Neoproterozoic crustal growth and reworking of the Northwestern Yangtze Block: Constraints from the Xixiang dioritic intrusion, South China

Lithos 120 (2010) 439–452

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Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s

Neoproterozoic crustal growth and reworking of the Northwestern Yangtze Block: Constraints from the Xixiang dioritic intrusion, South China Jun-Hong Zhao a,b,⁎, Mei-Fu Zhou b, Jian-Ping Zheng a, Shi-Ming Fang c a b c

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China Department of Earth Sciences, University of Hong Kong, Hong Kong Faculty of Resources, China University of Geosciences, Wuhan 430074, China

a r t i c l e

i n f o

Article history: Received 25 May 2010 Accepted 9 September 2010 Available online 17 September 2010 Keywords: Dioritic intrusion Petrogenesis Neoproterozoic Yangtze Block

a b s t r a c t Neoproterozoic magmatism along the northwestern margin of the Yangtze Block, South China, produced voluminous adakitic and normal arc felsic intrusions associated with many mafic–ultramafic bodies. Dioritic plutons are rare but are important for understanding the crustal growth and secular evolution of the region. The Xixiang dioritic intrusion at Hannan, on the northwestern margin of the Yangtze Block, provides a good opportunity to investigate the petrogenesis of intermediate plutons and to document the Neoproterozoic continental crustal evolution of the region. The pluton has a zircon U–Pb age of 764 ± 9 Ma. Rocks from the pluton have relatively low SiO2 (59.3–63.0 wt.%) and K2O + Na2O (5.8–6.2 wt.%) and high MgO (2.3–3.1 wt.%) and Al2O3 (15.8–17.1 wt.%). They have chondrite-normalized REE patterns enriched in LREE (La/Ybn = 7.2–14.7) with negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILE and depletion of HFSE (Nb and Ta) with positive Pb and negative Ti anomalies. They have a narrow range of initial 87Sr/86Sr ratios (0.7033 to 0.7035) with positive εNd values (+1.94 to +3.16). Their zircon εHf values range from +5.5 to +9.8 with an average of +7.32. These isotopic data are similar to those of the Neoproterozoic mafic–ultramafic intrusions in the Hannan region, suggesting that the diorites were generated by partial melting of newly emplaced basaltic rocks. The diorites have Mg# values (50.3–54.4) higher than those of experimental melts of mafic igneous rocks. In addition, they contain Fe-rich clinopyroxene, suggesting that partial melts of the newly formed mafic lower crust interacted with the mafic residue. Based on its arc-related geochemical affinity and emplacement preceding voluminous adakitic magmatism, the Xixiang intrusion is proposed to have been generated in a Neoproterozoic arc setting during crustal growth and reworking that lasted from N950 Ma to b 730 Ma, a period of more than 200 m.y. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Dioritic rocks are widely distributed on the earth, and are thought to have originated from magmas that differentiated at deep crustal levels. Thus, they represent an important “window” into lower crustal magmatic processes (Marzouki et al., 1982; Coleman et al., 1995; Roberts et al., 2000). They are closely related to continental crustal evolution and contain important information on the interaction between crust and mantle (e.g. Arth et al., 1978; Stern and Hanson, 1991; Smithies and Champion, 2000). Neoproterozoic igneous rocks are widely distributed along the margins of the Yangtze Block, South China, and are characterized by voluminous Neoproterozoic felsic intrusions associated with many mafic–ultramafic bodies (Zhou et al., 2002a,b, 2006a,b; Li et al., 2003a, b; Wang et al., 2004; Zhao and Zhou, 2007a,b, 2009a,b). Two contrasting models, mantle plume model and arc model, have been ⁎ Corresponding author. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China. E-mail address: [email protected] (J.-H. Zhao). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.09.005

proposed to explain their petrogenesis. The mantle plume model suggests that the Neoproterozoic magmatism around the Yangtze Block resulted from a mantle plume which initiated the breakup of Rodinia (Li et al., 2003a,b). The arc model argues that the mafic– ultramafic and felsic rocks were formed in arc-settings related to oceanic slab subduction beneath margins of the Yangtze Block (Zhou et al., 2002a,b, 2006a,b; Yan et al., 2004; Sun et al., 2007; Zhao and Zhou, 2007a,b, 2008, 2009a,b). Wang et al. (2004) further proposed that the Neoproterozoic igneous rocks along the southeastern margin of the Yangtze Block were products of post-collisional magmatisms related to breakoff of the subducted slab. Although the mafic–ultramafic and felsic igneous rocks have been extensively studied (i.e. Zhou et al., 2002a,b, 2006a,b; Li et al., 2003a, b; Ling et al., 2003; Sun et al., 2007; Zhao and Zhou, 2007a,b), intermediate assemblages are lacking. In the Hannan region at the northwestern margin of the Yangtze Block, there are abundant Neoproterozoic mafic–ultramafic and felsic intrusions (Fig. 1), thought to be the products of partial melting of the lithospheric mantle related to subduction (Zhou et al., 2002a; Zhao and Zhou, 2009a), and melting of a newly formed thickened lower crust (Zhao

440

J.-H. Zhao et al. / Lithos 120 (2010) 439–452 O

106 30' E

Archean-Mesoproterozoic strata Neoproterozoic Xixiang Group

107 O00' E

O

O

108 00' E

107 30' E

Unconformity Fault

Bijigou

Wudumen

Sample location

Granodiorite

Hanzhong Wangjiangshan

Diorite

3 3 O0 0 ' N

O

33 00'N

Mafic intrusions

Erliba

Granite

Cambrian strata Phanerozoic strata

Beiba

Xixiang Tarim QDO

North China

3 2 O4 0 ' N

3 2 O4 0 ' N

Sinian strata

B

TP

Tianpinghe 0 106 O30' E

107 O00' E

20Km

0

107 O30' E

800km

YB South China

108 O00' E

Fig. 1. Simplified geological map showing the distribution of Neoproterozoic intrusions in the Hannan region at the northern margin of the Yangtze Block, South China (modified from RGSXP, 1990).

and Zhou, 2008), respectively. The Xixiang intrusion is the only dioritic body in the Hannan region and provides a good opportunity to examine its source region and generation dynamics in relation to oceanic slab subduction during the Neoproterozoic. 2. Geological background South China consists of the Yangtze Block to the northwest and the Cathaysia Block to the southeast (Fig. 1), which were welded together during Meso- to Neoproterozoic time (Chen et al., 1991; Li and McCulloch 1996; Wang et al., 2007). The Yangtze Block is bounded by the Tibetan Plateau to the west, and separated from the North China Block by the Qinling–Dabie–Sulu orogenic belt to the north, which was formed by collision between the North China Craton and the Yangtze Block (Ames et al., 1996; Hacker et al., 1996). The Yangtze Block consists of basement complexes overlain by a Sinian to Cenozoic cover sequence. The Archean to Early Neoproterozoic basement complexes consist of metamorphosed arenaceous to argillaceous sedimentary strata. The thick cover sequence (N9 km) of Late Neoproterozoic to Permian strata mainly consists of glacial deposits, clastic and carbonate rocks (Yan et al., 2003). Neoproterozoic mafic–ultramafic and felsic bodies intrude the basement strata and are unconformably overlain by Late Neoproterozoic to Sinian strata (Zhou et al., 2002a,b, 2006a,b; Li et al., 2003a,b; Zhao and Zhou, 2007a,b). In the Hannan region, the northwestern margin of the Yangtze Block, pre-Sinian (N750 Ma) rocks exposed in a region of more than 1200 km 2 are mainly composed of Archean–Paleoproterozoic gneisses, amphibolites and migmatites, and Meso- to Neoproterozoic metasedimentary and volcanic rocks (Zhang, 1991; Ling et al., 2003). The Neoproterozoic Xixiang Group is a metavolcanic-sedimentary succession with zircon U–Pb ages of 950 and 895 Ma for its lower and upper units, respectively (Ling et al., 2003).

