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Volume 31 Issue 2
Apr.  2020
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Tong Li, Liang Liu, Xiao-Ying Liao, Yong-Sheng Gai, Tuo Ma, Chao Wang. Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China. Journal of Earth Science, 2020, 31(2): 237-248. doi: 10.1007/s12583-020-1298-6
Citation: Tong Li, Liang Liu, Xiao-Ying Liao, Yong-Sheng Gai, Tuo Ma, Chao Wang. Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China. Journal of Earth Science, 2020, 31(2): 237-248. doi: 10.1007/s12583-020-1298-6

Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China

doi: 10.1007/s12583-020-1298-6
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  • Mafic dyke swarm is widely developed in Proterozoic continental lithosphere. The Gangou diabase dyke from the Xixia area, intruded into the Douling complex of the South Qinling belt, yields zircon U-Pb age of 731 Ma. It has low SiO2 content (49.02 wt.%-49.37 wt.%) and Mg# (34.0-37.7) and shows characteristics of subalkaline tholeiite series. They show high ΣREE (155.5×10-6-184.7×10-6), weak negative Eu anomaly (δEu=0.88-0.93), slight depletion of Nb and Ta, and enrichment of LILE. Their major and trace element, Sr-Nd-Pb isotope and clinopyroxene compositions indicate that it originated from the partial melting of the asthenospheric mantle within-plate extension setting and was slightly contaminated by crust compositions. The similarities of formation age, petrogenesis, source characteristic and tectonic setting for Gangou diabase with mafic dykes in Wudang Block and mafic volcanic rocks in Yaolinghe Group indicate that the South Qinling belt underwent strong continental extension in Neoproterozoic during 796-685 Ma most likely correspondence to the breaking-up of the Rodinia supercontinent.
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Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China

doi: 10.1007/s12583-020-1298-6
  • Author Bio:

  • Corresponding author: Liang Liu

Abstract: Mafic dyke swarm is widely developed in Proterozoic continental lithosphere. The Gangou diabase dyke from the Xixia area, intruded into the Douling complex of the South Qinling belt, yields zircon U-Pb age of 731 Ma. It has low SiO2 content (49.02 wt.%-49.37 wt.%) and Mg# (34.0-37.7) and shows characteristics of subalkaline tholeiite series. They show high ΣREE (155.5×10-6-184.7×10-6), weak negative Eu anomaly (δEu=0.88-0.93), slight depletion of Nb and Ta, and enrichment of LILE. Their major and trace element, Sr-Nd-Pb isotope and clinopyroxene compositions indicate that it originated from the partial melting of the asthenospheric mantle within-plate extension setting and was slightly contaminated by crust compositions. The similarities of formation age, petrogenesis, source characteristic and tectonic setting for Gangou diabase with mafic dykes in Wudang Block and mafic volcanic rocks in Yaolinghe Group indicate that the South Qinling belt underwent strong continental extension in Neoproterozoic during 796-685 Ma most likely correspondence to the breaking-up of the Rodinia supercontinent.

Tong Li, Liang Liu, Xiao-Ying Liao, Yong-Sheng Gai, Tuo Ma, Chao Wang. Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China. Journal of Earth Science, 2020, 31(2): 237-248. doi: 10.1007/s12583-020-1298-6
Citation: Tong Li, Liang Liu, Xiao-Ying Liao, Yong-Sheng Gai, Tuo Ma, Chao Wang. Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China. Journal of Earth Science, 2020, 31(2): 237-248. doi: 10.1007/s12583-020-1298-6
  • Diabase samples were collected from the Douling complex in Gangou (33°12'27.2" N, 111°30'15.6" E), southeast of Xixia area (Fig. 2). The diabase dyke exposed with a width of 4–5 m which intruded into the plagioclase-amphibole gneiss of Douling Group (Fig. 3a). The rocks are grayish green with dense massive structure, fine grain and ophitic texture, which has been subjected to a certain degree of weathering alteration (Fig. 4). Felsic veinlets of 1–5 mm that intruded late can be seen on the outcrop and hand specimen (Fig. 3b). The diabase consists of plagioclase (~50%), clinopyroxene (~45%) and minor biotite (< 5%). The accessory minerals are mainly titanite, magnetite, zircon and apatite. Plagioclases are generally subhedral, and range from 0.2 to 0.5 mm with polycrystalline twins, which has been partly altered to sericite and epidote. Clinopyroxenes are subhedral crystals with grain size about 0.2 mm and commonly distributed between plagioclase septum. The margin of some clinopyroxenes are replaced by amphiboles (Fig. 4).

