Geological significance of the former Xiong’er Volcanic Belt on the southwestern margin of the North China Craton

Abstract

The rock association of low-grade metasedimentary rocks and greenschists located within the Meso-Cenozoic Liupanshan Fault system on the southwestern margin of the North China Craton (NCC) is regarded as part of the Paleoproterozoic Xiong’er Group. These low-grade rocks are separated by normal faults, with the greenschist located in the hanging walls. Zircon LA–ICP–MS U–Pb ages of the greenschists range from 2455 to 423 Ma, suggesting that they are not Paleoproterozoic in age. The protolith ages (206–194 Ma) of the greenschists were determined by LA–ICP–MS U–Pb dating of zircons from two siltstone interlayers. The petrology and geochemistry of the greenschists reveal that their protolith was continental tholeiitic basalt that formed in an extensional environment such as a continental rift. Thus, it is proposed that the protolith of the greenschists was a mafic volcanic rock of Late Triassic–Early Jurassic age and was metamorphosed during the Jurassic due to tectonism within the Liupanshan tectonic belt. These results show that the greenschists should be reclassified and removed from the Xiong’er Group, and explains why they differ so much from those of typical Xiong’er Group successions in other areas. The formation of the mafic volcanic rocks under conditions of continental rifting differs from that of coeval granitic rocks in the western Qinling Orogen, where the extension occurred during a post-collisional stage in the Late Triassic, which further suggests that the southwestern margin of the NCC became an extensional setting after the Late Triassic.

Keywords

southwestern margin of the NCC / Paleoproterozoic Xiong’er Group / greenschist / detrital zircon U–Pb geochronology / Late Triassic to Early Jurassic

Introduction

The Paleoproterozoic Xiong’er Group, which crops out at the southern margin of the North China Craton (NCC), consists mainly of a thick succession of volcanic rocks, reflecting the intense tectono-magmatic events that occurred after cratonization of the NCC basement (^{Zhao et al., 2007}). However, previous studies of the Xiong’er Group have largely been confined to the Xiaoqinling- Xiong’ershan Region in the southern margin of the NCC. A succession of low-grade metamorphic rocks exposed in Fengjiashan along the southwestern margin of NCC has been classified as the Paleoproterozoic Xiong’er Group on the basis of stratigraphic correlations (Fig. 1; ^{Bureau of Geology and Mineral Resources of Shaanxi Province, 1989}), and is the westernmost occurrence yet discovered. The lower boundary of these rocks is not exposed as they are overlain by Mesoproterozoic Gaoshanhe Group platform carbonates, separated by a fault. This study investigates the nature of these westernmost outcrops of the Xiong’er Group which is comprised of two major rock types: 1) low-grade metasedimentary rocks, including gray–green, gray–red sericitic quartzschist, thinly bedded calcareous slate, reworked sandstone, and finely laminated limestone; and 2) greenschist-facies rocks, including epidote–actinolite schists and albite–actinolite schists. ^{Zhou et al. (1994)} obtained a Sm–Nd model age of 1672±81 Ma for these greenschists and classified them as part of the Xiong’er Group. However, the rock types and ages are markedly different from those of the typical Xiong’er Group exposed in the Xiong’ershan, Xiaoshan, and Xiaoqinling Mountains (Fig. 1(b)), consisting of a thick succession of volcanic rocks, ranging from basaltic andesite, andesite, and dacite to rhyolite with an age of 1.85–1.70 Ga (^{Lu et al., 2002}), 1.80–1.75 Ga (^{Zhao et al., 2002}, ²⁰⁰⁴, ²⁰⁰⁷), 1.84–1.77 Ga (^{Hou et al., 2008}), 1.78 Ga (^{Peng et al., 2008}, ²⁰¹⁵), 1.78–1.75 Ga (^{He et al., 2009}; ^{Zhao et al., 2009}), 1.76 Ga (^{Wang et al., 2010}), 1800–1770 Ma (^{Cui et al., 2011}), 1.80–1.45 Ga (^{Deng et al., 2013}), ~1.80–1.78 Ga (^{Zhai et al., 2015}) and 1.80–1.00 Ga (^{Zhang et al., 2015}). Field mapping of the Xiong’er Group in the study area has revealed that the greenschists and low-grade metasedimentary rocks are separated by normal faults, the greenschists occur in the hanging walls of the faults (Fig. 1(d); localities No. 1 and No. 2), and two siltstone layers are interbedded with the greenschists (Fig. 2). In this study, we focus on the volcanic rocks that are proposed to be part of the Xiong’er Group. To determine the environment of formation and age of the protolith of the greenschists, their geochemical characteristics were analyzed, and LA–ICP–MS zircon U–Pb ages were obtained for the greenschists and interbedded siltstones. Our results suggest that the greenschists do not belong to the Paleoproterozoic Xiong’er Group, and that the protoliths of the greenschists indicate continental rifting during the Late Triassic to Early Jurassic.

