0 Introduction

It has long been noticed that the Late Mesozoic volcanic rocks and other associated igneous rocks are widely occurred in the Transbaikalia, NE Mongolia and NE China. In NE China, the volcanic rocks cover ～100 000 km2 in the Great Xing’an Range (IMBGMR, 1991), with cumulative thickness of the successions up to ～4 km (Xie, 2000). During the past decades, the volcanic rocks have been widely employed to provide geodynamic constraints on the reconstruction of geohistory of the northeastern part of Asia continent. Although many geological, petrological and chronological studies were carried out, no consent has been reached by far on the geodynamic mechanism, though various models have been postulated to account for the petrogenesis of widespread volcanics, e.g. possible mantle plume activity or intraplate process (Dobretsov & Vernikovsky, 2001; Shao et al., 2001), a back-arc extensional regime related to the Mesozoic subduction of the Kula or Izanagi plate (Faure & Natal’in, 1992; Li & Shu, 2002; Sun et al., 2013), widespread lithospheric delamination in eastern China (Wang et al., 2006; Wu et al., 2005), and post-orogenic collapse of an over-thickened lithosphere after the closure of the Mongol-Okhotsk Ocean (Xu et al., 2013; Tang et al., 2015).

These conflicting interpretations complicated the understanding of the Early Cretaceous tectonic evolution of NE part of continental Asia, as well as the genesis, distribution and composition of the ore deposits in the Sino-Russia-Mongolia border area. However, the A-type rhyolite, one of the most important Late Mesozoic volcanic rocks in revealing Early Cretaceous tectonism, is a key to enhance our understanding of the Early Cretaceous tectonic evolution of the Sino-Russia-Mongolia border tract. In the Late Mesozoic Hailar Basin of NE China, Lin et al. (2000) and Ge et al. (2001) first reported the Early Cretaceous A-type rhyolites, they believed that depleted asthenosphere was important magma source of the A-type rhyolites and interpreted that the tectonism related to the rhyolites was anorogenic intraplate environment. Nevertheless, Li et al. (2014) believed that the A-type rhyolites were derived from juvenile felsic lower crust due to ongoing heating by underplating mantle-derived magma, and attributed the geodynamics to a retreating Paleo-Pacific trench. Since different opinions from previous studies, further expatiation on the petrogenesis and geodynamics of the A-type rhyolites is in need.

In this paper, the authors present the geochemical data of A-type rhyolites from the Late Mesozoic Hailar Basin and correlate the data with other coeval A-type rhyolites in the basin and typical A-type granites or rhyolites from Eastern China and Algeria. Combined with other geologic information, the petrogenesis and geodynamics associated with the rhyolites are subsequently constrained.

1 Geological background and description of samples

The Hailar Basin is located in NE China, which is one of the eastern segments of the Central Asian Orogenic Belt (Sengör et al., 1993; Jahn et al., 2000) (Fig.1a). It is situated to the south of the Mongol-Okhotsk Suture and the western flank of the northern Great Xing’an Range. The basin is a Mesozoic--Cenozoic continental sedimentary basin consisting of the Upper Jurassic, Cretaceous, Upper Pliocene and Quaternary successions (IMBGMR, 1991; Chen et al., 2007).

The study area and adjacent regions (northern Great Xing’an Range) are widely covered by Late Mesozoic volcanic rocks (Fig.1b). Based on the lithological associations and lava flow sequences, these rocks were subdivided, from bottom to top, into the Middle-Late Jurassic to Early Cretaceous Tamulangou (～163--120 Ma; Li et al., 2014; Zhang et al., 2008; Sun et al., 2011; Xu et al., 2013), Early Cretaceous Shangkuli (～136--111 Ma; Li et al., 2014; Zhang et al., 2008; Ge et al., 2001) and Early Cretaceous Yiliekede (～126--106 Ma; Zhang et al., 2008; Wang et al., 2006) formations (corresponding to the Manketouebo, Baiyingaolao and Meiletu formations in the southern Great Xing’an Range, respectively) (Xu et al., 2013) (Fig.1b).

