Lithos 113 (2009) 621–639

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Petrogenesis of Late Mesozoic mafic dykes in the Jiaodong Peninsula, eastern NorthChina Craton and implications for the foundering of lower crust

Shen Liu a,b,⁎, Ruizhong Hu a, Shan Gao b, Caixia Feng b, Bobin Yu c, Guangying Feng b, Youqiang Qi b,Tao Wang b, Ian M. Coulson d

a State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, Chinab State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, Chinac No. 7 Brigade of China Armed Police, Shandong Yantai 264004, Chinad Solid Earth Studies Laboratory, Department of Geology, University of Regina, Regina, Saskatchewan, Canada S4S 0A2

⁎ Corresponding author. State Key Laboratory of Ore Dof Geochemistry, Chinese Academy of Sciences, Guiyang5895187; fax: +86 851 5891664.

E-mail address: liushen@vip.gyig.ac.cn (S. Liu).

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.06.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 April 2009Accepted 23 June 2009Available online 8 July 2009

Keywords:Dolerite-porphyriesZircon U–Pb agePGEPetrogenesisJiaodongNorth China Craton

Late Mesozoic (122.5–127 Ma) mafic dykes (dolerite-porphyries) in the Beibo gold district, ShandongProvince, eastern North China Craton, can be subdivided into two groups (high-K and low-K) based upon K2Ocontent. They are characterised by enrichment in light rare earth element (LREE) and some large ionlithophile elements (LILEs) (e.g., Rb, Ba and Sr), similar concentrations of heavy rare earth elements (HREEs),high (87Sr/86Sr)i ranging from 0.7094 to 0.7101, low εNd(t) values from −15.3 to −13.9, 206Pb/204Pb=16.98–17.70, 207Pb/204Pb=15.47–15.55 and 208Pb/204Pb=37.61–37.89. In addition, the high-K dolerite-porphyries aredistinguished by negative Eu, Nb, Ta, P anomalies in chondrite-normalised REE and primitive-mantle-normalisedtrace element diagrams. These features suggest that they were derived from a common, enriched garnet-lherzolite mantle beneath the North China Craton, and that this was metasomatised by multiple enrichmentevents induced by hybridism of foundering lower crust at mantle depths. The low-K dolerite-porphyries wereproduced by 10–15% partial melting of the garnet-lherzolite source that had undergone metasomatism bycarbonate-rich fluids producing secondary phlogopite and amphibole, whereas the high-K groupwas derived byrelatively low-degree (3–5%) partial melting of a source that had experiencedmetasomatism by a carbonate-richfluid with crystallisation of only secondary amphibole. The Pd-subgroup platinum-group element (PPGE) (Rh, Ptand Pd) enrichment and Ir-subgroup platinum-group element (IPGE) (Ir and Ru) depletion of these mafic dykessuggest derivation from an S-unsaturated magma. Geochemical and PGE features indicate that fractionation ofolivine, clinopyroxene, hornblende, plagioclase, ilmenite, titanite, apatite, laurite and Os–Ir–Ru alloys wasimportant in the formation of the high-K mafic magma. Additionally, separation of all the above phases with theexception of plagioclase and apatite played an important role in the origin of low-K dolerite-porphyries. Minorcrustal contamination also occurred during ascent of these mafic magmas.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

It is generally accepted that more than 100 km of ancient litho-sphere beneath the eastern North China Craton was removed duringthe Late Mesozoic (Menzies et al., 1993; Griffin et al., 1998; Zheng andLu, 1999; Fan et al., 2000; Xu, 2001; Gao et al., 2002; Zhou et al., 2002;Wu et al., 2003a; Yang et al., 2003; Wilde et al., 2003; Zhang et al.,2004; Liu et al., 2006, 2008a), thus requiring an unusual mechanismto account for this thinned lithosphere. Several hypotheses, such asextension, delamination or foundering and thermal/chemical erosion,have been proposed (Zheng and Lu, 1999; Lu et al., 2000; Xu, 2001;Wu et al., 2003b; Gao et al., 2004). Based on investigations of themafic

eposit Geochemistry, Institute550002, China. Tel.: +86 851

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dykes in Luxi, eastern North China Craton (Liu et al., 2008a), weproposed that these rocks are the products of lithosphere destructionmainly induced by foundering of the lower lithosphere (mantle andlower crust) beneath the North China Craton. In addition, precisedating of the mafic dykes that are widespread throughout the NorthChina Craton can constrain the exact timing of the lithosphericdestruction.

Previous studies of Late Mesozoic, mantle-derived rocks from theeastern North China Craton indicate that their mantle domains areheterogeneous (Zhang et al., 2004). Nevertheless, because of a paucityof Mesozoic mantle-derived samples, the origin and the spatialvariation of the enriched mantle source(s) within Luxi and Jiaodong,eastern North China Craton remain controversial (e.g., Xu et al., 2004;Zhai et al., 2004). Hence, further investigation of the geochronological,geochemical and isotopic characteristics of the Late Mesozoic litho-spheric mantle to the eastern North China Craton is required.Mesozoic mafic dykes, including gabbro dolerite, dolerite-porphyry

622 S. Liu et al. / Lithos 113 (2009) 621–639

and lamprophyre are believed to have formed as the result of sig-nificant extension of the continental lithosphere (Hall, 1982; Hall andFahrig, 1987; Tarney and Weaver, 1987; Zhao and McCulloch, 1993).Investigation on the origin of these rift-related mafic dyke swarms,therefore, can provide valuable information about the Mesozoic litho-spheric evolution beneath the eastern North China Craton (Liu, 2004;Liu et al., 2004, 2006, 2008a,b).

In addition, the platinum-group elements (PGE) are known to besensitive indicators of the degree of partial melting, sulphide seg-regation and silicate fractionation (Wyman et al., 1995; Vogel andKeays,1997;Maier and Barnes,1999; Crocket, 2000; Maier et al., 2003;Crocket and Paul, 2004, 2008; Lightfoot and Keays, 2005; Qi and Zhou,2008, Qi et al., 2008). Hence, these elements are useful in examiningthe petrogenesis of mafic rocks (Crocket and Paul, 2004; Lightfoot andKeays, 2005). However, because of the difficulty in analysing PGE atlow concentrations in mafic rocks (Qi et al., 2004, 2007), only a fewstudies of these elements have been carried out; see, for example thestudies of mafic rocks from the Deccan Traps, the Siberian Traps, EastGreenland and the Emeishan large igneous province (Zhang et al.,1998; Momme et al., 2002, 2003; Crocket and Paul, 2004; Lightfootand Keays, 2005; Song et al., 2006; Qi and Zhou, 2008; Qi et al., 2008).This study provides a good opportunity to investigate the origin ofmafic dykes and the behavior of PGE in mafic dykes, such as thediolerite-porphyries in Beibo gold district, Laixi, Shandong Province(Fig. 1). In addition to PGEs, we also present LA-ICP-MS zircon U–Pbdates, petrographic, major and trace element, and Sr–Nd–Pb isotopicdata for the mafic dykes. We use this comprehensive dataset to con-strain the emplacement age(s), as well as to investigate the factorscontrolling the petrogenesis of the mafic dykes, including mantlemetasomatism, partial melting and subsequent crustal contaminationand crystal fractionation.

2. Geological setting and petrology

The Shandong area is divided in two by the Tanlu fault zone(Fig. 1a). The eastern part is termed the Jiaodong, and the westernpart, Luxi. The Jiaodong Peninsula (Fig. 1a) comprises two differentterrains bounded by the Wulian–Mishan fault (Zhai et al., 2000),namely, the Jiaobei terrain and the Sulu orogenic belt. The Jiaobeiterranes consist of Precambrian basement rocks (Zhai et al., 2000),in which the Yanshanian granites (Wang et al., 1998) and QingshanFormation volcanic rocks and mafic dykes are exposed (Yang et al.,2004). TheseMesozoic magmatic rocks are associated with the largestgold deposit in China (Wang et al., 1998; Yang and Zhou, 2001; Qiuet al., 2002; Yang et al., 2003, 2004) (Fig. 1b).

The Precambrian basement of Shandong is mainly composed ofthe Jiaodong Group of Late Archaean age. This contains both volcanicand sedimentary sequences that have been metamorphosed atamphibolite to granulite facies conditions (Yang et al., 2004). TheYanshanian granites, which intruded the basement, have been di-vided, based on petrography, geochemistry and isotopic compositions,into three major suites: namely, the Linglong (160–156 Ma), theGuojialing (130–126Ma,Wang et al., 1998; Zhang et al., 2003) and theKunyushan (135–130 Ma, Zhang et al., 1995a,b; Qiu et al., 2002; Yanget al., 2004).

The Qingshan Formation volcanic rocks (98–117Ma), which eruptedthrough the basement, are predominantly andesitic in composition(Tang et al., 2008), and these are located in the faulted basins, such asthe Laiyang and Taocun (BGMRS, 1991; Fig. 1b). Mafic dykes are wide-spread in the Jiaobei terrain, intruding the basement, granites andvolcanic basins, occurring as NE-NW-NS-trending swarms (Liu et al.,2004, 2006). Available K–Ar, Rb–Sr andAr–Ar ages for thesemafic dykesrange from 84–169Ma (Luo,1992; Ying,1996; Qiu et al., 2001; Liu et al.,2004, 2006; Yang et al., 2004). Abundant gold deposits (e.g., Linglong,Jiaojia, Xincheng, Hedong, Cangshang, Daliuhang, Majiayao, Jinqingdingand Mouping) are distributed in the Jiaobei terrain. Occurring through-

out the Precambrian basement, and located in or proximal to Mesozoicgranitoids, these deposits are associated with Late Jurassic–EarlyCretaceous magmatism (e.g., dolerite and lamprophyre), and theirages are mainly Early Cretaceous (130–110 Ma) (Yang et al., 2003) andconstrain a peak of metallogenesis at 120 Ma (Zhai et al., 2004).

