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Table of Contents
Abstract
1. Introduction
2. Geological Background And Petrography
3. Analytical Methods
3.1. Zircon U-Pb Dating And Hf Isotopes
3.2. Major And Trace Elements
4 Results
4.1. Zircon U-Pb Ages
4.2. Zircon Lu-Hf Isotopic Data
4.3. Major And Trace Elements
5. Discussion
5.1. Petrogenesis
5.1.1. Origin Of The Sr/Y Geochemical Signature
5.1.2. Petrogenetic Model
5.2. Tectonic Implications
5.2.1 Diversified Sources In Generation Of The Middle Permian Magmatism
5.2.2 Closure Of The Palaeo-Asian Ocean In Southeastern CAOB
6. Conclusions
Acknowledgments
Abstract
The high Sr/Y geochemical feature of granitoids can be attributed to various mechanisms, and elucidating genesis of high Sr/Y granitoids provides insights into the material recycling and magmatic processes at depth. In southeastern Central Asian Orogenic Belt (CAOB), many Middle Permian granitoids exhibit high Sr/Y ratios, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Palaeo-Asian ocean. Here we present new zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemical data for the Middle Permian high Sr/Y monzogranites from central Inner Mongolia, southeastern CAOB. LA-ICP-MS zircon U-Pb data shows that these high Sr/Y rocks were emplaced during 273–261 Ma. They are calc-alkaline, sodium-rich and metaluminous to weakly peraluminous, with enriched large-ion lithophile elements (Rb, Th, K and Pb) and depleted high field strength elements (Nb, Ta, P and Ti), suggesting a mafic lower crustal source rather than evolved potassic crustal materials. Their relatively low (Gd/Yb)N (1.1–2.0), (Dy/Yb)N (1.0–1.3), Nb/Ta (7.9–10.9) ratios and flat heavy rare earth element patterns are characteristics of derivation from a relatively shallow depth with amphibolite as dominant residue. They also have highly variable εHf(t) values (−8.2 to +10.0) and TDMC (1814 to 649 Ma), similar to those of the Early Palaeozoic high Sr/Y intrusions along the Bainaimiao arc belt. Combined with data from literatures, we suggest that the high Sr/Y monzogranites in this study were probably generated by reworking of the newly underplated juvenile high Sr/Y lower crust of the Bainaimiao arc belt. Moreover, taking into account the regional investigations, the sublinear distributed Middle Permian magmatic rocks in the southeastern CAOB were likely associated with the incipient slab break-off of the Palaeo-Asian oceanic lithosphere following initial collision between the North China craton and the South Mongolia terranes. ARTICLE HISTORY Received 11 October 2018 Accepted 2 February 2019
ARTICLE HISTORY Received 11 October 2018 Accepted 2 February 2019 KEYWORDS High Sr/Y granitoids; Bainaimiao arc belt; Collision; Middle Permian; Palaeo-Asian ocean; Slab break-off
1. Introduction
Granitoids constitute an essential component in generation of the Earth’s continental crust, extraction and emplacement of granitoids with diversely geochemical characteristics are the principal processes by which continental crust has become differentiated (e.g. Hawkesworth and Kemp 2006). Generally, the high Sr/ Y granitoids are attributed to high-pressure magmatic processes, involving (1) partial melting of subducted oceanic slab (Defant and Drummond 1990; Martin et al. 2005) or differentiation of basaltic magma (Macpherson et al. 2006; Castillo 2012) in subduction zone, and (2) partial melting of thickened or delaminated lower continental crust (e.g. Atherton and Petford 1993; Xu et al. 2002; He et al. 2011). However, melting experiments and modellings have shown that high Sr/Y melts can also be generated by low-pressure partial melting of the lower crust without eclogitic residues (Qian and Hermann 2013; Dai et al. 2017). In addition, numerous studies have demonstrated that source inheritance (e.g. Ma et al. 2012, 2015) and magma mixing (Streck et al. 2007) can also account for the generation of high Sr/Y magmas. Thus, deciphering the genesis of the high Sr/Y granitoids provides important insights into the material recycling and magmatic processes in the deep crust that caused by various geodynamic settings. The southeastern Central Asian Orogenic Belt (CAOB; Figure 1(a)) recorded significant information concerning the magmatic responses to the accretionary convergence CONTACT Shaocong Lai shaocong@nwu.edu.cn State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, 710069, China Supplemental data for this article can be accessed here. © 2019 Informa UK Limited, trading as Taylor & Francis Group between the South Mongolia terranes (SMT) and the North China craton (NCC) (Zhang et al. 2009a, 2016; Jian et al. 2010; Li et al. 2016b), driven by the subduction and closure of the Palaeo-Asian ocean since the Early Palaeozoic or Neoproterozoic (Xiao et al. 2003, 2015; Windley et al. 2007; Wilde 2015). Previous work has documented that the Carboniferous to Early Permian magmatic rocks in southeastern CAOB represent the prolonged arc magmatism in response to the subduction of the Palaeo-Asian ocean (Xiao et al. 2003; Zhang et al. 2009a, 2009b; Liu et al. 2013; Li et al. 2016b), and the Late Permian to EarlyMiddle Triassic magmatic rocks were mainly related to the terminal collision between the NCC and SMT along the Solonker suture zone (Xiao et al. 2009, 2015; Schulmann and Paterson 2011; Eizenhöfer et al. 2014; Li et al. 2016a, 2017). Recently, many Middle Permian intermediate-felsic intrusions were also identified along the south side of the Solonker suture zone (Figure 1; Table 1), which are considered to be generated by the magma mixing processes in MASH (melting, assimilation, storage, and homogenization) zone that induced by the southward subduction of the Palaeo-Asian ocean beneath the northern NCC (Zeng et al. 2011; Liu et al. 2014; Zhang and Zhao 2017), or by the mixing of metasomatized mantle-derived with various amounts of crust-derivedmagma during the post-orogenic extension (Jiang et al. 2013; Luo et al. 2013; Zhao et al. 2016) or in relation to the tectonic switching from the oceanic subduction to the terminal collision between the NCC and SMT (Liu 2010; Wang 2014). However, we have known little about the reworking of the juvenile lower crust in response to the final closure of the Palaeo-Asian ocean, which may have played crucial role in the Middle Permian magmatism in southeastern CAOB. In recent studies, many Middle Permian granitoids exhibiting high Sr/Y geochemical signature have been identified from the southeastern CAOB (e.g. Liu 2010; Zeng et al. 2011; Zhao et al. 2016). In this paper, we present new zircon U-Pb ages, Lu-Hf isotopes, major and trace element compositions for the Middle Permian Sr/Y monzogranites in the Bulitai area of central Inner Mongolia (Figure 1(b)). Together with data from previous studies in adjacent areas, we have identified that these Sr/Y rocks were products of the reworking of the juvenile high Sr/Y lower crust of Bainaimiao arc belt in response to the slab break-off of the Palaeo-Asian oceanic lithosphere in southeastern CAOB. Figure 1. (a) Simplified sketch map of the Central Asian Orogenic Belt (modified after Jahn 2004); (b) Sketch geological map of the central-western Xing-Meng Orogenic Belt (modified after Jian et al. 2008; Xiao et al. 2015), and distributions of the Permian intrusive rocks (based on the Solon-HolinGola region 1:500,000 geological map compiled by Tianjin Institute of Geology and Mineral Resource). Abbreviations: SMT = South Mongolia terranes; HOAC = Hegenshan ophiolite-accretionary complex; BAAC = Baolidao arc-accretionary complex; SSZ = Solonker suture zone; OSAC = Ondor Sum subduction-accretionary complex; CAF = Chagan’aobaoArongqi fault; XHF = Xilinhot fault; LXF = Linxi fault; XMF = Xar Moron fault; BCF = Bayan Obo-Chifeng fault. Literature data: Hao and Jiang (2010), Jiang et al. (2011), Liu (2010), Hao (2012), Zeng et al. (2011), Feng et al. (2013), Jiang et al. (2013), Luo et al. (2013), Liu et al. (2014), Wang (2014), Zhao et al. (2016), Zhang and Zhao (2017).
2. Geological background and petrography
The southeastern CAOB is regarded as a complex collage of arcs, microcontinents, remnants of oceanic crust and associated volcanic-sedimentary sequences that were amalgamated between the NCC and the SMT during the evolution of the Palaeo-Asian ocean (e.g. Xiao et al. 2003, 2009, 2015). From south to north, the southeastern CAOB consists of the Bainaimiao arc belt abutting the northern NCC (Zhang et al. 2014), the Ondor Sum subductionaccretion complex (Xiao et al. 2003; De Jong et al. 2006; Jian et al. 2008), the Solonker suture zone (Jian et al. 2010; Fu et al. 2018), the Baolidao arc-accretionary complex (Xu et al. 2013; Li et al. 2014), the Hegenshan ophioliteaccretionary complex (Miao et al. 2008; Jian et al. 2012; Zhou et al. 2015) and the SMT (Figure 1(b)). The Solonker suture zone is widely accepted to have marked the final closure of the Palaeo-Asian ocean in the southeastern CAOB (Xiao et al. 2003, 2015; Eizenhöfer et al. 2014, 2015; Wilde 2015). Ages of the tectonic mélanges and ophiolitic fragments distributed along the Solonker suture zone are consistently Early Permian (Jian et al. 2010; Song et al. 2015). The Ondor Sum subduction-accretion complex is characterized by ophiolitic fragments or mélanges with ages of ca. 490–450 Ma (e.g. Xiao et al. 2003; Jian et al. 2008; Shi et al. 2013), and Early Palaeozoic high-pressure metamorphic rocks that in relation to the southward subduction of the Palaeo-Asian ocean (De Jong et al. 2006). The Early Palaeozoic Bainaimiao arc belt is composed of metamorphic sedimentary and volcanic rocks and plenty of felsic plutons (Zhang et al. 2014), which are unconformably overlain by the Late Silurian to Early Devonian Xibiehe Formation continental molasse or quasi-molasse deposition (e.g. Zhang et al. 2010). Extensively Late Palaeozoic to Early Mesozoic magmatic activities occurred in the southeastern CAOB (Chen et al. 2009; Zhang et al. 2009a, 2016; Li et al. 2016a, 2017). The Carboniferous to Early Permian intermediate-acid