Geochemical and Sr–Nd–Pb isotopic compositions of mafic
dikes from the Jiaodong Peninsula, China: evidence for
vein-plus-peridotite melting in the lithospheric mantle
Jin-Hui Yang
a,*, Sun-Lin Chung
b, Ming-Guo Zhai
a, Xin-Hua Zhou
ca
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
b
Department of Geosciences, National Taiwan University, Taipei, Taiwan c
Key Laboratory for Tectonic Evolution of the Lithosphere, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Received 19 December 2002; accepted 9 December 2003
Abstract
Major and trace elements and Sr – Nd – Pb isotope data are reported for Cretaceous mafic dikes from the Jiaodong Peninsula, eastern China. These dikes range from medium-K and high-K calc-alkaline to shoshonitic or ultrapotassic rocks, which are characterized by high MgO (Mg#= 71 – 53) and Cr (177 – 1012 ppm) and low TiO2(0.55 – 0.90 wt.%), total Fe2O3(5.12 – 9.48
wt.%) and CaO (4.99 – 9.94 wt.%). Overall, they are enriched in the large ion lithophile elements (LILE, e.g., Rb, Ba, Sr) and light rare earth elements (LREE), depleted in the high field strength elements (HFSE, e.g., Nb, Ti, P), and possess uniform initial87Sr/86Sr (0.7094 – 0.7114) but relatively wide ranges of Nd [eNd(T) = 10.1 – 17.0] and Pb (206Pb/204Pb = 16.75 –
18.03) isotopic ratios, implying a magma origin from enriched but heterogeneous mantle sources. These geochemical and isotopic characteristics can be explained by the vein-plus-peridotite melting model, with amphibole- or phlogopite-bearing pyroxenite veins that reside in refractory lithospheric mantle beneath the North China Block. Such a vein-enriched mantle source formed by multiple metasomatic events, which we infer to have resulted from subduction-related processes that may have occurred in the Late Archean and Mesoproterozoic. The mafic dikes constitute a member of the widespread Mesozoic magmas emplaced in the North China Block as a result of regional lithospheric extension.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Mafic dike; Vein-plus-peridotite; Geochemistry; Lithospheric mantle; Jiaodong Peninsula, China
1. Introduction
The North and South China Blocks are generally believed to have collided in Triassic time as
man-ifested by the exhumation of ultrahigh-pressure meta-morphic (UHPM) rocks within the Qinling – Dabie – Sulu orogenic belt, the largest expanse of UHPM rocks in the world (cf.Cong, 1996). Studies of mantle xenoliths captured by Ordovician kimberlites and Cenozoic alkaline basalts suggested that beneath the North China Block (NCB) a large part of the ancient, refractory lithospheric mantle (>120 km) has been
0024-4937/$ - see front matterD 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2003.12.003
* Corresponding author. Tel.: 62007900; fax: +86-10-62010846.
E-mail address: jinhui@mail.igcas.ac.cn (J.-H. Yang).
www.elsevier.com/locate/lithos Lithos 73 (2004) 145 – 160
removed and replaced by young and more fertile mantle materials(Menzies et al., 1993; Menzies and Xu, 1998; Griffin et al., 1998; Xu, 2001; Gao et al., 2002), via regional postcollisional tectonomagmatic event(s) that occurred most likely in the Mesozoic(Li et al., 1998; Jahn et al., 1999; Fan et al., 2001).
Mafic dike swarms are a common expression of mantle-derived magma generation that is associated with extensional tectonics postdating continental col-lisions (e.g., Rock, 1991). The mafic dikes, in this circumstance, provide important information for un-derstanding not only magmatic genesis from the mantle but also tectonic evolution in the orogenic belts. In the Jiaodong Peninsula, which is located in the southeastern margin of the NCB (Fig. 1), mafic dikes are widespread and marked by extensive Early Cretaceous gold mineralization (e.g., Wang et al., 1998; Yang and Zhou, 2001; Zhang et al., 2003). In this paper, we report new results of K – Ar age, major and trace elements, and Sr – Nd – Pb isotope composi-tions for the Jiaodong mafic dikes (JMD) in the hope: (1) to document the geochemical characteristics of these rocks, (2) to address their magma source(s) and petrogenesis, (3) to discuss the evolution of litho-spheric mantle domains beneath North China, and (4) to explore implications for postcollisional tectonic history in the region.
2. Geological background
The Jiaodong Peninsula (Fig. 1)is located to the east of the Tanlu fault and made of two different terrains bounded by the Wulian – Mishan fault (Zhai et al., 2000), namely, the Jiaobei terrain and Sulu orogenic belt. The Sulu region represents an exhumed UHPM complex of the Yangtze Block (YB)(Ernst and Liou, 1995; Hacker et al., 1996, 1998; Li et al., 1999), which had been underthrusted northward beneath the NCB to as deep as >200 km(Xu et al., 1992; Ye et al., 2000). The identification of coesite- and diamond-bearing eclogites within the Sulu region(Jahn et al., 1996; Ye et al., 2000) led many workers to propose that the Tanlu Fault displaced left-laterally in the Cretaceous and transferred the Sulu region from Qin-ling – Dabie region for f 500 km (e.g., Xu et al., 1987; Okay and Sengo¨r, 1992). The Jiaobei terrain consists of Precambrian basement rocks (Zhai et al., 2000), in which Mesozoic granites(Wang et al., 1998), bimodal volcanic rocks (Fan et al., 2001)and mafic dikes are exposed. The Mesozoic magmas are associ-ated with the largest gold deposit in China(Wang et al., 1998; Yang and Zhou, 2001; Qiu et al., 2002).
