Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton

Accepted Manuscript Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton Wang Xu, Fulai Liu, M. Santosh, Pinghua Liu, Zhonghua Tian, Yongsheng Dong PII: DOI: Reference:

S1367-9120(18)30238-4 https://doi.org/10.1016/j.jseaes.2018.06.016 JAES 3546

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

17 February 2018 4 June 2018 4 June 2018

Please cite this article as: Xu, W., Liu, F., Santosh, M., Liu, P., Tian, Z., Dong, Y., Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes.2018.06.016

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Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton

Wang Xu

a*

, Fulai Liu

a, b*

, M. Santosh

c, d

, Pinghua Liu

a, b

, Zhonghua Tian

a, b

,

Yongsheng Dong e a

Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

b

Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Beijing 100037,

China c

School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing

100083, China d

Department of Earth Sciences, University of Adelaide, Adelaide SA 5005, Australia

e

College of Earth Science, Jilin University, Changchun 130061, China

* Corresponding author. Email address: [email protected] (Wang Xu); [email protected] (Fulai Liu)

Abstract The Paleoproterozoic Jiao-Liao-Ji Belt (JLJB) in the North China Craton, a major terrane boundary, has remained controversial with respect to its tectonic history. Here we report LA–ICP–MS U–Pb zircon ages, and whole-rock elemental and isotopic compositions of mafic rocks intruding into the North and South Liaohe groups, with the aim of evaluating their petrogenetic affinities and metamorphic

history, and their implications for the tectonic evolution of the JLJB. The mafic rocks in the North Liaohe Group dominantly composed of meta-diabase and meta-gabbro, whereas mafic rocks in the South Liaohe Group are mainly (garnet-) amphibolite. LA–ICP–MS zircon U–Pb analyses yield crystallization and metamorphic ages of ca. 2130 Ma and ca. 1880 Ma, respectively, suggesting that they are coeval, and were metamorphosed at the same time. Whole-rock elemental and isotopic compositions suggest tholeiitic affinity for the protoliths of both of them, with variable SiO2 and MgO concentrations and Mg# and εNd(t) values. They both display enriched mid-oceanic-ridge basalt (E-MORB)-like trace element patterns with enrichment in light rare earth elements and depletion in some high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). The data suggest derivation of the melts from a depleted asthenospheric

mantle,

coupled

with

fractional

crystallization

and

crustal

contamination. Some of the mafic rocks in the South Liaohe Group have lower HFSE (e.g., Nb, Ta, Zr, Hf, and Ti) concentrations, and La/Sm and Th/La ratios, and higher U/Th ratios than their counterparts in the North Liaohe Group, possibly suggesting enhanced metasomatism by subduction-related fluids and/or melts. The geochemical features of the mafic rocks from JLJB contrast with those of typical intra-continental rifts and volcanic arcs, and may have formed in a back-arc basin. Our study supports the presence of a ca. 2.2–2.1 Ga back-arc basin that formed due to northward subduction and subsequent closure of an oceanic plate, resulting in arc–continent collision and formation of the JLJB at ca. 1.9 Ga.

Keywords Mafic rocks; Liaohe Group; Jiao-Liao-Ji Belt; Back-arc basin; North China Craton

1. Introduction The North China Craton (NCC), one of the oldest cratons in the world, is composed of a number of major crustal blocks which were amalgamated during the Paleoproterozoic along three major collisional suture: the Trans-North China Orogen, the Khondalite Belt (or the Inner Mongolia Suture Zone) and the Jiao-Liao-Ji Belt (Fig 1a; e.g., Zhao et al., 2005, 2012; Kusky et al., 2007; Santosh, 2010; Zhao and Zhai, 2013; Yang and Santosh, 2017; Tang and Santosh, 2018). These Precambrian orogenic belts have attracted much attention as they provide important clues on cratonic assembly. However, the nature of the tectono-thermal history remains debated, particularly in the case of the JLJB, which records not only Paleoproterozoic orogenesis (e.g., Zhou et al., 2008; Liu et al., 2013a), but also Phanerozoic tectonothermal events related to North China–Yangtze collision (e.g., Yang et al., 2007a; Liu et al., 2017a) and the Pacific subduction (e.g., Yang et al., 2007b). The JLJB has been variably interpreted as: (1) an intra-continental rift, which divided a single crustal block into the Longgang and Langrim blocks and which closed at ca. 1.9 Ga (e.g., Zhang and Yang, 1988; Li et al., 2001, 2005; Zhao et al., 2005; Luo et al., 2008; Wang et al., 2016); and (2) an early magmatic arc that collided with Archean basement at ca. 1.9 Ga (e.g., Bai, 1993; Faure, 2004; Li and Chen, 2014; Meng et al., 2017). On the basis of a comprehensive review of previous studies, Zhao et al. (2012)

suggested that a ca. 2.2–1.9 Ga intra-continental rift divided the Eastern Block into the Longgang and Nangrim blocks, forming an ocean basin. Subsequently, subduction of an oceanic plate and continent–continent collision produced high-pressure (HP) pelitic granulites (Zhou et al., 2008; Tam et al., 2011), ultimately constructing the JLJB at ca. 1.9 Ga. Constraining the tectonic setting of the Paleoproterozoic sedimentary and volcanic successions (such as the North and South Liaohe groups), and associated granitic and mafic intrusions which form the major rock suites in the JLJB is key to reconstructing its tectonic evolution. Sedimentary and volcanic successions of the JLJB can be divided into two zones based on stratigraphy and metamorphic history. Rocks of the northern zone, which consist of the North Liaohe Group in Liaoning, the Laoling Group in Jilin, and the Fenzishan Group in Shandong, are characterized by a clockwise P–T–t path (Fig. 1b; e.g., He and Ye, 1998; Li et al., 2001; Zhao et al., 2005). In contrast, rocks in the southern zone, which comprise the South Liaohe Group in Liaoning, the Ji’an Group in Jilin, the Jingshan Group in Shandong, and the Wuhe Group in Anhui, record an anticlockwise P–T–t path (Fig. 1b; e.g., He and Ye, 1998; Li et al., 2001; Zhao et al., 2005; Lu et al., 2006; Luo et al., 2008; Zhou et al., 2008; Zhao et al., 2012). These differences suggest that the two zones, which are separated by ductile shear zones and faults (e.g., Li et al., 2005), may have a different origin and record a different tectonic evolution. Recent studies have reported mafic and pelitic granulites and garnet amphibolites from the southern zone that show mineral reactions consistent with near-isothermal decompression, suggesting that they record clockwise metamorphic

evolution similar to the rocks from the northern zone (e.g., Tam et al., 2012; Liu et al., 2013b, 2017b; Cai et al., 2017). However, it is unclear whether clockwise P–T–t paths are widely developed in the southern zone. Theoretically, the correlative units within the two zones should have closely similar geochronological, geochemical and isotopic fingerprints related to an intra-continental rift if the rift closure model is accepted. In this study, we compare the zircon U–Pb ages, and geochemical and isotopic compositions of the Paleoproterozoic meta-mafic rocks that intrude the North and South Liaohe groups (Fig. 1c), with the aim of evaluating their tectonic affinity and the nature of the JLJB.

2. Geological setting and samples The JLJB as one of the three large Paleoproterozoic orogenic belts in the NCC (Fig. 1a), is located between the Longgang Block to the north and the Nangrim Block to the south (Fig. 1a, b; Li et al., 2004, 2005; Zhao et al., 2005, 2012; Lu et al., 2006; Liu et al., 2015). The

Longgang

Block

is

dominated

by

Neoarchean

tonalite–trondhjemite–granodiorite (TTG) gneisses, with minor Eo- to Mesoarchean basement rocks (e.g., Wan et al., 2013; Wang et al., 2016). The Nangrim Block has long been considered to represent a single Archean basement in North Korea (Zhao et al., 2006, and references therein). However, recent studies on detrital zircon from rivers running through the Nangrim Block suggest that it is a Paleoproterozoic unit similar to the JLJB (Wu et al., 2016, and references therein).