intrusive and volcanic rocks are calc-alkaline or high-K calc-alkaline, metaluminous or weak peraluminous, representing the prolonged arc magmatism in response to the subduction of the Palaeo-Asian ocean in southeastern CAOB (e.g. Zhang et al. 2009a, 2009b; Liu et al. 2013; Li et al. 2016b). The latest Permian to Early-Middle Triassicmagmatismwas sublinear distributed along the Solonker suture zone, and was considered as collision-related products by many recent studies, such as the ca. 255–250 Ma E-MORB dolerite, sanukitoid, high-Mg diorite and anorthosite in the Solonker ophiolitic mélanges (Jian et al. 2010), the ca. 255–251 Ma calc-alkaline granodiorites in the southeastern Xilinhot area (Li et al. 2016a), and the ca. 251–245 Ma high Sr/Y granitoids in the Linxi area (Li et al. 2017). This study investigated the Dabusu pluton in the northern Bulitai, which is tectonically located at the Ondor Sum subduction-accretion complex (Figure 1 (b)). The Dabusu pluton with outcrop area over 500 km2 intrudes into the Late Carboniferous Benbatu Formation carbonate rocks and the Early Palaeozoic Ondor Sum Group metamorphic strata in the field (Figure 2). It is dominated by grey, medium- to finegrained monzogranite, with fine-grained syenogranite occurring in the centre and coarse-grained hornblende syenite occurring in northern periphery (Liu 2010). The central part and the north part of the Dabusu pluton are seriously intruded by a lot of NE-trending and NWtrending syenogranite veins, granite-porphyry veins and quartz veins, and are unconformably covered by the Jurassic-Quaternary sedimentary strata. Most surface outcrops of monzogranites, syenogranites and hornblende syenites show varying degrees of alteration. The monzogranite samples in this study were collected from the southwestern part of the Dabusu pluton, which processes relatively fresh outcrops and clear contacting relations (Figure 3(a)). The studied monzogranites are porphyritic with phenocrysts of plagioclase (25–30%) and K-feldspar (15–20%), the groundmass (45–55%) is composed of plagioclase, quartz, biotite, K-feldspar and hornblende (Figure 3(b)).
3. Analytical methods
3.1. Zircon U-Pb dating and Hf isotopes
Zircons from the studied monzogranites were selected by standard techniques of density and magnetic methods and further purified by hand-picking at the Langfang Diyan Mineral Separating Limited Company, Hebei Province, China. Cathodoluminescence (CL) images were taken at the Gaonian Navigation Technology Company (Beijing) to reveal the internal structures. Zircon U-Pb age determinations of the studied granitoid samples were conducted at the Tianjin Institute of Geology and Mineral Resources, Tianjin, China. The analyses were carried out by using a Finnigan Neptune ICP-MS equipped with a NewWave 193 nm excimer laser, and a 35 μm spot was used for the laser ablation of a single grain. Harvard zircon 91,500 was used as a standard, and NIST 610 was used to optimize the analytical results. Details of the analytical methodology are described by Li et al. (2009). The common Pb corrections were made following the method of Anderson (2002). Data processing was carried out using the Isoplot 3.0 program (Ludwig 2003). In situ zircon Lu-Hf isotope measurements were conducted by using a Nu Plasma II MC-ICP-MS coupled with a RESOLution M-50 193 nm laser at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. A 44 μm laser beam with a repetition rate of 6 Hz was used for the analysis. Detailed information of instrumental settings and analytical procedures were described in detail by Bao et al. (2017) and Yuan et al. (2008). The Harvard zircon 91,500 and MT standards yielded weighted 176Hf/177Hf ratios of 0.282264 ± 0.000030 and 0.282528 ± 0.000019, respectively. The initial 176Hf/177Hf ratios and εHf(t) values were calculated based on the chondritic 176Hf/177Hf ratio of 0.282785 and 176Lu/177Hf ratio of 0.0336 (Bouvier et al. 2008). The depleted mantle model ages (TDM) were calculated with reference to the depleted mantle source with 176Hf/177Hf ratio of 0.28325 and 176Lu/177Hf of 0.0384 (Griffin et al. 2000). The crustal model ages (TDMC) were calculated assuming a 176Lu/177Hf ratio of 0.015 for the average continental crust (Griffin et al. 2002).
3.2. Major and trace elements
Fresh chips of whole rock samples were carefully crushed and then powdered through a 200-mesh screen using a tungsten carbide ball mill. The major and trace element compositions were determined at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology (BRIUG), China. Major elements were analysed using a Philips PW2404 X-ray fluorescence spectrometry with analytical errors of <5%. The FeO content was determined using the classical wet chemical method. Trace elements were analysed using a Perkin-Elmer Sciex ELAN DRC-e inductively coupled plasma mass spectrometer (ICPMS) and the analytical uncertainties were better than 5%. Detailed information on the analytical procedures and instrument parameters are described by Gao et al. (2007).
