Emplacement of the Late Triassic granitic Dushan pluton 

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Tectonic setting of the North China Block and its adjacent area

The NCB is one of the most important continental terranes that form the East Asian continent (e.g. Davis et al., 2001; Yin and Nie, 1996). It is separated from the Siberia Craton in the north by the Central Asian Orogenic Belt and from the South China Block in the south by the Qinling-Dabie-Sulu orogen, respectively (Figure 1.1). The North China Craton formed a stable craton from Mesoproterozoic to Paleozoic, after its assembly through the collision of the Eastern, Intermediate (or Fuping), and Western blocks during the Paleoproterozoic (Zhao et al., 2001; Faure et al., 2007; Li and Zhao, 2007; Li et al., 2012). The basement of the stable craton was overlain by a Mesoproterozoic to Permian sedimentary cover, separated by a regional unconformity between the Middle Ordovician and Upper Carboniferous–Permian strata (Li et al., 2016). The northern NCB is located to the south of the eastern segment of the CAOB (Figure 1.1). The evolution of the eastern segment of the CAOB is dominated by the late Paleozoic Paleo-Asian Ocean domain in the south and Mesozoic Mongol-Okhotsk Ocean domain in the north (Wang and Liu, 1986; Sengör and Natal’in, 1996; Xu and Chen, 1997; Davis et al., 2001; Xiao et al., 2003; Donskaya et al., 2013). The eastern segment of the CAOB is the result of a complex multi-period tectonic evolution during late Paleozoic to Mesozoic, with the subduction and closure of the Paleo-Asian Ocean and the Mongol-Okhotsk Ocean (Zonenshain et al., 1990; Sengor and Natal’in, 1996; Yin and Nie, 1996; Kravchinsky et al., 2002; Xiao et al., 2003; Windley et al., 2007; Zhang et al., 2007; Xiao and Kusky, 2009; Zhang et al., 2009; Han et al., 2012; Zhang et al., 2012; Zhang et al., 2014). In the south, it is characterized by the amalgamation of several microcontinents including, from west to east, the Erguna, Xing’an, Songnen, Jiamusi, and Khanka massifs (Sengör et al., 1993; Xu et al., 2013). Timing and process of the closure of the Paleo-Asian Ocean are still debated, 1) the late Devonian closure of the Paleo-Asian ocean occurred during the middle Paleozoic, with two opposite subductions and collisions along Ondor Sum in the south and Sunid Zuoqi in the north (Xu and Chen, 1997; Xu et al., 2013), 2) a Late Permian to Early Triassic collision, between the Tuva-Mongolia microcontinent and NCB, occurred with long-lived multiple southward and northward subductions from 530 to 250 Ma (Chen et al. 2000, 2009), 3) the final amalgamation of the CAOB occurred during latest Permian to mid-Triassic by termination of the accretionary processes (Xiao et al., 2003, 2015), and 4) an early to mid-Paleozoic paired orogens with a Permian intra-oceanic arc trench system and a sequence of tectono-magmatic events from 299 Ma to 260 Ma were responsible for the CAOB accretion (Jian et al. 2008, 2010). Numerous authors suggested the final closure of the Paleo-Asian Ocean was along the Solonker-Xra Moron-Changchun suture during Permian-Early Triassic (Sengör et al., 1993; Yin and Nie, 1996; Chen et al., 2000; Xiao et al., 2003; Shang, 2004; Li, 2006; Lin et al., 2008; Jian et al., 2010; Eizenhöfer et al., 2014; Li et al., 2014). In the eastern CAOB, it is closely related to the closure of the Mongol-Okhotsk Ocean in the north (Xu et al., 2009; Wu et al., 2011; Xiao et al., 2015). The Mongol-Okhotsk Ocean subducted northward beneath the Siberia Craton, resulting in the collision between the Siberia Craton and the Amurian supertarrane (Vander Voo et al., 