The Precambrian basement is mainly composed of the late Archean Jiaodong Group, which contains volcanic and sedimentary sequences that have been
Fig. 1. Simplified geologic map of the Jiaodong Peninsula showing the sample localities and gold deposits. Inset shows regional tectonic environment of the North China Block and the location of the Jiaodong Peninsula.
J.-H. Yang et al. / Lithos 73 (2004) 145–160 146
metamorphosed into amphibolite to granulite facies. SHRIMP U – Pb dating of zircon indicates that the protolith of the amphibolite was formed at 2530 F 17 Ma and underwent metamorphism at 1852 F 37 Ma. (Zhang et al., 2003). The Mesozoic plutonic rocks, which intruded the basement, have been divided based on petrography, geochemistry and isotopic composi-tion into three major suites, namely, Linglong, Guojial-ing and Kunyushan(Qiu et al., 2002). The Linglong and Kunyushan suites consist of medium-grained met-aluminous to slightly permet-aluminous biotite – granite, and the Guojialing suite of porphyritic hornblende-biotite granodiorite with large K-feldspar phenocrysts. Their emplacement ages are 160 – 156 Ma for the Linglong suit(Wang et al., 1998; Zhang et al., 2003), 135 – 130 Ma for the Kunyushan suit (Zhang et al., 1995)and 130 – 126 Ma for the Guojialing suit(Wang et al., 1998; Zhang et al., 2003). Contemporaneous volcanism occurred along the Sulu UHP metamorphic belt, mainly in the Laiyang basin(Fig. 1), marked by bimodal compositions (Fan et al., 2001). Fan et al. (2001)proposed that the postcollisional bimodal vol-canism was derived from an enriched lithospheric mantle, which might have undergone a fluid-related metasomatism by the subducted YB continent in the Triassic. The thickness of the volcanic sequences varies from several thousands to tens of meters, decreasing from the center to margin of the Basin, whose forma-tion has been interpreted as related to extensional tectonism(Xu et al., 1987; Wang et al., 1998). Thus, the magma generation and associated gold mineraliza-tion are ascribed to thermal perturbamineraliza-tion affiliated with lithospheric extension owing to large-scale displace-ment along the Tanlu fault system during late Mesozoic time.
Two main phases of deformation that took place during the Mesozoic are identified in the Jiaodong Peninsula (Wang et al., 1998). The first phase was characterized by northwest – southeast compression that is manifested by prominent northeast-trending brittle – ductile shear zones showing sinistral slip move-ments. The second phase involved the development of NNE- and NE-trending extensional brittle structures, accompanied by widespread intrusions of dikes and hydrothermal gold mineralization (e.g., Wang et al., 1998; Yang and Zhou, 2001; Zhang et al., 2003). Most of the mafic dikes occur as swarms that strike NE 20 – 40j and NNE 60 – 80j with steep dip angles (about E or
W 60 – 80j), and range from 0.2 70 to 1 1500 m2in dimension. They are generally undeformed and show little sign of metamorphism.
3. Samples and petrography
Samples for this study, comprising dolerite, horn-blende dolerite and lamprophyre (minette), were col-lected from mine shafts near the Xincheng, Linglong and Mouping areas. All samples were collected far away from the gold lodes to avoid the effect of later hydrothermal activity. They show holocrystalline, ophitic and/or porphyritic-seriate textures, with pheno-cryst contents of 10 – 30%. The phenopheno-crysts consist dominantly of clinopyroxene with subordinate plagio-clase in the dolerites and phlogopite with minor ortho-pyroxene in the minettes. Olivine, hornblende, biotite and Ti-magnetite appear in the matrix of the dolerites and are always subordinate to plagioclase and clino-pyroxene. In contrast, opaque minerals are rare in more basic dikes. Sporadic orthopyroxene, as a microphe-nocryst phase, is present. The mineral assemblage of the groundmass is similar to that of the phenocrysts but has a higher population of opaque minerals. Petro-graphic examinations show that even highly porphy-ritic rocks do not have cumulate textures, so that the samples can be used to reflect magma compositions.