4 Results
4.1. Zircon U-Pb ages
Five monzogranite samples from the Dabusu pluton were selected for zircon U-Pb dating. The results are given in Supplementary Table 1. Representative zircon CL images and corresponding U-Pb Concordia diagrams are shown in Figure 4. Zircons from all the samples are subhedral to euhedral, colourless or buff to transparent, with crystal length of 70–200 μm. Most zircon grains exhibit clearly oscillatory zoning in the CL images (Figure 4), which is typical feature of magmatic zircon. The studied zircons have varying Th and U abundances with Th/U ratios ranging from 0.06 to 2.17. These features are consistent with a magmatic origin (Hoskin and Schaltegger 2003). The analyses of 36 zircons from sample Db8-b1 yield 206Pb/238U ages ranging from 261 to 273 Ma, with a weighted mean age of 266 ± 1 Ma (MSWD = 2.3, n = 36) (Figure 4(a)). For sample Db9-b1, excluding one analysis with an older age of ca. 1729 Ma, the other 34 analyses gave a range of 206Pb/238U ages of 257 to 267 Ma, with a weighted mean age of 261 ± 1 Ma (MSWD = 2.5, n = 34) (Figure 4(b–c)). Twenty-six analyses of zircons from sample Db11-b1 yield 206Pb/238U ages between 255 Ma and 269 Ma, with a weighted mean age of 263 ± 1 Ma (MSWD = 2.5, n = 26) (Figure 4(d)). Twenty-five analyses were obtained from sample Db15-b1 and yielded 206Pb/238U ages of 271 to 274 Ma, with a weighted mean age of 273 ± 1 Ma (MSWD = 0.1, n = 25) (Figure 4(e)). A similar weighted mean age (272 ± 1 Ma) was also obtained from sample Db15-b10 (MSWD = 0.1, Figure 3. Representative field photographs and micrographs of the studied monzogranite from the Dabusu pluton. Mineral abbreviations: Pl = plagioclase; Kfs = K-feldspar; Bt = biotite; Qtz = quartz. n = 25) (Figure 4(f)), which yields 206Pb/238U ages ranging from 271 to 273 Ma.
4.2. Zircon Lu-Hf isotopic data
Representative zircons from the above five dated samples were also selected for in-situ Lu-Hf isotopic analysis, and the results are presented in Supplementary Table 2 and Figure 5. The initial 176Hf/177Hf ratios and εHf(t) values were calculated according to their 206Pb/238U ages. Twenty-six analyses of zircons from sample Db8-b1 have εHf(t) values of −0.1 to +7.3 and TDMC of 1295 to 824 Ma. Seventeen analyses of zircons from sample Db9-b1 yielded εHf(t) values of −2.4 to +10.0, corresponding to a TDMC of 1442 to 649 Ma. Twenty analyses of zircons from sample Db11-b1 yielded εHf(t) values and TDMC ranging from −3.5 to +7.5 and 1516 to 810 Ma, respectively. Nineteen analyses of zircons from sample Db15-b1 yielded εHf(t) values of +1.3 to +8.1, with TDMC ranging from 1212 to 776 Ma. Fifteen analyses of zircons from sample Db15-b10 yielded εHf(t) values of −8.2 to +9.0, corresponding to a TDMC from 1814 to 772 Ma.
4.3. Major and trace elements
The studied monzogranite samples are characterized by relatively uniform major and trace element compositions, and assay 68.85 to 72.18 wt.% SiO2 with Mg# Figure 4. Cathodoluminescence (CL) images of representative zircons and Concordia diagrams of the studied samples. Red and yellow circles indicate the locations of U-Pb dating and Hf analyses, respectively. values 40.7 to 48.3 (Supplementary Table 3). These rocks plot in the monzogranite field in a QAP diagram (Figure 6). They are varying from granite to granodiorite in the TAS diagram (Figure 7(a)) and belonging to the calc-alkaline series (Figure 7(b)). The studied monzogranites also show sodium-rich affinity with high Na2O/K2 O ratios (2.07–4.39) (Figure 7(c)). They also display moderate Al2O3 concentrations (14.42–15.38 wt.%), variable A/CNK values (molar Al2O3/(CaO + Na2O + K2O) of 0.97–1.20 (Figure 7(d)) and are moderately fractionated as suggested by their DI (differentiation index) values (76–87). The monzogranites have moderate total rare earth element (REE) contents (39.4–74.0 ppm), they are enriched in light REEs (LREEs) (Figure 8(a)) with relatively low (La/Yb)N values (4.0–11.8), (Gd/Yb)N values (1.1–2.0) and slightly negative to slightly positive Eu anomalies (δEu = 0.85–1.28). Their normal mid-ocean ridge basalt-normalized (N-MORB-normalized) patterns are characterized by positive large-ion lithophile element (LILE) anomalies (e.g. Rb, Th, U, K and Pb) and negative high field strength element (HFSE) element anomalies (e.g. Nb, Ta, P and Ti) (Figure 8(b)). Moreover, these monzogranite samples have relatively high Sr (285–413 ppm) contents and Sr/Y ratios (29–44), and low Y (8.94–12.80 ppm) and Yb (1.10–2.03 ppm) concentrations, which are comparable to those of the high Sr/Y or adakitic-like rocks (Figure 9(a,b)).