The studied area is located in the Jiaobei terrane, north of the Tanlufault zone (Fig. 1a and b). Mafic rocks are widely distributed in thisregion as dyke swarms (e.g., BGMRS, 1991; Liu, 2004). The dolerite-porphyries were mainly collected from the Beibo lode-gold deposit ofLaixi County. They occur as dykes with NW-, NS- and NE-strikeorientations, which intrude the Yanshanian granites and Precambrianbasement rocks of the gold ore body. Single dykes can range between50 and 70 m in width (the widest part) and 0.6–1.0 km in length(Fig. 1c). All of the dolerite-porphyries are porphyritic with pre-dominant phenocrysts of plagioclase (0.6–3.0 mm), clinopyroxene(0.6–3.0 mm), K-feldspar (1.0–2.0 mm), hornblende (0.5–3.0 mm),biotite (0.6–3.5 mm) and minor olivine (1.0–5.0 mm) (B4-1). Thegroundmass consists of fine-grained (b 0.5 mm grain size) plagioclase(An40), alkali feldspar, biotite, clinopyroxene and minor authigeniccarbonate (B1-3, Fig. 2). Accessory minerals include apatite andopaques (e.g., magnetite) (Fig. 2).

3. Analytical methods

3.1. Zircon LA-ICP-MS U–Pb dating

Zircon was separated from five dolerite-porphyries (N40 kg)(B1ZA, B3ZA, B4ZA, B5ZA and B6ZA) using conventional heavy liquidand magnetic techniques at the Langfang Regional Geological Survey,Hebei Province, China. Representative zircon grains were hand-pickedunder a binocularmicroscope andmounted in an epoxy resin disc, andthen polished and coated with a gold film. Zircons were documentedwith transmitted and reflected light as well as cathodoluminescenceimages to reveal their external and internal structures at the StateKey Laboratory of Continental Dynamics, Northwest University. Laserablation techniques were used for zircon age determinations(Table 1). The analyses were conducted with an Agilent 7500a ICP-MS equippedwith 193 nm excimer lasers, which is housed at the StateKey Laboratory of Geological Processes and Mineral Resources, ChinaUniversity of Geoscience inWuhan, China. Zircon 91500 was used as astandard and NIST 610 was used to optimise the results. The spotdiameter was 24 µm. Analytical methodology is described in detailin Yuan et al. (2004). Common-Pb corrections were made using themethod of Andersen (2002). Data were processed using the GLITTERand ISOPLOT (Ludwig, 2003) programs. Errors on individual analysesby LA-ICP-MS are quoted at the 95% (1σ) confidence level.

3.2. Major and trace elemental analyses

Eighteen samples were selected for major and trace elementdeterminations and Sr–Nd–Pb isotopic analyses. Whole-rock sampleswere trimmed to remove altered surfaces, and were cleaned withdeionized water, crushed and powdered with an agate mill.

Major elements were analysed with a PANalytical Axios-advance(Axios PW4400) X-ray fluorescence spectrometer (XRF) at the StateKey Laboratory of Ore Deposit Geochemistry, Institute of Geochem-istry, Chinese Academy of Sciences (IGCAS). Fused glass discs wereused and the analytical precision as determined on the ChineseNational standard GSR-1 and GSR-3 was better than 5% (Table 2). LossOn Ignition (LOI in Table 2) was obtained using 1 g of powder heatedup to 1100 °C for 1 h.

Trace element concentrations were determined with a POEMS ICP-MS at the National Research Center of Geoanalysis, Chinese Academyof Geosciences, following procedures described by Qi et al. (2000).The discrepancy between triplicate analyses is less than 5% for all

Fig. 1. (a) Simplified tectonic map of eastern China (modified after Xu, 2002); (b) Geological map of the Jiaobei area and the distributions of the Precambrian basement rocks,Yanshanian granites, Qingshan Formation volcanic rocks and gold deposits (modified after Yang et al., 2003); (c) Simplified geological map of the granites and dolerite-porphyries(modified after BGMRS, 1991).

623S. Liu et al. / Lithos 113 (2009) 621–639

Fig. 2. Representative photomicrographs of the studied dolerite-porphyries. cpx: clinopyroxene, ol: olivine, pl: plagioclase, Bi: biotite, Hb: hornblende, Kf: K-feldspar, Mt: magnetite.

624 S. Liu et al. / Lithos 113 (2009) 621–639

Table 1LA-ICP-MS U–Pb isotopic data for the zircons from the studied dolerite-porphyries.

Spot Th U Pb Th/U

238U/232Th

Isotopic ratios Age (Ma)207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ

B1ZA1 351 255 8.15 1.37 0.73 0.0471 0.0051 0.1226 0.0132 0.0189 0.0004 54 229 117 12 121 22 268 195 5.99 1.38 0.73 0.0586 0.0042 0.1499 0.0099 0.0192 0.0004 551 110 142 9 123 23 189 144 4.38 1.31 0.76 0.0592 0.0029 0.1537 0.0075 0.0192 0.0003 574 77 145 7 122 24 324 175 6.01 1.86 0.54 0.0584 0.0031 0.1505 0.0074 0.0193 0.0003 545 79 142 7 123 25 309 210 6.39 1.47 0.68 0.0503 0.0033 0.1306 0.0085 0.0189 0.0004 207 111 125 8 121 26 260 195 6.51 1.34 0.75 0.0461 0.0047 0.1221 0.0123 0.0192 0.0004 211 117 11 123 27 374 211 7.13 1.77 0.56 0.0462 0.0041 0.1196 0.0105 0.0188 0.0003 10 194 115 10 120 28 455 244 8.40 1.87 0.54 0.0551 0.0029 0.1438 0.0071 0.0194 0.0003 416 86 136 6 124 29 360 252 8.0 1.43 0.70 0.0461 0.0034 0.1208 0.0087 0.0190 0.0003 162 116 8 121 210 511 315 11.0 1.62 0.62 0.0461 0.0034 0.1249 0.0091 0.0197 0.0003 164 120 8 126 211 387 266 8.4 1.45 0.69 0.0518 0.0025 0.1383 0.0065 0.0198 0.0003 275 79 132 6 126 212 270 212 6.40 1.27 0.79 0.0484 0.0024 0.1289 0.0062 0.0196 0.0003 118 82 123 6 125 213 167 116 3.77 1.44 0.69 0.0571 0.0067 0.1538 0.0178 0.0195 0.0004 497 266 145 16 125 214 207 173 5.23 1.20 0.83 0.0543 0.0039 0.1485 0.0111 0.0197 0.0005 384 127 141 10 126 215 251 195 6.06 1.28 0.78 0.0556 0.0029 0.1516 0.0077 0.0202 0.0003 435 84 143 7 129 216 253 208 6.3 1.22 0.82 0.0588 0.0032 0.1574 0.0085 0.0197 0.0003 560 93 148 7 126 217 1011 453 17.3 2.23 0.45 0.0537 0.0021 0.1496 0.0058 0.0202 0.0002 358 68 142 5 129 118 247 193 5.79 1.28 0.78 0.0574 0.0028 0.1520 0.0069 0.0196 0.0003 506 70 144 6 125 2

B3ZA1 317 223 7.18 1.42 0.70 0.0558 0.0027 0.1508 0.0069 0.0200 0.0003 445 73 143 6 127 22 334 283 8.76 1.18 0.85 0.0532 0.0026 0.1454 0.0066 0.0203 0.0003 336 76 138 6 129 23 354 280 8.45 1.27 0.79 0.0542 0.0025 0.1422 0.0061 0.0194 0.0003 379 73 135 5 124 24 434 451 13.4 0.96 1.04 0.0474 0.0021 0.1336 0.0056 0.0205 0.0002 68 74 127 5 131 15 163 148 4.40 1.10 0.91 0.0461 0.0027 0.1210 0.0068 0.0191 0.0003 126 116 6 122 26 142 117 3.68 1.22 0.82 0.0476 0.0074 0.1221 0.0186 0.0186 0.0005 79 287 117 17 119 37 231 223 7.68 1.04 0.97 0.0461 0.0025 0.1222 0.0062 0.0192 0.0003 117 117 6 123 28 219 178 5.67 1.23 0.81 0.0461 0.0037 0.1265 0.0097 0.0199 0.0004 174 121 9 127 29 375 229 7.71 1.63 0.61 0.0554 0.0040 0.1501 0.0105 0.0199 0.0003 428 128 142 9 127 210 225 151 4.94 1.49 0.67 0.0521 0.0038 0.1358 0.0093 0.0197 0.0004 289 122 129 8 126 211 196 153 4.83 1.29 0.78 0.0479 0.0054 0.1281 0.0142 0.0194 0.0004 94 238 122 13 124 212 334 304 9.32 1.10 0.91 0.0469 0.0035 0.1302 0.0097 0.0202 0.0003 43 168 124 9 129 213 178 144 4.64 1.24 0.81 0.0520 0.0039 0.1429 0.0098 0.0205 0.0004 286 119 136 9 131 314 282 217 7.20 1.30 0.77 0.0564 0.0054 0.1669 0.0149 0.0203 0.0004 469 180 147 13 129 215 217 174 5.77 1.25 0.80 0.0542 0.0071 0.1484 0.0191 0.0199 0.0005 377 295 141 17 127 316 435 273 9.55 1.59 0.63 0.0542 0.0069 0.1405 0.0176 0.0188 0.0003 380 285 133 16 120 217 253 213 6.47 1.19 0.84 0.0589 0.0109 0.1525 0.0244 0.0198 0.0003 563 329 144 22 126 218 301 219 6.89 1.38 0.73 0.0553 0.0029 0.1491 0.0074 0.0199 0.0003 425 81 141 7 127 219 340 303 8.55 1.12 0.89 0.0534 0.0035 0.1424 0.0091 0.0201 0.0005 347 101 135 8 129 3