Some of the dolerites contain visible carbonate veins and display alteration features such as chloritiza-tion along the rims of primary phenocrysts. Olivine and orthopyroxene are often partially replaced by green or brown phyllosilicates. Plagioclase is usually slightly sericitized but totally replaced by sericite, saussurite and chlorite in seriously altered dikes, the latter were excluded from this study. Clinopyroxene that is gener-ally fresh is replaced by chlorite in some altered
Table 1
Whole rock K – Ar dates for mafic dikes from the Jiaodong Peninsula Sample no. K (%) 40 Arrad (mol/g) 40 Arrad (%) Age (Ma) 1r (Ma) XC-M02 1.91 4.391e 10 97.19 127.9 2.4 JQ-M02 1.82 4.003e 10 93.43 122.6 2.4 LL-M06 2.01 4.469e 10 92.17 123.9 2.5 DK-M04 1.18 2.815e 10 93.14 132.5 2.6 MP-M06 3.02 6.496e 10 97.81 120.0 1.1
Table 2
Major (wt.%) and trace (ppm) element, and Sr – Nd – Pb isotopic data of mafic dikes from Jiaodong Peninsula, eastern China
XC-M01 XC-M02 XC-M04 XC-M09 DK-M04 DK-M07 JQ-M02 JQ-M03 LL-M02 LL-M06
Rock type Qz dolerite Qz dolerite Qz dolerite Qz dolerite Dolerite Dolerite Dolerite Dolerite Hb dolerite Dolerite
Locality Xincheng Xincheng Xincheng Xincheng Linglong Linglong Linglong Linglong Linglong Linglong
SiO2 56.59 57.85 58.50 57.33 44.08 51.75 48.15 49.15 53.17 46.26 TiO2 0.63 0.65 0.62 0.66 0.76 0.82 0.86 0.90 0.77 0.80 Al2O3 12.47 12.58 14.78 13.30 13.28 14.92 14.14 15.36 14.81 13.52 Fe2O3 6.96 6.92 5.12 6.63 8.10 6.83 9.32 9.48 5.72 8.19 MnO 0.18 0.20 0.10 0.15 0.24 0.12 0.17 0.16 0.12 0.18 MgO 8.51 8.19 4.33 8.69 9.35 6.50 10.23 7.33 4.42 8.85 CaO 5.89 5.32 4.99 5.66 8.86 6.80 8.84 8.78 5.19 8.26 Na2O 2.95 2.73 3.69 2.80 1.72 2.84 1.71 2.50 4.01 1.95 K2O 2.20 2.40 2.97 2.22 1.20 2.18 1.36 1.53 2.75 2.53 P2O5 0.19 0.18 0.25 0.18 0.35 0.29 0.15 0.15 0.34 0.37 LOI 3.49 2.96 4.43 2.21 12.03 7.01 5.31 4.34 9.38 9.24 SUM 100.06 99.98 99.78 99.83 99.97 100.06 100.24 99.68 100.48 100.15 Mg#a 70.97 70.29 62.85 72.39 69.76 65.55 68.71 60.73 60.72 68.35 Cr 771 855 302 1012 358 265 556 201 177 486 Co 41 42 28 41 48 28 37 48 23 37 Rb 138 165 88 82 44 50 42 56 82 66 Sr 1071 1044 1517 1097 880 958 781 1023 1070 1079 Y 18 17 17 20 17 17 17 20 15 20 Zr 212 201 301 200 140 143 137 140 212 167 Nb 16 16 12 17 7.1 10 6.2 5.8 13 10 Cs 5.6 6.1 3.1 3.1 1.4 0.6 2.0 3.5 2.2 2.4 Ba 762 767 4381 1381 1479 1486 893 1094 2601 1866 La 26.3 29.9 74.6 37.7 57.7 44.6 24.1 35.7 82.5 48.7 Ce 50.2 51.5 143 76.2 118 93.6 51.3 76.0 157 99.8 Pr 5.87 5.59 16.2 8.55 13.7 10.8 5.84 8.92 16.7 11.5 Nd 23.5 23.9 58.6 31.7 49.9 38.7 23.2 35.8 57.4 45.6 Sm 5.05 4.46 8.74 6.01 8.03 6.87 3.93 6.56 8.27 7.84 Eu 1.49 1.59 3.22 2.24 2.66 2.42 1.62 2.16 2.77 2.73 Gd 5.26 5.02 7.41 6.36 7.32 6.39 4.37 6.09 7.38 7.10 Tb 0.61 0.59 0.78 0.76 0.78 0.81 0.60 0.80 0.68 0.83 Dy 3.31 3.26 3.45 4.12 3.64 3.70 2.88 4.29 3.26 3.85 Ho 0.65 0.64 0.65 0.76 0.68 0.72 0.73 0.84 0.55 0.74 Er 1.91 1.97 1.82 2.14 1.84 1.80 1.85 2.30 1.48 2.05 Tm 0.24 0.31 0.26 0.28 0.24 0.32 0.27 0.32 0.19 0.27 Yb 1.66 1.75 1.42 2.01 1.56 1.44 1.53 1.56 1.26 1.90 Lu 0.26 0.25 0.21 0.27 0.23 0.25 0.23 0.27 0.18 0.27 Hf 5.55 5.00 7.16 5.61 3.05 3.50 3.58 3.14 4.76 3.99 Ta 2.91 1.25 1.68 3.38 2.32 1.88 2.31 2.38 1.72 4.74 Pb 1.6 1.9 23.7 24.2 19.0 – – 1.8 85.7 4.2 J.-H. Y ang et al. / Lithos 73 (2004) 145–160 148