1999). The suture extends from the middle Mongolia to the Mongol-Okhotsk Sea (Tomurtogoo et al., 2005). This is controversial with various estimates as to the timing of its closure, i.e., Middle Jurassic (Zorin, 1999; Yarmolyuk et al., 2000; Tomurtogoo et al., 2005; Sun et al., 2013), Late Jurassic (Zonenshain et al., 1990), and Late Jurassic-Early Cretaceous (Parfenov et al., 2001; Kravchinsky et al., 2002; Cogné et al., 2005; Metelkin et al., 2010; Pei et al., 2011). However, the collision between the Siberia Craton and the Amurian supertarrane occurred after the Middle Jurassic, evidenced by the Triassic-Middle Jurassic marine deposits (Enkin et al., 1992). The majority of scholars suggested that the closure of the Mongol-Okhotsk Ocean was diachronous but generally became younger eastward (Zonenshain et al., 1990; Yin and Nie, 1996; Kravchinsky et al., 2002; Xiao and Kusky, 2009; Berzina et al., 2014), for instance, the Middle Jurassic in the west (Zorin, 1999; Parfenov et al., 2001) and the Late Jurassic or the Early Cretaceous in the east (Yarmolyuk et al., 2000; Daoudene et al., 2013). Paleomagnetic studies suggested that the Mongol-Okhotsk Ocean subducted southward and collided with the Amurian supertarrane (Enkin et al., 1992).
In the south of the NCB, it is separated the South China Block by the Qinglin-Dabie-Sulu Orogenic belt (Figure 1.1), which underwent a complex multi-period tectonic evolution related to the Proto-Tethys Ocean and the Paleo-Tethys Ocean (e.g., Mattauer et al. 1985; Hacker et al., 1995, 1996, 1998, 2000; Wang et al., 1995; Faure et al., 1999, 2003; Meng and Zhang 2000; Ratschbacher et al., 2003, 2006; Lin et al., 2005, 2009; Dong et al. 2011; Wu and Zheng 2013; Dong and Santosh 2016). Generally, the amalgamation of the South Qinling and the NCB occurred in the middle Paleozoic, forming the North Qinling (e.g., Mattauer et al. 1985; Meng and Zhang 2000). In the Late Triassic, the final amalgamation of the North and South China Blocks was along the Mianlue suture (e.g., Mattauer et al. 1985; Hacker et al., 1995, 1996, 1998, 2000; Wang et al., 1995; Meng and Zhang 2000; Ratschbacher et al., 2003, 2006). The continental deep subduction of the Yangtze Block occurred beneath the NCB, forming the HP–UHP Dabie-Sulu orogenic belt (Okay et al. 1989; Wang et al. 1989; Xu et al. 1992; Faure et al., 1999, 2003; Lin et al., 2005, 2009; Dong et al., 2011; Wu and Zheng, 2013; Dong and Santosh, 2016).
During the late Mesozoic, it transited to the Pacific tectonic regime from the Paleo-Tethys tectonic regime in eastern China, corresponding to the evolution of the Paleo-Pacific Plate (Izanagi Plate). However, timing and process of the subduction beneath the East Asian continent remain controversial. The borehole encountered the early Early Jurassic volcanics on the continental shelf of the East China Sea. Due to the nature of continental margin arc, the volcanics were considered to be related to the onset of the Paleo-Pacific Plate subduction beneath the South China Block (198-195Ma; Xu et al., 2017). In the Korean Peninsula, the Early Jurassic synorogenic foreland deposits and high-k calc-alkaline volcanic rocks were supposed to a result of the Paleo-Pacific Plate subduction in the active continental margin (184~167 Ma; Han et al., 2006; Kim et al., 2011). The Early Jurassic volcanics in the Yanshan belt of the NCB recorded the onset of the Paleo-Pacific Plate subduction (Hao et al., 2020). The Middle-Late Jurassic fold and thrust belt, and extensive Jurassic magmatism was also considered as a result of the Paleo-Pacific Plate subduction (Zhou and Li, 2000; Li and Li, 2007; Li et al., 2018).