B4ZA1 479 390 11.8 1.23 0.81 0.0484 0.0040 0.1262 0.0102 0.0189 0.0003 119 185 121 9 121.0 22 351 306 9.70 1.15 0.87 0.0461 0.0029 0.1262 0.0076 0.0199 0.0003 136 121 7 127.0 23 314 207 6.81 1.52 0.66 0.0534 0.0063 0.1407 0.0165 0.0191 0.0004 346 272 134 15 122.0 24 422 260 8.35 1.62 0.62 0.0558 0.0026 0.1445 0.0067 0.0191 0.0003 446 79 137 6 122.0 25 204 148 4.42 1.38 0.72 0.0552 0.0071 0.1396 0.0176 0.0184 0.0004 419 292 133 16 117.0 36 355 324 9.22 1.09 0.91 0.0525 0.0022 0.1366 0.0056 0.0190 0.0002 308 70 130 5 122.0 27 433 316 9.93 1.37 0.73 0.0504 0.0025 0.1368 0.0065 0.0198 0.0002 214 88 130 6 127.0 28 345 204 6.47 1.69 0.59 0.0586 0.0045 0.1548 0.0126 0.0190 0.0006 552 121 146 11 121.0 49 233 192 5.87 1.21 0.82 0.0521 0.0026 0.1402 0.0068 0.0197 0.0003 291 83 133 6 126.0 210 227 179 5.41 1.27 0.79 0.0582 0.0045 0.1502 0.0112 0.0191 0.0004 538 132 142 10 122.0 211 453 394 12.4 1.15 0.87 0.0514 0.0040 0.1355 0.0104 0.0191 0.0002 259 178 129 9 122.0 112 521 360 11.8 1.45 0.69 0.0554 0.0058 0.1391 0.0144 0.0182 0.0003 426 238 132 13 116.0 213 340 274 8.63 1.24 0.81 0.0520 0.0022 0.1424 0.0060 0.0201 0.0003 284 72 135 5 129.0 214 401 348 11.7 1.15 0.87 0.0545 0.0074 0.1453 0.0195 0.0193 0.0003 392 306 138 17 123.0 215 243 189 5.78 1.29 0.78 0.0544 0.0024 0.1419 0.0059 0.0192 0.0003 386 67 135 5 123.0 216 425 328 10.1 1.30 0.77 0.0543 0.0023 0.1428 0.0059 0.0192 0.0002 383 71 136 5 122.0 117 235 177 5.52 1.33 0.75 0.0461 0.00472 0.1181 0.0119 0.0186 0.00036 211 113 11 119.0 2

B5ZA1 380 269 8.25 1.41 0.71 0.0461 0.0033 0.1187 0.0082 0.0187 0.0003 156 114 7 119 22 225 171 5.47 1.31 0.76 0.0482 0.0050 0.1335 0.0137 0.0201 0.0004 107 230 127 12 128 23 234 169 5.15 1.39 0.72 0.0461 0.0030 0.1212 0.0076 0.0191 0.0004 143 116 7 122 24 285 220 6.86 1.29 0.77 0.0461 0.0050 0.1188 0.0128 0.0187 0.0002 219 114 12 119 15 228 171 5.22 1.34 0.75 0.0461 0.0039 0.1209 0.0100 0.0191 0.0003 186 116 9 122 26 767 605 18.9 1.27 0.79 0.0506 0.0017 0.1401 0.0045 0.0201 0.0002 224 53 133 4 129 17 186 115 3.85 1.61 0.62 0.0490 0.0067 0.1274 0.0173 0.0189 0.0004 147 278 122 16 121 38 287 228 6.91 1.26 0.79 0.0474 0.0038 0.1249 0.0099 0.0191 0.0003 70 180 120 9 122 29 238 175 5.38 1.36 0.74 0.0482 0.0044 0.1261 0.0114 0.0190 0.0003 108 207 121 10 121 210 471 447 13.1 1.06 0.95 0.0505 0.0019 0.1381 0.0050 0.0199 0.0002 218 62 131 4 127 111 168 96 3.22 1.74 0.57 0.0522 0.0097 0.1359 0.0250 0.0189 0.0005 296 365 129 22 120 312 130 102 3.10 1.28 0.78 0.0517 0.0066 0.1376 0.0172 0.0193 0.0005 274 284 131 15 123 313 207 173 5.07 1.20 0.84 0.0515 0.0062 0.1340 0.0159 0.0189 0.0004 261 272 128 14 121 2

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625S. Liu et al. / Lithos 113 (2009) 621–639

Table 1 (continued)

Spot Th U Pb Th/U

238U/232Th

Isotopic ratios Age (Ma)207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ

B5ZA14 206 147 4.75 1.40 0.72 0.0474 0.0050 0.1268 0.0130 0.0194 0.0004 71 225 121 12 124 215 264 203 6.16 1.30 0.77 0.0504 0.0027 0.1333 0.0073 0.0192 0.0003 214 100 127 7 122 2

B6ZA1 508 356 11.6 1.43 0.70 0.0510 0.0020 0.1427 0.0057 0.0203 0.0003 241 70 135 5 130 22 351 260 8.18 1.35 0.74 0.0524 0.0046 0.1404 0.0120 0.0194 0.0003 305 199 133 11 124 23 373 150 6.48 2.49 0.40 0.0461 0.0033 0.1233 0.0086 0.0194 0.0003 157 118 8 124 24 494 338 11.1 1.46 0.68 0.0516 0.0020 0.1425 0.0056 0.0202 0.0003 266 66 135 5 129 25 359 269 8.42 1.33 0.75 0.0559 0.0027 0.1498 0.0067 0.0199 0.0003 448 74 142 6 127 26 448 331 10.5 1.35 0.74 0.0534 0.0024 0.1469 0.0066 0.0202 0.0003 345 77 139 6 129 27 498 344 11.4 1.45 0.69 0.0461 0.0035 0.1271 0.0096 0.0200 0.0003 1 171 121 9 128 28 432 311 9.69 1.39 0.72 0.0519 0.0023 0.1388 0.0058 0.0197 0.0003 281 70 132 5 126 29 421 350 10.8 1.20 0.83 0.0505 0.0023 0.1401 0.0063 0.0203 0.0003 219 81 133 6 129 210 623 485 14.6 1.29 0.78 0.0495 0.0020 0.1324 0.0052 0.0195 0.0002 170 70 126 5 125 111 241 105 4.3 2.30 0.43 0.0543 0.0106 0.1430 0.0278 0.0191 0.0004 385 399 136 25 122 312 457 392 11.5 1.17 0.86 0.0461 0.0029 0.1214 0.0076 0.0191 0.0002 140 116 7 122 113 3374 998 43.8 3.38 0.30 0.0525 0.0016 0.1404 0.0044 0.0193 0.0002 309 51 133 4 123 114 461 400 12.2 1.15 0.87 0.0487 0.0019 0.1359 0.0051 0.0204 0.0003 134 64 129 5 130 215 305 255 7.68 1.20 0.84 0.0520 0.0024 0.1405 0.0061 0.0202 0.0003 285 73 133 5 129 216 580 516 15.2 1.12 0.89 0.0493 0.0019 0.1339 0.0050 0.0199 0.0002 160 65 128 4 127 117 690 565 16.9 1.22 0.82 0.0485 0.0019 0.1347 0.0051 0.0203 0.0002 125 65 128 5 130 218 337 278 8.08 1.21 0.82 0.0507 0.0026 0.1338 0.0066 0.0193 0.0003 226 89 127 6 123 219 431 463 13.3 0.93 1.07 0.0487 0.0042 0.1336 0.0114 0.0199 0.0003 131 196 127 10 127 220 582 485 15.8 1.20 0.83 0.0517 0.0025 0.1405 0.0065 0.0199 0.0003 272 80 133 6 127 221 681 343 12.6 1.99 0.50 0.0531 0.0039 0.1490 0.0128 0.0197 0.0004 333 155 141 11 126 322 496 358 11.1 1.38 0.72 0.0461 0.0023 0.1214 0.0060 0.0191 0.0002 110 116 5 122 123 380 322 9.47 1.18 0.85 0.0508 0.0023 0.1367 0.0062 0.0195 0.0002 231 82 130 6 124 224 448 363 11.1 1.24 0.81 0.0513 0.0022 0.1405 0.0062 0.0201 0.0003 253 76 133 5 128 2

626 S. Liu et al. / Lithos 113 (2009) 621–639

elements. Analyses of international standards OU-6 and GBPG-1 are inagreement with recommended values (Table 3).

3.3. Platinum-group elements (PGE)

PGE were measured by isotope dilution (ID)-ICP-MS after diges-tion of samples using a modified Carius tube technique (Qi et al.,2007) at the State Key Laboratory of Ore Deposit Geochemistry, IGCAS.The details of the analytical procedure have been described by Qi andZhou (2008) and Qi et al. (2008). The results for reference standards,WGB-1 (gabbro) and TDB-1 (diabase), are shown in Table 4. PGE

Table 2Major oxides (wt.%) of the studied dolerite-porphyries.