Many Neoproterozoic plutons intrude the Archean to Mesoproterozoic strata and are unconformably overlain by the Sinian strata, which is in turn overlain by Cambrian strata (Fig. 1). Mafic–ultramafic bodies include the Wangjiangshan, Bijigou, Luojiaba and Baiba intrusions which have SHRIMP zircon U–Pb ages ranging from 820 to 746 Ma (Zhou et al., 2002a; Zhao and Zhou, 2009a). The Wudumen and Erliba intrusions, which consist of granodiorite with adakitic geochemical characteristics, have SHRIMP zircon U–Pb ages of 735 ± 8 and 730 ± 6 Ma, respectively (Zhao and Zhou, 2008). These two felsic intrusions were proposed to have been generated by melting of a thickened mafic lower crust (Zhao and Zhou, 2008). The Tianpinghe intrusion is an I-type arc-related granite in the region. Ling et al. (2006) reported an ELA-ICPMS zircon U–Pb age of 863 Ma. However, Zhao and Zhou (2009b) reported a younger SHRIMP zircon age of 762 ± 4 Ma. The Xixiang pluton intrudes the Archean to Mesoproterozoic strata and the Xixiang Group. Along the margin of the pluton, the sedimentary rocks are metamorphosed to hornfels. It was in turn intruded by the Erliba and Wudumen intrusions to the west (Fig. 1). Rocks from the Xixiang pluton are pink in the field, and are distinct from the white to grey rocks of the Erliba and Wudumen intrusions. The rocks in this study were sampled along the road from Shahekan to Tongcheba (Fig. 1). 3. Analytical methods 3.1. EMP mineral analysis Mineral compositions were determined using a JXA-8100 electron microprobe at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS). The quantitative analyses were performed with an accelerating voltage of 15 kV, a specimen current of 3.0 × 10− 8 A, and a beam size of 2 um. The analytical uncertainties are generally less than 2%. Representative mineral compositions are listed in electronic Appendix 1.

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

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3.2. Zircon U–Pb isotopic analyses

3.4. Rb–Sr and Sm–Nd isotopic analyses

3.2.1. SHRIMP zircon U–Pb dating Zircon grains were separated using conventional heavy liquid and magnetic techniques, mounted in epoxy, polished, coated with gold, and photographed in transmitted and reflected light to identify grains for analysis. U–Pb isotopic ratios of zircon separates were measured using the SHRIMP II at the Institute of Geosciences, Chinese Academy of Sciences, China. The measured isotopic ratios were reduced off-line using standard techniques (see Claoue-Long et al., 1995) and the U–Pb ages were normalized to a value of 564 Ma determined by conventional U–Pb analysis of zircon standard CZ3. Common Pb was corrected using the methods of Compston et al. (1984). The 206Pb/238U and 207Pb/235U ratios were corrected for uncertainties associated with the measurements using the data from the CZ3 standard. The 206Pb/ 238 U ages given in Table 1 and Fig. 3a are independent of the standard analyses.

Rb–Sr and Sm–Nd isotopic analyses were performed on a VG-354 thermal ionization magnetic sector mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Chemical separation and isotopic measurement procedures are described in Zhang et al. (2001). Mass fractionation corrections for Sr and Nd isotopic ratios were based on values of 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Uncertainties in Rb/Sr and Sm/Nd ratios are less than ± 2% and ± 0.5% (relative), respectively.

3.2.2. Laser ablation ICP-MS zircon U–Pb dating U–Pb isotope compositions of zircon grains were also analyzed with an Agilent Q-ICP-MS connected to a 193 nm excimer laser ablation system at the Institute of Geosciences, Chinese Academy of Sciences, China. The GeoLas PLUS 193 nm excimer ArF laser ablation system is the upgrade product of GeoLas CQ made by Lambda Physik in Germany. Helium carrier gas transported the ablated sample materials from the laser ablation cell via a mixing chamber to the ICP-MS after mixing with Ar gas. Every 5 sample analyses were followed by one standard zircon 91500 and one NIST SRM 610 measurement. Details of the analytical techniques are described in Wu et al. (2006). The U–Pb ages were calculated using the U decay constants recommended by Steiger and Jager (1977) and IsoplotEx 3 software (Ludwig, 2003). Individual analyses are presented with a 1δ error in Table 2 and Fig. 3b and c, and age uncertainties are quoted at the 95% level. 3.3. Whole-rock geochemical analyses Major element abundances were obtained at the University of Hong Kong using X-ray fluorescence (XRF) on fused glass beads. Trace elements, including REE, were analyzed on a VG PQ Excell ICP-MS also at the University of Hong Kong. Closed beakers in high pressure bombs were used to ensure complete digestion, which is particularly suitable for analyzing Zr and Hf because all zircon is completely dissolved with a recovery of nearly 100% (Qi et al., 2000). Pure elemental standards for external calibration, and BHVO-1 and SY-4 as reference materials were used. Accuracies of the XRF analyses are estimated to be 1% for SiO2 and 2% for other major elements. The ICPMS analyses for trace elements yield accuracies better than 5%.

3.5. Zircon Hf isotopic analyses In situ zircon Hf isotopic analyses were carried out at the Institute of Geology and Geophysics, Chinese Academy of Sciences using a Neptune MC-ICP-MS with an ArF excimer laser ablation system. A spot size of 63 um and a laser repetition rate of 10 Hz with 100 mJ were used in the analyses. During analyses, 176Hf/177Hf and 176Lu/177Hf ratios of standard zircon (91500) were 0.282312 ± 15 (2δ, n = 15) and 0.000283, respectively approximately equal to the commonly accepted 176Hf/177Hf ratio of 0.282284 ± 3 (1δ) measured using the solution method (Goolaerts et al., 2004; Woodhead et al., 2004). We adjusted our data to their reference values. 4. Analytical results 4.1. Mineral chemistry The Xixiang intrusion consists of dioritic rocks composed mainly of microcline (35–40%), plagioclase (10–15%), amphibole (25–30%) and quartz (b20%), with minor Fe–Ti oxides, clinopyroxene, apatite and zircon (Fig. 2a–c). The microcline occurs as subhedral grains, 1–4 mm long, with Or values ranging from 88 to 99 and Ab values less than 11 (Appendix 1). Plagioclase grains are smaller, 0.2–0.8 mm in length, and consist chiefly of albite. They are tabular in shape and locally enclosed within microcline (Fig. 2b). Amphiboles occur as anhedral to subhedral grains, commonly 1– 2.8 mm long (Fig. 2c). They have relatively homogeneous compositions regardless of their shape and morphology. The amphiboles have (Ca + Na)B ≥ 1.00, NaB b 0.50 and CaB N 1.50, hence they belong to the calcic group and are further classified as edenite and magnesiohornblende associated with minor ferro-edenite according to the criteria of Leake et al. (1997). They have relatively constant Mg# values ranging from 0.51 to 0.60. Anhedral clinopyroxene grains are normally surrounded by amphiboles (Fig. 2c). They have Mg# varying from 0.70 to 0.74 and

Table 1 SHRIMP zircon U–Pb analytical results for the Xixiang intrusion in the Hannan region, South China. Spot

Concentration (ppm) Common

206

Pb

Sample XX16 (a diorite from the Xixiang XX-16.1 0.31 XX-16.2 0.43 XX-16.3 0.33 XX-16.4 0.25 XX-16.5 0.29 XX-16.6 0.38 XX-16.7 0.39 XX-16.8 0.08 XX-16.9 0.13 XX-16.10 0.24 XX-16.11 0.29 XX-16.12 0.23 XX-16.13 0.53

Age (Ma) U pluton) 122 100 78 141 95 92 146 86 129 103 95 175 71

Radiogenic 13.3 10.7 8.59 15.6 10.1 10.3 15.3 9.47 14.1 10.8 10.2 19 7.66

206

Pb

232

Th/238U

1.16 1.29 0.83 0.60 1.26 1.31 0.72 1.22 1.20 1.28 1.06 1.07 0.87

206

Pb/238U

773 751 772 779 751 786 740 777 772 741 764 767 758

δ

207

Pb/206Pb

±16 ±19 ±19 ±16 ±16 ±17 ±17 ±18 ±16 ±16 ±16 ±16 ±19

756 1008 869 669 740 656 563 814 782 590 306 419 645

δ

Discordant

± 48 ± 93 ± 63 ± 100 ± 60 ± 150 ± 67 ± 68 ± 56 ± 80 ± 130 ± 77 ± 100

−2 25 11 − 16 −1 − 20 − 31 4 1 − 26 − 150 − 83 − 17

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J.-H. Zhao et al. / Lithos 120 (2010) 439–452

Table 2 LA-ICP-MS zircon analytical results for the Xixiang intrusion in the Hannan region, South China. Analysis

207

Pb/206Pb

δ

206

Pb/238U

δ

207

Pb/235U

δ

Age (Ma) 207

Pb/206Pb

δ

206

Pb/238U

δ

207

Pb/235U

δ

Sample XX05 XX05-01 XX05-02 XX05-03 XX05-04 XX05-05 XX05-06 XX05-07 XX05-08 XX05-09 XX05-10 XX05-11 XX05-12 XX05-13 XX05-14 XX05-15 XX05-16 XX05-17 XX05-18 XX05-19 XX05-20

