The Paleoproterozoic bimodal magmatism in the SW Yangtze block: Implications for initial breakup of the Columbia supercontinent

Lithos 332–333 (2019) 23–38

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The Paleoproterozoic bimodal magmatism in the SW Yangtze block: Implications for initial breakup of the Columbia supercontinent Kang Liu a,b, Guimei Lu a,⁎, Zizheng Wang c, Sifang Huang a, Erkun Xue a, Wei Wang a,⁎ a b c

State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China Chengdu Center of China Geological Survey, Chengdu, China

a r t i c l e

i n f o

Article history: Received 19 October 2018 Accepted 25 February 2019 Available online 26 February 2019 Keywords: Paleoproterozoic Bimodal magmatism Petrogenesis Intracontinental rift Yangtze Block Columbia supercontinent

a b s t r a c t The fragmentation of the Columbia supercontinent is thought to have begun as early as ~1.7 Ga; however, the breakup of its core domain occurred only at ~1.5–1.25 Ga. This study presents the integrated petrological and chemical characteristics of gabbro and granite porphyry intrusions in the Haizi region of China, which record the oldest magmatism associated with the initial breakup of Columbia in the southwestern Yangtze Block. The dating of gabbro and granite porphyry samples using laser ablation–inductively coupled plasma–mass spectrometry zircon U\\Pb geochronology gives crystallization ages of 1754 ± 14 Ma and 1743 ± 4 Ma, respectively. The gabbro is characterized by low La/Ta, Th/Y, (La/Yb)N, and (Dy/Yb)N ratios, negligible Nb\\Ta anomalies, moderately depleted Nd isotope signatures (εNd (t) = +2.4 to +5.3), and enriched zircon Hf isotopes (εHf(t) = −1.8 to −7.8), indicating derivation of the primary magma from ambient asthenospheric mantle and complex petrogenetic processes before emplacement. Geochemical modeling indicates a variable but low degree of partial melting (1%–15%) and minor crustal contamination during magma ascent. The gabbro samples have relatively high TiO2 (1.94–3.73 wt%) and Zr (95–271 ppm) contents, typical of rocks formed in an intraplate tectonic setting. The granite porphyry samples display high FeOT/MgO and Ga/Al ratios, high contents of high-fieldstrength elements, and low CaO–Sr contents, resembling A-type granites. Compared with the gabbros, these porphyries show wider geochemical gaps, more variable ratios between incompatible elements, and lower radiogenic Nd isotopic compositions (εNd(t) = +0.3 to +2.7 and TDM2(Nd) = 2226– 2028 Ma). However, the porphyries have more radiogenic zircon Hf isotopes (εHf(t) = 0 to +3.7 and TDM2(Hf) = 2421–2189 Ma) than those of the gabbros, suggesting their derivation from the partial melting of a Paleoproterozoic crustal source triggered by an upwelling basaltic magma in an intraplate extensional setting. The ~1.75 Ga bimodal magmatism reported in this study, together with the ~1.75–1.65 Ga mafic magmatism documented in other cratonic blocks, suggests a possible spatial linkage between the Yangtze Block, northern Australia, northwestern Laurentia, and southern Siberia in the late Paleoproterozoic Columbia supercontinent. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The Yangtze Block in southern China is considered to have been part of the Paleo- to Mesoproterozoic Columbia supercontinent (Cawood et al., 2018; Furlanetto et al., 2016; Wang et al., 2016; Yin et al., 2013; Zhou et al., 2014); however, the limited exposure of Paleoproterozoic rocks within the block has hindered our understanding of its early Paleoproterozoic tectonothermal evolution and its role in the assembly and breakup of the supercontinent. Previous studies have reported an extensive 1767–1650 Ma within-plate mafic magmatism with a volumetrically subordinate one as young as 1486 Ma occurred in the southwestern Yangtze Block (Chen et al., 2013; Fan et al., 2013; Wang et al., ⁎ Corresponding authors. E-mail addresses: [email protected] (G. Lu), [email protected] (W. Wang).

https://doi.org/10.1016/j.lithos.2019.02.021 0024-4937/© 2019 Elsevier B.V. All rights reserved.

2019; Zhao et al., 2010). These findings point to the occurrence of intracontinental extensional magmatism in the southwestern Yangtze Block during the early fragmentation of the Columbia supercontinent (Lu et al., 2019). Nevertheless, bimodal magmatism, which is commonly associated with continental rifting, is relatively rare in the southwestern Yangtze Block. Although the pulse of mafic magmatism was most likely related to continental rifting, the accompanying felsic rocks signify the earliest extensional event associated with the partial melting of continental crust initiated by the first influx of hot mantle-derived mafic magma. Mafic intrusions and coeval granitic rocks (ca. 1.75 Ga) have recently been recognized in the southwestern Yangtze Block (Yang et al., 2015) and represent the earliest occurrence of bimodal magmatism in the region. These rocks should provide valuable information on Paleoproterozoic tectonothermal events, the early fragmentation of the

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K. Liu et al. / Lithos 332–333 (2019) 23–38

Columbia supercontinent, and the separation of the Yangtze Block from the supercontinent. However, the petrogenetic history and geodynamic setting of these rocks, as well as their temporal and spatial associations with the large-scale ~1.75 Ga bimodal magmatism associated with the breakup of the supercontinent, remain poorly constrained. In this contribution, we present an integrated study of the petrology, zircon U\\Pb ages, zircon Hf isotope and whole-rock Sr\\Nd isotope compositions, and geochemical characteristics of gabbro and granite porphyry from the southwestern Yangtze Block. The dataset is interpreted and synthesized to establish the petrogenesis and geodynamic setting of this bimodal magmatism and to constrain the paleogeography of the Yangtze Block in the Columbia supercontinent. 2. Geological background 2.1. Regional geology The South China Craton comprises the Yangtze Block in the northwest and the Cathaysia Block in the southeast. These blocks are considered to have amalgamated along the Jiangnan Orogenic Belt during the early to middle Neoproterozoic (Fig. 1a). The Yangtze Block is separated from the North China Craton to the north by the Qinling–Dabie orogenic belt and is bounded by the Tibetan Plateau and Indochina Block to the west and south, respectively (Fig. 1a). Archean–Paleoproterozoic crystalline basement rocks are exposed only in the northern and

southwestern portions of the Yangtze Block (Gao et al., 2011; Wang et al., 2016). The oldest exposed rocks are those of the ~3.45–2.90 Ga Kongling Complex in the northern Yangtze Block, comprising tonalitic– trondhjemitic–granitic (TTG) intrusions and associated sedimentary rocks (Gao et al., 2011; Guo et al., 2014). The Kongling Complex underwent upper-amphibolite- to granulite-facies metamorphism at 2.03–1.97 Ga and was later intruded by ~1.85 Ga A-type granites and mafic dikes (Li et al., 2016; Peng et al., 2009; Yin et al., 2013). More recent studies have reported the occurrence of late Archean to early Paleoproterozoic magmatic rocks, namely the Zhongxiang (2.85–2.65 Ga), Douling (2.5 Ga), Yudongzi (2.80–2.48 Ga), and Houhe (2.08 Ga) complexes in the northern Yangtze Block (Wang et al., 2018; Wu et al., 2012). In contrast, the southwestern Yangtze Block is characterized by widespread Paleo- to Mesoproterozoic sedimentary sequences and igneous rocks, represented by the Paleoproterozoic to early Mesoproterozoic (~1.73–1.50 Ga) Dahongshan, Dongchuan, and Hekou groups and the Tongan Formation (Chen et al., 2013; Greentree and Li, 2008; Lu et al., 2019; Wang and Zhou, 2014; Zhao et al., 2010). All of these Proterozoic rocks underwent greenschist-facies metamorphism and were fragmented by major NNE-trending faults (Fig. 1b). These old sequences show a faulted contact with the late Mesoproterozoic to early Neoproterozoic strata (e.g., the Kunyang, Julin, and Huili groups). The early Neoproterozoic rocks were metamorphosed at grades up to the lower greenschist facies and are overlain by Cryogenian to Ediacaran sequences (Fig. 1b).