Sample Rock type Series SiO2 Al2O3 Fe2O3 MnO

B6-7 Dolerite-porphyry Low-K 48.39 14.67 11.26 0.15B6-6 Dolerite-porphyry Low-K 48.28 14.59 11.41 0.16B6-5 Dolerite-porphyry Low-K 47.5 13.85 11.82 0.16B6-4 Dolerite-porphyry Low-K 47.1 14.20 11.95 0.18B6-3 Dolerite-porphyry Low-K 47.09 13.92 11.82 0.17B6-2 Dolerite-porphyry Low-K 46.32 14.2 11.61 0.10B6-1 Dolerite-porphyry Low-K 48.7 14.33 11.91 0.15B5-2 Dolerite-porphyry High-K 50.62 16.37 6.91 0.15B5-1 Dolerite-porphyry High-K 50.9 16.48 8.96 0.15B4-3 Dolerite-porphyry High-K 50.69 16.20 9.03 0.15B4-2 Dolerite-porphyry High-K 50.52 16.14 9.12 0.15B4-1 Dolerite-porphyry High-K 50.66 16.17 9.65 0.18B3-3 Dolerite-porphyry High-K 50.37 16.08 9.01 0.16B3-2 Dolerite-porphyry High-K 50.69 16.06 8.98 0.15B3-1 Dolerite-porphyry High-K 50.83 16.09 8.96 0.15B1-3 Dolerite-porphyry High-K 50.63 15.94 9.43 0.12B1-2 Dolerite-porphyry High-K 49.32 14.68 10.74 0.11B1-1 Dolerite-porphyry High-K 50.1 15.47 9.59 0.11GSR-3 RV⁎ 44.64 13.83 13.4 0.17GSR-3 MV⁎ 44.68 13.98 13.37 0.17GSR-1 RV⁎ 72.83 13.4 2.14 0.06GSR-1 MV⁎ 72.76 13.43 2.16 0.06

LOI=Loss on Ignition. RV⁎: recommended values;MV⁎: measured values; the values for GSR-1 fatomic ratio.

contents for WGB-1 and TDB-1 agree well with the recommendedvalues reported by Meisel and Moser (2004).

3.4. Sr–Nd–Pb isotopic analyses

For Rb–Sr and Sm–Nd isotopic analysis, sample powders werespiked with mixed isotope tracers, dissolved in Teflon capsules withHF+HNO3 acids, and separated by conventional cation-exchangetechniques. Isotopic measurements were performed on a FinniganMAT-262 thermal ionizationmass spectrometer (TIMS) at the IsotopicGeochemistry Laboratory of Yichang Institute of Geology andMinerals

MgO CaO Na2O K2O P2O5 TiO2 LOl Total Mg#

8.26 9.15 3.41 1.3 0.3 1.62 1.19 99.71 628.32 9.19 3.34 1.28 0.25 1.63 1.48 99.93 628.44 9.31 2.74 1.0 0.34 1.61 2.66 99.42 618.39 9.42 2.74 1.14 0.40 1.6 2.92 100.04 618.28 9.22 2.7 1.08 0.35 1.62 3.78 100.03 617.84 8.59 2.79 0.59 0.22 1.44 3.83 99.54 607.7 8.57 2.58 0.58 0.21 1.4 3.25 99.39 595.2 6.93 3.43 3.59 0.69 1.26 2.52 99.68 565.17 7.00 3.44 3.66 0.70 1.27 2.13 99.85 565.12 7.04 3.11 3.62 0.69 1.29 2.98 99.91 565.35 7.18 3.02 3.39 0.71 1.3 2.62 99.50 565.27 7.09 3.09 3.4 0.68 1.25 2.33 99.77 555.19 7.09 2.99 3.33 0.69 1.27 2.89 99.06 565.06 6.84 3.09 3.26 0.69 1.28 3.12 99.22 555.41 7.0 3.01 3.27 0.69 1.29 3.05 99.75 575.57 7.25 3.19 3.48 0.69 1.25 1.77 99.32 575.97 7.95 3.38 3.64 0.68 1.26 1.73 99.47 556.1 7.90 2.85 3.64 0.67 1.28 2.01 99.73 587.77 8.81 3.38 2.32 0.95 2.37 2.24 99.887.75 8.82 3.26 2.31 0.96 2.36 2.15 99.810.42 1.55 3.13 5.01 0.09 0.29 0.7 99.620.43 1.57 3.16 5.02 0.1 0.29 0.71 99.69

romGovindaraju (1994) andGSR-3 fromWang et al. (2003).Mg#=100⁎Mg/(Mg+∑Fe)

Table 3The trace elements analysis results (ppm) for the studied dolerite-porphyries.

Sample B6-7 B6-6 B6-5 B6-4 B6-3 B6-2 B6-1 B5-2 B5-1 B4-3 B4-2 B4-1 B3-3 B3-2 B3-1 B1-3 B1-2 B1-1 OU-6 (RV⁎) OU-6 (MV⁎) GBPG-1 (RV⁎) GBPG-1 (MV⁎)

Sc 20.6 14.4 21.4 20.6 20.6 21.0 20.3 14.7 14.2 14.7 15.9 15.4 14.7 15.1 15.3 20.1 20.1 20.5 22.1 21.5 13.9 14.3V 174 123 100 162 182 178 166 138 135 139 142 141 138 138 141 153 152 150 129 129 96.5 101Cr 248 136 220 212 211 216 206 139 134 144 150 152 158 141 143 450 460 481 70.8 72.7 181 180Co 47.5 27.8 44.6 47.1 43.7 45.2 44.8 21.9 20.9 22.5 23.4 22.9 22.3 22.5 22.7 33.2 32.2 33.1 29.1 28.6 19.5 20.1Ni 181 912 154 144 150 150 145 43.2 42.9 48.8 49.8 50.5 59.1 47.8 48.7 161 159 164 39.8 38.7 59.6 59.7Ga 15.8 16.3 16.6 16.5 16.0 17.6 16.0 17.1 16.7 16.1 16.5 16.9 16.2 17.1 16.8 14.7 14.5 14.1 24.3 24.1 18.6 19.2Rb 20.6 43.2 25.6 6.0 23.9 16.2 12.8 83.5 84.0 86.4 80.1 79.0 76.2 75.9 73.5 73.5 101 103 120 I09 56.2 55.8Sr 517 502 539 388 515 486 362 1030 1003 1145 1209 1162 1091 1086 1102 1559 779 801 131 128 364 367Y 16.6 14.6 18.7 17.3 17.7 18.0 17.1 22.7 21.7 21.7 22.0 22.1 21.2 22.6 21.7 16.8 16.5 16.0 27.4 26.9 18.0 18.4Zr 112 93 126 84 115 114 85 227 222 217 213 220 214 219 211 149 146 142 174 176 232 232Nb 24.4 19.6 27.4 9.6 26.3 19.6 10.2 17.1 16.8 15.9 15.8 16.1 15.6 16.0 15.6 8.08 7.75 7.68 14.8 14.7 9.93 10.2Cs 0.84 1.63 1.49 0.26 1.69 0.96 0.97 1.31 1.25 3.16 2.73 2.75 4.29 2.45 2.47 4.46 6.93 6.98 8.02 7.79 0.32 0.32Ba 434 1089 483 297 504 413 321 2396 2369 2541 2643 2476 2687 2420 2437 1889 3404 3333 477 468 908 916La 18.9 16.1 20.6 10.1 20.2 17.3 11.1 62.9 62.0 58.9 58.9 59.3 59.9 58.9 59.6 56.1 55.7 55.0 33.0 31.5 53.0 55.0Ce 37.5 29.8 40.4 21.1 40.1 34.0 22.7 127 128 120 123 122 123 123 123 110 107 106 74.4 73.0 103 98.7Pr 4.27 3.53 4.58 2.64 4.62 3.97 2.79 13.9 13.9 13.2 13.4 13.4 13.6 13.6 13.48 11.7 11.5 11.5 7.80 7.60 11.5 11.9Nd 17.2 14.1 18.6 11.7 18.7 16.4 12.0 51.2 51.8 49.1 50.0 50.1 50.3 49.8 50.8 43.3 42.3 42.4 29.0 27.8 43.3 43.1Sm 3.72 3.13 4.11 3.09 3.99 3.74 3.15 8.38 8.34 8.05 8.16 8.15 8.18 8.05 8.42 7.04 6.91 7.03 5.92 5.83 6.79 6.97Eu 1.17 1.15 1.25 1.05 1.26 1.10 1.04 1.62 1.50 1.41 1.40 1.46 1.44 1.47 1.51 1.45 0.96 0.96 1.36 1.32 1.79 1.84Gd 3.37 2.90 3.71 3.32 3.62 3.46 3.27 4.75 4.84 4.87 4.68 4.72 4.71 4.62 4.88 4.14 4.61 4.28 5.27 5.24 4.74 4.75Tb 0.56 0.45 0.60 0.56 0.59 0.59 0.56 0.77 0.74 0.73 0.75 0.75 0.74 0.74 0.77 0.61 0.59 0.57 0.85 0.83 060 O.63Dy 3.41 2.76 3.65 3.51 3.69 3.67 3.39 4.51 4.51 4.29 4.38 4.44 4.34 4.44 4.43 3.45 3.40 3.46 4.99 4.85 3.26 3.18Ho 0.64 0.53 0.70 0.67 0.69 0.72 0.67 0.83 0.83 0.80 0.82 0.83 0.82 0.82 0.82 0.63 0.62 0.62 1.01 1.04 0.69 0.71Er 1.74 1.46 1.87 1.82 1.80 1.94 1.82 2.37 2.38 2.27 2.32 231 2.32 2.32 2.32 1.79 1.74 1.75 2.98 2.89 2.01 2.03Tm 0.23 0.29 0.24 0.24 0.24 0.26 0.24 0.33 0.31 0.30 0.31 0.31 0.31 031 0.31 0.23 0.24 0.23 0.44 0.41 0.30 0.31Yb 1.44 1.28 153 1.48 1.53 1.68 1.52 2.03 2.05 1.97 1.98 2.04 1.99 1.99 2.04 1.50 1.50 1.52 3.00 2.97 2.03 2.10Lu 0.20 0.19 0.21 0.22 0.22 0.24 0.23 0.30 0.31 0.29 0.29 0.29 0.30 0.31 0.30 0.23 0.23 0.22 0.45 0.44 0.31 0.31Hf 2.68 2.39 2.95 2.30 2.91 2.93 2.32 4.94 4.99 4.81 4.76 4.83 4.81 4.88 4.77 3.56 3.39 3.54 4.70 4.82 6.07 5.92Ta 1.45 1.20 1.61 0.61 1.61 1.22 0.63 0.98 1.01 0.96 0.95 0.96 0.96 0.94 0.95 0.49 0.46 0.49 1.06 1.02 0.40 0.39Pb 10.6 17.9 4.4 2.6 10.4 10.6 13.3 13.2 18.0 15.1 14.6 13.7 16.3 13.8 12.8 14.4 13.5 16.1 28.2 27.9 14.1 13.6Th 3.17 2.54 3.47 1.43 3.42 3.02 1.63 9.79 10.07 9.81 9.90 10.16 10.01 10.15 9.82 9.15 9.02 9.18 11.5 10.7 11.2 11.4U 0.61 0.95 0.63 0.45 0.64 0.70 0.31 1.57 1.59 1.53 1.53 1.59 1.65 1.63 1.52 1.40 1.38 1.41 1.96 1.95 0.90 0.87Eu/Eu⁎ 1.00 1.16 0.98 1.01 1.02 0.93 0.99 0.78 0.72 0.69 0.69 0.72 0.71 0.73 0.72 0.82 0.52 0.53Nb/Ta 16.7 16.4 17.0 15.9 16.4 16.1 16.1 17.5 16.6 16.6 16.6 16.8 16.3 16.9 16.4 16.6 16.7 15.8Zr/Hf 41.9 39.1 42.6 36.6 39.7 39.0 36.5 46.1 44.6 45.1 44.6 45.5 44.5 44.9 44.2 42.0 43.0 40.1Ti/Y 584 668 517 554 550 479 492 332 350 357 355 339 359 340 365 447 458 480Th/Y 0.19 0.17 0.19 0.08 0.19 0.17 0.10 0.43 0.46 0.45 0.45 0.46 0.47 0.45 0.45 0.55 0.55 0.57Ta/La 0.08 0.07 0.08 0.06 0.08 0.07 0.06 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01Rb/Sr 0.04 0.09 0.05 0.02 0.05 0.03 0.04 0.08 0.08 0.08 0.07 0.07 0.07 007 0.07 0.05 0.13 0.13