0.0654 0.0661 0.0674 0.0686 0.0596 0.0637 0.0627 0.0643 0.0671 0.0633 0.0646 0.0666 0.0686 0.0643 0.0590 0.0639 0.0674 0.0695 0.0776 0.0632

0.0037 0.0032 0.0038 0.0038 0.0034 0.0040 0.0035 0.0042 0.0039 0.0041 0.0030 0.0048 0.0027 0.0033 0.0031 0.0024 0.0032 0.0034 0.0032 0.0035

0.1278 0.1281 0.1258 0.1196 0.1329 0.1281 0.1262 0.1277 0.1255 0.1281 0.1267 0.1270 0.1248 0.1263 0.1280 0.1247 0.1254 0.1284 0.1276 0.1244

0.0046 0.0044 0.0045 0.0042 0.0047 0.0047 0.0045 0.0048 0.0045 0.0048 0.0043 0.0050 0.0041 0.0044 0.0044 0.0040 0.0043 0.0044 0.0042 0.0044

1.1516 1.1673 1.1682 1.1311 1.0912 1.1244 1.0909 1.1310 1.1610 1.1168 1.1280 1.1658 1.1815 1.1199 1.0403 1.0989 1.1656 1.2296 1.3651 1.0832

0.0621 0.0530 0.0630 0.0589 0.0589 0.0667 0.0570 0.0705 0.0635 0.0688 0.0503 0.0791 0.0441 0.0547 0.0522 0.0389 0.0524 0.0569 0.0538 0.0576

787 810 849 887 588 730 697 750 841 717 760 824 888 751 566 739 850 913 1136 713

115 97 114 110 119 127 113 133 116 132 96 142 79 105 110 77 95 97 81 114

775 777 764 728 804 777 766 774 762 777 769 771 758 767 776 757 762 779 774 756

26 25 26 24 27 27 25 28 26 28 25 29 23 25 25 23 24 25 24 25

778 785 786 768 749 765 749 768 782 761 767 785 792 763 724 753 785 814 874 745

29 25 30 28 29 32 28 34 30 33 24 37 21 26 26 19 25 26 23 28

Sample XX10 XX10-01 XX10-02 XX10-03 XX10-04 XX10-05 XX10-06 XX10-07 XX10-08 XX10-09 XX10-10 XX10-11 XX10-12 XX10-13 XX10-14 XX10-15 XX10-16 XX10-17 XX10-18 XX10-19 XX10-20

0.0626 0.0658 0.0649 0.0642 0.0631 0.0647 0.0651 0.0625 0.0647 0.0690 0.0627 0.0645 0.0651 0.0657 0.0635 0.0639 0.0667 0.0662 0.0654 0.0634

0.0035 0.0026 0.0030 0.0032 0.0026 0.0026 0.0031 0.0038 0.0028 0.0030 0.0032 0.0046 0.0034 0.0035 0.0027 0.0026 0.0034 0.0031 0.0032 0.0030

0.1261 0.1281 0.1299 0.1274 0.1258 0.1253 0.1284 0.1251 0.1262 0.1253 0.1260 0.1187 0.1275 0.1267 0.1254 0.1264 0.1261 0.1259 0.1259 0.1241

0.0044 0.0042 0.0044 0.0044 0.0041 0.0041 0.0044 0.0046 0.0041 0.0042 0.0043 0.0046 0.0044 0.0044 0.0041 0.0041 0.0044 0.0042 0.0043 0.0042

1.0883 1.1624 1.1629 1.1284 1.0942 1.1170 1.1517 1.0780 1.1249 1.1910 1.0888 1.0551 1.1445 1.1471 1.0982 1.1124 1.1597 1.1500 1.1346 1.0839

0.0571 0.0442 0.0505 0.0540 0.0425 0.0422 0.0523 0.0620 0.0455 0.0496 0.0522 0.0712 0.0565 0.0579 0.0449 0.0426 0.0561 0.0504 0.0532 0.0482

694 801 772 749 712 763 776 692 763 898 698 757 778 795 726 737 828 814 786 721

113 82 93 103 84 82 97 124 87 88 104 143 106 108 89 83 103 94 101 96

766 777 787 773 764 761 779 760 766 761 765 723 773 769 761 767 766 765 765 754

25 24 25 25 23 23 25 26 24 24 25 27 25 25 24 23 25 24 25 24

748 783 783 767 751 762 778 743 765 796 748 731 775 776 753 759 782 777 770 746

28 21 24 26 21 20 25 30 22 23 25 35 27 27 22 20 26 24 25 23

Na2O from 0.60 to 0.75 wt.%. Therefore, the clinopyroxenes range from augite to diopside with Wo ranging from 47.7 to 51.3, En from 34.0 to 38.4 and Fs from 13.4 to 14.8. Accessory minerals include titanite, Fe–Ti oxide, biotite and zircon. 4.2. Zircon dating results SHRIMP zircon U–Pb dating results are listed in Table 1. Zircons from diorite sample XX16 have oscillatory zoning, igneous morphologies and relatively low U concentrations ranging from 71 to 146 ppm. Thirteen analyses on 13 grains form a coherent group on concordia, yielding a weighted mean 206Pb/238U age of 764 ± 9 Ma (Fig. 3a). Two additional samples from the Xixiang intrusion were selected for LA-ICP-MS zircon U–Pb dating. Sample XX05 has 206Pb/ 238 U ages ranging from 728 Ma to 804 Ma (Table 2). Excluding the three analyses that deviate from concordia, the remaining seventeen analyses yield a mean weighted 206Pb/238U age of 768 ± 12 Ma (Fig. 3b). Sample XX10 yields similar 206Pb/238U ages ranging from 723 Ma to 787 Ma. Nineteen analyses give a mean weighted age of 767 ± 11 Ma (Fig. 3c). These three samples have similar 206Pb/238U ages as determined by SHRIMP and LA-ICP-MS methods. We consider that the Xixiang intrusion was emplaced at around 764 Ma. This age is consistent with the field relationships and magmatic activity in the Hannan region (Fig. 1).

4.3. Major and trace elements Rocks from the Xixiang intrusion mainly plot in the diorite and monzonite fields in the SiO2 vs. Na2O + K2O diagram (Fig. 4a). These rocks have relatively low SiO2 (59.3–63.0 wt.%), K2O (1.8–3.1 wt.%) and Na2O (3.0–4.8 wt.%), and high MgO (2.3–3.1 wt.%), Fe2O3 (4.6–5.4 wt.%), CaO (3.1–6.2 wt.%), Al2O3 (15.8–17.1 wt.%), TiO2 (0.53–0.73 wt.%) and P2O5 (0.12–0.15 wt.%) (Table 3). Thus, they are calcic–alkalic in composition (Fig. 4b).They are also metaluminous with A/CNK values ranging from 0.82 to 1.13 and A/CN values from 1.54 to 1.94. Rocks of the Xixiang intrusion have chondrite-normalized REE patterns rich in LREE with La/YbN ratios from 7.2 to 14.7, and pronounced negative Eu anomalies (Eu/Eu* = 0.66–0.97) (Fig. 5a). In the primitive mantle-normalized trace element diagram (Fig. 5b), they are enriched in Rb, Ba, Th and U, and depleted in Nb, Ta and Ti. They have relatively low Sr (304–664 ppm) but high Y concentrations (12.9–28.8 ppm), yielding low Sr/Y ratios ranging from 18 to 50. They also have relatively high compatible trace elements, such as V (95–137 ppm) and Cr (32–49 ppm) (Table 3). 4.4. Sr–Nd–Hf isotope compositions Whole-rock Sr–Nd isotopes are listed in Table 4. The initial Sr–Nd isotopes were recalculated to 764 Ma. Six samples from the Xixiang

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

443

a

a

0.14 XX16

820

0.13

Am

780

740

0.12

206

Pb/238U

Ab

700

0.11

206

Qtz

238

Mean Pb/ U weight age of 13 spots = 763.8 9.4 (95% conf.) MSWD=0.76

660 620

0.10 0.6

0.8

1.0

1.2

207

b

1.4

1.6

Pb/235U

b 0.15 880

XX05

0.14

Pb/238U

Ab

0.13

206

800

Mic

Mic

840

0.12

760 720

Ab 680

0.11

206

238

Mean Pb/ U weight age of 17 spots = 768 12 (95% conf.) MSWD=0.10

640

0.10

c

0.8

1.0

1.2 207

1.4

1.6

Pb/235U

c 0.14

Cpx

840 XX10

800

Am

Am

760

0.12

720

206

Cpx

Pb/238U

0.13

680

0.11

206

640 Fig. 2. Photomicrographs of the rocks from the Xixiang intrusion in the Hannan region, South China. (a) Subhedral to anhedral albites which have relatively homogeneous compositions. (b) Anhedral albite enclosed within microcline that shows typical cross-hatch twinning. (c) Anhedral clinopyroxenes are surrounded by amphibole. Am= amphibole, Cpx=Clinopyroxene, Ab= albite, Mic= Microcline, Qtz= quartz. The scale bar is 0.5 mm.