Fig. 1. Regional geological map of western Yangtze Block (modified after Wang et al., 2014). (a) Simplified tectonic map displaying the location of study area in southwestern Yangtze Block; (b) Geological sketch map showing distribution of Late Paleoproterozoic to early Mesoproterozoic strata in southwestern Yangtze Block.

K. Liu et al. / Lithos 332–333 (2019) 23–38

2.2. Dongchuan group and descriptions of samples The distribution of the Dongchuan Group is controlled by a narrow fault belt and is N300 km long and up to 35 km (Fig. 1b). This group comprises (from base to top) the Yinmin, Luoxue, Etouchang, and Luzhijiang formations (Wang et al., 2014; Zhao et al., 2010). The Dongchuan Group has traditionally been considered the lower part of the late Mesoproterozoic Kunyang Group. However, more precise geochronological studies of mafic dikes/intrusions and volcanic rocks have confirmed the initiation of Dongchuan Group sedimentation as early as ~1.76 Ga (Yang et al., 2015). A suite of bimodal magmatic intrusions in the lower Dongchuan Group is known as the Haizi bimodal intrusions. The age of the intrusions has been determined at ca. 1.76–1.73 Ga by zircon LA–ICP–MS method (Yang et al., 2015). For the present study, 10 gabbro and 7 granite porphyry samples from the lower Dongchuan Group near Haizi (Fig. 2a, b) were collected and analyzed. The gabbro intrusions occur peripherally to the granite porphyry (Fig. 2a). These gabbros display a massive structure and gabbroic texture (Fig. 3a), and are composed chiefly of plagioclase (~65% modal), clinopyroxene (~25% modal), hornblende (~10% modal), and minor Fe\\Ti oxide minerals. Variable degrees of sericitization and kaolinization of plagioclase, and chloritization and epidotization of pyroxene indicate significant alteration in the gabbros (Fig. 3a, b). The granite porphyry samples show a typical porphyritic texture, with alkali feldspar being the dominant phenocryst phase in a

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groundmass of quartz with minor sericite, chlorite, and opaque minerals (Fig. 3c, d). The accessory minerals in the granites include ilmenite and zircon. 3. Analytical techniques Of the 17 samples analyzed, 4 each of gabbro and granite porphyry were analyzed using Sr\\Nd isotope methods. Gabbro (YM16–01) and granite porphyry (YM16–13) were selected for zircon U\\Pb age dating and Hf isotope analysis. 3.1. LA–ICP–MS Zircon U\\Pb dating Zircon grains from samples YM16–01 (gabbro) and YM16–13 (granite porphyry) were separated using conventional heavy liquid and magnetic techniques. Individual grains were handpicked under a binocular microscope and embedded in epoxy resin. All zircons were subsequently studied in transmitted and reflected light micro-images, as well as in cathodoluminescence (CL) images obtained using a Gatan Mono CL4+ detector attached to a Zeiss Sigma 300 field emission scanning electron microscope at the State Key Laboratory of Geological Process and Mineral Resources (SKLGPMR), China University of Geosciences, Wuhan, China. These images were used to determine the internal textures of zircons and to choose optimal sites (free of cracks and inclusions) for in situ analyses of U–Pb–Hf isotopes.

Fig. 2. Simplified geological map and cross section profile with sample locations in the Dongchuan Group (modified after Yang et al., 2015).

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K. Liu et al. / Lithos 332–333 (2019) 23–38

a

b Cpx Pl

Pl Cpx

Cpx

Pl Chl

Hb

Opq

Hb

Hb Pl

Cpx Pl c

d Kfs Pl

Opq Pl

Kfs

Pl Qz Qz

Fig. 3. Representative photomicrographs of the Haizi bimodal magmatic rocks from the lower Dongchuan Group. Mineral abbreviations: Pl–plagioclase; Kf-potassic feldspar; Q–quartz; Px–pyroxene; Hb–hornblende; Chl–chlorite; Opq–opaque mineral.

The zircon U\\Pb dating for sample YM16–01 was conducted using a Nu Instrument multi-collector inductively coupled with plasma-mass spectrometry (MC-ICP-MS), attached to the Resonetics Resolution M-50-HR Excimer Laser Ablation System, at the University of Hong Kong. All of the analyses were conducted with a beam diameter of 30 μm at a repetition rate of 6 Hz. Data acquisition started with a 30 s measurement of a gas blank during the laser warm-up period. The typical ablation time was 40 s for each analysis, resulting in pits of 30–40 μm depth. The isotopes of 232Th, 208Pb, 207Pb, 206Pb, and 204Pb were analyzed simultaneously in static-collection mode. External corrections were applied to all unknowns, and reference zircons 91,500 and GJ-1 were used as the external standard and the unknown, respectively, for quality control. The determined concordant ages of zircon 91,500 and GJ-1 were 1063.4 ± 1.7 Ma (n = 12, MSWD = 0.68) and 603.4 ± 1.4 Ma (n = 8, MSWD = 0.18), respectively (Table S1), which are within error of the recommended values (Jackson et al., 2004). The zircon U\\Pb dating and trace-element analyses for sample YM16–13 were performed simultaneously using LA–ICP–MS at the Hubei Geological Experiment Test Center, Wuhan, China, following the procedure described by Liu et al. (2010). Zircon grains were ablated with a laser spot size of 32 μm using a fluence of 8 J/cm−2 at a repetition rate of 6 Hz. Zircon 91,500 was used as an external standard to normalize U\\Pb isotopic fractionation, and GJ-1 was analyzed as an unknown to monitor data quality. The determined concordant ages of zircon 91,500 and GJ-1 were 1063.5 ± 4.5 Ma (n = 8, MSWD = 0.27) and 597.4 ± 2.7 Ma (n = 6, MSWD = 1.9), respectively (Table S1), which are within error of recommended values (Jackson et al., 2004). Sample trace-element compositions were normalized using multiple reference materials (BCR-2G, BHVO-2G, and BIR-1G), and 29Si was used for internal standardization. Raw data reduction was performed off-line using ICPMSDataCal software (Liu et al., 2010). Common lead was corrected using the EXCEL program for common lead correction of Andersen (2002).

Individual analyses are reported at the 1σ error level, and weighted mean ages are quoted at the 2σ error level. Concordia plots and weighted mean ages were calculated using Isoplot/EX_ver3.7 (Ludwig, 2003). The U\\Pb isotope and trace-element analytical data for zircon grains are presented in Tables S1 and S2. 3.2. Major- and trace-element analyses Major elements were analyzed by X-ray fluorescence at SKLGPMR. The Chinese National standards GSR-1, GSR-2, GSR-3, and GSR-10 were used for analytical quality control. The reported analytical precisions are better than 5% (Table S3). Trace-element analysis was conducted using an Elan DRC-II ICP–MS instrument at the University of Science and Technology of China (USTC), Hefei, China. About 50 mg of powder of each sample was dissolved in a mixture of HF + HNO3 in high-pressure Teflon bombs at 190 °C for 48 h. The soluble residue was mixed with Rh solution as an internal standard. USGS rock standards BHVO-2, AGV-2, W-2, and GSP-2 were chosen as reference materials to monitor the analytical quality. The analytical precision for most trace elements is better than 5%–10%. The whole-rock major- and trace-element data together with the data for the USGS rock standards are listed in Tables 1 and S3. 3.3. Whole-rock Sr\\Nd isotope analysis Whole-rock Sr\\Nd isotopic compositions were measured at USTC using a Finnigan MAT-262 mass spectrometer and following the methods described by Chen et al. (2007, 2009). For each sample, about 100 mg of powder was digested in an HF + HNO3 mixture in Teflon bombs. Standard ion exchange chromatography techniques were used for separating and purifying the elements. The mass fractionation correction for Sr and Nd isotopic ratios was based on values of 86 Sr/87Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The Nd

Table 1 Whole-rock major and trace element analyses for the Haizi bimodal intrusions from the Dongchuan Group. Sample No

