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622 Science in Ser. D Earth Sciences 2005 Vol.48 No.5 622—634

Petrogenesis and dating of the complex, Province

CHEN Yuelong1, LUO Zhaohua1, ZHAO Junxiang, LI Zhihong, ZHANG Hongfei & SONG Biao3

1. Faculty of Earth Sciences and Natural Resources, China University of Geosciences, Beijing 100083, China; 2. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; 3. Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Correspondence should be addressed to Chen Yuelong (email:[email protected])

Received July 14, 2003 Abstract An analysis of major and trace element geochemistry and SHRIMP U-Pb zircon dating of mafic, intermediate, and felsic rocks from the Mianning metamorphic complex of Si- chuan Province was conducted. One of our conclusions is that a majority of the zircons crystal- lized between 721 and 773 Ma in a magmatic regime. The complexes contain early to late Pa- leo-proterozoic relict zircons from ancient continental crust. The oldest age, 2468 Ma, may rep- resent the basement of the Yangtze block. Overgrowth zircon rims and palingenetic zircons have been observed and dated, and are believed to have formed after crystallization, during Paleozoic and Mesozoic metamorphic-anatexis. We also conclude that depletion of Nb, Ta, HREE, in a spider-diagram normalized to primitive mantle and calc-alkaline association, implies that they formed in island-arc or under-plating settings. Keywords: Kangding complex, zircons, SHRIMP, tectonic setting.

DOI: 10.1360/03yd0312

The Kangding group, also known as the Kangding evolution of the Yangtze block. It has been proposed complex, includes granulites, amphibolites, felsic that the complex was formed in the Neo-proterozoic gneisses and gneissic granites that are distributed (1200―706 Ma), primarily on the basis of a Sm-Nd along a belt from Kangding, Mianning to , isochron consisting of granulite, amphibolitic and to- in Sichuan Province. The complex has long been nalitic rocks from the Shaba, Mianning County, Si- thought to represent the crystalline basement of the chuan Province[5]. Xu et al. argued that granulitic Yangtze block. On the basis of U-Pb and Pb-Pb whole metamorphism took place shortly after formation of [1,2] rock ages , and of similarities in metamorphic facies the protolith, based on a Sm-Nd isochron age of 1140 and association of metamorphic rocks with typical Ma obtained from mineral separates of the Shaba Archean high grade terrains, the complex has been granulite and its initial Nd isotopic ratio[6]. Zhou et al. [3] widely believed to be Archean , although many zir- dated the Kangding complexes and some granites by con U-Pb ages from the complex are between 753― the SHRIMP U-Pb zircon method, which indicated 828 Ma[4]. Therefore, whether or not the complexes formation ages of between 796―797 Ma. They con- were formed during the Archean has become a crucial cluded that the granites and their metamorphic prod- issue in understanding the history of formation and ucts originated from island-arc magmatism[7]. There- Copyright by Science in China Press 2005 转载 中国科技论文在线 http://www.paper.edu.cn Petrogenesis and dating of the Kangding complex, Sichuan Province 623

