2. College of Earth Sciences, Jilin University, Changchun 130061, China;
3. Shenyang Tests Research Centre, Northeast China Coal Field Geology Bureau, Shenyang 110016, China
The central Asian orogenic belt (CAOB) is the largest accretionary orogenic belt in the world. It stretches from the Urals in the west to the circum-Pacific orogenic belt in the east, and is bounded to the north by the Siberian Craton and to the south by the Tarim-North China Craton (Safonova et al., 2011, 2009; Xiao et al., 2010; Jian et al., 2008; Windley et al., 2007; Xiao et al., 2003; Badarch et al., 2002; Jahn et al., 2000). The tectonic history of the CAOB reflects the evolution and final closure of the Paleo-Asian Ocean over the course of the Phanerozoic (Xiao et al., 2015, 2010). The belt that records the breakup of Rodinia and the subsequent amalgamation of the Tarim-North China and Siberian Craton (Han et al., 2015); thus, its geological feature is complex and diverse, comprising a mélange of microcontinents and crustal blocks, ophiolite and blueschist belts, and rocks originating in magmatic arc and back-arc and fore-arc basin settings (Fig. 1; Xiao et al., 2015).
This study examined the central part of the accretionary orogenic belt located along the northern margin of the North China Craton (NCC; Figs. 1, 2). Geologically, the study area is located within the Early Paleozoic Bainaimiao Arc Complex, which is bound to the north by the Chifeng-Bayan Obo fault and to the south by the Xar Moron fault. North of the Chifeng-Bayan Obo fault is the Early Paleozoic Onder Sum subduction-accretion complex, and south of the Xar Moron fault is the Precambrian North China Craton (Jian et al., 2008; Xiao et al., 2003). The area of Sonid Right Banner, Which is the foucus of the present study, is dominated by the highly metamorphosed and deformed Onder Sum and Bainaimiao groups. It is generally considered that the Onder Sum Group is a metamorphic-accretionary complex belonging to the Onder Sum-Kedanshan ophiolite belt, which is formed during the Cambrian to Middle Silurian (Jian et al., 2008; Wu et al., 1998; Zhang and Wu, 1998; Nie et al., 1994; Tang, 1990; Wang, 1985). Some authors have suggested that the metamorphosed volcanic rocks of the Bainaimiao Group belong to the Early Paleozoic Bainaimiao Island Arc (Xu et al., 2013; Jian et al., 2008; Xiao et al., 2003; Tang et al., 1995; Shao, 1991; Hu et al., 1990), whereas others have suggested that they are remnants of a Meso-Proterozoic landmass or a rifted passive continental margin (Zhang and Wu, 1999; Zhang et al., 1996; Nie et al., 1995, 1993). Based on zircon ages obtained from a rhyolite and two andesites from the Bainaimiao Group (which range from 474 to 436 Ma), and the age of a low-P/T metamorphic complex (462–437 Ma), Zhang et al. (2013) proposed that the Bainaimiao Group represents the remnants of an Early Paleozoic continental arc in the southern part of the CAOB. These previous explanations for the origin of the Bainaimiao Group are based mostly on the existing interpretations of the regional geology and tectonic evolution, rather than the characteristics of the volcanics themselves. This is due to a lack of systematic and detailed field observations and geochemical data relating to the Bainaimiao Group meta-volcanics, which hamper our understanding of their origin and evolution. In addition, the Early Paleozoic tectonic evolution of the northern margin of the NCC remains poorly constraint (Pei et al., 2016; Wang et al., 2016; Zhang et al., 2014). Thus, this study aimed to provide insights into the tectonic evolution of the CAOB in the Bainaimiao region by integrating detailed field studies, geochemical data, and zircon U-Pb dating of a typical section of the Bainaimiao Group in the Sonid Right Banner area.1 GEOLOGICAL BACKGROUND
Sonid Right Banner is located at the north of the Xuniwusu fault zone and the south of the Onder Sum suture zone, within central Inner Mongolia. The Bainaimiao Group east-west orientation and is widely exposed in the Bainaimiao mining area and also at Guodawusu and Baogedeaobao (Fig. 2).