YM16–01

YM16–05

YM16–06

YM16–07

YM16–08

YM16–09

YM16–10

YM16–11

YM16–12

YM16–13

YM16–14

YM16–15

YM16–16

YM16–17

YM16–18

YM16–19

Major oxide (wt%) SiO2 47.3 TiO2 2.17 Al2O3 12.9 T Fe2O3 15.4 MnO 0.19 MgO 5.81 CaO 10.0 Na2O 3.02 K2O 0.42 P2O5 0.16 LOI 1.98 TOL 99.30

46.5 3.60 11.2 18.9 0.10 5.10 6.39 4.84 0.37 0.31 1.66 98.92

47.6 3.73 12.1 14.8 0.05 5.22 8.00 4.75 0.84 0.33 1.61 98.96

44.1 3.51 10.5 18.9 0.08 5.18 8.94 3.83 0.88 0.23 2.34 98.50

47.1 2.13 12.9 15.4 0.19 5.59 10.2 3.00 0.51 0.19 1.74 98.96

50.0 3.64 11.2 17.0 0.04 3.61 5.57 6.08 0.44 0.50 0.87 98.95

46.4 1.94 12.6 15.3 0.13 6.23 10.2 3.23 0.76 0.13 1.58 98.55

48.2 2.16 13.5 13.7 0.08 6.02 8.66 3.86 0.87 0.22 2.20 99.42

49.8 2.75 11.8 13.9 0.06 4.91 7.71 5.39 0.60 0.28 1.86 99.14

48.5 3.53 11.5 13.2 0.06 4.95 7.63 5.19 0.81 0.13 3.84 99.36

69.6 0.53 10.7 11.7 0.02 0.22 0.12 5.74 0.05 0.07 0.85 99.63

71.4 0.49 11.2 8.39 0.05 0.22 0.09 6.00 0.10 0.05 1.59 99.63

67.6 0.61 12.6 9.47 0.05 0.25 0.13 6.87 0.13 0.07 1.04 98.91

71.1 0.51 10.6 9.78 0.01 0.16 0.12 5.86 0.04 0.06 0.93 99.11

74.2 0.45 10.8 7.07 0.02 0.15 0.13 6.20 0.03 0.08 0.74 99.86

66.0 0.54 12.3 10.9 0.05 0.38 0.13 6.58 0.07 0.06 1.37 98.45

75.4 0.17 12.6 2.45 0.10 0.20 0.15 6.92 0.15 0.06 1.11 99.30

Trace element (ppm) Li 18.3 Be 0.91 Sc 43.9 V 590 Cr 271 Ni 88.3 Cu 151 Zn 164 Ga 20.8 Rb 19.9 Sr 257 Y 22.8 Zr 126 Nb 11.0 Cs 0.18 Ba 307 La 10.4 Ce 23.6 Pr 3.39 Nd 16.4 Sm 3.93 Eu 1.44 Gd 4.74 Tb 0.80 Dy 4.90 Ho 0.93 Er 2.74 Tm 0.37 Yb 2.21 Lu 0.35 Hf 3.35 Ta 0.72 Pb 20.7 Th 1.78 U 0.34

25.3 0.94 42.7 613 131 60.0 253 54.2 22.4 9.93 51.9 29.8 207 18.6 0.24 42.7 27.7 62.0 8.03 36.0 7.68 2.57 7.77 1.25 6.57 1.23 3.61 0.49 3.19 0.49 5.53 1.29 1.10 2.61 0.62

59.6 2.39 45.2 599 186 44.3 62.1 50.9 25.7 33.9 59.2 42.8 268 27.7 1.15 93.0 32.6 76.7 10.1 44.7 10.3 3.07 10.57 1.68 9.57 1.74 4.76 0.67 4.20 0.60 6.87 1.75 0.83 3.43 0.92

49.3 1.02 48.8 801 109 71.8 1109 61.4 21.5 27.9 31.5 22.1 125 13.9 0.34 115 4.12 11.1 1.75 8.78 2.77 0.90 3.48 0.70 4.59 0.94 2.62 0.38 2.58 0.40 3.73 0.94 2.59 1.28 0.28

19.3 1.01 44.1 588 221 84.1 176 122 21.6 18.4 269 22.7 131 11.3 0.23 332 9.36 22.8 3.26 15.8 4.08 1.52 4.53 0.81 4.83 0.95 2.57 0.36 2.40 0.34 3.85 0.77 16.8 1.83 0.34

7.13 3.54 38.4 454 105 18.3 21.6 38.4 24.5 18.9 32.8 49.2 271 30.5 0.40 30.5 19.6 49.7 7.06 34.5 9.21 2.92 9.88 1.80 10.4 2.09 5.68 0.82 5.04 0.74 7.13 1.88 1.09 4.18 1.13

44.0 0.69 44.4 560 221 88.7 102 46.4 20.7 26.8 264 18.0 95.2 8.19 0.54 178 8.34 19.7 2.80 12.9 3.26 1.42 3.79 0.74 3.75 0.79 2.13 0.33 1.87 0.34 4.48 0.87 5.58 1.36 0.30

38.1 1.28 44.3 554 228 76.7 19.5 82.5 23.5 33.8 245 25.6 156 12.8 0.71 142 15.9 33.7 4.35 20.5 5.07 1.87 5.58 0.93 5.26 1.07 3.00 0.39 2.64 0.38 4.31 0.87 1.79 2.16 0.72

31.6 1.37 41.1 513 148 48.6 11.2 39.2 21.2 16.5 41.9 30.5 210 18.5 0.10 92.6 11.3 27.0 3.54 16.9 4.44 1.34 5.29 0.97 6.18 1.24 3.69 0.53 3.39 0.54 5.79 1.26 1.50 2.82 0.44

21.1 1.84 54.7 580 130 64.3 15.9 56.3 19.9 28.6 26.0 26.9 136 17.0 0.63 67.3 4.52 11.3 1.75 9.88 3.86 1.29 4.43 0.86 5.55 1.20 3.27 0.46 2.90 0.47 4.31 1.07 1.27 1.12 0.42

2.55 2.27 4.85 175 111 11.3 4.77 13.7 20.7 2.64 13.2 55.4 783 40.3 0.06 16.8 1.77 4.75 0.79 4.34 2.75 0.92 4.77 1.27 9.23 2.19 7.10 1.13 7.69 1.25 19.9 3.04 1.77 23.1 6.65

7.29 2.79 9.99 36.0 110 25.8 47.2 14.3 27.8 1.80 12.0 67.7 767 37.0 0.05 34.4 10.3 18.9 3.46 17.5 7.17 1.89 8.53 1.85 12.9 2.75 8.23 1.17 8.13 1.31 21.1 2.37 1.68 20.8 7.52

6.30 2.14 10.9 114 102 22.3 12.1 14.3 24.9 5.83 17.3 73.8 915 51.3 0.18 32.9 76.1 232 17.5 63.6 11.3 2.26 12.6 2.15 14.5 3.02 9.51 1.44 9.96 1.47 23.7 3.54 3.16 31.7 6.51

2.88 2.49 9.22 125 103 25.1 18.7 10.5 23.5 4.75 9.37 66.6 734 38.8 0.07 66.2 3.67 5.52 2.17 13.6 7.48 2.17 10.0 2.19 13.3 2.73 7.86 1.06 7.21 1.12 20.8 2.97 3.80 24.8 6.96

3.08 1.28 11.3 37.6 110 23.2 35.6 8.65 27.2 0.84 10.7 55.1 682 37.6 0.04 37.1 31.2 54.4 7.80 32.8 7.81 1.49 8.33 1.51 9.72 2.13 6.67 0.99 6.91 1.10 19.6 2.98 1.37 12.2 3.41

6.95 3.02 5.48 119 98.9 32.1 210 16.3 20.3 1.50 10.3 77.5 873 39.9 0.07 30.4 2.97 9.75 1.53 9.36 5.79 2.06 9.80 2.24 15.2 3.11 8.58 1.22 7.90 1.19 23.8 2.63 1.65 20.2 5.72

4.00 1.67 5.82 36.8 107 17.9 10.4 6.94 16.5 5.29 18.9 35.6 711 30.0 0.11 36.1 39.8 39.9 9.13 39.9 8.81 1.84 8.39 1.24 6.58 1.42 4.54 0.82 5.74 1.06 20.3 7.47 1.25 15.4 4.69

K. Liu et al. / Lithos 332–333 (2019) 23–38

YM16–04

Note: the repeated analysis, international rock standards and assocaited elemental ratios are given in details in Table S3.