The JLJB extends approximately NE–SW and consists of greenschist to amphibolite facies metasedimentary and metavolcanic successions with associated granitic and mafic intrusions. The sedimentary and volcanic successions include the Macheonayeong Group in North Korea, the Laoling and Ji’an groups in southern Jilin, the North and South Liaohe groups in eastern Liaoning, the Fenzishan and Jingshan groups in eastern Shandong, and the Wuhe Group in Anhui (Fig. 1b). These rocks were deposited between 2.05 and 1.95 Ga as inferred from the minimum peak age of igneous zircons and the maximum peak age of metamorphic zircons (Luo et al., 2004, 2008; Li et al., 2015; Liu et al., 2015; Meng et al., 2017; Wang et al., 2017a, b). The widely distributed igneous intrusions in the JLJB have ages falling into two main groups. Intrusions emplaced at ca. 2.2–2.1 Ga are interpreted to have formed in an intra-continental rift (e.g., Li et al., 2006; Wang et al., 2016, 2017c) or during oceanic plate subduction (e.g., Li and Chen, 2014; Meng et al., 2014; Yuan et al., 2015; Xu et al., 2018). Those emplaced at ca. 1.85 Ga have compositions consistent with a post-collisional or post-orogenic extensional setting (e.g., Li et al., 2006; Yang et al., 2007, 2017; Liu et al., 2017a). In addition, recent studies have reported high- and medium- pressure pelitic and mafic granulites in the Ji’an, South Liaohe, and Jingshan groups that were metamorphosed at ca. 1.95–1.85 Ga, and which are considered to be the products of continent–continent collision (Zhou et al., 2008; Tam et al., 2012; Liu et al., 2013b; Liu et al., 2015; Cai et al., 2017). The Paleoproterozoic mafic rocks from the Liaodong Peninsula were mainly emplaced into the North Liaohe Group as sills, dykes, and veins (Fig. 2a). A small

volume of mafic rocks intrud into, or are distributed throughout, the South Liaohe Group as veins (Liu et al., 2017b) or tectonic lenses (Fig. 2b). Generally, these mafic rocks show variable degrees of alteration and greenschist to amphibolite facies metamorphism. The mafic rocks in the North Liaohe Group are composed of meta-diabase and meta-gabbro (Fig. 2c) with minor amphibolite, and exhibit an increasing degree of metamorphism from north to south (Xu et al., 2017, 2018). The mafic rocks in the South Liaohe Group consist of amphibolite (Fig. 2d), and some samples contain garnet surrounded by plagioclase corona, indicating near-isothermal decompression (Liu et al., 2017b).

3. Analytical methods 3.1 Zircon U–Pb dating

Three representative mafic samples in the North Liaohe Group (one amphibolite and two meta-gabbros) and three from the South Liaohe Group (three amphibolites) within the JLJB were selected for zircon U–Pb analyses. Zircon CL (cathodoluminescence) imaging and U–Pb isotopic analyses for mafic rocks in the North Liaohe Group were conducted at Beijing Createch Testing Technology Co. Ltd. The instrument and relevant operating principles of zircon preparation, CL imaging and dating for mafic rocks in the North Liaohe Group have been described in detail by Xu et al. (2018). LA–ICP–MS zircon U–Pb analyses for mafic rocks in the South Liaohe Group were performed on transparent zircon grains using a pulsed (GeoLas) 193 nm ArF Excimer (Lambda Physik, Göttingen Germany)

laser ablation system (50 mJ/pulse; 6 Hz) coupled to an Agilent 7500a ICP–MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan, China. The ablation protocol used a spot diameter of 32 μm and zircon 91500 was the external standard to normalize isotopic discrimination. Detailed data processing, including off-line raw data selection, time-drift correction and quantitative calibration, used ICPMSDataCal 8.0 (Liu et al., 2010). Common lead was corrected following Andersen (2002). Concordia diagrams, weighted average age calculation and probability density plotting were conducted using Isoplot/Ex_ver 3.27 (Ludwig, 2003). The zircon U–Pb data for mafic rocks in the North and South Liaohe groups are listed in the supplementary data 1.

3.2 Major and trace element analyses

Fifteen mafic samples in the North Liaohe Group and fourteen from the South Liaohe Group were selected for whole-rock major and trace element analyses at the National Research Center for Geoanalysis of the Chinese Academy of Geological Sciences, Beijing, China. Major element data were determined using X-ray fluorescence (XRF) (PW4400; PANalytical, Almelo, Holland), with analytical uncertainty within 3%. Trace element analyses were performed using inductively coupled plasma-mass spectrometry (ICP-MS) (PE300D) with analytical uncertainty of 5%. The whole-rock major and trace elements data for mafic rocks in the North and South Liaohe groups are listed in Table 1.

3.3 Sm–Nd isotopes

Six mafic samples from the North Liaohe group and nine from the South Liaohe Group were analysed for whole-rock Sm–Nd isotopes using a Finnigan MAT-262 multi-collector mass spectrometer (Bremen, Germany) at the Laboratory for Radiogenic Isotope Geochemistry, University of Science and Technology of China (USTC). Sm–Nd and REEs were isolated on quartz columns. The operating principles and analytical procedures have been described in detail by Chen et al. (2000, 2007). 143

Nd/144Nd values were corrected for mass fractionation by normalization to

146

Nd/144Nd = 0.7219. Analytical uncertainties of

147

Sm/144Nd and Nd isotopic ratios

were 0.5% and 0.003%, respectively. The whole-rock Sm-Nd isotope data for mafic rocks in the North and South Liaohe groups are listed in Table 2.

4. Results 4.1 Zircon U–Pb ages

4.1.1 Mafic rocks in the North Liaohe Group

Two representative meta-gabbro samples (16KD68-1 and D9001-1) and one amphibolite sample (D2066-11) from the North Liaohe Group in the Helan area were selected for LA–ICP–MS zircon U–Pb analyses (Table S1; Fig. 3a–c). Most zircon grains from mafic samples in the North Liaohe Group are subhedral to euhedral and stubby or platy in shape, with crystal lengths of 50–100 μm and length-to-width ratios of 3:2 to 2:1. They are almost transparent in transmitted light images (TL) and gray in

color, and display internal growth zoning in CL images. Generally, these zircons have variable Th (99–4105 ppm) and U (136–1680 ppm) contents with high Th/U ratios (mostly >0.50), suggesting a magmatic origin. In spite of variable lead loss, three samples (16KD68-1, D9001-1 and D2066-1) yielded weighted mean 207Pb/206Pb ages of 2188.2 ± 8.5 Ma (1σ; MSWD = 0.78), 2118.6 ± 6.3 Ma (1σ; MSWD = 0.54) and 2083 ± 13 Ma (1σ; MSWD = 0.91) (Fig. 3a–c), which we interpret as the crystallization ages of the magmas. A few zircon grains from mafic samples in the North Liaohe Group are anhedral to subhedral, with no internal structure in TL and CL images. Some zircon grains from sample D9001-1 have low Th (3.68–76.2 ppm) and U (99.4–134 ppm) concentrations with Th/U ratios mostly <0.05. The other grains from sample D2066-11 have highly variable Th (2.95–5363 ppm) and U (48.2–6199 ppm) contents, and Th/U ratios (0.03–1.69) similar to those of the igneous zircons, implying incomplete transformation from igneous to metamorphic zircons. All these features indicate that these few zircon grains are of metamorphic origin. Zircon grains in two samples (D9001-1 and D2066-11) yielded weighted mean

207

Pb/206Pb ages of

1858 ± 21 Ma (1σ; MSWD = 0.21) and 1921 ± 20 Ma (1σ; MSWD = 2.4) (Fig. 3b, c), interpreted to record the timing of metamorphism.

4.1.2 Mafic rocks in the South Liaohe Group

Three representative amphibolite samples (SJZ07-5, SJZ11-1, and 16KD55-1-1) from the South Liaohe Group in the Sanjiazi area were chosen for LA–ICP–MS zircon U–Pb analyses (Table S1; Fig. 3e–g). A minority of zircon grains from these

amphibolite samples have the same features as those of magmatic grains from mafic samples in the North Liaohe Group described above. Moreover, these zircons have highly variable Th (22.4–3443 ppm) and U (77.8–3006 ppm) concentrations, and high Th/U (mostly 0.25–1.80) ratios. All these features indicate a magmatic origin. Two samples (SJZ11-1 and 16KD55-1-1) yielded weighted mean

207

Pb/206Pb ages of 2119

± 19 Ma (1σ; MSWD = 0.34) and 2063 ± 23 Ma (1σ; MSWD = 0.89) (Fig. 3f, g), respectively, interpreted as magmatic crystallization ages. The majority of zircons in these samples are anhedral and equant in shape, and gray in color with no internal growth zoning in TL and CL images. In addition, these zircons are characterized by low Th (0.53–12.58 ppm) and U (mostly 22.7–79.4 ppm) contents, and variable Th/U ratios (0–2.31, mostly <0.21), consistent with a metamorphic origin. Zircon grains in two samples (SJZ07-5 and 16KD55-1-1) yielded weighted mean

207

Pb/206Pb ages of

1899 ± 18 Ma (1σ; MSWD = 0.57) and 1894 ± 23 Ma (1σ; MSWD = 0.69) (Fig. 3e, g), interpreted as the age of metamorphism.