5. Discussion
5.1. Petrogenesis
5.1.1. Origin of the Sr/Y geochemical signature
The monzogranites from the DBS pluton have U-Pb ages ranging from 272 Ma to 261 Ma, share similar geochemistry and zircon Lu-Hf isotopic compositions (Figures 5–8), and show geochemical features of high Sr/Y-type granitoids, which are generally characterized by high Sr, low Y and Yb with high Sr/Y ratios (Figure 9). As stated above, different petrogenetic models have been proposed for the origin of high Sr/Y rocks, including slab melting (e.g. Defant and Drummond 1990), high-pressure partial melting of thickened or delaminated lower crust (Atherton and Petford 1993; He et al. 2011), magma mixing (Streck et al. 2007), assimilation and fractional crystallization (AFC) processes of parental basaltic magmas (Castillo et al. 1999; Castillo 2012), and partial melting of an intrinsically high Sr/Y source (Qian and Hermann 2013; Ma et al. 2015). Figure 5. Zircon εHf(t) vs. U-Pb age diagram for the high Sr/Y monzogranites from the Dabusu (DBS) pluton. Data source for the Middle Permian Guangxingyuan (GXY) high Sr/Y rocks and alkaline intrusions in the southeastern CAOB are from Zhao et al. (2016). Data source for the coeval low Sr/Y granitoids in the southeastern CAOB are from Zhang and Zhao (2017) and Luo et al. (2013). Data source for the early Palaeozoic (Early Pz.) magmatic rocks in the Bainaimiao arc (BA) are from Zhang et al. (2014) and Zhou et al. (2018). High Sr/Y granitoids formed by AFC processes of basaltic magmas generally contain clinopyroxenes and amphiboles with complex compositional zonation and show systematic geochemical correlations (e.g. MgO, Cr, Ni concentrations) (Castillo et al. 1999; Macpherson et al. 2006). However, the high SiO2 contents (68.85–72.1 wt. %), the relatively uniform geochemistry, as well as the absence of clinopyroxenes in the high Sr/Y monzogranites in this study, preclude their derivation from primary basaltic magma by AFC processes. Moreover, the lack of mafic xenoliths/enclaves, mingling textures, together with the absence of contemporaneous basaltic rocks in the study area, are inconsistent with the mixing model between felsic and basaltic magmas. In water-rich (≥2 wt.% H2O) magmas, high H2O contents would suppress fractional crystallization of plagioclase until after mafic minerals (hornblende), resulting in relatively high Sr/Y ratios (Moore and Carmichael 1998; Richards and Kerrich 2007; Blatter et al. 2013). However, hornblende fractionation usually produces U-shaped REE patterns due to the high partition coefficients of middle REEs (MREEs) in hornblende (e.g. Garrison and Davidson 2003), which was not observed in the high Sr/Y rocks in this study. Generally, high Sr/Y granitoids generated by high-pressure (>45 km) partial melting of the lower continental crust are characterized by high (Gd/Yb)N Figure 7. (a) TAS diagram (Irvine and Baragar 1971; Middlemost 1994), (b) K2O vs. SiO2 diagram (Rickwood 1989), (c) K2O/Na2O vs. SiO2 diagram and (d) A/NK vs. A/CNK diagram (Peccerillo and Taylor 1976) showing the compositional variation of the high Sr/Y monzogranites from the DBS pluton. Data source for the DBS monzogranites are from Xiong et al. (2013) and this study. Previously reported coeval GXY (Zhao et al. 2016) and Chehugou (CHG) (Zeng et al. 2011) high Sr/Y rocks, alkaline intrusions (Zhao et al. 2016) and low Sr/Y rocks in the southeastern CAOB (Hao 2012; Jiang et al. 2013; Liu et al. 2014; Wang 2014; Zhao et al. 2016; Zhang and Zhao 2017), as well as the Early Palaeozoic magmatic rocks from the BA (Zhang et al. 2014; Zhou et al. 2018) are plotted for comparison. Figure 8. (a) Chondrite-normalized REE diagram and (b) N-MORB-normalized trace element diagram. The values of chondrite and primitive mantle are from Boynton (1984) and Sun and McDonough (1989), respectively. Data sources are the same as in Figure 6. ratios (up to 5.8) due to the significantly higher partition coefficients of heavy REEs (HREEs) in garnet (Moyen 2009; Huang and He 2010), but the studied high Sr/Y monzogranites have low (Gd/Yb)N ratios of 1.1–2.0, indicating that they are unlikely to have been formed by high-pressure partial melting of a lower crust source. Besides, their relatively low Nb/Ta ratios, high Zr/Sm ratios (Figure 10(a)), and flat HREEs patterns (Figure 8 (a)) also support an amphibolite melting, rather than the eclogite melting (Martin et al. 2005; He et al. 2011). Oceanic slab-derived high Sr/Y rocks usually exhibit high Mg#, Cr and Ni values because of the inevitable interaction between the primary magmas and the overlying mantle peridotite during magma ascent (e.g. Martin et al. 2005). The high Sr/Y monzogranites in this study process relatively low Mg# values (40.7–48.3), MgO (0.82–1.55 wt.%), TiO2 (0.33–0.52 wt.%), Cr (20.60–61.60 ppm) and Ni (3.00–24.10 ppm) contents, precluding a direct oceanic slab melting mechanism in their petrogenesis. Besides, their SiO2 contents (68.85–72.1 wt.