Tectonic evolution of the Triassic-earliest Cretacous basins

In the NCB, the Triassic strata are mainly distributed in the Ordos Basin, e.g., the Helanshan, the Liupanshan, and the Alxa Block in its west (Deng and Li, 1998). The Ordos Basin was a cratonic basin with convergence plate margins during the late Paleozoic-Early Triassic (Yang et al., 2015). In the Early Triassic, sand and mudstone in fluvial and lacustrine facies were deposited in the Ordos Basin. During the Middle Triassic, red conglomerate and mudstone were deposited in its eastern margin, and gray green mudstone with intercalated coal seams was deposited in the middle. The Triassic structural deformation was sporadically documented in the Yanshan belt. In the Yanshan belt, the ductile shear zone concerns the E-W trending Chicheng-Fengning, Fengning-Longhua, and Damiao-Niangniangmiao ductile shear zones in the north (Wang et al., 2013; Zhang et al., 2014; Figure 2.1), and the E-W trending folds and thrusts include the Unnamed fault, Malanyu anticline, and Jixian thrust fault in the south (Davis et al., 2001; Ma et al., 2007).

Intermediate phase of the Yanshanian Orogeny

Wong (1928) firstly described “Intermediate phase” as the volcanism stage, characterized by the volcanics in the upper and the conglomerates in the lower (Figure 2.4). As mentioned above, the Longmen Formation and Jiulongshan Formation, distributed locally in the Yanshan belt, might be the molasse after the Event A of the Yanshanian Orogeny. Alternatively, both the Longmen Formation and the Jiulongshan Formation belong to one set of conglomerates beneath the volcanics (Zhang et al., 2013). The Intermediate phase has been defined as the volcanism stage of the Tiaojishan Formation. The Tiaojishan Formation consists of dark purple, purple-brown, and gray-green andesite, amphibole andesite, pyroxene andesite, andesitic lava breccia, andesitic breccia, andesite agglomerate with purplish red, grayish brown and grayish-green sandstone, conglomerate, siltstone and mudstone (Li et al., 2014). Several rifts have been distinguished in the Yanshan belt, e.g., Diaoe graben and Houcheng half-graben (Qi et al., 2015). A N-S striking graben was developed in the Jurassic strata in the hanging wall of the Chengde thrust (Davis et al., 2001). In the Niuyingzi-Dengzhangzi basin, the Middle Jurassic Guojiadian half-graben was dominated by the normal fault (Davis et al., 2009). Besides, the top-to-the NE detachment fault was developed in the Kalaqin MCC (156-150 Ma; Lin et al., 2014). However, the volcanics of the Tiaojiashan Formation are only distributed in the Yanshan belt and the north of the Taihangshan (Figure 2.3). In the western Liaoning, it corresponds to the Lanqi Formation (Figure 2.4). The coeval extensional basins are also distributed in the Yanshan belt (Figure 2.3). As mentioned above, the conglomerates below the volcanics are widely distributed in other areas (Figure 2.4). In the Shiguai basin of the Daqingshan, the Event A of the Yanshanian orogeny is characterized by the unconformity between the Zhaogou Formation and the Changhangou Formation (Figure 2.4), and the growth strata of the Changhangou Formation. Above it, the growth strata of the Daqingshan Formation overlie the Changhangou Formation (Wang et al., 2017; Figure 2.4). Therefore, the volcanism and coeval extensional basins in the intermediate phase could only occur in the eastern NCB (Figure 2.3).