The lack of intermediate rock types (Daly gap), the large volume of felsic rocks relative to that of the mafic types in the Hailar region, and the negative Eu, Ba and Sr anomalies are consistent with the origin of A-type rhyolites by partial melting. For the A-type rhyolites in the study area, the residual granulite melting' model (Collins et al., 1982; Clemens et al., 1986; Whalen et al., 1987) cannot explain some geochemical characteristics, such as high FeOt/(FeOt+MgO) (Creaser et al., 1991; Frost & Frost, 1997), and partial melting of quartz-feldspathic igneous sources with a metasedimentary component (Dall’Agnol & Oliveira, 2007) failed to explain the features of predominantly metaluminous (Frost & Frost, 1997; Anderson & Morrison, 2005). Different parental magma with I-type and mafic volcanic rocks discussed above indicates that partial melting of I-type granites in the shallow crust (Skjerlie & Johnston, 1993; Douce & Alberto, 1997) and lower crustal tholeiitic series rocks (Frost & Frost, 1997; Dall’Agnol & Oliveira, 2007) are impracticable as well.

2 Geochemistry of Early Cretaceous rhyolites

2.1 Analytical techniques and methodology

由表6可见，胶凝剂添加量为1.20%时，果冻的感官评分最高。此时，产品色泽呈现黄色，柔软度刚好，入口细腻，弹性及咀嚼感均好。因此，当胶凝剂添加量为1.20%时，果冻的整体效果为最佳。

而变频调速的特性以及无可比拟的节能功效在机械调速领域脱颖而出。因此，怎样更好地把变频调速技术引入并应用到起重机械行业中是一个值得探究的重要领域[1]。本文所研究的变频工况下的起重机械起升机构动力学仿真与研究，就是在这个背景下提出的。

2.2 Major elements

The compositions of major elements from the Early Cretaceous rhyolites in the study area have the following characteristics:

(1) Enriched in silicon, w(SiO2)=69.60%--76.58%(Fig.2a), and the silicon content is similar to the average value of A-type granite [w(SiO2)=73.81%] which confirmed by Whalen et al.(1987).

(2) Enriched in alkali and potassium with w(Na2O+K2O)=7.07%--9.63% and K2O/Na2O=1.14--1.98, respectively. AI index=0.83--1.07, same to the average value of A-type granite (AI=0.95) that confirmed by Whalen et al. (1987). AR index=2.43--5.45, and on a SiO2 versus AR diagram (Fig.2b), the samples fall into the alkaline series.

(3) Enriched in iron and depleted in magnesium with FeOt/(FeOt+MgO)=0.77--0.99. Plotting these samples on a SiO2 -FeOt/(FeOt+MgO) diagram (Fig.2c) indicates that they are ferroan. On discrimination diagrams of calc-alkaline, oxidized A-type and reduced A-type granite, the samples fall mainly in the reduced A-type area, and are identical in features to Baicha (NW Beijing) and Yaokeng (Fujian-Zhejiang coastal area) A-type granites in eastern China (Fig.2d, e).

(4) Depleted in aluminum and calcium, with w(CaO)=0.17%--1.15%, w(Al2O3)=8.97%--13.88%,similiar to the alkaline granitoids from the Djebel Drissa ring complex in Algeria (Kahoui & Mahdjoub, 2004), and the calcium content are relatively higher than the Yaokeng and Baicha A-type granites in eastern China (Table 2). A/CNK=0.86--1.06 and on a A/CNK versus A/NK diagram (Fig.2f), the samples fall into the metaluminous to weakly peraluminous and peralkaline.

从有理数到实数，是数系的第二次扩充.教师在实际教学时，应合理利用教材中的计算表格(见图5)估计近似值，借用有理数来不断逼近无理数，从而引入无理数的概念，完成数系的自然扩充.教师要引导学生先借助计算器得到近似值，在计算过程中巧妙渗透逼近思想，让学生意识到无理数不是有限小数，再借助计算机的计算结果认识无理数也不是无限循环小数，从而正确建立无理数的概念——无限不循环小数.

关于爆品，对于市场人来说，永远要记得去研究、分析，总结那些成功的爆款产品，或内容背后的道理和方法，而不是将其视为偶然、幸运。今天笔者想谈谈“爆品的逻辑”！这里“爆品”的“品”放在农药行业，指品类而非互联网行业里的单一产品概念。

According to data in Table 1, the Early Cretaceous rhyolites in research area have total REE abundances (ΣREE) of (176.4--347.6)×10-6, obviously lower than granitoids formed in continental rifting and breakup, such as Brandberg and Paresis complex in the Damaraland of west-central Namibia (Schmitt et al., 2000; Trumbull et al., 2004) and Corsica peralkaline granites in France (Poitrasson et al., 1995), but similar to that of post-collisional granitoids, such as alkaline granitoids from the Djebel Drissa ring complex of Algeria (Kahoui & Mahdjoub, 2004), and Baicha and Yaokeng granites in eastern China (Wang, 2009; Xiao et al., 2007). The main characteristics of the samples show LREEs enrichment and HREEs depletion with (La/Yb)N ratios of 6.63--17.56. The fractionations of LREEs are higher than HREEs with (La/Sm)N ratios of 3.33--4.94 and (Gd/Yb)N ratios of 1.50--2.36. Significant negative Eu anomalies with δEu=0.09--0.58 suggest that samples underwent intense fractional crystallization of plagioclase. Chondrite-normalized REE diagram shows uniform REE pattern and is similar to that of the other coeval A-type rhyolites in the Hailar Basin, Baicha and Yaokeng A-type granites, and the Djebel Drissa ring complex of Algeria (Fig.3a; Table 2).