Petrological features of the greenschists

The greenschists and low-grade metasedimentary rocks in the study area are exposed in the banks of the Qianhe River (localities No. 1 and No. 2; Figs. 1(c), 1(d), and 2). The contacts between these two lithological units are normal faults, and the greenschists formed in the hanging walls (Figs. 3(a)–3(c)). Fault breccias are weakly cemented, indicating that they formed in a near-surface, low-temperature structural setting (Figs. 3(a) and 3(c)). The greenschists are overlain by Quaternary loess.

The greenschists are green in color, show weak deformation, and contain quartz veins that are concordant with schistosity in the greenschists, which formed during greenschist-facies metamorphism. The orientation of schistosity in the greenschists varies from 168°∠28° to 179°∠43°, and 200°∠41° (Fig. 2). The greenschists contain chlorite, epidote, albite, and actinolite; the chlorite and albite show fibroblastic and lepidoblastic textures (Fig. 4). This mineral assemblage, together with the occurrence of interbedded siltstones, suggests that the protolith of the greenschist was an intermediate–basic volcanic rock.

The interbedded siltstones, which are exposed at localities No. 4 and No. 5 (Figs. 1(d) and 2), are aqua-colored and have well-developed lamellae oriented parallel to the schistosity. The siltstones consist of three to five thin lamellae that are each 7–10 cm thick (Figs. 3(e) and 3(f)).

Analytical methods and results

Analytical methods

Major and trace element analyses were conducted at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. The samples were washed and crushed in a jaw crusher to 60 mesh. Sixty grams of the sample were then milled in a WC Mill (T1-100, CMT) to 75 mm for whole-rock analysis. Major element oxides were measured using a Rigaku RIX 2100 XRF with Li-borate glass disks. Trace and rare earth elements were measured using an Agilent 7500a ICP–MS. Samples (50 mg) were dissolved in a sealed high-temperature and high-pressure bomb using equal parts of HF and HNO₃. International rock standards BHVO-1 and AGV-1 (from USGS) were used to calibrate major and trace element analyses, to ensure that precision and accuracy were within 5%.

Detrital zircons were analyzed by LA–ICP–MS. The zircons were extracted from the siltstones using heavy liquid and magnetic separation, hand-picked under a binocular microscope, mounted in epoxy resin, and polished until the grain centers were exposed. Cathodoluminescence (CL) imaging was done using a Quanta 400FEG environmental scanning electron microscope, equipped with an Oxford energy dispersive spectroscopy system and a Gatan CL3+ detector. The U–Pb analyses were conducted on an Agilent 7500a ICP–MS instrument equipped with a 193 nm ArF-excimer laser, and a homogenizing, imaging optical system. A fixed spot size of 30 mm and a laser repetition rate of 6 Hz were applied for all analyses, and helium was used as the carrier gas. The standard silicate glass NIST 610 was used to optimize the instrument and obtain maximum signal intensity (²³⁸U signal intensity>460 cps/(mg·g⁻¹)) and low oxide production (ThO/Th<1%). The U/Th ion-signal intensity ratio was measured to monitor complete zircon vaporization (Günther and Hattendorf, 2005). A single spot was measured in each zircon. The ICP–MS measurements were conducted using time-resolved analysis operating in fast peak jumping mode of 20 ms per peak, and in DUAL detector mode using a short integration time. ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U, and ²⁰⁸Pb/²³²Th values were calculated using GLITTER 4.0 (Macquarie University). The ratios were corrected using Harvard zircon 91500 as an external standard, with a recommended ²⁰⁶Pb/²³⁸U age of 1065.4±0.6 Ma (^{Wiedenbeck et al., 1995}, ²⁰⁰⁴), to correct for both instrumental mass bias and depth-dependent elemental and isotopic fractionation. Concordia diagrams and weighted mean calculations were plotted using Isoplot (v. 2.49; ^{Ludwig, 2001}). Uranium, Th, and Pb concentrations were calibrated using ²⁹Si as an internal standard and NIST SRM610 as an external standard. Zircon standards 91500 and GJ-1 were analyzed as unknowns.