dioritic pluton have relatively homogeneous initial 87Sr/86Sr ratios ranging from 0.703306 to 0.703522 and positive εNd values from + 1.94 to+ 3.16 (Table 4 and Fig. 6). Lu–Hf isotope analyses for zircon grains from three samples of the Xixiang intrusion are presented in Table 5 and are plotted in Fig. 7. 176 Lu/177Hf ratios range from 0.000337 to 0.000965 and present-day 176 Hf/177Hf ratios from 0.282471 to 0.282552. Initial 176Hf/177Hf ratios vary from 0.282465 to 0.282547. εHf values range from + 5.5 to + 9.8 with an average of +7.32 ± 0.22 (Fig. 7a). Single-stage model ages for

0.10 0.8

0.9

238

Mean Pb/ U weight age of 19 spots = 767 11 (95% conf.) MSWD=0.10

1.0

1.1 207

1.2

1.3

1.4

Pb/235U

Fig. 3. SHRIMP (a) and LA-ICP-MS zircon U–Pb ages (b and c) for the Xixiang intrusion in the Hannan region, South China.

these samples range from 0.97 to 1.09 Ga and two-stage model ages from 1.09 to 1.30 Ga. The weighted means of the single-stage and two-stage model ages are 1.04 ± 0.01 Ga (MSWD = 6.3) and 1.22 ± 0.01 Ga (MSWD = 6.3), respectively (Fig. 7b). 5. Discussion Igneous rocks of intermediate composition are normally considered to have been produced by differentiation of mantle-derived magmas

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J.-H. Zhao et al. / Lithos 120 (2010) 439–452

a

albite is magmatic in origin, and that the rocks from the Xixiang intrusion did not undergo significant low-temperature albitization. Al, Ca and Mg (e.g., Beswick, 1982), high field strength elements (HFSE), such as Th, Zr, Hf, Nb, Ta, Ti, Y and REE including the Sm–Nd isotopic system, are typically immobile during low-temperature alteration (Barnes et al., 1985). Although Na, K, Rb and Sr can be easily changed during alteration, the rocks from the Xixiang intrusion have nearly constant Na2O (3.0–4.8 wt.%) and K2O contents (1.8– 3.1 wt.%), suggesting that they were not significantly modified by low-temperature alteration. In addition, the rocks show good correlations for K2O and Sr against CaO, suggesting that K2O and Sr were mainly controlled by Ca-bearing minerals and not modified by alteration. Therefore, the geochemical compositions of the Xixiang intrusion are believed to approximately reflect their source characteristics.

9

N a 2O + K 2O ( w t . % )

Quartz Monzonite

Monzonite

8

Granite

7 Monzodiorite

6 5 4

Diorite

Granodiorite

Gabbroic Diorite

3 50

55

60

65

70

75

80

SiO2 (wt.%)

b N a 2O + K 2O - C a O ( w t . % )

8

Alk

4

0

Al

k

-c ali

Ca -4 50

alc

a lc55

ali

c

ic

lka

lic

Ca 60

lci

c

65

70

SiO2 (wt.%) Fig. 4. Plots of total alkalis vs. SiO2 (Middlemost, 1994) and Na2O + K2O–CaO vs. SiO2 (Frost et al., 2001) for the rocks from the Xixiang intrusion in the Hannan region, South China.

(e.g. Arth et al., 1978), or melting of the lower mafic crust (e.g. Tepper et al., 1993; Muir et al., 1995), with or without magma mixing and interaction (e.g. Appleby et al., 2008). Identification of the processes occurring within the crust is of critical importance given that evolved magmas create the continental crust which is the differentiated endmember of the solid Earth (e.g. Rudnick, 1995; Tatsumi and Suzuki, 2009). 5.1. Alteration effects Both low-grade metamorphism and alteration can modify mineral and whole-rock compositions (e.g. Lee and Parsons, 1997; Holness, 2003). Rocks from the Xixiang intrusion are mainly composed of relatively fresh microcline, amphibole and plagioclase (Fig. 2). Lack of deformation and low-grade metamorphic minerals, such as epidote and chloritoid, suggests that the rocks of the Xixiang intrusion are unmetamorphosed. Rocks from the Xixiang intrusion have LOI less than 4 wt.% (Table 3), indicating that they underwent slight low-temperature alteration. However, albites in the Xixiang intrusion are tabular in shape and have uniform compositions (Appendix 1). Most of the albites occur as fine- to medium-grained, commonly subhedral crystals, which are locally enclosed within the microline grains (Fig. 2a and b), suggesting that the albite crystallized early or simultaneously with the microcline. In addition, the absence of Carich plagioclase suggests that the albite was not produced by albitization of Ca-plagioclase (Fig. 2). Therefore, we infer that the

5.2. Differentiation of mantle-derived mafic magma Diorites produced by differentiation of mantle-derived magmas are usually comagmatic with mafic and felsic igneous rocks and show a continuous range of compositions (e.g. Arth et al., 1978; Roberts et al., 2000; Litvinovsky et al., 2002). Mantle-derived diorites are characterized by high Mg#, Cr and Ni, and are exemplified by Archean high Mg diorites (e.g. Stern and Hanson, 1991; Smithies and Champion, 2000). In the Hannan region, there are numerous Neoproterozoic mafic–ultramafic intrusions, which may have been precursors of the Xixiang intrusion (Fig. 1). Rocks from the Xixiang intrusion have whole-rock εNd values and zircon Hf isotopic compositions that are similar to those of the Neoproterozoic mafic– ultramafic rocks in the Hannan region (Figs. 6 and 8). This suggests that the Xixiang diorites and the mafic–ultramafic rocks may have been derived from the same source region, and that the Xixiang diorites could have been generated by differentiation of mantlederived mafic magmas without significant crustal contamination. The mafic–ultramafic rocks from the Hannan region are calcic in composition (Zhou et al., 2002a; Zhao and Zhou, 2009a) and have plagioclase with An values ranging from 52.8 to 89.3 (Su, 2004). However, feldspars from the Xixiang intrusion consist of albite and microcline with no transitional phases (Appendix 1), and are unlikely to have formed by differentiation of the mantle-derived calcic magmas. In addition, pyroxenes in equilibrium with melts similar in composition to their host rock have not been identified (Fig. 9a). Thus, it is unlikely that the Xixiang diorites were produced by differentiation of mantle-derived mafic magmas. The Xixiang diorites have relatively low MgO (2.3–3.1 wt.%) and Cr concentrations (32–49 ppm) compared with those of mantle-derived, high Mg diorites (Stern and Hanson, 1991; Smithies and Champion, 2000). In addition, the Xixiang diorites have relatively high La, Sm and Th and low Sc concentrations, which cannot be achieved by high degrees of fractional crystallization from mafic magmas. For example, removal of 80% mafic minerals from the proposed parental magma, represented by the average composition of the mafic–ultramafic intrusions from the Hannan region, would increase Th from its initial concentration of 0.6 ppm to 3.0 ppm (Zhao and Zhou, 2009b). Th concentrations of the Xixiang diorites (3.3–9.7 ppm) are significantly higher than 3.0 ppm, arguing against derivation by differentiation of mantle-derived magmas. This conclusion is also supported by the compositional gap between the Xixiang diorites and the mafic– ultramafic rocks. 5.3. Dehydration melting of mafic crust under low pressure Alternatively, the Xixiang diorites may have been generated by melting of mafic crustal material. Sill-like intrusions are thought to be a major component of mid- to lower crustal regions (Bohlen, 1987). Partial melting of mafic intrusions newly emplaced into the lower crust can produce voluminous silicic magmas with initial isotopic

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

445

Table 3 Major and trace elements for the rocks from the Xixiang intrusion in the Hannan region, South China. Sample

XX01

XX02

XX03

XX04

XX05

XX06

XX07

XX08

XX09

XX10

XX11

XX12

XX13

XX14

XX15

XX16

XX17

XX18

Major elements (wt.%) SiO2 61.14 62.35 TiO2 0.53 0.63 Al2O3 16.23 16.39 Fe2O3 4.99 4.99 MnO 0.06 0.06 MgO 2.89 2.74 CaO 3.58 3.13 Na2O 3.03 3.47 K2O 3.12 2.90 P2O5 0.14 0.14 LOI 3.91 3.04 Total 99.62 99.83