    Figure 2.  Simplified geological map and the sampling localities of the Douling complex in SQB (modified from Nie et al., 2016).

    Figure 3.  (a) Photograph showing the field occurrences of the Gangou diabase in Xixia area (the surrounding rocks are amphibole plagioclase gneiss of Douling complex); (b) photograph of the specimen that showing the felsic veinlets of the Gangou diabase in Xixia area.

    Figure 4.  Photomicrographs of the Gangou diabase in Xixia area. (a) The diabasic texture of the sample; (b) microstructure of euhedral clinopyroxene and strongly altered plagioclase. Cpx. Clinopyroxene; Pl. plagioclase.

  • The analytical methods used in this study include whole rock major and trace element analyses, Sr-Nd-Pb isotope analyses, major mineral elements analyses, zircon cathodeluminescence (CL) imaging and zircon U-Pb dating. The Sr-Nd-Pb isotope analyses were completed in Nanjing FocuMS Technology Co. Ltd. and other analyses were completed in the State Laboratory of Continental Dynamics, Northwest University, China.

  • Whole rock geochemical samples were coarsely crushed and then finely grounded to less than 200 mesh. Analysis of major and trace element compositions were performed using X-ray fluorescence (XRF) (Rigku RIX 2100) and inductively coupled plasma mass spectrometry (ICP-MS) (PE 6100 DRC), respectively. XRF method was used for major element analysis. Trace elements were analyzed by ICP-MS. The analytical precision and accuracy of major and trace elements are generally better than 5% and 10%, respectively. In addition, one sample from every five samples was randomly selected for analyzing twice to check the precision of the analyses.

  • The Sr-Nd-Pb isotope pretreatment and mass spectrometry were completed in Nanjing FocuMS Technology Co. Ltd. Sr, total REE and Pb were separated by cationic-strontium resin after high pressure digestion. Nd was separated from the total REE components by Ln resin. The content of Sr, Nd and Pb of the samples were determined on Agilent 7700x quadrupole ICP-MS, and the isotope ratio of the samples were determined on Nu Plasma II MC-ICP-MS. 86Sr/88Sr=0.119 4, 146Nd/144Nd=0.721 9 and 205Tl/203Tl=2.388 5 were used for the determination of isotope ratios of samples Sr, Nd and Pb, respectively, to calibrate mass fractionation. NIST SRM 987, JNdi-1 and NIST SRM 981 were used as external standards to correct instrument drift.

  • Mineral compositions were determined using a JXA-8230 electron microprobe. Quantitative analyses were performed using a specimen current of 1×10-8 A, an accelerating voltage of 15 kV, with a beam diameter of 1 μm. The analytical precision is generally better than 2%. Mineral standard samples provided by SPI company, different mineral samples are used to calibrate different elements, for example, quartz/jadeite-Si, jadeite/plagioclase-Al, jadeite/albite-Na, diopside-Ca, olivine-Mg, diopside-K, ilmenite-Fe, rhodonite-Mn, rutile-Ti.

  • Euhedral zircons without inclusions and cracks were separated under binocular microscope, and then fixed on epoxy to be polished to the two-thirds exposed. The internal texture of zircons were obtained using CL imaging. The zircon U-Pb dating was performed on a Agilent 7500a type laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Standard zircon 91500 was used for isotope ratio fractionation correction as the external standard, which was analyzed every five sample points. Element concentration was calculated by using NIST610 as external standard and 29Si as internal standard. U-Th-Pb data of zircons was calculated using GLITTER (ver4.0). The concordia plots and were calculated using ISOPLOT (ver3.0) (Ludwig, 2003). Compositions of the common Pb were corrected according to Andersen Tom's (2002) 3D coordinate method, as shown in the references (Yuan et al., 2008).

  • The whole rock major and trace element compositions of the Gangou diabase in Xixia area are shown in Table S1.