RV⁎: recommended values; MV⁎: measured values. The values for GBPG-1 from Thompson et al. (2000), and for OU-6 from Potts and Kane (2005).

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Table 4PGE analysis results for the representative dolerite-porphyries.

Sample no. B1-1 B1-2 B3-1 B3-2 B4-1 B4-2 B5-1 B5-2 B6-1 B6-2 WGB-1 (MV⁎) WGB-1 (RV⁎) TDB-1 (MV⁎) TDB-1 (RV⁎)

Average N=6 Average N=6

Ir (ng/g) 0.008 0.011 0.003 0.035 0.026 0.005 0.015 0.006 0.038 0.028 0.18±0.02 0.211 0.078±0.01 0.075Ru (ng/g) 0.022 0.021 0.016 0.043 0.019 0.026 0.029 0.022 0.029 0.013 0.14±0.01 0.144 0.21±0.02 0.198Rh (ng/g) 0.025 0.021 0.010 0.041 0.014 0.015 0.012 0.013 0.011 0.006 0.21±0.02 0.234 0.48±0.02 0.471Pt (ng/g) 0.519 0474 0,166 0.645 0.227 0.264 0.280 0.267 0.091 0.066 6.26±0.58 6.1 5.12±0.23 5.01Pd (ng/g) 0.534 0.736 0419 4.427 2.451 0.457 1.447 0.559 1.646 1.197 133±1.2 13.9 23.4±1.1 24.3Ni (ppm) 164 159 487 47.8 50.5 49.8 42.9 43.2 145 150Cu (ppm) 324 31.6 24.4 21.6 21.6 21.8 22.4 21.7 58.3 55.4Cu/Ni 0.20 0.20 0.50 0.45 0.43 0.44 0.52 0.50 0.40 0.37Cu/Pd 60,716 42,880 58,183 4877 8800 47,409 15,501 38,836 35,430 46,327Pd/Ir 667 66.9 140 125 94 91.4 96.5 93.2 43.3 43Pt/Pd 0.97 0.64 0.44 0.15 0.09 0.58 0.19 0.48 0.06 0.05

RV⁎: recommended values; MV⁎: measured values; WGB-1 and TDB-1 are from Meisel and Moser (2004).

628 S. Liu et al. / Lithos 113 (2009) 621–639

resources. Procedural blanks were b200 pg for Sm and Nd and b500 pgfor Rb and Sr. The mass fractionation corrections for Sr and Nd isotopicratios were based on 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219,respectively. Analyses of standards during the period of analysis areas follows: NBS987 gave 87Sr/86Sr=0.710246±16 (2σ); La Jolla gave143Nd/144Nd=0.511863±8 (2σ). Lead was separated and purified byconventional cation-exchange techniques (AG1×8, 200–400 resin)with dilutedHBr as the eluant. Analyses of NBS-981 during the period ofanalysis yielded 204Pb/206Pb=0.0896±15, 207Pb/206Pb=0.9145±8,and 208Pb/206Pb=2.162±2. Our analytical results for Sr–Nd–Pbisotopes are presented in Table 5.

4. Results

4.1. Zircon cathodoluminescence images and U–Pb data

Zircon is relatively abundant in the dolerite-porphyries. Prior toLA-ICP-MS zircon U–Pb dating, the surfaces of the grain mountswerewashed in dilute HNO3 and pure alcohol to remove any potentiallead contamination. Zircon is relatively abundant in the dolerite-porphyries. Prior to LA-ICP-MS zircon U–Pb dating, the surfaces of thegrain mounts werewashed in dilute HNO3 and pure alcohol to removeany potential lead contamination. Zircons selected from five dolerite-porphyries samples (B1ZA, B3ZA, B4ZA, B5ZA and B6ZA) are euhedral,

Table 5Sr–Nd–Pb isotopic ratios for the representative dolerite-porphyries.

Sample no. B6-4 B6-1 B5-2 B5-1 B4-3

Sm (ppm) 3.09 3.15 8.36 8.33 7.97Nd (ppm) 11.6 11.9 51.5 51.8 50.1147Sm/144Nd 0.1610 0.1607 0.0981 0.0972 0.09143Nd/144Nd 0.511841 0.511849 0.511788 0.511784 0.5112σ 12 10 9 9 10(143Nd/144Nd)i 0.511555 0.511564 0.511706 0.511703 0.511εNd(t) −15.0 −14.8 −15.0 −15.1 −15Rb (ppm) 6.02 12.8 82.8 83.9 85.7Sr (ppm) 390 364 1026 1005 113987Rb/86Sr 0.0446 0.1016 0.2332 0.2413 0.2187Sr/86Sr 0.709672 0.709601 0.709895 0.709924 0.702σ 11 14 10 12 13(87Sr/86Sr)i 0.709593 0.709421 0.709474 0.709489 0.70206Pb/204Pb 17.537 17.728 17.498 17.551 17.59207Pb/204Pb 15.542 15.549 15.481 15.476 15.4208Pb/204Pb 38.109 37.921 37.933 37.928 37.9(206Pb/204Pb)i 17.329 17.700 17.351 17.441 17.47(207Pb/204Pb)i 15.532 15.548 15.474 15.471 15.4(208Pb/204Pb)i 37.890 37.872 37.633 37.701 37.7

Chondrite Uniform Reservoir (CHUR) values (87Rb/86Sr=0.0847, 87Sr/86Sr=0.7045,λRb=1.42×10−11 year−1 (Steiger and Jäger, 1977); λSm=6.54×10−12 year−1 (Lugmair

colourless and transparent, mostly elongate-prismatic, and range upto 100 µm in diameter. The majority exhibit oscillatory or planar zoningunder cathodoluminescence, a typical feature of magmatic zircon.Selected zircon cathodoluminescence images are given in Fig. 3. Somegrains exhibit zoning with apparently rounded or irregular cores,mantled by euhedral overgrowths which also have oscillatory zoning(not shown). The studied zircons all have relatively high Th/U ratios(0.96–3.38), suggestive of a magmatic origin. On the basis ofcathodoluminescence images and Th/U ratios, an igneous origin forthe zircons is evident. The U–Pb zircon data are presented in Table 1.Analyses of zircon grains with oscillatory structures were concordantand yielded the weighted mean 206Pb/238U age of 124.8±1.5 Ma(n=18) for B1ZA, 126.9±1.7 Ma (n=19) for B3ZA, 122.5±1.5 Ma(n=17) for B4ZA, 123.7±2.2 Ma (n=15) for B5ZA, and 125.4±1.1 Ma(n=25) for B6ZA (Fig. 3). These ages are interpreted as the crystal-lisation ages of the dolerite-porphyries.

4.2. Major and trace elements

Geochemical data for the dolerite-porphyries are listed in Tables 2and 3. The mafic dykes are characterised by slight variation in SiO2

(47.1–50.83 wt.%), Na2O (2.58–3.44 wt.%) and TiO2 (1.25–1.63 wt.%),whereas they have large range of Al2O3 (13.85–16.48 wt.%), Fe2O3

(8.91–11.95 wt.%), MnO (0.11–0.18 wt.%), MgO (5.06–8.44 wt.%),

B4-1 B3-3 B3-1 B1-3 B1-1

8.13 8.18 8.41 7.03 6.9650.2 49.6 51.2 43.4 42.1

62 0.0979 0.0997 0.0993 0.0979 0.0999785 0.511781 0.511774 0.511779 0.511846 0.511843

11 8 8 9 10708 0.511610 0.511693 0.511698 0.511672 0.511761.1 −15.2 −15.3 −15.2 −13.9 −14.0

78.5 76.3 78.3 73.2 1011161 1093 1098 1559 798

74 0.1954 0.2017 0.2061 0.1357 0.36589802 0.709791 0.709803 0.709811 0.710323 0.710318

10 10 11 8 109422 0.709450 0.709447 0.709448 0.710082 0.7096683 17.656 17.755 17.742 17.130 17.08796 15.508 15.513 15.509 15.493 15.50191 38.072 38.054 38.047 37.861 37.8581 17.516 17.632 17.597 17.013 16.98190 15.501 15.507 15.502 15.487 15.49635 37.779 37.810 37.741 37.610 37.632

147Sm/144Nd=0.1967 and 143Nd/144Nd=0.512638) are used for the calculation.and Harti, 1978).