4.2 Geochemistry

4.2.1 Mafic rocks in the North Liaohe Group

The mafic rocks in the North Liaohe Group show considerable range in SiO2 (46.81–53.94 wt.%), MgO (2.99–11.32 wt.%) and total Fe2O3 (9.32–22.23 wt.%) contents. They are characterized by low TiO2 (mostly 0.51–1.67 wt.%), Al2O3 (mostly 9.64–15.95 wt.%), and CaO (5.46–10.82 wt.%) concentrations. On the Nb/Y–Zr/TiO2 diagram (Winchester and Floyd, 1977; Fig. 4a), all samples plot in the subalkaline

basalt field. Furthermore, almost all samples define a typical tholeiitic trend according to plots in the Zr–Y diagram (MacLean and Barrett, 1993; Fig. 4b). In general, they have low total REE concentrations [ΣREE = 44.01–79.05 ppm, except for sample 3037-1 (116.4 ppm)] and are slightly enriched in light rare earth elements (LREE) with (La/Yb)N values of 1.68–3.93. All of these mafic samples exhibit flat middle to heavy REE (MREE to HREE) chondrite-normalized REE patterns with (Dy/Yb)N ranging from 1.06 to 1.19, except for sample 16SMT06-1 [(Dy/Yb)N = 1.35] (Fig. 5a). In primitive mantle (PM)-normalized trace element diagrams (Fig. 5c), they are depleted in Nb, Ta, P, and Ti. In addition, the mafic rocks in the North Liaohe Group have highly variable (143Nd/144Nd)i ratios (0.509768–0.510078) and εNd(t = 2130 Ma) values (+3.9 to –2.2) (Table 2). Our geochemical results are similar to those reported from other parts of the North Liaohe Group (Figs. 1c, 4a, b, and 5a, c; Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Xu et al., 2017, 2018).

4.2.2 Mafic rocks in the South Liaohe Group

The mafic rocks in the South Liaohe Group exhibit similar geochemical compositions to their counterparts in the North Liaohe Group, albeit showing less variability (Fig. 6; Table 1). They have mafic to intermediate compositions (SiO2 = 45.90–52.10 wt.%, MgO = 5.22–9.04 wt.%, total Fe2O3 = 10.36–16.35 wt.%) with low abundance of TiO2 (mostly 0.68–1.46 wt.%) and Al2O3 (12.98–14.89 wt.%), and plot in the subalkaline (Fig. 4a) and tholeiitic (Fig. 4b) fields. Furthermore, they are slightly enriched in LREE with (La/Yb)N values of 1.36–3.67 and (Dy/Yb)N values of

1.01–1.23

with

no

apparent

Eu

anomalies

(Eu/Eu*

=

0.80–1.19)

in

chondrite-normalized REE plots (Fig. 5b). In the PM-normalized trace element diagram (Fig. 5d), they show depletion in some high field strength elements (HFSEs), notably Nb, Ta, P, Zr, and Ti. These mafic samples display considerable variation in (143Nd/144Nd)i ratios (0.509725–0.510159) and εNd(t = 2125 Ma) values (+5.4 to –3.1) (Table 2). These results are consistent with previous investigations of mafic rocks in the South Liaohe Group (Figs. 1c, 4a, b, and 5b, d; Gao et al., 2017; Li et al., 2017).

5. Discussion 5.1 Similar crystallization and metamorphic ages for mafic rocks in the North and South Liaohe groups

Previous studies on zircon and baddeleyite from mafic rocks in the North Liaohe Group have constrained the emplacement age of these rocks as ca. 2.1 Ga (Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Xu et al., 2018). However, the timing of formation of mafic rocks in the South Liaohe Group has not been systematically evaluated (Gao et al., 2017; Liu et al., 2017b). In addition, the relationship between mafic rocks in the North and South Liaohe groups, which occur as bodies of different sizes (Fig. 1c) and exhibit different degrees of metamorphism (Fig. 2c, d), remains unconstrained. In this study, zircon grains from three mafic samples of the North Liaohe Group in the Helan area, and from three mafic samples of the South Liaohe Group in the Sanjiazi area were analyzed. Abundant igneous and metamorphic zircons were

identified based on the grain morphology and internal structures, Th and U contents, and Th/U ratios (Fig. 3; Section 4.1). Zircons from three mafic samples in the North Liaohe Group yielded weighted mean magmatic crystallization ages ranging from 2188.2 ± 8.5 Ma to 2083 ± 13 Ma, and metamorphic ages from 1921 ± 20 Ma to 1858 ± 21 Ma (Fig. 3a–c). Zircons from three mafic samples in the South Liaohe Group yielded weighted mean crystallization ages ranging from 2119 ± 19 Ma to 2063 ± 23 Ma, and metamorphic ages from 1899 ± 18 Ma to 1894 ± 23 Ma (Fig. 3e–g). These ages, combined with previous data (Yu et al., 2007; Li and Chen, 2014; Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Gao et al., 2017; Liu et al., 2017b; Xu et al., 2018), are consistent with a magmatic age peak at ca. 2130 Ma (n = 277) and a metamorphic age peak at ca. 1880 Ma (n = 142) for mafic rocks in the North Liaohe Group (Fig. 3d), and a magmatic age peak at ca. 2125 Ma (n = 38) and metamorphic age peak at ca. 1875 Ma (n = 141) for mafic rocks in the South Liaohe Group (Fig. 3h). Thus, these data suggest that mafic rocks in the North and South Liaohe groups share statistically indistinguishable Paleoproterozoic magmatic and metamorphic histories.

5.2 Similar geochemical compositions of mafic rocks in the North and South Liaohe groups

Although the mafic rocks in the South Liaohe Group show less variable major element concentrations than mafic rocks in the North Liaohe Group, their geochemical compositions are similar (Section 4.2; Figs. 4–6). In addition, the mafic

rocks in the North and South Liaohe groups have both undergone extensive fractional crystallization of olivine, clinopyroxene, and plagioclase, and accumulation of Fe–Ti oxides (e.g., magnetite and titanite) as described in detail by Xu et al. (2018) (Fig. 6). The petrographic observations and loss on ignition (LOI) values (Table 1) suggest that these mafic rocks in the North and South Liaohe groups have undergone varying degrees of alteration and metamorphism. The Paleoproterozoic age of emplacement (ca. 2130 Ma), combined with subsequent alteration and metamorphism, suggests that mobile elements [e.g., large-ion lithophile elements (LILEs)] were redistributed within these mafic rocks (Wang et al., 2016; Xu et al., 2018). For this reason, only the immobile elements (e.g., HFSEs) are used here to compare the geochemical compositions of mafic rocks in the North and South Liaohe groups. The mafic rocks in the North and South Liaohe groups are broadly geochemically similar. For example, they both have compositional characteristic of tholeiitic basalts (Fig. 4) with considerable variation in εNd(t) values (Table 2). In addition, both exhibit slightly enriched LREE and relatively flat MREE to HREE chondrite-normalized REE patterns (Fig. 5a, b), and have negative Nb–Ta–Ti anomalies in PM-normalized trace element diagrams (Fig. 5c, d). In the Ti–Sm–V diagram (Fig. 7a), which is a useful discriminant of incompatible elements in mafic rocks (Vermeesch, 2006), most mafic samples in the North and South Liaohe groups fall into the MORB field. A similar origin is suggested in a Ti–V plot (Shervais, 1982; Fig. 7b), in which almost all samples have MORB-like compositions, although the Ti and V contents are highly variable due to extensive Ti–V oxide accumulation, as shown by negative correlations

between TiO2 (Fig. 6b), V (not shown), and Mg#. Plots in the Nb/Yb–Th/Yb diagram (Pearce, 2008; Fig. 7c) indicate that the magmas were widely affected by crustal contamination. The Zr–Zr/Y diagram (Pearce and Norry, 1979; Fig. 7d), brings out some of the geochemical distinctions between mafic rocks in the North and South Liaohe groups. Most of these mafic samples plot within the overlapping region between MORB and island-arc basalt (IAB), whereas some mafic samples in the North Liaohe Group with high Zr contents have within-plate basalt (WPB) compositions. Some mafic rocks in the South Liaohe Group with low Zr contents fall in the IAB field. The high Zr contents of some mafic samples in the North Liaohe Group can be interpreted to reflect the addition of zirconium-rich minerals (e.g., zircon) based on the increase in Zr concentration with decreasing Mg# value (not shown). The low Zr concentrations of some mafic samples in the South Liaohe Group may have been inherited from a mantle source that witnessed metasomatism by subduction-related fluids and/or melts rather than by crustal contamination, as the continental crust is usually enriched in Zr (average upper crust = 193 ppm; average lower crust = 68 ppm; Rudnick and Gao, 2014). In summary, the mafic rocks in the North and South Liaohe groups have similar geochemical compositions and likely formed in a tectonic environment similar to a mid-ocean ridge or back-arc basin. The mantle source of mafic rocks in the South Liaohe Group probably underwent more intense metasomatism by subduction-related fluids and/or melts than did the mantle source of mafic rocks in the North Liaohe Group.