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Event B of the Yanshanian Orogeny

The Event B of the Yanshanian Orogeny occurred after the volcanism of the Tiaojishan Formation, characterized by the large-scale fold-and-thrust belts (Wong, 1928). In the Yanshan belt, the widely distributed conglomerates in the Tuchengzi/Houcheng Formation overlie the volcanics (Zhao et al., 1990; Cope et al., 2007; Li et al., 2016; Figure 2.4). The Event B corresponds to the unconformity between the Tuchengzi/Houcheng Formation and the overlying Zhangjiakou Formation (Figure 2.4). The growth strata of Tuchengzi/Houcheng Formation consist of syn-tectonic conglomerates during the Event B of the Yanshanian orogeny (Fu et al., 2018; Liu et al., 2018).
The deformation of the Event B was widely distributed in the Yanshan belt. E-W striking fold and thrust belts were widely developed in the middle and western segments of the Yanshan belt, for instances, the Xuanhua fault, the Xiahuayuan fault, and the Jimingshan fault in the Xiaohuayuan basin (Zhang et al., 2006), the Xiaosuangou-Jiuchaigou fault and the Shangyi fault in the southeastern Shangyi basin (He et al., 1999), the Qianjiadian fault, the Shaliangzi fault, and the Tanghekou fault (<158Ma), the thrust nappe in the Shisanling (151-127Ma; Davis et al., 2001), the Sihetang thrust (151-143Ma) and Shanggu-pingquan fault (145-135Ma) and the Gubeikou fault (148-132Ma) (Davis et al., 2001). The paleostress along the dextral strike-slip faults is NW-SE direction (Lin et al., 2020). It is characterized by NE or NNE striking thrust and fold belts in the western Liaoning, e.g., the Lingyuan-Dongguanyingzi fault, the Nangongyingzi-Beipiao fault, the Jianchang-chaoyang fault and the Jingzhougou fault. These faults thrust onto the Lanqi/Tuchengqi Formation, and were overlaid by the Early Cretaceous Yixian Formation (Zhang et al., 2002; Davis et al., 2009; Hu et al., 2010).
Although the Tiaojishan/Lanqi Formation was only distributed in the Yanshan belt, the conglomerates in the Tuchengzi/Houcheng Formation and their counterparts were distributed in the whole NCB (Figure 2.4). In the Shiguai basin of the Daqingshan, the Late Jurassic Daqingshan Formation, synchronous with the NW–SE compression, underlay the growth strata of the Changhangou Formation related to the Event A (Wang et al., 2017). In the western margin of the Ordos basin, the Middle Jurassic Anding Formation, involved in fold and thrust belt, was covered by the syn-tectonic conglomerates in the Fenfanghe Formation (Zhang et al., 2011). In the northern segment of the Taihangshan, the syn-tectonic conglomerates were developed in the footwall of the NNE striking Nanyangzhai-Heishiling fault (Wang et al., 2016). The unconformity between the Upper Jurassic Tuchengzi Formation and the lower Cretaceous Zhangjiakou Formation was developed in the Hunyuan basin (Li et al., 2015).