Results

Major and trace elements in the greenschists

The major and trace element abundances of greenschists from the Xiong’er Group are listed in Table 1. The loss on ignition (LOI) of the samples ranges from 3.1% to 11.0% (average LOI= 4.9%), suggesting that the samples are highly altered and consistent with the greenschist facies metamorphism of the samples which have been taken into consideration where major elements have been used in calculations. Silica values of the greenschists are low (46.2%–51.7%; average SiO₂ = 48.7%), indicating they are basic. Titanium contents range from 1.4% to 2.1% (average TiO₂ = 1.9%), higher than values of oceanic tholeiites (1.4%) and island arc tholeiites (0.8%), and lower than oceanic island basalts (OIB, 2.2%; Pearce, 1982). Total alkali (ALK= K₂O+Na₂O) contents range from 1.3% to 3.3% (average ALK= 2.6%), and Na₂O>K₂O. The samples are classified as basalt in the TAS diagram for volcanic rocks (Fig. 5), and thus it is likely that the protolith of the greenschist was basaltic. In addition, Mg# = 34.1–44.7 (average Mg# = 40.8), which is lower than that of primitive basaltic magma (68–75; Frey et al., 1978), suggesting that the basaltic protolith was formed from the fractionation of a primitive basaltic magma.

Total REE (SREE) range from 46.45 to 69.20 ppm (parts per million) (average SREE= 64.95 ppm), and LREE are slightly enriched relative to HREE [(La/Yb)_N = 1.34–1.59]. The chondrite-normalized REE trends of the greenschists are similar to that of E-MORB (Fig. 6(a)), and they have no Eu anomalies (dEu= 0.94–1.04). The variability shown in REE trends (Fig. 6(b)) suggests that the basaltic protolith was subjected to crustal contamination. Large-ion lithophile (LILE) and high-field-strength elements (HFSE) are relatively enriched, but Nb, Ta, and P are relatively depleted. The geochemical characteristics of the greenschists are consistent with those of continental tholeiite. Greenschist samples fall into the sub-alkaline field in the SiO₂–ALK diagram and into the tholeiite field in an AFM diagram, which indicates that the protolith of the greenschists was a sub-alkaline tholeiitic basalt (Figs. 7(a) and 7(b)). Samples fall into the “intracontinental rift and continental marginal rift tholeiite” field in log–log Th/Hf–Ta/Hf and La/Zr–Nb/Zr discrimination diagrams, which suggests that the greenschist protolith was tholeiitic and formed in an intracontinental or continental marginal rift (Fig. 8).

Zircon U–Pb geochronology of the greenschists

Zircon LA–ICP–MS U–Pb dating was performed on the greenschist (sample 16LP05) exposed at outcrop No. 6 (Fig. 1(d)). CL images show that the zircons are variable in size and that some have rounded margins (Fig. 9), possibly caused by abrasion or melting. Some zircons have inner cores (e.g., zircon grains of spot analyses 6, 15, and 20 in Fig. 9), and some show clear growth zoning, revealing their complex history. Most zircons are characterized by columnar and euhedral shapes, clear magmatic zoning, and Th/U>0.1, which all indicate an igneous origin. A small number of zircons have Th/U<0.1, indicating a possible metamorphic origin.

Thirty-six analyses were conducted on 36 zircon grains from greenschist sample 16LP05, with 21 analyses producing reliable data. The U–Pb data are presented in Table 2 and U–Pb concordia plots are shown in Fig. 10. The concordant or nearly concordant ages range from 2455 to 423 Ma. Given that the youngest zircon age of the greenschist sample is much older than the youngest age obtained from the detrital zircons in the clastic interlayers, the zircon greenschist ages do not represent the formation age of the greenschist protolith. However, it is clear that the many young zircon ages are inconsistent with the rocks that comprise the Xiong’er Group.

