2. Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China;
3. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
The Central Asian orogenic belt (CAOB) is an ancient accretion-type orogenic belt located between the Siberian Plate and the North China Plate (Jian et al., 2008), and it presents a series of island arcs as well as forearc and back-arc basins, an ophiolitic belt, and microcontinents formed from the Neoproterozoic to the Mesozoic (Xu et al., 2013; Glorie et al., 2011; Kröner et al., 2011, 2010, 2007; Demoux et al., 2009; Xiao et al., 2009, 2003; Li, 2006; Buslov et al., 2004; Khain et al., 2003, 2002; Badarch et al., 2002; Şengör and Natal'in, 1996; Mossakovsky et al., 1994; Şengör et al., 1993; Hsu et al., 1991). In addition, the massive juvenile crust materials were formed during this period (Ma et al., 2017; Xu et al., 2013; Wang et al., 2012; Han et al., 2011, 1997; Jahn et al., 2009, 2000a, b; Hong et al., 2004, 1995). However, the tectonic evolution of this region during the Late Paleozoic and the closing time of the Paleo-Asian Ocean remain unclear.
One hypothesis asserts that the Paleo-Asian Ocean closed because of a collision between the Siberian and the North China plates, which occurred before the Late Devonian (Shao et al., 2014; Xu et al., 2014; Xu and Chen, 1997; Hong et al., 1994; Tang, 1992, 1990). However, another hypothesis states that the collisional orogenesis continued until the Late Permian–Early Triassic (Zhang and Zhai, 2010; Li et al., 2007; Zhang et al., 2006) and occurred concurrently with an Andeans-type continental margin.
In this paper, the latest data on two genetic types of granite found in the Xilinhot area were obtained based on regional geological surveys as well as petrographic, geochemical and zircon U-Pb and Lu-Hf isotope analyses of the granites, and we provide new evidence on the tectonic evolution of the CAOB.1 GEOLOGIC SETTING AND PETROGRAPHY
Based on the demarcation of geotectonic units, the Xilinhot area belongs to the suture zone that developed after the subduction and collision between the Siberian Plate and the North China Plate. The suture zone extends from the Solonker Mountains along the Sonidzuoqi and continues eastward to the Xilinhot area, and this zone represents the location of the Paleo-Asian Ocean subduction (Xu and Chen, 1997). After performing detailed surveys and field research, we discovered multistage intrusive rocks from the Carboniferous Period to the south and southeast of Xilinhot City, including A-type K-feldspar granite, monzonitic granite, granodiorite, and gabbro.
The Early Carboniferous A-type K-feldspar granites, which were mainly distributed on the northwest side of the Daituojika Mountain (Fig. 1), intruded into the schists of the Ordovician Bulongshan Group and were covered by glutenites of the Lower Jurassic Hongqi Group. These Early Paleozoic glutenites were then intruded by granodiorites (Zhou et al., 2016) of the Late Carboniferous and diorites of the Early Permian Period. The monzonitic granites of the Early Carboniferous Period were output as stocks, bosses, and dykes, and they intruded the biotite-plagioclase gneiss of the Early–Late Proterozoic. Subsequently, these granites were overlain by an unconformity of volcanic rocks of the Meiletu Group of the Lower Jurassic Period and then intruded by the gabbro and granodiorite (Zhou et al., 2016) of the Late Carboniferous Period. In the Yintou Valley, Saqinhua, Shihuiyao, and Gongsiari areas, prominent outcrops can be observed.
The A-type K-feldspar granites have a fresh pink color, exhibit subhedral-anhedral granules, and are fine-to-medium grained (Figs. 2a, 2b). These granites consist of K-feldspar (63%–66%; anhedral granules; 0.3–0.5 mm in diameter; and internal microcrystals of plagioclase), quartz (19%–22%; anhedral granules; 0.5–1 mm in diameter; and internal microcrystals of plagioclase and K-feldspar that constitute a graphic texture with K-feldspar), and plagioclase (15%; subhedral tabular granules; 0.3–1.5 mm; albite twins; and Np'^(010)=13º, which represents oligoclase).