The main lithological unit in the Xuniwusu-Maogaitu area is the Neoproterozoic Bayan Obo Group, which is widely distributed across the northern margin of the NCC. In the study area, this group has been juxtaposed against the Silurian Xibiehe Formation along a thrust fault (Liu et al., 2014). Early Paleozoic rocks in the study area include those of the Cambrian to Middle Silurian Onder Sum and Bainaiomiao groups, the Middle Silurian Xuniwusu Formation and the Upper Silurian Xibiehe Formation. The Onder Sum Group is divided into the Sangdalaihuduge and Haerhada formations. The former consists of greenschist rocks with a basaltic protolith (Li W B et al., 2012; Nie et al., 1993), whereas the latter comprises quartzites and quartz schists. Magmatic rocks in the Wulanhada-Tulinkai area comprise the Deyanqimiao amphibolite and adakitic rocks, including quartz diorite, trondhjemite, anorthosite, and dacite (Wang et al., 2015; Liu et al., 2003; Xu, 1988). The Bainaimiao Group consists of a volcano-sedimentary rock sequence that has undergone greenschist facies metamorphism. The meta-magmatic rocks that crop out in the southern part of the Bainaimiao Arc are dominated by gneissic quartz diorite, and these rocks host the Bainaimiao Cu-Au-Mo Deposit (Zhang et al., 2013; Li C D et al., 2012). The Bainaimiao Group is unconformably overlain by the Middle Silurian Xuniwusu Formation, which comprises pebbly sandstone, sandstone, siltstone, and mudstone. Upper Paleozoic rocks in the study area belong to the Carboniferous Benbatu and Amushan formations, and the Permian Sanmianjing Formation. Mesozoic volcano-sedimentary rocks unconformably overlie rocks of Paleozoic (and older) age. The intrusive rocks can be divided into two groups. The first group consists of Early Paleozoic granitoids, including the Tulinkai adakite (Liu et al., 2003), the Bainaimiao gneissic quartz diorite (Tong et al., 2010), and the Guodawusu-Chaganaobao tonalite-trondhjemite-granodiorite (TTG) suite. These rocks are distributed mainly between the Wulanhada and Xuniwusu faults. The second group comprises Early Permian granites, including syenitic granite, porphyritic granite, and quartz diorite. These rocks are found north of the Xuniwusu fault (Fig. 2).2 GEOLOGICAL CHARACTERISTICS 2.1 Distribution of Rocks within the Bainaimiao Group
The extensively studied Bainaimiao Group is well exposed in the Bainaimiao mining area (Fig. 3). It is unconformably overlain by the Xuniwusu Formation in the southern part of the area, and intruded by the Bainaimiao gneissic quartz diorite in the northern part. Based on detailed field observations of rock associations and spatial distributions, as well as geologic relationships, we constructed a cross-section through the Bainaimiao Group (Fig. 4).
Based on the structures and lithologies observed in the cross-section, we divided the Bainaimiao Group into an upper and a lower formations (Table 1). The lower formation comprises biotite-quartz schist, two-mica quartz schist, chlorite-plagioclase schist, chlorite-actinolite schist, garnet-plagioclase gneiss, biotite leptynite and plagioclase leptynite. The protolith is estimated to have been a suite of weakly acidic volcanic rocks > 672 m thick, comprising a thick basal unit of shaley feldspar-quartz sandstone and rhyolite overlain by basalt, followed by rhyolite interspersed with thin beds of argillaceous-arenaceous sandstone. These varying facies indicate a transition from a sedimentary depositional environment to an active volcanic environment during the generation of the formation. The alternation of sedimentary and volcanic rocks reflects the intermittent nature of volcanic activity in the area.
The upper formation is composed of metamorphosed andesitic tuff, epidote-chlorite schist, sericite-chlorite schist, chlorite-actinolite schist, and chlorite-sericite schist, interspersed with thin layers of quartz schist and lenticular crystalline limestone. The protolith is inferred to have been a suite of intermediate-basic volcanoclastic rocks and lavas, with a total thickness exceeding 819 m, comprising mainly basalt, basaltic-andesitic tuff, volcanic breccia, and tuffaceous fine-grained sandstone and siltstone, interspersed with thin beds of rhyolite and lenticular limestone. Our field observations and laboratory analyses suggest that the basaltic rocks were the products of effusive lava flows, whereas the andesitic-dacitic rocks were the products of pyroclastic flows associated with explosive eruptions. The basaltic rocks are characterized by blastoporphyritic textures and contain relic amygdaloidal-vesicular structures (Fig. 5a). The volcanic breccia and tuff units contain a wide range of clast sizes, suggesting that they are products of highly explosive eruptions. We infer that four eruptive cycles took place, based on the distribution of the limestone and argillaceous sandstone units, which would have been deposited during periods of volcanic quiescence. The second eruption cycle, which is relatively intact, shows a regular cyclicity.