U–Pb geochronology of detrital zircons from interbedded siltstones

U–Pb dating was performed on detrital zircons from interbedded siltstones (samples 14LP04 and 14LP03) exposed at outcrops No. 4 and No. 5, respectively (Figs. 1(d) and 2). The zircons are variable in size and some have rounded margins (Fig. 11), indicating a detrital origin. However, some are columnar and euhedral, suggesting they have not been transported far from their source. Individual zircon grains (e.g., analyses 13, 19, and 21 in sample 14LP03; Fig. 11) have cores, and some (e.g., analyses 1, 20, 29, 30, 33, and 40 in sample 14LP04; Fig. 11) show clear growth zoning, revealing a complex history. Magmatic zoning is also clear in most CL images, and their Th/U values are greater than 0.1, indicating an igneous origin.

Thirty-four analyses were conducted on 33 zircon grains from sample 14LP03, with 24 yielding reliable ages. Forty analyses were performed on 34 grains from sample 14LP04, 33 of which were retained. The U–Pb data are listed in Table 2 and concordia plots are shown in Fig. 12. The concordant zircons from both siltstone samples yield similar ages: sample 14LP03 yields concordant U–Pb ages ranging from 2462 to 206 Ma, while sample 14LP04 yields concordant U–Pb ages from 3039 to 194 Ma. Both samples show age peaks at 2000 and 400 Ma. The youngest ages from both samples are similar (206 and 194 Ma). Sample 14LP04 also yields ages of 222, 228, and 229 Ma, which are close to the youngest. The youngest ages represent the maximum depositional age of the sediments, indicating that the greenschist protolith is younger than the Late Triassic or Early Jurassic.

Discussion

Age of the greenschist protolith

The age of the greenschist protolith cannot be determined using stratigraphic relationships; therefore, the ages of detrital zircons are used to indirectly determine this age. Figures 3(e) and 3(f) show that the siltstones have not been extensively metamorphosed, meaning it is unlikely that the youngest ages from samples 14LP03 (206±3 Ma) and 14LP04 (194±3 Ma) represent the timing of metamorphism. Furthermore, the detrital zircons clearly show internal magmatic zoning, and have magmatic Th/U values (0.55 and 0.45). Thus, these dates represent the ages of the source of the siltstones, indicating that the continental tholeiite protolith of the greenschists could not have been formed earlier than the siltstones (194–206 Ma).

It is proposed that the protolith of the greenschists was formed in the Late Triassic to Early Jurassic based on the following observations.

1) The zircons yielding Late Triassic to Early Jurassic ages are characterized by euhedral crystals and do not display extensive abrasion. This indicates that the source area of the zircons was near the area of sedimentation and that the zircons may have crystallized in the protolith of the greenschists itself.

2) Based on field observations, the protolith could not have formed before 194–206 Ma, nor could it have formed much later, such as in the Cenozoic, as younger clastic rocks of the Lower Cretaceous Liupanshan Group, which are widely distributed along the southwestern margin of the NCC, show no evidence of metamorphism. In the western Qinling Orogen, bimodal volcanism, comprised of kamafugite, carbonatite, shoshonite, and rhyolite, erupted between 23 and 7.1 Ma in an extensional environment (^{Yu et al., 2011}); therefore, the study area and its surrounds likely experienced the same extension. Since the protolith was metamorphosed to conditions of greenschist facies, it must pre-date the bimodal volcanism.

3) The Late Triassic to Early Jurassic extensional setting of the study area favored the formation of the protolith of the greenschists. The sedimentary record preserved on the southwestern margin of the NCC extends into the Late Triassic (^{Zhao et al., 2006}; ^{Deng et al., 2008}). Upper Triassic gray–green sandstone and variably colored shale of the Yanchang Formation occured in the study area (Fig. 1(b)). While Late Triassic to Early Jurassic magmatic rocks have not been identified on the southwestern margin of the NCC. A series of Late Triassic granitoids occurred in the western Qinling Orogen, and a lamprophyre of similar age occurred in the central part of the Shangdan Suture (Fig. 1(b); Table 3). These sedimentary and magmatic occurrences indicate that extension in the region occurred during the Late Triassic.

4) No Early Jurassic magmatic rocks have been identified along the southwestern margin of the NCC or surrounds, suggesting that the source of the 194±3 Ma zircons was the protolith of the greenschist.

