Abstract

Latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatic rocks outcrop in the Dunhuang terrane, represented by A-type granites and mafic (basaltic) rocks that have metamorphosed into amphibolites. The A-type granites, emplaced at ca. 1.79–1.77 Ga, are geochemically characterized by high Na2O + K2O, Fe2O3T, Zr, Nb, and Ce contents, as well as high Fe2O3T/(Fe2O3T + MgO) and Ga/Al ratios. Furthermore, they have Nb/Ta, Y/Nb, Rb/Nb, and Sc/Nb ratios of 12.10–15.56, 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, showing affinity to A2-type granite. The A-type granites have negative εNd(t) values (−5.4 to −4.8) with Neoarchean depleted mantle (TDM2) ages (2591–2494 Ma), corresponding to coupling between εHf(t) values (−4.85 to -0.92) and TDM2 ages (2817–2556 Ma) of zircons. Therefore, the A-type granite pluton was mostly generated by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex in a postcollisional tectonic setting following a late Paleoproterozoic continent-continent collisional event. The metamafic rocks have a protolith age of 1605 ± 45 Ma and metamorphic age of 317 ± 20 Ma, indicating a Paleozoic tectonic event. The metamafic rock samples are geochemically characterized by relatively high alkali (Na2O + K2O = 4.39–4.81 wt%) contents and low Nb/Y (0.63–0.66) ratios, and they show steep rare earth element (REE) patterns with light REE enrichment and insignificant Eu anomalies and Nb-Ta, Zr-Hf, and Ti anomalies, resembling subalkaline oceanic-island basalt affinity. They have positive εNd(t) values (+0.8 to +1.8) close to the chondrite evolutionary line and variable εHf(t) values (-1.09 to +9.06) of zircons. Hence, the protolith of the metamafic rocks may have been produced by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source during the initial rifting stage in an extensional setting, completing the formation of the Precambrian Dunhuang Complex. Considering the ca. 1.85–1.80 Ga regional metamorphism in the Dunhuang terrane, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A2-type granitic magmatism and mafic magmatism documented the postorogenic to initial rifting processes following the global-scale late Paleoproterozoic collisional event, which is comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, but different from that of the Tarim craton.

INTRODUCTION

The assembly, accretion, and breakup history of the Columbia supercontinent is very significant for understanding early Precambrian geodynamic processes and thus has caused hot debates during the last decades (Condie, 2002; Rogers and Santosh, 2002; Zhao et al., 2002a, 2004; Ernst et al., 2008). It has been widely accepted that the amalgamation of the Columbia supercontinent was completed through a global-scale collisional event ca. 2.1–1.8 Ga, forming a series of Paleoproterozoic orogens (Zhao et al., 2002a, 2003, 2004), while the breakup of this supercontinent occurred at ca. 1.6–1.2 Ga, as indicated by continental rifting, anorogenic magmatism, and mafic dike swarms in cratonic blocks across Columbia (Zhao et al., 2002a, 2003, 2004; Gao et al., 2013; Li et al., 2015; Gibson et al., 2018). Therefore, latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatism may have played an important role in initiating the transition from assembly to breakup of the Columbia supercontinent and can provide key clues for understanding the origin of microcontinents formed during the breakup of the Columbia supercontinent.

A-type granites, characterized by high Na2O + K2O contents, Ga/Al and Fe2O3T/(Fe2O3T + MgO) ratios, and high field strength element (HFSE; i.e., Zr, Nb, Ce, Y) concentrations, are usually generated in extensional tectonic settings, such as a postcollisional environment or anorogenic setting (e.g., Loiselle and Wones, 1979; Collins et al., 1982; Whalen et al., 1987; Eby, 1992; Bonin, 2007). They were rarely produced before the middle Paleoproterozoic but were extensively developed since the latest Paleoproterozoic (Anderson and Bender 1989; Dall’Agnol et al., 2012). Additionally, A-type granites are genetically related to continental crust and probably indicate a geodynamic transition (Loiselle and Wones, 1979; Collins et al., 1982; Bonin, 2007). They can also be used to constrain the termination of syncollisional compression during an orogeny (Loiselle and Wones, 1979; Whalen et al., 1987; Eby, 1992; Bonin, 2007). Basaltic rocks are one of the most widespread rocks on Earth, and they can erupt in various tectonic settings, such as intra-oceanic, convergent margin, passive margin, and intracontinental settings, indicative of different geodynamic environments (Pearce and Cann, 1973; Pearce and Norry, 1979; Wood et al., 1979; Shervais, 1982; Meschede, 1986; Pearce, 2008; Safonova et al., 2016). Incompatible and geochemical elements are critical in determining the tectonomagmatic environments of basaltic rocks, and several discrimination diagrams based on these elements have been proposed (e.g., Pearce and Cann, 1973; Pearce and Norry, 1979; Wood et al., 1979; Pearce, 2008). The integrated study of geochronology, petrogenesis, and tectonic setting of basaltic rocks can provide significant evidence for understanding regional geodynamic evolution.

The Dunhuang terrane witnessed a complicated geological history during the Mesoarchean–Paleoproterozoic and Paleozoic Eras, as indicated by the Precambrian Dunhuang Complex and Paleozoic granitoid-metamafic rocks, respectively. Recent studies have mostly focused on Archean crustal evolution (Zhang et al., 2013; Zong et al., 2013; Zhao et al., 2013, 2015a, 2015b), as well as Paleozoic tectonic evolution (Zong et al., 2012; He et al., 2014; Zhao et al., 2016; Wang et al., 2016a, 2016b, 2016c, 2017a, 2017b, 2017d, 2018; Shi et al., 2018; Xu et al., 2019). However, the geodynamic mechanism and tectonic setting of the latest Paleoproterozoic magmatism have rarely been discussed, and both the A-type granites formed at ca. 1.77 Ga and the metamafic rock with a protolith age of ca. 1.6 Ga are considered to be products of the breakup of the Columbia supercontinent, leading to some confusion (He et al., 2013; Yu et al., 2014; Wang et al., 2014). Additionally, the formation and affinity of the Precambrian Dunhuang Complex still lack direct evidence. A key issue associated with the termination of the Paleoproterozoic orogeny can be addressed by constraining the formation of the Precambrian Dunhuang Complex. Geologically, the latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatism is developed in the Dunhuang terrane, which is significant for understanding the above scientific issues.

In this contribution, we present systematic geochronological, geochemical, zircon Lu-Hf, and whole-rock Sm-Nd isotopic data for the latest Paleoproterozoic (ca. 1.8–1.6 Ga) A-type granites and basaltic intrusions in the Dunhuang terrane to (1) confirm their crystallization ages; (2) clarify their petrogenesis, magma sources, and tectonic settings; and (3) provide clues for understanding the affinity of the Precambrian Dunhuang Complex.

