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Volume 31 Issue 1
Jan.  2020
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Geochronological and Geochemical Constraints on the Petrogenesis of Early Paleoproterozoic (2.40-2.32 Ga) Nb-Enriched Mafic Rocks in Southwestern Yangtze Block and Its Tectonic Implications

  • Recent geological survey has identified the Early Paleoproterozoic meta-mafic intrusions in the southwestern Yangtze Block. We present geochronological,whole-rock geochemical and Nd isotopic data for these meta-mafic rocks to better address the tectonic evolution of the Yangtze Block during the Early Paleoproterozoic Period. Geochronological data show that the meta-mafic rocks have zircon ages of 2 395-2 316 Ma. They have high TiO2 contents of 1.40 wt.%-3.66 wt.% and Nb concentrations of 13.7 ppm-45.5 ppm,thus aregrouped as Nb-enriched mafic rocks. These mafic rocks are characterized by tholeiitic compositions with enrichment of LREEs and LILEs,and can be divided into two groups. Group 1 samples display E-MORB-like geochemical characteristics. Group 2 samples have positive εNd(t) values of 4.0-5.0. Geochemical data indicate that all meta-mafic rocks were likely derived from a depleted asthenospheric mantle. REE modeling indicates lower degree of partial melting for Group 2 samples (3%-10%) relative to Group 1 samples (15%-20%). Taking into account contemporaneous post-collisional granitoids in southwestern Yangtze Block,we propose that these meta-mafic rocks were formed in a post-collisional extension setting. These meta-mafic rocks can be compared with those in Africa,South America and Europe,and might be linked with the Arrowsmith orogenic belt.
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Geochronological and Geochemical Constraints on the Petrogenesis of Early Paleoproterozoic (2.40-2.32 Ga) Nb-Enriched Mafic Rocks in Southwestern Yangtze Block and Its Tectonic Implications

    Corresponding author: Xin Qian, qianx3@mail.sysu.edu.cn
  • 1. Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
  • 2. Yunnan Institute of Geological Survey, Kunming 650216, China
  • 3. School of Resource Environment and Earth Sciences, Yunnan University, Kunming 650091, China
  • 4. Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai 519082, China
  • 5. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

Abstract: Recent geological survey has identified the Early Paleoproterozoic meta-mafic intrusions in the southwestern Yangtze Block. We present geochronological,whole-rock geochemical and Nd isotopic data for these meta-mafic rocks to better address the tectonic evolution of the Yangtze Block during the Early Paleoproterozoic Period. Geochronological data show that the meta-mafic rocks have zircon ages of 2 395-2 316 Ma. They have high TiO2 contents of 1.40 wt.%-3.66 wt.% and Nb concentrations of 13.7 ppm-45.5 ppm,thus aregrouped as Nb-enriched mafic rocks. These mafic rocks are characterized by tholeiitic compositions with enrichment of LREEs and LILEs,and can be divided into two groups. Group 1 samples display E-MORB-like geochemical characteristics. Group 2 samples have positive εNd(t) values of 4.0-5.0. Geochemical data indicate that all meta-mafic rocks were likely derived from a depleted asthenospheric mantle. REE modeling indicates lower degree of partial melting for Group 2 samples (3%-10%) relative to Group 1 samples (15%-20%). Taking into account contemporaneous post-collisional granitoids in southwestern Yangtze Block,we propose that these meta-mafic rocks were formed in a post-collisional extension setting. These meta-mafic rocks can be compared with those in Africa,South America and Europe,and might be linked with the Arrowsmith orogenic belt.

0.   INTRODUCTION
1.   GEOLOGICAL SETTING AND PETROGRAPHY
  • The South China Craton, one of the three main Precambrian cratons in China, includes the Yangtze Block in the northwest and Cathaysia Block in the southeast, which are separated by the Jiangnan orogenic belt (Fig. 1; Cawood et al., 2018; Zhao, 2015; Wang et al., 2013), and forms one of the main Precambrian cratons in southeastern Asia. The Yangtze Block is bounded by the Qinling-Dabie-Sulu orogenic belt to the north, and by the Longmenshan fault and Ailaoshan zone to the west and southwest. Archean-Paleoproterozoic basement and igneous rocks are mainly distributed in the northern and western parts of the block, including the Paleoarchean-Early Paleoproterozoic Kongling complex, and Neoarchean- Paleoproterozoic Zhongxiang, Douling, Yudongzi and Houhe complexes in the north (Fig. 1; Hu et al. 2018, 2013; Hui et al., 2017; Wu et al., 2014, 2012; Cen et al., 2012; Gao et al., 2011; Zhang et al., 2001), and Paleoproterozoic-Early Mesoproterozoic Dongchuan, Hekou and Dahongshan groups and Tongan Formation and Baoban complex in the southwest (Fig. 1; Cui et al., 2019; Lu et al., 2019; Zhang et al., 2018; Kou et al., 2017; Wang et al., 2014; Wang and Zhou, 2014; Zhao et al., 2010;). The Dahongshan and Dongchuan groups contain tuffaceous layers with zircon ages of ~1 700 Ma (Chen et al., 2013; Zhao et al., 2010). These Paleoproterozoic-Early Mesoproterozoic rocks in southwestern Yangtze Block underwent upper greenschist to lower amphibolite facies metamorphism (Chen et al., 2013; Zhao et al., 2010; Greentree and Li, 2008). The Dahongshan Group has been regarded as the oldest lithostratigraphic sequence in southwestern Yangtze Block (Zhao et al., 2010; Greentree and Li, 2008), which is discordantly overlain by greenschist facies metamorphosed rocks of the Dongchuan Group and by Triassic‒Jurassic clastic rocks (Qian and Shen, 1990). Cui et al. (2019) and Kou et al. (2017) reported zircon ages of 2 012 Ma for the ore-hosting metavolcanic rocks in the Dahongshan ore district and 2 363-2 359 Ma for the granitoids in the Laochanghe Formation of the Dahongshan Group, respectively. Lu et al. (2019) recently reported a zircon age of 2 299±17 Ma for the dolerite dykes in the Tongan Formation.