61.64 0.60 15.96 4.61 0.06 2.65 4.07 3.61 2.99 0.13 3.29 99.59

59.77 0.64 16.23 5.36 0.09 2.71 6.23 3.96 1.87 0.15 2.39 99.37

60.66 0.59 16.66 5.06 0.09 2.71 5.19 4.26 2.25 0.13 1.89 99.49

61.31 0.61 15.90 4.82 0.09 2.58 5.11 3.64 3.26 0.13 1.61 99.06

59.46 0.58 16.61 5.04 0.09 2.78 5.08 4.22 2.29 0.14 2.34 98.62

66.51 0.37 15.08 2.71 0.06 1.46 2.72 2.49 6.21 0.08 1.56 99.24

61.01 0.63 16.95 5.00 0.08 2.73 5.08 4.23 1.93 0.13 2.05 99.82

60.08 0.62 16.24 5.44 0.09 3.00 5.20 3.79 2.24 0.15 2.07 98.92

62.97 0.59 16.02 4.06 0.05 2.26 3.83 4.57 2.66 0.12 2.21 99.36

61.45 0.63 15.85 4.70 0.05 2.52 3.67 4.05 3.07 0.13 3.14 99.25

60.72 0.61 16.62 4.84 0.07 2.63 5.10 4.75 2.05 0.14 2.17 99.68

59.41 0.64 16.91 4.63 0.06 2.48 5.13 4.79 2.22 0.13 2.98 99.38

59.69 0.61 16.50 4.59 0.07 2.73 5.00 4.63 2.37 0.13 2.62 98.93

60.66 0.71 16.53 5.06 0.07 2.76 4.98 4.58 1.92 0.15 2.15 99.58

59.25 0.73 16.49 5.42 0.08 3.09 5.38 4.41 2.09 0.15 2.07 99.16

59.52 0.72 17.06 5.17 0.07 2.95 5.56 4.58 1.77 0.15 1.93 99.47

Trace elements (ppm) Ga 19.5 18.9 Cs 2.24 2.02 Sc 17.6 15.8 V 120 116 Cr 45.5 43.2 Co 16.9 16.7 Cu 30.7 39.7 Zn 57.1 55.5 Rb 104.9 104.1 Sr 304 332 Y 18.5 14.6 Zr 152 151 Nb 6.03 5.74 Ba 653 663 La 19.6 23.8 Ce 44.8 49.6 Pr 5.36 4.88 Nd 22.6 18.4 Sm 4.70 3.68 Eu 1.13 0.95 Gd 4.22 3.36 Tb 0.61 0.46 Dy 3.26 2.39 Ho 0.68 0.50 Er 1.80 1.43 Tm 0.26 0.19 Yb 1.69 1.25 Lu 0.26 0.20 Hf 3.85 3.40 Ta 0.43 0.34 Pb 5.36 8.69 Th 4.98 9.67 U 1.84 1.26

18.1 1.29 17.3 112 37.8 16.0 40.5 54.1 81.8 481 16.9 211 5.91 772 16.4 38.2 4.61 18.5 4.29 1.01 3.76 0.53 2.85 0.61 1.60 0.23 1.48 0.23 4.92 0.46 9.09 3.72 0.86

20.8 1.07 22.5 130 49.4 17.8 28.7 54.3 58.5 664 17.4 220 6.44 426 31.4 61.9 6.08 22.5 4.66 1.07 4.39 0.58 2.86 0.63 1.72 0.24 1.53 0.24 5.04 0.39 11.36 9.03 0.89

19.6 1.76 26.9 119 36.9 17.4 36.6 55.2 70.3 542 18.0 285 5.87 533 19.6 42.7 4.96 19.0 4.11 1.01 3.94 0.56 2.92 0.62 1.67 0.24 1.53 0.25 6.32 0.48 9.67 5.74 0.90

19.5 1.42 20.8 124 38.8 15.5 105.6 53.5 80.2 518 28.8 228 12.01 839 50.4 100.2 10.72 37.8 7.96 1.81 6.97 0.93 5.09 1.06 2.83 0.42 2.69 0.40 5.59 1.74 14.92 24.56 3.44

19.8 1.11 23.2 125 40.1 16.9 43.2 56.8 63.6 567 17.9 163 5.54 613 19.2 43.9 5.11 21.4 4.61 1.14 4.11 0.59 3.19 0.68 1.79 0.25 1.57 0.26 3.84 0.44 10.02 6.41 1.21

14.3 1.90 16.2 64.8 20.80 7.92 181 30.0 120 426 18.5 120 7.53 1346 22.7 53.1 5.76 21.9 4.52 1.35 4.40 0.61 3.07 0.66 1.90 0.27 1.73 0.28 2.83 0.90 20.93 17.28 5.86

20.1 1.56 14.5 118 37.5 16.3 72.8 60.1 75.0 536 13.2 204 5.09 289 18.3 35.9 3.89 15.7 3.39 0.75 3.20 0.45 2.38 0.49 1.27 0.17 1.14 0.17 4.35 0.25 8.51 7.44 0.76

20.8 1.18 19.7 133 46.2 17.5 49.0 61.3 67.9 551 21.2 180 5.84 595 25.5 53.0 6.09 24.7 5.38 1.12 4.73 0.66 3.54 0.72 2.01 0.28 1.73 0.26 4.03 0.33 9.17 6.06 0.61

16.8 1.20 15.0 95.1 38.2 13.3 35.3 46.8 67.3 431 13.4 164 4.51 669 12.4 29.3 3.53 14.5 3.20 0.87 3.04 0.42 2.18 0.49 1.28 0.17 1.23 0.18 3.71 0.25 7.48 3.32 0.89

18.6 2.16 16.4 115 43.7 15.2 35.4 56.9 98.9 380 15.7 173 4.55 645 13.9 31.8 3.75 15.8 3.70 0.80 3.38 0.50 2.65 0.57 1.55 0.22 1.34 0.20 3.90 0.27 8.94 3.79 1.38

18.6 1.17 17.0 116 42.9 15.5 29.1 53.3 61.3 509 14.7 275 4.80 578 17.1 34.7 4.08 15.7 3.44 1.03 3.21 0.47 2.42 0.51 1.42 0.20 1.36 0.22 5.42 0.33 8.28 4.33 1.52

18.4 1.32 14.9 114 32.2 14.9 44.3 52.5 63.9 557 13.5 132 4.58 598 17.8 36.7 3.93 15.7 3.38 0.97 3.20 0.43 2.29 0.49 1.31 0.18 1.21 0.18 2.83 0.28 9.81 4.83 0.66

18.4 1.57 17.6 102 44.6 15.0 33.4 51.3 73.3 559 14.2 160 3.86 638 16.7 35.6 4.03 16.6 3.41 1.01 3.33 0.45 2.57 0.52 1.34 0.19 1.35 0.20 3.51 0.22 7.46 4.17 0.81

18.9 1.10 17.1 126 33.9 16.5 45.5 56.3 56.2 528 12.9 201 4.30 593 18.3 36.6 3.79 16.2 3.52 0.97 3.26 0.43 2.28 0.49 1.31 0.16 1.15 0.18 4.15 0.25 8.18 6.76 1.21

19.9 0.96 17.9 137 45.4 19.0 30.9 65.6 67.7 599 13.4 180 4.72 659 17.6 38.2 3.91 15.2 3.42 1.03 3.41 0.41 2.23 0.46 1.21 0.18 1.21 0.17 3.77 0.26 8.61 4.30 0.87

20.1 0.88 17.8 123 42.1 17.3 41.5 60.0 50.4 645 12.9 156 4.12 589 16.3 34.5 3.72 15.3 3.02 1.01 3.31 0.42 2.05 0.45 1.20 0.16 1.11 0.16 3.08 0.23 8.85 3.97 1.07

values the same as their mafic sources. The occurrence of numerous mafic–ultramafic intrusions in the Hannan region implies that the Neoproterozoic was an important period of continental crust growth (Zhao and Zhou, 2009a). The Erliba and Wudumen adakitic intrusions have Sr–Nd–Hf isotopic values similar to those of the mafic– ultramafic intrusions and are therefore proposed to have been generated by melting of a thickened lower mafic crust, which was formed by previous underplating of mantle-derived magma (Figs. 1 and 6) (Zhao and Zhou, 2008). Rocks from the Xixiang intrusion have whole-rock εNd and zircon Hf isotopic values similar to those of the Neoproterozoic mafic–ultramafic intrusions (Figs. 6 and 8), suggesting that they may have been generated in the same way as the adakitic intrusions. Melting of basaltic rocks under high pressure (N1.2 GPa) produces adakitic melts (Sen and Dunn, 1994; Rapp and Watson, 1995), which are characterized by high Sr/Y (N40) and (La/Yb)n ratios (N10) and low HREE contents due to the presence of garnet in the source regions (Defant and Drummond, 1990; Drummond and Defant, 1990). The rocks from the Xixiang intrusion have Sr/Y ratios ranging from 16.4 to 50.0 and (La/Yb)n ratios from 7.2 to 14.7 which are lower than those of typical adakites, indicating melting under moderate to low pressures.