5.3 Geochemical comparison with mafic rocks from typical intra-continental rifts

Previous studies on the tectonic evolution of the JLJB considered the opening and closure of an intra-continental rift, and according to this model, a Paleoproterozoic (ca. 2.2–2.0 Ga) rifting event divided a single Archean block into the Longgang and Nangrim blocks, and then closed to form the JLJB (Zhang and Yang, 1988; Li et al., 2005, 2012; Li and Zhao, 2007; Luo et al., 2008; Zhao et al., 2012; Zhao and Zhai, 2013; Wang et al., 2016, 2017). If this model is valid, the associated magmatic rocks (e.g., mafic rocks in the North and South Liaohe groups) that formed at ca. 2.2–2.0 Ga should have geochemical features similar to those of typical intra-continental rifts. To evaluate this model, we compare the JLJB mafic rock data with those on mafic rocks from typical intra-continental rifts. Two types of mafic rocks from these intra-continental rifts are identified: one shows OIB-like patterns (large igneous province; e.g., the East African Rift, the Antarctica Rift, and the Gulf of Suez Rift, http://georoc.mpch-mainz.gwdg.de/georoc/) and the other type corresponds to MORB-like patterns (break-up of supercontinent; e.g., Rodinia, Li et al., 2008, and Columbia, Zhang et al., 2012). Generally, the mafic rocks from the JLJB have similar HREE concentrations to those from large igneous provinces in intra-continental rifts (Fig. 8a). However, the primitive mantle-normalized multi-element patterns of the JLJB mafic rocks are more akin to those of E-MORB (Sun and McDonough, 1989), exhibiting relatively flat LREE to HREE patterns, distinct depletions in Nb and Ta, and low Zr and Ti contents. In contrast, the mafic rocks from large igneous provinces have OIB-like

PM-normalized incompatible trace element patterns (Sun and McDonough, 1989) with significant enrichment of LREE, generally positive Nb and Ta anomalies, and high Zr and Ti concentrations (Fig. 8a). These distinctions suggest that the JLJB mafic rocks have different sources and/or formed in a tectonic setting different to an intra-continental rift producing large igneous province. The Lu/Hf vs. Sm/Nd diagram (Fig. 8b) offers a good opportunity to test this inference, because these ratios can filter the effects of fractional crystallization effectively, and evaluate the source characteristics (e.g., geochemical composition) and petrogenesis (e.g., degree of partial melting) of these mafic rocks (Zhu et al., 2008). In general, the JLJB mafic rocks have high Lu/Hf and Sm/Nd ratios. Most samples, which define a positive correlation between Lu/Hf and Sm/Nd, may record mixing between a depleted asthenospheric mantle source with high εNd(t) and continental upper crust with low εNd(t) (e.g., continental detritus), as suggested by Xu et al. (2018), except for a few samples with higher Lu/Hf and Sm/Nd ratios similar to N-MORB and PM (Figs. 7c and 8b). In contrast, the mafic rocks from large igneous provinces related to intra-continental rifts mostly have low Lu/Hf and Sm/Nd ratios, and the majority plot in the field between OIB and SCLM; with only rare examples showing similarity to MORB. This suggests that the formation of mafic rocks in large igneous provinces related

to

intra-continental

rifts

witnessed

interaction

between

enriched

asthenospheric mantle (e.g., mantle plume and HIMU-like mantle) and lithospheric mantle, with subordinate depleted asthenospheric mantle, but that the effect of crustal contamination is limited, consistent with the lack of negative Nb and Ta anomalies

(Fig. 8a) (e.g., Martin et al., 2013; Aviado et al., 2015; Feyissa et al., 2017; Santosh et al., 2018). These observations suggest that the geochemistry of the JLJB mafic rocks differs from that of mafic rocks in large igneous provinces related to typical intra-continental rift systems, indicating a distinction in the nature of mantle sources. Some of the mafic rocks related to break-up of supercontinent (Fig. 8c, e.g., Columbia, Zhang et al., 2012; Rodinia, Li et al., 2008) show relative depletion in incompatible elements, and they were derived from depleted asthenospheric mantle (e.g., Zhang et al., 2012). These mafic rocks have MORB-like PM-normalized incompatible trace element patterns similar to those of the JLJB mafic rocks (Fig. 8c). Therefore, the JLJB mafic rocks may have similar mantle source and tectonic setting to these MORB-like mafic rocks from break-up of supercontinent. We note that these mafic rocks usually lack depletion in Zr and Hf (c.f. Li et al., 2008; Zhang et al., 2012); on the contrary, the least-contaminated sample of the JLJB mafic rocks is depleted in Zr and Hf, suggesting that the mantle source was metasomatized by subduction-related fluids and/or melts (Xu et al., 2018). On the other hand, the mafic rocks related to break-up of supercontinent, generally show OIB and/or within-plate basalt (WPB) affinities, such as those from the break-up of Rodinia which fall in the OIB field in the Ti–Sm–V diagram (Fig. 8d; Li et al., 2008), and those from break-up of Columbia plotting within the WPB field in the Ti–Zr–Y diagram (Fig. 8d; Zhang et al., 2012). In addition, almost all the mafic rocks related to break-up of supercontinent are characterized by high Zr concentrations and Zr/Y ratios, and they fall within the field of WPB in the Zr–Zr/Y diagram (Fig. 7d). However, the JLJB mafic rocks show

MORB and arc affinities, and they deviate clearly from the OIB and WPB fields in these discrimination diagrams (Figs. 7d and 8d). These features may imply that the JLJB mafic rocks and those related to break-up of supercontinent were formed in different tectonic environments. In summary, the Paleoproterozoic (ca. 2.2–2.1 Ga) mafic rocks in the JLJB did not form in an intra-continental rift.

5.4 Tectonic implications

5.4.1 Back-arc setting for the JLJB

As the major Paleoproterozoic orogenic belt in the Eastern Block of the NCC (Fig. 1a), the JLJB has attracted much attention, and its tectonic history remains equivocal. Two alternative geodynamic models have been proposed to explain the evolution of the JLJB: the closure of an intra-continental rift (Zhang and Yang, 1988; Peng and Palmer, 1995; Li and Zhao, 2007; Luo et al., 2008; Li et al., 2012; Zhao et al., 2012; Zhao and Zhai, 2013; Wang et al., 2016) and arc–continent collision (Bai et al., 1993; Faure et al., 2004; Li et al., 2014; Meng et al., 2014, 2017). The intra-continental rift opening and closing model requires a ca. 2.2-2.0 Ga rifting event to separate the Longgang and Nangrim blocks, allowing deposition of the volcano-sedimentary sequences (e.g., Liaohe Group) and emplacement of associated granitic (e.g., Liaoji granites) and mafic (e.g., mafic rocks in the North and South Liaohe groups) intrusions (e.g., Zhao and Zhai, 2013, and references therein). However, the new geochronological and geochemical data of mafic rocks in the North