    The study area is located in the southwestern part of Yuxi City. The volcano-sedimentary sequences of the region include mainly Paleoproterozoic amphibolite, quartzite, granulite, chlorite schist, metasandstone and phyllite of the Laochanghe and Manganghe formations, Mesoproterozoic argillaceous slate, quartzite, sandstone, dolomitic limestone and marble limestone with thin tuff beds of the Kunyang Group, which are overlain by Triassic muddy limestone, sandstone, shale with rare thin coal seam and conglomerate (1 : 50 000 geological maps of the Yangwu and Qinglongchang areas; Fig. 2). All these sequences are in fault contact with each other. The Laochanghe and Manganghe formations have been considered as the bottom of the Paleoproterozoic Dahongshan Group in southwestern Yangtze Block (Cui et al., 2019; Kou et al., 2017; Qian and Shen, 1990). Felsic and meta-mafic rocks are distributed discontinuously in the ductile shear belts of the Laochanghe and Manganghe formations (Fig. 2). The felsic intrusions can be subdivided into the Neoproterozoic Cuoke unit consisting of K-feldspar granite and porphyritic granite and the Paleoproterozoic Andichong unit composed of gneissic monzogranite and granodiorite (Cui et al., 2019).

    The meta-mafic rocks are distributed sporadically in the Cuoke area, and intrude the monzogranites and metasedimentary rocks of the Laochanghe Formation (Figs. 2 and 3). They occur as dikes or small blocks with schistose structure (Figs. 3a and 3b). Samples (Yjhlc-16, Yjyw-33 and Yjyw-36) locally display blastogabbroic texture, with mineral assemblage consisting of clinopyroxene, plagioclase and chlorite with minor quartz and Fe-Ti opaque minerals. Secondary chlorite and Fe-Ti oxides in the matrix show replacement features (Figs. 3d and 3e). The sample Yjyw-07 with porphyritic texture is green to gray-green, and contains clinopyroxene, chlorite and minor amounts of plagioclase, albite and Fe-Ti opaque minerals (Fig. 3f). Chlorite is mainly present in the groundmass and formed through replacement/alteration of clinopyroxene.

    Figure 3.  Field photos and microscopic photographs for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block. Pl. Plagioclase; Cpx. clinopyroxene; Chl. chlorite.

2.   ANALYTICAL METHODS
  • Cathodoluminescence (CL) images for the zircon grains from the samples (Yjck-07, Yjhl-16, Yjyw-33, Yjyw-36) were obtained on a JEOL JXA-8100 electron microprobe. Zircon dating was analyzed on a LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences in Wuhan (CUG). The detailed analytical procedure follows Yuan et al. (2004). Off-line selection and integration of background, analyte signals, time-drift correction, and quantitative calibration were conducted by ICPMSDataCal (Liu et al., 2010a, b ). Common Pb correction was in accordance with the method of Andersen (2002). Concordia U-Pb diagrams were prepared by ISOPLOT3.00 (Ludwig, 2003). The results are listed in Table 1.