Th 9.1 9.4 13.6 12.5 7.0 6.2 3.5 3.9 13.5 7.5 U 3.1 2.6 3.0 3.2 1.3 1.1 0.7 0.8 3.0 1.4 206 Pb/204Pb 18.026 17.735 17.483 18.001 17.367 17.176 17.300 17.281 17.063 17.439 207 Pb/204Pb 15.531 15.300 15.444 15.372 15.534 15.443 15.489 15.472 15.287 15.496 208Pb/204Pb 38.402 37.734 37.989 37.845 37.991 37.717 37.971 37.847 37.297 37.953 87Rb/86Sr 0.3635 0.4563 0.1741 0.2186 0.2128 0.1428 0.1453 0.1452 0.2217 0.1590 87Sr/86Sr 0.710825 0.710885 0.710419 0.710288 0.710397 0.709839 0.710165 0.710110 0.711076 0.709572 2r ( 10 6) 19 17 12 20 14 17 31 14 29 18 147Sm/144Nd 0.1046 0.1055 0.0980 0.1010 0.0993 0.0972 0.1100 0.1212 0.0910 0.1029 143Nd/144Nd 0.512006 0.512006 0.512041 0.511984 0.511806 0.511680 0.511790 0.511807 0.511684 0.511788 2r ( 10 6) 11 9 8 15 9 11 16 9 7 10 ISr(125 Ma) 0.71018 0.71007 0.71011 0.70990 0.71002 0.70959 0.70991 0.70985 0.71068 0.70929 eNd(125 Ma) b 10.9 10.9 10.1 11.2 14.7 17.1 15.2 15.0 16.9 15.1 TDM(Ga) b 1.60 1.61 1.46 1.57 1.79 1.92 1.99 2.20 1.82 1.87 fSm/Nd 0.47 0.46 0.50 0.49 0.50 0.51 0.44 0.38 0.54 0.48 LL-M08 MP-M01 MP-M02 MP-M04 MP-M05 MP-M06 MP-M07 MP-M08 MP-M09
Rock type Dolerite Dolerite Dolerite Dolerite Dolerite Minette Minette Minette Minette
Locality Linglong Mouping Mouping Mouping Mouping Mouping Mouping Mouping Mouping
SiO2 46.61 47.35 46.49 47.34 50.64 47.84 48.31 49.01 49.21 TiO2 0.79 0.83 0.83 0.79 0.61 0.59 0.65 0.55 0.63 Al2O3 12.62 14.50 14.50 13.39 14.84 14.51 14.66 13.34 14.67 Fe2O3 7.42 7.09 7.82 7.42 5.53 5.53 7.89 8.25 7.14 MnO 0.16 0.11 0.18 0.21 0.10 0.14 0.13 0.17 0.16 MgO 8.49 7.75 5.99 8.49 4.67 5.01 5.32 4.72 4.85 CaO 7.51 7.78 9.94 8.97 5.93 6.69 5.98 6.89 6.37 Na2O 2.22 1.83 2.67 2.27 1.57 0.41 – – – K2O 2.10 2.18 1.63 1.94 3.51 4.13 3.50 2.81 3.59 P2O5 0.34 0.37 0.39 0.32 0.29 0.28 0.31 0.26 0.33 LOI 11.49 10.69 9.69 9.15 12.30 14.81 13.51 14.25 13.52 SUM 99.75 100.13 100.13 100.29 99.99 99.94 100.26 100.25 100.47 Mg# 69.58 64.66 60.52 69.59 62.80 64.46 57.42 53.35 57.59 Cr 444 306 329 499 276 242 363 209 225 Co 44 38 43 47 30 26 23 36 33 Rb 58 50 34 40 89 96 128 62 77 Sr 1023 1494 1760 1084 803 1162 314 317 244 Y 19 19 21 18 15 10 18 10 11 Zr 176 167 212 184 160 100 223 104 94 Nb 10.2 11.0 13.8 9.9 9.3 7.1 10.0 6.6 8.1 Cs 1.1 1.6 0.60 0.7 1.5 3.8 4.8 3.3 3.2 Ba 3205 1087 1255 1169 1119 1588 985 410 1074 La 65.1 26.1 35.6 31.4 30.7 39.5 47.5 43.9 44.6 Ce 127 53.8 66.3 60.0 60.5 75.4 98.0 81.4 78.5
(continued on next page)
J.-H. Y ang et al. / Lithos 73 (2004) 145–160 149
Table 2 (continued)
LL-M08 MP-M01 MP-M02 MP-M04 MP-M05 MP-M06 MP-M07 MP-M08 MP-M09
Rock type Dolerite Dolerite Dolerite Dolerite Dolerite Minette Minette Minette Minette
Locality Linglong Mouping Mouping Mouping Mouping Mouping Mouping Mouping Mouping
Pr 14.0 6.19 7.66 6.98 6.91 8.18 11.2 8.72 8.43 Nd 50.8 24.3 30.7 28.8 26.9 28.3 40.3 31.2 29.8 Sm 8.32 5.14 5.77 4.39 4.39 4.67 6.23 4.83 4.43 Eu 2.93 1.57 2.28 1.76 1.60 1.69 2.19 1.35 1.72 Gd 7.62 5.11 6.45 5.12 4.94 4.05 6.41 4.24 4.19 Tb 0.77 0.64 0.74 0.65 0.58 0.44 0.69 0.42 0.48 Dy 3.89 3.42 3.99 3.59 2.33 2.05 3.82 2.17 2.25 Ho 0.75 0.69 0.81 0.73 0.58 0.37 0.73 0.39 0.45 Er 2.01 1.69 2.27 1.77 1.48 1.04 1.80 1.04 1.04 Tm 0.26 0.26 0.34 0.27 0.24 0.15 0.25 0.16 0.21 Yb 1.52 1.70 1.97 2.14 1.42 1.10 1.61 1.29 1.16 Lu 0.24 0.23 0.29 0.29 0.24 0.17 0.29 0.14 0.18 Hf 3.98 4.45 5.13 4.41 4.12 3.15 5.60 2.79 2.82 Ta 1.04 2.14 5.05 2.70 1.40 3.89 5.19 1.18 3.92 Pb 19.4 – 0.8 – 14.2 21.2 32.5 17.8 21.8 Th 8.2 3.0 4.0 5.2 4.5 5.2 6.8 4.6 5.0 U 1.4 0.7 0.8 0.9 1.7 0.9 1.8 1.5 1.2 206Pb/204Pb 17.117 17.208 17.129 17.093 16.987 16.811 16.745 16.940 16.968 207Pb/204Pb 15.414 15.415 15.405 15.376 15.437 15.298 15.243 15.383 15.424 208Pb/204Pb 37.485 37.445 37.367 37.352 37.349 36.920 36.755 37.167 37.433 87Rb/86Sr 0.1536 0.0963 – 0.1073 0.3336 0.2382 1.2462 0.5897 0.9451 87Sr/86Sr 0.709357 0.709421 – 0.709502 0.709495 0.709727 0.711431 0.710512 0.711075 2r ( 10 6) 10 16 – 23 25 20 20 18 23 147Sm/144Nd 0.1005 0.1050 – 0.1016 0.0904 0.0920 0.0935 0.0880 0.0927 143Nd/144Nd 0.511838 0.511926 – 0.511731 0.511690 0.511711 0.511696 0.511698 0.511694 2r ( 10 6) 10 11 – 8 7 9 11 6 12 ISr(125 Ma) 0.70908 0.70925 – 0.70931 0.70890 0.70930 0.70922 0.70946 0.70940 eNd(125 Ma) 14.1 12.4 – 16.2 16.8 16.4 16.7 16.6 16.8 TDM(Ga) 1.76 1.71 – 1.92 1.80 1.80 1.84 1.76 1.83 fSm/Nd 0.49 0.47 – 0.48 0.54 0.53 0.52 0.55 0.53
aMg#= atomic 100 (Mg/Mg + Fe2 +), in which FeO = 0.9Fe 2O3.