The dykes that cross-cut the Bainaimiao Group have a range of lithologies, including diorite, porphyritic diorite, and porphyritic granite. They exhibit the characteristics of syntexis-type granites, suggesting that they were formed in an island arc setting (Nie et al., 1994).2.2 Petrological Characteristics of the Bainaimiao Group
The metamorphic rocks of the Bainaimiao Group exhibit a wide range of characteristics, and we have grouped them into four types, as described below. They are all of low metamorphic grade, and as such, primary structures are still clearly visible in most of the rocks. Locally, some of the rocks have experienced further hydrothermal alteration and dynamic metamorphism subsequent to the initial phase of metamorphism.
The first type of metamorphic rock is metamorphic volcanic tuffs (Figs. 6a, 6b), mainly in the form of meta-andesitic tuff, meta-andesitic crystal tuff, and meta-andesitic crystal tuff containing debris fragments. In the field, these rocks exhibit the directional growth of sericite and chlorite, and stretched feldspar crystals are aligned with the schistosity. In addition, palimpsest tuff textures are observed locally. The tuffs contain a minor amount of detrital terrigenous material, indicating that they are sedimentary tuffs. In the meta-andesitic tuff containing debris and crystal fragments, clasts of quartzite debris (~5% of the total rock content) are semi-rounded, with grain sizes of 1.0–2.5 mm. Plagioclase crystal fragments (10%–40% of the total rock content) are angular and have been carbonatized and saussuritized. Hornblende crystal fragments (5%–30% of the total rock content) have mostly been transformed to actinolite and exhibit a range of shapes and sizes, with aqua to yellow colours. Both the plagioclase and hornblende crystal fragments are aligned with the schistosity. The volcanic ash component of the matrix has been metamorphosed to produce actinolite, chlorite and biotite.
The second type of metamorphic rock is felsic in composition, comprising chlorite-sericite schist, garnet-bearing plagioclase gneiss, biotite-quartz schist, and biotite leptynite (Figs. 5e, 5f). These rocks are interbedded with volcanic tuffs and greenschist facies rocks within every member of the Bainaimiao Group. It is interpreted that the protolith was a combination of rhyolite, sedimentary tuff, siltstone, and argillaceous rocks.
The third type of metamorphic rock is of the greenschist facies (Figs. 6d, 6e). The main constituent minerals are actinolite, chlorite, and epidote, along with minor albite and quartz. These rocks are greyish-green to dark grey in colour with a schistose structure, and exhibit schistose-columnar blastic textures and/or granular lepidoblastic textures. The actinolite, plagioclase, and flaky minerals (e.g., chlorite and micas) are strongly aligned, defining lineations and foliations. The actinolite schist contains plagioclase phenocrysts and relic volcanic breccias that occur in lenses with long axes oriented parallel to the schistosity and contain clasts of 1–5 cm in diameter (Fig. 5d). In some areas, the actinolite schist has been pegmatitized, exhibiting 1 cm diameter euhedral actinolite phenocrysts (Fig. 5c). The protolith of the greenschist facies rocks is inferred to have consisted of basalt, andesitic basalt, and basaltic and andesitic tuffs.
Finally, the fourth type of metamorphic rock comprises crystalline limestones, which appear repeatedly throughout the Bainaimiao Group due to tectonic deformation. These rocks occur both as lenses and as continuous beds interspersed within greenschist-facies rocks (Fig. 5b). The crystalline limestones consist of carbonate with minor (<5%) clastic quartz and are characterized by a fine crystalline texture and massive structure.
The samples used for dating were taken from acidic meta-volcanic rocks from the two formations of the Bainaimiao Group. Sample 16TW42 is from the chlorite-sericite schist (Fig. 6d) found close to the top of the upper formation (112°31′08″N, 42°11′49″E). It is light yellow in colour, with a leipidoblastic and blastoporphyritic texture and schistose structure. Blastorhyolitic structure is locally observed. The schistosity is strongly developed and is locally deformed into asymmetric folds. The main constituent minerals are sericite (40%), quartz (25%), feldspar (25%) and chlorite (10%). Sericite and chlorite occur as fine scaly aggregations, whereas the quartz is granular and present either as distributed fragments or augen. The feldspar crystals are tabular and sericitized.