    Analytical spot Concentration (ppm) Isotopic ratio Calculated apparent age (Ma)
    Th U Th/U 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U
    Ratio 1σ Ratio 1σ Ratio 1σ Age 1σ Age 1σ Age 1σ
    Yjck-07-1 321 446 0.72 0.131 866 0.001 376 6.312 161 0.091 245 0.346 616 0.004 959 2 124 19 2 020 13 1 918 24
    Yjck-07-2 904 909 0.99 0.139 684 0.002 109 6.723 671 0.123 437 0.348 071 0.004 527 2 233 26 2 076 16 1 925 22
    Yjck-07-3 91 172 0.53 0.146 972 0.001 712 8.667 309 0.129 611 0.426 164 0.004 929 2 311 20 2 304 14 2 288 22
    Yjck-07-4 94 474 0.20 0.147 400 0.003 043 8.406 991 0.299 053 0.405 115 0.007 246 2 317 35 2 276 32 2 193 33
    Yjck-07-5 158 132 1.19 0.152 025 0.002 173 9.575 038 0.159 374 0.456 019 0.006 613 2 369 25 2 395 15 2 422 29
    Yjck-07-6 149 273 0.55 0.153 534 0.001 980 9.401 654 0.126 279 0.442 046 0.004 965 2 387 22 2 378 12 2 360 22
    Yjck-07-7 144 59 2.44 0.154 268 0.002 032 9.595 811 0.154 130 0.451 120 0.005 673 2 394 22 2 397 15 2 400 25
    Yjck-07-8 89 118 0.75 0.154 611 0.002 266 9.562 751 0.154 850 0.448 489 0.005 207 2 398 30 2 394 15 2 389 23
    Yjck-07-9 78 108 0.73 0.156 283 0.001 837 9.778 427 0.155 608 0.451 714 0.006 468 2 417 20 2 414 15 2 403 29
    Yjhlc-16-1 497 1 465 0.34 0.147 079 0.001 713 8.613 064 0.117 482 0.425 292 0.005 474 2 322 20 2 298 13 2 284 25
    Yjhlc-16-2 569 1 382 0.41 0.148 291 0.001 589 7.700 580 0.122 812 0.375 349 0.005 092 2 328 19 2 197 14 2 055 24
    Yjhlc-16-3 284 781 0.36 0.148 530 0.001 769 8.749 991 0.120 788 0.427 363 0.005 777 2 329 20 2 312 13 2 294 26
    Yjhlc-16-4 312 823 0.38 0.149 437 0.001 707 9.012 225 0.136 472 0.436 075 0.005 227 2 339 19 2 339 14 2 333 23
    Yjhlc-16-5 526 1 280 0.41 0.149 733 0.001 684 8.832 169 0.126 858 0.427 170 0.005 141 2 343 19 2 321 13 2 293 23
    Yjhlc-16-6 400 1 172 0.34 0.149 994 0.001 589 8.941 648 0.114 446 0.431 665 0.004 696 2 346 19 2 332 12 2 313 21
    Yjhlc-16-7 532 1 617 0.33 0.150 685 0.001 667 8.281 243 0.113 876 0.398 518 0.004 720 2 354 19 2 262 13 2 162 22
    Yjhlc-16-8 442 1 043 0.42 0.151 295 0.001 721 9.053 230 0.113 386 0.433 868 0.004 489 2 361 19 2 343 12 2 323 20
    Yjhlc-16-9 203 570 0.36 0.151 337 0.001 783 8.644 116 0.133 084 0.413 608 0.005 723 2 361 19 2 301 14 2 231 26
    Yjhlc-16-10 256 820 0.31 0.151 763 0.001 741 9.121 860 0.127 016 0.436 441 0.005 712 2 366 21 2 350 13 2 335 26
    Yjhlc-16-11 700 1 067 0.66 0.152 066 0.001 609 8.969 774 0.124 913 0.427 457 0.005 117 2 369 18 2 335 13 2 294 23
    Yjhlc-16-12 209 588 0.35 0.152 106 0.001 597 9.054 892 0.112 556 0.431 518 0.004 560 2 369 18 2 344 11 2 313 21
    Yjyw-33-1 250 775 0.32 0.147 592 0.004 215 9.004 416 0.275 709 0.442 376 0.012 673 2 318 49 2 338 28 2 361 57
    Yjyw-33-2 387 1 589 0.24 0.144 562 0.004 076 7.552 061 0.241 987 0.377 760 0.011 226 2 283 49 2 179 29 2 066 53
    Yjyw-33-3 153 403 0.38 0.147 651 0.004 169 8.860 229 0.259 487 0.435 480 0.012 191 2 320 48 2 324 27 2 330 55
    Yjyw-33-4 530 803 0.66 0.153 097 0.004 222 9.364 618 0.272 287 0.442 922 0.012 208 2 381 47 2 374 27 2 364 55
    Yjyw-33-5 232 1 812 0.13 0.144 643 0.004 009 8.428 299 0.286 475 0.422 225 0.013 928 2 284 48 2 278 31 2 271 63
    Yjyw-33-6 156 1 484 0.10 0.145 785 0.004 041 8.111 374 0.236 752 0.403 115 0.011 281 2 298 47 2 244 26 2 183 52
    Yjyw-33-7 183 923 0.20 0.148 752 0.004 156 8.680 153 0.252 239 0.422 757 0.011 729 2 332 53 2 305 27 2 273 53
    Yjyw-33-8 108 693 0.16 0.147 427 0.004 129 8.819 604 0.255 944 0.433 143 0.011 889 2 316 16 2 320 27 2 320 53
