Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China

Accepted Manuscript Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China Shuyan Yang, Peng Peng, Zhaoyuan Qin, Xinping Wang, Chong Wang, Jing Zhang, Taiping Zhao PII: DOI: Reference:

S0301-9268(17)30196-1 https://doi.org/10.1016/j.precamres.2017.10.004 PRECAM 4902

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Precambrian Research

Received Date: Revised Date: Accepted Date:

14 April 2017 24 September 2017 4 October 2017

Please cite this article as: S. Yang, P. Peng, Z. Qin, X. Wang, C. Wang, J. Zhang, T. Zhao, Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China, Precambrian Research (2017), doi: https://doi.org/10.1016/j.precamres.2017.10.004

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Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China Shuyan Yang a, b, Peng Peng*a, b, Zhaoyuan Qin c, Xinping Wang a, b, Chong Wang a, b, Jing Zhang a, Taiping Zhaod

a

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese

Academy of Sciences, Beijing 100029, China; b

School of Earth Sciences, the University of Chinese Academy of Sciences, Beijing 100049,

China c

College of Earth Sciences, Chengdu University of Technology, Chengdu, Sichuan 610059, China

d

Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese

Academy of Sciences, Guangzhou 510640, China e-mail: [email protected]

ABSTRACT The 1780 Ma dykes and coeval volcanics constitute a large igneous province in the North China Craton; however, whether these dykes and volcanic rocks of varying compositions originated from one or multiple sources remains controversial. The ca. 1780 Ma Lvliang dykes are characterized by widespread E-W-oriented dykes and minor N-W-oriented diabase dykes. The E-W-oriented dykes are bimodal and comprise two subgroups: one is dominated by acidic high-silica dykes (SiO2 >63 wt.%), while the other is dominated by mafic to intermediate relatively low-silica dykes (SiO2 <63 wt.%). The ca. 1780 Ma volcanics (in the Xiaoliangling

Group) in the Lvliang area are also bimodal and dominated by rhyolite to dacite volcanics and basalt to andesite volcanics with few clastic interlayers. SIMS U-Pb dating on zircons from one high-silica dyke and a rhyolite volcanic layer yields 207

Pb/206Pb ages of 1783 ± 7 Ma and 1776 ± 6 Ma, respectively, and SIMS U-Pb dating on

baddeleyites from a low-silica dyke yields a 207Pb/206Pb age of 1789 ± 5 Ma. They are all tholeiitic in composition (MgO: 0.3-6.1 wt.%; SiO2: 51-74 wt.%), enriched in light rare earth elements ((La/Yb)N = 7.9–20.2) and show negative anomalies in Nb, Ta, Sr and Eu (Eu/Eu* = 0.3–0.9). They also have similar in-situ Hf isotopes of baddeleyite/zircon (εHf values of -14.5 to -1.8) and bulk rock εNd (-6.7 to -3.0) isotopes. Furthermore, immiscible textures are identified from the Xiaoliangling Group volcanics, which are characterized by some silica-rich melts globules in the Fe-rich magma. These spatiotemporal affinities and geochemical similarities, as well as the identified immiscible textures, reveal that the low-silicate dykes are conduits for the basalt-andesite with significant fractionation of clinopyroxene and plagioclase; while the high-silica dykes are conduits for the rhyolite-dacite with distinct immiscible segregation prior to eruption. In addition, there is a sharp change of εHf values at 1780–1730 Ma, and their Hf TDM ages are ~2750–1950 Ma. As these rocks originated from the subcontinental lithospheric mantle, this may indicate that the subcontinental lithospheric mantle was metasomatized during this time period, possibly by a paleo-plume event. Keywords: North China Craton; Paleoproterozoic; Lvliang dykes; Xiaoliangling Group; Immiscibility

1. Introduction The North China Craton (NCC) is one of the cratons in the world with a crust record back to 3800 Ma (Liu et al., 2008; Wu et al., 2008), and it has experienced multistage crust growth events, e.g., a 2.9–2.7 Ga major growth event and a 2.6–2.5 Ga crust reworking-plus-growth event (Diwu et al., 2010, 2011; Wan et al., 2014, 2015, 2016; Wu et al., 2005; Zhai, 2004, 2014). Then the NCC experienced widespread metamorphism at 1950-1800 Ma (e.g., Guo et al., 2005, 2012; Kusky and Li 2003; Kusky et al., 2007; Liu et al., 2005; Peng et al., 2014; Zhai et al., 1996; Zhai and Peng 2007; Zhao, 2014; Zhao et al., 2001) followed by a series of intra-continental rifts from 1800 Ma and intermittently continued to ca. 800 Ma (Peng, 2016; Zhai et al., 2014). The ~1780 Ma dyke swarm and coeval Xiong'er Volcanic Province (XVP) are generally believed to constitute a large igneous province (LIP) in the NCC (Fig. 1a; Peng et al., 2005, 2008; Peng, 2015; Pirajno and Chen, 2005; Zhao et al., 2002). Some researchers have suggested that they were from the same source (e.g., Hou et al., 2008 a, b; Peng et al., 2007, 2008; Peng, 2015) as they are contemporaneous (Cui et al., 2010; Peng et al., 2007, 2008), and exhibit closely spatial correlations and overlapped bulk rock trace element patterns and Nd-isotopic compositions (Peng et al., 2008). However, some have suggested that they were from different parents (e.g., He et al., 2008, 2009; Wang et al., 2004, 2008; Zhao et al., 2009) mainly based on their different compositions which are difficult to be interpreted using a simple one-source model. Specially, the Taihang dykes are dominated by basic compositions (e.g. Peng et al., 2008, 2015), whether they were evolved in a subduction or orogenic belt environment (e.g. Wang et al., 2004; Zhao et al., 1998) or a continental rift setting (e.g. Halls et al., 2000; Hou et al., 2001; Kusky and Li, 2003;

Qian and Chen, 1987) has been long discussed. Nevertheless, the XVP is dominated by bimodal basic-intermediate (basaltic-andesite to andesite) and acidic (dacite to rhyolite) compositions, within which ~1/3 of the volcanics are silicic component (He et al., 2008, 2009; Peng et al., 2008; Pirajno and Chen, 2005; Wang et al., 2010; Zhao et al., 2002, 2009). Some have suggested that the XVP originated in a continental margin environment (e.g., He et al., 2008, 2009; Zhao et al., 2009); while some suggested that they developed in the interior of a paleo-continent (e.g., Peng et al., 2008; Wang et al., 2010; Zhao et al., 2002). The Lvliang area contains outcrops of both the 1780 Ma dykes (also called the Lvliang dykes) and coeval volcanic rocks (the Xiaoliangling Group/Formation) (Fig. 1b, c). Most dykes have a similar E-W-trending and are dominated by bimodal basic-intermediate and acidic compositions (Peng et al., 2008). The Xiaoliangling Group volcanics are also bimodal, dominated by rhyolite to dacite volcanics and basalt to andesite volcanics with few clastic interlayers (Qiao et al., 1983; Xu et al., 2007). Peng et al. (2015) identified immiscible textures from a rhyolite layer of the Xiaoliangling Group. In this study, we investigate the genetic relationship between the Taihang dykes and the Xiong’er volcanic province by comparing their geological occurrences and magmatic processes in the Lvliang area.