fore, when the complexes formed, whether there are 1 Geological setting and sample description relict zircons from a continental crust or whether they The western margin of Yangtze block connects originated in an island-arc environment, or whether the Ganzi-Songpan orogenic belt, and has been known the inconsistencies of past U-Pb ages were caused by as the Sichuan-Yunnan geo-axis. There are the Peng- the mixing of different original zircons, SHRIMP tech- guan and Micangshan complexes to the north, and the nology may provide the answers to these issues. Mianning, Mopangshan-Miyi, Tongde, Dukou (Pan- Recently, Li et al. suggested that South China (in- zhihua) complexes to the south around the Kangding cluding the Yangtze block) is the lost part of North complex, in Sichuan Province[16], with a total outcrop- America from between the Australian and Laurentian ping area of 2000 km2. The Kangding group/ complex continents in the light of: 1) contemporary Neo-pro- is thought to be the metamorphic basement in the area. terozoic mafic rocks (gabbros, mafic dykes and sheets) The group is divided into different formations, from in the Yangtze and Cathaysia blocks with Australian the bottom to top, according to the earliest strati- Gairdner swarms, and 2) similarities in tectonic set- graphic correlation, such as the Zali Formation of the tings and their tectonostrata[8,9]. They concluded that lower part, which is composed of plagioclase amphi- the mafic swarms are indicators of the breakup of bolites, tonalitic-trondjemitic-granodioritic (TTG) gn- Rodinia from mantle plume activity. Li et al.[10] also eisses. Next is the Lengzuguan Formation of the in- concluded that the Precambrian tectonics of East Asia, termediate-upper part, characterized by associations of and its relevance to super-continent evolution, i.e., biotite-plagioclase gneisses, gneissic kali-granites[1]. Rodinia, occurred during late Meso-proterozoic to The Kangding group, from Mianning in the north to Neo-proterozoic (>900 Ma), while the breakup of Dukou in the south, was metamorphosed to a granu- Rodinia started as early as 850 Ma to ca. 750―700 litic facies[17]. During recent geological mapping, at a Ma. scale of 1:50000, the complexes were treated as gneissic plutons in Mianning1), for instance, the Ze- Geochronologic studies have shown that main yuan gneiss and the Langhuan gneiss, corresponding metamorphic basement outcroppings, in the eastern to the Zali and Lengzuguan formations, respectively. margin of the Yangtze block (Zhejiang and Fujian Amphibolite and granulite are lenticules at different provinces), formed in 1.6 to 2.8 Ga[11]. Nd model ages scales in TTG gneisses at Shaba, Mianning County. of a clastic sedimentary profile, from Sinian to Juras- The numbers of amphibolitic lenticules decrease in sic at , Sichuan Province, indicate that the biotite-plagioclase gneisses of the intermediate-upper main part of the Yangtze block should have formed at part. The samples used in this study include granitic [12] about 2.1Ga . Recent Sm-Nd and zircon SHRIMP gneiss (98922-5-3), amphibolite (98921-4-1), and U-Pb geochronological studies indicate that the rocks fine-grained granulite (98922-4-1). The granitic gneiss of the Kongling group, from the middle segment of the shows a mylonitic texture, which is composed mainly northern Yangtze block, formed between 1.99 and 3.28 of plagioclases, microclines, and quartzs, with minor [13 ― 15] Ga . This suggests that the basement of the biotites, and displays significant orientation. The fine- Yangtze block may have a rather complicated history. grained granulite, with an equigranular texture, is a The Kangding complexes should not represent the lenticule in the granitic gneiss, and consists of horn- basement of the Yangtze block, whose formation age blendes, plagioclases with minor components of garnet, and tectonic environment are key to understanding the hypersthene, biotite, microcline and magnetite. The history of formation and evolution of the Yangtze amphibolite, with a medium-grained subhedral, granu- block as a whole. lar texture, is composed of plagioclases, hornblendes,

1) The Panxi Geological Survey Team of Sichuan Bureau of Geology and Mineral Resources, Geological Map and Notation of the People’s Republic of China, Scale of 1:50000, the Lugu, Salian Breadths, 1995.

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and biotites. The latitudes, longitudes and Nd model tra-standard solution. The sample solutions were di- ages of the samples are listed in Table 1. Fig. 1 shows luted to ca. 40 g and oscillated. The analyses were the sample localities and a geological sketch of the conducted by PQ2 Turbo ICP―MS of VG, British. studied area. Dynamic linear range of the instrument is 8 orders of 2 Samples preparation and analytical methods magnitude. Reference material used in this study is GSR-1 of the National Standard Reference Materials. The samples were powdered into less than 200 Deviations of the measured values from the recom- meshes by a cleaned vibration crusher, or agate mortar, mended values are less than 4% for most elements and using purified ethanol. Major and trace element com- less than 12% for other elements. positions were analyzed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, by X-ray For zircon separation, the samples were crushed at fluorescence spectrum and ICP-MS, respectively. the Langfang Laboratory of Geophysical Exploration, Relative errors estimated from duplicate samples are Geological Exploration Bureau of Hebei Province, less than 4% for major compositions of the samples. Ministry of Land and Mineral Resources. After being For trace element measurements, sample powders of elutriated by an oscillation bed, or panned out, zircon ca. 100 mg were digested in a Teflon bomb at 200℃ grains were hand-picked under a binocular microscope for dating. for 48 h with purified HNO3+HF. After complete di- gestion, the sample solutions were dried twice with The zircons from both samples and standards (TEM, −1 purified HClO4. Then, the samples were extracted, 417Ma; SL13, 572 Ma with 238 µg · g of U) were using purified HNO3 and H2O in closed bombs for cast in an epoxy mount, which was then polished ac- over 12 h at 200℃. The sample solution cooled down cording to the method described by Song et al.[18] Zir- to room temperature, weighed, and then transferred cons were examined for morphology and internal into Teflon bottles with 1 mL of 1 µg · g−1 In in- structure using optical microscopy and cathodelumi-

Table 1 The Coordinates and Nd model ages of samplesa)

No./Lithology Longitude/(°E) Latitude/(°N) εNd/(770 Ma) tDM/Ga 98922-4-1/grey-black fine-grained granulite 102.0402 28.2355 −0.95 1.56 98921-4-1/amphibole leptite 102.0633 28.4439 1.33 1.33 98922-5-3/granitic gneiss 102.0301 28.2438 1.21 1.29 a) Data are from Chen, Luo, Liu[5].

