sr)ai eNd(t)a,b TDMa,b fSm / Nd JH-07 Biotite granite 189 88.0 6.312 0.724871 14 8.87 50.0 0.1074 0.511897 5 0.71357 13.1 1792 0.45 JH-08 Biotite granite 199 89.9 6.426 0.724965 14 9.92 59.1 0.1015 0.511887 6 0.71346 13.2 1712 0.48 JH-09 Biotite granite 195 91.2 6.189 0.724698 13 8.29 48.2 0.1039 0.511885 11 0.71361 13.3 1752 0.47 JH-15-1 Biotite granite 195 91.8 6.143 0.724303 10 10.5 61.1 0.1042 0.511873 6 0.71330 13.6 1773 0.47 JH-17-1 Biotite granite 196 92.7 6.129 0.724322 12 9.00 50.5 0.1078 0.511909 9 0.71335 12.9 1781 0.45 FW01-162 Biotite granite 189 91.1 6.024 0.724178 16 9.63 59.3 0.0982 0.511897 10 0.71339 13.0 1650 0.50 JH-27 Hornblende granite 132 15.9 24.15 0.770192 16 9.21 60.7 0.0918 0.511743 6 0.72694 15.9 1755 0.53 JH-28 Hornblende granite 128 22.2 16.76 0.758347 15 12.3 94.5 0.0789 0.511739 16 0.72833 15.8 1592 0.60 JH-29 Hornblende granite 128 21.7 17.20 0.756847 13 9.01 62.5 0.0871 0.511738 5 0.72605 15.9 1696 0.56 JH-31 Hornblende granite 130 21.2 17.87 0.758142 14 22.2 175 0.0765 0.511700 5 0.72614 16.5 1607 0.61 FW01-164 Hornblende granite 125 20.2 18.05 0.753824 16 17.1 144 0.0720 0.511817 16 0.72151 14.1 1432 0.63 JH-10 MME 178 241 2.132 0.716645 12 17.8 86.6 0.1242 0.511887 6 0.71283 13.6 2143 0.37 JH-13 MME 206 316 1.886 0.714726 9 10.6 49.6 0.1291 0.511929 5 0.71135 12.9 2191 0.34 JH-14 MME 169 274 1.783 0.714974 9 10.9 60.9 0.1081 0.511915 14 0.71178 12.8 1778 0.45 JH-15-2 MME 177 83.9 6.112 0.724662 12 13.3 65.0 0.1236 0.511879 10 0.71372 13.8 2142 0.37 JH-16 MME 136 235 1.667 0.713782 7 9.71 56.5 0.1040 0.511932 6 0.71080 12.4 1688 0.47 JH-17-2 MME 78.9 173 1.325 0.715729 14 7.74 44.8 0.1045 0.511875 15 0.71336 13.5 1775 0.47 JH-20 MME 112 607 0.5358 0.710886 15 7.54 45.1 0.1010 0.511848 6 0.70993 14.0 1756 0.49 JH-22 MME 106 789 0.3902 0.710431 11 6.30 39.1 0.0976 0.511830 7 0.70973 14.3 1728 0.50 JH-23 MME 191 225 2.453 0.717002 7 13.3 66.6 0.1211 0.511912 11 0.71261 13.1 2031 0.38 JH-25 MME 212 149 4.131 0.719675 6 8.59 48.6 0.1070 0.511960 5 0.71228 11.9 1696 0.46 JH-30 MME 124 118 3.044 0.720206 11 9.95 51.5 0.1168 0.511835 7 0.71475 14.5 2061 0.41 a: t = 126 Ma. b: The143_{Nd /}144_{Nd and}147_{Sm /}144_{Nd ratios of chondrite and depleted mantle at present day are 0.512638 and 0.1967, 0.51315 and}

hybrids. The fine-grained, igneous-textured mafic to intermediate enclaves from I- and A-type granites have been considered to represent remnants of a mafic com-ponent added to intermediate or felsic magma chambers (e.g.,Vernon et al., 1988; Holden et al., 1987; Be´dard, 1990; Didier and Barbarin, 1991; Bonin, 2004; Yang et al., 2004b). In addition, the Hf isotopic ratios are higher in the zircon cores than the rims, produced by high

176

Hf /177Hf zircon growth in a low 176Hf /177Hf mag-matic environment (Fig. 9). Also, the Zr contents (191 to

10,270 ppm) and Zr / Hf ratios (37 to 448) increase with increasing SiO2and decreasing MgO from the MMEs to

hornblende and biotite granites (Table 1), indicating a magma mixing process in the petrogenesis of the Qian-shan suite. Therefore, the enclaves represent additional components involved in the generation of the host A-type granites, a view that is compatible with the petro-graphic, geochemical and isotopic data.