2. Research background 2.1. The 1780–1730 Ma dyke Swarm and the Xiong’er volcanic province 2.1.1 The ca. 1780 Ma Taihang dykes The 1780 Ma Taihang dyke swarm is one of the largest dyke swarms in North China, which extends throughout the central NCC for over 1000 km, encompasses an area of >0.1 Mkm2 and

contains dominantly N-NW-oriented dykes, as well as a few E-W-oriented dykes (Fig. 1; Halls et al., 2000; Hou et al., 2001, 2008a, b; Peng, 2015, 2016; Peng et al., 2004, 2007, 2008; Qian and Chen, 1987; Wang et al., 2004, 2008). The E-W-trending dykes, also grouped as the Lvliang swarm, are mostly distributed in the Lvliang Mts., with some in the Wutai and Zhongtiao Mts. (Peng, 2015). Published ages indicate that the Taihang dykes emplaced at a limited time range of ~1780–1770 Ma (Halls et al., 2000; Han et al., 2007; Peng, 2015; Peng et al., 2005). Reconstruction of some rotations among active blocks of NCC reveals a fanning/radiating geometry for the Taihang dykes with a magma center located in the southern margin (Hou et al., 2008a, b; Peng et al., 2008). In the Yinshan area, the dykes were uplifted and exhumed from crustal levels up to 20 km deep (Hou et al., 2001; Peng et al., 2008).

2.1.2 The ca. 1780 Ma Xiong’er volcanics The XVP is distributed throughout the southern NCC, i.e., in the Huashan Mts., Wangwu Mts., Zhongtiao Mts., Waifang Mts. and Xiaoshan Mts. It centers in the middle of a triple conjugation, in which two rift branches extend along the southern margin of the NCC and the third one extends as far as into the heart of the NCC, represented by the Xiaoliangling Group in the east flank of Lvliang Mts. (Fig. 1; Peng et al., 2008; Qiao et al., 2014; Zhao et al., 2007). The XVP has an outcrop area of over 0.06 M km2 and a thickness of 3–7 km (Zhao et al., 2002). These volcanics unconformably overlie the Neoarchean to Paleoproterozoic crystalline basement (Chen and Zhao, 1997; Zhang et al., 2013) and are overlain by Mesoproterozoic terrigenous sandstones, limestones and calc-silicate rocks (Chen et al., 2004).

2.1.3 The ca. 1730 Ma Miyun dykes The ca. 1730 Ma Miyun dykes encompasses an area of 500 × 300 km2, and consist of N-NW trending dykes in the central NCC and N-E-trending dykes in the north and east of NCC (Fig. 1a; Peng, 2016). The individual dykes are generally 10–50 m wide and are several kilometers long. They are typically diabase in composition (Peng, 2016; Peng et al., 2012b). Peng et al. (2012b) suggested that the ca. 1730 Ma Miyun dykes are the latest pulse of a 1780–1730 Ma plume-associated large igneous province, while the main pulse formed the Taihang dykes and the Xiong’er volcanic province.

2.2 Regional geology of the Lvliang Mt. The Lvliang Complex is located in the central-western of Shanxi Province and exposed as the westernmost part of the Trans-North China Orogen (Zhao et al., 1999a, b, 2005a). It is predominantly composed of supracrustal rocks, multistage granitoid intrusions, minor metamorphosed mafic dykes and unmetamorphosed bimodal dykes. In this region, the late Paleoproterozoic volcanics are also well developed (Geng et al., 2000; Wan et al., 2000; Wang et al., 2014).

2.2.1 The ca. 1780 Ma Lvliang dykes The ca. 1780 Ma Lvliang dykes are characterized by widespread E-W-trending dykes and minor N-W-oriented dykes (Fig. 1). The individual dykes are mostly 5-50 m wide and are up to 20 km long, with most of them over 3 km long (Peng et al., 2008; Wang et al., 2014). Most dykes have a similar E-W-trending and are dominated by bimodal basic-intermediate and acidic

compositions, including gabbro, diabase, diabase-porphyrite, granite porphyry, quartz-orthophyre, ivernit and granodiorite-porphyry (SBGMR, 1989; Peng et al., 2008; Wang et al., 2014). The E-W-oriented dykes are relatively high in Si-content and intruded the uppermost crustal levels, whereas the N-W-oriented dykes are high in Fe-, Ti- and P-contents and intruded the relatively deeper-level crust (Peng et al., 2015). Wang et al. (2014) reported LA-ICP-MS zircon U-Pb ages of 1775 ± 16 Ma, 1779 ± 15 Ma, 1786 ± 16 Ma and 1781 ± 21 Ma from four mafic dykes in the Lvliang area. Peng (2015) reported a TIMS baddeleyite age of 1789 ± 28 Ma from an E-W-oriented dyke (Xiaolouze dyke).

2.2.2 The ca. 1780 Ma Xiaoliangling Group volcanics The Late Paleoproterozoic volcanics are restricted to a small area in the Lvliang Mts., such as the Hangaoshan Group to the west flank of Lvliang Mts. and the Xiaoliangling Formation (we call it as the Xiaoliangling Group in this paper as it is a unique sequence in the area) to the east flank of Lvliang Mts. (Fig. 1b, c; SBGMR, 1989). It is widely accepted that the Xiaoliangling Group is the counterpart of the Xiong’er Group in the Lvliang area, based on their roughly similar ages, as well as petrographic and geochemical characteristics (Fig. 2; Qiao et al., 2014; Xu et al., 2007; Zhao et al., 2007). The Xiaoliangling Group is mainly distributed in the Baijiatan area over an area of >3.5 km2. It is more than 460 m thick and extends up to 5 km (SBGMR, 1989). The volcanic rocks unconformably overlie metamorphosed basement, the Lvliang Complex, and are dominated by bimodal basalt-andesite and dacite-rhyolite compositions, with a thin layer of fuchsia shale in its upper section (Fig. 2; Qiao et al., 1983; Xu et al., 2007). The distribution of vesicles and the color

of the volcanic rocks suggest that the Xiaoliangling Group was erupted in more than ten cycles, with massive to amygdaloidal rocks in each cycle (SBGMR, 1989). Xu et al. (2007) has obtained a LA-ICP-MS U-Pb zircon age of 1779 ± 20 Ma from a rhyolite layer in the Xiaoliangling Group; A SHRIMP U-Pb zircon age of 1778 ± 20 Ma were also obtained from the Xiaoliangling basaltic andesite volcanics (Qiao et al., unpublished data). Previous studies of the geochemical characteristics of the Lvliang volcanics have suggested that they were formed in a continental rift environment (Peng et al., 2008; Xu et al., 2007).