%) and Rb/Sr ratios (0.1–0.3) are also much higher than those of the typical slab-derived high Sr/Y magma (Figure 10(b–f)). It is also crucial to examine whether the high Sr/Y geochemical signature of the rock samples are directly inherited from their source rocks (He et al. 2011; Ma et al. 2015). The high Sr/Y monzogranites in this study are calc-alkaline and sodium-rich with low K2O/Na2O ratios (0.23–0.48) (Figure 7(c)) and high Mg# values (40.7–48.3), these geochemical features are similar to those of the normal-arc series rocks generated in subduction zones (Rushmer and Jackson 2006; Zhu et al. 2017). The arcrelated affinity is also indicated by their trace element characteristics, such as enrichment in LILEs (e.g. Cs, Rb, Ba, Th, and U) and depletion in HFSEs (e.g. Nb, Ta, and Ti) (Figure 8(b)). It is noteworthy that the Early Palaeozoic (mainly Ordovician to Silurian) magmatic rocks in the Bainaimiao arc belt, which constitute a major component of the juvenile lower crust source of the Bainaimiao arc belt, are also characterized by high contents of Sr and low contents of Y and Yb with high Sr/Y ratios (up to 88.7) (Figure 9(a)) (Zhang et al. 2014; Zhou et al. 2018). These high Sr/Y rocks are composed of gabbroic diorites and diorites, and belong to the calc-alkaline, sodium-rich, metaluminous to weak peraluminous series (Figure 7). More importantly, a large majority of these Early Palaeozoic magmatic rocks have variable Hf isotopic compositions similar to those of the Middle Permian high Sr/Y monzogranites in this study (Figure 5). Partial melting of the lower crust with high Sr/Y ratios with normal crustal thickness generally generates high Sr/Y melts, and the lower the degree of partial melting of this source, the higher the Sr/Y ratios would be produced in the melt (Moyen 2009; Ren et al. 2018). Thus, we may suggest that the high Sr/Y geochemical signature of the high Sr/Y monzogranites in this study might be inherited from an intrinsically high Sr/Y crustal source, which was likely dominated by the Early Palaeozoic newly underplated magma beneath the Bainaimiao arc belt. A more detailed petrogenesis is described in the following section.
5.1.2. Petrogenetic model
The high Sr/Y monzogranites in this study have slightly negative to slightly positive Eu anomalies (δEu = 0.85–1.28), suggesting that plagioclase is minor or absent in their source residual. As pointed out above, these rocks were likely generated at relatively low pressures with amphibolite as dominant residual phases (Figure 10(a)). Their low (Dy/Yb)N ratios (1.0–1.3) also agree with a low-pressure melting condition, because melting of high Sr/Y crustal materials at low pressures with amphibole as the dominant residual phases would produce magmas with Figure 9. Plots of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b) for the studied monzogranite samples. Fields for adakite and arc calcalkaline rocks are based on Defant and Drummond (1990) and Petford and Atherton (1996). The crystal fractionation paths of the primary minerals in Figure 9(a) are from Castillo et al. (1999). The batch partial melting trends in Figure 9(b) are based on Petford and Atherton (1996). substantially low (Dy/Yb)N ratios (Macpherson et al. 2006; S.B. Zhang et al. 2009; He et al. 2011). In the (La/Yb)N vs. YbN diagram (Figure 9(b)), the high Sr/Y monzogranites in this study plot closed to the 7%-Grt amphibolite melting curve, further indicating a relatively shallow melting depth (around 40 km; Rapp et al. 1999) for their generation. According to Zhang et al. (2014), most Early Palaeozoic high Sr/Y magmatic rocks in the Bainaimiao arc belt are characterized by low initial 87Sr/86Sr ratios Figure 10. Discrimination diagrams of the high Sr/Y rocks. Note that Nb/Ta vs. Zr/Sm diagram (a) is from Martin et al. (2005); Mg# vs. SiO2, MgO vs. SiO2, Cr vs. SiO2 and Ni vs. SiO2 diagrams are modified after Long et al. (2015, and references therein); Th vs. Rb/Sr diagram is from Huang et al. (2009, and references therein). (0.70492–0.70765), and variable εNd(t) values (−4.9–5.2) and Nd isotopic model ages (0.78–2.39 Ga). Combined with their adakitic-like geochemical characteristics, high Nb/Ta and low Zr/Sm ratios (Figure 10(a)), as well as variable εHf(t) values and TDMC, these high Sr/Y rocks were suggested to be derived from the partial melting of the subducted slab of the Palaeo-Asian ocean with involvements of the overlying mantle wedge and the ancient crustal materials (Zhang et al. 2014a; Zhou et al. 2018). This means that the lower crust of the Bainaimiao arc belt was obviously subduction-modified and relatively heterogeneous during the Early Palaeozoic. Although the Middle Permian high Sr/Y rocks in this study exhibit similar major and trace element compositions, their relatively large variations in εHf (t) values (−8.2 to +10.0) and TDMC (1814 to 649 Ma) (especially