The monzonitic granites are off-white in color at the fresh surfaces, and under a polarizing microscope, the minerals are subhedral and fine-to-medium grained (Figs. 2c, 2d). The monzonite granites consist of plagioclase (45%; subhedral tabular granules; 0.5–1.5 mm; albite twins; Np'^(010)=12º–15º, which represents oligoclase or andesine; and sericitization), K-feldspar (25%; anhedral granules; and 0.5–1.5 mm in diameter), quartz (25%; anhedral granules; and 0.5–1.5 mm in diameter), biotite (3%; flaky; ~0.8 mm; and little chloritization), and muscovite (2%; thin film; and 0.5–1 mm).2 ANALYTICAL METHODS 2.1 Zircon Morphology
The sample processing for the zircon separation involved crushing, initial heavy liquid separation, and subsequent magnetic separation. The representative zircons were manually selected and mounted on adhesive tape, embedded in epoxy resin, polished to approximately half their size, and then photographed in reflected and transmitted light (Song et al., 2002). The zircon structure and origin were studied by cathodoluminescence (CL) imaging of the zircon grains using a Quanta 400FEG environmental scanning electron microscope equipped with an Oxford spectrometer and a Gatan CL3+ detector at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. The accelerating voltage was 10 kV, and the beam current was 240 μA.
Zircon LA-ICP-MS U-Pb dating was conducted at the Isotope Laboratory of Tianjin Center, China Geological Survey, using a neptune MC-ICP-MS with a UP193-FX ArF-excimer laser-ablation system. The data were collected with a 35 μm spot, and the ablation depth was 20–40 μm. Helium was used as a carrier gas to enhance the transport efficiency of the ablated material. Zircon 91500 was the external standard for the age calculation (Wiedenbeck et al., 1995), NIST SRM 610 silica glass was the external standard for the concentration calculation (Pearce et al., 1997), and Si was the internal standard. The collection mode of the sample was an individual spot ablation, and the data were collected using one massive peak to one spot (peak jumping). We measured NIST SRM610 twice after each round of 15–20 zircon sample spots. The off-line selection and integration of the background and analysis signals and the time-drift correction and quantitative calibration for the trace element analyses and U-Pb dating were performed using ICPMSDataCal (Liu et al., 2009, 2008), and common Pb was corrected according to Andersen (2002). The reported ages were calculated, and concordia diagrams were constructed and weighted mean calculations were performed using the software Isoplot/Ex_Ver3 (Ludwig, 2003).2.2 Major and Trace Element Analyses
The whole-rock samples were crushed in a corundum jaw crusher (to a size of 60 mesh). Approximately 60 g was powdered in an agate ring mill to a size of less than 200 mesh. The major element contents of the samples were determined via the X-ray fluorescence analysis of fused glass beads using a spectrometer (3080E1; Rigaku, Tokyo, Japan), and the trace elements were determined by the ICP-MS (X. Series, Thermo Fisher Scientific, Germany) at the Wuhan Rock and Mineral Analysis Center, Ministry of Land and Resources Research, China. The analytical precision and accuracy of these methods were better than 5% for most elements. The ferric-ferrous iron proportions were determined by wet chemistry.2.3 Zircon Hf Isotopic Analysis
The experiments were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 ArF-excimer laser ablation system (Lambda Physik, Göttingen, Germany) that was hosted by the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. The energy density of the laser ablation used in this study was 5.3 J/cm2. A wire signal smoothing device is included in this laser ablation system to ensure the production of smooth signals even at low laser repetition rates of 1 Hz (Hu et al., 2012). Helium was used as the carrier gas within the ablation cell, and it was merged with argon (makeup gas) after the ablation cell. As we previously demonstrated (Hu et al., 2008a), a consistent 2-fold signal enhancement for the 193 nm laser was achieved in helium, which was more effective than argon gas. We used a simple Y-junction downstream from the sample cell to add small amounts of nitrogen (4 mL·min-1) to the argon makeup gas flow (Hu et al., 2008b). Compared with the standard arrangement, the addition of nitrogen in combination with the use of the newly designed X skimmer cone and the Jet sample cone in the Neptune Plus improved the signal intensity of Hf, Yb and Lu by a factor of 5.3, 4.0 and 2.4, respectively. All of the zircon data were acquired in single spot ablation mode at a spot size of 44 μm. Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of the ablation signal acquisition. The detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument along with the analytical methods are described in Hu et al. (2012).3 RESULTS 3.1 Zircon U-Pb Data
The zircon U-Pb analysis results for the two genetic types of granites are listed in Table 1. The zircon samples of the K-feldspar granite acquired from the Daituojika Mountain are mostly euhedral and shortly prismatic, 150–300 μm long, present length/width ratios of 1.5–2.7, and show common and clearly internal oscillatory zoning in the CL images (Fig. 3). Twenty spots of 18 zircons yielded Th/U ratios of 0.56–1.40, which is a distinct feature of magmatic zircons (Wu and Zheng, 2004; Möller et al., 2003; Rubatto and Gebauer, 2000). Except for one analysis, which determined a relatively old age for the sample (368±2 Ma), the other 19 spots showed consistent age results of 324–339 Ma (Fig. 4a) and a weighted mean 206Pb/238U age of 332±2 Ma, with a MSWD of 0.19 (Fig. 4b). For the monzonitic granite sample from the Nagawula area in Xilinhot, all of the zircons present a euhedral prismatic structure, are approximately 130–190 μm long, and have length/ width ratios ranging from 2 to 3 (Fig. 5). All of the spots have high Th/U ratios of 0.78–1.71, which suggests that these zircons formed from magma. Apart from the 5 sample spots that were not consistent or yielded relatively old ages of 348–387 Ma, the estimated ages of 15 of the 20 tested zircon grains were consistent (Fig. 6a) with the 206Pb/238U ages ranging of 323-329 Ma. The weighted mean 206Pb/238U age is 323±2 Ma, with a MSWD of 1.9 (Fig. 6b).