bThe143Nd/144Nd and147Sm/144Nd of chondrite and depleted mantle at present day are 0.512638 and 0.1967, 0.51315 and 0.222, respectively. :Not determined and/or below detecting limit ( – ). J.-H. Y ang et al. / Lithos 73 (2004) 145–160 150
samples. The four minette samples analyzed do not show apparent alteration features but contain tiny pellets and/or veins of igneous carbonates, which have been measured for stable isotope (C, H, O) ratios suggestive of upper mantle origin(Sun et al., 2001). Emplacement of such carbonatite-bearing minettes is interpreted to be the result of ‘‘comagmatic metasoma-tism’’ owing to high volatile contents, i.e., a process coined as gas phase metasomatism (cf.Rock, 1991), which may largely account for the high loss on ignition (LOI) concentrations observed in these rocks (see below).
4. Analytical methods
Conventional whole-rock K – Ar age determinations were carried out at the Institute of Geology, Chinese State Seismological Bureau. The analytical procedures
are similar to those described by Chen and Chen
(1997).
Major and trace elements and Sr – Nd – Pb isotope data were obtained at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Major elements were analyzed by X-ray Fluorescence (XRF) method combining with wet chemical method (MgO, Na2O, P2O5 and LOI). Inductively Coupled
Plasma Mass Spectrometry (ICP-MS) was used to analyze trace element contents. Analyzed uncertain-ties are F 3 – 5% for major elements and better than 5 – 8% for trace elements(Ren, 1995).
Samples for isotopic analysis were dissolved in Teflon bombs after being spiked with 84Sr, 87Rb,
150
Nd and 147Sm tracers prior to HF + HNO3(with a
Fig. 2. Plots of (a) Total alkali vs. SiO2, and (b) K2O vs. SiO2for mafic dikes from the Jiaodong Peninsula. The nomenclature fields are fromLe Maitre et al. (1989).
Fig. 3. Chondrite-normalized REE patterns for mafic dikes from the Jiaodong Peninsula.
ratio of 2:1) dissolution. Rb, Sr, Sm and Nd were separated using conventional ion exchange proce-dures and measured using a VG-354 multicollector mass spectrometer (Qiao, 1988). Procedural blanks were < 100 pg for Sm and Nd and < 500 pg for Rb and Sr. 143Nd/144Nd were corrected for mass frac-tionation by normalization to 146Nd/144Nd = 0.7219, and 87Sr/86Sr ratios normalized to 86Sr/88Sr = 0.1194. Typical within-run precision (2r) for Sr and Nd was estimated to be F 0.00002 and F 0.000015, respec-tively. The measured values for the La Jolla Nd standard and NBS-607 Sr standard were143Nd/144Nd = 0.511853 F 7 (2r, n = 12) and87Sr/86Sr = 1.20042 F 2 (2r, n = 12), respectively, during the period of data acquisition. Pb isotope data were corrected by refer-ence to the analyses of NBS981 Pb standard that indicate a mass fractionation averaging 0.1% per amu.(Yang, 2000).
5. Analytical results 5.1. K – Ar dating result
Our K – Ar data (Table 1) yielded a magmatic
duration of 132.5 – 120.0 Ma for mafic dikes from the Xincheng, Linglong and Mouping areas. The emplacement of the dikes took place synchronously with formations of the Guojialing granodioritic suite (130 – 126 Ma,Wang et al., 1998; Zhang et al., 2003) and gold mineralization in the Jiaobei terrain(Wang et al., 1998; Yang and Zhou, 2001; Zhang et al., 2003). This is consistent with the K – Ar results reported by Li and Yang (1993)andSun et al. (1995)and zircon SHRIMP U – Pb ages reported byZhang et al. (2003) and confirms previous notion that the dike swarm occurred in the early Cretaceous.
5.2. Major and trace element data
Results of major and trace element analyses are listed in theTable 2and plotted inFigs. 2 – 4.
Whole-Fig. 4. Primitive mantle (PM)-normalized trace element variation patterns for mafic dikes from the (a) Xincheng, (b) Linglong and (c) Mouping areas, respectively. (d) Plots of averaged values of the dikes in comparison with the (upper) continental crust(Rudnick and Fountain, 1995). Normalizing data of the PM are fromSun and McDonough (1989).
J.-H. Yang et al. / Lithos 73 (2004) 145–160 152
rock silica contents (SiO2) range from 56.59 to 58.50
wt.% in the Xincheng, 44.08 – 53.17 wt.% in the Linglong and 46.49 – 50.64 wt.% in the Mouping areas (Table 2), which overall range from basalt to andesite with minor trachyandesites according to the nomenclature of Le Maitre et al. (1989). All dikes belong to subalkaline rocks based on the alkali vs. silica plot (Fig. 2a). They are characterized by low
TiO2 (0.55 – 0.90 wt.%), total Fe2O3 (5.12 – 9.48
wt.%) and CaO (4.99 – 9.94 wt.%), and high Mg numbers [Mg number = Mg/(Mg + 0.9FeT) = 53 – 71]
and Cr contents (177 – 1012 ppm, with most >200 ppm). Using K2O vs. SiO2nomenclature ofLe Maitre
et al. (1989), these dikes are classified as medium-K, high-K calc-alkaline to shoshonitic rocks (Fig. 2b). The four minette samples from the Mouping area
Fig. 5. Initial87Sr/86Sr vs. e
Nd(T) diagram of mafic dikes (b), compared with volcanic rocks, granites and Cenozoic basalts (a) in the Jiaodong Peninsula. Data sources include: (1) the NCB lower and upper crusts and the YB lower crust fromJahn et al. (1999), (2) the Jiaodong volcanic rocks fromFan et al. (2001), granites fromYang (2000), and kimberlites fromZheng et al. (1998), (3) Cenozoic basalts fromPeng et al. (1986), Basu et al. (1991),Tatsumoto et al. (1992)andChung (1999).