  • Samples have 49.02 wt.%–49.37 wt.% SiO2, 3.15 wt.%–3.56 wt.% TiO2, 11.87 wt.%–12.36 wt.% Al2O3, 2.52 wt.%–2.81 wt.% Na2O, 0.26 wt.%–1.02 wt.% K2O and 2.99 wt.%–3.56 wt.% total alkalis (Na2O+K2O), with the ratios of Na2O/K2O > 1. All samples exhibit a tholeiitic trend in the FAM diagram (Fig. 5a). Considering that Na and K are active elements that are susceptible to alteration, Zr/TiO2-Nb/Y diagram (Fig. 5b) is adopted for classification. All samples plot into the area of sub-alkaline basalt. Therefore, the Gangou diabase has the evolution trend of subalkaline tholeiite basalt series.

    Figure 5.  Lithological classification diagrams of the Gangou diabase in Xixia area. (a) FAM (Na2O+K2O-FeOT-MgO) diagram; (b) Nb/Y-Zr/TiO2 diagram (after Winchester and Floyd, 1977; Irvine and Baragar, 1971, respectively).

  • The total amount of rare earth element (ΣREE) of Gangou diabase in Xixia area is relatively high (155.5×10-6–184.7×10-6), with an average value of 171.0×10-6, which enriched in LREE relative to HREE (ΣLREE/ΣHREE=1.55–1.57) with a weak fractionation between LREE and HREE ((La/Yb)N=3.66–3.90). On the diagram of chondrite-normalized REE patterns (Fig. 6a), it shows a gentle right-dipping curve with less negative Eu anomaly (δEu=0.88–0.93), which reflect a slight fractional crystallization of plagioclase in the magmatic source. Above all, the Gangou diabase has a slightly enriched REE type with the characteristics of high REE a (a) Diagram of chondrite-normalized REE patterns, (b) MORB-normalized trace element spider diagram (chondrite normalization values after Boynton, 1984; MORB normalization values after Pearce and Norry, 1979; OIB, N-MORB, E-MORB values after Sun and McDonough, 1989; continental tholeiite (CT) values after Wood et al., 1979). mount, weak differentiation and less negative Eu anomaly.

    MORB-normalized trace element spider diagram (Fig. 6b) shows "humped" pattern. The first half of the curve shows enrichment of incompatible elements, while the second half is relatively flat. There is obvious negative Sr anomaly, weak depleted of Nb and Ta, and the differentiation degree of HFSE is also weak. Sr and Eu are preferred to be selectively concentrated in plagioclase, which further indicates the Gangou diabase experienced the fractional crystallization of plagioclase. Consequently, they are geochemically characterized by the transition between ocean island basalt and continental tholeiite. In addition, the Th/Ta (2.02–2.44) and Ta/Hf ratio (0.185–0.200) are also consistent with the continental within-plate setting (Th/Ta > 1.6, Ta/Hf < 0.1), and different from the oceanic witinin-plate setting (Th/Ta < 1.6, Ta/Hf > 0.1) (Wang et al., 2001). In general, the trace element geochemical characteristics of the samples are more similar to those of continental tholeiite.

  • Compositions of clinopyroxene from the Gangou diabase in Xixia area were analyzed by electron probe with the results shown in Table S2. The content of MgO, CaO, FeO, Al2O3, TiO2 in the 16 clinopyroxenes tested are 12.33 wt.%–15.20 wt.%, 15.18 wt.%–18.91 wt.%, 11.46 wt.%–15.16 wt.%, 1.91 wt.%–3.56 wt.%, 0.78 wt.%–1.42 wt.%, respectively.

  • Whole-rock Sr-Nd isotopic data for the Gangou diabase are presented in Table S3. The Pb isotopic data are presented in Table S4. The Nd isotope ratio is relatively constant (εNd(t)=2.4–3.5), nevertheless, the ratio of Sr isotope changes greatly ((87Sr/86Sr)i=0.703 5–0.706 5). It can be seen from Table S4 that the Ganou diabase has low Pb isotopic composition with the (206Pb/204Pb)i values ranging from 17.99 to 18.12, (207Pb/204Pb)i values from 15.52 to 15.54 and (208Pb/204Pb)i values from 38.34 to 38.64.