5) Late Triassic to Early Jurassic basalts occurred at Rujigou on the northwestern margin of the NCC (Fig. 1(b)) and, as both the study area and Rujigou basalt lie in the western margin of the NCC, it is possible that similar extensional processes occurred in the study area at this time. The Rujigou basalt (36.5 m thick) overlies clastic rocks of the Upper Triassic Yanchang Formation and is overlain by Middle Jurassic sedimentary rocks. It has been extensively altered, with alteration decreasing in intensity towards the central parts of the basalt, and containing abundant chlorite (^{Yang et al., 2010}). Although several studies have obtained a range of ages (229±15 Ma, ^{Bureau of Geology and Mineral Resources of Ningxia Hui Autonomous Region, 1990}; 193–229 Ma, ^{Wang et al., 2005}) for the Rujigou basalt, its contacts with adjacent strata indicate an age of Late Triassic to Early Jurassic. The continental tholeiitic Rujigou basalt (^{Yang et al., 2010}) was formed in a continental rift environment (^{Wang et al., 2005}), revealing that rifting occurred along the northwestern margin of the NCC during the Late Triassic and Early Jurassic. The ages of the Rujigou basalt are similar to the youngest zircon ages obtained in this study; therefore, we propose that the basalt and the protolith of the greenschists in the study area are coeval. This hypothesis is supported by the occurrence of the Upper Triassic Yanchang Formation in both areas (Fig. 1(b)). The protolith was metamorphosed under greenschist-facies conditions by extensive faulting compression and burial related to movement along the Liupanshan Fault system during the Late Jurassic and Early Cretaceous, reflected by the thick deposits of coeval conglomerates. However, the Rujigou basalt was only weakly metamorphosed as it lies distant from the Liupanshan Fault system. In addition, tuff intervals of Upper Triassic Yanchang Formation are laterally widespread in the Ordos basin, suggesting the coeval volcanic activity around the Ordos block (^{Qiu et al., 2014}).

Reassignment of the greenschists

The youngest zircon ages of the siltstones interbedded with the greenschists indicate that the formation age of the protolith of the greenschists cannot be earlier than 193–206 Ma; therefore, the greenschists should not be considered part of the Paleoproterozoic stratigraphic unit. In addition, detrital zircons from blastolithic sandstones of the low-grade metasedimentary rocks of the former Xiong’er Group yield concordant U–Pb ages of 1091–2854 Ma (to be discussed in a forthcoming article). Consequently, the sedimentation could not have pre-dated 1091 Ma and the Xiong’er Group may not have developed on the southwestern margin of the NCC. This would account for the differences in lithological associations and ages of deformation between the Xiong’er Group on the southwestern margin of the NCC, and the group located in the Xiong’ershan, Xiaoshan, and Xiaoqinling mountains (Fig. 1(b)).

Provenance of siltstones interbedded with the greenschists

The ages (412–480 Ma) of siltstones interbedded with the greenschists (Fig. 12) correspond to the ages of several basic and acidic magmatic rocks (413–462 Ma) (^{Zhang et al., 2006}; ^{Chen et al., 2007}; ^{He et al., 2007}; ^{Pei et al., 2007a}, ^b; ^{Wei et al., 2012}; ^{Wang, 2013}; ^Wei,2013) in the Longshan area, west of the present study area (Table 4). Therefore, it is possible that the magmatic suites in the Longshan area were the source of the siltstones.

The age peak of 2000 Ma (Fig. 12) is similar to that of the NCC basement and to granite–gneisses of the Longshan Complex (1.90 Ga–1.95 Ga; ^{He et al., 2005}). Therefore, this age likely represents the age of amalgamation of the Columbia Supercontinent.

Late Triassic to Early Jurassic tectonic environment along the southwestern margin of the North China Craton

The Late Triassic to Early Jurassic continental tholeiite protolith of greenschists in the Xiong’er Group indicates that the tectonic regime at the time was dominated by extension. This tectonic setting was also responsible for the distribution of Late Triassic sediments in the area (^{Zhao et al., 2006}; ^{Deng et al., 2008}). The Late Triassic mafic or alkaline rocks and intrusions occurred on the northern and southern margins of the North China Craton and Xingmeng Orogenuc belt, implying that intensified extension occurred across the entire region of North China (^{Liu et al., 2015}). ^{Han et al. (2014)} proposed that Late Triassic sedimentation occurred in a region of steep topography caused by NE-trending compression in the Qinling Orogen (^{Feng et al., 2009}), and that the source of sediments was close to their sites of deposition. ^{Liu et al. (2013)} proposed that the Late Triassic continuous subsidence and deposition within the Ordos basin were predominantly related to the thrust load of the East Qilian–Qinling belt and inferred mantle flow effects associated with paleotethys plate subduction. The present results show that even though Late Triassic regional compression was unlikely to have occurred along the southwestern margin of the NCC, regional extension did occur.