possess K2O>3 wt.% and K2O/Na2O>2 wt.%,
togeth-er with MgO>3 wt.% and high Cr (>200 ppm), belonging to the ultrapotassic rocks defined byFoley et al. (1987) and thus similar to the composition of potassic rocks from Central Italy (Peccerillo, 1990). Note that these samples have low Na2O ( < 0.4 wt.%),
similar to certain minettes from Peru, the eastern Andean Cordillera (Carlier et al., 1997) and eastern Antarctica(Hoch et al., 2001).
Chondrite-normalized REE patterns for the dikes are marked by (1) an enrichment in the LREE, (2) less variation in the heavy REE (HREE), and (3) minor or absent positive Eu anomalies(Fig. 3). Overall speak-ing, these samples display a large variation in REE abundance levels (total REE = 122 – 340 ppm), with variable (LaN/SmN) ratios (3.3 – 6.5) and (LaN/YbN)
ratios (10.5 – 46.8). In the primitive mantle (PM) normalized trace element variation diagram(Fig. 4), all dikes show very distinctive negative anomalies in the HFSE (Nb and Ti), coupled with enrichments in the LILE relative to LREEs (e.g., Ba/La = 9 – 59) and HFSE (e.g., Ba/Nb = 47 – 367, La/Nb = 1.6 – 8.1). A significant feature to note is that the abundance levels of LREE and LILE of shoshonitic and high-K calc-alkaline rocks are apparently higher than those of medium-K magmas from the same area (Figs. 3 and 4).
5.3. Sr – Nd – Pb isotope data
Initial 87Sr/86Sr ratios of the JMD are relatively uniform (0.70890 – 0.71017; Table 2), whereas their Nd isotopes are heterogeneous among samples from three localities. The eNd(125 Ma) values are 10.1 –
11.2, 14.1 – 17.1 and 12.5 – 16.8 for
sam-ples from the Xincheng, Linglong and Mouping areas, respectively(Table 2). In the Sr – Nd isotopic correla-tion diagram(Fig. 5b), the Xingcheng samples exhibit higher initial 87Sr/86Sr and 143Nd/144Nd ratios that plot away from samples from the other two areas. Pb isotopic ratios of the JMD are also distinctive, with
206
Pb/204Pb ratios of the Xincheng, Linglong and Mouping samples ranging from 17.48 to 18.03, 17.06 – 17.44 and 16.75 – 17.21, respectively (Table 2 andFig. 6). Note that the Pb isotope compositions of Xincheng samples plot close to or within the field of Cenozoic alkaline basalts from the same region, which represent within plate magmas derived mainly from
the asthenospheric mantle(Peng et al., 1986; Basu et al., 1991; Chung, 1999). All samples except two from the Xincheng area (XC-M04 and XC-M02) plot above
the Northern Hemisphere Reference Line (Hart,
1984).
6. Discussion
6.1. Petrogenesis: crustal assimilation vs. source enrichment
The JMD have generally high MgO (max. 10.23 wt.%; Mg#= 71) and Cr (max. 1012 ppm) contents, indicating a dominant magma source from the upper mantle. However, these dikes are also marked by ‘‘crustal-like’’ trace element and isotopic features, e.g., the enrichments in the LILE and LREE, deple-tions in the HFSE (Table 2;Fig. 3), and high initial
Fig. 6. Plots of206Pb/204Pb vs. (a)207Pb/204Pb and (b)208Pb/204Pb ratios of mafic dikes from the Jiaodong Peninsula. The NHRL is fromHart (1984). Data sources for Cenozoic basalts are same as Fig. 5.
J.-H. Yang et al. / Lithos 73 (2004) 145–160 154
87
Sr/86Sr and low eNd(T) values (Table 2;Figs. 5 and
6). It is important to note that their overall geochem-ical characteristics resemble those of postcollisional lavas emplaced in the Tethyan orogenic belts, such as Spanish lamproites(Nelson et al., 1986)and Tibetan shoshonitic rocks (Turner et al., 1996; Miller et al., 1999), which are widely considered to have originated from enriched lithospheric mantle sources.
There are at least three likely processes to account for the JMD geochemistry. These are: (1) crustal assimilation, i.e., mantle-derived melts that assimilat-ed wall rocks during magma ascent; (2) relatively old source metasomatism, i.e., enrichment in the mantle source region via geodynamic processes such as subduction; and (3) binary mixing between mantle-and crustal-derived magmas. Crustal assimilation may produce some trace element and isotopic variations observed inFigs. 3 – 6. It, however, does not explain the very high concentrations of Ba (max. of 4381 ppm) and Sr (max. of 1760 ppm) of the JMD (Table 2; Fig. 4d), which are much higher than the continental crust values (Ba = 390 ppm; Sr = 325 ppm; Rudnick and Fountain, 1995), and hence these data exclude crustal assimilation to have played a significant role in the petrogenesis. Besides, crustal assimilation coupled with fractional crystallization (AFC) is unlikely as this would result in progressive decreases in Cr, Ni, Co, and Mg numbers with concomitant increase in initial
87
Sr/86Sr ratios and decrease in eNd (T) values,
fea-tures that are not observed in the JMD. The third scenario, i.e., magma mixing, is also not favored because this should generate mixing curves in the isotopic correlation diagrams and in plots between isotopic ratios and certain elements (e.g., MgO or SiO2), which are not observed either(Figs. 5 – 7). Two
groups of dikes are identified, as shown in Fig. 7a, one with high MgO (Mg#>68) and Cr (>300 ppm) and the other with low MgO (Mg# < 68) and Cr ( < 400 ppm). These rocks do not display mixing curves(Fig. 7a). Moreover, the initial87Sr/86Sr ratios of each group are rather uniform over a wide range of MgO contents(Fig. 7b).