Fig. 1. The geological sketch map and distribution of samples in -Mianning area, Sichuan Province (revised after the geological map in the regional geological chronography[1]). 1, Himalayan tectonostrata; 2, Yanshanian-Indochina strata; 3, Variscan tectonostrata; 4, Caledonian tec- tonostrata; 5, Precambrian tectonstrata; 6, granite; 7, diorite; 8, deep and great fault; 9, city or county; 10, sampling locality.

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nescence imaging. Then the mount was re-coated with under a microscope through transmission light. The a 50Å layer of high-purity gold (99.999%) after sizes of zircon grains vary from 37 to 127 µm in re-polishing. Measurements of U, Th, and Pb (Table 3) length, and 13 to 46 µm in width, based on statistical were conducted using SHRIMP II at the Beijing Ion results of 89 grains (Table 3). The averaged length, Probe Center. After measurement, dated grains were width, and ratio of length to width of zircons, from the observed for their cathodoluminescence images at the granitic gneiss, are 56 µm, 30 µm, and 1.89, respec- Institute of Geology and Geophysics, Chinese Acad- tively. The zircons of the amphibole leptite are char- emy of Sciences (Fig. 2). acterized by an averaged length of 68 µm, width of 26 3 Petrochemistry µm, and ratio of 2.69 for length to width. The zircons The concentrations of major and trace elements, and of grey-black fine-grained granulite are averaged at a calculated normal mineral contents of Q, An, Ab, and length of 63 µm, width of 24 µm, and ratio of 2.75 Or of the three samples, by CIPW method, are listed in between length and width. Therefore, the zircons from Table 2. They belong to the calc-alkaline series, based the amphibole leptite and granulite show elongated on their petrochemistry. The grey-black fine-grained prisms. In contrast, the zircons of the granitic gneiss granulite (98922-4-1) is basaltic-gabbroic. Amphibole display relatively short prisms. Zonation structure can leptite (98921-4-1) is andesitic-dioritic. Sample be observed for most zircon grains in CL images. Most 98922-5-3 is granitic gneiss. of them are indicative of a magmatic origin. Bright or Figure 3(a) shows REE patterns of these samples dark overgrowth margins can be observed for some normalized by a chondrite[19]. The granitic gneiss is zircons around central cores, which are shown as 1, 4, characterized by strong fractionation between LREE 6, 7, 8, 9, 10, 11 grains of 98922-5-3 and 2, 12 grains and HREE, with no prominent Eu anomaly. Fractiona- of 98922-4-1 as well as grain 8 of 98921-4-1 in Fig. 2. tion of amphibole leptite, between LREE and HREE, The U-Pb isotopic compositions for zircons from is stronger than that of the grey-black fine-grained the granitic gneiss and grey-black granulite are plotted granulite. The grey-black fine-grained granulite is in a U-Pb concordia diagram (Fig. 4). characterized by the lowest fractionation, between LREE and HREE, along with an inconspicuous posi- The results shown in Table 4 and Fig. 4 demonstrate tive Eu anomaly among the three samples. that the zircons, from the granitic gneiss and grey-black granulite, have experienced a very compli- Figure 3(b) shows a spider-diagram normalized by a cated history. Apart from one group of 206Pb/238U ages primitive mantle[20]. All three samples are character- between 696 to 808 Ma, there are also some data ized by depletion of Nb, Ta, and HREE in the diagram. points close to concordia, with 207Pb/206Pb ages of The difference between these samples is the depletion — of Sr, P, and Ti for the granitic gneiss and marked de- 1884 2171 Ma from relict zircons and of early Her- pletion of Zr, Rb, Th, and U and enrichment of Sr for cynian to Indosinian, late Yanshanian palingenetic the grey-black fine-grained granulite. zircons in the grey-black fine-grained granulite. The young ages are not obtained from overgrowth rims of 4 Zircon SHRIMP U-Pb analyses detrital zircons, but are from independent grains. The Intact zircon crystals exhibit di-tetragonal di- pyra- majority of zircons in the granitic gneiss are ca. 770 mids and prisms, di-prisms and di-pyramids, with mi- Ma. One grain with an apparent overgrowth rim was nor tabular-prisms, whose prisms are {100}, {110} dated for 3 spots to its core (spots 7.1, 7.2, 7.3 of and pyramids are {111}, partly {211}. The zircons are 98922-5-3 in Fig. 2), whose concordia age is 2450 Ma euhedral and translucent. Some grains display granular with indistinguishable 207Pb/206Pb ages of 2378±11 Ma, forms because of short prisms. Fissures and mineral 2468±11 Ma, 2373±12 Ma, respectively. Because spot inclusions can be observed in rounded zircon grains 7.2 shows the highest concordia degree, its 207Pb/206Pb