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isotopic ratios of USGS reference materials AGV-2, BCR-2, and BHVO-2 were also determined to monitor analytical quality, yielding 143 Nd/144Nd ratios of 0.512794 ± 0.000007 (2σ), 0.512627 ± 0.000004 (2σ), and 0.512981 ± 0.000007 (2σ), respectively (Table S4), which are within error of the recommended values of 0.512790 ± 0.000016 (2σ, n = 8), 0.512634 ± 0.000012 (2σ, n = 11), and 0.512981 ± 0.000010 (2σ, n = 13) (Weis et al., 2006). The whole-rock sample Sr\\Nd isotopic compositions, together with the Nd isotopes for the USGS reference materials, are reported in Table S4. 3.4. Zircon Lu\\Hf isotopes Zircon Lu\\Hf in situ isotope analyses were performed using a Neptune Plus MC–ICP–MS instrument (Thermo Fisher Scientific, Germany) coupled to a Geolas 193 nm ArFexcimer LA system, housed at SKLGPMR, CUG, Wuhan, China. To accurately calculate initial Hf isotope ratios and model ages based on the zircon crystallization age, the Hf isotope analyses were performed on the same zircon sites as those where the U\\Pb isotope data had been obtained. A spot size of 44 μm was used for the analyses. The isobaric interference and instrumental mass bias were calibrated following the method of Liu et al. (2010). Zircon 91,500 was used as the external standard and zircon GJ-1 was employed as an unknown to control analytical quality, yielding weighted mean 176 Hf/177Hf ratios of 0.282307 ± 0.000013 (2σ, n = 10) and 0.282015 ± 0.000012 (2σ, n = 8), respectively (Table S5). These results are within error of the recommended values (Elhlou et al., 2006; Woodhead and Hergt, 2005). Raw data were processed off-line using ICPMSDataCal (Liu et al., 2010), and all the Lu\\Hf isotope analysis results are reported at the 2σ error level. The Lu\\Hf isotopic compositions for zircon grains from samples YM16–01 and YM16–13 are listed in Table S5. 4. Results 4.1. Zircon U\\Pb ages and trace-element compositions 4.1.1. Gabbro Zircon grains from the gabbro sample (YM16–01) are euhedral to anhedral, have aspect ratios of 1:1 to 3:1, and display light luminescence in CL images with no obvious oscillatory zoning (Fig. 4c). Sixteen analyzed zircon grains gave Th/U ratios of 0.16–3.38 (Table S1), consistent with a magmatic origin. Analytical spots 07, 10, 15, and 16 were excluded from the age calculations as they yielded much older 207Pb/206Pb apparent ages (2008–2541 Ma) than those of the other spots, indicative of xenocrystic grains. The remaining 12 spots have variable 207Pb/206Pb ages (1771–1613 Ma) owing to lead loss, and they constitute a welldefined discordia line with an upper-intercept age of 1750 ± 36 Ma (MSWD = 3.1). Of these spots, six analyses define a single group and plot close to the upper intercept, yielding a weighted mean 207Pb/206Pb age of 1754 ± 14 Ma (MSWD = 2.7). Hence, we interpret 1754 ± 14 Ma as the best estimate of the crystallization age of the gabbro. 4.1.2. Granite porphyry Zircon grains separated from the granite porphyry sample (YM16–13) are euhedral to subhedral, have aspect ratios of 1:1 to 2:1, and exhibit light to dark luminescence in CL images with no oscillatory zoning (Fig. 4a). Sixteen zircon grains were analyzed and yielded Th/U ratios of 0.73–1.27 (Table S1). Four of these 16 analyses (spots 03, 05, 13, and 14) gave old 207Pb/206Pb ages of 2028 to 1801 Ma, indicating inheritance. The other 12 analyses yield a concordia age of 1742.9 ± 3.7 Ma (MSWD = 0.73), within error of the weighted mean 207 Pb/206Pb age of 1740 ± 11 Ma (MSWD = 1.6). We interpret the former age as the crystallization age of the granite porphyry. The total rare-earth element (REE) concentrations of zircon grains from the granite porphyry (YM16–13) are variable and range from 461 to 2271 ppm (Table S2). All analyses display coherent chondritenormalized REE distribution patterns, showing depletion in light REEs

(LREEs) and enrichment in heavy REEs (HREEs) as well as positive Ce and negative Eu anomalies (Ce/Ce* = 1.19–42.88, Eu/Eu* = 0.16–0.43) (Fig. 4d), suggesting a magmatic origin. 4.2. Whole-rock geochemistry The gabbro samples have low SiO2 (44.1–50.0 wt%) contents, low to moderate MgO (3.61–6.23 wt%) contents, and Mg# values of 33.1–50.7. These gabbros show relatively low Al2O3 (10.5–13.5 wt%), high TiO2 (1.94–3.73 wt%), and variable Fe2O3T (13.2–18.9 wt%) and CaO (5.57–10.2 wt%) contents (Tables 1 and S3; Fig. 5). These rocks also have high total alkali contents (3.44 to 6.51 wt%) with K2O/Na2O ratios of 0.07 to 0.23, indicating sodic alteration (Table S3). Given the high and variable loss-on-ignition (LOI) values (0.87–3.84 wt%) and mineral alteration features of these rocks, Zr/TiO2–Nb/Y and FeOT/MgO–SiO2 diagrams using relatively immobile elements were preferred for classification purposes (Miyashiro, 1974; Winchester and Floyd, 1977). The low Nb/Y (b0.75) and high FeOT/MgO (N2) ratios of the gabbros indicate these rocks are tholeiitic basalt (Fig. 6a, b). The gabbro samples are characterized by moderate Cr (105–271 ppm) and Ni (18.3–88.7 ppm) contents, and variable REE concentrations (ƩREE = 45–211 ppm) (Table 1). In the chondrite-normalized REE diagram, the gabbros exhibit slightly LREE-enriched and flat HREE patterns with (La/ Yb)N and (Gd/Yb)N ratios of 1.1–6.2 and 1.11–2.08, respectively (Fig. 7a). The samples do not show significant Eu anomalies (Eu/Eu* = 0.85–1.24). In a primitive-mantle-normalized trace-element diagram, the rocks show enrichment in Rb, Th, U, and Ti (Ti/Ti* = 1.21–2.55), and depletion in Sr (Fig. 7b). Furthermore, the rocks show slight negative to obvious positive Nb, Ta, Zr, and Hf anomalies (Nb/Nb* = 0.74–2.57, Hf/ Hf* = 0.8–1.9) (Table S3; Fig. 7b). The granite porphyry samples have variable SiO2 (66.0–75.4 wt%) and Fe2O3T (2.5–11.7 wt%) contents, and uniform Al2O3 (10.6–12.6 wt%), TiO2 (0.17–0.61 wt%), MgO (0.15–0.38 wt%), and CaO (0.09–0.15 wt%) contents (Tables 1 and S3; Fig. 5). These porphyries have moderate to high total alkali concentrations (5.77–7.07 wt%) with K2O/Na2O ratios of 0.01–0.02, implying strong sodic alteration. The samples plot mostly in the rhyolite + dacite field in a Zr/TiO2 versus Nb/Y classification diagram (Winchester and Floyd, 1977) (Fig. 6a). These rocks are characterized by variable REE contents (ƩREE = 50–457 ppm) (Table 1). In a chondritenormalized REE diagram, four of seven samples exhibit significant LREEdepleted patterns with (La/Yb)N ratios of 0.17–0.91 (Fig. 7c), possibly resulting from the fractionation of a phosphorous-bearing mineral (monazite or apatite) during magma evolution, as suggested by their marked negative P anomalies and the negative correlations between SiO2 and P2O5 (Figs. 5f and 7d). The remaining samples display LREEenriched patterns with (La/Yb)N ranging from 3.24 to 5.48 (Fig. 7c). In addition, all samples have moderate to pronounced negative Eu anomalies (Eu/Eu* = 0.56–0.84). The primitive-mantle-normalized trace-element patterns of the granite porphyries show enrichment of Th, U, Zr, and Hf relative to neighboring elements. Sr, P, Ti, and the Nb\\Ta group (Nb/Nb* = 0.35–0.86), except for three samples (Nb/Nb* = 1.38–2.13), show marked negative anomalies (Fig. 7d; Table S3). 4.3. Whole-rock Sr\\Nd isotopes The gabbro samples have measured and initial 87Sr/86Sr ratios of 0.707802–0.726620 and 0.705446–0.698450, respectively. These mafic rocks also have measured 143Nd/144Nd ratios of 0.512285–0.512320, corresponding to εNd(t) values of +2.4 to +5.3 and depleted mantle model ages (TDM1) of 1857–2282 Ma (Table S4). The measured 87Sr/86Sr and 143Nd/144Nd ratios of the granite porphyry samples range from 0.725286 to 0.727230 and from 0.511694 to 0.513137, respectively. Apart from one sample (YM16–16) with the highest 87Rb/86Sr ratio (1.46687) and an unrealistic initial 87Sr/86Sr value of 0.68982, probably resulting from alteration, the remaining samples with low 87Rb/86Sr ratios (0.57697–0.81119) show a restricted