Sample 16TW3 is biotite leptynite (Fig. 6f) from the middle part of the lower formation (112°31′25″N, 42°12′50″E), beside a river in the northern part of the Bainaimiao mining area. It is light red in colour, with a schistose and granoblastic texture. The main constituent minerals are biotite (20%), plagioclase (45%), and quartz (35%). The biotite is flaky, exhibits a preferred orientation, and is locally subjected to strong chloritization. The plagioclase and quartz are both granular and the plagioclase has been strongly sericitized.3 ANALYTICAL METHODS
Zircon grains were picked from the samples at the Heibei Regional Geological and Mineral Survey, Langfang, China. The grains were selected based on well-developed crystal habit and being free of cracks and inclusions when inspected under a binocular microscope. After mounting the zircons in resin and polishing their surfaces, they were imaged and analysed using transmitted and reflected-light microscopy and cathodoluminescence (CL). Zircon U-Pb analyses were conducted at the Tianjin Institute of Geology and Mineral Resources, Tianjin, China, where laser ablation-multi collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) was used to determine in situ U-Pb isotope ratios. For details of the instrument configuration and analytical process, see Li et al.(2010, 2007). The external standard GJ-1 was used as a reference material for U-Pb isotope analyses (Jackson et al., 2004). The data were processed using ICPMS DataCal, developed by Liu et al. (2009) at China University of Geosciences, and Isoplot software (Ludwig, 2003).
The initial samples were first pulverized and then cleaned to remove impurities such as amygdales. Major and trace element analyses were performed by ALS Analytical Testing, Guangzhou, China. Major oxide concentrations were determined using X-ray fluorescence spectroscopy (ME-XRF06). The analytical uncertainty for major element oxides is < 0.5%. Trace element concentrations were determined using inductively coupled plasma-mass spectrometry (ICP-MS) (Agilent 7700x) following sample dissolution in acid using teflon bombs. The analytical uncertainties for the trace element data are 1%–5% for abundances of > 1 ppm, and 5%–10% for abundances of < 1 ppm.4 RESULTS 4.1 Zircon U-Pb Ages
The results of the zircon U-Pb isotope analyses and derived zircon ages are listed in Table S1. The CL images in Fig. 7 show the typical characteristics of the analysed zircons. Figure 8 shows the U-Pb data obtained from the zircons, plotted on concordia diagrams.4.1.1 Sample 16TW42
Sample 16TW42 (chlorite-sericite schist) has a rhyolitic protolith. The sizes of the zircon crystals vary from 50 to 150 μm, and they typically exhibit aspect ratios of 2 : 1. Most zircons are euhedral to subhedral, occurring as elongate columnar crystals, and exhibit clear oscillatory growth zoning in CL images (Fig. 7). These features, combined with their high Th/U ratios (0.32–0.78), suggest that the zircons have an igneous origin (Belousova et al., 2002; Koschek, 1993). A total of 19 zircon analyses yielded 206Pb/238U ages of 472 to 484 Ma and a weighted mean age of 478±2 Ma (MSWD = 1.4, n = 19; Fig. 8a). This is interpreted as the age of formation of the protolith, based on the magmatic petrographic properties of the zircons.4.1.2 Sample 16TW3
The protolith of the biotite leptynite is also rhyolite. The CL images of the zircons (Fig. 6) show that they are idiomorphic to subhedral, with elongate columnar habits. The sizes of the zircons are between 50 and 250 μm, with aspect ratios of 2 : 1. Again, they demonstrate magmatic characteristics in the form of clear, narrow, oscillatory zoning, and they do not exhibit metamorphic rims. Their Th/U ratios are higher than 0.4 (0.42–0.77). All these characteristics suggest a magmatic origin. A total of 21 zircon analyses yielded 206Pb/238U ages from 493 to 504 Ma and a weighted mean age of 499±2 Ma (MSWD = 0.64, n = 21; Fig. 8b). This age is interpreted as the crystallization age of the rhyolite.4.1.3 Age of the Bainaimiao Group
Our U-Pb data indicate that the age of formation of the protolith of the biotite leptynite found in the middle part of the lower formation of the Bainaimiao Group is 499±2 Ma, corresponding to the Late Cambrian. The age of formation of the protolith of the chlorite-sericite schist in the upper part of the upper formation is 478±2 Ma, corresponding to the Middle Ordovician. The protolith of the meta-andesite of the Bainai-miao Group has been dated at 450±4 Ma (LA-MC-ICP-MS method; Gu et al., 2012), corresponding to the Late Ordovician. In addition, Zhang et al. (2013) used sensitive high-resolution ion microprobe (SHRIMP) analyses to obtain U-Pb zircon ages for the protoliths of the meta-rhyolite (474±7 Ma) and meta- andesites (453±7 and 436±9 Ma) of the Bainaimiao Group, and Li et al. (2015) reported U-Pb zircon ages of 465±1 Ma for meta-volcanic rocks of the Bainaimiao Group. Finally, zircon ages (derived using LA-ICP-MS) of 493±3 and 489±2 Ma have been obtained for the chlorite-plagioclase schist and the chlorite-actinolite-plagioclase schist of the Bainaimiao Group, respectively (Zhou et al., 2017).