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. 4) ble a T e listed in which ar ed, easur m e ains and 41 spots wer 34 gr ons and dated spots ( c zir ages of inescence im Cathodolum

2. Fig.

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Table 2 The composition of major and trace elements Sample No. 98922-4-1 98921-4-1 98922-5-3 Sample No. 98922-4-1 98921-4-1 98922-5-3

SiO2(%) 48.27 57.59 74.61 Eu 1.07 1.31 0.87

TiO2 0.98 0.79 0.21 Gd 2.84 5.51 2.80

Al2O3 18.75 17.23 13.39 Tb 0.47 0.94 0.45

Fe2O3 4.82 2.81 1.57 Dy 2.65 5.01 2.11 FeO 5.47 4.38 0.86 Ho 0.51 0.96 0.42 MnO 0.2 0.14 0.07 Er 1.42 2.66 1.07 MgO 6.18 3.77 0.26 Tm 0.21 0.38 0.14 CaO 11.07 7.11 1.43 Yb 1.35 2.61 0.76

Na2O 2.37 3.18 3.77 Lu 0.20 0.39 0.12

K2O 0.19 1.46 3.14 Rb 0.60 45.0 68.9

P2O5 0.27 0.18 0.04 Ba 108 458 768 LOI 1.24 0.93 0.47 Th 0.38 3.16 2.93 Q(CIPW) 2.24 12.25 37.29 U 0.07 0.20 0.15 An 39.96 28.43 6.83 Nb 1.38 5.70 2.44 Ab 20.05 26.91 31.9 Ta 0.20 0.60 0.09 Or 1.12 8.63 10.56 Sr 971 531 147 La/µg·g−1 10.5 26.9 25.0 Y 12.7 24.9 10.1 Ce 24.0 60.4 46.1 Zr 24.2 121 120 Pr 3.35 7.51 4.99 Hf 0.87 3.74 4.01

Nd 15.1 30.12 17.7 (La/Yb)N 5.29 6.98 22.2 Sm 3.23 5.90 3.04 δ Eu 1.14 0.74 0.96

Fig. 3. Chondrite-normalized REE patterns (a) and primitive mantle-normalized incompatible elements spider-diagrams (b).

Table 3 Morphological parameters of the zircons Length/µm Width/µm Length to width ratio Sample No. range average range average range average 98922-4-1(grey-black fine-grained granulite) 39—102 63 13—39 24 1.71—5.86 2.75 98921-4-1(amphibole leptite) 41—127 68 16—39 26 1.25—4.88 2.69 98922-5-3(granitic gneiss) 37—77 56 21—46 30 1.15—2.82 1.89 age of 2468±11 Ma may represent its crystallization imply that the zircon formed during the Neo-pro- age, approximately. terozoic, and has no relict portion. The U-Pb composi- tions of the majority of zircons from the 3 samples are The consistent zircon ages of the amphibole leptite plotted in fig. 5. The concordia age of 6 grains from