Experimental melts of water-saturated basaltic rocks under low pressure (3–6.9 Kb) are strongly peraluminous felsic liquids, rich in Ca and poor in Fe, Mg, Ti, and K, whereas dehydration–melting experiments yield mildly peraluminous to metaluminous granodioritic to trondhjemitic melts which have been regarded as the main sources of arc-affinity granites (Beard and Lofgren, 1991). Rocks from the Xixiang intrusion are metaluminous and slightly enriched in MgO (2.26 to 3.09 wt.%) and TiO2 (0.53 to 0.73 wt.%). They plot in the field of the experimental melts of Rapp and Watson (1995) (Fig. 10), whose major element concentrations were produced in hightemperature runs between 1000 and 1100 °C. Thus, it is likely that the Xixiang diorites were generated by dehydration melting of basaltic rocks at high-temperature. Experiments by Rapp and Watson (1995) further reveal that low-degrees of dehydration melting (5–10%) of basaltic rocks produces high silica melts, whereas 20–40% melting produces high Al2O3 silicic to intermediate melts (trondhjemitic– tonalitic, granodioritic, quartz dioritic and dioritic), which form beyond the amphibole-out phase boundary. Rocks from the Xixiang intrusion are dioritic in composition and have low SiO2 (59.3–63.0 wt.%) and Al2O3 (15.8–17.1 wt.%), implying that they may have been produced by 20–40% dehydration melting of basaltic rocks.

446

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

a

10 500

MORB (t=0 Ma) MORB (t=750 Ma)

100

Nd

Sample/Chondrite

5

0 Adakites

10 -5

1

La Ce Pr Nd

OIB

Tianpinghe granite

-10 0.702

Mantle array

0.704

0.706

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

0.708

0.710

87

Sr /86Sri

b

Fig. 6. Plot of Sr–Nd isotopes for the rocks from the Xixiang intrusion. The fields for MORB and corrected MORB (T= 750 Ma) are from Zimmer et al. (1995). The Neoproterozoic mafic–ultramafic intrusions, adakites and Tianpinghe granites in the Hannan region are also shown for comparison (Zhao and Zhou, 2008, 2009a,b).

500

Sample/Primitive mantle

Neoproterozoic mafic intrusions in Hannan region

100

10

1 Rb Th Ta La Pb Sr Zr Sm Ti Tb Y Er Yb Ba U Nb Ce Pr Nd Hf Eu Gd Dy Ho Tm Lu Fig. 5. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for the rocks from the Xixiang intrusion. Normalization values are from Sun and McDonough (1989).

The rocks from the Xixiang intrusion plot outside the melt field for dehydration–melting of natural, low-K, calcic amphibolite (67.4% hornblende, 32.5% anorthite) at 10 kbar and 750–1000 °C (Wolf and Wyllie, 1994) (Fig. 10). They have relatively high K concentrations compared with the experimental melts of low-K olivine tholeiite and low-K Archaean greenstone (Fig. 11), suggesting that the source region of the Xixiang diorites was moderately K-enriched. In addition, they are different from experimental melts of alkali-rich basalt due to relatively low Na concentrations, precluding alkali basaltic sources. However, the chemical compositions of the Xixiang diorites are similar to those of magmas produced by melting of high-Al basalts (Fig. 11). Therefore, the protolith of the Xixiang diorites was probably K-rich, high-Al, calcic basalts, which are chemically equivalent to the Neoproterozoic mafic– ultramafic intrusions in the Hannan region (Zhou et al., 2002a; Zhao and Zhou, 2009a). Such geochemical characteristics of the basaltic protolith are typically exemplified by mantle-derived rocks in active continental margins (Miyashiro, 1974).

Amphiboles from the Xixiang dioritic rocks have SiO2 ranging from 38.0 to 48.3 wt.% and TiO2 from 0.8 to 1.6 wt.%, typical of magmatic varieties, and significantly different from secondary amphiboles, which have high SiO2 contents (N50 wt.%) (Rock, 1991). The Si contents of amphiboles are normally related to the Si contents of the magmas from which they crystallize (Giret et al., 1980). In amphiboles of the Xixiang dioritic rocks, Si correlates positively with Mg/(Mg + Fe2+) ratios and negatively with Fe2+ contents (Fig. 9b). These variations reflect progressive Fe depletion of the magma by abundant magnetite precipitation under oxidizing conditions (Kawakatsu and Yamaguchi, 1987). In summary, the Xixiang diorites are believed to have been produced by dehydration melting of the mafic portion of the continental crust under oxidizing condition and at relatively high temperatures (~1000 °C) but relatively low pressures (b1.2 GPa). Their protolith was an underplated basaltic layer derived from a mantle wedge which had been previously modified by subduction components.

5.4. Melt interaction with the residual mafic protolith Dehydration melting of the continental mafic crust normally produces silicic rocks which have low MgO contents (Beard and Lofgren, 1991; Sen and Dunn, 1994; Rapp and Watson, 1995; Rapp et al., 1999, 2002). Rocks from the Xixiang intrusion have relatively high MgO contents compared with experimental melts at the same SiO2 contents (Fig. 12). They also have higher MgO and lower SiO2 than the adakites in the Hannan region (Fig. 12), suggesting that the Xixiang dioritic rocks were not generated by melting of mafic rocks alone. Their relatively high Mg#s imply that additional mafic materials were involved in their primitive intermediate melts (Fig. 11).

Table 4 Sr–Nd isotopes for the rocks from the Xixiang intrusion in the Hannan region, South China. Sample No.

Rb (ppm)

Sr (ppm)

87

Rb/86Sr

XX4 XX7 XX10 XX11 XX14 XX17

54.1 65.3 65.2 72.5 61.3 62.3

640 553 504 454 548 561

0.2446 0.3415 0.3741 0.4626 0.3238 0.3213

87

Sr/86Sr

2δ

87

Sr/86Sr(i)

0.706193 0.707103 0.707495 0.708479 0.706930 0.706816

0.000009 0.000011 0.000014 0.000013 0.000011 0.000013

0.703522 0.703373 0.703409 0.703426 0.703394 0.703306

Sm (ppm)

Nd (ppm)

147

Sm/144Nd

3.98 3.53 4.71 3.11 2.69 2.53

21.3 17.4 23.7 15.5 13.7 13.1

0.1130 0.1227 0.1201 0.1213 0.1181 0.1170

143

Nd/144Nd

0.512347 0.512417 0.512353 0.512378 0.512374 0.512400

2δ

εNd

0.000009 0.000014 0.000012 0.000013 0.000013 0.000013

+ 2.51 + 2.95 + 1.94 + 2.30 + 2.54 + 3.16

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

447

Table 5 Zircon Hf isotopic compositions for the rocks from the Xixiang intrusion in the Hannan region, South China. Grain

176

Yb/177Hf

176

Lu/177Hf

176

Hf/177Hf

2δ

Hfi

εHf

δ

T1 (Ga)

δ

T2 (Ga)

δ

0.282510 0.282476 0.282513 0.282534 0.282552 0.282552 0.282487 0.282472 0.282513 0.282515 0.282488 0.282488 0.282515 0.282509 0.282509 0.282482 0.282523 0.282493 0.282510 0.282525

0.000019 0.000016 0.000016 0.000015 0.000018 0.000017 0.000017 0.000017 0.000015 0.000016 0.000015 0.000015 0.000017 0.000016 0.000017 0.000017 0.000016 0.000017 0.000015 0.000018

0.282502 0.282469 0.282507 0.282527 0.282547 0.282547 0.282480 0.282466 0.282507 0.282506 0.282481 0.282480 0.282509 0.282501 0.282503 0.282476 0.282518 0.282486 0.282504 0.282520