Tong et al. (2010) dated the quartz diorite (459±3 and 454±14 Ma) that intrudes granitoid rocks in the Bainaimiao Group (associated with late-stage arc formation) in the northeast of the Bainaimiao mining area, providing a Middle Ordovician upper age limit for the granitoid rocks. In addition, Zhao et al. (2013) conducted Re-Os isotope analyses of molybdenite samples from the Bainaimiao Copper Deposit and obtained a weighted mean age of 447±4 Ma (Late Ordovician). Regionally, the Bainaimiao Group is unconformably overlain by, and therefore must pre-date, the Middle Silurian Xuniwusu Formation. Thus, the Bainaimiao Group is inferred to have formed between the Cambrian and the Middle Silurian.4.2 Geochemical Characteristics of the Meta-Volcanic Rocks
The major and trace element compositions of 11 representative samples of the Bainaimiao Group meta-volcanic rocks (greenschists and felsic metamorphic rock) collected in the Sonid Right Banner area are listed in Table S2. The analysed samples are of low metamorphic grade, with well-preserved primary rock structures. It was concluded from petrographic and geochemical analyses that the samples all had volcanic protoliths.
The samples have experienced varying degrees of chloritization and carbonation, and show high loss on ignition (LOI). The greenschist samples are strongly altered. After accounting for LOI, the major element compositions of the samples were as follows: SiO2 = 46.85 wt.%–78.95 wt.%, Al2O3 = 11.08 wt.%–19.76 wt.%, MgO = 0.71 wt.%–13.95 wt.%, Ti2O = 0.13 wt.%–0.94 wt.%, and Na2O+K2O = 2.01 wt.%–5.73 wt.%. As the meta-volcanic rocks of the Bainaimiao Group have undergone significant alteration, we used a Nb/Y vs. Zr/TiO2 diagram (Fig. 9), based on high-field-strength elements (HFSEs), to classify the rock compositions. This reveals that the protoliths were predominantly basalt and rhyolite with minor andesite. Similarly, the Hf/3 vs. Th vs. Nb/16 diagram (Fig. 11c), based on HFSEs with a level of activity, indicates that the protoliths of the meta-volcanic rocks of the Bainaimiao Group have a calc-alkaline affinity.
On Harker diagrams (unpublished data), the greenschist samples display increasing Na2O and P2O5, and decreasing MgO, CaO and TFeO contents with increasing SiO2. The felsic meta-volcanic rocks display increasing Na2O and decreasing Al2O3, K2O, and P2O5 contents with increasing SiO2. These trends imply that fractional crystallization occurred during magmatic evolution.
The chondrite-normalised rare earth element (REE) diagram (Fig. 10a) shows that the Bainaimiao Group rocks are enriched in LREEs relative to HREEs. In the greenschist rock samples, ΣREE = 40.54 ppm–168.12 ppm, LREE/HREE = 3.4–8.8, (La/Yb)N = 3.08–10.9, (La/Sm)N = 1.82–3.95, (Gd/Yb)N = 0.98–2.01, and δEu = 0.77–1.37. In the felsic meta-volcanic rock samples, ΣREE = 63.87 ppm –133.43 ppm, LREE/HREE = 3.9–10.4, (La/Yb)N = 2.92–9.89, (La/Sm)N = 2.81–5.43, (Gd/Yb)N = 0.69–1.11, and δEu = 0.57–1.05. A mid-ocean ridge basalt (MORB) normalized trace element spidergram (Fig. 10b) shows that the greenschist samples contain an obvious enrichment in the large-ion lithophile elements (LILEs) K, Rb, and Ba, and depletion in the HFSEs Nb and Ta, and that the Cr content is highly variable (23.5 μg/g–1 530 μg/g). The felsic meta-volcanic rocks are characterized by enrichments in the LILEs Rb and Ba, and in the HFSEs Zr and Hf. However, they exhibit depletions of the LILE Sr and of the HFSEs Nb and Ta, and also of the compatible elements Cr and Ni (Cr = 3.1 μg/g–22.1 μg/g, Ni = 1.58 μg/g–9.65 μg/g).5 DISCUSSION 5.1 Rock Formation and Tectonic Environment
On Harker diagrams (unpublished data), Na2O and P2O5 contents in the greenschist rocks increase with SiO2 content, whereas the MgO, CaO, and TFeO contents decrease. In the felsic meta-volcanic rocks, Na2O content increases with increasing SiO2, whereas Al2O3, K2O and P2O5 contents decrease. These geochemical trends indicate that fractional crystallization of ferric minerals, such as pyroxene, and re-melting of phosphorous-containing minerals, such as apatite, occurred within the basic magma, and that fractional crystallization of apatite occurred within the acidic magma. In addition, the depletion in TiO2 suggests the fractional crystallization of Ti-containing minerals, such as sphene. These inferences are consistent with our petrographic observations.