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Table 4 U-Th- Pb compositions of zircons −1 −1 −1 206 206 238 207 206 207 235 Sample No. Spot U/µg· g Th/ µg· g Th/U 206Pb*/µg· g Pbc(%) Pb/ U t206/238(±1σ)/Ma Pb/ Pb t207/206(±1σ)/Ma Pb/ U t207/235(±1σ)/Ma 98922-4-1, grey-black fine-grained granulite 1.1 181 159 0.88 17.9 0.75 0.1140 696 14 0.0634 720 69 1.00 704 27 2.1 718 282 0.39 137.9 1.60 0.2198 1281 23 0.1330 2138 126 4.03 1640 123 2.2 796 183 0.23 140.8 0.13 0.2055 1205 21 0.1153 1884 10 3.27 1474 29 3.1 227 484 2.13 22.9 0.39 0.1168 712 13 0.0652 780 50 1.05 729 22 4.1 153 140 0.92 16.6 1.04 0.1250 759 15 0.0613 650 87 1.06 734 33 5.1 370 113 0.31 107.5 0.23 0.3369 1872 32 0.1223 1990 22 5.68 1928 44 5.2 525 187 0.36 141.1 0.04 0.3130 1755 30 0.1356 2171 24 5.85 1954 47 6.1 267 176 0.66 14.2 1.41 0.0609 381 8 0.0487 132 148 0.41 349 23 6.2 232 156 0.67 12.8 1.00 0.0634 397 8 0.0509 236 139 0.45 377 24 7.1 288 183 0.64 9.80 1.42 0.0391 248 5 0.0482 110 170 0.26 235 18 8.1 456 266 0.58 15.4 1.31 0.0388 245 5 0.0541 374 101 0.29 259 13 9.1 207 340 1.64 23.9 0.39 0.1335 808 16 0.0632 716 58 1.16 828 31 9.2 139 156 1.13 15.1 1.96 0.1240 754 15 0.0614 652 146 1.05 729 52 10.1 366 296 0.81 15.9 0.98 0.0500 314 6 0.0501 199 101 0.35 305 15 11.1 262 352 1.34 26.3 0.59 0.1160 708 13 0.0624 689 53 1.00 704 23 12.1 2464 1830 0.74 33.0 0.67 0.0155 99 2 0.0626 694 73 0.13 124 5 13.1 1070 665 0.62 79.3 0.46 0.0859 531 10 0.0553 424 48 0.65 509 15 98921－4－1, amphibole leptite 1.1 285 644 2.26 30.5 2.24 0.1246 757 14 0.0645 758 33 1.11 758 19 2.1 359 919 2.56 38.2 0.35 0.1234 750 14 0.0651 778 32 1.11 758 19 3.1 80 61 0.76 9.01 1.22 0.1292 784 16 0.0625 692 109 1.11 758 42 (To be continued on the next page)

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(Continued) −1 −1 −1 206 206 238 207 206 207 235 Sample No. Spot U/µg· g Th/ µg· g Th/U 206Pb*/µg· g Pbc(%) Pb/ U t206/238(±1σ)/Ma Pb/ Pb t207/206(±1σ)/Ma Pb/ U t207/235(±1σ)/Ma 4.1 364 914 2.51 39.6 0.09 0.1264 767 14 0.0664 821 26 1.16 782 18 5.1 86 129 1.50 9.49 1.89 0.1279 776 16 0.0664 819 87 1.17 787 37 6.1 339 858 2.53 38.2 1.50 0.1314 796 15 0.0662 814 26 1.20 801 18 7.1 312 730 2.34 35.1 1.11 0.1306 791 16 0.0650 774 29 1.17 787 20 8.1 291 673 2.31 31.6 0.25 0.1262 766 14 0.0668 830 29 1.16 782 19 9.1 83 62 0.74 10.0 1.94 0.1395 842 18 0.0782 1152 69 1.50 930 39 9.2 197 372 1.89 21.9 1.63 0.1283 778 15 0.0607 628 65 1.07 739 27 10.1 431 1172 2.72 47.1 0.46 0.1268 769 14 0.0636 728 34 1.11 758 19 98922－5－3, granitic gneiss 1.1 660 372 0.56 71.7 0.81 0.1264 767 14 0.0684 880 17 1.19 796 17 2.1 2342 2096 0.89 262 0.21 0.1303 790 14 0.0646 762 9 1.16 782 16 3.1 984 632 0.64 110 0.58 0.1306 791 14 0.0700 930 15 1.26 828 17 4.1 417 120 0.29 45.6 0.43 0.1274 773 14 0.0671 840 27 1.18 791 18 5.1 2126 284 0.13 232 0.29 0.1269 770 14 0.0676 855 10 1.18 791 16 6.1 411 329 0.80 43.8 0.33 0.1242 755 14 0.0662 814 22 1.13 768 17 7.1 71 53 0.74 26.3 1.05 0.4305 2308 41 0.1528 2378 18 9.07 2345 56 7.2 162 40 0.25 66.7 0.12 0.4787 2521 41 0.1612 2468 11 10.6 2492 52 7.3 86 50 0.59 30.4 0.19 0.4103 2216 39 0.1524 2373 17 8.62 2299 53 8.1 314 254 0.81 33.4 1.17 0.1238 752 14 0.0680 868 36 1.16 782 20 9.1 1096 469 0.43 118 1.10 0.1257 763 14 0.0739 1038 44 1.28 837 24 10.1 758 559 0.74 82.8 0.77 0.1272 772 14 0.0697 919 16 1.22 810 17 11.1 1778 442 0.25 195 1.41 0.1275 774 14 0.0717 979 36 1.26 828 22