    Yjyw-33-9 274 997 0.27 0.148 051 0.004 175 8.941 409 0.261 140 0.437 260 0.012 044 2 324 48 2 332 27 2 338 54
    Yjyw-33-10 319 3 249 0.10 0.144 712 0.004 102 7.583 998 0.227 222 0.379 467 0.010 757 2 284 49 2 183 27 2 074 50
    Yjyw-33-11 203 625 0.32 0.145 920 0.004 208 8.568 898 0.257 029 0.425 053 0.011 838 2 298 49 2 293 27 2 283 54
    Yjyw-33-12 440 1 238 0.36 0.147 505 0.004 171 8.645 668 0.258 675 0.424 209 0.011 963 2 317 16 2 301 27 2 280 54
    Yjyw-33-13 160 666 0.24 0.148 659 0.004 180 8.666 606 0.255 229 0.421 917 0.011 679 2 331 53 2 304 27 2 269 53
    Yjyw-33-14 142 537 0.27 0.147 489 0.004 147 8.995 114 0.261 089 0.441 567 0.012 051 2 317 48 2 338 27 2 358 54
    Yjyw-33-15 192 534 0.36 0.147 643 0.004 116 8.753 083 0.255 685 0.429 532 0.011 979 2 320 48 2 313 27 2 304 54
    Yjyw-33-16 429 909 0.47 0.148 041 0.004 089 8.397 608 0.244 223 0.410 915 0.011 431 2 323 48 2 275 26 2 219 52
    Yjyw-33-17 133 925 0.14 0.144 077 0.004 004 7.782 995 0.224 068 0.391 060 0.010 560 2 277 48 2 206 26 2 128 49
    Yjyw-33-18 217 871 0.25 0.153 120 0.004 367 8.217 106 0.261 433 0.388 361 0.011 336 2 381 48 2 255 29 2 115 53
    Yjyw-33-19 115.5 1 214 0.10 0.135 566 0.004 970 7.019 826 0.239 812 0.355 358 0.011 098 2 172 65 2 114 30 1 960 53
    Yjyw-33-20 74 583 0.13 0.147 344 0.004 232 8.569 635 0.256 870 0.421 468 0.011 676 2 317 50 2 293 27 2 267 53
    Yjyw-33-21 318 3 578 0.09 0.131 762 0.003 790 5.381 563 0.166 698 0.295 355 0.008 279 2 121 51 1 882 27 1 668 41
    Yjyw-33-22 87 589 0.15 0.150 520 0.004 268 9.119 462 0.269 932 0.438 357 0.012 029 2 352 48 2 350 27 2 343 54
    Yjyw-33-23 67 359 0.19 0.137 933 0.003 970 7.582 670 0.227 207 0.397 488 0.010 914 2 202 50 2 183 27 2 157 50
    Yjyw-33-24 182 682 0.27 0.147 220 0.004 320 7.854 669 0.250 490 0.385 689 0.011 372 2 314 51 2 214 29 2 103 53
    Yjyw-36-1 113 936 0.12 0.149 281 0.003 650 8.924 659 0.222 446 0.430 412 0.007 447 2 339 42 2 330 23 2 308 34
    Yjyw-36-2 2 165 3 342 0.65 0.149 297 0.003 990 9.459 429 0.274 509 0.457 110 0.007 276 2 339 46 2 384 27 2 427 32
    Yjyw-36-3 390 1 940 0.20 0.147 904 0.003 504 9.098 510 0.227 279 0.440 440 0.006 541 2 322 41 2 348 23 2 353 29
    Yjyw-36-4 559 1 523 0.37 0.147 803 0.003 782 8.578 257 0.224 912 0.416 908 0.006 989 2 321 44 2 294 24 2 246 32
    Yjyw-36-5 2 820 13 596 0.21 0.146 607 0.004 744 7.790 169 0.286 505 0.384 695 0.008 331 2 306 56 2 207 33 2 098 39
    Yjyw-36-6 420 1 578 0.27 0.146 248 0.003 750 8.961 177 0.247 279 0.439 416 0.007 771 2 303 44 2 334 25 2 348 35
    Yjyw-36-7 360 1 264 0.28 0.151 775 0.003 930 9.058 092 0.250 974 0.430 626 0.007 306 2 366 44 2 344 25 2 309 33
    Yjyw-36-8 2 289 10 385 0.22 0.142 698 0.003 536 8.529 358 0.223 825 0.430 349 0.006 698 2 261 43 2 289 24 2 307 30
    Yjyw-36-9 280 515 0.54 0.140 439 0.003 665 7.786 716 0.217 423 0.401 689 0.007 482 2 232 45 2 207 25 2 177 34
    Yjyw-36-10 558 2 291 0.24 0.140 148 0.004 469 8.521 089 0.277 313 0.437 870 0.008 983 2 229 55 2 288 30 2 341 40
    Yjyw-36-11 2 989 24 261 0.12 0.139 503 0.003 566 6.725 033 0.234 299 0.346 795 0.009 000 2 221 44 2 076 31 1 919 43
    Yjyw-36-12 1 403 2 213 0.63 0.193 920 0.005 617 14.688 634 0.449 941 0.544 137 0.010 597 2 776 53 2 795 29 2 801 44
    Yjyw-36-13 266 1 307 0.20 0.192 357 0.005 866 14.490 223 0.463 195 0.539 954 0.009 993 2 763 45 2 782 30 2 783 42
    Yjyw-36-14 519 1 037 0.50 0.190 759 0.006 416 14.919 028 0.546 083 0.563 500 0.013 753 2 750 55 2 810 35 2 881 57