  • The typical zircon CL images of the Gangou diabase presented in Fig. 7. Most of them are subhedral-euhedral columnar, with a particle size of 60–120 μm and length-width ratio from 1 : 1–2 : 1. The images show wide and uniform magmatic oscillatory zoning, which consistent with the characteristics of zircons in mafic rocks (Koschek, 1993), while a few zircons have obvious magmatic oscillatory zoning. Moreover, zircons with residual core can be also found in this study.

    Figure 7.  Representative CL images of zircons from the Gangou diabase in Xixia area.

    Results for U-Pb single zircon dating of the 24 analyses are presented in Table S5 and Fig. 8. 207Pb/206Pb age was used for older zircons (> 1 000 Ma), while the younger zircons (< 1 000 Ma) used 206Pb/238U age. The results show that the ages of most zircons range from 725–736 Ma with a 206Pb/238U weighted average age of 731±12 Ma (n=9, MSWD=0.032) (Fig. 8), representing the formation age of Gangou diabase. Furthermore, two zircons whose ages range from 105 to 110 Ma are speculated to be the intrusive age of felsic veins in Gangou diabase, which is consistent with the regional research results (Xue, 2018; Hu et al., 2012). Zircons with residual core are of relatively scattered ages (837 to 1 911 Ma, Fig. 8) and speculated that were captured.

    Figure 8.  Concordia plots of LA-ICP-MS zircon U-Pb data and the weighted average ages of the Gangou diabase in Xixia area.

  • Partial melting and fractional crystallization are two main mechanisms of magmatism and evolution. The concentration ratio of hypermagmatophilic elements (such as Ta, Th, La, Ce and so on) and magmophilic elements (such as HREE, Zr, Hf etc.) to hypermagmatophilic elements can distinguish fractional crystallization from partial melting. In the La-La/Sm and La-La/Zr discriminant diagrams (Fig. 9), the horizontal linear distribution of samples indicates fractional crystallization, and the oblique distribution of samples indicates partial melting. All the samples of Gangou diabase were distributed horizontally, indicating that the influence of fractional crystallization. The Gangou diabase have low SiO2 content (49.02 wt.%–49.37 wt.%), Mg#=34.0–37.7 and Ni=31.7×10-6–44.1×10-6. All of them are lower than the value of primary magma (Mg# > 65, Ni > 235×10-6) (Deng, 2004; Hess, 1992; Ringwood, 1975), also implying that the Gangou diabase underwent fractional fractionation from primary magma with mantle source.

    Figure 9.  (a) Diagram of La vs. La/Sm and (b) diagram of La vs. La/Zr from the Gangou diabase in Xixia area.

    Contamination and fractional crystallization usually are very common within the continental setting during the invasion of mantle-derived magma (Yan et al., 2019; Spera and Bohrson, 2004; Thorpe et al., 1984). Rocks contaminated by crustal materials or subduction fluids are characterized by significant depleted of Nb, Ta and Ti (Xiong et al., 2020; Ernst, 2014; Rudnick and Gao, 2003; Frey et al., 2002). In the spider diagram of trace elements (Fig. 6b), the samples show weak negative Nb and Ta anomalies, implying that they were weakly contaminated by crustal materials. Nb/La ratio and (Th/Nb)PM ratio can indicate whether crustal material was added during magma evolution (Xia, 2014; Kieffer et al., 2004; Saunders et al., 1992). Very high (Th/Nb)PM ratios (> > 1) and low Nb/La (< 1) are generally considered reliable trace element indicators for crustal contamination. As shown in Fig. 10a, the Gangou diabase from the Xixia area are characterized by Nb/La=0.79–0.87 and (Th/Nb)N=1.10–1.28, reflecting that they are subject to very weak crustal contamination. The Gangou diabase from Xixia area has similar trace element characteristics with mafic dykes in the Wudang Block (7 samples, according to Ling et al., 2002a; Zhang et al., 1999a) and mafic volcanic rocks of the Yaolinghe Group (6 samples, according to Ling et al., 2002a). (Th/Nb)PM-(La/Nb)PM diagram (Fig. 10b) can distinguish whether the contaminated material comes from the upper crust or the lower crust (Frey et al., 2002; Fitton et al., 1998a, b). The upper crust is rich in La and Th elements, while the lower crust is relatively depleted in Th (Bader et al., 2013). In Fig. 10b, the Gangou diabase fall between the oceanic basalt and the lower crust, which indicates that may be weakly contaminated by lower crust materials during the formation process.