* Mass fraction of radiogenetic206Pb, subscript c denotes mass fraction of common 206Pb in total lead. Three ages are corrected by 204Pb.

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Fig. 4. U-Pb Concordia diagram for zircons from the granitic gneiss (98922-5-3) and grey-black fine-grained granulite (98922-4-1). the grey-black fine-grained granulite is 721 Ma. The The formation age (721 Ma) of major zircons in concordia ages of the granitic gneiss (98922-5-3, ex- grey-black fine-grained granulite is consistent with the cept for 7.1, 7.2, 7.3 spots being an apparently relict Sm-Nd isochron of 706±36 Ma[5]. zircon) and amphibole leptite (98921-4-1) are 772±15 Early to late Paleo-proterozoic zircons may indicate Ma, 773±11 Ma, respectively. Because three samples a contribution of early continental crustal components. are of fitting probabilities greater than 0.15, and Previous studies show that the Yangtze block had ac- MSWD less than 2.5, their ages are credible and errors [21] creted during three main periods around >3.3 Ga, 2.0 may be analytical errors . Thus, the rock suite Ga, and 1.0 Ga[12]. Early to late Paleo-proterozoic zir- should have formed in the Neo-proterozoic rather than cons may represent the continental basement of the the Archean, as has been speculated previously. Yangtze block. Sm-Nd isochron and zircon U-Pb ages, [13―15] 5 Discussion 1.99―3.28Ga , are indicative of the old continent for the Huanglin complex from the central segment of Generally, the Th/U ratio of a zircon may indicate the Northern Yangtze margin. Many relict zircon U-Pb its magmatic or metamorphic genesis, i.e., the Th/U ages, 1.8―2.5 Ga[10], also demonstrate the existence ratio of metamorphic zircons is generally less than 0.1, of an old continental basement in Fujian and Zhejiang whereas that of igneous zircons is commonly between provinces, of the eastern Yangtze block. Averaged Nd 0.1 and 1[7]. U and Th contents of the zircons are plot- model ages of 1.2 Ga for the three samples, similar to ted as Fig. 6. All Th/U ratios are greater than 0.1, other rocks in this region, suggest that they would not which is indicative of a magmatic origin, whether their have been separated from the upper mantle to the crust ages are Neo-proterozoic or late Yanshanian. From the before late Meso-proterozoic. internal structures of the zircons shown in their CL images, overgrowth rims may represent metamor- As the rock suite formed, during 721―773 Ma phic-alteration events after the crystallization of Neo- principally, concurrently with the zircons[22] of 751― proterozoic zircons. Most overgrowth rims are too 768 Ma (obtained from SHRIMP dating in granodio- narrow (less than spatial resolution required by rites, diorites, and granites from the Kangding-Luding SHRIMP II) to be dated. But Zhou dated the over- and Panzhihua regions), note that they are younger growth rim of a zircon in a Gezong granite that gives than the crystallization age of 795―797 Ma[7] dated an age of 177 Ma. This indicates the existence of a by SHRIMP U-Pb for zircons from gneissic granites in [7] metamorphic event . the Kangding-Luding-Danba region. On the basis of