Table 2 presents the results for the major, trace, and rare earth element analyses for the two genetic types of granites from the Xilinhot area. The K-feldspar granites are highly enriched in SiO2 (72.94 wt.%–76.66 wt.%), highly alkaline (7.86–8.97), low in Fe and Mg (Fe2O3=0.16 wt.%–0.84 wt.%, FeO=0.20 wt.%-0.97 wt.%, MgO=0.09 wt.%–0.29 wt.%), and low in CaO (0.23 wt.%–0.98 wt.%). The average K2O/Na2O ratio is 0.93, but the average FeOT/MgO ratio is higher at 13.6. The samples are mainly ferroan (Fig. 6c) and fall within the alkali field on the alkali-lime index plot (Fig. 6d). In the TAS diagram of intrusive rocks, all 8 samples fall in the sub-alkaline granite field (Fig. 7). Then content of Al2O3 presents a restricted range (12.70 wt.%–13.97 wt.%), with A/NK=1.03–1.19 and A/CNK=1.03–1.14. These values suggest that the K-feldspar is peraluminous (Fig. 8). Similar to the results for the trace elements, the K-feldspar granites present high values of total rare earth elements (REEs), are strongly enriched in light rare earth elements (LREEs), have LaN/YbN ratios ranging from 3.08 to 9.53, and present obvious negative Eu anomalies (δEu: 0.29–0.53), which is typical of A-type granites. The LREEs show an obvious rightward incline, whereas the HREEs appear as adecline in the rock/chondritic diagram (Fig. 9). In the rock/ primitive mantle diagram (Fig. 10), the rocks indicate enrichment in both large ion lithophile elements (LILE), such as Rb, K, and Ba, and in the high field strength elements, such as Th, Nd, Zr, and Hf. However, the Nb, Ta, P, Ti, and Sr elements show obviously negatively anomalies.
The analyses for the major, trace, and rare earth elements of the 8 samples of the monzonitic granites are presented in Table 3. The monzonitic granite samples are characterized by high SiO2 (68.62 wt.%–75.42 wt.%), high alkalinity (K2O+Na2O=6.51 wt.%–7.97 wt.%), high Na and low K, and the average K2O/Na2O ratio is 0.88. All of the samples fall into the sub-alkaline granite field in the TAS diagram of intrusive rocks (Fig. 7). The monzonitic granites present high Al2O3 (12.99 wt.%–15.45 wt.%), with A/CNK=0.99–1.14 and A/NK= 1.48–1.87, and these results reflect the characteristics of peraluminous rock (Fig. 8). In addition, the high average FeOT/MgO (4.15) ratio is also a feature of monzonitic granites. In the rock/chondritic, the values of the total REEs range of 83.12-151.51, which are obviously lower than those of the K-feldspar granites (Fig. 9). In general, the samples show an enrichment of LREEs (LaN/YbN=2.92–9.24; LREE/HREE: 4.17–9.24), stable amounts of HREEs, and negative Eu anomalies (δEu=0.43–0.75); however, the degree of enrichment is obviously less than that of the K-feldspar granites. For the trace elements (Fig. 10), the monzonitic granites display a distinct enrichment of LILE (K, Rb, Ba) and HFSE (Zr, Hf) but show obvious negative abnormities of the Nb, Ta, Sr, P and Ti elements. The degree of the negative abnormity for the Ti element is less than that of the K-feldspar granites.