GEOLOGICAL BACKGROUND

The Dunhuang terrane, located between the Tarim craton and the North China craton, is a broadly triangular-shaped terrane that was initially regarded as a stable part of the Tarim craton (Fig. 1A). It is confined by the Beishan orogen to the north, the Alxa block to the east, and the Altyn Tagh–Aketashtage to the southwest (Fig. 1B). To the west and the southeast, the Qiemo-Xingxingxia fault and the Altyn Tagh fault separate the Dunhuang terrane from the Tarim craton and the Qilian orogen–Quanji Massif, respectively (Fig. 1B). The latest studies have confirmed that the Dunhuang terrane is constituted by the Precambrian Dunhuang Complex (Zhang et al., 2013; Zong et al., 2013; Zhao et al., 2013, 2015a, 2015b, 2017; Wang et al., 2013b; Zhao and Sun, 2018) and a Paleozoic magmatic-metamorphic complex (Zong et al., 2012; He et al., 2014; Zhao et al., 2015c, 2016, 2017; Wang et al., 2016a, 2016b, 2016c, 2017a, 2017b, 2017d, 2018). It is now considered to be a mobile belt that underwent intensive reworking during a Paleozoic orogeny (Zhao et al., 2016; Wang et al., 2016a, 2017a, 2017b; Shi et al., 2018).

The Precambrian Dunhuang Complex is mainly composed of Archean tonalite-trondhjemite-granodiorite (TTG) gneisses and Paleoproterozoic supracrustal rocks that underwent amphibolite- to granulite-facies metamorphism and strong deformation, as well as a small amount of Paleoproterozoic migmatites, and granodiorite-granite plutonic and mafic magmatic rocks. The TTG rocks were predominantly generated in three stages of ca. 3.1 Ga, ca. 2.7–2.6 Ga, and ca. 2.6–2.5 Ga, and metamorphosed at ca. 2.0–1.8 Ga and ca. 450–310 Ma (Zhang et al., 2013; Zong et al., 2013; Zhao et al., 2013, 2015a, 2015b). The Paleoproterozoic supracrustal rocks are composed mainly of quartzite, (garnet-/sillimanite-bearing) mica-quartz schist, biotite–plagioclase gneiss, and marble with a depositional age of ca. 2.03–1.86 Ga and metamorphic age of ca. 1.83–1.80 Ga (Wang et al., 2013b; Zhao et al., 2019). The Paleoproterozoic magmatism occurred in two periods of ca. 2.05–1.82 Ga (Zhang et al., 2013; Zhao and Sun, 2018) and ca. 1.80–1.60 Ga (Yu et al., 2014; Wang et al., 2014). The ca. 2.05–1.82 Ga magmatism produced arc- and back-arc–related granitoids and basaltic rocks (Zhao and Sun, 2018), combined with a ca. 2.0–1.8 Ga metamorphic age of TTG and supracrustal rocks and ca. 1.85–1.83 Ga granulite-facies metamorphism (Zhang et al., 2012a), possibly suggesting assemblage of the Dunhuang Complex to the Columbia supercontinent. The ca. 1.80–1.60 Ga magmatism, which will be discussed in detail in this contribution, was previously described as a response to the breakup of the Columbia supercontinent (Yu et al., 2014; Wang et al., 2014).

The Paleozoic magmatic-metamorphic rock series were produced generally through orogenic process related to the closure of the southernmost Paleo–Asian Ocean, representatively composed of ca. 440–360 Ma arc-related granitoids (Wang et al., 2016b, 2016c; Zhao et al., 2017), (garnet-bearing) amphibolite/high-pressure mafic granulite with a metamorphic age of ca. 440–350 Ma (Zong et al., 2012; He et al., 2014; Zhao et al., 2016; Wang et al., 2016a, 2017a, 2017b, 2017d, 2018), and ca. 335–315 Ma postcollisional potassic granites (Zhao et al., 2017). Recently, however, based on the latest paleogeographic study, Xu et al. (2019) suggested that the Paleo–Asian Ocean between the Tarim craton and the Dunhuang terrane was still open in the Early Permian (ca. 290–280 Ma).

In this paper, our study areas were Huoyanshan and Hongliuxia (Figs. 2A and 2B). The Huoyanshan area is located northeast of the Sanweishan area (Fig. 1C), with E-W– and NE-SW–trending faults and outcropping Paleoproterozoic mica-quartz schist, marble, and granite pluton, as well as Paleozoic granitoids (Fig. 2A). The Hongliuxia area is located in the southernmost margin of the Dunhuang terrane (Fig. 1C), with NW-SE– and NNE-SSW–trending faults and outcrops composed of Precambrian Dunhuang Complex and Paleozoic granites (Fig. 2B; Zhao et al., 2019).

SAMPLE DESCRIPTION

The identified A-type granite (samples HYS09 and 18HYS09) and metamafic rock (samples HLX26 and 18HLX26) outcrop in the Huoyanshan area (global positioning system [GPS] location: 40°04′56.7″N, 94°56′17.2″E) and the Hongliuxia area (GPS location: 39°43′22.69″N, 95°23′29.08″E), respectively (Figs. 1C, 2A, and 2B). Six granite samples and five metamafic rock samples were selected for whole-rock major- and trace-element analyses. Two representative samples of each kind of rock were chosen for whole-rock Sm-Nd isotopic analyses. One representative sample of each rock was used for zircon U-Pb dating and zircon Lu-Hf analyses.

The Huoyanshan granite, showing massive and gneissic structures, emplaced into the Paleoproterozoic supracrustal rocks of the Precambrian Dunhuang Complex and was intruded by late mafic dikes (Figs. 3A and 3B). It is composed of medium- to coarse-grained minerals, i.e., K-feldspar (∼40 vol%), plagioclase (∼30 vol%), quartz (∼25 vol%), and biotite (∼5 vol%), with accessory minerals of zircon, apatite, and Ti-Fe oxide (Fig. 3C). The K-feldspar is dominated by perthite, the plagioclase is often identified by its polysynthetic crystal twinning, and the biotite is generally flake-like in shape and has been partially altered to chlorite (Fig. 3C). Based on field and microscopic characteristics, this kind of granite could be named as alkali-feldspar granite.

The metamafic rocks occur as lenses within deformed supracrustal rocks (garnet-bearing mica-quartz schist/mica-quartz schist) of the Precambrian Dunhuang Complex that were intruded by late Carboniferous potassic granite, showing gneissic structure (Figs. 3D and 3E). Based on field observation, the metamafic rock has documented two phases of deformation. The first phase occurred before emplacement of late Carboniferous granite, and the second phase occurred at the same time as when the late Carboniferous granite emplaced. It is made up of amphibole (∼62 vol%), plagioclase (∼35 vol%), and minor quartz (<3 vol%), with accessory minerals of zircon, apatite, magnetite, and titanite (Fig. 3F). The coarse-grained amphibole is green in color, shows oblique cleavage planes, and is strongly elongated and orientated, and the plagioclase is xenomorphic granular in shape with zircon inclusions (Fig. 3F). This kind of metamafic rock is amphibolite, according to the mineral assemblage.