3. Occurrence and petrography As previously mentioned, the ca. 1780 Ma dykes in the Lvliang area are characterized by widespread E-W-trending dykes and minor N-W-trending dykes. The compositions of E-W-trending dykes vary from mafic to acidic, while the N-W-trending dykes are dominated by diabase and are relatively high in Fe-, Ti- and P-contents (SBGMR, 1989). In this text, we cite data of N-W-trending dykes (TiO2 >3% wt.%) from Peng et al. (2004) as an end-member for comparison. The samples were collected from 6 representative E-W-trending dykes and the Xiaoliangling Group volcanics (Fig. 1). The dykes are classified into two groups based on their SiO2 contents: one is dominated by acidic high-silica dykes (HS dykes; SiO2 >63 wt.%), while the other is dominated by mafic to intermediate, relatively low-silica dykes (LS dykes; SiO2 <63 wt.%). The Xiaoliangling Group volcanics are also bimodal, dominated by rhyolite to dacite volcanics (RD volcanics) and basalt to andesite volcanics (BA volcanics) with few clastic interlayers (Fig. 2).

3.1. The Lvliang dykes 3.1.1 The low-silica (LS) dykes The Xiaolouze diabase dyke is ~50 m wide, NW-W-trending (290°) (Fig. 3a) and is vertical to subvertical, slightly dipping to the south. It has a typical gabbroic texture with clinopyroxene-to-plagioclase mineral volumes of 2:3. Most clinopyroxene grains have undergone alteration of epidotization and most plagioclases have undergone alteration of sericitization (Fig. 3b, c, Fig. 5). The Dushihe andesitic porphyrite dyke is E-W-trending (270°) and ~50 m wide, and outcrops continuously for ~20 km (Fig. 3d). The southern chilled margin is in sharp contact with the surrounding granitic rocks. This dyke shows a porphyritic-like texture and contains a mineral assemblage of andesine-labradorite (40%–50%), orthoclase (10%–20%), augite-pigeonite (∼25%), with minor amounts of magnetite and quartz. Most clinopyroxenes and plagioclases have undergone alteration (Fig. 3e, f, Fig. 5). The Shixianggou area contains outcrops of many small scale mafic dykes, which are basically E-W-trending (250°–280°) and 3–8 m wide (Fig. 3g). Compositionally, the dykes are diabase-porphyrite and contain a mineral assemblage of andesine-labradorite, augite and amphibole, with minor amounts of alkali feldspar, ilmenite, pyrite and quartz. Most dykes have undergone strong alteration (Fig. 3h, i, Fig. 5).

3.1.2 The high-silica (HS) dykes The Tadigou area outcrops two porphyritic felsic dykes, which are basically E-W-trending (280°) and 17–20 m wide (Fig. 3j). These dykes show a porphyritic-like texture in which

orthoclase, albite and minor augite occur as phenocrysts. The phenocrysts are up to 4 mm long, and the matrix contains a mineral assemblage of quartz, orthoclase, albite, augite, needle-shaped magnetite and muscovite. Graphic and myrmekitic textures are common in these rocks. Most dykes have undergone strong alteration (Fig. 3k, l, Fig. 5). It is noteworthy that mafic dykes outcrop just adjacent (physically contact, with no indication of chronologic relationship) to the porphyritic felsic dykes in the Tadigou area. Besides the mafic dykes, the Shixianggou area also outcrops several felsic dykes, which are basically E-W-trending (Fig. 3m). These dykes show a porphyritic-like texture and contain a mineral assemblage of quartz, orthoclase, albite, epidote and chlorite, with minor amounts of sphene, augite and magnetite. Graphic and myrmekitic textures are common in these rocks. Most dykes have undergone strong alteration (Fig. 3n, o, Fig. 5).

3.2 The Xiaoliangling group volcanics The basalt-andesite layers are gray-green and contain relatively fewer vesicles and amygdaloids (Fig. 4a, d). The volcanic rocks show an andesitic texture, in which labradorite occurs as phenocrysts. The matrix contains a mineral assemblage of plagioclase and clinopyroxene, with minor amounts of sphene, quartz and needle-shaped magnetite. Two samples (824BJT, 823YT1) were collected from basaltic andesite layers with uniform composition. Most rocks have undergone strong alteration (Fig. 4a-f, Fig. 5). The rhyolite layer is fuchsia with a fluidal structure (Fig. 4g-i). Rhyolite phenocrysts are mainly composed of alkali feldspar and minor altered clinopyroxene, which are up to 3 mm in size. The matrix is mainly composed of quartz, alkali feldspar, plagioclase and magnetite. Graphic and

myrmekitic textures are common in these rocks. Most rocks have undergone strong alteration. More importantly, the rhyolite layer records immiscible textures, as previously reported by Peng et al. (2015).

4. Results 4.1. Pb-Pb baddeleyite ages and U-Pb zircon ages 4.1.1 Sample 819XLZ (Xiaolouze diabase dyke) The Xiaolouze dyke outcrops in the Xiaolouze village, Loufan. Sample 819XLZ was collected from the central part of this dyke (Fig. 3a-c; GPS: 37°57'10"N, 111°48'10" E). Baddeleyites from this sample typically occur as thin and platy crystals that are 30–150 µm in length and light to medium brown (Fig. 6a). A total of 25 spot analyses are performed on this sample. Four spots (spots 13, 15, 19, 21) yield 207Pb/206Pb ages with large errors (185–969 Ma), whereas the other 21 spots yield 207 Pb/206Pb ages ranging from 1747 Ma to 1821 Ma, yielding an average 207Pb/206Pb age of 1789 ± 5 Ma (2σ, n = 21, MSWD = 1.3; Fig. 6a; Supplementary Table 2) that represents the crystallization age of the Xiaolouze dyke.