and South Liaohe groups presented in this paper, exclude this possibility. If the ca. 2.2–2.1 Ga mafic rocks formed in an intra-continental rift, they should mainly show OIB and/or WPB affinities, as do those from typical intra-continental rift systems (Figs. 7d and 8). This is not the case, and the ca. 2.2–2.1 Ga mafic rocks from the JLJB have different geochemical compositions (Fig. 8), and were derived from a depleted asthenospheric mantle that was contaminated by continental crust (Figs. 7c and 8b; Xu et al., 2018). Thus, we suggest that the formation of the JLJB was related to Paleoproterozoic oceanic plate subduction and subsequent arc–continent collision. Recent studies have discussed some of the details of the arc–continent collision model, such as the location of the magmatic arc. Some workers propose that the ca. 2.2–2.1 Ga mafic rocks exhibit similar geochemical features to arc basalts and thus represent a magmatic arc (Faure et al., 2004; Li et al., 2014; Yuan et al., 2015). Others believe that the volcano-sedimentary sequences and mafic intrusions with arc-like geochemical composition formed in a back-arc setting (Wang et al., 2011; Meng et al., 2014). However, several lines of evidence argue against the presence of a magmatic arc, as summarized by Xu et al. (2018): (1) the magmatic rocks of the JLJB consist mainly of a bimodal series of mafic and granitic intrusions (e.g., Zhang and Yang, 1988; Sun et al., 1993) rather than andesitic rocks, which are typical volcanic arc assemblages (Wilson, 1989); (2) the ca. 2.2–2.1 Ga mafic rocks are tholeiitic (Fig. 4), which is inconsistent with the calc-alkaline nature of typical volcanic arc basalts; and (3) most of the ca. 2.2–2.1 Ga mafic rocks exhibit a depletion in Nb–Ta–Ti, but not Zr and Hf (Fig. 5c, d), which could be interpreted to indicate contamination by upper

continental crust (Figs. 7c and 8b; Xu et al., 2018). Moreover, these features are different from the negative Nb–Ta–Zr–Hf–Ti anomalies that are diagnostic features of typical arc basalts. Therefore, these mafic rocks cannot have formed in a volcanic arc, and more likely formed in a back-arc setting, consistent with limited metasomatism from subduction-related fluids and/or melts, as recognized in the least-contaminated mafic samples in the North Liaohe Group (Xu et al., 2018).

5.4.2 Constraints from sedimentary and metamorphic rocks

Although the precise depositional ages of the Northern zone (i.e., the Laoling, North Liaohe and Fenzishan groups) and the Southern zone (i.e., the Ji’an, South Liaohe, and Jingshan groups) rocks remain debated, it is widely accepted that these two tectonic entities were deposited simultaneously (Lu et al., 2006; Luo et al., 2008; Li et al., 2015a; Liu et al., 2017b; Tian et al., 2017; Wang et al., 2017b; Xu et al., 2018). Detailed U–Pb dating and Hf isotopic analyses on detrital zircons from these two zones show identical populations of ca. 2.1 Ga and ca. 2.5 Ga, and comparable Hf signatures. This suggests that the rocks were derived from a common source, namely the Paleoproterozoic granites (e.g., Liaoji Granites) and Archean basement rocks (e.g., Luo et al., 2004, 2008; Lu et al., 2006; Meng et al., 2017; Wang et al., 2017a, b). This conclusion is supported by the similar geochemical compositions of the North and South Liaohe groups (Fig. 9b, c; Li et al., 2015b; Meng et al., 2017). These observations further suggest that the protoliths of these two zones formed in the same tectonic setting. From this point of view, deposition in a single back-arc basin can

reasonably explain the similarities between these two zones. Recently, several researchers reported medium- and high-pressure metamorphic rocks in the southern zone that were metamorphosed at ca. 1.95–1.85 Ga, including pelitic and mafic granulites, and garnet amphibolite (Zhou et al., 2008; Tam et al., 2012; Liu et al., 2013b, 2017b; Cai et al., 2017). All the metamorphic rocks record a clockwise P–T–t path that is consistent with oceanic plate subduction and subsequent continent–continent collision, to bring the pelitic and mafic rocks to the depths of the middle–lower crust. From this perspective, subduction and the development of a back-arc basin offers a plausible geodynamic setting for the formation of these metamorphic rocks.

5.4.3 Subduction polarity of the oceanic plate

Typical arc magmatism, as evidence for the arc–continent collision model, has not been identified in the study area. Consequently, the polarity of the subducting Paleoproterozoic oceanic plate is debated. Some researchers prefer south-directed subduction (e.g., Faure et al., 2004; Li et al., 2014; Chen et al., 2016), whereas others favor north-directed subduction (e.g., Yang et al., 2015; Yuan et al., 2015), although strong evidence is lacking. On the basis of the geochemical similarities between the mafic rocks in the North and South Liaohe groups, we offer a possible interpretation. The mafic rocks in the South Liaohe Group have similar formation ages and geochemical and isotopic compositions to mafic rocks in the North Liaohe Group (Figs. 3–8), suggesting the

similar nature of the mantle source (e.g., asthenospheric mantle) and magmatic evolution (e.g., fractional crystallization and continental crustal contamination). It has been shown that strong linear correlations between LREE/MREE, LREE/HREE, and εNd(t) is effective in evaluating the effects of contamination by continental crust (e.g., Yang et al., 2007a). The mafic rocks in the North Liaohe Group show the influence of a mantle source on the upper crust (Fig. 9a; Xu et al., 2018). In contrast, some mafic samples in the South Liaohe Group with highly variable εNd(t) values, but relatively low and constant La/Sm ratios (Fig. 9a), indicate a different petrogenesis, possibly reflecting variable degrees of partial melting (e.g., Aldanmaz et al., 2000) or a different evolution (e.g., the input of a third end-member). Furthermore, the Th/La, Nb/La, and Sm/La ratios, which usually are not affected by fractionation and can trace the effects of assimilation effectively, even in subduction zones (Plank, 2014), suggest that this discrepancy could reflect the variable assimilation of crustal materials between the mafic rocks in the North and South Liaohe groups. In Sm/La and Nb/La vs. Th/La diagrams (Fig. 9b, c), all of the wall rocks of these mafic rocks, including the sedimentary–volcanic sequences of the Liaohe Group, the ca. 2.2–2.1 Ga granitic rocks, and the ca. 2.5 Ga Archean basement as the provenance of the Liaohe Group (Luo et al., 2008; Liu et al., 2015; Wang et al., 2017a, b), coincide with the linear correlation defined by mafic rocks in the North Liaohe Group. However, they do not correspond with the trend defined by mafic rocks in the South Liaohe Group, which have low Th/La ratios similar to Island arc basalts (Kelemen et al., 2014). This observation excludes a specific rock type as the third end-member involved in the

formation of mafic rocks in the South Liaohe Group. Thus, we infer that the mafic rocks in the South Liaohe Group were metasomatized by subduction-related fluids and/or melts to a greater degree than mafic rocks in the North Liaohe Group. This inference is supported by two lines of evidence: (1) some mafic samples in the South Liaohe Group have low Th, Nb, Ta, Zr, Hf, and Ti concentrations comparable to subduction-zone basalts (Tatsumi and Eggins, 1995), which are lower than those of mafic samples in the North Liaohe Group (Figs. 7d and 8a); and (2) the mafic samples in the South Liaohe Group generally have higher U/Th ratios than mafic samples in the North Liaohe Group (Fig. 9d), which could be attributed to saline (e.g., NaCl) fluids that, in arc melts, can produce U/Th ratios several times higher than those in MORB (Bali et al., 2011). In other words, the mantle source of mafic rocks in the South Liaohe Group was possibly closer to an island arc than that of mafic rocks in the North Liaohe Group, suggesting northward subduction of the oceanic plate (Fig. 10). Although this inferred subduction polarity is based on geochemical data, more work is needed to verify the interpretation. In the present study, a petrogenetic model involving opening and closure of a back-arc basin is proposed. Northward subduction of an oceanic plate induced the opening of a ca. 2.2–2.1 Ga back-arc basin, and continuous subduction and subsequent arc–continent collision closed the back-arc basin and formed the JLJB at ca. 1.9 Ga.

6. Conclusions On the basis of geochronological, geochemical, and isotopic data from mafic rocks in the North and South groups of the Jiao-Liao-Ji Belt, we draw the following conclusions. (1) The mafic rocks in the North and South Liaohe groups were both emplaced at ca. 2130 Ma and metamorphosed at ca. 1880 Ma. (2) They exhibit similar geochemical and isotopic features, indicating that they were derived from the same mantle source and underwent a similar magmatic evolution. (3) The mafic rocks in the Liaohe Group display different geochemical composition as compared the mafic rocks of typical intra-continental rifts and volcanic arcs, and possibly formed in a back-arc setting. (4) A ca. 2.2–2.1 Ga back-arc basin opened via northward subduction of an oceanic plate, and subsequently closed to form the Jiao-Liao-Ji Belt at ca. 1.9 Ga.