5.3. Sources

Although radiogenic isotope equilibrium is attained more rapidly than chemical equilibrium in the course of magma mixing (Lesher, 1990; Poli et al., 1996), the mafic enclaves and host hornblende granites have dis-tinct Sr and Nd isotopic compositions. This indicates that Sr and Nd isotopes were not equilibrated during mafic–felsic magma mixing and can be used to trace the sources of the mafic and felsic magmas. This is also reflected by the zircon Hf isotopes. Host granites are highly enriched in Zr, up to 10,270 ppm (Table 1), and concentrations of Zr in mafic enclaves range from 189 to 340 ppm, showing an increase with increasing K2O,

Rb and SiO2(Table 1). It seems likely that there was a

gain of Zr into the mafic magma during magma mixing, indicating that some Zr was transferred from the gra-nitic magma during the mixing process. However, the variable Hf isotopic compositions of zircons from MMEs and host granites (Fig. 10) and higher Hf iso-topic ratios of zircon cores compared to the rims (Fig. 9b) show that zircon Hf isotopes can also be used to trace the sources of mafic and felsic magmas.

Fig. 9. Cathodoluminescence (CL) images of representative zircons from enclaves and host Qianshan granites. Circles indicate the location of LA-MC-ICPMS Hf analyses and the numbers in circles refer to the eHf(t) values. Spot numbers are labelled near the circles. (a) CL image of zircon in

hornblende granite; (b) CL image of zircon in biotite granite; and (c) and (d) CL images of zircons in enclaves. All eHf(t) values of enclave and host

granites were calculated at ~126 Ma, the emplacement age of the Qianshan pluton. Fig. 8. eNd(t) vs. (

87

Sr /86Sr)iplot of enclaves and host granitoids of

the Qianshan pluton, Liaodong Peninsula. The data for Jurassic two-mica granites are fromWu et al. (2005b). The data for Early Creta-ceous I-type granites and enclaves are fromYang et al. (2004b). The source end-members are fromYang et al. (2004b). The Sr and Nd isotopic compositions of lithospheric mantle represented by mafic enclaves in the I-type granites were obtained fromYang et al. (2004b).

In Fig. 8, the Sr and Nd isotopic data from the Qianshan granites and their mafic enclaves may be interpreted as a result of two-component mixing be-tween mafic [(87Sr /86Sr)ib 0.7097, and eNd(t) N 11.9]

and felsic [(87Sr /86Sr)iN 0.7283, and eNd(t) b 16.5]

isotopically evolved end-members, which are repre-sented by the isotopic compositions of the mafic dykes or mafic enclaves in the Gudaoling I-type gran-ites near the Qianshan pluton (Yang et al., 2004b) and hornblende granites, respectively.

The hornblende granites have high initial 87Sr /86Sr ratios, high Rb / Sr ratios and negative eNd(t) values at

126 Ma, indicating that they were derived from a crustal source with more radiogenic Sr and less radiogenic Nd isotopic ratios. Although it has been proposed that A-type granites originated from the partial melting of F- and/or Cl-enriched dry, granulitic (or charnockitic) residual material from which a granitoid melt was pre-viously extracted (Collins et al., 1982; Clemens et al., 1986; Creaser et al., 1991; King et al., 1997), this model cannot explain the geochemical characteristics of the hornblende granites. In the Liaodong Peninsula, the synchronous I-type granites have geochemical and iso-topic features distinct from the A-type granites (Yang et al., 2004b), indicating they cannot be derived from a common source. Geochemical characteristics of the hornblende granites, including low Al and Ca contents (Fig. 4) and high FeO* / MgO and (Na2O + K2O) / Al2O3