ANALYTICAL METHODS

Whole-Rock Major and Trace Elements

Fresh chips of bulk-rock samples were powdered to 200 mesh using a tungsten carbide ball mill and then dried in an oven at 105 °C for 2 h. In total, 0.7 g of sample powder was mixed with 5.2 g of Li2B4O7, 0.4 g of LiF, 0.3 g of NH4NO3, and minor LiBr in a platinum pot, and the mixture was fused into glass beads prior to major-element analyses (Wang and Liu, 2016). Major-element contents were measured using a Rigaku RIX 2100 X-ray fluorescence (XRF) spectrometer at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, with analytical uncertainties generally lower than 5% for all the elements. Trace-element concentrations were determined using an Agilent 7700a inductively coupled plasma–mass spectrometer (ICP-MS) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, and at Nanjing FocuMS Technology Co., Ltd., employing international rock standards (BHVO-2, AGV-2, BCR-2, and GSP-1) of the U.S. Geological Survey (USGS). For most trace elements, the relative deviation and relative standard deviation were lower than 5% (Liu et al., 2007).

Whole-Rock Nd Isotopes

Whole-rock high-precision Nd isotope measurements were made using an Agilent Technologies 7700x quadrupole ICP-MS (Hachioji, Tokyo, Japan) at Nanjing FocuMS Technology Co., Ltd. Neodymium was purified from the same digestion solution by two-step column chemistry: The first exchange column with BioRad AG50W-X8 was used to separate rare earth elements (REEs), and Nd was separated from the other REEs on the second column with Ln Spec-coated Teflon powder. Raw data of isotopic ratios were corrected for mass fractionation by normalizing to 146Nd/144Nd = 0.7219. International isotopic standard JNdi-1 was periodically analyzed to correct instrumental drift. Geochemical reference materials USGS BCR-2, AGV-2, and BHVO-2 were used as quality control, with determined 146Nd/144Nd ratios of 0.512636, 0.512797, and 0.512986, respectively. The εNd(t) values were calculated using present-day chondritic 147Sm/144Nd and 143Nd/144Nd ratios of 0.1967 and 0.512638, respectively (Wasserburg et al., 1981). Whole-rock Nd depleted mantle model ages TDM1 and TDM2 were calculated using present-day 147Sm/144Nd = 0.2136 and 143Nd/144Nd = 0.51315 for depleted mantle (Liew and Hofmann, 1988).

Zircon U-Pb Dating and Trace-Element Analyses

Zircon grains were separated using conventional heavy liquid and magnetic techniques, and then they were handpicked under a binocular microscope at the Institute of Regional Geology and Mineral Resources Survey, Langfang City, Hebei Province, China. Representative zircon grains were selected and mounted in an epoxy resin and then were polished until their centers were exposed. Prior to zircon U-Pb dating, cathodoluminescence (CL) images of zircons were taken using a Gatan Mono CL 3+ fluorescence spectrometer to reflect their internal texture. Laser-ablation (LA) ICP-MS zircon U-Pb isotopes and trace elements were measured using an Agilent 7500a ICP-MS instrument equipped with a 193 nm ArF-excimer laser and a homogenizing, imaging optical system at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. A fixed spot size of 32 mm for zircons from granite samples and 24 mm for zircons from metamafic samples, and a laser repetition rate of 6 Hz were adopted throughout the analysis. Helium was used as the carrier gas to provide efficient aerosol delivered to the torch. Detailed analytical procedures were described by Yuan et al. (2004). The standard silicate glass NIST 610 was used to optimize the instrument to obtain a maximum signal intensity and low oxide production. Isotopic ratios 207Pb/206Pb, 207Pb/235U, 206Pb/238U, and 208Pb/232Th were calculated using the GLITTER 4.0 program. The Harvard zircon 91500 with a recommended 206Pb/238U age of 1064.2 ± 1.7 Ma was used as the external standard to correct both instrumental mass bias and depth-dependent elemental and isotopic fractionation. Trace-element concentrations of U, Th, and Pb were calibrated using 29Si as an internal standard and NIST SRM 610 as an external standard. Concordia diagrams and age calculations were completed using Isoplot 3.0 software (Ludwig, 2003).

Zircon Lu-Hf Isotopic Analyses

In situ zircon Lu-Hf isotopes were analyzed using a Nu Plasma high-resolution multicollector (MC) ICP-MS (Nu Instrument Ltd., Wrexham, UK) equipped with a GeoLas 193 nm excimer laser-ablation system at State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. A beam size of 44 μm and a repetition rate of 10 Hz were adopted during the analysis. The isobaric interference of 176Lu on 176Hf was corrected by measuring the intensity of an interference-free 175Lu isotope, and a recommended 176Lu/175Lu ratio of 0.02655 was used to calculate the 176Lu/177Hf ratio. The interference of 176Yb on 176Hf was corrected by measuring an interference-free 172Yb isotope, and the 176Hf/177Hf ratio was calculated using a 176Yb/172Yb ratio of 0.5886 (Chu et al., 2002). Time-dependent drifts of Lu-Hf isotopic ratios were corrected using a linear interpolation according to the variations of zircon standards 91500, Monastery, and GJ-1. Detailed information about analytical procedure was described by Yuan et al. (2008). The present-day chondritic ratios of 176Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332, as reported by Blichert-Toft and Albarède (1997), were adopted to calculate εHf values at the time when the zircons crystallized from magma. Single-stage zircon Hf model ages (TDM1) were calculated based on a depleted mantle source with a present-day 176Hf/177Hf ratio of 0.28325 and 176Lu/177Hf ratio of 0.0384 (Griffin et al., 2000). Two-stage Hf model ages (TDM2) were calculated by projecting the initial 176Hf/177Hf ratio of zircon back to the depleted mantle growth curve using 176Lu/177Hf = 0.015 for the average continental crust (Griffin et al., 2002).

ANALYTICAL RESULTS

Whole-Rock Major- and Trace-Element Compositions

Six granite samples (HYS09–1, HYS09–2, 18HYS09–3, 18HYS09–4, 18HYS09–5, and 18HYS09–6) from northeast of the Sanweishan area and five metamafic rock samples (HLX26, 18HLX26–1, 18HLX26–2, 18HLX26–3, and 18HLX26–4) from the Hongliuxia area were collected for whole-rock geochemical analyses. The major- and trace-element compositions are listed in Table 1.