2.3 Trace elements

The major elements of Early Cretaceous rhyolites in the study area are characterized by enrichment in silicon, alkali and iron, and depletion in magnesium, aluminum and calcium, which meet the criteria of typical A-type granite according to Whalen et al. (1987) or Eby (1990) and similar to that of Baicha and Yaokeng A-type granites in eastern China (Table 2).

The compositions of trace elements indicate that the samples are enriched in Rb, Th and Pb, and depleted in Ba, Sr, P and Ti (Fig.3b). Ga shows a high content with 10 000 Ga/Al=1.8--3.4, and on the discrimination diagram based on 10 000 Ga/Al ratio (Fig.4b), the samples all fall into the A-type fields. Furthermore, the Ga, Zr, Nb, Y and Rb abundances of the rhyolites in the study area are similar to that of the Baicha and Yaokeng A-type granites, but Sr, Ba abundances are relatively higher than them and more approximate to granites in Djebel Drissa (Table 2).

Samples for geochemical analysis were crushed in an agate mill to ～200 mesh after the removal of altered rims and amygdules under a magnifier. Both major element oxide analyses and trace element analyses were conducted at the Laboratory of the Fifth Geological Survey of Jilin Province, Changchun, China. Major element oxides were determined by atomic absorption spectroscopy (AAS) using a GGX-610 (Beijing Haiguang Instrument). The detailed analytical procedure follows GB/T 14506 (2010). The analytical errors were estimated to be less than 5%. Trace elements were analyzed with X Series-2 ICP-MS (Thermo Scientific, America). Three duplicates and two international standards (BHVO-1 and AGV-1) were prepared using the same procedure to monitor the analytical reproducibility. Analytical precision for all elements is better than 10%, and for most elements is <5%. The detailed analytical procedure follows that reported by Liu et al. (2007).

3 Discussion

3.1 Petrogenesis of Early Cretaceous A-type rhyolites

Major and trace element data along with the correlation are the main clues to understand the nature of the volcanic rocks. The Early Cretaceous rhyolites of the Shangkuli Formation in the Hailar Basin exhibit petrologic and geochemical characteristics of typical A-type granites (Whalen et al., 1987; Eby, 1992). The rhyolites have a high FeOt/(FeOt+MgO) ratio and pertain to reduced A-type granites (Fig.2d, e). Furthermore, plotting the rhyolites on the Pearce diagrams (Fig.4c) indicates that they are within plate granites (WPG) and Nb is relatively more enriched than Y. On Nb-Y-Ce and Nb-Y-3Ga triangle diagrams (Fig.4d, e), these samples fall into A1 field. Therefore, understanding the petrogenesis of Early Cretaceous A-type rhyolites in Hailar Basin need to explain the important features of: (1) enrichment in iron and depletion in magnesium with high FeOt/(FeOt+MgO) ratio; (2) reduced A-type granites; (3) depletion in aluminum and calcium; (4) within plate and A1-type trace elements characteristics.

Generally, magmatic rocks derived from fractional crystallization of mantle-derived intermediate and mafic magma, AFC processes or magma mixing should exhibit a continuous compositional trend from mafic through intermediate to felsic rocks (Frost & Frost, 1997; Kim et al., 2006). However, the Late Mesozoic volcanic rocks in the Hailar Basin and adjacent areas show bimodal assemblage and do not display a continuous compositional trend in the A-type rhyolites. Moreover, the rhyolites have high FeOt/(FeOt+MgO) which are obviously higher than contemporaneous mafic magmatic rocks (Li et al., 2014; Ge et al., 2000) in the Hailar Basin and adjacent areas, indicating that they could not be derived by fractionation involving the crystallization of Fe-Ti oxides. Their high Zr contents (generally ～400×10-6, Table 1 and 2) also suggest that they are not derived from a mafic magma that was undersaturated with respect to Zr, because once Zr becomes saturated in a magma, high Zr contents cannot be reached via fractional crystallization (King et al., 1997). In addition, their ‘reduced A-type granites’ feature also indicates that they could not be derived by fractionation (Anderson & Morrison, 2005; Dall’Agnol et al., 2005; Frost & Frost, 1997).

