Age and provenance of the Ergunahe Group and the Wubinaobao Formation, northeastern Inner Mongolia, NE China: implications for tectonic setting of the Erguna Massif

Here, we present the results of U–Pb dating of detrital zircons from the Ergunahe Group and the Wubinaobao Formation, within northeastern Inner Mongolia, NE China, with the aim of constraining the tectonic setting of the Erguna Massif. The majority of detrital zircons from five samples collected from the Ergunahe Group and the Wubinaobao Formation are magmatic, although some zircons have metamorphic growth rims. Zircons in two mica schists and in feldspar–quartz sandstone from the Ergunahe Group yield age populations that cluster around 738, 760, 792, 837, 890, 964, and 1050 Ma, whereas zircons from two quartz sandstones within the Wubinaobao Formation yield age populations that cluster at 466, 484, 515, 738, 795, 837, and 894 Ma. These data, combined with detrital zircon age populations (ca. 712 Ma) from the adjacent Xinghuadukou Group, and the fact that the Ergunahe Group intruded by Caledonian gabbros is overlain by upper Silurian units, indicate that the Ergunahe Group formed at 738–712 Ma (i.e. during the Neoproterozoic). In addition, the Wubinaobao Formation is subdivided into two units: a calcareous siltstone unit within the western part of the study area and a quartz sandstone within the eastern part. The calcareous siltstone formed at 712–795 Ma, similar to the Ergunahe Group, whereas the quartz sandstone formed between the 466 Ma and late Silurian. The age spectra of detrital zircons from the Ergunahe Group and the Wubinaobao Formation indicate that sediment in both of these units was derived from terranes that outcrop around the basin. The widespread occurrence of Neoproterozoic detrital zircons within both the Ergunahe Group and the Wubinaobao Formation suggests that Precambrian terranes are present within the Erguna Massif and that the massif has an affinity to the Siberian Craton.


Introduction
The Erguna Massif is located in the eastern segment of the Central Asian Orogenic Belt (CAOB) (Figure 1(a)), an area that underwent tectonism dominated by the Palaeozoic amalgamation of microcontinental massifs, including, from west to east, the Erguna, Xing'an, Songnen-Zhangguangcai Range, Jiamusi, and Khanka massifs (Li et al. 1999;Wu et al. 2002Wu et al. , 2007Li 2006;Xu et al. 2009). Palaeozoic and Mesozoic granites and Mesozoic volcaniclastic sediments are widespread in the central and western parts of the Erguna Massif (Figure 1(b)). However, few studies have examined the Precambrian geology and sedimentology of the eastern section of the CAOB, in contrast to the numerous studies on Palaeozoic and Mesozoic units in this area (Ge et al. 2005(Ge et al. , 2007Wu et al. 2005Wu et al. , 2011Gou et al. 2013;Tang et al. 2013;Xu et al. 2013). Recent zircon U-Pb dating indicates that the so-called Precambrian sediments in this area actually formed during the Palaeozoic and early Mesozoic; this is exemplified by a detrital zircon study that determined that the Zhangguangcailing Group within the Songnen-Zhangguangcai Range and the Fengshuigouhe Group within the Xing'an Massif (HBGMR 1993) formed in the Palaeozoic-early Mesozoic, rather than in the Precambrian (Wang et al. 2012a(Wang et al. , 2012bXu et al. 2012). This is also the case for the Xinghuadukou Group within the Erguna Massif, which was thought to have formed in the Palaeoproterozoic, but actually formed in the Palaeozoic (Miao et al. 2007;Wu et al. 2012). The Mashan Group within the Jiamusi Massif also formed in the early Palaeozoic (Wilde et al. 2000(Wilde et al. , 2003. These revised ages suggest the need to reconsider the ages of the Ergunahe Group, one of the Precambrian sedimentary units within the Erguna Massif of NE China, and of the Wubinaobao Formation, a unit previously thought to be Ordovician and located in the same area as the Ergunahe Group. The provenance of these sedimentary units is also unclear, as is their relationship to the regional tectonic history of the area, as well as the tectonism recorded within the Erguna Massif. Here, we report new zircon U-Pb dating of the Ergunahe Group and the Wubinaobao Formation within the Erguna Massif, with the aim of constraining the timing of formation of these units, their sedimentary provenances, and the tectonic affinity of the Erguna Massif.