K. Liu et al. / Lithos 332–333 (2019) 23–38

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5.0

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0.1 La

Ce

Pr

Nd

Sm Eu

Gd Tb

Dy Ho

Er

Tm Yb

Lu

235

Pb/ U

Fig. 4. (a–c) U\ \Pb Concordia plots of zircon crystals from gabbro and granite porphyry; (d) Chondrite-normalized REE patterns (normalization values from Sun and Mcdonough, 1989) for zircon grains from granite porphyry sample (YM16–13). Morphology and internal structure of typical zircons are also shown.

range of initial 87Sr/86Sr values (0.70688–0.71081). Two samples (YM16–13 and YM16–16) have much higher 147Sm/144Nd ratios (0.33431 and 0.38288, respectively) and much lower εNd(t) values (−32.1 and − 22.5) than those of the other samples (YM16–15 and YM16–19; 147Sm/144Nd = 0.10226 and 0.13342, respectively; εNd(t) = 0.3–2.7) (Table S4). Furthermore, samples YM16–13 and YM16–16 have anomalous Nd model ages (Table S4), indicating the Sm\\Nd isotope system was disturbed by post-magmatic alteration. 4.4. Zircon Hf isotope compositions A total of 21 zircon grains from samples YM16–01 (gabbro) and YM16–13 (granite porphyry) were analyzed for Hf isotopes. The measured 176Lu/177Hf and 176Hf/177Hf ratios of 9 zircon grains from the gabbro range from 0.000587 to 0.000939 and from 0.281481 to 0.281644, respectively. Values of εHf (t = 1750 Ma) for zircon grains from the gabbro vary from −1.8 to −7.8, and TDM1 ages range from 2472 to 2239 Ma (Table S5; Fig. 8d). In contrast, 12 zircon grains from the granite porphyry sample have variable 176Lu/177Hf ratios (0.000514–0.001973) and relatively restricted 176Hf/177Hf ratios (0.281704–0.271826), corresponding to εHf (t = 1745 Ma) values of 0 to +3.7, TDM1 ages of 2169 to 2027 Ma, and TDM2 ages of 2421 to 2189 Ma (Table S5; Fig. 8d). 5. Discussion 5.1. Alteration and element mobility Our petrographic investigation identified variable levels of alteration in gabbro and granite porphyry, further supported by their high LOI

values (0.87–3.84 and 0.74–1.59 wt%, respectively). It is therefore necessary to evaluate the effects of alteration on element mobility prior to using element contents and patterns in petrogenetic interpretations. Zirconium has been widely used as an alteration-independent index because of its immobility during low-grade metamorphism and hydrothermal alteration (Polat and Hofmann, 2003). Elements such as SiO2, TiO2, MgO, Yb, Y, Nb, Th, and U display linear correlations with Zr, and the large-ion lithophile elements (LILEs) (e.g., Rb) are broadly correlated with Zr in all the samples (Fig. 9), indicating a general immobility of these elements in the studied rocks. The gabbro samples show a linear correlation between LREEs (e.g., La and Sm) and Zr, whereas the granite porphyry samples display scatter in the Zr–LREE plot (Fig. 9). This is attributed to possible hydrothermal alteration of the granite porphyry that also affected the LREEs (Li et al., 2018). Given the above, only the relatively immobile major oxides (e.g., SiO2 and TiO2) and trace elements (including the REEs and high-field-strength elements (HFSEs)) were considered for petrogenetic interpretations and discussion regarding the gabbro. For the granite porphyry, only the major oxides (e.g., SiO2 and TiO2) and trace elements (including the HREEs and HFSEs) were used. 5.2. Petrogenesis of the ~1.75 Ga gabbro 5.2.1. Crustal contamination and crystal fractionation Crustal contamination and crystal fractionation are crucial processes in the petrogenetic evolution and chemical modification of mantlederived magmas during magma ascent and emplacement in the shallow crust. Crustal contamination would be expected to produce enrichment in Zr and Hf and depletion in Nb, Ta, and Ti, which would usually be

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Fig. 5. (a–f) Bivariate diagrams against SiO2, (g–h) Plots of Th/Zr vs Nb/Zr; Zr/Nb vs Lu/Hf for gabbro and granite porphyry. The data of ~1.76 Ga Haizi dolerite and ~1.73 Ga A-type granites are from Yang et al. (2015).

marked by significant negative Nb\\Ta and Ti, and positive Zr\\Hf anomalies in multi-element spider diagrams. In a primitive-mantlenormalized multi-element spiderplot, the studied gabbros define coherent distribution patterns without negative Nb\\Ta anomalies (Fig. 7b). Furthermore, most of the samples retain relatively constant Nb/La ratios and show a crystal fractionation trend in a Nb/La versus Mg# plot (Fig. 10a). However, the gabbro data show a broadly negative correlation in an Nd/Sm versus Zr/Nb diagram (Fig. 10b). All of these observations preclude substantial crustal contamination during magma evolution.

The gabbro samples have MgO, Cr, and Ni contents significantly lower than those of primary melts in equilibrium with mantle peridotite (MgO = 11.39–16.40 wt%, Cr = 300–500 ppm, Ni N 300 ppm; Frey et al., 1978), indicating the evolved nature of the magma prior to crystallization. The negative correlations of SiO2 with Fe2O3T, MgO, and CaO (Fig. 5b, d, and e), as well as the positive correlations with Cr, Ni, and Sc (Fig. 10c, d), suggest the dominance of crystal fractionation with olivine and clinopyroxene as the main crystallizing phases. The absence of any notable Eu anomaly in the chondrite-normalized REE patterns rules out the role of plagioclase as a significant fractionating phase.

K. Liu et al. / Lithos 332–333 (2019) 23–38 1

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a)

Alkali Rhyolite

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s it Ande

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0.1

1

10

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0 40

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45

50

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SiO 2 (wt.%)

Fig. 6. Rock classification diagrams for gabbro and granite porphyry. (a) Zr/TiO2 × 0.0001 vs Nb/Y diagram (after Winchester and Floyd, 1977); (b) SiO2 vs FeOT/MgO diagram (after Miyashiro, 1974).