4.1.2 Sample 827TDG1 (Tadigou granite porphyry dyke) Sample 827TDG1 was collected from a granite porphyry dyke near Tadigou village, Gujiao city (Fig. 3j-l; GPS: 37°53'19"N, 111°47'50"E). Zircon grains from this sample are 100–200 µm in diameter and vary in their shapes (Fig. 6b). A total of 22 spot analyses were performed on different zircon grains from this sample. Except for ten discordant and scattered analyses, the rest twelve are more coherent and have U-content of 130–493 ppm, Th-content of 104–973 ppm, and

Th/U ratios of 0.7–2.0, yielding a

207

Pb/206Pb weighted average age of 1783 ± 7 Ma (n = 12,

MSWD = 1.2) and a concordia age of 1783 ± 7 Ma (n = 12, MSWD = 0.0081) (Fig. 6b; Supplementary Table 2), which represent the crystallization time of the Tadigou dyke.

4.1.3 Sample 823YT2 (Yatou rhyolite) Sample 823YT2 was collected from the middle part of the Xiaoliangling Group in Yatou village, Loufan (Fig. 4g-I; GPS: 37°56'5"N, 111°58'35"E). Zircons from 823YT2 are 100–200 µm in diameter, are columnar in shape and display clear and wide zoning (Fig. 6c). In addition, they record Th-content of 38–403 ppm, U-content of 63–363 ppm and Th/U ratios of 0.61–1.21. 207

Pb/206Pb ages from 20 spots range from 1748 Ma to 1798 Ma, yielding an average age of 1776

± 6 Ma (2σ, n = 20, MSWD = 0.58) and a concordia age of 1771 ± 6 Ma (n = 20, MSWD = 2.5) (Fig. 6c; Supplementary Table 2), which represent the crystallization age of the Yatou rhyolite.

4.2 Geochemistry 4.2.1 The low-silica (LS) dykes In general, the LS dykes record 50.66–58.38 wt.% SiO2, 1.20–1.70 wt.% TiO2, 13.78–15.62 wt.% Al2O3, 9.21–9.94 wt.% FeOT (total iron), 2.8–6.1 wt.% MgO, 5.23–8.07 wt.% CaO, 2.75–2.99 wt.% Na2 O, 2.03–3.29 wt.% K2O, and 0.31–0.56 wt.% P2 O5 (Fig. 7; Supplementary Table 3). Calculated Mg# values (Mg-number, calculated as 100*Mg/ (Mg+Fe2+) in molecular) range from 24 to 37. These dykes record slight enrichment in the light rare earth elements (LREE) ((La/Yb)N = 7.9–12.1). They also exhibit slightly negative Eu anomalies (δEu = 0.65–0.87, δEu =

EuN/(SmN*GdN)1/2) on chondrite-normalized REE patterns. They are enriched in large ion lithophile elements (LILE) (e.g., Rb, Ba, and K) but are depleted in Nb, Ta and Sr relative to their neighboring elements on the spidergram (Fig. 8; Supplementary Table 3). The

87

Sr/86Srt (t = 1780 Ma) ratios and εNd (t, t = 1780 Ma) values of the LS dykes range

from 0.703 to 0.706 and -6.7 to -3.0, respectively. The calculated Nd-depleted mantle model ages (TDMNd) vary from 2490 to 2720 Ma (Fig. 9a, b; Supplementary Table 4). Baddeleyites from sample 819XLZ yield εHf (t, t = 1780 Ma) values ranging from -14.5 to -1.8. In addition, data from four ca. 1780 Ma dolerite dykes in the Lvliang area from Wang et al. (2014) were also plotted (Fig. 9c, d; Supplementary Table 5).

4.2.2 The high-silica (HS) dykes The HS dykes have 67.83–74.48 wt.% SiO2, 0.37–0.84 wt.% TiO2 , 13.22–13.71 wt.% Al2O3, 2.50–4.91 wt.% FeOT , 0.4–0.7 wt.% MgO, 0.45–2.84 wt.% CaO, 2.75–2.99 wt.% Na2O, 4.73–5.79 wt.% K2O, and 0.05–0.24 wt.% P2O5 (Fig. 7; Supplementary Table 3). The Mg# values vary from 11 to 14. These samples show slight enrichment in LREE ((La/Yb)N = 12.4–17.0). They also have moderately negative Eu-anomalies (δEu = 0.27–0.48) on chondrite-normalized REE patterns (Fig. 8). Although they are enriched in LILE, they record negative anomalies in Nb, Ta and Sr compared with their neighboring elements on spidergrams (Fig. 8; Supplementary Table 3). The only two analyzed samples yield 87Sr/86Srt (t =1780 Ma) values of 0.682 and 0.705, εNd (t, t = 1780 Ma) values varying from -5.6 to -5.7 and model Nd ages (TDMNd) of 2530 Ma to 2570 Ma (Fig. 9a, b; Supplementary Table 4). The exceptionally low

87

Sr/86Srt (t =1780 Ma) value

(0.682) could be due to late-stage erosion that may cause radiogenic Sr-loss. Zircons from sample 827TDG1 yield εHf (t, t = 1780 Ma) values varying from -10.9 to -6.4 (Fig. 9c, d; Supplement Table 5).

4.2.3 The basalt-andesite (BA) volcanics In general, the two basaltic andesite samples have 53.13–56.26 wt.% SiO2, 1.24–2.23 wt.% TiO2, 13.95–14.45 wt.% Al2O3, 9.12–10.13 wt.% FeOT, 4.1–4.5 wt.% MgO, 5.46–6.66 wt.% CaO, 2.29–2.30 wt.% Na2 O, 2.38–3.32 wt.% K2O, and 0.38–0.90 wt.% P2 O5 (Fig. 7; Supplementary Table 3). The Mg# values vary from 34 to 35. These samples record slight enrichment in LREE ((La/Yb)N = 9.7–12.7) and slight negative Eu anomalies (δEu = 0.65–0.68) on chondrite-normalized REE patterns. They are enriched in LILE but show negative anomalies in Nb, Ta and Sr, compared with their neighboring elements on spidergrams (Fig. 8; Supplementary Table 3). Sample 824BJT yields an 87Sr/86Srt (t = 1780 Ma) value of 0.703, an εNd (t, t = 1780 Ma) value of -4.3, and a model Nd age (TDMNd) of 2550 Ma (Fig. 9a, b; Supplementary Table 4).