Fig. 3. Representative cathodoluminescence images and the LA-ICP-MS concordia ages for the zircon grains from the mafic dykes. The numbers correspond to the spot analyses givenin Table 1.

629S. Liu et al. / Lithos 113 (2009) 621–639

630 S. Liu et al. / Lithos 113 (2009) 621–639

Mg#=55–62, K2O (0.58–3.66 wt.%) and P2O5 (0.21–0.71 wt.%)(Table 2). The dolerite-porphyries can be divided into two groups(high-K and low-K) in a K2O vs. Na2O diagram (Fig. 4). In detail, thehigh-K group dolerite-porphyries include samples B1-1-3, B3-1-3, B4-1-3 and B5-1-2, while the low-K dolerite-porphyries comprise sam-ples B6-1, B6-2, B6-3, B6-4, B6-5, B6-6 and B6-7 (Table 2).

On Harker diagrams, the two groups plot in separate fields (Fig. 5).Within the high-K dolerite-porphyries, positive correlations betweenMgO and Fe2O3, CaO, CaO/Al2O3 and other compatible elements like Ni(Fig. 5d–e, j) and negative correlations of MgO vs. Al2O3, Sr, Zr, Th andSiO2 are observed (Fig. 5a, c, g–i). The low-K dolerite-porphyries haveMgO varying negatively with SiO2 (Fig. 5a), positively with CaO, CaO/Al2O3 and TiO2 (Fig. 5b, e and f); no correction with respect to MgO isexhibited by Al2O3, Fe2O3, Sr, Zr, Th and Ni (Fig. 5c–d, g–i, j).

The two groups of dolerite-porphyries have distinct and differentchondrite- and primitive-mantle-normalised trace element patterns(Fig. 6). However, they are all significantly enriched in LREE and someLILEs, such as Rb, Ba and Sr, together with relatively constant in HREEs(i.e., Ho, Er, Tm, Yb and Lu) (Fig. 6). Furthermore, the high-K dolerite-porphyries are characterised by moderate or small negative Euanomalies (Eu/Eu⁎= 0.78–0.52, Table 3) (Fig. 6c), and depletion inU and some high field-strength elements (HFSEs), such as Nb, Ta andP, in primitive-mantle-normalised trace element patterns (Fig. 6d).All studied samples show sub-chondritic Nb/Ta ratios (15.8–17.5)(chondritic ratio: 19.9±0.6, Münker et al., 2003) and Zr/Hf frac-tionation with super-chondritic Zr/Hf ratios (36.6–46.1) (chondriticratio: 34.3±0.3, Münker et al., 2003).

4.3. Cu, Ni and PGE

The studied dolerite-porphyries have a wide range of Cu, Ni andPGE contents, with Cu=16.0–58.3 ppm, Ni=42.9–164 ppm,Ir=0.003–0.38 ppb, Ru=0.013–0.043 ppb, Rh=0.006–0.025 ppb,Pt=0.066–0.519 ppb and Pd=0.328–4.427 ppb. Accordingly, theyshow large variation in Cu/Ni (0.2–0.52), Cu/Pd (4,877–60,718), Pd/Ir(43–140) and Pt/Pd (0.05–1.11) ratios (Table 4). In general, the low-Kdolerite-porphyries contain higher Cu, Ni, Ir and Pd, but lower Ptcontents than the high-K dolerite-porphyries, resulting in their rela-tively high Cu/Pd and low Pd/Ir and Pt/Pd ratios (Table 4; Fig. 7a–c). Allthe rocks show depletion (negative anomalies) of the Ir-subgroup PGE(IPGE) (Ir and Ru) relative to the Pd-subgroup PGE (PPGE) (Rh, Pt andPd) in the primitive-mantle-normalised chalcophile element diagram(Fig. 8). Furthermore, except for two samples (B3-2, B4-1) with higherPd contents, each group exhibits relatively smooth, steeply slopingpatterns (Fig. 8).

Fig. 4. Na2O vs. K2O plots for rock series classification.

4.4. Sr, Nd and Pb isotopes

Strontium, Nd and Pb isotopic data have been obtained fromrepresentative samples of the dolerite-porphyries (Table 5). Thesedykes have relatively constant initial 87Sr/86Sr ratios (0.709 to 0.710),and negative εNd(t) values (−15.3 to −13.9), which suggest acommon source region. In addition, the Sr–Nd isotopic ratios aresimilar to those of the Fangcheng basalts (Zhang et al., 2002), all plotwithin the mantle array and the field of the mafic dykes fromShandong Province (Liu, 2004; Yang et al., 2004; Liu et al., 2006) in aplot of εNd(t) vs. (87Sr/86Sr)i (Fig. 9).

The dolerite-porphyries have relatively constant Pb isotopic ratios(206Pb/204Pb=16.98–17.70, 207Pb/204Pb=15.47–15.55, 208Pb/204Pb=37.61–37.89). The Pb isotopic ratios are significantly differentfrom those of the mafic rocks from Yangtze Craton (Yan et al., 2003),but are comparable to those of mafic rocks from the North ChinaCraton (Zhang et al., 2004; Xie et al., 2006) (Fig. 10a, b).

5. Discussion

5.1. Crustal contamination

Crustal contamination might cause significant depletion in Nb–Taand highly enriched Sr–Nd isotopic signatures in basaltic rocks (Guoet al., 2004). As the high-K dolerite-porphyries are characterised bynegative Nb–Ta anomalies, high Sr isotopic composition and negativeεNd(t), this could imply a crustal component to the magma genesis ofthe mafic dykes. Further support for this is given in their low Ta/Laratios (0.01–0.02) (cf. Ta/La=0.06 for primitive mantle; Wood et al.,1979). Nevertheless, crustal assimilation would cause significantvariation in Sr–Nd isotopes, and result in a positive correlationbetween MgO and εNd(t) values, and negative correlation betweenMgO and (87Sr/86Sr)i ratios, features not observed in these dolerite-porphyries. This suggests that they have not been significantlyaffected by crustal contamination. In contrast, the low-K dolerite-porphyries have relative enrichment of Nb and Ta that cannot beexplained by crustal contamination as crustal materials are typicallypoor in Nb and Ta (Rudnick and Gao, 2003). In addition, the low-Kdolerite-porphyry rocks have relatively uniform Ta/La ratios (0.06–0.08) with primitive mantle also indicating insignificant crustalcontamination (Song et al., 2006). This is consistent with theirpositive Ti anomalies, as well as lower Rb (6–43 ppm), Th (1.63–3.47 ppm) and U (0.31–0.95 ppm) relative to the upper crust(Rb=84 ppm, Th=10.5 ppm, U=2.7 ppm; Rudnick and Gao,2003). In summary, the geochemical and isotopic signatures of thetwo groups of dolerite-porphyry dykes appear mainly to be derivedfrom an enriched mantle source.

5.2. Source regions and partial melting

The dolerite-porphyry dykes have low SiO2 (47.1–50.83 wt.%)suggesting derivation from an ultramafic mantle source rather thanfrom the crust; partial melting of any of the crustal rocks (e.g.,Hirajima et al., 1990; Yang et al., 1993; Zhang et al., 1994, 1995a,b; Katoet al., 1997) and lower crustal intermediate granulites (Gao et al.,1998a,b) present in the deep crust would have produced highly-siliceous magmas (i.e., granitoid liquids; Rapp et al., 2003). Further-more, the high initial 87Sr/86Sr ratios, negative εNd(t) values and Pbisotopic compositions (Table 5) for the dolerite-porphyries indicatethat these rocks originated from partial melting of an enriched,lithospheric mantle source beneath the North China Craton, ratherthan an asthenospheric mantle source with a depleted Sr–Nd isotopiccomposition, such as mid-ocean ridge basalt (MORB).

The studied dolerite-porphyries have La/Sm and Sm/Yb ratiosconsistent with derivation by partial melting of a garnet-lherzolite(Fig. 11a, b). This is further supported by their high Ti/Y ratios (332–

Fig. 5. Variation diagrams for major oxides and trace elements vs. MgO contents for the studied dolerite-porphyries.

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Fig. 6. Chondrite-normalised rare earth element patterns and primitive-mantle-normalised multi-element diagrams for the mafic dykes. REE abundances for chondrites and traceelement abundances for primitive mantle are after Sun and McDonough (1989).

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668) (Johnson, 1998). Furthermore, the high-K group was derivedfrom relatively low degrees of partial melting (3–5%) of garnet-lherzolite mantle; whereas the low-K dolerite-porphyries originatedfrom melts that possibly underwent 10–15% partial melting of thesource (Fig. 11a and b). Relatively high degrees of partial melting forthe low-K dolerite-porphyries are also supported by their high MgO,Cr and Ni contents (Tables 2 and 3).

It has been proposed that low degrees of partial melting canproduce an Rh-, Pd- and Pt-enriched melt with a relatively high Pd/Irratio, whereas higher degrees of melting produce magmas with lowerPd/Ir ratios (Crocket and Teruta, 1977; Alard et al., 2000). Accordingly,the low-K dolerite-porphyries with higher degrees of melting shouldhave relatively lower Pd/Ir ratios than high-K dolerite-porphyry dykesthat originated by lower degrees of mantle partial melting. Thisconfirms the modeling results on the basis of REE content and ratios(Fig. 11a and b).