the early Middle Permian samples) might be indicative of a relatively heterogeneous magma source. Thus, we argue that the variations in Hf isotopic composition of these high Sr/Y rocks were likely in relation to the intrinsical heterogeneity of the subduction-modified juvenile high Sr/Y lower crust beneath the Bainaimiao arc belt. Partial melting of crustal materials at relatively shallow depth would inevitably require an external heat source. In southeastern CAOB, the final closure of the Palaeo-Asian ocean was suggested to be started with the initial collision/amalgamation between the NCC and the SMT in the Middle Permian, which led to cessation of the prolonged arc magmatism, disappearance of the marine sedimentation, and formation of a regional angular unconformity in central Inner Mongolia (Jian et al. 2010; Eizenhöfer et al. 2014; Xiao et al. 2015; Li et al. 2014, 2016a, 2016b, 2017). Combined with the presence of the Middle Permian peralkaline-alkaline intrusions in the southeastern CAOB (Zhao et al. 2016), an extensional regime related to the ocean closure should be taken into account for the petrogenesis of the high Sr/Y monzogranites in this study. We suggest that slab break-off of the Palaeo-Asian oceanic lithosphere following the collision between the NCC and the SMT was a plausible mechanism. Because the tensile stresses between the buoyant continental lithosphere and the previously subducted oceanic lithosphere would lead to the detachment of the oceanic slab and formation of a slab window soon after the collision, which would enable upwelling of the asthenosphere through the slab window (Davies and von Blanckenburg 1995; Van Hunen and Allen 2011). Then, the heat from the underplated magma in response to the asthenosphere upwelling would trigger a linear zone of magmatism with limited spatial distribution (Li et al. 2016a), which is consistent with the linear distributed Middle Permian magmatism in southeastern CAOB (Figure 1(b)). Thus, we propose that the partial melting of the source materials of the high Sr/Y rocks in this study was likely triggered by the asthenosphere upwelling during the slab break-off of the subducted Palaeo-Asian oceanic lithosphere following the final ocean closure.
5.2. Tectonic implications
5.2.1 Diversified sources in generation of the middle Permian magmatism
In southeastern CAOB, the Middle Permian was previously regarded as a short magmatic hiatus caused by the initial collision between the NCC and the SMT (e.g. Li et al. 2016a, 2016b). However, more and more Middle Permian magmatic rocks were identified along the south side of the Solonker suture zone in recent years, they are composed of high Sr/Y calc-alkaline granitoids, low Sr/Y high-K calc-alkaline rocks and peralkaline-alkaline intrusions (Figure 1; Table 1). As discussed in the preceding sections, generation of the Middle Permian high Sr/Y monzogranites in this study was likely related to the reworking of the juvenile lower crust of the Bainaimiao arc belt. In comparison, the coeval high Sr/Y tonalites from the Guangxingyuan area (Zhao et al. 2016) have lower Mg# values, MgO, Cr, Ni contents (Figure 10(b–e)), as well as lower Na2O/K2O ratios and higher Yb contents than those of the high Sr/Y rocks in this study (Figure 9(b)). We suggest that these geochemical variations might be caused by the heterogeneous nature of the juvenile lower crust of the Bainaimiao arc belt. The high Sr/Y rocks from the Chehukou deposit in Chifeng area (Zeng et al. 2011) are dominantly high-K calc-alkaline and potassic-rich, with prominently enriched LREEs, high Sr/Y and (La/Yb)N ratios, low Y and YbN values (Figures 7–9). Considering that the Chehukou high Sr/Y rocks are exposed at the northern margin of the NCC, we propose that they were probably generated in relation to the partial melting of the ancient lower crust of the NCC at deeper crustal levels or higher pressures. The Middle Permian peralkaline-alkaline intrusions in southeastern CAOB were suggested to be formed though the partial melting of the subduction-modified lithospheric mantle with mixing of different amounts of the ancient crustderived magma during the asthenosphere upwelling (Zhao et al. 2016). The Middle Permian low Sr/Y magmatic rocks in southeastern CAOB are dominantly high-K calcalkaline, potassic-rich, showing large variations in major and trace element compositions, as well as Hf isotopic compositions (Figures 5, 7 and 8). It seems plausible that variable amounts of the ancient crust materials beneath the Bainaimiao arc belt might have been involved in the formation of those low Sr/Y magmatic rocks. The above inferences suggest that the magma source of the Middle Permian magmatism in southeastern CAOB is relatively complex, materials from the juvenile lower crust, ancient lower crust components and subduction-modified lithospheric mantle were all likely to be involved in themagma generation.