The protoliths of the Bainaimiao Group, which comprise basalts, rhyolites, and minor andesite, belong to the calc-alkaline series of volcanic rocks. According to Fig. 11a, the compositional data all fall into the area that corresponds to a calc-alkaline basaltic composition. The REE patterns (Fig. 10a) and trace element spidergrams (Fig. 10b) of the meta-volcanic rocks of the Bainaimiao Group are most akin to those of Andean-type calc-alkaline arc volcanic rocks. Compared with typical island arc volcanic rocks, the rocks of the Bainaimiao Group exhibit the characteristics of an active continental margin arc, displaying a strong affinity to continental crust.
The greenschist rocks are enriched in LILEs (Rb, Ba, and K) and depleted in HFSEs, which are both characteristics of subduction-related magmatism. According to Figs. 11b, 11c, the Bainaimiao Group data plot within the areas corresponding to calc-alkaline basalts and subduction-zone basalts, again implying that the meta-volcanic rocks of the Bainaimiao Group are related to subduction. According to Fig. 11d, the data plot within the area corresponding to volcanic arc basalts, further suggesting that the Bainaimiao Group rocks initially formed in a continental margin arc environment. Partial melting and fractional crystallization generally do not have a significant effect on incompatible element ratios. However, assimilation of continental crust during magma ascent tends to increase the magmatic concentrations of SiO2, K2O, Zr, Hf, Th, Cs, Rb, and Ba, along with a rise in La/Nb and Zr/Nb ratios, and a fall in Ti/Yb and Ce/Pb ratios (MacDonald, 2001; Baker et al., 1997). The La/Nb ratio positively correlates with the SiO2 content of the greenschist rocks, indicating that material derived from continental crust contributed to their magmatic evolution. Therefore, the basaltic magma is inferred to have been a product of mantle wedge metasomatism within the subduction zone of an active continental margin.
Rhyolites generally form via partial melting of crustal material in the presence a geothermal anomaly (Deng et al., 1987). The felsic meta-volcanic rocks in the Bainaimiao Group are widely distributed. Their relatively high SiO2, Al2O3 and Na2O contents, and low MgO, Cr and Ni contents indicate a predominantly crustal source. Their Eu and Sr anomalies also indicate that the magmatic source lay within the stability field of plagioclase. These data imply that the protolith of the felsic meta-volcanic rocks formed via partial melting due to a high geothermal gradient in the upper-crustal portion of a subduction zone.5.2 Early Paleozoic Tectonic Evolution of the Sonid Right Banner Area
A regional geological survey conducted by the Geological Bureau of the Inner Mongolia Autonomous Region in 1958, at a scale of 1 : 1 000 000, named the ferriferous metamorphic rocks outcropping at Onder Sum-Tulinkai the Onder Sum Group, and compared it with copper-bearing metamorphic rocks of the Bainaimiao area. An 40Ar/39Ar isotopic age of 446±15 Ma has been obtained for glaucophane within the greenschist rocks of the Sangdalaihuduge Formation (Tang, 1992). In addition, de Jong et al. (2006) obtained 40Ar/39Ar isotopic ages of 453±2 and 449±2 Ma for phengite within mylonites of the Haerhada Formation in northern Wulangou. It is inferred that these ages, which are obtained from high-grade metamorphic minerals, correspond to a stage of high-pressure metamorphism during subduction and accretion. An age of 480±2 Ma has been obtained for a meta-gabbro that cross-cuts greenschist rocks in the Tulinkai area (Jian et al., 2008).
Li C D et al. (2012) obtained LA-MC-ICP-MS U-Pb zircon ages for both the meta-andesite of the upper Sangdalaihuduge Formation (470±2 Ma) and the Haerhada Formation (445–480 and 424–438 Ma). Consequently, it is inferred that the age of formation of the Onder Sum Group is Cambrian to Middle Silurian. The Early Paleozoic strata and rock associations in the Bainaimiao area have been studied extensively since the "Bainaimiao Group" was identified by the Geological Bureau of the Inner Mongolia Autonomous Region in 1965. The flysch sediments of the Middle–Late Silurian Xuniwusu Formation were first identified by Hu (1988) to the south of Bainaimiao. These are uncomfortably overlain by molasse sediments of the Late Silurian Xibiehe Formation, indicating that the northern margin of the NCC experienced a period of relative tectonic quiescence during the Early Paleozoic.