The studied granite samples have relatively high contents of SiO2 (70.69–72.35 wt%), K2O (4.63–5.81 wt%), Na2O (3.24–4.04 wt%; Fig. 4A), and Fe2O3T (2.86–3.99 wt%) and relatively low contents of Al2O3 (13.12–13.82 wt%), CaO (1.17–1.26 wt%), and MgO (0.12–0.17 wt%), with total alkali (Na2O + K2O) contents of 8.52–9.32 wt% and Mg# values of 9.03–10.89. They are characterized by potassium-rich features in the K2O versus SiO2 diagram (Fig. 4B) and mostly belong to metaluminous rock series (Fig. 4C), with A/CNK (= Al2O3/[CaO + Na2O + K2O]) and A/NK (= Al2O3/[Na2O + K2O]) ratios of 0.96–0.99 and 1.15–1.19, respectively. The rock samples are ferruginous, with Fe2O3T/(Fe2O3T + MgO) ratios varying from 0.94 to 0.96 (Fig. 4D). Major element compositions of metamafic rock (Fig. 5) will be described later. These granite samples have high REE contents (∑REE = 370–586 ppm) and are characterized by moderately fractionated chondrite-normalized REE patterns (light REE/heavy REE [LREE/HREE] = 9.64–15.3, [La/Yb]N = 10.0–18.9, and [Gd/Yb]N = 1.44–1.93) and significantly negative Eu anomalies (δEu = 0.37–0.46; Fig. 6A). In the primitive mantle–normalized trace-element variation spidergrams, similar to A-type granites, they are characterized by remarkable depletion of Ba, Nb, Ta, Sr, and Ti but enrichment of Rb, La, Pb, Nd, and Sm (Collins et al., 1982; Whalen et al., 1987), with most samples displaying insignificant Zr-Hf anomalies (Fig. 6B). As shown in Figures 4 and 6, geochemical signatures of the studied samples are similar as those of Sanweishan A-type granite as described by Yu et al. (2014).

The studied metamafic rocks have SiO2 = 48.07–50.01 wt%, Fe2O3T = 16.95–17.78 wt%, TiO2 = 3.49–3.83 wt%, MnO = 0.22–0.25 wt%, Al2O3 = 12.21–12.67 wt%, MgO = 4.13–4.43 wt%, CaO = 6.40–7.48 wt%, Na2O = 3.05–3.25 wt%, K2O = 1.18–1.76 wt%, and P2O5 = 0.38–0.46 wt%, with low Mg# and A/CNK values of 36.22–36.89 and 0.61–0.66, respectively. The higher TiO2 contents and moderate MnO contents suggest that they are ortho-amphibolite (Fig. 5A). The low to moderate Nb/Y ratios (0.63–0.66) and low contents of SiO2 and low ratios of Zr/TiO2 indicate subalkaline basaltic rock series for these metamafic rocks (Figs. 5B and 5C). The higher Fe2O3T and TiO2 contents but lower MgO and Al2O3 contents in these metamafic rocks show an affinity to high-Fe tholeiite (Fig. 5D). Different from previously published data, the metamafic rock samples have relatively higher REE abundances (∑REE = 193–224 ppm), displaying fractionated REE patterns (LREE/HREE = 5.52–6.01, [La/Yb]N = 5.89–6.66, and [Gd/Yb]N = 2.49–2.63) and insignificant Eu anomalies (δEu = 0.90–0.96; Fig. 6C). They are characterized by oceanic-island basalt (OIB)–like patterns in the primitive mantle–normalized trace-element spidergrams, showing slightly positive Nb-Ta anomalies and slightly negative Zr-Hf anomalies, with insignificant Ti anomalies (Fig. 6D). Concentrations of compatible elements (Cr, V, and Ni) within these metamafic rocks are relatively higher (Table 1). As shown in Figures 5 and 6, the geochemical features of the studied metamafic rock samples are different from those of ca. 1.6 Ga Hongliuxia amphibolites as described by Wang et al. (2014).

Nd Isotopic Characteristics

The whole-rock Sm-Nd isotopic data and calculated initial isotopic ratios for representative samples are listed in Table 2. Age values of 1786 Ma and 1605 Ma were assigned to calculate the Nd radiogenic isotopic composition of granite and metamafic rock samples, respectively, at the time of magma crystallization.

The granite samples (18HYS09–3 and 18HYS09–4) have 147Sm/144Nd and 143Nd/144Nd ratios of 0.094169–0.103324 and 0.511266–0.511187, respectively, with initial 143Nd/144Nd ratios ranging from 0.510052 to 0.510081. The age-calculated εNd(t) values vary from −5.4 to −4.8, corresponding to single-stage Nd model age (TDM1) and two-stage Nd model age (TDM2) of 2591–2494 Ma and 2715–2670 Ma, respectively (Fig. 7), consistent with the data published by Yu et al. (2014).

The metamafic rock samples (18HLX26–1 and 18HLX26–3) have 147Sm/144Nd and 143Nd/144Nd ratios of 0.137035–0.142750 and 0.512099–0.512111, respectively, with initial 143Nd/144Nd ratios of 0.510604–0.510653. Their age-calculated εNd(t) values range between +0.8 and +1.8, corresponding to TDM1 of 2229–2087 Ma (Fig. 7).

Zircon U-Pb Geochronology

Zircon grains separated from representative samples of granite (HYS09) and metamafic rock (HLX26) were taken for geochronological analyses. Representative CL images of zircons are shown in Figure 8. Zircon U-Pb dating results are listed in Table 3 and shown in Figure 8.

Zircons from the granite sample (HYS09) are brown in color, transparent, and subhedral-euhedral in shape, with lengths up to 120–300 mm and aspect ratios varying from 1:1 to 4:1, respectively (Fig. 8A). CL images revealed that all the zircon grains have clear planar/oscillatory zoning (Fig. 8A). Totally 29 spots were analyzed. All the age data yielded 207Pb/206Pb ages ranging from 1689 ± 34 Ma to 1833 ± 24 Ma and plot on a well-defined discordant line with an upper-intercept age of 1786 ± 20 Ma (mean square of weighted deviates [MSWD] = 2.6), indicative of radiogenic Pb loss (Fig. 8B). These zircons showed apparent oscillatory zoning and high Th/U ratios of 0.44–0.70, indicating a typical magmatic origin (Rubatto, 2002; Belousova et al., 2002; Hoskin and Schaltegger, 2003). Thus, the upper-intercept age of 1786 ± 20 Ma is interpreted as the crystallization age of the granite pluton.