Therefore, we argue that the geochemical and isotopic characteristics of the JMD are, similar to the Tethyan orogenic lavas (Nelson et al., 1986; Turner et al., 1996; Miller et al., 1999), derived from enriched domains or metasomes(Menzies et al., 1993) in the lithospheric mantle beneath the NCB. Such
domains appear to be heterogeneous and are believed to have resulted from multiple metasomatic events (see below).
6.2. Characteristics of the mantle sources
Mafic dikes from the Xincheng have relatively higher eNd (T) values ( 10 – 11) and 206Pb/204Pb
ratios (17.48 – 18.03) than those of the Linglong and Mouping dikes(Figs. 5 and 6). Distinctions can also be observed in plots of eNd(T) values with MgO, K2O and
Sm/Nd ratios (Fig. 8), in which the Xincheng rocks define one magmatic evolution trend whereas the Linglong and Mouping lavas delineate a second. These can not result from a single source but require involve-ment of multiple mantle components. For each group, there are at least two ‘‘end-members’’, i.e., a high-Mg,
Fig. 7. (a) Cr vs. Mg# diagram for mafic dikes from the Jiaodong Peninsula, through which two groups of dikes, one with higher Mg# (Mg# > 68) and Cr (Cr > 300 ppm) and the other with lower Mg# (Mg # < 68) and Cr (Cr < 400 ppm), are grouped. (b) Initial87Sr/86Sr ratios vs. MgO diagram.
low-K component and a low-Mg, high-K component (Fig. 8a and b). This observation is consistent with the vein-plus-wall-rock melting model proposed byFoley (1992). Under the framework of that model, melting of the veins (pyroxenite) would form the shoshonitic and high-K calc-alkaline melts with lower MgO contents that are more enriched in incompatible trace elements, whilst partial melting of the wall-rock (peridotite)
produces the high-Mg melts with lower K2O and less
enriched incompatible elements.
The high-Mg (Mg#>68) dikes, with high Cr (358 – 1012 ppm) but low total Fe2O3and CaO(Table 2), are
likely to have originated from a refractory mantle source that had experienced previous extraction of basaltic melts. This notion is supported by plots of Fe2O3 vs. TiO2 (Fig. 9a), in which these high-Mg
dikes fall in the field defined by the experimental melts from depleted peridotite(Falloon et al., 1988). Such a refractory mantle may be represented by the lithospheric mantle of the NCB.
There are two types of ‘‘vein’’ component, marked by different isotopic compositions (Fig. 8), the low-Mg, high-K characteristics furthermore point to po-tassium-rich phases (e.g., phlogopite, amphibole) in
Fig. 8. Plots of eNd (T) vs. (a) MgO, (b) K2O, and (c) Sm/Nd indicating two evolution trends of mafic dikes from the Jiaodong Peninsula. See text for detailed discussion.
Fig. 9. (a) Plots of TiO2vs. total Fe2O3for mafic dikes with Mg number > 68 in comparison with fields of the peridotitic melts reported byFalloon et al. (1988). (b) Rb/Sr vs. Ba/Rb diagram for all mafic dikes from the Jiaodong Peninsula.
J.-H. Yang et al. / Lithos 73 (2004) 145–160 156
the vein sources. Melts in equilibrium with phlogopite are expected to have higher Rb/Sr (>0.1) and lower Ba/Rb ( < 20) ratios than those from amphibole-bear-ing sources(Furman and Graham, 1999). In Fig. 9b, high-K lavas from the Xincheng and Linglong areas display higher Ba/Rb (>30) and lower Rb/Sr ( < 0.1), suggesting an amphibole-bearing vein source. In con-trast, shoshonitic dikes from the Mouping area exhibit lower Ba/Rb ( < 20) and higher Rb/Sr (>0.1), implying phlogopite to have been involved in the magma generation. Note that dikes from the Linglong and Mouping areas appear to be derived from ‘‘veins’’ that contain different types of K-rich phases, i.e., amphi-bole and phlogopite, respectively, although they de-lineate similar evolution trends inFig. 8.