5.2.2. Nature of the mantle source and melting conditions To minimize the potential effect of partial melting and crystal fractionation processes, only the incompatible element ratios, Sr\\Nd isotopes and zircon Hf isotopes, were considered for evaluating the nature of the mantle source of the gabbroic rocks. The gabbros display slight enrichment in Rb and Ba (LILEs) and in LREEs (Fig. 7a and b), similar to E-MORB. These features suggest a geochemically enriched mantle source. Possible reasons for the enrichment include (1) the involvement of lithospheric mantle that had been previously modified by subduction-derived materials (Zhao and Zhou, 2009), (2) a source derived directly from a mantle plume (Ernst, 2014), or (3) mixing (assimilation) with crustal materials during ascent of the magma (Kerr et al., 1995). In a Th/Yb versus Ta/Yb diagram, the rocks plot within the MORB– OIB array, ruling out the possibility of interaction with subductionrelated materials (Fig. 11a). This finding is also confirmed in an (Hf/Sm)N versus (Ta/La)N diagram (Fig. 11b). Furthermore, partial

melts of a subduction-modified lithospheric mantle would show an arc affinity, indicated by Nb and Ta depletion, which is inconsistent with the negligible negative Nb\\Ta anomalies observed for the gabbro (Fig. 7b). Asthenospheric mantle-derived mafic rocks have much lower La/Ta ratios than those derived from lithospheric mantle (Leat et al., 1988). All the gabbro samples plot within the field of asthenospheric mantle melts in an εNd(t) versus La/Ta diagram (Fig. 11c), indicating an asthenospheric mantle source. The low Th/Y ratios (0.04–0.09) of the gabbros are typical of asthenospheric-mantle-derived rocks (Th/Y b 0.14; Sun and McDonough, 1989)). In addition, the (La/Yb)N and (Dy/Yb)N ratios of the gabbros, which range from 1.12 to 6.22 and from 1.19 to 1.53, respectively, are much lower than those of plumederived rocks ((La/Yb)N = 2.01–695, (Dy/Yb)N = 1.14–1.9) (Harpp and White, 2001). The measured 87Sr/86Sr and 143Nd/144Nd ratios of the gabbros are also different from those of typical modern-day EM-1, EM-2, and HIMU mantle end-members, which are considered as the potential sources of OIB (Fig. 8a).

Fig. 7. (a,c) Chondrite-normalized REE distribution patterns, (b,d) Primitive-mantle normalized multi-element spider diagrams for gabbro and granite porphyry. Normalization data are from Sun and Mcdonough (1989).

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0.5135

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O 0.5130

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Fig. 8. (a)143Nd/144Nd vs 87Sr/86Sr diagram showing fields for MORB, EM-I, EM-II and HIMU after GEOROC database: http://georoc.mpch-mainz.gwdg.de/georoc/; (b) εNd(t) vs Age plot; (c) 176Hf/177Hf vs 176Yb/177Hf diagram and (d) εHf (t) vs Age plot.

The gabbro samples display depleted Nd isotopic compositions (εNd(t) = +2.4 to +5.3) and ‘crust-like’ zircon Hf isotopic signatures (εHf(t) = −1.8 to −7.8) (Tables S4 and S5; Fig. 8b and d). The decoupling between whole-rock Nd and zircon Hf isotopic characteristics implies that the gabbroic magma underwent a complex petrogenetic process before emplacement/crystallization. Magma mixing between isotopically depleted mafic and enriched felsic magmas could be an option in the open-system process(es), based on the assumption that mantle-derived mafic magmas are isotopically more depleted than crust-derived felsic magmas (Jiang et al., 2018). The ‘crust-like’ zircon Hf isotopic signatures can be reconciled with minor crustal input into the mafic magma. This interpretation is further supported by the presence of several xenocrystic zircons in gabbro sample YM16–01. After zircon crystallization, the hybrid magma was replenished by voluminous isotopically depleted mantle-derived mafic magma, and the recharged and resident magmas were thoroughly mixed, resulting in the depleted whole-rock Nd isotopic compositions. Considering all the foregoing geochemical and isotopic features, we surmise that the parental magma for these gabbros was derived from moderately Nd-isotopedepleted ambient asthenospheric mantle and underwent a complex petrogenetic process involving the mixing (or assimilation) of minor crustal material and crystal differentiation before emplacement. REEs are useful indicators of mantle melting conditions (e.g., source lithologies and degree of melting) as they behave differently in typical mantle aluminous phases (e.g., plagioclase, spinel, and garnet) (Aldanmaz et al., 2000). Mafic rocks have characteristically flat HREE patterns (Fig. 7a), indicating an absence of refractory garnet. The nonmodal batch-melting model proposed by Shaw (1970) was used to quantitatively estimate the melting conditions of gabbro. In an Sm/Yb versus Sm diagram (Fig. 11d), the gabbros plot between the fields of spinel lherzolite and garnet-bearing spinel lherzolite, indicating that the parental magma was derived from relatively shallow depths

(b85 km). In addition, the mantle source is suggested to have undergone a variable but low degree of partial melting (1%–15%) (Fig. 11d). 5.3. Petrogenesis of the granite porphyry 5.3.1. Chemical characteristics and origin of the granite porphyry The granite porphyry samples have high (10,000 × Ga)/Al (2.48–4.77) and FeOT/MgO (11.1–53.7) ratios, and high Zr (682–915 ppm), Nb (30.0–51.3 ppm), and Y (35.6–77.5 ppm) concentrations, all characteristic of A-type granites (Frost and Frost, 2011; Whalen et al., 1987) (Fig. 12a, b). A-type granitic melts can be generated by several mechanisms, including the extreme differentiation of mantle-derived melts (with or without crustal assimilation) (Namur et al., 2011), the re-melting of granulitic crustal residue (either metasedimentary or metaigneous) (Huang et al., 2018), the hybridization melting of upper-crustal components (DTTG) (Pankhurst et al., 2011), and the mixing of two end-member materials (Kemp et al., 2005). In general, granitic rocks derived from the crystal fractionation of mantle-derived mafic magmas or from AFC processes should present a trend of continuously evolving composition from mafic through intermediate to felsic end-members (Wang et al., 2010). The studied granite porphyry shows a wide chemical composition gap compared with the gabbro (Figs. 5a–f and 9), and there is an absence of intermediate counterparts in the region. In addition, the granite porphyry samples have much higher Th/Zr (0.018–0.035) and Zr/Nb (17.8–23.7) ratios, and lower Lu/Hf (0.050–0.063) ratios than those of the gabbros (Th/Zr = 0.008–0.015, Zr/Nb = 8.0–12.2, and Lu/Hf = 0.075–0.108) (Fig. 5g and h), and these two rock types also show contrasting wholerock Nd and zircon Hf isotope characteristics (Tables S4 and S5; Fig. 8b and d). These features argue for a magma source of the granite porphyry distinct from that of the gabbro. There is no field evidence for magma mixing, such as the occurrence of mafic enclaves. In addition, the granite porphyry has Y/Nb ratios in the range of 1.2 to 1.9, indicating an A2

K. Liu et al. / Lithos 332–333 (2019) 23–38

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Zr (ppm)

Fig. 9. Zr vs selected major and trace element plots for studied rocks to evaluate the alteration effect on elemental mobility.

anorogenic granite affinity in the Nb–Y–Ga diagram of Eby (1992) (Fig. 12c), suggesting a crustal source. The positive εNd(t) (+0.3 to +2.7) and εHf(t) (0 to +3.7) values might reflect the partial melting of previously underplated basaltic rocks, given that the possibility of magma mixing has been ruled out. In addition, the granite porphyry samples have high Zr (682–915 ppm) and Nb (30.0–51.3 ppm) concentrations (Table 1), consistent with a partial melting origin. Most major oxides, such as Al2O3, TiO2, Fe2O3T, and P2O5, exhibit negative linear correlations with SiO2 (Fig. 5a–f), underlining the role of the fractionation of plagioclase, Fe\\Ti oxides, and phosphorous-bearing minerals (apatite or monazite). Considering all of the above observations, we argue that the original magma for the granite porphyry was most likely derived from the partial melting of Paleoproterozoic mafic rocks underplated in the lower crust that experienced plagioclase, Fe\\Ti oxide, and phosphorous mineral fractionation. However, the possibility of minor assimilation of the middle–upper crust during magma ascent is indicated by the depletion of Nb, Ta, and Ti (Fig. 7d).