Zircons from the metamafic rock sample (HLX26) are colorless, transparent, and rounded-ovoid grains, with lengths and width/length ratios of 80–100 mm and 1:3–1:1, respectively. All the grains showed core-rim structures in CL images (Fig. 8C). Most of the dark zircon cores are characterized by high and variable Th and U contents of 488–4269 ppm and 516–1336 ppm, respectively, with Th/U ratios mainly ranging between 0.49 and 3.20 (Table 3). The bright zircon rims had low Th and U contents of 2.37–12.4 ppm and 48–189 ppm, respectively, with Th/U ratios mostly varying from 0.02 to 0.09 (Table 3). It is notable that the zircon rims have obviously low Th/U ratios than zircon cores, possibly indicating fluid metasomatism during metamorphism. In summary, the zircon cores are possibly magmatic in origin, and the zircon rims are metamorphic in origin (Hoskin and Black, 2000; Rubatto, 2002; Belousova et al., 2002; Corfu et al., 2003; Hoskin and Schaltegger, 2003). In total, 28 spot analyses were conducted on 21 zircon grains, of which 20 age data on zircon cores yielded a discordant line with an upper-intercept age of 1605 ± 45 Ma (MSWD = 5.1; Fig. 8D), interpreted as the crystallization age of the protolith of this metamafic rock sample. Another eight analytical data spots conducted on zircon rims plot on a discordant line with a lower-intercept age of 317 ± 20 Ma (MSWD = 0.57; Fig. 8D). This age is interpreted to be the metamorphic age of the studied metamafic rock sample, which is consistent with the crystallization age of potassic granite intruding into the metamafic rock, within error (Zhao et al., 2017).

Zircon Lu-Hf Isotopic Composition

Zircon Lu-Hf isotopic analyses were performed on the same or partially superimposed position of the same grains that have been taken for U-Pb chronological analyses (Figs. 8A and 8C). The analytical results are listed in Table 4 and shown in Figure 9.

In total, 29 analyses were implemented on zircons from the granite sample (HYS09). These analyzed spots showed homogeneous Hf isotope compositions, giving a narrow range of 176Hf/177Hf ratios from 0.281543 to 0.281644 (Table 4). Their age-corrected εHf(t) values varied from −4.85 to -0.92 calculated at 1786 Ma, corresponding to TDM1 and TDM2 ages of 2433–2266 Ma and 2817–2556 Ma, respectively, with the TDM2 age showing a peak at ca. 2.74 Ga (Fig. 9). This data set is slightly different from that published by Yu et al. (2014), which showed negative to positive εHf(t) values and relatively older TDM2 ages compared to the studied samples.

Twenty-eight Lu-Hf isotopic analyses were conducted on zircons from the metamafic rock sample (HLX26), of which eight zircon rims had 176Hf/177Hf ratios varying from 0.281930 to 0.282065, consistent with those of the zircon cores, ranging between 0.281860 and 0.282059 (Table 4). This may indicate that the Lu-Hf isotopic compositions of the zircon rims remained unchanged, though their U-Pb system had been reset during late metamorphism. Thus, the age of 1605 Ma was assigned to calculate εHf(t) values and zircon Hf model ages of the analyzed spots. Their age-corrected εHf(t) values ranged between -1.09 and +9.06, corresponding to variable TDM1 ages of 2108–1694 Ma (Fig. 9).

DISCUSSION

Petrogenesis

A2-Type Signature of Granite

The studied granite samples are characterized by relatively high SiO2 (70.69–72.35 wt%), Na2O + K2O (8.52–9.32 wt%), and Fe2O3T (2.86–3.99 wt%), but low MgO (0.12–0.17 wt%) contents (Table 1), with an average Fe2O3T/(Fe2O3T + MgO) ratio of 0.96, belonging to the ferroan rock series (Frost et al., 2001). They have high large ion lithophile element (LILE) concentrations (e.g., Rb, Ba, Th, and U), have low CaO and P2O5 contents, and are strongly depleted in Eu, Sr, and Ti (Figs. 6A and 6B), resembling A-type granite (e.g., King et al., 1997; Pankhurst et al., 2013). Moreover, they have relatively high contents of Zr + Nb + Ce + Y (626–973 ppm), Zr (358–646 ppm), and Nb (26.1–47.4 ppm), as well as high 10,000 × Ga/Al (3.67–4.24) ratios (Table 1; Fig. 10), comparable with typical A-type granite featured by enrichment of Na2O + K2O and incompatible elements such as Zr, Nb, and Ce, and high Fe2O3T/(Fe2O3T+MgO) and 10,000 × Ga/Al ratios (Loiselle and Wones, 1979; Collins et al., 1982; Whalen et al., 1987; Creaser et al., 1991; Bonin, 2007). A-type granitic rocks usually formed, separated, or crystallized under high temperatures (e.g., Creaser et al., 1991; King et al., 2001), and bulk-rock zircon saturation temperatures (TZr) could provide a useful estimate of initial magma temperature of the source (Miller et al., 2003; Boehnke et al., 2013). It has been accepted that TZr provides minimum estimates of temperature if the magma was undersaturated, but maximum estimates if it was saturated (Miller et al., 2003). Zirconium contents of the studied granite samples varied from 358 to 646 ppm, corresponding to calculated zircon saturation temperatures of 845–902 °C, with an average of 871 °C (Table 1). No inherited cores were observed in the zircon CL images, suggesting that the initial magma of the granite might have been undersaturated in zircon (Miller et al., 2003). Accordingly, the average temperature of 871 °C could be considered as a minimum temperature of initial magma, consistent with typical A-type granites (Creaser et al., 1991; King et al., 2001). All these clues support the interpretation that the studied granite is typical ferroan and high-temperature A-type granite.

Eby (1992) subdivided A-type granitoids into two chemical groups. The A1 type is characterized by element ratios consistent with those of OIB, representing differentiates of magmas derived from OIB-like sources but emplaced in continental rifts or during intraplate magmatism. The A2 type is characterized by element ratios varying from those of continental crust to those of island-arc basalt (IAB), representing magmas derived from continental crust or underplated mafic crust that has been through a cycle of continent-continent collision or island-arc magmatism (Eby, 1992). The studied granite samples have Nb/Ta and Zr/Hf ratios of 12.10–15.56 and 38.35–40.57, respectively, identical with those of continental crust (Nb/Ta ≈ 11–17.5; Zr/Hf ≈ 36.7; Taylor and McLennan, 1985; Wedepohl, 1995; Gao et al., 1998), but different from those of OIB (Nb/Ta ≈ 17.78; Zr/Hf ≈ 35.9; Sun and McDonough, 1989). In addition, their Y/Nb (1.45–1.79), Yb/Ta (1.73–2.56), and Ce/Nb (4.33–7.83) trace-element ratios are higher than those of OIB, but lower than those of IAB (Eby, 1992). Furthermore, they have Y/Nb, Rb/Nb, and Sc/Nb ratios of 1.45–1.79, 3.52–6.51, and 0.11–0.19, respectively, mostly plotting within the A2-type field in rock type discrimination diagrams (Figs. 11A and 11B; Eby, 1992). In the Nb-Y-Ce and Nb-Y-3×Ga triangular diagrams, the studied granite samples also mostly fall within the A2-type field near the boundary of A1 and A2 types (Figs. 11C and 11D). In the tectonic setting discrimination diagrams, all the samples plot in the post-collisional granite field, rather than the within-plate granite field (Fig. 12). These geochemical features most likely indicate that the studied granite belongs to A2-type granite.