7.6 6.4 7.5 7.4 9.8 9.2 6.6 6.3 7.4 7.7 6.7 6.7 7.4 7.3 7.6 6.2 7.8 7.1 7.6 7.8

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.04 1.08 1.03 1.00 0.97 0.97 1.07 1.09 1.03 1.03 1.07 1.07 1.03 1.04 1.04 1.07 1.02 1.06 1.03 1.01

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

1.21 1.28 1.20 1.18 1.09 1.11 1.26 1.29 1.21 1.20 1.26 1.26 1.20 1.22 1.21 1.28 1.18 1.24 1.20 1.18

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.000508 0.000408 0.000370 0.000582 0.000965 0.000529 0.000517 0.000576 0.000462 0.000733 0.000382 0.000510 0.000722 0.000412 0.000371 0.000824 0.000703 0.000502 0.000415 0.000472

0.282516 0.282471 0.282477 0.282476 0.282507 0.282532 0.282480 0.282526 0.282511 0.282510 0.282537 0.282483 0.282530 0.282530 0.282500 0.282527 0.282487 0.282529 0.282508 0.282516

0.000024 0.000016 0.000015 0.000017 0.000019 0.000017 0.000016 0.000016 0.000017 0.000017 0.000016 0.000016 0.000019 0.000016 0.000015 0.000016 0.000017 0.000016 0.000016 0.000015

0.282509 0.282465 0.282472 0.282468 0.282493 0.282524 0.282473 0.282517 0.282504 0.282499 0.282531 0.282476 0.282519 0.282524 0.282495 0.282515 0.282476 0.282522 0.282502 0.282509

7.6 6.3 6.8 6.3 7.0 8.0 6.6 7.8 7.4 7.2 8.4 5.5 8.1 8.2 7.0 7.9 6.4 8.0 7.3 7.3

0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

1.03 1.09 1.08 1.09 1.05 1.01 1.08 1.02 1.03 1.04 1.00 1.07 1.02 1.01 1.05 1.02 1.08 1.01 1.04 1.03

0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

1.20 1.29 1.27 1.29 1.24 1.17 1.27 1.18 1.21 1.22 1.15 1.30 1.17 1.16 1.23 1.18 1.27 1.17 1.22 1.21

0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.000699 0.000813 0.000412 0.000840 0.000546 0.000794 0.000473 0.000799 0.000788 0.000726 0.000692 0.000792 0.000420

0.282489 0.282497 0.282512 0.282511 0.282495 0.282547 0.282527 0.282506 0.282545 0.282502 0.282511 0.282490 0.282536

0.000020 0.000023 0.000019 0.000018 0.000020 0.000020 0.000019 0.000020 0.000021 0.000019 0.000021 0.000020 0.000022

0.282479 0.282485 0.282506 0.282499 0.282487 0.282535 0.282520 0.282495 0.282533 0.282492 0.282502 0.282479 0.282530

6.7 6.4 7.6 7.5 6.5 9.0 7.4 7.3 8.6 6.5 7.3 6.6 8.2

0.3 0.4 0.3 0.3 0.4 0.3 0.3 0.4 0.4 0.3 0.4 0.3 0.4

1.07 1.06 1.03 1.04 1.06 0.99 1.01 1.05 1.00 1.05 1.04 1.07 1.00

0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02

1.26 1.26 1.20 1.21 1.26 1.13 1.19 1.22 1.14 1.25 1.22 1.27 1.15

0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03

Sample XX5 XX05-01 XX05-02 XX05-03 XX05-04 XX05-05 XX05-06 XX05-07 XX05-08 XX05-09 XX05-10 XX05-11 XX05-12 XX05-13 XX05-14 XX05-15 XX05-16 XX05-17 XX05-18 XX05-19 XX05-20

0.015419 0.013623 0.010547 0.013748 0.009357 0.008936 0.012053 0.010598 0.010170 0.017691 0.011589 0.015796 0.011169 0.016126 0.010323 0.011938 0.009285 0.011531 0.009987 0.009802

0.000552 0.000501 0.000393 0.000487 0.000349 0.000337 0.000449 0.000398 0.000382 0.000640 0.000447 0.000584 0.000411 0.000603 0.000398 0.000474 0.000364 0.000442 0.000391 0.000387

Sample XX10 XX10-01 XX10-02 XX10-03 XX10-04 XX10-05 XX10-06 XX10-07 XX10-08 XX10-09 XX10-10 XX10-11 XX10-12 XX10-13 XX10-14 XX10-15 XX10-16 XX10-17 XX10-18 XX10-19 XX10-20

0.014297 0.011432 0.010136 0.016517 0.027548 0.014730 0.014175 0.016308 0.012741 0.020956 0.010169 0.013861 0.020326 0.011034 0.009707 0.022776 0.019199 0.013019 0.011205 0.012590

Sample XX16 XX16-01 XX16-02 XX16-03 XX16-04 XX16-05 XX16-06 XX16-07 XX16-08 XX16-09 XX16-10 XX16-11 XX16-12 XX16-13

0.016901 0.020128 0.009654 0.020432 0.013370 0.019406 0.011142 0.020005 0.019575 0.018129 0.017084 0.019552 0.010026

Rocks from the Xixiang intrusion are approximately similar to that of the hybridised melt and sanukitoid in terms of SiO2, Mg# and Sr/Y ratios (Fig. 12), suggesting that high Mg# values of the Xixiang diorites may have resulted from hybridization by minor mafic magmas. For example, hybridization of the adakitic rocks in the Hannan region by the mantle-derived magma can produce intermediate magmas which are chemically similar to the Xixiang diorites (Figs. 6 and 12). Alternatively, the high Mg# values of the Xixiang diorites were probably resulted from interaction between their parental intermediate melts and mafic residues. If the Xixiang diorites were products of hybridization of intermediate melts by basaltic magmas, some primitive minerals should be identified and their compositions should have equilibrated with melts

similar in composition to their host rocks (e.g. Feeley and Dungan, 1996). However, clinopyroxenes in equilibrium with their host rocks have not been identified. Clinopyroxenes from the Xixiang intrusion are too rich in Fe to have crystallized at liquidus conditions in equilibrium with melts similar in composition to their host rocks (Fig. 9a). In addition, they are chemically similar to those of the Neoproterozoic mafic–ultramafic intrusions in the Hannan region (Fig. 9a). These mineralogical and geochemical evidence suggest that the clinopyroxenes were more likely inherited from their mafic protolith, not from the mantle-derived magma. This conclusion is consistent with the feldspar compositions of the Xixiang intrusion. The coexistence of albite and microcline in the rocks from the Xixiang intrusion suggests that they crystallized from magmas with different

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J.-H. Zhao et al. / Lithos 120 (2010) 439–452

a

a 12

1

0.3 Mean=7.32+/-0.22 MSWD=7.1 95% conf.

Number

8

(Fe/Mg) cpx (Fe/Mg) host rock

Kd=

0.9

Clinopyroxene Mg#

10

6 4

0.2 0.8

0.7

Mafic-utramafic intrusions in the Hannan region

2 0.6 0 4

5

6

7

8

9

10

11 0.5 0.75

Hf

b

1.25

1.5

1.75

2.25

2.5

2.0

Whole rock FeO*/MgO 10

b

Mean=1.22+/- 0.01 MSWD=6.3 95% conf.

0.60

6

4

2

0 1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

Amphibole Mg/(Mg+Fe2+)

8

Number

1.0

0.55

Increasing f O2

0.50

0.45

T2 (Ga) Fig. 7. Histograms of zircon εHf values (a) and two-stage model Hf ages (b) for the Xixiang intrusion in the Hannan region, South China.

0.40 6.5

7.0

7.5

8.0

8.5

Amphibole Si chemical compositions. The albite may have crystallized from Na-rich magmas which were produced by melting of the basaltic crust (Petford and Atherton, 1996). The microcline probably crystallized in the final stage after the magma interacted with the mafic residues and the upper crust. The Xixiang diorites are therefore believed to have been produced by relatively high degree melting of the newly formed mafic crust followed by interaction with the mafic residues. Thus, besides magma

Fig. 9. Plots of Clinopyroxene Mg# vs. whole-rock FeO*/MgO (a) and amphibole Si vs. Mg/ (Mg + Fe2+) ratios (b). Lines in Fig. 9a represent equilibrium between minerals and whole-rock compositions (Peeley and Dungan, 1996). Clinopyroxene compositions of the Neoproterozoic mafic–ultramafic intrusions in the Hannan region are from Su (2004).

mixing and mafic magma differentiation (e.g. Arth et al., 1978; Rapp and Watson, 1995), interaction between the intermediate magmas and the mafic residue is another important process in the formation of the Mg-rich dioritic igneous rocks.