The Deyanqimiao amphibolite, found to the south of Onder Sum, is related to the Onder Sum subduction zone. It represents the remnants of an oceanic island arc that formed by asthenospheric convection following ocean-ocean collision. It has similar characteristics, in terms of mineralogy, rare earth element distribution, isotopic age, spatial distribution, and protolith type, to the amphibolitic xenolith found at the base of the Bainaimiao Group. The Deyanqimiao amphibolite yields a zircon U-Pb SHRIMP age of 490±5 Ma (Wang et al., 2015).
The Sonid Right Banner area is located within the Early Paleozoic orogenic belt along the northern margin of the NCC, which lies north of the Xuniwusu fault zone and south of the Onder Sum suture zone. The nature of Early Paleozoic strata and igneous rocks in the Sonid Right Banner area suggests that the region experienced intense tectonic activity during this period.
The low-P/T metamorphic complex located close to the Bainaimiao mining area yields SHRIMP U-Pb ages that indicate two anatectic/crustal melting events at 462±11 (sillimanite gneiss) and 437±5 Ma (plagioclase-hornblende gneiss; Zhang et al., 2013). The gneissic quartz diorite in the north of the Bainaimiao mining area formed broadly contemporaneously at 459–454 Ma (according to LA-MC-ICP-MS zircon analyses; Tong et al., 2010), and exhibits geochemical properties typically associated with strongly deformed arc volcanic rocks (Jian et al., 2008). The ages of these rocks correspond with the age of formation of the glaucophane in the greenschist rocks of the Onder Sum Group, suggesting that they all formed contemporaneously during a regional tectono-metamorphic event.
The Tulinkai adakite rock association in the southern part of the Onder Sum region contains quartz diorite, trondhjemite, anorthosite, and dacite, indicating Paleozoic subduction in the region (Liu et al., 2003). Zircon U-Pb ages measured using SHRIMP analyses indicate that the quartz diorite was emplaced first at ~467±13 Ma, followed by high-grade metamorphism and partial melting at ~451±18 Ma, the eruption of dacite at ~459±8 Ma, and trondhjemite formation at ~451±7 Ma. The anorthosite dyke was the last to form at ~429±7 Ma. Jian et al. (2008) obtained similar ages for quartz diorite, dacite, and an anorthosite dyke of 454±3,458±3, and 425±2 Ma, respectively. In addition, a zircon age of 419±10 Ma has been derived from a granodiorite that intrudes the Bainaimiao Group and has been associated with regional collision (Zhang et al., 2013). During our field investigations, we identified Early Silurian (432–442 Ma) TTG rocks at Guodawusu-Chaganaobao in the eastern part of the Bainaimiao mining area. This indicates that Paleozoic subduction in the region occurred over a prolonged period of time.
The ages of the above-mentioned granitic rocks are comparable with the ages of formation of the Onder Sum and Bainaimiao groups. Field studies conducted by Li W B et al. (2012) found that the Onder Sum Group is not only an ophiolite association, but also contains basaltic, basalt-andesite, and andesitic rocks generated in an intra-oceanic arc setting, implying that ocean-ocean collision occurred in the Onder Sum oceanic basin during the Ordovician. Therefore, the Onder Sum Group should be described as an accretionary complex that contains oceanic crustal and intra-arc rocks with different modes and timings of genesis. It is inferred from these observations that the Bainaimiao and Onder Sum groups represent an archipelago system, given their similar ages and their close connection in space, which indicate the concurrence of multiple subduction zones. With regard to the two metamorphic belts developed to the north and south of Sonid Right Banner, the northern belt is inferred to represent an intra-oceanic arc subduction zone and the southern belt is inferred to represent the Bainaimiao volcanic arc subduction zone. In addition, structural studies of the Onder Sum ophiolitic mélange show that it consistently exhibits a top-to-the-NW sense of shear, and south-dipping folds with L/L > S lineations reflect the southward subduction of the Paleo-Asian Ocean (Wang, 2014; Shi et al., 2013). Considering that the Onder Sum subduction-accretion complex represents the final stage of closure of the Paleo-Asian Ocean (Xiao et al., 2003), we infer that the Ordovician adakites at Tulinkai, the quartz diorite at Bainaimiao, and the Guodawusu-Chaganaobao TTG rocks can then be attributed to the Caledonian orogenic stage in the region. Thus, the geological record preserved in the Sonid Right Banner area documents a prolonged period of subduction of the Paleo-Asian oceanic plate beneath the northern margin of the NCC, transforming the northern margin of the NCC into an active continental margin. It is inferred that orogenesis ceased during the Middle–Late Silurian, as an east-west trending foreland basin of this age occurs in the Bainaimiao area, hosting a sedimentary sequence that comprises flysch overlain by molasse (Tang, 1992; Hu et al., 1990). In addition, the sequence includes a basal conglomerate, which contains pebbles of underlying litho-tectonic units. Thus, magmatism and accretionary tectonics must have terminated around this time. This interpretation is supported by an undeformed pegmatite dated at 411±8 Ma, which cuts the low-P/T metamorphic complex in the Bainaimiao area (Zhang et al., 2013). Following the end of orogenesis, a Carboniferous limestone platform was deposited across the Sonid Right Banner area (BGMRIM, 1991).