4.2.4 The rhyolite-dacite (RD) volcanics The rhyolite sample 823YT2 has 72.65 wt.% SiO2, 0.6 wt.% TiO2 , 10.95 wt.% Al2 O3, 5.71 wt.% FeOT, 0.5 wt.% MgO, 0.90 wt.% CaO, 2.12 wt.% Na2O, 3.81 wt.% K2O, and 0.16 wt.% P2O5 ( Fig. 7; Supplementary Table 3). The Mg# value is 13. The sample shows a slight enrichment in LREE ((La/Yb)N = 12.9) and a moderately negative Eu-anomaly (δEu = 0.51). It is enriched in LILE but record negative anomalies in Nb, Ta and Sr (Fig. 8; Supplementary Table 3).

The rhyolite sample 823YT2 yields an 87Sr/86Srt (t = 1780 Ma) value of 0.683, an εNd (t, t = 1780 Ma) value of -5.7, and a model Nd age (TDMNd) of 2610 Ma (Fig. 9a, b; Supplementary Table 4). The exceptionally low 87Sr/86 Srt (t =1780 Ma) value (0.683) could be due to late-stage erosion that caused radiogenic Sr-loss. Zircons from the sample also records

176

Lu/177Hf values

ranging from 0.00060 to 0.00219, 176 Hf/177 Hf values of 0.281387 to 0.281554, and εHf (t, t = 1780 Ma) values varying from -10.5 to -5.9 (Fig. 9c, d; Supplementary Table 5).

5. Discussion 5.1. Ages of the igneous suites The Taihang dyke swarm and the Xiong’er volcanics were both active during a limited time interval, 1790–1760 Ma (with their peak at ca. 1780 Ma) (Cui et al., 2010; Halls et al., 2000; Han et al., 2007; He et al., 2009; Peng et al., 2005, 2006, 2012b, 2015; Pirajno and Chen, 2005; Wang et al., 2004, 2010, 2016a; Xu et al., 2007; Zhao et al., 2004, 2005b). In this paper, we obtained an age of 1783 ± 7 Ma from a HS dyke, an age of 1789 ± 5 Ma from a LS dyke, and an age of 1776 ± 6 Ma from a rhyolite layer in the Xiaoliangling Group (Fig. 6). Similarly, Wang et al. (2014) reported ages of 1775 ± 16 Ma and 1779 ± 15 Ma from two HT dykes, 1786 ± 16 Ma and 1781 ± 21 Ma from two LS dykes in the Lvliang area; Xu et al. (2007) also obtained a U-Pb zircon age of 1779 ± 20 Ma (LA-ICP-MS) from a rhyolite layer in the Xiaoliangling Group; A SHRIMP U-Pb zircon age of 1778 ± 20 Ma were also obtained from the Xiaoliangling basaltic andesite volcanics (Qiao et al., unpublished data). These ages indicate that the Lvliang dykes and Xiaoliangling volcanics were both formed within a limited time interval (1790-1770 Ma), mostly around ca. 1780 Ma.

5.2. Relationships between the LS (low-SiO2) dykes and BA (basalt-andesite) volcanics As seen on the Nb/Th vs. Nb/La diagram, all LS dyke and BA volcanic samples (except for sample 828SXG1) record similar Nb/La ratios (Fig. 10a), which indicates that little crustal contamination occurred during magma ascent, as otherwise the Nb/Th and Nb/La ratios would systematically decrease with increasing degrees of contamination. Similarly, on the Ta/La vs. La/Yb diagram, all samples (except for sample 828SXG1) record similar Ta/La ratios (Fig. 10b), which also indicates that only minor crustal contributions occurred. The BA volcanics record mineral compositions and whole-rock compositional variations (in major elements, trace elements and Sr isotopes) that are well within those of the LS dykes (Fig. 5, 7, 8, 9). Moreover, they both record consistent variations in major elements: for example, with decreasing MgO-contents, SiO2-, K2O- and Na2O-contents systematically increase and CaO-, Al2O3-, TiO2- and P2 O5-concentrations decrease; however, FeOT-contents do not record any systematic variations (Fig. 11). Since the BA and LS groups are mainly composed of clinopyroxene and plagioclase, fractional crystallization of these two major phases may have controlled the compositional evolution of the magma. The presence of plagioclase megacrysts also indicates that plagioclase-fractionation occurred during the ascent of magma. The fractionation of plagioclase could have significantly decreased the CaO and Al2O3 in the liquid and produced negative Eu- and Sr-anomalies (Fig. 8, 11). A significant decrease in MgO with only limited variation of Al2O3 (Fig. 11) indicates that a mafic mineral phase also underwent differentiation. Clinopyroxene is likely the fractionated phase as it is the only mafic phenocryst in the LS dykes and BA volcanics.

In addition, Zhao et al. (2004) has reported a SHRIMP U-Pb zircon age of 1773 ± 37 Ma from a diabase dyke which intruded into the Xiong’er volcanic layers. Although the field relationship between the LS dykes and BA volcanics in the Lvliang area is not well verified, their contemporaneous formation ages and similar geochemical characters indicate a genetic relationship between the two groups, in which the LS dykes are the conduits for the BA volcanic rocks.

5.3. Relationships between the HS (high-SiO2) dykes and RD (rhyolite-dacite) volcanics and their origins: evidence of immiscibility The bulk compositions of the HS dykes and RD volcanics record similar major and trace element patterns as well as Nd and Hf isotopic compositions (Fig. 7, 8, 9). Moreover, they have similar mineral assemblages, as both groups are mainly composed of K-feldspar, albite and clinopyroxene. In addition, their clinopyroxene crystals have commonly undergone chloritization and epidotization alteration (Fig. 3, 4). All these features indicate that the HS dykes were the conduit for the RD volcanics; that is, the RD volcanics represent the erupted equivalents of the HS dykes, and their compositional variations could be the result of fractional crystallization. However, the relationship between the LS-BA pair and the HS-RD pair remains unclear. Peng et al. (2015) proposed that plagioclase- and clinopyroxene-dominated fractional crystallization and density-driven mineral sorting caused these liquids to be poor in Ca and Al but rich in Fe, Ti, P and K, thus making them chemically immiscible. In addition, there are conjugate interstitial granophyric and ilmenite-rich intergrowths and reactive microstructures (especially olivine coronas) in the dykes, and Si-/Fe-Ti-rich globules in the volcanics, which support an immiscibility