5.3.2. Conditions of melting The granite porphyry samples have high TZr values, ranging from 901 to 929 °C with a mean of ~918 °C (Table S3), indicating derivation from a high-temperature magma. The presence of inherited zircon grains shows that the granitic magma was saturated in zircon, thereby providing maximum estimates of magma temperature calculated from whole-rock compositions (Miller et al., 2003). In addition, the magma temperatures calculated using the modified Ti-in-zircon thermometer have a wide range from 666 to 923 °C with a mean of ~818 °C (Ferry and Watson, 2007) (Table S2; Fig. 12d). Thus, the parental magma of the granite porphyry was most likely formed within a temperature range of 818–918 °C. The element Ce in zircon is sensitive to the oxidation state of magma and can therefore be used to estimate the oxygen fugacity (fo2) of magma. The oxygen barometer designed by Smythe and Brenan (2016) was used to quantitatively calculate the fo2 of the granite porphyry. For the calculations, the H2O content was set at 0, 1, and 3 wt%. The calculations for the granite porphyry rocks yielded high log(fo2)

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5

6

a)

b)

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Haizi dolerite (1.76 Ga)

0.1 1

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0.1 1

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Cr (ppm)

Fig. 10. Discrimination diagrams for gabbro to evaluate crustal contamination and fractional crystallization. (a) Nb/La vs Mg#; (b) Nd/Sm vs Zr/Nb; (c,d) plots of Ni vs Cr, Sc vs Cr for gabbros; sample YM1601 is considered to represent the parental magma composition, the lines with numbers are fractional crystallization (FC) modeling. Partition coefficients of individual trace elements are from GERM Partition Coefficients (kd) Database (https://earthref.org/). The data of ~1.76 Ga Haizi dolerite are from (Yang et al., 2015).

values, ranging from −14.7 to −10.2 (1 wt% water, clean zircon with La ≤ 0.1 ppm), corresponding to ΔFMQ values of 2.8 to 3.3 (Table S2), implying a high fo2 (Fig. 12d). However, the granite porphyry samples have high FeOT/(FeOT + MgO) values, suggesting a reduced A-type granite affinity (Fig. 12f), which is at odds with the results of the Ce-in-zircon oxybarometer (Fig. 12d). A clear power-law relationship between La/Sm and Ce/Ce* is evident (Fig. 12e), indicating a strong dependence of the Ce/Ce* anomalies on the variation in La and Pr abundances in zircon (Miles et al., 2014). The Ce-in-zircon oxybarometer of Smythe and Brenan (2016) is independent of REE concentrations, and therefore the results yielded by the oxybarometer are not affected by La or Pr contents in zircon (Zou et al., 2019). The spurious fo2 results can be explained in terms of the whole-rock composition of sample YM16–13, which is considered the geochemical equivalent of the melt in equilibrium with the zircon grains. As mentioned above, the parental magma of granite porphyry underwent minor middle- to upper-crust assimilation and crystal fractionation during magma ascent as well as post-crystallization hydrothermal alteration. All of these processes can potentially modify the geochemical composition of parental magma. Thus, the Ce-in-zircon oxybarometer, using the whole-rock composition of YM16–13, is unable to reliably reflect the oxidation state of the host magma from which the zircon grains crystallized. Therefore, we suggest that the granite porphyry crystallized from a hightemperature magma under relatively reducing conditions at shallow crustal levels.

post-orogenic extension environments (Brewer et al., 2004; Moraes et al., 2003; Wang et al., 2012). The studied gabbro is classified as tholeiite with high HFSE contents (e.g., Zr and Ti) and E-MORB-like geochemical features, typical of within-plate basalts (Pearce, 1982). The gabbro samples also show negligible Nb\\Ta anomalies and moderate positive εNd(t) values. These various lines of evidence argue against a back-arc or post-orogenic extension environment for the studied gabbros, as the mantle source beneath such tectonic settings would have been modified by subduction-related materials, resulting in an arc-type geochemical affinity (e.g., negative Nb and Ta anomalies and calc-alkaline composition) (Wang et al., 2012). Moreover, voluminous 1.73–1.65 Ga mafic magmatism in the southwestern Yangtze Block has been attributed to intracontinental rifting (Chen et al., 2013; Greentree and Li, 2008; Lu et al., 2019; Zhao et al., 2010). Considering all the above-mentioned arguments, we infer an intracontinental extensional setting for the studied mafic rocks, in which the upwelling of basaltic magma from ambient asthenospheric mantle and the subsequent partial melting of mafic crustal materials generated the parental magma of the Paleoproterozoic granite porphyry (Fig. 13). If this interpretation is correct, the ~1.75 Ga bimodal igneous rocks reported in this study represent the earliest record of an intracontinental rift event in the southwestern Yangtze Block.

5.4. Tectonic setting of the ~1.75 Ga bimodal magmatic rocks

The Yangtze Block, with records of ~2.1–2.0 Ga subduction, ~2.0–1.83 Ga upper-amphibolite- to granulite-facies metamorphism, and ~1.95–1.93 Ga syn-collision I-type granite emplacement in its northern part, has been widely accepted as a component of the Paleo-

Bimodal magmatism is commonly associated with extensional tectonic regimes, including continental rifting, back-arc extension, and

6. Implications for the paleogeographic reconstruction of the Yangtze block in the Columbia supercontinent

K. Liu et al. / Lithos 332–333 (2019) 23–38

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a) IAB

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Fig. 11. Inter-element ratio plots for gabbro samples to illustrate the nature of their mantle source and melting conditions. (a)Th/Yb vs Ta/Yb diagram (after Pearce, 1982); (b) (Hf/Sm)N vs (Ta/La)N diagram (after La Flèche et al., (1998)); (c) εNd(t) vs La/Ta diagram (after Lawton and Mcmillan, 1999); (d) Sm/Yb vs Sm diagram, The melt curves are calculated based on the non-modal batch melting model: CL/C0 = 1/[D + F*(1-P)] (Shaw, 1970), where CL is the concentration of a trace element in the melt, C0 is its concentration in the unmelted source, D is the bulk partition coefficient and F is the weight fraction of melt produced. The starting component is primitive mantle and depleted mantle (Sun and Mcdonough, 1989; Workman and Hart, 2005). Curves for spinel-Iherzolite (with modal Ol0.578 + OPx0.27+ Cpx0.119 + Sp0.033 and melt model Ol0.06 + OPx0.28 + Cpx0.67 + Sp0.11) and garnet lherzolite (with modal Ol0.598 + OPx0.211 + Cpx0.076 + Grt0.11 and melt modal Ol0.03+ OPx0.16+ Cpx0.88+ Sp0.09) are after Mckenzie and O'Nions (1991) and Aldanmaz et al. (2000). Partition coefficients of individual trace element are from GERM Partition Coefficients (kd) Database (https://earthref.org/).