5.2.2 Closure of the Palaeo-Asian ocean in southeastern CAOB
Slab break-off of the subducted oceanic lithosphere is believed to be a natural consequence after the ocean closure due to the attempted subduction of continental lithosphere (Wortel and Spakman 2000; Atherton and Ghani 2002). Continental collision is not an instantaneous process, the evolution from initial collision to complete detachment of the oceanic slab may cost 5–40 Ma (e.g. Royden 1993; Andrews and Billen 2009; Van Hunen and Allen 2011). In southeastern CAOB, after the long-lived subduction of the Palaeo-Asian ocean since the Neoproterozoic (Xiao et al. 2003, 2015; Wilde 2015), the NCC and the SMT eventually collided following the ocean closure in the Middle Permian (Jian et al. 2010; Xiao et al. 2015; Li et al. 2016a, 2017). The available palaeomagnetic data also suggest that the NCC and SMT were very close during Early Permian (Zhang et al. 2014c). The collision between the NCC and SMT led to the cessation of the arc magmatism and marine sedimentation, accompanied by a short magmatic hiatus and a regional angular unconformity in the XMOB (e.g. Eizenhöfer et al. 2014; Li et al. 2014, 2016b). Furthermore, Li et al. (2016a, 2017) proposed that the collision between the NCC and SMT can be further divided into three stages, including the initial collision during Middle Permian, the slab break-off during Late Permian and the intracontinental contraction during Early-Middle Triassic. The Middle Permian high Sr/Y monzogranites in this study were likely derived from the partial melting of the juvenile lower crust beneath the Bainaimiao arc belt during slab break-off of the Palaeo-Asian oceanic lithosphere. The presence of this slab break-off process is also supported by many recent studies, for example, Jian et al. (2010) argue that the Late Permian to earliest Triassic igneous rocks (255–250 Ma) in the Mandula forearc mélange were derived from the decompression melting of the mantle peridotite as the result of the asthenosphere upwelling during the slab break-off of the Palaeo-Asian oceanic lithosphere. Li et al. (2016a) also suggested that the latest Permian magmatic flareup in the southeastern Xilinhot area was formed in response to the slab break-off. Slab break-off generally has three evolutionary stages, including (1) lithospheric tearing, (2) slow sinking of the detaching root associated with downward dragging of the overriding lithosphere, and (3) complete detachment with faster slab sinking and the overriding lithosphere rebounding (e.g. Ferrari 2004; Yuan et al. 2010). In this case, the Middle Permian high Sr/Y monzogranites in this study, as well as the contemporaneous magmatic rocks in southeastern CAOB, were more likely corresponding to the early stage of slab break-off (lithospheric tearing) (Figure 11), which indicates that slab break-off of the Palaeo-Asian oceanic lithosphere might have already started in Middle Permian. The more intense Late Permian magmatism in southeastern CAOB might be related to the late stage of the slab break-off (Jian et al. 2010; Li et al. 2016a). Afterwards, a sublinear distributed Early-Middle Triassic high Sr/Y magma belt was formed probably in response to the moderately thick stacking of crustal rocks due to the subsequently ongoing weak compression between the NCC and the SMT (Wang et al. 2015; Li et al. 2017), which marks the termination of Palaeo-Asian ocean. The regionally post-collisional extension probably occurred until the Early Mesozoic, as evidenced by the Triassic metamorphic core complex in the Sonid Zuoqi area (Davis et al. 2004), and the widespread Mesozoic A-type granitic rocks and alkaline complexes along the northern NCC (Wu et al. 2011; Zhang et al. 2012, 2014b).
6. Conclusions
(1) Zircon U-Pb dating indicates that the high Sr/Y monzogranites from the Dabusu pluton in central Inner Mongolia were emplaced at 273–261 Ma. (2) They are calc-alkaline, sodium-rich and LILEs enriched with variable εHf(t) values (−8.2 to +10.0) andwere likely formed by reworking of the juvenile lower crust of Bainaimiao arc belt at a relatively shallow melting depth. (3) These high Sr/Y rocks were associated with the slab break-off of the Palaeo-Asian oceanic lithosphere following the collision between NCC and SMT. Article highlights ● Presence of the Middle Permian high Sr/Y monzogranites in the southeastern CAOB. ● Reworking of the juvenile lower crust of the Bainaimiao arc belt. ● Magmatic response to the slab break-off of the Palaeo-Asian oceanic lithosphere
Acknowledgments
We are grateful to Haoran Wang, Zhen Wang, and Zhong Wang for their help with the field investigation. We thank Yu Zhu, Fangyi Zhang and Zezhong Zhang for their laboratory assistance. We also appreciate Yuan Yuan and Yaoyao Zhang for their insightful suggestions. Critical and constructive comments from the Editor and the anonymous reviewers significantly improved the quality of this manuscript. Disclosure statement No potential conflict of interest was reported by the authors. Funding This work was jointly supported by the China Geological Survey [1212011085490] and National Natural Science Foundation of China [41421002].
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