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Furthermore, the recent research of Li et al. (2014) indicates that the volcanic rocks within the Hailar Basin mainly include A-type rhyolites, I-type rhyolites and mafic volcanic rocks, and A-type rhyolites represent a very rare rock-type comparing to the abundant I-type rhyolites. Therefore, the geochemical relationship of the A-type rhyolites, I-type rhyolites and mafic volcanic rocks of the Hailar Basin is important to reveal the petrogenesis of A-type rhyolites. The data of our samples and other coeval volcanic rocks from Hailar Basin (Li et al., 2014; Ge et al., 2000, 2001) were plotted in Harker and trace-element rariation diagrams, which show small number of intermediate compositions, indicating that Hailar magmatic activity has bimodal component characteristics.The distinct Na2O, FeOt, Al2O3 and P2O5 variation in felsic and mafic rocks (Fig.5a,b,d,e) show unrelated magma fractionation trends resulting from compositionally different parental magma batches. Also, the enrichment of the LREE relative to the HREE during magma fractionation of the I-type rhyolites and mafic volcanic rocks, as shown by the increasing (La/Yb)N ratio with SiO2 (Fig.5f), produces a compositional gap relative to the A-type rhyolites that cannot be reconciled with a common origin by crystal fractionation. Finally, the SiO2 versus Nb/La diagram (Fig.5c) and arguments previously made for the genesis of I-type rhyolites and mafic volcanic rocks (Li et al., 2014; Ge et al., 2000, 2001) indicate that I-type rhyolites and mafic volcanic rocks have similar large negative Nb-anomalies, whereas all the A-type rhyolites show only small negative Nb-anomalies. There is no obvious process or mineral to account for the drastic decrease of the Nb anomaly in the A-type rhyolites, without changing the degree of magma fractionation. Therefore, existing evidence indicate that A-type rhyolites, I-type rhyolites and mafic volcanic rocks were formed from different sources and the models of fractional crystallization of mantle melts, AFC and magma mixing are unable to explain the petrogenesis of A-type rhyolites.

The continuous outcrop of the Early Cretaceous Shangkuli Formation exceeds 4 000 m in the western margin of the Hailar Basin (the western lakeshore of the Lake Hulun), and consists of rocks with typical volcanic lithologies and lithofacies of 136--125 Ma (Ge et al., 2001; Li et al., 2014). The rocks mainly comprise three parts, i.e. the lower part of pyroclastic rock, the central part of flow banding rhyolite, lithophysa rhyolite, and pyromeride, and the upper part of pyroclastic lava, cryptoexplosive breccia, pyromeride and glassy lava. These three parts constitute a complete volcanic eruption cycle. Among which, blocks or lapilli within pyroclastic rocks often occur as pyromeride. The rhyolite is the most important component of the outcrop, having a spherulitic texture or porphyritic texture with phenocrysts in a glassy matrix. The phenocrysts are mainly orthoclase, quartz, plagioclase, and biotite. The glassy matrix consists of fine grained plagioclase and hornblende and opaque oxide minerals with devitrificated quartz and feldspar. Regarding to the structure, flow banding, lithophysa and perlitic structures are most common. The pyroclastic lava usually exhibits welded texture and pseudo-fluidal structure. 17 representative felsic rock samples were analyzed from this continuous outcrop.

Conventional magmatic underplating and partial melting of lower crustal quartz-feldspathic igneous sources (Anderson & Morrison, 2005) can explain the characteristics of high SiO2 content, reduced A-type rhyolites, aluminum, calcium, Sr and Eu depletion, and Ga and Pb enrichment. But it seems implausible for the A1-type trace elements features. The A-type rhyolites in the study area are produced along the Erguna-Hulun Fault (Nie et al., 2011; Wu et al., 2010) (Fig.1b). Zheng et al. (2015) indicated that the fault is an Early Cretaceous large-scale extensional ductile shear zone which deformed the lower lithosphere. The formation of A-type rhyolites in the study area and the Erguna-Hulun Fault is coupled. Therefore, the petrogenesis of A-type rhyolites is bound to the lithospheric mantle material during the extensional deformation of the fault, thereby the rhyolites show A1-type trace elements features. Depleted asthenosphere was not important as magma source during lithospheric extension. This petrogenesis mechanism is similar to that of the Yaokeng A-type granite in the Fujian-Zhejiang coastal area of eastern China which is situated along the famous Changle-Nan’ao deep fault (Xiao et al., 2007). Furthermore, the geochemical similarity of the rhyolites with the Baicha and Yaokeng granites and the Djebel Drissa ring complex (Figs. 2, 4; Table 2) suggests that the rhyolites formed in a post-orogenic setting.