Table 4 presents the Lu-Hf isotope analysis of the two genetic types of granites in Xilinhot. The initial 176Hf/177Hf ratio of the K-feldspar granites is relatively high and ranges of 0.282 930–0.282 980, and the values of εHf(t) are strongly positive (Fig. 11) and range from +5.4 to +10.7, with a mean value of +13.6. The two-stage model ages (TDM2) range of 420–503 Ma, with an average age of 453 Ma.
In contrast, the initial 176Hf/177Hf ratio of the monzonitic granites ranges of 0.282 719–0.282 878 and is obviously lower than that of the K-feldspar granites; however, the monzonitic granite samples present a positive εHf(t) that ranges from +5.4 to +10.7 (Fig. 11) and a mean value of +7.4. These values are distinctly lower than that of the K-feldspar granites except for the TDM2 value, which ranges from 615 to 927 Ma and has an average age of 798 Ma.4 DISCUSSION 4.1 Petrogenesis
The two genetic types of granites in the Xilinhot area of Inner Mongolia display a distinct relationship in which they act as intrusions and include intrusions. The 9 Ma interval of these two plutons and the younger intrusion age of the more acidic pluton (the K-feldspar granites) indicate that the two genetic types of granites may not have formed from the fractional crystallization of the same batch magma and suggest that they belong to different batches from the same magma origin region or were formed from diverse magmatic sources.
The chondrite-normalized REE distribution patterns (Fig. 9) were compared with the primitive mantle-normalized trace element spidergrams of the two genetic types of granites (Fig. 10), and the higher value of the total REEs and the more negative anomalies of Sr and Eu in the K-feldspar granites indicate that a lower degree of partial melting occurred at the magmatic source, where the plagioclase may be remnant. Thus, the K-feldspar granites could not have been derived from mantle sources (in extreme conditions, strong crystal fractionation of mantle magma can generate alkaline rocks with limited or low Sr and Eu elements; however, massive mafic rocks would be observed). In addition, both of the granites present negative anomalies of Ti, P and Nb elements, which may have been caused by the remnants of rutile, apatite, or other accessory minerals in the lower crust source region or by the occurrence of fractional crystallization of accessory minerals during the rising stage of the magma.
To distinguish the A-type granite from other types of granites, the diagrams of Whalen et al. (1987) are widely used. All of the samples of the K-feldspar granites are characterized by high values of 104×Ga/Al and an average value of 3.02, and they fall in the field of A-type granite (Fig. 12). Because of their high alkali contents, elevated Fe/Mg and Ga/Al, low abundance of Ba and Sr, and high concentrations of Zr, Nb, these granites are exclusively distributed into the A-type field in the discrimination diagrams of Whalen et al. (1987). Furthermore, the K-feldspar granites are ferroan, alkali and peraluminous in terms of the A-type granitoid classification scheme of Frost et al. (2001). In contrast, 6 of the 8 samples of the monzonitic granites fall into the non-A-type granite field. Moreover, based on the concentrations that falls in the A2 field on the Nb-Y-Ga (Fig. 13a) and Nb-Y-Ce diagrams (Fig. 13b) (Eby, 1992) and the PA field on the R1-104×Ga/Al diagram (Fig. 13c), the Xilinhot K-feldspar granites exhibit a post-orogenic extensional tectonic setting. In the R1-R2 multicationic scheme (Batchelor and Bowden, 1985), the K-feldspar granites are concentrated in the field of a post-orogenic setting (Fig. 14), although the monzonitic granites fall in the region of a syn-collision. The results cannot be interpreted to show that the monzonitic granites are formed with the tectonic setting of syn-collision; rather, the results indicate that they occurred as a hybridization of crust materials before the magma emplacement.
The distinct degree of compatibility between the Lu and Hf elements in the zircons causes the enrichment of the Hf element, and the lack of Lu element causes the zircons to always have a low ratio of Lu/Hf. Thus, radiogenic Hf is scarcely produced during the decay process of the Lu element; moreover, the high sealing temperature and the strong ability to resist the reformation of the hydrothermal fluids indicate that the measured ratio of 176Hf/177Hf can represent the Hf isotope composition during the formation of the isotopic system (Wu et al., 2007; Kinny and Maas, 2003). Studying the isotope system of Hf represents a reasonable method of tracing the origin of the magma. The ionic radius and electrovalence of Lu allow this element to stay in the magmatic residual phase, which is inconsistent with the characteristics of Hf during the process of partial melting. The εHf(t) values indicate that the juvenile crust and residual mantle evolved toward different directions (the juvenile crust moves toward negative values, and the residual mantle moves toward positive values), and the zircon Hf isotope composition can be used to provide information on the magmatic source.