Geological background and sample descriptions
The study area is located within the eastern CAOB and is located east of the Erguna River (Figure 1(c)). Outcropping sediments in the study area include the previously believed Cambrian Ergunahe Group, which is defined based on regional rock associations (IMBGMR 1991), the Ordovician Wubinaobao Formation, defined based on biostratigraphy (IMBGMR 1991), and widespread Mesozoic volcanics (Meng et al. 2011;Xu et al. 2012Xu et al. , 2013. The Ergunahe Group is subdivided into the following four sections ( Figure 2) (from bottom to top): a dolomitic and/or siliceous marble unit that contains lenses of metamorphosed calcareous quartz sandstone (herein referred to as the first section); a crystalline limestone unit (second section); a unit containing crystalline limestone, white marble, carbonaceous crystalline limestone, and grey-white slate (third section); and a metamorphosed fine-grained feldspar-quartz sandstone and grey-black to grey-white schist unit (fourth section). This group is unconformably overlain by the Wubinaobao Formation, which is dominated by light grey, light green, and yellow-grey silty slates, grey metamorphic siltstones, and metamorphosed finegrained feldspar-quartz sandstones. These outcrops (i.e. the Ergunahe Group and Wubinaobao Formation) occur as monoclinal strata and experienced late-stage metamorphism of the lower greenschist facies. The study area also contains widespread Neoproterozoic, Palaeozoic, and Mesozoic granitoids (Figure 1(c)) (Wu et al. 2011;Wang et al. 2012b;Tang et al. 2013).
The sampling locations visited during this study are shown in Figure 1(c), and the samples are described in the following paragraphs.
Sample 11ER8-2 is a silicified quartz sandstone from the Wubinaobao Formation and was collected from a mountain located to the east of Zhengyang (119°37′08.6″ E, 50°41′25.9″N). The sample is grey-white, has a clastic texture, is massive (Figure 3(d)), and contains quartz (~90%) and feldspar (~10%). The quartz is rounded, and silicification is intensely developed along fracture surfaces.

Analytical methods
Zircons were extracted from whole-rock samples using standard techniques of density and magnetic separation, and then by handpicking under a binocular microscope, at the Langfang Regional Geological Survey, Hebei Province, China. The handpicked zircons were examined under transmitted and reflected light with an optical microscope. To reveal their internal structures, cathodoluminescence (CL) images were obtained using a JEOL scanning electron microscope housed at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. Distinct domains within the zircons were selected for analysis based on the CL images. LA-ICP-MS zircon U-Pb dating and trace element analyses were performed using an Agilent 7500a ICP-MS equipped with a 193 nm laser, housed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. The zircon 91500 was used as an external standard for age calibration, and the NIST SRM 610 silicate glass was applied for instrument optimization. The crater diameter was 32 μm during the analyses. For details on instrument settings and analytical procedures, see Yuan et al. (2004). The ICPMSDataCal (Ver. 6.7; Liu et al. 2008Liu et al. , 2010aLiu et al. , 2010b and Isoplot (Ver. 3.0; Ludwig 2003) programs were used for data reduction. Correction for common Pb was made following Anderson (2002). Errors on individual analyses by LA-ICP-MS are quoted at the 1σ level, while errors on pooled ages are quoted at the 95% (2σ) confidence level. The dating results are presented in Table 1

Analytical results
A total of five representative samples were analysed by U-Pb dating, and we focus on these new zircon U-Pb ages combined with the results of trace element analyses of the samples. The internal structure of zircons was determined by CL imaging, and CL images of zircons from the five samples are shown in Figure 4.

Wubinaobao Formation
Sample 11ER8-2: silicified quartz sandstone Zircons separated from sample 11ER8-2 are generally long, columnar, and subhedral. The majority of these zircons have distinct core-rim structures with rims that contain fine-scale oscillatory growth zoning visible during  CL imaging (Figure 4(d)). A total of 38 analyses of zircons from this sample yielded 206 Pb/ 238 U ages of 465 ± 6 to 2267 ± 18 Ma (Table 1) (e, f)). These zircons are short, columnar, and subhedral, and have fine-scale oscillatory growth zoning visible during CL imaging. These textures, combined with Th/U ratios of 0.21-1.16, suggest an igneous origin for the zircons (Figure 4(e)). The detrital zircons from two samples in the Wubinaobao Formation show negative Eu anomalies (Eu/Eu* = 0.01-0.77), positive Ce anomalies (Ce/ Ce* = 1.13-132), and HREE enrichment relative to LREEs (Figure 6(b)).