ratios compared to calc-alkaline granitoids, the enrich-ment in high field strength eleenrich-ments, such as Zr, Nb, and Y, as well as the Ga / Al ratio (Fig. 5), and the depletion in Eu and Sr, are all consistent with the results from exper-imental melts (Skjerlie and Johnston, 1993; Patin˜o Douce, 1995, 1997, 1999) produced by dehydration melting of calc-alkaline granitoids at low pressure ( P V 4 kbar). Therefore, dehydration melting of horn-blende and biotite-bearing assemblages with a plagio-clase-rich residual assemblage in the shallow crust (at depths of 15 km or less) is a likely origin for the blende granite, as evidenced by the fact that the horn-blende granites have high Rb / Sr ratios and concave-up REE patterns with strongly negative Eu anomalies (Fig. 6a). However, attaining the required melting tempera-tures (N900 8C) so close to the Earth’s surface requires the participation of hot mafic magmas in the origin of A-type granites. The variable Sr, Nd and zircon Hf isotopic compositions indicate that a mafic magma with high Nd and Hf isotopic ratios, corresponding to the parental magma to the MMEs, is involved in the origin of horn-blende granites. Thus, we propose that the hornhorn-blende granites were mainly derived from partial melting of a hornblende- and/or biotite-bearing source, with a plagio-clase-rich residual assemblage. The with force was the emplacement of mafic magma into the upper crust, with which the granitic magmas interacted.

The source of the biotite granites (at 126 Ma) had more radiogenic Nd isotopic ratios and much less

ra-Fig. 10. Histograms of eHf(t) values for zircons in (a) hornblende

granites, (b) biotite granites and (c) enclaves. All eHf(t) values of

enclaves and host granites were calculated at ~126 Ma, the emplace-ment ages of the Qianshan pluton. The eHf(t) value ( 9.0 F 0.7) for

enriched mantle is represented by mafic dykes obtained by Yang (2005, unpublished data).

diogenic87Sr /86Sr ratios than the hornblende granites, ruling out magma derivation solely by partial melting of an upper crustal source. Therefore, sources other than the upper crust must also be involved. It is unlikely that the lower crust was the source for the biotite granites, because the lower crust beneath the Liaodong Peninsula has much less radiogenic Nd (eNd(t) = 17 to 20) and

Sr (initial 87Sr /86Sr = 0.7055–0.7116) isotopic ratios (Yang et al., 2004b). Neither derivation from lower crust or upper crust nor any combination of the two can account for the isotopic composition of the Qian-shan biotite granites. Importantly, the biotite granites have similar whole rock Sr and Nd and zircon Hf isotopic compositions to the mafic enclaves, indicating they were derived from a common source.

Mafic enclaves have relatively higher Mg-numbers (19.7–54.0) than the host granites. Sample JH-22, with the highest Mg-number (54.0), has the lowest silica content (SiO2= 60%) without negative Ba, Sr and Eu

anomalies, indicating that a component with a high MgO content was involved in the genesis of this enclave. Although it has a highly negative eNd(t) value ( 14.3),

this is because isotopic equilibrium was attained more rapidly than chemical equilibrium in the course of magma mixing (Lesher, 1990; Poli et al., 1996). The source component should have lower SiO2

concentra-tions, higher MgO, more radiogenic Nd isotopic ratios (eNd(t) N 12) and less radiogenic

87

Sr /86Sr ratios (b0.7097) than either the granites or the mafic enclaves. Such geochemical features and isotopic compositions are similar to those of magmas derived from lithospheric mantle, represented by mafic enclaves hosted by Early Cretaceous I-type granites or Early Cretaceous mafic dykes in the Liaodong Peninsula (Yang et al., 2004b). The enclaves in I-type granites and mafic dykes have low to moderate SiO2contents (51–57%), high Mg-numbers

(64–72), initial 87Sr /86Sr ratios of 0.7058–0.7073, eNd(125 Ma) of 7.2 to 8.0, eHf(125 Ma) of 2.1,

and are enriched in LILEs and LREEs and depleted in high field strength elements (HFSEs), indicating they were derived from enriched lithospheric mantle (Yang et al., 2004b). However, mafic enclaves in the Qianshan granites have variable major and trace element contents (Table 1,Figs. 4 and 7) and are enriched in K2O, Rb and

Zr, indicating mixing with granitic magmas. 5.4. Genesis of the Qianshan pluton

The hornblende granites were derived by partial melting of upper crust at low pressure, but involving lithospheric mantle-derived materials. They plot close to the theoretical upper crust end-member in the

eNd(t) vs. (87Sr /86Sr)i diagram (Fig. 8), whereas, the

biotite granites and mafic enclaves plot on the binary mixing line between lithospheric mantle and upper crust. Two-component mixing calculations suggest that as much as 10% to 20% hybridization with man-tle-derived melts accounts for the Sr and Nd isotopic

Fig. 11. (a) Eu / Eu*, (b) Rb / Sr and (c) SiO2vs. Sr diagrams showing

plagioclase- and biotite-dominated fractionation in the evolution of an evolved mafic magma. Partition coefficients are from Rollinson (1993). Symbols are: pl = plagioclase; hb = hornblende; cpx = clinopyr-oxene; opx = orthopyrclinopyr-oxene; bi = biotite.

heterogeneity of the A-type granites and the mafic enclaves.