    Figure 10.  (a) Diagram of (Th/Nb)N vs. Nb/La; (b) diagram of (La/Nb)PM vs. (Th/Nb)PM (data of primary mantle (PM) are from Sun and Mcdonough (1989); data of UC-upper crust, MC-middle crust, LC-lower crust are from Rudnick and Gao (2003); data of oceanic basalts are from Frey et al. (2002); data of mafic dykes in Wudang Block are from Ling et al. (2002a) and Zhang et al. (1999a); data of mafic volcanic rocks in Yaolinghe Group are from Ling et al. (2002a).

    The Gangou diabase has variable initial 87Sr/86Sr ratios (0.703 5–0.706 5) and positive εNd values (2.4–3.5). In Fig. 11, most of samples fall in or near OIB area within the continental lithospheric mantle. In addition, εNd values showed a tendency of migration to the continental lithospheric mantle with the increase of (87Sr/86Sr)i (Fig. 11). The Sr-Nd characteristics of Gangou diabase are similar to that of mafic dykes in the Wudang Block and mafic volcanic rocks of the Yaolinghe Group. Therefore, the primary magma of Gangou diabase should come from an asthenospheric mantle source and influenced by lithospheric composition. The Gangou diabase samples have lower Pb isotopic compositions, and relatively concentrated in the left of the zero geochron line (Fig. 12a) that fall in the area between the DUPAL anomalous oceanic island basalt and the lower crustal trend above the northern hemisphere reference line (NHRL) (Fig. 12b). The ∆208Pb/204Pb values ranged from 96.3 to 119.5, and ∆207Pb/204Pb values varied from 7.13 to 10.28 (Table S3) that similar to the mafic dykes in the Wudang Block and roughly equivalent to the deviation of southern DUPAL anomaly province (Hart, 1984), suggesting that their magma source is similar to the southern DUPAL anomaly ocean island basalt source. Moreover, the correlation diagram of ∆208Pb/204Pb-∆207Pb/204Pb (Fig. 12c) shows that these samples are mainly distributed along the trend of DMM and EMI, indicating an anomaly mantle source that originated from depleted asthenospheric mantle mixed with EMI, which is consistent with the magma source region reflected by Sr-Nd isotope characteristics.

    Figure 11.  Variation of (87Sr/86Sr)i vs. εNd(t) diagram of the Gangou diabase in Xixia area (after Wilson, 1989; the data of mafic dykes in Wudang Block are from Zhang et al., 1999b, and the data of mafic volcanic rocks in Yaolinghe Group are from Zhang, 2002, Huang and Wu, 1990).

    Figure 12.  Variation diagrams of Pb isotope of the Gangou diabase in Xixia area (the legend is the same as Fig. 11). (a) Correlation diagram of (206Pb/204Pb)i-(207Pb/204Pb)i; (b) correlation diagram of (206Pb/204Pb)i-(208Pb/204Pb)i; (c) diagram of ∆208Pb/204Pb-∆207Pb/204Pb (after Hart, 1988).

    In conclusion, the Gangou diabase in Xixia area originated from partial melting of asthenospheric spinel mantle peridotite with slightly contaminated by the lower crust.

    Since the fractional crystallization has little effect on Tb/Yb, it can be used to determine the depth of mantle magma source, i.e., garnet phase (TbN/YbN > 1.8) or spinel phase (TbN/YbN < 1.8) (Wang et al., 2002). The weakly HREE fractionation, no negative anomaly of Y and Yb, and the lower ratio of (La/Yb)N (3.66–3.90) and TbN/YbN (1.50–1.60) suggests that primary magma of the Gangou samples were originated from spinel mantle peridotite. In La/Sm-Sm/Yb (Wilson, 1989) and Nb/Yb-Dy/Yb (Workman and Hart, 2005) diagrams (Figs. 13a, 13b), the Gangou diabase also originated from the partial melting of the spinel mantle peridotite.

    Figure 13.  (a) Diagram of La/Sm-Sm/Yb, data of spinel peridotite and garnet peridotite are from Wilson (1989); (b) diagram of Nb/Yb-Dy/Yb (the legend is the same as Fig. 11; data of source composition of MORB (DMM) are from Workman and Hart, 2005).