OIB-Type Signature and High Fractional Crystallization of the Protolith of Metamafic Rock

The geochemical affinity of the metamafic rocks can sometimes be difficult to define because of chemical modifications during metamorphism. However, it is generally accepted that REEs and HFSEs are immobile elements during metamorphism and thus can be used to discriminate petrogenesis, magma source, and tectonic setting of the protolith of metamafic rocks (Rudnick et al., 1985; Kerrich et al., 1999). The studied metamafic rock samples have Zr/TiO2 ratios and Ni concentrations of 0.0062–0.0099 and 39.8–47.5 ppm, respectively (Table 1), and in the discrimination diagram of TiO2 versus MnO, they plot in the ortho-amphibolite field (Fig. 5A), suggesting a magmatic origin of their protolith (Misra, 1971; Winchester, 1984). This conclusion is further supported by the field occurrences where the amphibolites outcrop as lenses within the Paleoproterozoic deformed supracrustal rocks, implying that the protolith of the studied metamafic rocks intruded into metasedimentary rocks.

The metamafic rock samples are characterized by relatively high alkali contents (Na2O + K2O = 4.39–4.81 wt%), and steep REE patterns with enrichment in LREE and insignificant Eu anomalies, resembling typical OIB-like basalts (Fig. 6C). The insignificant Nb-Ta and Ti anomalies together with the insignificant to slightly negative Zr-Hf anomalies of the metamafic rock samples are also typical features of OIB-like basalts (Fig. 6D). In the diagrams of SiO2 versus Nb/Y and Zr/ TiO2 versus Nb/Y, all the studied samples plot in the subalkaline basalt field but close to the alkali basalt field (Figs. 5B and 5C), showing OIB affinity. In the Nb/Yb versus Th/Yb diagram by Pearce (2008), all the samples plot between the enriched mid-ocean-ridge basalt (E-MORB) and OIB fields (Fig. 13A). Additionally, they have Th/Nb and TiO2/Yb ratios of 0.08–0.10 and 0.99–1.21, respectively (Table 1), consistent with OIB and displaying closer affinity to oceanic-island basalt–modified lithosphere (OML; Pearce et al., 2017). The studied samples possess low MgO contents (4.13–4.43 wt%) and Mg# values (36.22–36.89), implying that the compositions are not primary magma. The insignificant Eu anomalies (Fig. 6C) may indicate that there was insignificant plagioclase remaining in the source region, whereas the negative Sr anomalies may have been caused by fractionation of clinopyroxene. They have relatively high TiO2 (3.49–3.83 wt%), Fe2O3T (16.95–17.78 wt%), and P2O5 (0.38–0.46 wt%) contents, suggestive of high fractional crystallization. Thus, the protolith of Hongliuxia metamafic rock was highly fractionally crystallized OIB-like basaltic rock formed at ca. 1.6 Ga that was then metamorphosed at ca. 320 Ma.

Magma Source

A-Type Granite: Partial Melting of Neoarchean Continental Crust

Several petrogenetic models have been proposed for possible magma sources of A-type granites: (1) fractional crystallization of mantle-derived basaltic magma (e.g., Turner et al., 1992; Smith et al., 1999; Anderson et al., 2003); (2) partial melting of granulite-facies metasedimentary rocks under high temperature (Collins et al., 1982); (3) further differentiation or crustal contamination of syenite magma formed by mixing of mantle-derived alkaline magma and crustal materials (Dickin, 1994; Charoy and Raimbault, 1994); (4) partial melting of continental crust of tonalitic to granodioritic composition (Creaser et al., 1991); (5) partial melting of lower continental crust caused by thermal upwelling of mantle (Anderson and Bender, 1989; Frost and Frost, 1997; Wu et al., 2002); and (6) mixing of mantle-derived mafic and crustal-derived granitic magmas (Yang et al., 2006). Our granite samples have high alkaline but relatively low Al2O3 contents with metaluminous characteristics, excluding the metasedimentary-melting origin. Mafic microgranular enclaves (MMEs) in nearly all granites, as well as contemporaneous mafic rocks, were not found in the field. In addition, the studied samples are characterized by homogeneous whole-rock Sm-Nd and zircon Lu-Hf isotopic compositions (Figs. 7 and 9), with extremely low Mg# values (9.03–10.89), and Cr (1.25–6.79 ppm), and Ni (0.55–4.24 ppm) contents, precluding the magma mixing process. These A-type granite samples have high contents of Fe2O3T (2.86–3.99 wt%). Additionally, as mentioned before, their zircon saturation temperatures of 845–902 °C are higher than that of fractionated granite (with an average of 780 °C). These lines of evidence eliminate significant magma fractional crystallization of the A-type granite, and their geochemical signatures were probably controlled by partial melting of the magma source. Their chondrite-normalized REE patterns display remarkably negative Eu, Sr, and Ti anomalies, flat and relatively high HREE patterns, and low ratios of [La/Yb]N (10.0–18.9), Gd/Yb (1.74–2.34), and Dy/Yb (1.58–1.85), reflecting the residue of Ca-rich plagioclase, amphibole, and rutile, rather than garnet in their sources. These values suggest that the partial melting occurred at a shallow level less than 8 kbar (Zhang et al., 2011), and the magma source could not have been mafic lower continental crust or underplated mafic rocks in the lower crust. Thus, calc-alkaline intermediate basement rocks of tonalitic to granodioritic compositions are likely to have been the magma source, having generated the A-type melt at 0.4–0.8 GPa and a high temperature of 950 °C (Patiño Douce, 1997). The upwelling of asthenosphere or underplating of mafic magmas could have provided the required high temperature for partial melting. Additionally, they have low Mg# values (9.03–10.89), as well as extremely low contents of Cr (<7 ppm) and Ni (<5 ppm), also suggestive of continental crust origin.