20.0

5.5. Geodynamic implications 15.0 Depleted mantle

Hf

10.0 5.0 0.0

Neoproterozoic mafic-ultramafic intrusions in the Hannan region

-5.0 -10.0

Crustal residence time (1.09-1.30 Ga)

CHUR

0

500

1000

1500

Age (Ma) Fig. 8. εHf values corrected to the crystallization ages of zircons for the Xixiang diorites. Reference lines representing meteoritic Hf evolution (CHUR) and depleted mantle are from Blichert-Toft and Albarede (1997) and Griffin et al. (2000), respectively. εHf values for the Neoproterozoic mafic–ultramafic intrusions in the Hannan region are also shown for comparison (Zhao and Zhou, 2009a).

Although the mantle plume, as proposed by Li et al. (2003a,b), can initiate melting of the lithospheric mantle and continental crust, evidence for a Neoproterozoic mantle plume is lacking (such as voluminous basalts, radiating dike swarm and crustal doming) (Zhao and Zhou, 2007a). The mafic–ultramafic, intermediate and felsic rocks in the Hannan region can be best interpreted as having been emplaced in a subduction-related environment, further ruling out the possibility that a mantle plume existed beneath the Yangtze Block. The 950–895 Ma volcanic rocks of the Xixiang Group have arcaffinity geochemical features, suggesting formation in an environment related to oceanic slab subduction beneath the northern margin of the Yangtze Block (Ling et al., 2003). Their eruption was followed by emplacement of mafic–ultramafic intrusions with SHRIMP zircon U–Pb ages of 820–746 Ma (Zhou et al., 2002a; Zhao and Zhou, 2009a). These intrusions are believed to have been produced by partial melting of a mantle wedge which was modified by subduction components (Zhou et al., 2002a; Zhao and Zhou, 2009a). Therefore, the Neoproterozoic was an important period of continental crustal growth in the Hannan region,

J.-H. Zhao et al. / Lithos 120 (2010) 439–452

100

Mg#=molar MgO/((MgO+FeO)

TiO2 (wt.%)

3

2

1

0 50

449

55

60

65

Sanukitoid

60

Hybridised melts(4GPa)

Mantle melts

40

20

0 45

70

High-Mg andesites

80

Slab melts at 1-4 GPa (Residue=garn+cpx+/-amph)

50

55

60

SiO2 (wt.%)

Hannan adakites

65

70

75

80

SiO2 (wt.%) 1000

8 Rushmer (1991) Rapp and Watson (1995)

Slab melts (1-4 GPa)

Beard and Lofgren (1991)

6

100

Hannan adakite

Sanukitoid

Sr/Y

MgO (wt.%)

Wolf and Wyllie (1994)

Hybridised melts(4GPa)

4

Hannan adakites

10

High-Mg andesites

2

0 50

55

60

65

1 20

70

SiO2 (wt.%)

which occurred on an active continental margin that persisted for more than 200 m.y. (Zhao and Zhou, 2009a,b). Melting of newly formed mafic crust was also an important mechanism of continental crustal differentiation during the Neoproterozoic in the Hannan region. The 762-Ma Tianpinghe I-type granites were produced by dehydration melting of newly underplated basaltic rocks at temperatures above 780 °C (Zhao and Zhou, 2009b). Similarly, the Xixiang diorites were generated by melting of newly K

MgO Alkali-rich basalt High-Al basalt Low-K olivine tholeiite Low-K Archean greenstone

K 2 O+Na 2 O

Na

40

50

60

70

80

Mg#=molar MgO/((MgO+FeO)

Fig. 10. Plots of TiO2 and MgO against SiO2 for the rocks from the Xixiang intrusion. The background references fields are from Jung et al. (2002) and references therein. The Neoproterozoic adakites in the Hannan region are also shown for comparison (Zhao and Zhou, 2008).

Fe 2 O 3

30

Ca

Fig. 11. Ternary A–F–M and molar Na–K–Ca diagrams for the rocks from the Xixiang intrusion. The experimental melts for different protoliths are from Rapp and Watson (1995). The Xixiang diorites plot within the field for dehydration melts of high-Al basalts, but are slightly richer in MgO.

Fig. 12. Plots of SiO2 vs. Mg# and Mg# vs. Sr/Y for the rocks from the Xixiang intrusion. The reference fields for mantle melts, high Mg andesites, hybridised melts, sanukitoid and experimental melts under 1–4 GPa are from Souza et al. (2007) and references therein. The Hannan adakites are also shown for comparison (Zhao and Zhou, 2008). The Xixiang diorites are chemically different from the experimental melts and mantlederived melts, but similar to the hybridised melts and sanukitoid.

emplaced basaltic rocks followed by interaction with the mafic residue. Clearly, anatexis of a continental crust under high temperatures within continental collision zones requires significant heat input through under- or intraplating by mantle-derived basaltic magmas (e.g. Clemens, 1990; Vielzeuf et al., 1990). One-dimensional conductive heat transfer calculations reveal that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800 °C (Tepper et al., 1993). Both the Tianpinghe and Xixiang intrusions show normal arc-like geochemical compositions, suggesting that they were generated at depths of less than 50 km, which is the minimum depth for producing adakitic melts in the eclogitized lower crust (e.g. Rapp et al., 1991; Kay and Kay, 2002). Stype granites have not been identified in the Hannan region, suggesting that the thermal gradient was not high enough to melt the upper felsic continental components. The 735-Ma adakitic intrusions in the Hannan region were generated by melting of a thickened lower mafic crust, marking the beginning of continental crustal thinning (Zhao and Zhou, 2008). The magmatism between 950 Ma and 735 Ma produced a distinctive magmatic assemblage related to continental crustal growth and differentiation which resulted from oceanic slab subduction beneath the northern margin of the Yangtze Block. We therefore summarize the petrogenetic model for the Neoproterozoic igneous intrusions in the Hannan region as follows (Fig. 13): (1) Melts from the subduction modified lithospheric mantle were emplaced into the upper and

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Mafic-ultramafic intrusions (820-740 Ma)

Erruption

Basalt+andesite (950-890 Ma)

Xixiang diorite (762 Ma)

Underplating

Low pressure melting

High pressure melting

Tianpinghe I-type granite (762 Ma)

Hannan adakite (735 Ma)

Crustal delamination

Newly formed lower mafic crust

Crustal reworking

Subduction modified lithospheric mantle

Crustal growth

Differentiation

Fig. 13. An integrated model for the formation of the igneous intrusions and secular continental crustal evolution in an active continental margin during the Neoproterozoic in the Hannan region, South China.

lower crust. Magmas emplaced into the upper crust experienced strong differentiation, and formed mafic–ultramafic intrusions, basalts and andesites (Zhou et al., 2002a; Ling et al., 2003; Zhao and Zhou, 2009a). At the same time, underplating of the mantle-derived magma thickened the lower crust (e.g. Haschke and Günther, 2003), forming a relatively new lower crust and providing the heat for melting of the lower crustal rocks. (2) Intraplating or underplating of mantle-derived magmas under low pressure locally initiated melting of previously emplaced rock and generated the Tianpinghe I-type granites. Interaction of felsic magmas with mafic residue resulted in the Xixiang dioritic magma. These felsic and intermediate intrusions were the result of crustal thickening at an active continental margin. (3) At ~735 Ma, the continental crust reached a thickness of more than 50 km. Melting of such newly formed thickened lower mafic crust generated the Hannan adakitic intrusions marking the beginning of continental crust extension.

6. Conclusions The 764-Ma Xixiang intrusion is dioritic in composition and was produced by high degree melting of the newly emplaced basaltic rocks. Rocks from the Xixiang intrusion have higher Mg# values than the experimental melts, suggesting that the primitive intermediate melts interacted with the mafic residues. The intrusion resulted from intraplating of mantle-derived magmas during crustal thickening, suggesting that the Neoproterozoic was an important period of continental growth and reworking in the Yangtze Block. The igneous assemblage in the Hannan region records subduction of an oceanic slab and continental crustal growth during the Neoproterozoic. Supplementary data to this article can be found online at doi:10.1016/j.lithos.2010.09.005.

Acknowledgments This work was substantially supported by the National Nature Science Foundation of China (40873027, 90714002), the Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan) and a 973 project matching grant from the University of Hong Kong. We are grateful to Xiao Fu and Gao Jianfeng for geochemical analyses. Reviews by Nelson Eby, Paul T. Robinson and an anonymous referee improved an early draft of this paper and are gratefully acknowledged.

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