Acknowledgements We are grateful to Lishuang Liu, Lei Ji, Fang Wang, Jia Cai, Hong Yang, and Lei Zou for help during the field work, and Fukun Chen for assistance in Sm–Nd isotopic analyses. We thank Associate Editor Jinhui Yang and two anonymous reviewers for their constructive reviews and comments. This work was supported by the National Natural Science Foundation of China (Grant 41430210), Chinese Geological Survey Bureau project (Grant DD20160121), and Basic Scientific Foundation of CAGS

(Grant YYWF201703).

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Figure captions Fig. 1 (a, b) Tectonic framework of NCC and JLJB (modified from Zhao et al., 2005); (c) Simplified geological map of central Liaodong Peninsula illustrating the distribution of Paleoproterozoic mafic rocks in the Liaohe Group (modified from LBGMR, 1975a, b, c, 1976). The rough boundary between NLH and SLH is after Wang et al. (2011). NCC—North China Craton; KB—Khondalite Belt; TNCO— Trans North-Central Orogen; JLJB—Jiao-Liao-Ji Belt. Previous data are from Meng et al. (2014); Li and Chen. (2014); Yuan et al. (2015); Wang et al. (2016); Li et al. (2017); Xu et al. (2017, 2018); Gao et al. (2017)

Fig. 2 (a) Meta-diabase showing concordant intrusive contact with marble from the Dashiqiao Formation, North Liaohe Group; (b) Amphibolite as the tectonic lenticle in marble from Dashiqiao Formation, South Liaohe Group; (c) Photomicrograph of representative meta-gabbro with plagioclase undergoing variable degrees of sericitization, North Liaohe Group; (d) Photomicrograph of representative amphibolite, South Liaohe Group. Pl-plagioclase; Hbl-hornblende; Act-actinolite

Fig. 3 (a–c) Representative mafic samples in the North Liaohe Group with concordia plots and cathodoluminescence (CL) images. (e–g) Representative mafic samples in the South Liaohe Group with concordia diagrams and CL images. (d, h) Binned frequency histograms of zircon ages from this study and precious works for mafic rocks in the North and South Liaohe groups to show their formation ages and metamorphic ages. Sources of previous data: mafic rocks in the North Liaohe Group (Yu et al., 2007; Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Xu et al., 2018); mafic rocks in the South Liaohe Group (Li and Chen, 2014; Gao et al., 2017; Liu et al., 2017b)

Fig. 4 (a) Nb/Y-Zr/TiO2 diagram for geochemical classification (Winchester and Floyd, 1977). (b) Zr-Y diagram distinguishing tholeiitic and calc-alkaline series (MacLean and Barrett, 1993). Sources of previous data: mafic rocks in the North Liaohe Group (Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Xu et al., 2017, 2018); mafic rocks in the South Liaohe Group (Li et al., 2017; Gao et al., 2017)

Fig. 5 (a, b) Chondrite-normalized REE patterns for mafic rocks in the North and South Liaohe groups. (c, d) Primitive mantle (PM) normalized trace element diagrams for mafic rocks in the North and South Liaohe groups. The elements are arranged in order of increasing incompatibility from right to left. The chondrite and primitive mantle values are from Sun and McDonough (1989). Previous data are the same as

those in Fig. 4

Fig. 6 Various oxide plots against Mg# for mafic rocks in the North and South Liaohe groups, Liaodong Peninsula: (a) SiO2 ; (b) TiO2; (c) Al2O3 ; (d) TFe2O3; (e) CaO; (f) P2O5. Previous data are the same as those in Fig. 4

Fig. 7 Tectonic discrimination diagrams for mafic rocks in the North and South Liaohe groups. (a) Ti-Sm-V diagram (Vermeesch, 2006). (b) Ti-V diagram (Shervais, 1982). (c) Nb/Yb-Th/Yb diagram (Pearce, 2008). (d) Zr-Zr/Y diagram (Pearce and Norry, 1979). Mafic rocks related to break-up of Columbia and Rodinia are from Zhang et al. (2012) and Li et al. (2008), respectively. OIB—oceanic-island basalt; CFB— continental flood basalt; WPB —within-plate basalt; E-MORB—enriched mid-oceanic-ridge basalt; N-MORB — normal mid-oceanic-ridge basalt; IAB — island-arc basalt. Previous data are the same as those in Fig. 4

Fig. 8 (a) Primitive mantle (PM)-normalized incompatible trace element spidergrams and (b) Lu/Hf vs. Sm/Nd diagram of the JLJB mafic rocks for geochemical comparison with the mafic rocks showing OIB-like patterns that are from large igneous province in intra-continental rift systems (i.e., East African, Antarctica and Gulf of Suez rifts) in the world. (c) Primitive mantle (PM)-normalized incompatible trace element spidergrams and (d) Ti–Sm–V (Vermeesch, 2006) and Ti–Zr–Y (Pearce and Cann, 1973) diagrams of the JLJB mafic rocks for geochemical comparison with

the mafic rocks showing MORB-like patterns that are related to break-up of supercontinent (e.g., Columbia and Rodinia). Data sources: East African, Antarctica and Gulf of Suez rifts (http://georoc.mpch-mainz.gwdg.de/georoc/); PM, OIB, E-MORB and N-MORB (Sun and McDonough, 1989); SCLM (subcontinental lithospheric mantle) (McDonough, 1990); LC (lower crust) and UC (upper crust) (Rudnick and Gao, 2014); Mafic rocks related to the break-up of Columbia (Zhang et al., 2012) and Rodinia (Li et al., 2008). CAB — calc-alkali basalt. The other abbreviations as in caption for Fig. 7. Previous data are the same as those in Fig. 4

Fig. 9 (a) La/Sm vs. εNd(t), (b) Nb/La and (c) Sm/La vs. Th/La, and (d) La/Yb vs. U/Th diagrams for mafic rocks from the JLJB. Data sources: mafic rocks in the North Liaohe Group (This study; Meng et al., 2014; Yuan et al., 2015; Wang et al., 2016; Xu et al., 2017, 2018); mafic rocks in the South Liaohe Group (This study; Li et al., 2017; Gao et al., 2017); North Liaohe Group (Meng et al., 2017); South Liaohe Group (Li et al., 2015b); ca. 2.2–2.1 Ga granite (Yang et al., 2015; Wang et al., 2017); ca. 2.5 Ga Archean basement (Wan et al., 2015)

Fig. 10 Schematic illustration showing that the northward subduction of oceanic plate opened a ca. 2.2–2.1 Ga back-arc basin and subsequently formed the Liaohe Group and the mafic intrusions reported in this study.

Table captions Table 1 Major (wt%) and trace (ppm) elements for mafic rocks in the North and South Liaohe groups

Table 2 Sm–Nd isotopic composition of mafic rocks in the North and South Liaohe groups, Liaodong Peninsula

Table 1 Major (wt%) and trace (ppm) elements for mafic rocks in the North and South Liaohe groups Mafic rocks in the North Liaohe Group Sample