hypothesis (Peng et al., 2015). Our new data from these HS dykes and RD volcanics are also similar to those for immiscible felsic volcanic rocks. The ternary diagram CaO vs. Al2O3 vs. TiO2 + P2O5 + Na2O + K2O can be used to define the field of liquid immiscibility in silicate melts. The alignment of immiscible melts defines the bimodal curves that separate the one-liquid field from the two-liquid field (Charlier and Grove, 2012) (Fig. 12a). This diagram reveals that the LS group and BA volcanics in the study area also approach the bimodal curve along the proposed liquid line of descent, whereas the HT and HS dykes extend into the two-liquid fields (Fig. 12a). Furthermore, the FeOT vs. SiO2 diagram exhibits an obvious compositional gap (Fig. 12b), representing two distinct composition end members. Further proof of this model is the presence of a few volcanic layers containing Si-rich globules and Fe-Ti-rich globules; In macro-scale, the Fe-Ti-rich globules are in sharp contact with the Si-rich globules or the matrix, and the abundance of Fe-Ti-rich globules decreases in the younging direction (Fig. 13 a, b). In fact, the Fe-Ti-rich globules contain both Fe-Ti and Si-K content, as seen on the XRF maps of a rhyolite sample showing the coexistence of K+Si and Fe+Ti elements (Fig. 13 c, d). The Si-K content is clearly unevenly dispersed and is thus present as globules, which probably represent the state of the melt before segregation, in which an incomplete segregation of two types of immiscible components or droplets has occurred. This provides direct evidence for the presence of immiscibility. Based on the chemical features of the HS dykes and RD volcanics (Fig. 7, 8, 9, 12), as well as the immiscible textures observed in the volcanic layers (Fig. 13), we suggest that the HS dykes and RD volcanics were possibly segregated from an immiscible melt. However, the LS dykes and

BA volcanics are not the counterparts of the HS dykes and RD volcanics, which should be low in SiO2 but high in TiO2, FeOT and P2 O5. It is thus believed that the HT dykes, as reported by Peng et al. (2015), should represent the “missing” conjugated components of the HS dykes and RD volcanics. The HT dykes are mostly found in the Fengzhen area and less commonly found in Wutai Mts. (Peng et al., 2015), but are not found in the Lvliang region. We believe that the Lvliang Mts. were only exposed from a very shallow depth since the ca. 1780 Ma volcanics are preserved; however, as the HT dykes are denser than the HS dykes and RD volcanics due to their greater volumes of denser magnetite and ilmenite, they may have existed at deeper crustal levels and rarely been erupted. This hypothesis is consistent with the observation that there are more HT dykes in the north (e.g., Fengzhen) than in the south (e.g., Wutai Mts.), as paleomagnetic data indicate that the northern region was exposed from a deeper crustal level (Halls et al., 2000; Hou et al., 2008a, b). This also suggests that the Lvliang dykes are part of the Taihang dyke swarm; however, the reason for their differing major orientations (i.e., N-W vs. E-W) remains unclear. One possible explanation is that the E-W dykes occur only near the surface in a different stress field due to the emplacement of the N-W dykes and the uplift of the region, as suggested by Peng et al. (2008).

5.4. Magma source(s) of the ca. 1780 Ma igneous event and the evolution of the subcontinental lithospheric mantle It has been widely accepted that the ca. 1780 Ma dykes and BA group volcanics originated from the subcontinental lithospheric mantle (SCLM, Hou et al., 2001, 2008a, b; Peng et al., 2004, 2007, 2008; Wang et al., 2004, 2008, 2010). This is consistent with the rocks’ geochemical data,

such as their negative Nd isotopes (-6.7 to -3.0) and model Nd ages (TDMNd) (2.5–2.7 Ga) (Fig. 9a, b; Supplementary Table 4), εHf(t) values (-14.5 to -1.8) and model Hf ages (TDMHf; 2.3–2.8 Ga) (Fig. 9c, d; Supplementary Table 5). However, it remains controversial whether their enriched trace element characteristics (e.g., enrichment in LILE and depletion in HFSE) were produced by the contamination of the juvenile crust in a subduction setting (e.g., Wang et al., 2004, 2008) or inherited from the SCLM (e.g., Peng et al., 2004, 2007; Wang et al., 2010). Baddeleyites are believed to crystallize in SiO2-undersaturated magma; therefore, they can represent the compositions of magma that have experienced little crustal contamination. The εHf(t) values of baddeleyite from the Xiaolouze LS dyke vary from -14.5 to -1.8, and their corresponding model Hf ages (TDMHf) range from 2276 Ma to 2760 Ma (Fig. 9c, d; Supplementary Table 5). These ~2700 Ma model ages are quite similar to the age at which the crust of the Eastern North China Craton was formed (Diwu et al., 2010; Wan et al., 2014, 2015, 2016). In addition, the mafic rocks formed at ~2.5–1.8 Ga mostly record similar trace element patterns, such as depleted in high field-strength elements while enriched in large ion lithophile elements; these include the ~2500 Ma Huangbaiyu dykes (Li et al., 2010b), the ~2060 Ma Hengling sills (Peng et al., 2012a), the ~2115 Ma Haicheng sills (Wang et al., 2016b; Yuan et al., 2015), and the ~1970 Ma Xiwangshan dykes (Peng et al., 2012a). We argue that all of these samples were originated from the SCLM, as all of them have TDMNd ages of ~2.7 Ga (Li et al., 2010b; Peng et al., 2012a; Wang et al., 2016b; Wu et al., 2005; Yuan et al., 2015) and thus formed around the time when the major parts of the crust of the Eastern North China Craton were differentiated from the mantle. Therefore, based on the Hf isotopic data of baddeleyites and whole-rock chemical compositions, we suggest that the Taihang dykes and the Xiaoliangling Group volcanics were originated from the SCLM, which was