Most granites are considered to have originated from crustal sources. If the two-stage model age (TDM2) is much larger than the emplacement age, then the granites might have formed by crustal melting. If the TDM2 is close to the emplacement age, then the granites might have formed from the juvenile crust. Although both granites show positive zircon εHf(t) values, significant differences were observed in the zircon Hf isotope composition between the two genetic types of granites. The earlier emplaced K-feldspar granites (332 Ma) have a higher εHf(t) value than the centralized younger TDM2 granites (453 Ma) (Fig. 11). However, the later emplaced monzonite granites (323 Ma) present lower εHf(t) values and older separate TDM2 ages (798 Ma) (Fig. 11). A strongly positive εHf(t) value and young TDM2 indicates that the Xilinhot area may have been formed in the Early Paleozoic as a juvenile lower crust. The dispersed age of the monzonite granites and the older average TDM2 (798 Ma) reflect that a different degree of contamination occurred in the older continental crust above the juvenile lower crust.4.2 Tectonic Evolution
It is still uncertain when the Paleo-Asian Ocean closed. The Baolidao island-arc type granite series (414–418 Ma) and the Bayanhalatu collision granite series (363 Ma) are closely related to the subduction and collision process of the Paleo-Asian Ocean (Chen and Xu, 1996). On a regional scale, the Devonian–Early Carboniferous molasses and the epi-continental carbonate deposits do not uniformly overlie an imbricated oceanic rock suite bordering the suture zone (Tang, 1990). In addition, an unconformable surface occurs under the Upper Devonian Seribayanaobao Group (Shao et al., 2015). The lower part is a greenschist series with medium pressure orogenic metamorphism, and the upper part is generally non-metamorphic and has thick-bedded basal conglomerate. This evidence indicates that the subduction of the Paleo-Asian Ocean was not complete until at least the Late Devonian and suggests that collision and orogeny had already started at this point. The Xilinhot K-feldspar granites with representative post-orogenic A-type granite characteristics indicate that the region represents a post-orogenic extension stage. During the Late Devonian–Early Carboniferous, the collision and orogeny were complete and the subduction slab broke off, and these changes led to the thermal upwelling of the asthenosphere and the resulting lithospheric extension and melting of the juvenile crust, which formed during the oceanic subduction. In addition, the rising magma was contaminated by the overlying ancient crustal material. The coetaneous ultrabasic-basic rocks (354–333 Ma) with high εNd(t) values (Jian et al., 2012) in the Xiaobaliang area and the gabbros (319 Ma) (Zhou et al., 2016) with high εHf(t) values in the Xilinhot area also support the asthenosphere upwelling and melting of the juvenile crust.
Recent studies on sedimentation and paleo-magnetic reconstructions (Zhao et al., 2013) demonstrate that the inland sea developed on a welded continent after the collision between the North China Plate and the northern blocks during the Late Carboniferous Period (Zhao et al., 2016).5 CONCLUSIONS
(1) The U-Pb ages indicate that the two species of granite are Lower Carboniferous, with the K-feldspar granite emplacing at 332 Ma and showing typical A-type granite geochemical characteristics in the post-collision stage, and the emplacement age of the monzonite granite is 323 Ma.
(2) The two granites reveal higher εHf(t) values, with the K-feldspar granite at 13.6 and the monzonite granite at 7.4 on average. The two-stage model ages of these granites are 453 and 798 Ma. These Hf isotope features reflect that the magma originated from the partial melting of the juvenile crust, and the monzonite granite was derived from mantle contaminated by ancient crustal material.
(3) The above results indicate a more recent post-orogenic period and demonstrate that the Paleo-Asian Ocean had closed before the Early Carboniferous Period, and they show that the Xilinhot area experienced its initial post-orogenic evolution in an extensional tectonic environment in the Early Carboniferous Period.
The authors are indebted to Prof. Defan Xie from the School of Earth Sciences, China University of Geosciences, Wuhan for his guidance during field work, and thanks also go to Jianzhen Geng and Yongqing Zhang for the zircon U-Pb dating by LA-ICP-MS at the Tianjin Center of China Geological Survey. This study was supported by the China Geological Survey (Nos. 1212010510507, 1212010811005, 1212011220448) and the Fundamental Research Funds for the Central Universities, China University of Geosciences, Wuhan (No. CUGL150816). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0942-2.
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