Depositional age of the Ergunahe Group
The depositional age of the Ergunahe Group remains controversial. The Ergunahe Formation, the precursor of the Ergunahe Group, was established in 1979(HBG 1977. The Ergunahe Group was proposed in 1981 and assigned to the Cambrian (HBG 1977). A similar age was reported in a 1:200,000 scale regional geological survey (IMBGMR 1985a(IMBGMR , 1985b and in a study of the regional geology of Inner Mongolia (IMBGMR 1991). However, a Neoproterozoic depositional age for the Ergunahe Group has also been suggested (IMBGMR 1991). These contrasting ages reflect a paucity of age-diagnostic fossils and a lack of precise ages for the various units within the Ergunahe Group. In addition, it is clear that the previous methods of dating, using metamorphic grades and regional rock associations without precise ages, are no longer considered reliable. This study addresses these shortcomings by presenting new U-Pb ages for detrital zircons within the Ergunahe Group of the Erguna Massif. In addition, we use the contact relationships between different sedimentary units within the region to further constrain the timing of deposition of the Ergunahe Group.
The detailed fieldwork and sampling undertaken during this study focused on the second and fourth sections of the Ergunahe Group; these units are rare and only form a small volume of the western Erguna Massif (Figure 1(c)). Barring some rounded-subrounded zircons, the majority of zircons from the Ergunahe Group are euhedral-subhedral with fine-scale oscillatory growth zoning and exhibit negative Eu anomalies, positive Ce anomalies, and HREE enrichment relative to LREEs, indicative of a magmatic origin (Koschek 1993;Belousova et al. 2002;Corfu et al. 2003). In addition, some zircons have inherited cores with narrow structureless rims.
It is generally accepted that the youngest concordant detrital zircon age within a sediment is indicative of the maximum depositional age of that sediment. The biotite schist sample obtained during this study (ER26-1) contains a youngest zircon population that yielded an age of 738 ± 8 Ma ( Figure 5(a)), indicating formation from a sedimentary protolith that was deposited after ca. 738 Ma. Similarly, the mica-quartz schist sample obtained during this study (ER26-2) has a youngest zircon population that yielded an age of 761 ± 10 Ma ( Figure 5(b)), suggesting that the protolith for this schist was deposited after ca. 761 Ma. In addition, the youngest reliable age of 796 ± 16 Ma ( Figure 5(c)) for a calcareous siltstone from the Wubinaobao Formation that crops out in the west of the study area is again indicative of the maximum depositional age of this sediment, suggesting that the Wubinaobao Formation, previously considered to be deposited during the Early-Middle Ordovician, should in fact be assigned to the Ergunahe Group. These data indicate that the Ergunahe Group was deposited after 738 Ma; however, the minimum depositional age of the Ergunahe Group is still unclear, although it is constrained by the depositional ages of overlying sediments and the detrital zircon geochronological record of this region. The report of a 1:200,000 regional geological survey (IMBGMR 1991) indicates that the overlying Wubinaobao Formation was deposited later than the Ergunahe Group. In addition, Early-Middle Ordovician gabbros and granitoids (Wu et al. 2011) and Cambrian granitoids (ca. 510 Ma;Zhou et al. 2011), which intruded into and cross-cut Neoproterozoic granitoids (Tang et al. 2013), have also been identified within the Erguna Massif. These early Palaeozoic igneous rocks were a significant source of the majority of clastic material within the Xinghuadukou Group (Wu et al. 2012). However, zircons with early Palaeozoic ages have not been identified within the Ergunahe Group, indicating that this group must have been deposited before ca. 510 Ma. In addition, detrital zircon U-Pb ages for metasedimentary rocks within the Ergunahe Group are near-identical to those of the Xinghuadukou Group (Zhou et al. 2011;Wu et al. 2012), suggesting that these U-Pb ages are reliable. Recent research has identified igneous Neoproterozoic detrital zircons with ages around ca. 712 Ma from the biotiteplagioclase leptynite (Zhou et al. 2011). These zircons are common both in previous studies and within the samples discussed here, suggesting that ca. 712 Ma magmatism occurred within the Erguna Massif (Zhou et al. 2011;Wu et al. 2012). However, this ca. 712 Ma magmatism did not provide any of the clastic material within the Ergunahe Group, thereby indicating that deposition of this group began before ca. 712 Ma.
In conclusion, deposition of the Ergunahe Group occurred between 712 and 738 Ma, indicating a Neoproterozoic age for this group.