to Mesoproterozoic Columbia supercontinent (Wang et al., 2015; Wu et al., 2012; Yin et al., 2013). The presence of ~1.85–1.79 Ga postorogenic A-type granites, together with coeval mafic dikes in the northern Yangtze Block, is generally considered evidence for fragmentation of the Yangtze Block from the Columbia supercontinent (Deng et al., 2017). Clearly, these post-orogenic A-type granites and coeval mafic dikes not only pre-dated the final breakup of the Columbia core at 1.5–1.35 Ga (Pisarevsky et al., 2014) but also the early fragmentation of the supercontinent at ~1.7 Ga, thereby raising doubts regarding the genetic link between extensional magmatic events and supercontinent breakup. The ~1.75 Ga bimodal magmatism in the southwestern Yangtze Block, together with the rift-related sedimentary basin that was formed at ~1.75 Ga (Wang and Zhou, 2014), was synchronous with the early failed breakup of the Columbia supercontinent. Therefore, it is argued that the ~1.75 Ga bimodal magmatic rocks and coeval sedimentary sequences might represent the first event within the Yangtze Block during the early breakup of the supercontinent and can thus be used to reconstruct the location of the Yangtze Block in the Columbia supercontinent. Paleomagnetism is the most quantitative method for supercontinent reconstructions (Zhang et al., 2012), but precise paleomagnetic data from the Yangtze Block are limited. Therefore, we used geological records, namely the timing and characteristics of magmatism, metamorphism, deformation, and sedimentation, as well as the links among these events (Wang et al., 2016), to configure and correlate cratonic

blocks and to explore the possible paleoposition of the Yangtze Block in the Columbia supercontinent. Previous studies have suggested the possible involvement of the Yangtze Block in the 2.50–2.28 Ga Arrowsmith orogenic events (Pehrsson et al., 2013), resulting in the Yangtze Block together with the Anbar Block accreting to the Rae Craton of Laurentia (Wang et al., 2016). Subsequently, these units experienced high-grade metamorphism during the 2.0–1.95 orogeny, corresponding to the assembly of the Columbia supercontinent (Hoffman, 2014; Yin et al., 2013). Furthermore, the peripheral blocks were fragmented and broke away from the Columbia supercontinent, accompanied by the development of various intracontinental extensional sedimentary basins and mafic magmatism, such as the Leichhardt– Calvert–Isa basin in northern Australia, the Wernecke–Horny Bay– Athabasca basin in northwestern Laurentia, the Mukun and Uchur groups in Siberia, and the Dongchuan, Dahongshan, and Hekou groups in the southwestern Yangtze Block. All of these basins and groups share similar sedimentary characteristics, including sedimentary facies, detrital zircon age distribution patterns, and whole-rock Nd isotopes (Cawood et al., 2018; Wang and Zhou, 2014). In addition, voluminous late Paleoproterozoic to early Mesoproterozoic mafic magmatic events have been reported in these cratonic blocks. The numerous events include (1) the ~1.75 Ga Timptaon–Algamaisky and Chaiskii dike swarms, 1.74–1.70 Ga Bilyakchan-Ulkan bimodal magmatism, and 1.67–1.65 Ga mafic dike intrusion in the southern Siberian Craton (Didenko et al., 2015; Gladkochub et al., 2010; Larin, 2014); (2) the 1.76–1.75 Ga Bulonga

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Fig. 12. (a–b) FeOT/MgO and Nb vs 10000*Ga/Al (Whalen et al., 1987) diagram; (c) Nb–Y–3*Ga diagram showing A-type signatures for granite porphyry (Eby, 1992); (d) logfO2 vs T (C°) diagram, showing oxygen fugacity range for granite porphyry in the southwestern Yangtze Block. Values of log fO2 were calculated using the Ce-in-zircon oxygen barometer (Smythe and Brenan, 2016); T (°C) was calculated based on Ti content in zircon (Ferry and Watson, 2007). Buffer curves are from Frost (1991), abbreviations used: MH = magnetite–hematite buffer; NNO = nickel–nickel oxide buffer; FMQ = fayalite–magnetite–quartz buffer; IW = iron–wustite buffer; QIF = quartz–iron–fayalite buffer; (e) La/Sm vs Ce/Ce* for zircons, Ce/Ce* = Ce/(La × Pr)1/2; (f) FeOT/(FeOT + MgO) vs Al2O3 diagram displaying the fields of reduced A-type granite, oxidized A-type granite and calc-alkaline granite (Dall'Agnol and Oliveira, 2007).

and Marraba bimodal magmatic events, ~1.73–1.70 Ga Fiery Creek and Peters Creek mafic magmatic events, and 1.69–1.65 Ga mafic magmatism in the northern Australian craton (Baker et al., 2010; Neumann et al., 2009); (3) the 1. 75 Ga bimodal Kivalliq Igneous Suite, ~1.71 Ga Bonnet Plume River intrusions, and 1.66 Ga bimodal magmatism in northwestern Laurentia (Bowring and Ross, 1985; Peterson et al., 2015; Thorkelson et al., 2001); and (4) the ~1.75 Ga Haizi bimodal magmatism together with ~1.73–1.65 Ga mafic intrusions/dikes in the southwestern Yangtze Block (Chen et al., 2013; Lu et al., 2019; Zhao et al., 2010). The foregoing mafic magmatic events within different cratonic blocks are typical products of within-plate geodynamic settings. In summary, these correlations suggest a possible spatial linkage of the Yangtze Block with northern Australia, northwestern Laurentia, and southern Siberia during the late Paleoproterozoic.

7. Conclusions (1) The studied gabbro and granite porphyry were emplaced at 1754 ± 14 Ma and 1743 ± 4 Ma, respectively, representing the earliest bimodal magmatism in the southwestern Yangtze Block. (2) The gabbro originated from an ambient asthenospheric mantle source that underwent variable low-degree partial melting (1%–15%). The magma evolved through a complex process involving minor crustal assimilation and crystal fractionation. The granite porphyry has an A-type granite affinity, was derived from the partial melting of Paleoproterozoic mafic rocks underplated in the lower crust, and experienced plagioclase, Ti–Fe-oxide, and apatite fractionation.

K. Liu et al. / Lithos 332–333 (2019) 23–38

Fig. 13. Cartoon showing the geodynamic setting for generation of the ~1.75 Ga bimodal magmatism in the southwestern Yangtze Block.

(3) The ~1.75 Ga bimodal magmatism reported in this study, together with the ~1.75–1.65 Ga mafic magmatism documented in northern Australia, northwestern Laurentia, and southern Siberia, suggest a late Paleoproterozoic spatial linkage of the Yangtze Block with these crustal units in the Columbia supercontinent. (4) The following are the supplementary data related to this article. Supplementary data to this article can be found online at https://doi. org/10.1016/j.lithos.2019.02.021. Acknowledgement This study was supported by the National Science Foundation of China (NSFC 41572170), “Thousand Youth Talents Plan” grant to Wei Wang, Fundamental Research Fundsforthe Central Universities, China University of Geosciences (Wuhan) (CUGCJ1709) and MOST Special Fund from the State Key Laboratory of Geological Process and Mineral Resources (MSFGMR 01-1). We would like to thank Zufan Lan and Liang Qiao for help in field investigation and sample collection, Dan Zhu for the LA-ICP-MS analysis, and Liang Li and Liangrong Tian for the whole-rock geochemical analyses. Prof. Pandit Manoj is thanked for his careful correction on English language. We are grateful to two anonymous reviewers and Editor Prof. Andrew Kerr for their thoughtful and constructive comments and suggestions that helped in improving the presentation and interpretations significantly. References Aldanmaz, E., Pearce, J.A., Thirlwall, M.F., Mitchell, J.G., 2000. Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey. Journal of Volcanology and Geothermal Research 102, 67–95. Andersen, T., 2002. Correction of common lead in U–Pb analyses that do not report 204 Pb. Chemical Geology 192, 59–79. Baker, M.J., Crawford, A.J., Withnall, I.W., 2010. Geochemical, Sm–Nd isotopic characteristics and petrogenesis of Paleoproterozoic mafic rocks from the Georgetown Inlier, North Queensland: implications for relationship with the Broken Hill and Mount Isa Eastern succession. Precambrian Research 177, 39–54. Bowring, S.A., Ross, G.M., 1985. Geochronology of the Narakay Volcanic complex: implications for the age of the Coppermine Homocline and Mackenzie igneous events. Canadian Journal of Earth Sciences 22, 774–781. Brewer, T.S., Åhäll, K.I., Menuge, J.F., Storey, C.D., Parrish, R.R., 2004. Mesoproterozoic bimodal volcanism in SW Norway, evidence for recurring pre-Sveconorwegian continental margin tectonism. Precambrian Research 134, 249–273. Cawood, P.A., Zhao, G., Yao, J., Wang, W., Xu, Y., Wang, Y., 2018. Reconstructing South China in Phanerozoic and Precambrian supercontinents. Earth-Science Reviews 186, 173–194.

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