Whole-rock Nd isotope analyses reveal negative εNd(t) values (−5.4 to −4.8) with TDM1 and TDM2 ages of 2591–2494 Ma and 2715–2670 Ma, respectively (Fig. 7), implying that the studied A-type granite was generated by partial melting of Neoarchean continental crust extracted from depleted mantle at ca. 2.7 Ga. The negative εHf(t) values (−4.85 to -0.92) of zircons, together with TDM1 (2433–2266 Ma) and TDM2 (2817–2556 Ma) ages also demonstrate a magma source consisting of Neoarchean crustal materials. Moreover, Wang et al. (2017c) obtained normal δ18O values (7.6‰–8.6‰) from Paleoproterozoic (ca. 1.73 Ga) A-type granite in the Dahongshan area, within the northwestern Dunhuang terrane, also suggesting a crustal magma source of low maturity, if the Huoyanshan A-type granite has the same origin as the Dahongshan granite. Therefore, it can be concluded that the studied A-type granite most likely formed by partial melting of Neoarchean tonalitic to granodioritic basement rocks of the Dunhuang Complex, consistent with identification of Neoarchean TTG gneisses in the Dunhuang terrane (Zhang et al., 2013; Zhao et al., 2015a).

Protolith of Metamafic Rock: Magma Mixing of a Depleted Mantle Source with a Metasomatized Lithospheric Mantle Source

The new geochemical data from the Hongliuxia metamafic rocks allow more sophisticated interpretations of melt generation than previously considered in Wang et al. (2014). The [La/Sm]PM and [Gd/Yb]PM values for the Hongliuxia metamafic rocks are 1.64–1.85 and 2.49–2.63, respectively (Table 1), implying that the garnet stability zone was the most likely source region for partial melting, and the magma source contained an enriched component. The relatively high [La/Sm]PM values also suggest low degrees of partial melting or high crustal material–like input. The continental crust is enriched in many incompatible elements and has [La/Sm]PM >1 (Niu, 2009); however, it is also characterized by marked depletion in Nb, Ta, and Ti. The studied metamafic rock samples displayed insignificant Nb-Ta and Ti anomalies, suggesting that continental crust could not be the enriched component of the magma source. The relatively low Th/Nb ratios (0.08–0.10) also suggest that the crustal input had negligible influence on the petrogenesis of the protolith of the studied metamafic rock. Thus, crustal contamination is ruled out, and the enriched component was most likely associated with low-degree melt metasomatism.

Zircons from the metamafic rocks showed variable εHf(t) values (-1.09 to +9.06), with TDM1 ages ranging from 2.10 Ga to 1.69 Ga (Fig. 9), indicating that the parental magma was probably derived from a depleted mantle source but assimilated with some components of an enriched source during magma migration through the lithospheric mantle. Nd isotopic signatures reveal that the metamafic rock samples have positive εNd(t) values (+0.8 to +1.8), with TDM1 ages of 2.22–1.08 Ga (Fig. 7), suggesting that the basaltic magmas for the ca. 1.6 Ga tectono-thermal event were produced by magma mixing of an E-MORB–like depleted mantle source (with εNd[t] value of +5) with a metasomatized (upper crust–like REE) lithospheric mantle source (with εNd[t] value of −6).

Therefore, the previous discussion on REE chemistry, element ratios, εNd(t), and zircon εHf(t) isotopic values suggests a magma mixing process between a depleted mantle source and a metasomatized lithospheric mantle source for the parent magma of the protolith of studied metamafic rock in the Hongliuxia area.

Tectonic Setting

A-Type Granite: Postcollisional Tectonic Setting

Although several models have been proposed to interpret petrogenesis of A-type granite, it is generally accepted that typical A-type granite is generally associated with an extensional environment, including both anorogenic (e.g., rift and mantle plume) and postcollisional (postorogenic) tectonic settings (Whalen et al., 1987; Eby, 1992; Pitcher, 1997; Wu et al., 2002; Bonin, 2007). As discussed before, the Huoyanshan granite samples belong to A2-type granite that has been through a cycle of continent-continent collision or island-arc magmatism (Eby, 1992), suggesting a postcollisional setting. High-K calc-alkaline granitoids are usually abundant in the orogenic belts related to continental collision, particularly at the time when collision is ending, and they could occur either during periods of relaxation that separate periods of culmination within a collision event, or during a transition from a compressional regime to a tensional regime (Barbarin, 1999). The Huoyanshan granite samples are characterized by geochemical features of high-K calc-alkaline rock series (Figs. 4A and 4B), implying that they were likely produced in a postcollisional setting. In the tectonic discrimination diagram of Ta versus Yb (Pearce et al., 1984), the granite samples plot in the within-plate field (Fig. 12A); in the Rb versus Y + Nb diagram (Pearce, 1996), they fall into the postcollisional granite field (Fig. 12B), further demonstrating the above conclusion. Furthermore, in the Dunhuang terrane, Paleoproterozoic arc-related magmatism (ca. 1.89–1.83 Ga) and a collisional event (ca. 1.83–1.80 Ga) characterized by granitoids in the Sanweishan-Dongshuigou area and high-pressure granulite in the Hongliuxia area, respectively, have been identified (Zhang et al., 2012a; Zhao and Sun, 2018). The intrusion age of Huoyanshan A-type granite is ca. 1.79–1.76 Ga, i.e., slightly later than the regional collisional event. Coeval anorthosite, rapakivite, and alkaline magmatic rocks have not been discovered in the region. All these data, therefore, suggest that the Huoyanshan A-type granite was emplaced in a postcollisional tectonic setting following a continent-continent collisional event rather than in an anorogenic regime. It is different from features of magmatism related to breakup of a supercontinent.

Protolith of Metamafic Rock: Initial Rifting in an Extensional Setting

As emphasized before, the studied metamafic rock from the Hongliuxia area is geochemically related to OIB (Figs. 6C and 6D) and distributes along the trend representing the MORB-OIB array (Fig. 13A), suggesting decompressional melting of upwelling asthenospheric mantle, which possibly interacted with the overlying lithospheric mantle during lithospheric extension (Chung et al., 1995). Several established discrimination diagrams have been applied to distinguish the tectonic environment of the studied metamafic rock samples. On the diagrams of Ti/1000 versus V and Zr versus Zr/Y (Shervais, 1982; Pearce and Norry, 1979), all the samples plot in the within-plate basalt (WPB) field (Figs. 13B and 13C), suggestive of an extensional tectonic setting. This result can also be obtained from the triangular diagram of Hf/3-Th-Ta (Wood et al., 1979), on which all the samples fall into the E-MORB/WPB field (Fig. 13D). Additionally, on the triangular diagrams of Ti/100-Zr-Sr/2 and Ti/100-Zr-Y×3 (Pearce and Cann, 1973), all the samples plot in the ocean-floor basalt (OFB) field and WPB field, respectively (Figs. 14A and 14B), further providing evidence for an extensional setting. However, though the studied samples have relatively high Na2O + K2O contents and Nb/Y ratios of 4.39–4.81 wt% and 0.63–0.66, respectively, they are still of subalkaline series, implying that magmatism associated with lithospheric extension mostly began at ca. 1.6 Ga. More importantly, in the field, the basaltic magmatism of this stage is limited, and the rift deposits, as well as iconic rifting-related magmatic products of the same period, such as alkaline magmatic rocks, anorthosite, and rapakivite, were not discovered in the Dunhuang terrane, which also supports the above conclusion. Therefore, the ca. 1.6 Ga basaltic magmatism most likely represents magmatic products of the initial stage of a rift environment.