either enriched by contamination of the subducted juvenile crust in a subduction setting (e.g., Wang et al., 2004, 2008) or inherited from the homogenized SCLM with little assimilation (e.g., Peng et al., 2004, 2007; Wang et al., 2010). The Hf isotope data of baddeleyite and zircon from the 2150–1730 Ma dykes and sills are plotted in Fig. 9c, d. Among these, the ~2115 Ma Haicheng sills (Wang et al., 2016b), ~2090 Ma Zanhuang sills (Peng et al., 2017), ~2060 Ma Yixingzhai dykes (Peng et al., 2012a) were believed to have originated from the SCLM whereas the 1730 Ma Miyun dykes may have incorporated melts from the asthenosphere or paleo-plumes (Peng et al., 2012b). It is quite clear that the 1730 Ma Miyun dykes record more depleted characteristics (εHf = -0.6–+5.2; TDMHf = 1953 Ma–2184 Ma; Fig. 9c; Supplementary Table 5). From 1780 Ma to 1730 Ma, the Hf isotopic values jump from negative to positive (Fig. 9c), which is consistent with a similar sharp change observed in the Nd values (Peng, 2015). Moreover, during 1800–1765 Ma (Fig. 9d), the εHf values (ranging, on average, from -8.27 to -4.28) increase. We suggest that this indicates that the compositions of the SCLM of the Eastern North China Craton became more depleted from 1800 to 1730 Ma, although it is unclear if this feature occurred throughout the entire mantle or just locally. This hypothesis is consistent with the Nd isotopic data (Li et al., 2015b; Peng, 2015). This rejuvenation of the SCLM could be due to significantly melting of the SCLM or alternatively through mantle upwelling and metasomatism at ca. 1730 Ma (Peng et al., 2008). Li et al. (2015) have evaluated the first possibility using model calculation and concluded that to complete such a jump of Nd isotopes of the SCLM within 200 Ma, vast igneous rocks (i.e., an amount comparable to the whole upper crust) are supposed to be produced. Moreover, the trace element compositions of the SCLM may also have been enriched during this time period (1800 to 1730 Ma) based on the trace element patterns

of the Miyun dykes (which record less depletion of the HFSE) (Li et al., 2015b). This possibility suggests that the 1780 Ma igneous events may have originated from a paleo-mantle plume (e.g., Zhao et al., 2002; Peng et al., 2008), which could have significantly modified or metasomatized the SCLM during 1800–1730 Ma.

6. Conclusions (1) The Lvliang dykes in the study area are mostly E-W-oriented. Based on their SiO2 content, they can be separated into two groups: one is dominated by acidic high-silica dykes (HS dykes), and the other is dominated by mafic to intermediate low-silica dykes (LS dykes). The Xiaoliangling Group volcanics are dominated by rhyolite-dacite volcanics (RD volcanics) and basalt to andesite volcanics (BA volcanics), with few clastic interlayers. Both suites comprise bimodal compositions. (2) SIMS U-Pb dating on zircons from one HS dyke and a rhyolite volcanic layer yields 207

Pb/206Pb ages of 1783 ± 7 Ma and 1776 ± 6 Ma, respectively, and SIMS U-Pb dating on

baddeleyite from a LS dyke yields a 207Pb/206Pb age of 1789 ± 5 Ma. These ages indicate that the dykes and volcanic rocks were formed within a limited time interval (1790 Ma–1770 Ma). (3) These spatiotemporal affinities, geochemical similarities, as well as the identified immiscible textures, suggest that the dykes are conduits for the volcanics: specifically, the LS dykes are conduits for the BA volcanics, and the HS dykes are conduits for the RD volcanics. The LS dykes and BA volcanics have both experienced significant fractionation of clinopyroxene and plagioclase. There are immiscible textures recorded in the RD rocks, thus indicating the occurrence of immiscible segregation. It is thus possible that the HS dykes and RD volcanics were

segregated from an immiscible melt that may have originated from the same source as the LS dykes and BA volcanics. (4) All the dykes and volcanics have enriched Sr-Nd isotopic characteristics (87Sr/86Srt = 0.7026–0.7055, εNd = -6.7 to -3.0) and ancient Nd depleted mantle model ages (2500–2700 Ma). There is a sharp change of εHf values at 1780–1730 Ma (ranging, on average, from -8.2 to1.5), and their Hf T DM ages are ~2750–1950 Ma. As these rocks were likely originated from the subcontinental lithospheric mantle, this indicates that the subcontinental lithospheric mantle should have been metasomatized during this time period, possibly caused by a paleo-plume event.

Acknowledgments Two anonymous reviewers are thanked for their constructive comments and suggestions. This study was supported by NFSC Project (41630211), the Major State Research-Development Project (2016YFC0600109), the CAS priority strategic program ‘Linking the earth Interior Processes and Surface evolution (LIPS)’ (XDB18030205) and the National High-Level Talents Special Support Plan project. We are grateful to Prof. Jinghui Guo for his constructive suggestion. We thank Guoqiang Tang, Jiao Li, Hongxia Ma, Xiaoxiao Ling and Saihong Yang for SIMS U–Pb isotope analyses, and Yueheng Yang for MC-ICP-MS Hf isotope analyses, and Chaofeng Li, Hongyan Li, Weiyi Li and Youlian Li for Sr-Nd isotopic analyses, and Wenjun Li, Bingyu Gao, Hongyue Wang, Dingshuai Xue and Yanhong Liu for whole-rock major and trace element analyses, and Jujie Guo and Qian Guo for sample preparation, and Di Zhang for EPMA analyses. We also thank Shuai Sun for improving an early version of the paper and Xiangdong Su for his help in the field. This is a special contribution to show honor to Prof. Mingguo Zhai, the founder

of the Precambrian Geology Group in IGGCAS (Beijing).

Supplementary: Analytical methods All the analyses were performed at Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The data are listed in Supplementary Tables 1-5. Major-element analyses for the minerals were performed using JXA-8100 electron microprobe. Quantitative analyses were performed using wavelength-dispersive spectrometers (WDS), which was also used to acquire the Back Scattered Electron (BSE) images. Data was corrected using the ZAF procedure (Supplementary Table 1). Baddeleyite and zircon grains were extracted using a water-based mineral separation technique (Söderlund and Johansson, 2002) and then they were mounted on epoxy resin disks together with the baddeleyite Phalaborwa age standard (SIMS

207

Pb/206Pb age = 2060 Ma) and

zircon U-Pb standards Qinghu and Plesovice. Afterwards the cathodoluminescence (CL) images, the transmitted and reflected photomicrographs of the zircon and baddeleyite grains were taken to reveal their internal structure. After being coated with ~30 nm of high-purity gold, U-Pb dating was conducted on a CAMECA SIMS 1280 machine (Li et al., 2009, 2010a). The analyzed ion beam was 20 µm × 30 µm in diameters. Common Pb in zircon was corrected by the determined 204

Pb (Stacey and Kramers, 1975). The data are given in Supplementary Table 2. FeO content was obtained by wet chemical techniques and other major element

determinations were performed by X-ray fluorescence (XRF). Trace element analyses were determined using an ELEMENT ICP-MS after HNO3+HF digestion of whole rock powders. The accuracy and reproducibility were monitored by the Chinese national standard sample GSR1 and