6.3. Multiple mantle metasomatic events
The identification of different types of the vein component implies that there were different types of metasomatism at different times, i.e., multiple meta-somatic events, in lithospheric mantle of the NCB. The general similarity in incompatible element pat-terns between the JMD and upper continental crust (Fig. 4d) tends to support the contention that attrib-utes recycled continental crustal materials to explain the generation of postorogenic potassic lavas (e.g., Nelson, 1992; Peccerillo, 1999). To account for the ‘‘crustal-like elemental and isotopic signatures’’ ob-served in Cretaceous mafic – ultramfic intrusions from the northern Dabie complex, which are temporally and geochemically comparable with the JMD, Li et al. (1998) and Jahn et al. (1999) envisioned a recycled component composed of the YB lower and/or middle crust to have been subducted via the Triassic continental collision processes and later involved in the magma generation. However, such a Triassic subduction/collision interpretation works on-ly in areas close to the Dabie – Sulu orogenic belt, whereas Cretaceous magmatism is widespread over the NCB, occurring extensively in the Liaoning
Province and Western Shandong Province (Chen
and Chen, 1997; Guo et al., 2001). We therefore favor a larger-scale and longer-lasting scenario that is multiple continental arc-type magmatic events in the Late Archean and Mesoproterozoic, a mechanism proposed to have caused mafic magma underplating around the crust – mantle boundary in the NCB(Yu et
al., 2003). Such magmatic events took place before/ during the final assemblage of the NCB that occurred at f 1.8 Ga in the Late Paleoproterozoic (Zhao et al., 2001). This interpretation is consistent with the observation that mafic granulite and pyroxenite xen-oliths have Sr – Nd – Pb isotopic compositions over-lapping with those of the JMD and contemporaneous magmas from the NCB (Zhou et al., 2002; Yu et al., 2003).
The ‘‘ancient’’ subduction-related enrichments above-described may have resulted in the amphibole and phlogopite-bearing pyroxenite veins involved in the JMD generation. These veins could have been imparted with the incompatible element features ob-served in the JMD and, with time, developed the radiogenic isotopic signatures (Foley, 1992; Schmidt et al., 1999). The Xincheng dikes have apparently higher eNd (T) values and Pb isotopic ratios than the
Linglong and Mouping ones, leading to the two distinct trends observed in Fig. 8. This may be explained by similar metasomatic events at different times. Relative to dikes from the other two areas that exhibit Nd isotopic model ages (TDM) between 1.7
and 2.2 Ga(Table 2), the Xincheng samples appear to have younger and restricted TDMages of f 1.5 – 1.6
Ga that is consistent with the interpretation of a younger enrichment in the mantle source region.
6.4. Tectonic implications
Magmatism in the Jiaodong Peninsula has been proposed as being produced under an intracontinental extension setting(Fan et al., 2001), in association with the development of rifting basins and major strike-slip movement of the Tanlu fault zone during the late Mesozoic (e.g.,Xu et al., 1987). Such intracontinental extensional magmatism marked by subduction geo-chemical fingerprints is not unusual in modern and ancient orogens (e.g., Turner et al., 1996; Romer et al., 2001). Studies focused on mantle xenoliths from
the NCB (Menzies et al., 1993; Menzies and Xu,
1998; Griffin et al., 1998; Xu, 2001; Gao et al., 2002; Zhou et al., 2002) repeatedly indicated that the cra-tonic lithosphere beneath has been removed for at least 120 km, although the precise timing and mech-anism of the removal remain highly debated. The collision between the North China and South China (or Yangtze) Blocks and the subduction of Pacific
Oceanic plate along the East Asia not only resulted in the UHPM rocks exposed in the Qinling – Dabie – Sulu orogenic belt, but also reactivated the eastern part of the NCB cratonic lithosphere as manifested by exten-sive basin formation and movement of the Tanlu fault since Jurassic time. The reactivation, affiliated most likely with the lithospheric removal and replacement by ascended asthenosphere, led to elevation of the geotherm and thus the widespread magmatic activity. Consequently, the NCB evolved from stable craton through contractional orogen to an extensional tecton-ic environment that is characterized by development of rifted basins and basaltic eruptions in the Cenozoic history (e.g.,Tian et al., 1992; Ren et al., 2002).
7. Concluding remarks
Our K – Ar dates indicate that the dikes from the Jiaodong Peninsula, eastern China occurred in the Early Cretaceous (135 – 115 Ma), broadly synchronous with the massive emplacement of granitic plutons and gold mineralization in the region. The dikes range in composition from medium-K and high-K calc-alkaline to shoshonitic or ultrapotassic rocks, whose overall geochemical and isotopic characteristics can be explained in terms of the vein-plus-wall-rock melting model (Foley, 1992), in which the veins consist of amphibole- or phlogopite-bearing pyroxenites and the wall-rock peridotite is refractory cratonic lithospheric mantle beneath the North China Block. The enriched mantle source may have resulted from multiple meta-somatic events imparted by subduction-related pro-cesses that occurred in the Late Archean and Mesoproterozoic before/during the accretion of the North China Block. It became involved in the magma generation when the Triassic continental collision and the subduction of Pacific plate along the East Asia reactivated the stable lithosphere of the North China Block. Therefore, the mafic dikes, analogous to post-collisional lavas from many orogens, represent intra-continental extension-induced magmas derived from the lithospheric mantle that was previously metasom-atized by subduction zone processes. In the Jiaodong Peninsula, the mafic dikes are associated with volcanic sequences and felsic plutons showing similar geo-chemical affinities. Together with contemporaneous lavas from the Dabie – Sulu orogenic belt and other
localities in the North China Block, these rocks con-stitute the Mesozoic magmatic province whose gener-ation and evolution bear important informgener-ation about the timing and mechanism of key tectonic events such as the lithospheric removal from below this region. Furthermore, detailed investigations of individual out-crops are hence urgently needed.
Acknowledgements
J.-H. Yang thanks Qi Zhang, Simon Wilde, Wei Liu, Hong-Rui Fan and Jing-Hui Guo for insightful discussion and help they kindly provided at various stages of this study, and benefited from a one-year visit in the Department of Geosciences, National Taiwan University, which allowed the completion of the manuscript. We thank journal reviewers, Profs. M. Roden and F.-Y. Wu, for their thoughtful comments and helpful suggestions that significantly improved the content and presentation of the manuscript. This study was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, and Chinese Academy of Sciences under grants NSFC-40132020, KZCX1-07 and 95-Yu-25, respectively.
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