    Table 1.  LA-ICP-MS zircon U-Pb dating results of the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block

    All elemental analyses were carried out at the GPMR, CUG. Major and trace elements were analyzed by a Shimadzu Sequential 1800 XRF spectrometer and an Aglient 7500a ICP-MS, respectively. Nd isotopic analyses were performed on a Finnigan MAT 261 thermal ionization mass spectrometer (TIMS). The detailed analytical techniques for Nd isotopes are in accordance with those described by Gao et al. (1999). The analytical results are shown in Table 2.

    Sample No. SiO2 TiO2 Al2O3 Fe2O3t MnO MgO CaO Na2O K2O P2O5 LOI Total Mg# FeOt Sc Cr Co Ni V Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U Eu/Eu* (La/Yb)N (Gd/Yb)N 143Nd/144Nd 2s 147Sm/144Nd εNd(t) TDM (Ga)
    Yjhlc-16-1 46.45 3.64 13.58 19.34 0.09 7.69 1.77 0.07 0.74 1.34 5.54 100.25 44 17.41 37.7 27 78.3 24.4 247 26.8 21.4 56.8 465 27.3 245 42.1 91.2 11.4 51.4 10.6 2.87 12.1 1.86 11.3 2.3 6.52 0.98 6.05 0.89 10 1.39 2.8 3.91 1.57 0.77 4.99 1.65
    Yjhlc-16-2 44.87 3.93 14.1 19.04 0.09 7.89 1.91 0.06 0.84 1.43 5.64 99.8 45 17.14 40.7 28 54.2 24.3 264 31.9 21.1 63.1 472 29.3 276 48.2 106 13.2 55.4 11.4 3.36 12.9 2.22 12.9 2.6 7.37 1.06 7.04 1.02 10.4 1.57 2.3 4.68 1.62 0.84 4.91 1.52
    Yjhlc-16-3 47 3.96 13.38 18.16 0.08 7.27 1.94 0.06 0.94 1.47 5.28 99.54 44 16.34 40.4 28 74.3 24 460 37.7 23.4 70.5 495 29.7 302 54.6 121 14.6 63.6 14.1 4.06 15.3 2.53 15.4 3.03 8.43 1.24 7.64 1.11 11.2 1.62 2.4 4.67 1.76 0.84 5.13 1.66
    Yjyw-33-2 44.35 3.51 12.33 20.59 0.28 4.48 6.39 1.37 2.14 0.49 5.4 101.33 30 18.53 45.3 9.8 55 16.1 273 104 193 54.2 224 28 379 30.6 66.6 8.48 35.8 8.77 2.81 9.59 1.53 9.54 1.89 5.32 0.77 4.91 0.73 5.61 1.6 17 2.93 1.03 0.93 4.47 1.62 0.512 153 0.000 007 0.147 9 5 2.3
    Yjyw-33-3 50.06 2.87 15.11 12.17 0.25 4.48 4.36 4.16 0.61 0.52 6.04 100.63 42 10.95 23.4 16.8 40 24.7 400 29.6 106 36.9 276 43.3 118 40.7 84.4 9.93 39.9 7.67 2.31 6.98 1.06 5.91 1.21 3.61 0.52 3.22 0.48 6.62 2.72 21.6 6.84 8.73 0.95 9.07 1.79
    Yjyw-33-4 48.44 2.83 12.35 17.34 0.25 4.93 7.29 2 1.46 0.43 3.96 101.28 36 15.61 39.9 64.2 64.2 57 314 64.4 196 46.5 208 16.9 658 21.2 45.5 6.08 27.8 7.46 2.33 8 1.32 8.13 1.66 4.53 0.64 4.18 0.63 5.26 1.19 7.8 3 0.86 0.92 3.64 1.58 0.512 317 0.000 002 0.162 1 4 2.45
    Yjyw-33 48.51 2.84 16.06 13.46 0.25 4.91 3.29 4.01 1.36 0.52 5.3 100.51 42 12.11 24.9 16.6 61.9 34.5 302 82.3 87.1 33.6 267 40.6 118 33.6 71.4 7.98 33.8 6.82 1.99 6.63 0.99 5.77 1.19 3.28 0.48 2.91 0.44 6.24 2.82 16.7 6.1 7.36 0.89 8.28 1.88
    Yjyw-36 44.4 1.7 16.36 13.66 0.19 6.9 6.87 2.37 1.61 0.22 6.14 100.42 50 12.29 34.6 40.3 92 72.4 219 73.8 80.2 25.5 97 12.2 161 15.3 30.7 3.89 16.8 4.14 1.67 4.72 0.73 4.59 0.91 2.45 0.35 2.23 0.33 2.53 1.06 8.12 1.58 0.43 1.15 4.92 1.75 0.512 139 0.000 003 0.148 9 4.4 2.37
    Yjyw-36-1 48.25 2.66 13.14 16.3 0.3 5.03 3.99 0.2 3.22 0.5 6.46 100.05 38 14.67 29.4 43 93.1 75.7 281 233 165 44.7 178 13.7 228 19 59.3 5.39 24.9 6.57 1.85 7.21 1.16 7.15 1.46 4.03 0.57 3.68 0.55 4.44 1.26 18.3 4.2 2.46 0.82 3.7 1.62 0.512 281 0.000 013 0.159 7 4 2.44
    Yjyw-36-2 45.44 3.63 13.38 18.16 0.27 5.12 8.06 3.95 0.32 0.79 0.9 100.01 36 16.34 44.1 26.5 42.2 25.1 247 6.99 127 77.3 338 45.5 117 38.3 80 13.5 58.8 14.2 4.15 14.2 2.59 16 3.13 7.86 1.4 9.13 1.22 10.5 2.6 10.2 4.54 1.03 0.88 3.01 1.29
    Yjck-07-06-1 40.45 1.23 14.9 9.02 0.37 6.74 9.82 2.93 2.02 0.16 11.39 99.03 60 8.12 36.5 1 154 66 267 203 131 203 24.4 101 15.4 372 14.3 29.4 3.65 16.1 4.25 1.35 4.79 0.72 4.44 0.92 2.69 0.37 2.21 0.28 2.5 0.94 9.5 1.43 0.36 0.91 4.64 1.79
    Yjck-07-06-2 40.85 1.26 15.15 9.21 0.36 6.74 9.49 2.91 2.13 0.17 11.07 99.34 59 8.29 37.3 1 147 64 274 215 134 201 25.2 102 15.7 390 13.8 28.7 3.66 17 4.09 1.51 4.96 0.72 4.56 0.91 2.66 0.37 2.2 0.3 2.6 0.96 9.4 1.42 0.35 1.02 4.5 1.87
    Yjck-07-06-3 40.24 1.25 15.09 9.24 0.37 6.68 9.9 2.95 2.1 0.17 11.43 99.42 59 8.32 37.6 1 166 64.7 284 208 134 205 25.3 103 15.4 379 14.5 29.6 3.73 16.9 4.16 1.44 4.65 0.71 4.54 0.95 2.81 0.38 2.31 0.33 2.7 0.95 9.7 1.51 0.37 1.00 4.5 1.67
    Yjck07-07-1 41 1.26 15.06 9.25 0.38 7.63 9.46 2.99 1.7 0.17 11.14 100.04 62 8.33 38.7 1 140 73.5 273 202 110 201 24.7 102 15.8 318 12.9 27.4 3.55 15.4 4.32 1.34 4.55 0.72 4.38 0.89 2.61 0.36 2.13 0.28 2.6 0.94 9.2 1.52 0.36 0.92 4.34 1.77
    Yjck07-07-2 40.87 1.26 15.59 9.33 0.37 8.06 8.81 3.12 1.64 0.16 10.82 100.03 63 8.4 38.4 1 160 77.4 274 206 108 191 23.7 104 15.8 305 12.3 26.4 3.52 15.9 3.87 1.32 4.43 0.73 4.6 0.95 2.65 0.38 2.33 0.32 2.6 0.96 9 1.49 0.38 0.97 3.79 1.57
    Fe2O3t represents total Fe-oxides; Mg#=molar Mg×100/(Mg+Fe); LOI. Loss on ignition; FeOt=0.899 8×Fe2O3t.

    Table 2.  Major (wt.%), trace elements (ppm) and Nd isotopic data of the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block

3.   RESULTS
  • Four samples (Yjck-07, Yjhlc-16, Yjyw-33 and Yjyw-36) were selected for zircon U-Pb dating. Zircon grains selected from these samples are transparent, light brown and prismatic. The grains are 60-100 μm long with elongation ratios ranging from 1.5 : 1 to 2 : 1, and display weak internal oscillatory or broad-banded zoning in CL images (Fig. 4).

    Figure 4.  LA-ICP-MS zircon U-Pb concordia diagrams and cathodoluminescence (CL) images of representative zircon grains for the dated samples from the Cuoke area in southwestern Yangtze Block. The sample locations are shown in Fig. 2.

    The Th and U concentrations of nine zircon grains from sample Yjck-07 range from 78 ppm to 904 ppm and from 59 ppm to 909 ppm, respectively, with Th/U ratios of 0.20-2.44. Five spots on five grains from this sample yield a weighted mean 207Pb/206Pb age of 2 395±22 Ma (MSWD=2.40) (Fig. 4a), interpreted as the formation age. The remaining four discordant spots are considered as reflecting post-crystallization Pb-loss. The Th/U ratios for the grains from sample Yjhlc-16 range from 0.31-0.66. Nine spots on 9 grains yield a weighted mean 207Pb/206Pb age of 2 354±11 Ma (MSWD=2.30) (Fig. 4b). The Th/U ratios of the zircon grains from samples Yjyw-33 and Yjyw-36 are in the range of 0.09-0.66 (mostly > 0.1). Twenty spots on 20 grains from sample Yjyw-33 and seven spots on 7 grains from sample Yjyw-36 yield weighted mean 207Pb/206Pb ages of 2 316±8 Ma (MSWD=1.04) and 2 329±17 Ma (MSWD=0.92), respectively, (Figs. 4c and 4d), representing the crystallization ages. The remaining zircon grains give older 207Pb/206Pb ages of 2 750-2 776 Ma, interpreted as inherited ages. The discordant analyses might be attributed to Pb-loss triggered by younger tectonothermal events.