The inferred Early Paleozoic tectonic evolution of the NCC in the Sonid Right Banner area is shown in Fig. 12. From the Late Precambrian to the Early Cambrian, the Onder Sum ophiolite was formed due to sea-floor spreading in the Paleo-Asian Ocean; it was then transported from north to south with the oceanic plate. The Bainaimiao area, located along the northern margin of the NCC, was being continuously subjected to extrusion and strain accumulation. From the Cambrian to the Ordovician, ocean-ocean collision occurred to the south of the Onder Sum ophiolite, resulting in the formation of accretionary complexes and arc volcanic rocks, and in the accretionary complexes at Onder Sum-Tulinkai being cross-cut by gabbro. Following this, the oceanic crust started to subduct beneath the continental crust of the NCC, resulting in the eruption of acidic magmas upon partial melting of the continental crust. The northern margin of the NCC thus became an active continental margin. In the southern part of the subduction zone, tephra deposits of the Bainaimiao Group were deposited in a terrestrial environment. As subduction progressed, metasomatism occurred in the mantle wedge, resulting in the eruption of basic volcanic rocks in the Bainaimiao area.
From the Ordovician to the Silurian, continued plate subduction and magmatic activity resulted in orogenesis. The collision between the Deyanqimiao Island Arc and the Bainaimiao Volcanic Arc caused metamorphism of, and intrusive activity within, the Bainaimiao Group, resulting in the low-P/T metamorphic rocks and gneissic quartz diorite intrusives that characterize the present-day Bainaimiao Group. At the same time, adakites and blueschists were formed in the Onder Sum area, and a fault was developed to the south of Tulinkai. Concurrent with subduction during the Middle–Late Silurian, flysch deposits of the Xuniwusu Formation accumulated in the southern part of the subduction zone. Orogenic activity led to the Xuniwusu Formation being laid down unconformably above the Bainaimiao Group. After a phase of tectonic movement during the Caledonian Period, molasse deposits of the Xibiehe Formation were rapidly laid down above the Onder Sum and Bainaimiao groups.6 CONCLUSION
(1) The Bainaimiao Group contains two different formations. The lower formation comprises mainly intermediate-felsic volcanic rocks, predominantly rhyolite, with a small amount of clastic rock and basalt. The upper formation comprises mainly intermediate-mafic volcanic rocks, including basalt, basaltic-andesitic tuff, volcanic breccia, lenticular limestone, and tuffaceous fine-grained sandstone and siltstone, interspersed with thin beds of rhyolite and lenticular limestone.
(2) The results of zircon LA-MC-ICP-MS dating (this study), along with previous data, confirm that the Bainaimiao Group was formed during the Cambrian to Middle Silurian. The group is sub-alkaline, exhibiting characteristics of the calc-alkaline series and is composed of basalt, rhyolite and minor andesite. The protolith of the greenschist rocks of the group is inferred to have originated from partial melting of sub-arc mantle wedge that had already been metasomatized by subducted-slab-derived fluids. The protolith of the felsic meta-volcanic rocks of the group is inferred to have been generated by partial melting of upper continental crust. Both of the protoliths were formed along an active continental margin.
(3) During the Early Paleozoic, the Paleo-Asian Ocean underwent south-directed subduction beneath the northern margin of the NCC, forming the Bainaimiao active continental margin arc. Ocean-ocean collision occurred along the northern part of the continental margin, resulting in the formation of an intra-oceanic island arc and the Onder Sum accretionary complexes. This produced an archipelagic arc-basin system with accompanying widespread volcanism.ACKNOWLEDGMENTS
This study was financially supported by the Nation Key R & D Program of China (No. 2017YFC0601305-01) and the China Geological Survey (Nos. DD20160048-04, DD20160343-08, DD20160343-09). We thank the reviewers and the editors for constructive comments. We are grateful to the staff of the Tianjin Institute of Geology and Mineral Resources, China, for their assistance with zircon U-Pb analyses. We also thank Wanqiong Wang, Zhiwei Fan, and Xinhui Bai for help in the field. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-1202-9.
Electronic Supplementary Materials: Supplementary materi-als (Tables S1, S2) are available in the online version of this article at https://doi.org/10.1007/s12583-018-1202-9.
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