Thus, it is concluded that the Early Cretaceous A-type rhyolites are most probably derived from magmatic underplating and partial melting of lower crustal quartz-feldspathic igneous rocks involving the lithospheric mantle material due to the post-orogenic extensional deformation of the Erguna-Hulun Fault.

3.2 Tectonic implication

Previously published data (Wang et al., 2006; Zhang et al., 2008; Ying et al., 2010) indicate that the first stage of volcanic activity is represented by the Tamulangou Formation, the last stage is represented by the Shangkuli and Yiliekede formations, and the Early Cretaceous was the major period of Late Mesozoic volcanism in the northern Great Xing’an Range and the Sino-Russia-Mongolia border tract. Moreover, the composition of the volcanic rocks show bimodal magmatism which is similar to that in other regions of an extensional lithosphere in NE China (Xu et al., 2013) and worldwide (Christiansen & Lipman, 1972; Griffin et al., 2010). This fact, together with the presence of coeval A-type rhyolites in the study area and the Erguna Massif (Meng et al., 2011; Xu et al., 2011; Ge et al., 2001; Li et al., 2014), suggests an extensional environment.

The rhyolites are in association with other regional geological phenomena that are linked to their origin and temporal-spatial distribution. A series of Late Mesozoic metamorphic core complexes exposed along a NE trend in the Transbaikalia-northeast Mongolia region. Zircon U-Pb ages for the intrusions, and biotite, muscovite and hornblende 40Ar/39Ar ages for the mylonites indicate that these metamorphic core complexes were formed between 150--112 Ma (Dongskaya et al., 2008; Wang et al., 2011). Besides, Late Mesozoic extensional volcanic fault basins are widely developed in this area, such as Hailar, Genhe, Yingen and East Gobi basins. Isotopic ages for these basins mainly range from 141--117 Ma (Gou et al., 2010; Wang et al., 2006). Furthermore, Early Cretaceous was an important time for mineralization in the metallogenic province in the Sino-Russia-Mongolia border tract. Geological and geochemical characteristics indicate that the deposits formed in an extensional background (Chabiron et al., 2003). The coeval regional geological phenomena therefore indicate a setting of crustal extension and thinning in the study area and adjacent regions. This interpretation is also supported by recent geophysical studies (Gao & Li, 2014).

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The Mongol-Okhotsk Suture is the most important tectonic belt in this area, which sutured in a scissor-like manner during the Triassic--Early Jurassic in the west and the Late Jurassic-Early Cretaceous in the east (Zonenshain et al., 1990, Tang et al., 2015). In the Early Cretaceous, the overthickened post-collisional crust gravitationally collapsed and extended (Zorin, 1999; Dongskaya et al., 2008; Zheng et al., 2015). Thus, the widespread bimodal volcanic rocks, metamorphic core complexes, volcanic fault basins and metallogenic province in the adjacent region of Hailar Basin are the results of the post-collisional extension of the Mongol-Okhotsk Suture.

4 Conclusions

(1) The Early Cretaceous A-type rhyolites within the Shangkuli Formation of the Hailar Basin exhibit geochemical characteristics of high silicon, alkali, Fe/Mg, Ga/Al, Zr, Pb, HFSEs, and REE contents but low Ca, Ba, Sr and Eu, which meet the criteria of typical reduced A-type granite. The features are similar to that of Baicha and Yaokeng A-type granites in eastern China.

(2) The A-type rhyolites are most probably derived from magmatic underplating and partial melting of quartz-feldspathic igneous sources of lower crust, and involved the lithospheric mantle material due to the extensional deformation of the Erguna-Hulun Fault. Depleted asthenosphere was not significant as magma source during lithospheric extension.

(3) The A-type rhyolites show A1-type trace elements characteristics, nevertheless, they were formed in a post-orogenic extensional background together with the widespread bimodal volcanic rocks, metamorphic core complexes, volcanic fault basins and metallogenic province in the Sino-Russia-Mongolia border tract. This extension event was related to the collapse of a thickened region of the continental crust after the closure of the Mongol-Okhotsk Ocean rather than the anorogenic intraplate mantle plume or the retreating Paleo-Pacific trench.

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