  • Major and trace element compositions are listed in Table 2. The analyzed samples in this study have various LOI contents of 0.90-11.43, suggesting varying degrees of alteration. Hence, only the least alteration-sensitive parameters can be used. Zr is often used as an index to test the mobility of other incompatible elements (Rolland et al., 2009). Based on their lithology and geochemical characteristics, our samples can be divided into two groups. The Group 2 samples with blastogabbroic texture show positive correlations between Zr and TiO2, Fe2O3t, P2O5, Nb, Th and La (Fig. 5), and negative correlations between Zr and Al2O3 and CaO (Figs. 5d and 5e). While no correlations are observed between of Zr and Na2O (Fig. 5f), Sr and Rb (not shown). Therefore, these immobile elements and Nd isotopic data can be used to discuss their petrogenesis. Data of major oxides are normalized to 100% (volatile-free), and the recalculated data are used in the diagrams and descriptions below. All samples exhibit variations in major oxide contents with SiO2 of 45.73 wt.%-52.92 wt.%, TiO2 of 1.40 wt.%-3.66 wt.% and Al2O3 of 12.69 wt.%-17.48 wt.%. The Mg# (molar 100×Mg/ (Mg+Fe) values for all samples range from 30-63. These samples mostly plot in the basalt field and belong to the tholeiitic series (Figs. 6a and 6b). All samples have significantly higher Nb (12.2 ppm-45.5 ppm) contents and high Nb/La (0.72-1.28) ratios in comparison with the intra-oceanic arc basalts (Nb < 2 ppm; Martin et al., 2005), and can be classified as Nb-enriched mafic rocks (Fig. 6c; Sajona et al., 1996, 1994).

    Figure 5.  Plots of (a) TiO2, (b) Fe2O3t, (c) P2O5, (d) CaO, (e) Al2O3, (f) Na2O, (g) Nb, (h) Th and (i) La vs. Zr for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block.

    Figure 6.  (a) Nb/Y vs. Zr/TiO2 (after Winchester and Floyd, 1977), (b) SiO2 vs. FeOt/MgO (after Miyashiro, 1974) and (c) MgO vs. Nb/La diagrams for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block. The fields of the island arc basalts and Nb-enriched basalts (NEBs) are from Kepezhinskas et al. (1996).

    The Group 1 samples exhibit enriched light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with high (La/Yb)N (N herein refers to chondrite- normalized value; e.g., Sun and Mcdonough, 1989) of 3.79-4.64 and (Gd/Yb)N of 1.57-1.87 (Fig. 7a), which are roughly parallel to the enriched mid-oceanic ridge basalts (E-MORB). The majority of these samples show negligible Eu anomalies (Eu/Eu*=0.91-1.02) but clearly positive Nb and Ta anomalies, similar to the ~2.30 Ga diorites from southwestern Yangtze Block (Fig. 7b; e.g., Lu et al., 2019). Such geochemical features resemble typical high-Nb basalts (HNBs; e.g., Hastie et al., 2011; Castillo, 2008). The Group 2 samples are strongly enriched in LREEs and display moderately sloping patterns (Fig. 7a), with high (La/Yb)N ratios of 3.01-9.07 and (Gd/Yb)N ratios of 1.29-1.88 (Fig. 7a). The Group 2 samples have weak Eu anomalies (Eu/Eu*=0.77-1.15). In spidergram (Fig. 7b), the Group 2 samples display enrichment in large ion lithophile elements (LILEs) and have high Nb-Ta concentrations with negative Sr anomalies, roughly resembling the ~1.7 Ga gabbros from southwestern Yangtze Block (Fig. 7b; e.g., Chen et al., 2013). Four samples from Group 2 were selected for Nd isotopic analysis (Table 2). The 143Nd/144Nd ratios range from 0.512 139 to 0.512 317, and εNd(t) values and Nd model ages range from 4.0-5.0 and 2.30-2.44 Ga, respectively.

    Figure 7.  (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element spidergrams for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block. The data for the ~1.70 Ga gabbros from the Hekou Group and the ~2.30 Ga dolerites from the Tongan Formation in southwestern Yangtze Block are form Chen et al. (2013) and Lu et al. (2019), respectively.

4.   DISCUSSION
  • All studied samples have low Th/Ce ratios of 0.04-0.09 and Th/La ratios of 0.09-0.22, which are significantly lower than the continental crust values (Th/Ce=~0.15 and Th/La=~0.30, respectively; e.g., Plank, 2005; Taylor and Mclennan, 1995). Although the presence of inherited zircon grains in sample Yjyw-36 indicates possible crustal contamination, there is no obvious correlation between La/Nb and Mg# for our samples (Fig. 8a), suggesting that the crustal contamination is insignificant.

    Figure 8.  Plots of (a) Mg# vs. La/Nb, (b) La vs. La/Sm and (c) La vs. La/Yb for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block.

    All samples have Mg# values ranging from 30 to 63, indicating that they were not primary magmas and experienced some fractional crystallization of olivine and clinopyroxene. On the plots of La vs. La/Sm and La vs. La/Yb, the Group 2 samples mainly plot along the trend of fractional crystallization (Figs. 8b and 8c). The correlations between V, Ni and Cr suggest that the parental magmas of the Group 2 samples have undergone varying degrees of clinopyroxene and olivine fractionation (Figs. 9a and 9b). The positive correlations between Zr and TiO2, Fe2O3t, P2O5 of the Group 2 samples might indicate the apatite and Fe-Ti oxides fractionations (Figs. 5a-5c). The Group 2 samples have weak Eu anomalies and obvious negative Sr anomalies (Fig. 7), indicating fractional crystallization of plagioclase. In contrast, the Group 1 samples have no obvious Eu anomalies (Fig. 7a), indicating little fractionation of plagioclase.