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regional distribution of the complexes in west Sichuan Madagascar, and South Africa[22]. Zheng concluded Province, the complexes age (from southern Panzhi- that a super mantle plume, resulting in the breakup of hua through central Mianning to northern Kang- the middle Neo-proterozoic super-continent and ex- ding-Luding-Danba within an extent of ca. 1000 km), tensive rifting magmatism, might have been a substan- 721 Ma, is the lowest zircon U-Pb concordia age from tial trigger for global glaciation and interglaciation[26]. the grey-black granulite. Combining the structure of As for the genesis of the suite, there are two views. CL and the result of a Th/U ratio >0.1, we conclude One is continental rifting, i.e., a super mantle plume that the ages of 721―773 Ma indicate the timing of magma crystallization in the region. The ages of 99― 531 Ma, obtained from some zircon grains and over- growth rims in the grey-black fine-grained granulite, suggest that the complexes have experienced strong metamorphism after crystallization, further indicating that the physical-chemical conditions during that pe- riod must have been suitable for zircon formation. Xu et al. argue that a Rb-Sr isochron of 218 Ma, on min- erals and whole rock, indicate thermo-disturbance of a Permian syeno- gabbroic intrusion[6]. Ma et al. found that the crystalline basements had been affected by late Permian to Himalayan dynamic metamorphism during regional geological investigation in the Panzhihua- Xichang region[23]. All these suggest that part of the region was at high temperature, at least until the In- dosinian-Yanshanian period[24]. We will reconstruct an evolutionary history of the rocks, according to Ar-Ar thermo-geochronology of biotites and amphiboles and internal mineral and whole rock isochrones of Rb-Sr and Sm-Nd. These ages are indistinguishable from those of the upper tuff (758 Ma[25]) of the Dieshuihe Formation, and tuff interlayer (766 Ma[26]) of the Liantuo Forma- tion in the Lower Nanhua series. Shen et al. showed accretion of nascent crust to the eastern segment of the Nanling, northwest Fujian Province, during the Neo-proterozoic (0.8―0.7 Ga), based on a systematic study of Sm-Nd isotopes from Meso-proterozoic to early Mesozoic strata[27]. Chen et al. dated zircons of 0.7―0.8 Ga in Taohang of the Dabie-Sulu and Hushan granitic gneisses by SIMS[28]. All of these suggest that there was intensive Neo-proterozoic magmatic activity in South China. The magmatism occurred not only in South China, but also in Adelaide, Australia; west Fig. 5. U-Pb Concordia diagram for zircons of the grey-black fine- grained granulite, granitic gneiss, and amphibole leptite (fitting and Australia, Tasmania, Laurentia, India, Seychelles, calculation by Ludwig’s Isoplot).

中国科技论文在线 http://www.paper.edu.cn 632 Science in China Ser. D Earth Sciences

Guangxi, Jiangxi, Yunnan, Anhui. But their Nd model ages are between 1.85 and 2.22 Ga[31], which are in- consistent with the Nd model age of 1.2 Ga from the north segment. Moreover, the samples dated, using zircon SHRIMP U-Pb method by Li et al. in the Kangding-Luding and Panzhihua regions, include in- termediate diorites[22] belonging to the same series as the amphibole leptite of this study, a member of the andesitic series. The Neo-proterozoic igneous rocks do not show bimodal characteristics, petrochemically. Therefore, the Kangding complexes may have been produced by assembling of a piece of nascent crust to Fig. 6. The Th vs. U plot of zircons. the northwestern Yangtze block during the Neo-pro- provided the driving force for the breakup of the terozoic period, whose tectonic setting would be char- [10,22] Rodinia super-continent, as suggested by Li , and acteristic of an Island-arc or under-plating. They might [29] Li et al. , on the basis of the bimodal association of have been formed by interaction between under-plat- granitic and mafic rocks and radiate mafic swarms and ing magma or subducted oceanic crust beneath the dikes from the Neo-proterozoic. The other is, as sug- northwestern Yangtze block after 830―795 Ma, dur- gested by Zhou et al.[7], that the rocks suite was ing the period of the Rodinia super-continent break-up. formed as part of an island-arc, since the time span of Thus, the formation process of the Yangtze block may 100 Ma, between the Miyi complex (760 Ma) and be very complicated, and may have consised of dif- Gezong granitic complex (865 Ma), would be too long ferent blocks assembled as late as the Neo-proterozoic. for a mantle plume, and that discrimination of volcanic arc in the Y vs. Nb and Y+Nb vs. Rb diagrams, i.e., As discussed above, the age of the crystallization of subduction of oceanic lithosphere eastward under the the Kangding complex is ca.721 Ma rather than 1.2 Ga. Yangtze Block. To explain the long duration of a man- This indicates that metamorphism to a granulitic facies tle plume, Li et al. proposed a model of two phases for may have occurred during the Paleozoic to Mesozoic. the mantle plume, i.e., the first one at ca. 830―795 Nd model age and Sm-Nd internal isochron, may time Ma and the second one at ca.780―745 Ma[22]. Al- the mixing between the Yangtze block’s basement and Neo-proterozoic nascent crust. Alternatively, they time though the geochemical characteristics (depletion of the mixing between partial melting products of the Nb, Ta, and HREE) of our samples also display simi- basement and nascent crust. This view is supported larities to those of Zhou’s[7], their evolutionary setting by a linear relationship of 143Nd/144Nd ratio vs. 1/Nd should be investigated further since only one mafic plot for similar rocks in the Kangding, Luding, and rock has been studied, and because of the uncertainty Mianning regions. in identifying tectonic settings through granitoids[25]. Nevertheless, there are obvious differences in Pb iso- 6 Conclusion topic compositions and Nd model ages between the On the basis of zircon SHRIMP U-Pb geochro- southern and northern segments in the western Yang- nological studies, and major and trace elemental char- tze margin, i.e., the northern segment is characterized acteristics of the Kangding complexes, in Mianning by lower radiogenic Pb and lower Nd model age than County, Sichuan Province, we present the following those of the southern segment[30]. If granitic rocks in conclusions: the Kangding complexes had formed by anatexis of the Yangtze block’s basement only, their Pb isotopic (1) The Kangding complexes, of Mianning County, compositions and Nd model ages would have been crystallized during the period from 721 to 773 Ma, consistent with those of granitoid rocks in northern which are the synchronous products of global