5.3. Mantle metasomatism

The picrites of the Emeishan large igneous province have very lowTh/Y ratios (0.003–0.006, Wang et al., 2007), which are interpreted asrepresentative of the source conditions for the primitive, depletedmantle-derived magma. In contrast, the dolerite-porphyries in thisstudy contain relatively high Th/Y ratios (0.1 to 0.6) (Table 3), imply-ing a metasomatised mantle origin. The dolerite-porphyry dykes havesub-chondritic Nb/Ta ratios (15.8–17.5) (chondritic ratio: 19.9±0.6,Münker et al., 2003) and super-chondritic Zr/Hf ratios (36.6–46.1)(chondritic ratio: 34.3±0.3, Münker et al., 2003) analogous to thoseof the tephrites from the Emeishan large igneous province (Qi et al.,2008), which are believed generated from a mantle source that hadbeen metasomatised by a carbonate-rich fluid. We suggest, therefore,that the primary magmas of the dolerite-porphyries were derivedfrom amantle sourcewhich had undergone carbonatemetasomatism;

an origin further supported by our petrological studies (Fig. 2). Alter-natively, amphibole and phlogopite as the twomost common volatile-bearing minerals may play a major role in creating lithosphericsources enriched in large ion lithosphile elements (e.g., K, Rb and Ba)(e.g., Foley et al., 1996; Ionov et al., 1997; Grégoire et al., 2000). Ingeneral, amphibole has Rb/Sr ratios lower than or close to that ofprimitive mantle; its presence cannot yield whole-rock Rb/Sr ratioshigher than the primitive-mantle value of ~0.03 (Ionov et al., 1997).On the other hand, phlogopite possesses a relatively high Rb/Sr ratio(0.13–60) (Ionov et al., 1997), so that phlogophite metasomatism canresult in much higher Rb/Sr ratios than that of primitive mantle.Accordingly, the high Rb/Sr ratios (0.05–0.13) of the high-K dolerite-porphyries suggest a predominance of phlogopite rather thanamphibole in the melt source. In contrast, phlogopite and amphibole(e.g., low-Mg, Tiepolo et al., 2000; Foley et al., 2002; Jörg et al., 2007)both are essential components in the source of the low-K dolerite-porphyry dykes that have low Rb/Sr ratios of 0.02–0.09 (Ionov et al.,1997).

5.4. Crystal fractionation

The low-K dolerite-porphyries are interpreted as originatingfrom a relatively primitive magma as evidenced by high MgO andcompatible elements like Cr, Co and Ni contents; whereas the high-Kgroup contains lower MgO, Cr, Ni and Co (Tables 2 and 3), implyingthat they were derived from a more evolved magma. Within the high-K dolerite-porphyries there are positive correlations between MgOand Fe2O3, CaO, CaO/Al2O3 and Ni (Fig. 5d–e, j) and negative corre-lations for MgO vs. SiO2, Al2O3 and Sr (Fig. 5c, g), suggesting olivine,clinopyroxene, hornblende and plagioclase fractionation. Likewise,the separation of plagioclase, Ti-bearing phases (rutile, ilmenite,titanite, etc.) and apatite could account for the observed negativeEu, Nb, Ta and P anomalies in chondrite-normalised REE patterns and

Fig. 7. Cu/Ni vs. Cu/Pd (a) and Pd/Ir (b), and Pt/Pd vs. Pd/Ir (c) diagrams showing thevariations of chalcophile elements for the studied mafic dykes.

Fig. 8. Primitive mantle-normalised PGE patterns of the studied dolerite-porphyries.The PGEs abundances for primitive mantle are from Sun and McDonough (1989).

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primitive-mantle-normalised trace element patterns (Fig. 6c and d).The low-K dolerite-porphyries have MgO which shows a negativecorrelation with SiO2 (Fig. 5a) and a positive correlation with CaO,CaO/Al2O3 and TiO2 (Fig. 5b, e and f). These trends are considered to berelated to fractionation of clinopyroxene, hornblende and Ti-bearingphases (e.g., rutile, ilmenite, titanite). Nevertheless, crystal fractiona-tion of rutile can be ruled out, as fractionation of even low amounts ofrutile would produce elevated Nb/Ta and Zr/Hf ratios with stronglydecreasing Nb and Zr concentrations (Jörg et al., 2007), which are notobserved. In addition, in plots of Sr vs. Ba and Rb (Fig. 12a, b),fractionation of plagioclase in magmas parental to high-K dolerite-porphyries and clinopyroxene, olivine and hornblende in parentalmelts to low-K dolerite-porphyries is observed.

PGE are generally subdivided into compatible IPGE (Os, Ir and Ru)and incompatible PPGE (Rh, Pd and Pt) elements during fractionationof mafic magmas (Qi and Zhou, 2008). The ratio of Cu/Pd is a usefulindicator to the degree of S-saturation of magmas, because the par-tition coefficient for Pd (about 3×104) is much higher than that for Cu

(about 4×103) between immiscible sulphide liquid and silicatemagma (e.g., Barnes and Maier, 1999). In general, the Cu/Pd ratiosshould increase if sulphide is fractionated from magma (Qi et al.,2008). The dolerite-porphyries have Cu/Pd ratios varying from 8800to 60,718, with one exception (Table 4), which is relatively highcompared to primitive mantle (~7000, Barnes and Maier, 1999), butare comparable with those of high-Ti basalts in the Emeishan largeigneous province, such as the rocks in Jinping (Cu/Pd: 19,000–90,000,Wang et al., 2007), the rocks in Longzhoushan (Cu/Pd: 8500–86,000,Qi et al., 2008), and the rocks in Heishitou (Cu/Pd: 6700–45,000, Qiand Zhou, 2008). On the basis of previous studies on these high-Tibasalts (Wang et al., 2007; Qi et al., 2008; Qi and Zhou, 2008), wepropose that the dolerite-porphyries also crystallised from S-under-saturated melts.

The crystallisation of laurite and Os–Ir–Ru alloys in mafic magmashas been confirmed by experimental work (Hiemstra, 1979; Merkle,1992; Brenan and Andrews, 2001; Bockrath et al., 2004; Righter et al.,2004). Generally, early precipitation of laurite and Ru–Os–Ir alloy thatare enclosed in chromite and olivine (Stockman,1984) will cause IPGEdepletion and PPGE enrichment in S-undersaturated melts (Qi andZhou, 2008). The steep primitive-mantle-normalised PGE patternsof the dolerite-porphyries clearly indicate the melts were S-under-saturated and may have undergone early fractionation of chromiteand/or olivine (Fig. 8). On the other hand, the crystallisation of lauriteor Os–Ir–Ru alloys during the early stages of fractionation may cause anegative Ir and Ru anomaly on primitive-mantle-normalised chalco-phile element diagrams (Qi and Zhou, 2008). All of the dolerite-porphyries have such negative Ir and Ru anomalies (Fig. 8), which isfurther suggestive of early fractionation of chromite or olivine in theprimary magma.

Crocket and Paul (2004) reported lower Pt/Pd ratios in the DeccanTraps and suggested that crystal fractionation would result in frac-tionation of Pt and Pd, resulting in lower Pt/Pd ratios. The studieddolerite-porphyries have lower Pt/Pd ratios (0.05–0.97, average in0.37) than primitive mantle (~1.82, McDonough and Sun, 1995),which can be interpreted as the result of early fractionation of olivinefrom S-undersaturated melts. Because Pt has a larger partition coef-ficient than Pd between silicate magma and olivine phenocrysts(Momme et al., 2002) or between the fractionating assemblage(olivine±PGE-phases) and a komatiitic melt (Puchtel and Humayun,2000, 2001), Pt will be preferentially removed from a S-under-saturated magma undergoing silicate mineral fractionation (Mommeet al., 2002). Accordingly, the lower Pt/Pd ratios in these rocks mayreflect olivine fractionation in S-undersaturated melts. Such a processis also evidenced by the negative correlations between Cu/Ni and Cu/Pb and the positive correlations between Cu/Ni and Pd/Ir (Fig. 7a, b),

Fig. 9. Initial 87Sr/86Sr vs. εNd(t) diagram for the studied mafic dykes. MORBs and OIBs (after Zhang et al., 2002 and the references therein), Mantle array is from Zhang et al. (2005),Palaeozoic lithospheric mantle beneath the eastern North China Craton are from Zheng and Lu (1999), Fangcheng basalts are from Zhang et al. (2002), Jinan gabbros are from Guoet al. (2001), mafic rocks from North China Craton are from Guo et al. (2001, 2003), mafic rocks from Yangtze Craton are from Chen et al. (2001) and Li et al. (2004), mafic rocks fromNorth Dabie are from Li et al. (1998), Jahn et al. (1999), Fan et al. (2004) and Wang et al. (2005), mafic rocks from Sulu belt are from Fan et al. (2001) and Guo et al. (2004, 2005),mafic dykes from Shandong Province are from Liu (2004, 2008a,b), Yang et al. (2004) and Liu et al. (2006, 2008a,b). Also plotted are trends to lower crust (after Jahn et al., 1999).

Fig.10. 208Pb/204Pb (a) and 207Pb/204Pb (b) vs. 206Pb/204Pb diagrams for themafic dykes,compared with those of Early Cretaceous mafic rocks from the North China and Yangtzecratons. Fields for I-MORB (Indian MORB) and P and N-MORB (Pacific and NorthAtlantic MORB), OIB, NHRL and 4.55 Ga geochron are after Barry and Kent (1998), Zou etal. (2000), and Hart (1984), respectively. Data of North China craton are from Zhang etal. (2004) and Xie et al. (2006), Yangtze mafic rocks are after Yan et al. (2003).

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because both Ni and Ir are more compatible than Cu and Pd duringolivine fractionation (Keays,1995). Furthermore, the strongly negativecorrelation of Pd/Ir and Pt/Pd ratios in the dolerite-porphyries (Fig. 7c)indicates that the Pt/Pd ratios may be related to fractionation of non-sulphide phases.