    Figure 9.  Plots of (a) Cr vs. V and (b) Cr vs. Ni for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block.

  • As mentioned above, all meta-mafic samples are characterized by high Nb concentrations, and can be classified as Nb-enriched mafic rocks (Sajona et al., 1996, 1994). Two main petrogenesis models have been proposed for the Nb-enriched mafic rocks including (1) partial melting of mantle wedge metasomatized by adakitic melts and (2) derivation from the asthenospheric mantle or magma mixing between a highly- enriched (OIB-like) and a relatively depleted (arc- or MORB-like) mantle components (e.g., Liu et al., 2017; Mazhari, 2016; Castillo, 2008; Wang et al., 2008; Castillo et al., 2007). The first model requires a young (≤25 Ma) and hot oceanic crust subducted beneath the mantle wedge as well as contemporaneous adakitic rocks (Wang et al., 2007; Prouteau et al., 2000). However, in southwestern Yangtze Block, neither occurrence of Paleoproterozoic adakitic rocks nor evidence for the Early Paleoproterozoic subduction has been reported. Thus, the first model is not feasible for our samples. Instead, a derivation from the asthenospheric mantle or magma mixing is more plausible explanation for our samples. The Group 2 samples are strongly enriched in LREEs and display moderately sloping HREE patterns. Their positive εNd(t) (4.0-5.0) values are close to the depleted mantle, indicating that the primary magmas were originated from a depleted mantle source. The high Zr concentrations and low La/Ta and Th/Ta ratios for the Group 2 samples indicate an asthenospheric mantle-derived melt (Figs. 10a and 10b). Besides, it is noteworthy that their εNd(t) values are similar to the ~1.70 and ~2.30 Ga mafic rocks in southwestern Yangtze Block, which have been suggested to have originated from a depleted asthenospheric mantle source (Fig. 10a; Lu et al., 2019; Chen et al., 2013). These signatures, along with the high Ta/Nd ratios and enrichment in LREEs (Figs. 7 and 10c), further support a depleted asthenospheric mantle source. Similarly, our Group 1 samples with E-MORB- like geochemical characteristics show positive Nb and Ta anomalies, roughly resembling the ~2.30 Ga diorites from southwestern Yangtze Block, which have been suggested to be derived from a depleted asthenospheric mantle source (Fig. 7; Lu et al., 2019). In Figs. 10b and 10c, the Group 1 samples plot along the plume-type influence trend. These signatures along with their La/Ta (< 22) and La/Nb (< 1.5) ratios might indicate an asthenospheric mantle source for the Group 1 samples. In the Sm vs. Sm/Yb diagram (Fig. 10d), all these meta-mafic rocks fall along the spinel+garnet lherzolite melting trend but with different melting degrees (Aldanmaz et al., 2000). The mantle source of the Group 2 samples likely has experienced lower degrees of melting (3%-10%) than that of the Group 1 samples (15%-20%).

    Figure 10.  Plots of (a) La/Ta vs. εNd(t) (after Chen et al., 2013), (b) Zr vs. Th/Ta (after Saccani et al., 2013), (c) Th/Nb vs. Ta/Nd (after Aldanmaz et al. 2008) and (d) Sm vs. Sm/Yb for the meta-mafic rocks from the Cuoke area in southwestern Yangtze Block. APS. Average pelitic sediments; UCC. upper continental crust. DMM is from Workman and Hart (2005), DM, PM, N-MORB and E-MORB are from Sun and McDonough (1989) and McKenzie and O'Nions (1991). The referred melt curves are after Aldanmaz et al. (2000) and references therein. The data for the ~1.70 Ga gabbros from the Hekou Group and the ~2.30 Ga dolerites from the Tongan Formation in southwestern Yangtze Block are form Chen et al. (2013) and Lu et al. (2019), respectively.

  • Condie et al. (2009) proposed a global tectonomagmatic shutdown hypothesis due to the apparent paucity of zircon ages between ~2.45-2.22 Ga in both magmatic and detrital records. However, many studies have shown that abundant Archean- Early Paleoproterozoic igneous rocks occur around the world (Fig. 12). The evidence for a globally contemporaneous ~2.40-2.30 Ga tectono-magmatic event further argues against the plate-tectonic shutdown or 'slowdown' model. Similarly, along the northern and western parts of the Yangtze Block and its southern part in northern Vietnam, sporadic Archean-Early Paleoproterozoic igneous rocks have been reported (Figs. 1 and 12; e.g., Cui et al., 2019; Lu et al., 2019; Zhao et al., 2019; Hui et al., 2017; Kou et al., 2017; Wang et al., 2016; Wu et al., 2014, 2012; Hu et al., 2013; Gao et al., 2011; Zhao et al., 2010; Nam et al., 2003; Zhang et al., 2001 and references therein). Furthermore, detrital zircon grains from the Irrawaddy, Salween, Mekong and Red rivers in southeastern Asia have three Paleoproterozoic age-peaks at 2.5, 2.3 and 1.9 Ga (e.g., Bodet and Schärer, 2000). All these studies indicate the existence of Archean-Early Paleoproterozoic basement in southwestern Yangtze Block, although these ages are mainly from the felsic rocks and metasedimentary rocks. Our zircon geochronology data show that the meta-mafic rocks from the Cuoke area in southwestern Yunnan formed at ~2.40-2.32 Ga, and confirm the presence of Early Paleoproterozoic mafic rocks within southwestern Yangtze Block. These newly identified meta- mafic rocks can be compared in time and space with the ~2.30 Ga granitoids in the study area (Fig. 1; Cui et al., 2019), and coeval ~2.30 Ga dolerites of the Tongan Formation (Lu et al., 2019), as well as the ~2.30-2.10 Ga granitic magmatism records of the Phan Si Pan complex in northern Vietnam (Fig. 1; Zhao et al., 2019; Wang et al., 2016).