Geological Implications

According to the discussion here, integrated with a relatively extensive record of Paleoproterozoic (ca. 1.9–1.8 Ga) subduction- and collision-related magmatic and metamorphic activities, it can be suggested that postorogenic extension in the Dunhuang terrane initiated during ca. 1.80–1.75 Ga (Fig. 15A), while the initial phase of rift-induced magmatism most likely commenced at ca. 1.6 Ga (Fig. 15B). The early postcollisional event should be taken into consideration as a possible mechanism for initial rifting during a transition stage of a late postcollisional–anorogenic regime (ca. 1.6 Ga), where the initial rifting event marked the complete formation of the Precambrian Dunhuang Complex. The studied latest Paleoproterozoic (ca. 1.8–1.6 Ga) A-type granite and mafic magmatism further provide evidence for Paleoproterozoic orogeny in the Dunhuang terrane.

It has been widely accepted that the Paleoproterozoic–Mesoproterozoic (ca. 2.1–1.2 Ga) period is a key period related to amalgamation and breakup of the Columbia supercontinent, marked by a global-scale ca. 2.1–1.8 Ga continental collision event and widespread ca. 1.6–1.2 Ga continental rifting, anorogenic magmatism, and mafic dike swarms in all cratonic blocks of Columbia (Zhao et al., 2002a, 2003, 2004; Rogers and Santosh, 2002; Ernst et al., 2008). In the North China craton, following the continent-continent collision between the Western and Eastern blocks and the final amalgamation of the North China craton at ca. 1.85 Ga (Guo and Zhai, 2001; Zhao et al., 2002b, 2003), postcollisional magmatism was widely developed along the southern and northern margins of the North China craton and the Trans–North China orogen. It is primarily indicated by ca. 1.80–1.76 Ga A2-type granites distributed in the Ningxia-Songshan-Lingbao areas in the southern margin of the North China craton (Zhao and Zhou, 2009; Gao et al., 2013; Deng et al., 2016a; Shi et al., 2017) and in the Lüliang area in the Trans–North China orogen (Zhao et al., 2018). In the northern margin of the North China craton, the postcollisional/postorogenic extensional setting is indicated by 1.78–1.67 Ga anorthosite, mangerite, alkali-feldspar granite, and charnockite (AMCG suite) in the Wayao-Chicheng-Changshaoying-Miyun-Longhua-Jianping areas (Zhang et al., 2007; Zhao et al., 2009; Wang et al., 2013b; Liu et al., 2016), accompanied by coeval mafic dikes in the Miyun area (Li et al., 2015). The most robust evidence in the North China craton for the Mesoproterozoic fragmentation of Columbia comes from the ca. 1.6–1.2 Ga Zhaertai–Bayan Obo–Huade–Weichang rift zone along the northern margin of the craton (Zhao et al., 2003, 2011; Liu et al., 2014), the ca. 1.35–1.31 Ga diabase sills in the Chaoyang area and bimodal magmatism in the Yanliao and Huade areas, which were emplaced into the Wumishan, Tieling, and Xiamaling Formations (Zhang et al., 2012b; Zhai et al., 2015), and 1.53 Ga anorogenic A1-type granite in the southern margin of the North China craton (Deng et al., 2016b), corresponding to an anorogenic tectonic setting. The magmatic event related to breakup of the Columbia supercontinent in the Tarim craton has been rarely reported. Wu et al. (2014) first reported Mesoproterozoic (ca. 1470 Ma) diabase sills resembling OIB affinity in the northern Tarim craton, and proposed that the Tarim craton documented Mesoproterozoic breakup of the Columbia supercontinent and it was close to Laurentia, Siberia, Greater Congo, and South China before dispersing from the Columbia supercontinent. According to petrographic and geochronologic studies on ca. 1784 Ma mafic dikes, ca. 1525 Ma bimodal volcanic rocks, and ca. 1117 Ma A-type granites from the southwestern Tarim craton, Zhang et al. (2019) proposed that the southern Tarim craton was possibly detached from West Africa initially at ca. 1.78 Ga, and completed breakup from the Columbia supercontinent at ca. 1.11 Ga, while the northern Tarim craton could be a continental fragment from the North China craton or/and India. It is noticeable that, though lacking Mesoproterozoic–Neoproterozoic sedimentation, the ca. 1.79 Ga A2-type granitic magmatism and ca. 1.60 Ga mafic magmatism in the Dunhuang terrane are comparable with ca. 1.80–1.67 Ga postcollisional and ca. 1.60–1.53 Ga anorogenic magmatism in the North China craton, respectively, but significantly different from those in the Tarim craton, having documented the postorogenic to initial rifting processes following the global-scale Paleoproterozoic collisional event.

CONCLUSIONS

  • (1) The Huoyanshan granite and the protolith of Hongliuxia metamafic rock emplaced at 1786 ± 20 Ma and 1605 ± 45 Ma, respectively, are indicative of a significant latest Paleoproterozoic magmatic event in the Dunhuang terrane.

  • (2) The Huoyanshan granite shows affinity to A2-type granite and was derived by partial melting of Neoarchean tonalitic to granodioritic rocks of the Dunhuang Complex, while the protolith of Hongliuxia metamafic rock displays OIB affinity and was generated by magma mixing processes between a depleted mantle source and a metasomatized lithospheric mantle source.

  • (3) The Huoyanshan granite originated under a postcollisional tectonic setting, while the protolith of Hongliuxia metamafic rock was produced during the initial rifting stage of an anorogenic tectonic setting.

  • (4) The latest Paleoproterozoic (ca. 1.8–1.6 Ga) magmatism in the Dunhuang terrane documented the postorogenic to initial rifting processes following a Paleoproterozoic collisional event.

ACKNOWLEDGMENTS

We are grateful to the editor and reviewers for their thoughtful and constructive comments, as well as smooth handling of the manuscript. We sincerely thank Jianqi Wang and Ye Liu from the State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, China, for their help with major- and trace-element analyses. We are also grateful to Huadong Gong from the same laboratory for his assistance with zircon U-Pb chronological analyses. This research work was supported by the National Natural Science Foundation of China (grants 41802199 and 41421002), the China Postdoctoral Science Foundation (grant no. 2017M623222), and the Shaanxi Postdoctoral Science Foundation.

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