5.5. Genetic model and process

Based on the above discussion, the mafic dykes in this study arederived from partial melting of an enriched mantle source. A dynamicmodel, however, is required to decipher the origin of the enrichedmantle. At least three competing mechanisms can be envisaged: (1)the enriched mantle beneath eastern North China Craton can beformed by addition of melts or melts and water from subductedYangtze crust (e.g., Zhang et al., 2004, 2005); (2) the melts fromsubducted ancient Pacific plate (i.e., Izanagi Plate) metasomatised andmodified the lithospheric mantle beneath eastern North China (e.g.,Chen et al., 2004); (3) lithospheric mantle beneath eastern NorthChina Craton was progressively enriched due to successive hybridismof foundered lower crust (e.g., Liu et al., 2008a,b).

The mafic dykes in this study have Pb isotopic characteristics(Table 5) that are distinct from those of the Yangtze Craton litho-sphere mantle (Yan et al., 2003; Fig. 10a, b), which rules out aninvolvement of Yangtze Craton lithosphere (Xie et al., 2006) in theirgenesis. In contrast, the distinctive Pb isotopic data suggest that themafic dykes were derived from the overlying North China Cratonduring the Late Mesozoic (Xie et al., 2006; Fig. 10). Additionally,evidence from C and O stable isotope geochemistry of the highpressure–ultra high pressure rocks in the Dabie–Sulu belt suggest thatvolatiles may not escape from the rocks during the rapid subduction ofthe Yangtze continental crust (Zheng et al., 2003; Zheng, 2005),therefore, the enrichedmantle beneath the North China Craton cannotbe formed by addition of melts or melts and water from a subductingYangtze crust. Hence, we suggest that the Sr, Nd and Pb isotopiccompositions of themafic dykes from the studied area are also not dueto the involvement of subducted Yangtze lithosphere.

In the case of subduction, the Pacific plate would likely releasefluids and/ormelts during its descent into themantle; as such it can beenvisaged that fluids/melts from the subduction of the palaeo-Pacific

Fig. 11. Plots of La/Sm vs. La and Sm/Yb vs. Sm showing melt curves (or lines) obtained using the non-modal batch melting equations of Shaw (1970). Melt curves are drawn forspinel-lherzolite (with mode and melt mode of ol0.530+opx0.270+cpx0.170+sp0.030 and ol0.060+opx0.280+cpx0.670+sp0.110; respectively; Kinzler, 1997) and for garnet-lherzolite(with mode and melt mode of ol0.600+opx0.200+cpx0.100+gt0.100 and ol0.030+opx0.160+cpx0.880+gt0.090; respectively; Walter, 1998). Mineral/matrix partition coefficients andDMM are from the compilation of McKenzie and O'Nions (1991, 1995); primitive mantle, N-MORB and E-MORB compositions are from Sun and McDonough (1989). Tick marks oneach curve (or line) correspond to degrees of partial melting for a given mantle source.

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Plate (i.e., the Izanagi Plate) metasomatised and modified the litho-spheric mantle beneath the eastern North China Craton (Chen et al.,2004). However, the Late Mesozoic was a period when the IzanagiPlate primarily moved towards the north or the north-northeast(Maruyama and Send, 1986; Kimura et al., 1990), thereby having littleinfluence on the origin of the mafic dykes in this study. Furthermore,no evidence has yet been presented suggesting a contribution of thepalaeo-Pacific Plate to the Mesozoic magmatism of eastern NorthChina Craton (Zhang et al., 2005). Moreover, recent U–Th disequili-brium studies of the Cenozoic potassium basalts from northeasternChina also argue against contributions from the palaeo-Pacific Plate(Zou et al., 2003). Work on Early Cretaceous mantle-derived rocksfrom the western North China Craton also indicates that the origin ofthe enriched lithospheric mantle sources for the Late Mesozoic rockswas unrelated to the subduction of palaeo-Pacific Plate (Wang et al.,2006; Ying et al., 2007).

Accordingly, foundering of continental lower crust is a most likelygenetic model for the origin of the mafic dykes in study area. Becauseof its higher density than that of lithospheric mantle peridotite by0.2–0.4 g cm−3 (Rudnick and Fountain, 1995; Jull and Kelemen,2001; Levander et al., 2006; Anderson, 2006), eclogite can be recy-cled into the mantle (Arndt and Goldstein, 1989; Kay and Kay, 1991;Jull and Kelemen, 2001; Gao et al., 2004). Eclogites have lowermelting temperatures than mantle peridotites (Yaxley, 2000; Kogisoet al., 2003; Sobolev et al., 2005, 2007), and so foundered, silica-

saturated eclogites can melt to produce silicic melts (tonalite totrondhjemite) that may hybridise variably with overlying mantleperidotite. Such reactions may produce an olivine-free pyroxenite,which, if subsequently melted, will generate basaltic melt (Kogisoet al., 2003; Sobolev et al., 2005, 2007; Herzberg, 2007; Gao et al.,2008).

The preference of delamination model is further supported by theintensive lithospheric thinning (Liu et al., 2008a,b,c,d), voluminouscoeval magmatism (130–120 Ma) (Wang et al., 1998; Guo, 1999; Guoet al., 2001; Qiu et al., 2001; Zhang et al., 2004; Yang et al., 2003, 2004;Liu et al., 2004, 2006, 2008a,b), large-scale mineralisation (Wanget al., 1998; Yang and Zhou, 2001; Qiu et al., 2002; Yang et al., 2003,2004) and the adakitic lavas observed in eastern North China Craton(Gao et al., 2004; Liu et al., 2008c,d), as lithospheric foundering wouldresult in all of these features. In addition, neither of the other twomodels can adequately explain the production of adakitic lavasfrom foundered lower crust coeval with the studied mafic dykes,as is observed in the Luxi and Jiaodong areas (Liu et al., 2008c,d).

We therefore suggest a model in which lower crustal delaminationcoincided with mafic magmatism (Fig. 13). During the Triassic (240–185Ma) (Zhang et al., 2005; Liu et al., 2008a,b), the continual collisionbetween the North China Craton and Yangtze Craton induced athickened crust (Fig. 13a) and eclogitisation of the lower portionsof this crust (Liu et al., 2008a,b); at about 185–165 Ma, founderingof eclogite from the lower, thickened crust occurred beneath the

Fig. 12. Plots of: (a) Ba vs. Sr, and (b) Rb vs. Sr for the dolerite-porphyries. Mineralfractionation vectors calculated using partition coefficients from Villemant et al. (1981)and McKenzie and O'Nions (1991). Tick marks indicate percentage of mineral phaseremoved, in 10% intervals. cpx: clinopyroxene, ol: olivine, pl: plagioclase, Bi: biotite, Hb:hornblende.

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Dabie–Sulu orogenic belt and eastern North China Craton (Li et al.,2002; Liu et al., 2008a,b) which triggered asthenospheric upwelling,sudden uplift of the Sulu belt, orogenic collapse and lithospheric

Fig. 13. Diagram illustrating tectonic evolution in southeastern North China Craton (modifiedCraton resulted in thick lithosphere (mantle and lower crust) and eclogitisation of the lowesubsequently, silicic melts originated bymelting of foundered eclogites which reacted extensthe hybridised lithospheric mantle producedmafic dykes, meanwhile, adakites originated froJiaodong.

extension and thinning. Subsequently, silicic melts, originated bymelting of the foundered eclogites and reacted extensively with theoverlying mantle peridotite (Fig. 13b). Between 130 and 120 Ma,decompressional melting of the hybridised lithospheric mantle pro-duced primary magma (basaltic melts), that underwent fractionationandminor contamination to produce mafic dyke swarms. The residualeclogitic, lower crust remaining from the delamination event becameheated at the base of the lithospheric mantle and melted to producethe adakitic lavas present in the Luxi and Jiaodong areas (Liu et al.,2008c,d) (Fig. 13c).

6. Conclusions

Zircon U–Pb dating has shown that the studied dolerite-porphyrieswere intruded between 122.5±1.5 Ma and 126.9±1.7 Ma. Two seriesof dolerite-porphyry dykes were derived from partial melting ofenriched mantle, induced by hybridism of foundered lower crust atmantle depths. One group, the low-K dolerite-porphyries originatedfrom melts that underwent 10–15% partial melting of a garnet-lherzolite source that had experienced metasomatism by carbonate-rich fluids producing secondary phlogopite and amphibole. In con-trast, the high-K dolerite-porphyry group was produced by relativelylow-degree partial melting (3–5%) of a garnet-lherzolite mantle,which had experienced metamorphism by a carbonate-rich fluid withcrystallisation of only secondary amphibole. Both mafic dyke suiteshave experienced minor, but unimportant, crustal contaminationduring magma ascent.

The relatively high PPGE and low IPGE concentrations of thesedolerite-porphyry dykes suggest derivation from S-undersaturatedmagmas. The high-K dolerite-porphyries were derived from a moreevolved magma, which had undergone olivine crystal fractionation.Clinopyroxene, hornblende, plagioclase, ilmenite, titanite, apatite,laurite and Os–Ir–Ru alloys may also have played an important role intheir origin. In contrast, the low-K group magma had experiencedseparation of olivine, clinopyroxene, hornblende, imenite, titanite,laurite and Os–Ir–Ru alloys prior to their emplacement.

after Liu et al., 2008a,b). (a) 240–185Ma: collision of the Yangtze block and North Chinar thickened crust; (b) 185–165 Ma: delamination of the eclogitic lower crust occurred,ively with the overlying mantle peridotite; (c) 130–120 Ma, decompressional melting ofm partial melting of residual lower crust in the lithospheric mantle occurred in Luxi and

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Acknowledgements

The authors would like to thank Andrew Kerr, Derek Wyman andone anonymous reviewer for their constructive reviews on an earlierversion of this manuscript. We also thank Ruud Koole for quickeditorial handling of the manuscript. This research was supported bythe National Nature Science Foundation of China (40673029,40773020, 90714010, 40634020 and 40521001), Chinese Ministry ofEducation (B07039). We are grateful to Gui-xiang Yu for help with theSr, Nd and Pb isotopic analysis, and Yong-sheng Liu and Zhao-Chu Huare thanked for help with zircon U–Pb dating. The authors also thankHu-jun Gong for helping with cathodoluminescence image handling.

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