GSR3. The relative standard deviation was better than 5%. The data are given in Supplementary Table 3. Analytical procedures for Rb-Sr and Sm-Nd isotopic analysis follow those described by Li et al (2015a, 2016). Whole rock powders for Sr and Nd isotopic analyses were dissolved in HF + HNO3 + HClO4 solution. Rb, Sr, Sm and Nd were separated using the classical two-step ion exchange chromatographic method before measurement. NBS-987 (Sr) and JNdi-1 (Nd) were employed as the international standard. The external precisions (2δ) of 87Rb/86Sr and147Sm/144Nd ratios were both better than 0.5%. The data are given in Supplementary Table 4. In-situ Hf isotope analyses of zircons were determined using a Neptune MC-ICP MS. A ~65 µm spot size was applied on zircons and a ~40 µm spot size was applied on baddeleyites. During analysis, the mean fractionation index proposed by Iizuka and Hirata (2005) was used to correct the isobaric interference of 176Yb on 176Hf. A value of 0.5886 was used for the

176

Yb/ 172Yb ratio

(Chu et al., 2002). In-situ Hf isotope analyses were also performed on baddeleyites from the 2115 Ma Haicheng dyke (Wang et al., 2016b) and 2090 Ma Zanhuang dyke (Peng et al., 2017) in this study, besides zircons and baddeleyites from the Xiaolouze dyke, the Tadigou dyke and the Yatou rhyolite. We calculated the depleted-mantle model ages (TDM) and average continental crust model ages (TCDM) using 176 Lu/177 Hf of 0.015 for the average continental crust (Griffin et al., 2004) and depleted mantle values proposed by Griffin et al. (2000). The data are given in Supplementary Table 5.

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Figure Captions

Fig. 1 (a) Simplified geological map showing the distribution of ca. 1780 Ma Taihang dyke swarm and the Xiong’er volcanics (modified after Peng, 2015); (b) Simplified geological map of Lvliang area showing distribution of the dykes and volcanic in the study area. (c) Simplified geological map illustrating the distribution of Xiaoliangling Group volcanics and associated dykes. Asterisks are sampling localities.

Fig.2 Columns of the Xiaoliangling Group, Hangaoshan Group and Guankou volcanics from Lvliang Area, and the Xiong’er Group from Xiong’er Mts. (modified after Qiao et al., 2014).

Fig. 3 The LS and HS dykes: (a-c) The Xiaolouze LS dyke; (d-f) The Dushihe LS dyke; (g-i) The Shixianggou LS dyke; (j-l) The Tadigou HS dykes; (m-n) The Shixianggou HS dykes.

Fig. 4 The BA and RD volcanics: (a-c) The Yatou basaltic andesite; (d-f) The Baijiatan basaltic andesite; (g-i) The Yatou rhyolite.

Fig. 5 Classification of Or-Ab-An diagram for feldspar and Wo-En-Fs diagram for clinopyroxene.

Fig. 6 Weighted average 207Pb/206Pb age and concordia U–Pb age diagrams plots. (a) Xiaolouze diabase dyke (translight, baddeleyite, sample 819XLZ); (b) Tadigou granite porphyry dyke (CL, zircon, sample 827TDG1); (c) Yatou rhyolite (CL, zircon, sample 8323YT2).

Fig. 7 (a) The TAS classification and (b) FeOT/MgO vs. SiO2 (wt.%) diagram (after Miyashiro, 1975) for the dykes and volcanics in the Lvliang area.

Fig. 8 (a-c) Chondrite-normalized REE patterns, and (d-f) primitive mantle-normalized trace element spidergram of the dykes and volcanics in the Lvliang area (chondrite normalized values and primitive mantle values of Sun and McDonough, 1989). The data of HT dykes are from Peng et al. (2004); the data of RD volcanics are from Wang et al. (2010), Zhao et al. (2012); the data of BA volcanics are from He et al. (2009), Peng et al. (2008), Wang et al. (2010) and Zhao et al. (2002).

Fig. 9 (a) 87Sr/86Sr (t = 1780 Ma) vs. εNd (t, t = 1780 Ma) diagram, and (b) εNd (t, t = 1780 Ma) vs. t (Ma) diagram of the Lvliang dykes and volcanics. Data sources: Chondrite (143Nd/144 Nd (t = 0) = 0.512638,

147

Sm/144Nd (t = 0) = 0.1967) after Goldstein et al. (1984), BSE (Bulk Silicate

Earth, 87Sr/86Sr (t = 0) = 0.7047, 87Rb/86Sr (t = 0) = 0.085) after Taylor and McLennan (1984); DM (Depleted Mantle): (1989);

87

143

Nd/144Nd (t = 0) = 0.51315, 147Sm/144Nd (t = 0) = 0.2137 after Peucat et al.

Sr/86Sr (t = 0) = 0.7026,

87

Rb/86Sr (t = 0) = 0.046 after Taylor and McLennan (1984).

(c-d) εHf (t) vs. t (Ma) diagram of the Lvliang dykes and volcanics. Data of the Pangquangou dyke (1786 Ma), the Shanshuicun dyke (1781 Ma), the Zhaishangcun dyke (1779 Ma) and the Shizhuang dyke (1775 Ma) are after Wang et al., 2014; Data of Yixingzhai dyke (2060 Ma) are after Peng et al., 2012a.

Fig. 10 (a) Nb/La vs. Nb/La diagram and (b) Ta/La vs. La/Yb diagram showing negligible crustal contamination for the LS dykes and BA volcanics.

Fig. 11 Plots of whole-rock MgO concentrations versus major element concentrations in whole-rocks. The data of HT dykes are from Peng et al. (2004).

Fig. 12 (a) CaO vs. Al2O3 vs. TiO2+P2 O5+Na2O+K2O (wt.%) diagram (after Charlier and Grove , 2012); (b) FeOT vs. SiO2 (wt.%) diagram. The data of HT dykes are from Peng et al. (2004).

Fig. 13 (a-b) Si-K-rich and Fe–Ti-rich globules in a rhyolite volcanic layer near Baijiatan village; (c-d) XRF map of a rhyolite sample showing the coexistence of K+Si and Fe+Ti content (authors’ own unpublished data).

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Highlights:


1780 Ma bimodal dykes and volcanics are identified in the Lvliang area.


The low-Si dykes are conduits for basalt-andesite while high-Si ones for rhyolite-dacite.


Both bimodal dykes and volcanics were originated from subcontinental lithospheric mantle.


The lithosphere was probably metasomatized by a paleo-plume at 1780-1730 Ma.