Besides magma mixing, all MMEs and biotite gran-ites plot on the trends of biotite and plagioclase frac-tionation in the Eu / Eu*, Rb / Sr and SiO2 vs. Sr

diagrams (Fig. 11). This indicates that the biotite gran-ites were generated by fractionation of both biotite and plagioclase in a mafic magma or formed as an evolved mafic magma derived from an enriched lithospheric mantle source.

Based on the petrography, major and trace element geochemistry and Sr, Nd and zircon Hf isotopic com-positions of mafic microgranular enclaves and their host granites, a complex, multi-stage process involving magma mixing, crystal fractionation and partial melting of upper crustal source rocks is proposed to explain the formation of the Qianshan A-type granites.

In the Liaodong Peninsula, the Qianshan pluton is coeval with formation of Early Cretaceous metamorphic core complexes (120 to 110 Ma), indicating an exten-sional regime at this time. This most likely resulted from lithospheric thinning and possible subduction of the Paleo-Pacific ocean plate (Menzies et al., 1993; Griffin et al., 1998; Wu et al., 2003b; Yang et al., 2003; Gao et al., 2004; Wu et al., 2005a). Furthermore, the formation of hornblende granites by partial melting of upper crust indicates a high temperature in the upper crust, evidenced by the formation of large-scale gold mineral-ization in the Liaodong Peninsula which was a result of thermal fluid activity at a shallow crustal level (Yang et al., 2003). Thus, we suggest that the Qianshan pluton was the result of lithospheric thinning that led to inten-sive underplating of mantle-derived magma and a high degree of crustal melting, thereby producing strong magma mixing and hybrid granitoids in a largely exten-sional tectonic setting.

6. Concluding remarks

Mafic microgranular enclaves and host A-type gran-ites of the Qianshan pluton in the Liaodong Peninsula of NE China provide direct evidence of the involve-ment of mantle-derived magmas in A-type granitoid genesis. The geochemical characteristics and isotopic compositions of the Qianshan A-type granites and their mafic enclaves imply that they were generated not via partial melting of lower crustal granulitic residues or by fractionation of alkali basalts, but by mixing of enriched lithospheric mantle-derived magma with upper crustal-derived magmas.

Basic magmas were emplaced penecontempora-neously with A-type granitoids and were contaminated

during their ascent by crustal materials. Although it is difficult to deduce precise estimates for the isotopic signature of the basaltic component, the available data suggest that magmas with high MgO, negative eNd(t)

and eHf(t), and high (87Sr /86Sr)i, i.e., partial melts of an

enriched lithospheric mantle and its evolved products, were involved in the hybridization process. Hornblende granites with high Rb / Sr and (87Sr /86Sr)i ratios and

negative eNd(t) and eHf(t) values are interpreted to

represent an upper crustal-derived magma.

The Qianshan plutonic association thus illustrates that hybridization of mantle- and crustal-derived mag-mas is a complex, multiple-stage process involving several different mechanisms (magma mixing, crystal fractionation, and crustal anatexis) which all operated together during Late Mesozoic extension in the Liao-dong Peninsula of NE China.

Acknowledgements

We thank Dr. Fu-Kun Chen and Chao-Feng Li for helping with Sr and Nd isotopic analyses, Dr. Qian Mao and Yu-Guang Ma for helping with CL imaging and Dr. Lie-Wen Xie for helping with zircon Hf isotopic anal-yses. We thank Stephen Foley (chief editor), Boris A. Litvinovsky and an anonymous reviewer for their con-structive comments. This study was supported by grants 40325006, 40203005 from the National Natural Sci-ence Foundation of China and bFunds for Hundred Outstanding Talents PlanQ.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, atdoi:10.1016/j.lithos. 2005.10.002.

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