16KD68-1-1

16KD68-1-2

16KD68-1-4

16KD68-1-5

D2066-11.1

D2097-1

D3037-1

D4021-1

D5019-1.1

D5053-1

D9001-1

D9002-3.1

D9002-8.1

D3019-1

16SMT06-1

Location

E123.31076°

E123.31076°

E123.31076°

E123.31076°

E123.37111°

E123.36500

E123.35277°

E123.39111°

E123.38833°

E123.49833°

E123.27444°

E123.47083°

E123.41638°

E123.41361°

E123.43000°

N40.85733°

N40.85733°

N40.85733°

N40.85733°

N40.93722°

N40.99305

N40.87666°

N40.95916°

N40.86638°

N40.96972°

N40.85277°

N40.92027°

N40.88527°

N40.92000°

N40.90111°

SiO2

53.94

53.11

53.23

53.16

50.55

47.40

47.53

51.19

49.4

46.81

49.03

49.43

50.29

50.57

49.35

TiO2

0.54

0.52

0.52

0.51

1.32

2.11

3.22

0.67

2.51

2.17

1.58

0.71

1.36

1.67

0.76

wt%

Al2O3

10.77

10.54

10.77

9.64

15.33

11.18

10.96

14.79

13.56

17.12

13.15

15.66

13.51

14.33

15.95

Fe2O3

2.70

3.15

2.9

2.61

3.11

9.20

3.73

2.03

3.35

4.65

5.30

1.88

3.11

2.69

1.78

FeO

6.38

6.02

6.09

6.45

10.54

9.47

16.65

6.56

13.78

8.68

8.53

7.24

10.4

10.76

8.14

MnO

0.17

0.18

0.17

0.17

0.21

0.24

0.29

0.15

0.23

0.19

0.20

0.12

0.21

0.21

0.13

MgO

10.37

10.54

10.46

11.32

4.61

6.19

3.64

8.58

2.99

3.88

6.57

9.34

5.91

6.64

10.81

CaO

9.73

10.41

9.86

10.31

9.39

8.20

7.87

10.53

7.58

7.94

9.15

10.66

10.82

5.46

6.26

Na2O

2.06

2.08

2.35

1.87

2.99

2.69

2.35

2.59

3.19

3.19

2.63

2.53

1.87

2.87

3.65

K2O

1.18

1.17

1.18

1.04

0.50

0.86

0.87

0.80

1.00

1.53

0.90

0.47

0.46

1.08

0.21

P2 O 5

0.07

0.07

0.06

0.06

0.12

0.14

0.23

0.06

0.28

0.10

0.14

0.07

0.12

0.15

0.07

LOI

1.23

1.24

1.41

1.22

0.54

0.38

0.07

1.36

0.22

1.92

1.16

0.98

0.82

1.47

1.99

Total

99.14

99.03

99.00

98.36

99.21

98.06

97.41

99.31

98.09

98.18

98.34

99.09

98.88

97.90

99.10

Mg#

68

68

68

70

38

38

24

65

24

35

47

65

44

47

66

ppm Li

4.61

4.37

4.91

3.70

5.25

14.2

5.25

9.29

5.57

20.7

12.1

6.25

8.25

12.1

30.4

Be

0.84

0.73

0.80

0.62

0.67

0.65

1.13

0.42

1.17

0.67

0.79

0.54

0.71

0.73

0.57

Sc

43.6

44.2

44.9

47.6

42.9

51.7

44.2

36.8

35.4

29.2

49.5

40.8

40.6

42.3

36.2

V

184

188

189

208

317

628

771

184

364

490

350

208

305

365

232

Cr

1324

1387

1353

1670

16.1

4.13

2.12

175

1.27

35.7

126

200

128

107

165

Co

41.0

41.0

43.5

44.4

50.7

66.9

72.0

43.7

43.8

50.1

55.3

47.2

50.2

48.5

44.3

Ni

160

172

194

167

29.2

39.3

11.3

23.7

17.4

36.9

40.9

27.9

46.1

71.4

27.0

Ga

12.3

12.1

12.3

11.5

21.4

19.6

25

14.2

23.9

26.2

20.3

17.1

19.2

21.8

11.7

Rb

33.0

30.6

35.1

27.8

23.8

30.4

16.6

26.4

36.4

70.2

57.3

11.2

11.1

34.6

4.59

Sr

130

172

155

108

364

117

99.7

446

191

385

140

400

270

160

229

Y

16.7

17.1

16.9

16.3

27.2

26.4

42.3

13.8

55.1

19.6

29.6

15.3

25.5

32.6

14.9

Zr

73.8

69.9

57.2

54.9

97.0

93.6

219

54.7

231

98.7

117

59.3

95.2

122

54.7

Nb

4.04

3.54

3.57

3.62

5.83

5.38

11.6

3.34

12.2

5.44

6.39

3.74

5.74

6.81

3.49

Cs

0.52

0.48

0.48

0.39

0.61

0.34

0.39

0.64

0.61

4.04

0.60

0.54

0.19

0.80

0.48

Ba

116

243

156

286

80.2

107

217

172

302

250

200

228

123

371

192

La

8.82

9.31

9.49

8.90

8.19

8.23

15.1

6.58

12.6

8.20

9.80

6.33

8.99

9.82

6.26

Ce

20.7

19.9

20.5

20.1

18.8

18.9

34.8

13.9

34.6

18.2

21.3

13.8

19.9

22.8

15.1

Pr

2.57

2.64

2.62

2.66

2.68

2.68

5.04

1.84

4.96

2.56

3.23

1.91

2.93

3.33

1.95

Nd

11.4

11.8

11.4

11.9

12.6

12.3

23.5

7.98

23.8

11.9

15.2

8.47

13.5

15.5

9.35

Sm

2.60

2.59

2.54

2.65

3.70

3.52

6.50

2.02

6.94

3.38

4.27

2.26

3.76

4.50

2.37

Eu

0.74

0.72

0.72

0.73

1.35

1.19

2.27

0.66

2.23

1.33

1.52

0.90

1.33

1.51

0.49

Gd

2.67

2.73

2.59

2.69

4.60

4.24

7.23

1.98

7.96

3.51

5.17

2.60

4.66

5.24

2.95

Tb

0.43

0.43

0.44

0.45

0.81

0.78

1.33

0.40

1.50

0.66

0.91

0.47

0.79

0.99

0.49

Dy

2.82

2.84

2.75

2.79

5.09

4.82

8.06

2.48

9.18

3.79

5.52

2.83

4.81

6.04

2.87

Ho

0.62

0.61

0.61

0.60

1.12

1.07

1.76

0.54

2.02

0.79

1.21

0.62

1.06

1.20

0.57

Er

1.73

1.75

1.75

1.72

3.10

2.96

4.79

1.49

5.62

2.05

3.29

1.69

2.90

3.62

1.62

Tm

0.26

0.25

0.25

0.25

0.47

0.44

0.71

0.23

0.84

0.30

0.49

0.25

0.44

0.54

0.22

Yb

1.78

1.77

1.73

1.67

3.01

2.86

4.58

1.40

5.37

1.89

3.11

1.62

2.79

3.42

1.42

Lu

0.27

0.27

0.26

0.25

0.49

0.46

0.73

0.22

0.87

0.29

0.50

0.26

0.45

0.54

0.20

Hf

2.25

2.15

1.84

1.79

2.67

2.56

5.38

1.50

5.64

2.67

3.18

1.63

2.67

3.46

1.71

Ta

0.33

0.27

0.28

0.29

0.49

0.45

0.93

0.30

1.00

0.43

0.52

0.32

0.49

0.45

0.26

Pb

6.34

14.3

9.14

6.02

5.20

0.99

2.40

3.23

2.42

3.43

1.35

5.13

3.44

4.13

3.06

Th

3.31

2.74

2.71

2.79

1.76

1.42

3.25

2.17

4.05

2.23

1.62

2.05

2.05

1.60

1.80

U

0.83

0.80

0.74

0.61

0.37

0.33

0.72

0.38

0.72

0.40

0.35

0.39

0.45

0.47

0.41

Mafic rocks in the South Liaohe Group Sample

SJZ06-1

SJZ07-3

SJZ07-5

SJZ10-1

SJZ11-1

SJZ23-1

16KD07-1

16KD55-1-1

16KD55-1-2

16KD55-1-3

16KD59-1

16KD61-3-3

16KD61-5-1

16KD61-5-2

location

E123.22305°

E123.22494°

E123.22494°

E123.18947°

E123.27670°

E123.44342°

E125.12442°

E123.26592°

E123.26592°

E123.26592°

E123.28496°

E123.27642°

E123.27542°

E123.27542°

N40.71901°

N40.70168°

N40.70168°

N40.71303°

N40.71737°

N40.75564°

N40.63336°

N40.73715°

N40.73715°

N40.73715°

N40.74067°

N40.71954°

N40.71640°

N40.71640°

SiO2

46.74

50.30

52.10

50.32

49.52

51.14

48.13

50.98

51.04

50.94

45.90

49.25

48.06

48.00

TiO2

1.29

0.69

1.17

0.68

1.98

0.90

1.01

0.88

0.88

0.94

1.46

1.86

1.37

1.33

wt%

Al2O3

14.12

14.04

13.05

14.31

13.02

14.74

13.64

14.70

14.89

14.51

13.17

12.98

14.48

14.41