  • The MORB-normalized diagram and ratios of Th/Ta and Ta/Hf show that the characteristics of trace elements are more similar to the geochemical characteristics of continental tholeiite. However, there are some limitations in distinguishing tectonic setting of mafic intrusive rocks by trace elements due to the influence of fractional crystallization (Wang et al., 2016; Xia et al., 2008; Li, 1992). During fractionation crystallization of melt, the way of metal ions entering the lattice of clinopyroxene is different due to different tectonic settings, which results in the obvious difference and differentiation of elements content. The ratio of percentage of tetrahedral sites occupied by Al (IVAl) to clinopyroxene octahedral sites occupied by Ti (IVAl/Ti) of cumulates in the island-arc setting is higher than that in the extensional setting. Thus, compositions of clinopyroxene can be used to identify different tectonic settings (Loucks, 1990). TiO2-IVAl% diagram (Fig. 14) can be obtained after the treatment of the EPMA results of clinopyroxene, which reflects that the IVAl/TiO2 ratios of clinopyroxene in these samples are similar to that of clinopyroxene formed at continental within-plate/rift tectonic setting. Therefore, the Gangou diabase dyke in Xixia area belongs to a continental within-plate rift setting, which is consistent with the recent research on the voluminous dioritic, granodioritic and granitic magma in Neoproterozoic that intruded into Douling complex (Bai et al., 2019; Zhang J et al., 2018; Nie et al., 2016; Yang et al., 2011; Zhang C L et al., 2004a).

    Figure 14.  Diagram of TiO2 vs. IVAl (after Loucks, 1990).

    The extensive development of mafic dykes indicates the existence of rigid or semi-rigid continental block with a considerable scale and stable consolidation, which is also an important sign of continental block break-up (Windley, 1989). The ca. 731 Ma Gangou dyke that intruded into the amphibole plagiogneiss of the Douling complex, has the similar geochemical and isotopic characteristics of mafic dykes from the ca. 782–755 Ma in the Wudang Block and ca. 796–685 Ma mafic volcanic rocks in the Yaolinghe Group (Ling et al., 2007, 2002a, b; Zhang, 2002; Zhou et al., 1998). These features show that the Douling complex, Wudang Block and Yaolinghe Group have formed in a unified solidified basement (at least before 731 Ma) and the SQB developed strong lithospheric extension or rifting during 796–685 Ma. The Neoproterozoic igneous rocks, which are widely istributed in the northern margin of the YZB and the SQB, are considered to be in response to the event of assembly and breakup of the Rodinia supercontinent (Bai et al., 2019; Nie et al., 2019; Wang et al., 2019; Zhang et al., 2018). The strong lithospheric extension or rifting setting indicated by the Gangou diabase is consistent in time and space with the results of regional studies. Therefore, we speculate that they may have experienced the breaking-up of the Rodinia supercontinent together and then developed into the Qinling Ocean Basin setting in Early Paleozoic.

  • (1) Zircon LA-ICP-MS U-Pb dating reveal that the Gangou diabase dyke in Xixia area was formed at 731 Ma.

    (2) Whole rock major and trace elements characteristics, Sr-Nd-Pb isotopic composition and composition of clinopyroxene indicate that the Gangou diabase dyke was originated from the partial melting of a heterogeneous spinel-bearing mantle source mixed by a slight-depleted asthenospheric mantle and an EMI component in a extensional setting of within-plate rifts.

    (3) There are similarities of formation age, petrogenesis, source characteristic and tectonic setting between the Gangou diabase, mafic dykes in the Wudang Block and mafic volcanic rocks in the Yaolinghe Group indicate that the SQB developed strong lithospheric extension or rifting during 796–685 Ma. The solidified basement of SQB may have experienced the breaking-up of the Rodinia supercontinent and then developed into the Qinling ocean basin setting in Early Paleozoic.

  • We are grateful to three anonymous reviewers for their constructive comments and suggestions. This study was financially supported by the National Nature Science Foundation of China (Nos. 41572049, 41902050, 41430209, 41421002), the Major State Basic Research Development Projects (No. 2015CB856103) and the State Key Laboratory of Continental Dynamics, Northwest University. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1298-6.

    Electronic Supplementary Materials: Supplementary materials (Table S1, S2, S3, S4, S5) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1298-6.

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