    Figure 11.  (a) Zr vs. Zr/Y (after Pearce and Norry, 1979) and (b) Yb vs. Th/Ta (after Gorton and Schandl, 2000) geochemical discrimination diagrams for the meta-mafic rocks from the Cuoke area in Yangtze Block.

    In general, the Nb-enriched mafic rocks are considered to have formed during regional extension (e.g., Liu et al., 2017; Straub et al., 2013 Castillo, 2009, 2008; Castillo et al., 2007, 2002). Our geochemical data indicate that the ~2.40-2.32 Ga Nb-enriched mafic rocks in southwestern Yangtze Block have high TiO2 contents and depleted Nd isotopic compositions, and were mainly derived from a depleted asthenospheric mantle. On the Zr vs. Zr/Y and Yb vs. Th/Ta tectonic discrimination diagrams (Fig. 11; e.g., Gorton and Schandl, 2000; Pearce and Norry, 1979), all samples fall in the field of within-plate basalts. In addition, Lu et al. (2019) proposed that the ~2.30 Ga dolerite dykes with E-MORB-like geochemical characteristics in southwestern Yangtze Block were formed in an intra-continent extensional setting. These signatures along with the synchronous post-collisional granitoids (~ 2.36 Ga) in the study area (Cui et al., 2019), might indicate a bimodal igneous assemblage. Therefore, the Nb-enriched mafic rocks in southwestern Yangtze Block possibly formed in a post-collisional extension setting.

    ~2.42-2.30 Ga rift- or LIP-related mafic dykes have been reported from several places around the world (Fig. 12; Partin et al., 2014 and references therein). The mafic rocks in the Dharwar and North Atlantic cratons have been linked with the widespread 2.37 Ga LIP/rifting event (Nilsson et al., 2013; Kumar et al., 2012; French and Heaman, 2010). Partin et al. (2014) recently proposed that the ~2.40 Ga Ringvassøy mafic rocks in the Fennoscandian Shield (Kullerud et al., 2006), ~2.42 Ga Scourie dykes in the Baltic Block (Heaman and Tarney, 1989), and ~2.42-2.41 Ga Widgiemooltha dyke swarm in the Yilgarn Craton (French et al., 2002; Nemchin and Pidgeon, 1998) all belong to an integral ~2.40 Ga extension-related event. In addition, the ~2.30 Ga Tulisaari dolerite dyke swarms in the Karelian Craton (HHölttä et al., 2000) and ~2.31 Ga mafic dykes in the Fuping complex in the North China Craton (Liu et al., 2002) might record a slightly younger and less extensive event. Our zircon ages of 2 395-2 316 Ma for the meta-mafic rocks in southwestern Yangtze Block can be compared with these reported ages of mafic rocks around the world (Fig. 12). Moreover, the ~2.36-2.29 granitic rocks in the Yangtze Block can also be compared with coeval counterparts in the North America, South America and Asia (Fig. 12), further indicating a global magmatic event during the Early Paleoproterozoic.

    It is noteworthy that the Arrowsmith orogeny in the Rae Craton of Laurentia is also characterized by the Early Paleoproterozoic tectonomagmatism, which produced 2.33-2.29 Ga syn- to post-collisional granitoids and subsequent 2.19-2.02 Ga post-orogenic complexes (Berman et al., 2013; Hartlaub et al., 2007). Cui et al. (2019), Zhao et al. (2019) and Wang et al. (2016) recently proposed a close spatial relationship between southwestern Yangtze Block and the Arrowsmith orogenic belt based on the newly identified 2.36-2.30 Ga post-collisional granitoids and 2.10 Ga extension-related A-type gneissic granites from the Cuoke area and Phan Si Pan complex, respectively (Fig. 12). Similarly, ~2.40-2.30 Ga metamorphic event (~700 ℃ and 0.65 GPa) in northwestern Vietnam (Nam et al., 2003) might represent a high-grade tectonothermal event related to the continent-continent or arc-continent collision (Cui et al., 2019). This metamorphic event has been compared with the amphibolite- to granulite-facies metamorphism (~700 ℃ and 0.5-0.6 GPa) in the Arrowsmith orogenic belt of the Rae Craton and the Anabar Block of Siberia Craton (Wang et al., 2016; Berman et al., 2013; Rosen, 2002). Collectively, these global correlations are compatible with reconstruction models in which the Yangtze Block was spatially close to the Rae and Siberia cratons during the Early Paleoproterozoic (Wang et al., 2016). The ~2.45 Ga metamorphic records of the Yudongzi complex in the northwestern Yangtze Block might also support this connection (e.g., Hui et al., 2017). Pehrsson et al. (2013) has proposed a Neoarchean supercraton named the Nunavutia, which includes the Rae, Gawler-Mawson, India, West Africa, Amazonia, North China and Sask cratons/blocks. Lu et al. (2019) recently suggested that the ~2.30 Ga E-MROB-like dolerite dykes in southwestern Yangtze Block might imply dispersing of the Yangtze Block from the Nunavutia supercraton. A synthesis of these data indicates that the ~2.40-2.30 Ga meta-mafic rocks in southwestern Yangtze Block might be linked with the Arrowsmith orogenic event.

5.   CONCLUSIONS
  • (1) Nb-enriched meta-mafic rocks from the Cuoke area in southwestern Yangtze Block yield zircon 207Pb/206Pb ages of 2 395-2 316 Ma.

    (2) The meta-mafic rocks were likely derived from a depleted asthenospheric mantle in a post-collisional extension setting.

    (3) The Cuoke Nb-enriched meta-mafic rocks in southwestern Yangtze Block might be linked with the Arrowsmith orogenic event.

ACKNOWLEDGMENTS
  • We are grateful to three anonymous reviewers for their critical and constructive comments on this paper. This work was jointly supported by the National Natural Science Foundation of China (Nos. U1701641, 41702230), the District Summary and Service Product Develop of Yunnan Region Geologic Survey (No. 121201102000150012-02), Yunnan Province Geological Survey Foundation (No. 2013HA001) and Natural Science Foundation of Guangdong Province (No. 2018B030312007). The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1260-7.

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