While widespread sedimentation occurred during the Early Jurassic, the occurrence of Lower Jurassic strata in the area was limited. This could reflect the deep burial of Lower Jurassic strata, or the extensive denudation of these rocks between the Late Jurassic and Early Cretaceous, which may have acted as the source of the widespread Upper Jurassic Fenfanghe Formation and Lower Cretaceous Liupanshan Group conglomerates along the southwestern margin of the NCC. The denudation from the Late Jurassic to Early Cretaceous not only explains the absence of Lower Jurassic strata, but also fragmented the Upper Triassic Yanchang Formation, which consists of fine-grained clastic rocks scattered throughout the study and adjacent areas.

The ages of detrital zircons in siltstones (400 Ma) correspond with those obtained from the magmatic rocks from the western Qinling Orogen; therefore, the tectonic environment in which the study area developed may have formed as a result of post-collisional extension associated with orogenesis, which continued throughout the Late Triassic and Early Jurassic. ^{Huang et al. (2013)} proposed that 229 Ma rhyolites, trachytes, and dacites in the western Qinling Orogen formed under regional conditions of crustal thickening and compression, and local shearing-related extension; however, numerous Late Triassic magmatic rocks (Table 3) provide evidence of regional extension.

A second mechanism that could account for Late Triassic to Early Jurassic extension is continental rifting resulting from geodynamic processes in the lower lithosphere. The greenschists in the study area have geochemical characteristics of continental tholeiitic basalt. We obtained e_Nd (t = 200 Ma) = 3.38, reflecting limited crustal contamination. The average ¹⁴³Nd/¹⁴⁴Nd value for the greenschist is 0.512835, which is near-chondritic, and the average e_Nd (t = 1672 Ma) is 3.78, suggesting that the protolith of the greenschists was derived from a depleted mantle source (^{Zhou et al., 1994}).

Figure 13 shows that the greenschist samples fall between N-MORB, E-MORB, and intraplate tholeiites in tectonic discrimination diagrams, suggesting that their protolith was contaminated by crustal material. Due to the absence of an oceanic environment in the study area during the Late Triassic to Early Jurassic, we propose that continental rifting was the dominant process responsible for the formation of the protolith of the greenschists. However, post-collisional extension in the western Qinling Orogen may have contributed to regional extension along the southwestern margin of the NCC.

The 193–229 Ma Rujigou basalt may be considered additional evidence for continental rifting in the study area. This basalt occurs between the Ordos Block in the east and the Alanshan Block in the west. Post-collisional extension, similar to that in the western Qinling Orogen, did not occur in this region at that time; therefore, the Rujigou basalt likely formed as a result of geodynamic processes in the lower lithosphere. Granodiorites (223±26 Ma) occur at the southern margin of the Helanshan Mountains (^{Bai and Lu, 2009}), near the Rujigou basalt. Together, these units may constitute a bimodal magmatic suite. Similarities in age, geochemistry, and location indicate that the Rujigou basalt and the protolith of the greenschists in the Xiong’er Group were generated by similar tectonic processes, particularly continental rifting (^{Wang et al., 2005}; ^{Yang et al., 2010}).

Conclusions

1) Detrital zircons from two siltstone interlayers within greenschists are proposed to be part of the Paleoproterozoic Xiong’er Group at the southwestern margin of the NCC yield concordant U–Pb ages ranging from 3039 to 194 Ma. Based on these ages and regional geological data, we propose that the protolith of the greenschists were formed between the Late Triassic and Early Jurassic (206–194 Ma); therefore, the greenschists should be reclassified and removed from the former Xiong’er Group.

2) The protolith of the greenschists was continental tholeiitic basalt that formed in a continental rift environment. This protolith was metamorphosed under conditions of the greenschist facies, likely related to extensive faulting in the Liupanshan Fault system during the Jurassic.

3) Late Triassic, post-collisional extension of the western Qinling Orogen may have contributed to regional extension along the southwestern margin of the NCC.

4) Late Triassic to Early Jurassic extension took place at the southwestern margin of the NCC, rather than subsidence of a foreland basin under regional compression.

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