Fe2O3

4.40

2.13

3.24

2.35

4.78

5.99

1.86

2.52

2.08

2.47

5.35

4.29

3.58

3.42

FeO

8.68

8.28

11.8

8.10

10.26

4.26

10.44

7.13

7.45

7.49

9.25

9.97

9.25

9.39

MnO

0.19

0.19

0.22

0.19

0.25

0.16

0.21

0.18

0.17

0.18

0.48

0.24

0.19

0.19

MgO

7.88

8.26

5.22

8.09

5.43

7.65

9.04

7.69

7.55

7.86

7.10

5.90

6.95

6.89

CaO

10.51

11.63

9.18

11.05

8.60

9.76

10.19

11.20

11.22

11.27

10.11

9.44

9.98

9.89

Na2O

2.67

1.17

1.64

2.60

2.16

2.79

2.38

2.37

2.45

2.28

2.30

2.27

2.73

2.64

K2O

1.03

0.89

0.72

0.57

0.94

0.89

0.57

0.46

0.47

0.45

1.57

0.82

0.98

0.99

P2 O 5

0.13

0.05

0.11

0.05

0.18

0.07

0.09

0.07

0.08

0.07

0.15

0.14

0.13

0.14

LOI

1.03

1.10

0.50

0.77

1.63

1.04

0.62

0.84

0.73

0.78

1.46

0.97

0.94

0.95

Total

98.67

98.73

98.95

99.08

98.75

99.39

98.18

99.02

99.01

99.24

98.30

98.13

98.64

98.24

Mg#

53

59

39

59

40

59

57

59

59

59

47

43

50

50

13.9

7.32

6.23

6.25

10.7

7.28

6.38

6.04

Be

0.62

0.71

2.02

0.95

0.77

0.52

0.47

0.38

0.42

0.38

0.76

0.64

0.64

0.75

Sc

42.2

44.5

45.9

46.4

46.6

37.7

43.3

37.4

42.0

43.6

40.6

43.9

38.0

38.9

ppm Li

V

336

272

352

269

501

220

275

252

254

267

312

366

302

302

Cr

167

198

12.1

58.9

88.9

60.9

157

152

152

161

107

32.6

161

158

Co

57.7

47.0

50.6

48.3

58.3

37.1

57.8

41.2

40.5

42.4

51.7

53.7

47.6

49.3

Ni

80.1

88.1

25.8

41.6

56.1

15.2

83.4

49.9

51.5

52.3

55.7

40.2

59.9

58.6

Ga

19.4

14.4

19.7

16.7

20.1

13.1

17.3

15.5

16.4

16.1

19.6

18.5

18.6

18.7

Rb

44.1

52.6

18.9

15.0

38.2

38.5

10.5

10.4

10.9

10.2

54.7

23.9

21.9

22.0

Sr

204

162

109

224

203

351

315

191

200

189

207

165

257

256

Y

21.1

18.3

29.3

12.2

29.1

16.3

22.0

15.4

15.5

16.8

26.1

32.0

23.7

24.4

Zr

81.3

30.0

72.2

32.1

113

71.8

61.3

49.3

48.4

49.2

97.1

103

83.7

86.9

Nb

4.18

1.45

3.22

1.64

6.75

3.65

4.27

2.75

2.88

3.13

6.22

6.92

5.19

5.08

Cs

0.82

1.70

0.65

1.01

0.62

1.19

0.23

0.32

0.32

0.40

0.53

0.34

0.57

0.64

Ba

167

145

82.9

133

213

229

121

125

125

127

369

180

149

180

La

8.21

4.87

6.54

3.80

11.1

9.71

6.38

4.79

5.05

5.07

10.0

10.2

8.08

8.41

Ce

18.3

8.36

14.0

8.77

26.3

21.0

15.3

11.2

11.6

12.0

23.1

24.6

19.3

20.5

Pr

2.56

1.35

2.13

1.26

3.60

2.77

2.22

1.64

1.67

1.74

3.23

3.5

2.83

2.98

Nd

12.0

6.25

10.3

5.86

16.6

12.0

10.9

8.15

8.10

8.62

15.9

17.4

13.6

14.9

Sm

3.42

2.20

3.53

1.78

4.84

3.10

2.92

2.13

2.17

2.28

3.96

4.61

3.47

3.70

Eu

1.27

0.76

1.28

0.75

1.65

0.87

1.10

0.91

0.88

0.93

1.42

1.71

1.21

1.28

Gd

4.15

3.17

5.16

2.35

5.98

3.55

3.36

2.58

2.62

2.82

4.41

5.41

4.23

4.26

Tb

0.71

0.57

0.91

0.41

1.04

0.59

0.57

0.44

0.42

0.45

0.75

0.89

0.69

0.67

Dy

4.08

3.45

5.50

2.41

5.75

3.48

3.68

2.80

2.76

3.00

4.73

5.73

4.19

4.41

Ho

0.93

0.81

1.28

0.57

1.29

0.78

0.80

0.60

0.59

0.64

0.99

1.22

0.87

0.91

Er

2.68

2.41

3.71

1.63

3.66

2.13

2.28

1.70

1.65

1.76

2.80

3.43

2.46

2.49

Tm

0.38

0.35

0.54

0.23

0.52

0.3

0.33

0.24

0.22

0.24

0.40

0.49

0.35

0.37

Yb

2.39

2.28

3.44

1.44

3.37

1.90

2.15

1.58

1.54

1.68

2.62

3.25

2.32

2.40

Lu

0.38

0.34

0.52

0.22

0.53

0.29

0.33

0.24

0.23

0.24

0.39

0.49

0.35

0.35

Hf

2.41

1.26

2.37

1.22

3.43

2.25

1.90

1.64

1.55

1.61

3.01

3.25

2.53

2.56

Ta

0.36

0.25

0.47

0.19

0.56

0.37

0.31

0.22

0.22

0.23

0.43

0.46

0.37

0.37

Pb

3.59

3.98

5.90

5.83

5.16

5.58

0.91

5.20

5.62

5.81

4.16

6.63

4.25

4.22

Th

0.86

0.71

1.47

0.55

1.38

1.50

0.85

0.66

0.71

0.75

1.18

1.64

1.00

1.04

U

0.82

0.71

0.75

0.14

0.29

0.22

0.21

0.16

0.15

0.17

0.41

0.46

0.66

0.68

Mg# = Mg/(Mg+Fe2+)

Table 2 Sm–Nd isotopic composition of mafic rocks in the North and South Liaohe groups, Liaodong Peninsula Sample

Age(Ma)

Sm(ppm)

Nd(ppm)

147

Sm/144Nd

143

Nd/144Nd

2δ

εNd(t)

TDM1 (Ma)

TDM2 (Ma)

Mafic rocks in the North Liaohe Group 16KD68-1-1

2130

2.55

11.20

0.1379

0.509844

10

-0.7

2743

2616

D2066-11.1

2130

3.67

12.49

0.1775

0.509881

7

0.0

3256

2557

D2097-1

2130

3.69

12.90

0.1730

0.509768

15

-2.2

3546

2734

D5053-1

2130

3.42

12.05

0.1717

0.510078

14

3.9

2395

2246

D9001-1

2130

4.50

16.03

0.1698

0.509887

8

0.2

3038

2547

D9002-8.1

2130

3.94

14.13

0.1684

0.509884

4

0.1

3021

2553

Mafic rocks in the South Liaohe Group 16KD07-1

2130

2.601

9.708

0.1619

0.510009

11

2.6

2547

2356

16KD55-1-1

2130

2.102

8.045

0.1580

0.510154

15

5.4

2126

2128

16KD59-1

2130

3.505

14.07

0.1506

0.510059

8

3.5

2352

2276

SJZ06-1

2130

3.216

11.29

0.1723

0.509774

10

-2.1

3502

2725

SJZ07-3

2130

2.09

5.938

0.2128

0.509718

13

-3.2

61658

2813

SJZ07-5

2130

3.3

9.629

0.2072

0.509868

13

-0.2

8576

2577

SJZ10-1

2130

1.5

4.938

0.1836

0.509752

13

-2.5

4125

2760

SJZ11-1

2130

4.847

16.62

0.1763

0.509771

11

-2.1

3660

2730

2130

3.081

11.93

0.1562

0.509775

10

-2.0

3117

SJZ23-1 The

147

144

Sm/

Nd and

143

144

Nd/

Nd ratios at the present time are 0.1967 and 0.512638 for chondrite, and 0.2137 and 0.51315 for depleted mantle, respectively. The

147

144

Sm/

Nd ratio is 0.118 for crust. λ

2724 147

Sm=6.54×10

-12 -1

a .

Highlights 1. ~2130 Ma mafic rocks both in the North and South Liaohe Groups of the Jiao-Liao-Ji Belt and these rocks have undergone metamorphism at ca. 1880 Ma. 2. Geochemically they are different from those formed in intra-continental rifts and volcanic arcs. 3. The Jiao-Liao-Ji belt was involved the opening of a back-arc basin above the northern subduction of oceanic plate and the closure of this basin at ca. 1900 Ma.

Graphical abstract