Formation age of the Wubinaobao Formation
The Wubinaobao Formation was established in 1979, and an Early Ordovician age for this formation was proposed by the 1:200,000 Regional Geological Survey (IMBGMR 1991), the Regional Geology of the Inner Mongolia Autonomous Region (IMBGMR 1991), and the Rock Formation in Inner Mongolia Autonomous Region (IMBGMR 1991) reports, in addition to the 1:200,000 regional geological map that was revised in 2000. This age was ascribed based on contacts observed in the field, lithostratigraphic relationships, and palaeontological evidence, without any precise isotope geochronology.
Zircons from silicified quartz sandstone (sample 11ER8-2) within the Wubinaobao Formation yielded several age populations, the youngest of which yielded an age of 466 ± 3 Ma ( Figure 5(d)); this age most likely represents the oldest age of sedimentation, indicating that deposition of the protolith for this unit began after 466 ± 3 Ma.
In addition, detrital zircon U-Pb dating of a pebblebearing sandstone (sample 12ER5-1) from the second rock section of the Erguna Group, in the south of the study area, yielded a youngest concordant age of 461 ± 6 Ma ( Figure 5(e, f)), consistent with and within error of dating of the Wubinaobao Formation. This indicates that not only should the so-called Cambrian Ergunahe Group be ascribed to the Wubinaobao Formation but also that deposition of the pebble-bearing sandstone began after 461 ± 6 Ma.
These age data indicate that the deposition of the Wubinaobao Formation began after 461 ± 6 Ma. This result, combined with the fact that the late Silurian strata overlain on the Wubinaobao Formation within the study area, suggests that the Wubinaobao Formation formed between the Middle Ordovician and the late Silurian, which is consistent with the biostratigraphical evidence (IMBGMR 1991).
Age peaks at 738, 761, and 791 Ma are widespread within the Ergunahe Group and are consistent with known Neoproterozoic magmatic events within the Erguna Massif (Tang et al. 2013). This relationship is also supported by a stratigraphic relationship whereby the Ergunahe Group overlies Neoproterozoic intrusive rocks in the study area. In addition, age peaks at 837, 890, 964, and 1050 Ma are widely reported in Palaeozoic sedimentary and igneous rocks within the Erguna Massif (Zhou et al. 2011;She et al. 2012;Gou et al. 2013;Tang et al. 2013), suggesting that these magmatic events occurred within the Erguna Massif and provided sedimentary material for the Ergunahe Group sediments. Detrital zircon age peaks for biotite and mica-quartz schists and for feldspar-quartz sandstones within the Ergunahe Group are consistent with known magmatic events in regions surrounding the study area (Gou et al. 2013;Tang et al. 2013). In addition, CL imaging indicates that these detrital zircons have low degrees of roundness and are generally euhedral (Figure 4), indicating minimal transport from the source area. This suggests that the sediments that formed these biotite and mica-quartz schists and feldspar-quartz sandstones were sourced from units that were exposed within the study area and adjacent regions, and that rapid sedimentation occurred soon after these units were exposed.
The youngest zircon population (ca. 466 Ma) within the Wubinaobao Formation is similar to the ages of gabbros and granites within the Erguna Massif (Sui et al. 2006;Ge et al. 2007;Wu et al. 2011). A ca. 484 Ma age peak is also consistent with the results of zircon U-Pb dating of granite and gabbro within the study area and adjacent regions, in addition to pyroxene-bearing trondhjemite and quartz-syenite units within the Sludyanskiy Complex of the southwest Lake Baikal region, located along the southern margin of the Siberian Craton (Salnikova et al. 1998). Magmatic events that have the same 515 Ma age peak are also widespread throughout the Erguna Massif (Wilde et al. 2000(Wilde et al. , 2003Wu et al. 2005Wu et al. , 2011Zhou et al. 2010Zhou et al. , 2011, indicating that the 738, 792, 837, and 890 Ma ages, similar to the detrital zircon peak ages of the Ergunahe Group, are consistent with widespread Neoproterozoic magmatism within the Erguna Massif (Wu et al. 2011;Zhou et al. 2011;She et al. 2012;Gou et al. 2013;Tang et al. 2013).
In summary, detrital zircon age peaks within silicified quartz sandstone and pebble-bearing sandstone samples of the Wubinaobao Formation are consistent with magmatic events within the study area and adjacent regions. In addition, CL imaging of these euhedral detrital zircons with a low degree of rounding ( Figure 4) suggests that these zircons have only been transported short distances, indicating that the Wubinaobao Formation sediments were sourced directly from exposed units within the study area and adjacent regions.