中国科技论文在线 http://www.paper.edu.cn Petrogenesis and dating of the Kangding complex, Sichuan Province 633

Neo-proterozoic magmatism. The complexes are in- geological significance, Geological Journal of China Univeristies trusions of a calc-alkaline series. Metamorphism(s) (in Chinese), 2002, 8(4): 399―406. after crystallization fixed the present characteristics of 7. Zhou, M. F.,Yan, D. P., Kennedy, A. K. et al., SHRIMP U-Pb zir- high-grade metamorphic rocks, whose zircons of 99 to con geochronological and geochemical evidence for Neoprotero- zoic arc-magmatism along the western margin of the Yangtze 531 Ma might have recorded the ages of said meta- Block, South China, Earth Planet Sci. Lett., 2002, 196: 51―67. morphic events. [DOI] (2) Early to late Paleo-proterozoic relict zircons, the 8. Li, Z. X., Li, X. H., Kinny, P. D. et al., The breakup of Rodinia: Did it start with a mantle plume beneath South China? Earth oldest of which is 2468 Ma, in the granitic gneiss and Planet Sci. Lett., 1999, 173: 171―181. [DOI] mafic granulite, may indicate the basement of the 9. Li, Z. X., Zhang, L. H., Powell, C. M., South China in Rodinia: Yangtze block. part of the missing link between Australia-East Antarctica and ― (3) The calc-alkaline association and depletion of Laurentia, Geology, 1995, 23: 407 410. [DOI] 10. Li, Z. X., Cho, M., Li, X. H., Precambrian tectonics of East Asia Nb, Ta, and HREE for the complexes suggest that they and relevance to supercontinent evolution, Precambrian Res, 2003, might be formed in an island-arc or under-plating set- 122: 1―6. [DOI] ting. 11. Zhou, X. H., Cheng, H., Zhong, Z. H., The source province of metamorphic rocks from southern Zhejiang―a case study, In Acknowledgements We thank Prof. Liu D.Y. for his help and support with the ion probe on the zircons at Beijing SHRIMP Center. Prof. Memoir of Lithospheric Tectonic Evolution Research (1), Zhang B. R., Prof. Xu R. H., and Prof. Wan Y. S. are thanked for thor- Laboratory of Lithosphere Tectonic Evolution, Institute of Ge- ough reviews and detailed suggestions to an early version of the manu- ology, Chinese Academy of Sciences, Beijing: Seismological script. Anonymous referees’ critical comments are helpful for consid- Publishing House, 1992, 114―119. erable improvement of the paper. We benefited from suggestions of 12. Chen, Y. L., Yang, Z. Y., Nd model ages of sedimentary profile Prof. Fang Z. of the Department of Geosciences, Nanjing University, during our study on the Kangding complexes. Dr. Zhu X. K. and Mark from the northwest Yangtze Craton, Guangyuan, Sichuan Prov- Schoonover are thanked for the polishing of the English version of the ince, China and their geological implication, Geochemical Journal, paper. Zheng Y. is thanked for drawing parts of the figures. This paper 2000, 34(4): 263―270. is partial result of the Projects of the National Natural Science Founda- 13. Lin, W. L., Gao, S., Zheng, H. 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