Tectonic implications
It is currently unknown whether Precambrian basement is present within microcontinental massifs in NE China . It was previously thought that Precambrian basement units were present in all of these microcontinental massifs, such as the Xinghuadukou Group within the Erguna Massif, the Mashan Group within the Jiamusi Massif, the Zhangguangcailing Group within the Songnen-Zhangguangcai Range Massif (HBGMR 1993), and the Fengshuigouhe and Zhalantun groups and the Palaeoproterozoic Xilinguole Complex within the Xing'an Massif (JBGMR 1988;IMBGMR 1991;HBGMR 1993). However, recent dating suggests that the majority of these so-called Precambrian terranes actually formed in the Palaeozoic or early Mesozoic (Wilde et al. 2000(Wilde et al. , 2003Miao et al. 2003Miao et al. , 2007Shi et al. 2004;Wu et al. 2007;Chen et al. 2009;Zhou et al. 2009;Wang et al. 2012aWang et al. , 2012bXu et al. 2012). This indicates that it is unclear whether Precambrian terranes are actually present within NE China; answering this question will provide important information on the geological history of this area. As described earlier, ages of detrital zircons, combined with contact relationships between different sediments and previously published data, indicate that Ergunahe Group sediments were deposited during the Neoproterozoic.
Neoproterozoic intrusive rocks have recently been identified within the Erguna Massif (Gou et al. 2013;Tang et al. 2013), and the present study provides new precise geochronological evidence of the existence of a Precambrian terrane in this region. Widespread Neoproterozoic detrital zircons within the study area suggest that Neoproterozoic magmatism could also have occurred within the Erguna Massif. This view is supported by contemporaneous magmatism within the Tuva Massif, located along the southern margin of the Siberian Craton (Kuzmichev et al. 2001;Kozakov et al. 2005;Gladkochub et al. 2007), suggesting in turn that the Erguna Massif may be related to the Siberian Craton. The occurrence of these Neoproterozoic magmatisms within the Erguna and Tuva massifs indicates that the break-up of the Rodinia supercontinent could have resulted in not only the separation of a number of the Precambrian massifs from the Siberian Craton, including the Tuva, Central Mongolia, and Erguna massifs, but also the development of a passive margin succession along the southern margin of the Siberian Craton (Wu et al. 2012;Tang et al. 2013). The Ergunahe Group was most likely deposited in a Neoproterozoic back-arc basin environment along the southern margin of the Siberian Craton.

Conclusions
New detrital zircon U-Pb ages for Ergunahe Group and Wubinaobao Formation units within eastern Inner Mongolia, combined with the relationships between units in this area, give rise to the following conclusions.
(1) The Ergunahe Group within the Erguna Massif formed at 712-738 Ma, indicating Proterozoic sedimentation, whereas the Wubinaobao Formation formed between 466 Ma and the late Silurian.
(2) The Ergunahe Group and Wubinaobao Formation sediments were sourced from exposed units within the study area during rapid sedimentation. (3) The detrital zircon ages presented here provide evidence of a Neoproterozoic magmatic event within the Erguna Massif, suggesting a link between this massif and the Siberian Craton.