Triassic-Jurassic magmatism

The Triassic-Jurassic igneous rocks were widely distributed in the NCB. According to the isotope chronology, four stages of magmatism have been identified, i.e., Late Permian to Middle Triassic (262–236 Ma), Late Triassic (231–199 Ma), Early to Middle Jurassic (189–176 Ma), and Late Jurassic (155–145 Ma) (Figures 2.1and 2.3).
The Late Permian to Middle Triassic intrusive rocks (262–236 Ma) were mainly distributed in the Yinshan-Yanshan fold and thrust belt. These intrusive rocks are mainly composed of monzogranite, syenogranite and monzonite, with minor mafic rocks and granodiorite (Zhang et al., 2014). Some volcanics were developed in the Yanshan belt, e.g., the andesitic volcanic rocks in the Shuiquangou Formation. Late Permian–Middle Triassic intrusive rocks were derived from mixing of coeval mantleand crust-derived melts, linked to the postcollisional/postorogenic lithospheric extension and asthenospheric upwelling after the final collision of the Mongolian arc terranes with the North China craton (e.g., Zhang et al., 2009; Zhang et al., 2012).
The EW-trending Late Triassic alkaline intrusive complexes (e.g., nepheline syenite, quartz syenite, and A-type granite) and associated mafic rocks were widely developed along the northern NCB (Zhang et al., 2012). The Late Triassic alkaline intrusive complexes were mainly derived from the melts of subduction-modified enriched lithospheric mantle and ancient lower crust with some depleted asthenospheric mantle components during an intracontinental extensional setting (Yang et al., 2012; Zhang et al., 2014). It points to an upwelling of the asthenospheric mantle during the early lithospheric thinning or modification of the northern NCB. In the southern NCB, the Late Triassic igneous rocks, including Hornblende monzonite, quartz monzonite, quartz diorite, syenite and monzogranite, are mainly distributed in the Xiaoqingling area (Ding et al., 2011; Zhang et al., 2014). These rocks were originated from the partial melting of thickened lower crust, suggesting the postcollisional lithospheric extension after the collision of the South and North China blocks. Early Jurassic–earliest Middle Jurassic igneous rocks are mainly distributed in the eastern NCB, including the Liaodong and Jiaodong peninsulas and the eastern Yanshan belt (Wu et al., 2005, 2011; Figure 2.3). The main rock types include granite, monzodiorite, monzonite, and syenite, as well as the volcanics in the Nandaling/Xinglonggou Formation. The volcanics are composed of basalt, andesite, andesite, dacite and a small amount of trachyte. The intrusive rocks were mainly produced by partial melting of the ancient lower crust, whereas the volcanics were derived mainly from the ancient enriched lithospheric mantle (Wu et al., 2005; Yang et al., 2010). One group of researchers considers that the Early Jurassic–earliest Middle Jurassic igneous rocks are related to the subduction of the Paleo-Pacific plate, and the volcanic rocks are originated from the back-arc extension (Wu et al., 2005; Gao et al., 2004; Yang et al, 2010). Some scholars believe that it is an ongoing postcollisional lithospheric extension after the Mongolia arc terranes and the NCB (e.g., Zhang et al., 2014).
The Late Jurassic intrusive rocks are widely distributed in the Yanshan belt, the Liaodong area and Jiaodong Peninsulas in the eastern NCB, and the southern Yanbian– Liaobei area in the northeastern NCB and the northern North Korea (Wu et al., 2011; Figure 2.3). These intrusive rocks include monzogranite, syenite, diorite, monzonite and granite. The simultaneous volcanics are composed of the Tiaojishan Formation in the mid-western Yanshan belt and the Lanqi Formation in the eastern Yanshan belt, including andesite, basaltic andesite, andesitic tuff and trachyandesite, and minor rhyolite. Both the intrusive rocks and the volcanics were derived from partial melting of the ancient lower crust (e.g., Zhang et al., 2014). It was considered that the Late Jurassic igneous rocks are related to the subduction of the Paleo-Pacific plate (e.g., Wu et al., 2019).

Table of contents :

Abstract
Chapter 1. General introduction 
1.1. Background and scientific issues
1.2. Research contents and methodology
1.2.1. Research purpose and contents
1.2.2. Methodology
1.3. Workload of the study
1.4. Major findings and innovations
Chapter 2. Regional geological outline of North China 
2.1. Tectonic setting of the North China Block and its adjacent area
2.2. Tectonic evolution of the Triassic-earliest Cretacous basins
2.2.1. Triassic
2.2.2. Early-Middle Jurassic
2.2.3. Event A of the Yanshanian Orogeny
2.2.4. Intermediate phase of the Yanshanian Orogeny
2.2.5. Event B of the Yanshanian Orogeny
2.3. Triassic-Jurassic magmatism
Chapter 3. Emplacement of the Late Triassic granitic Dushan pluton 
3.1. Introduction
3.2. Geological overview of the YFTB
3.3. Field Observations in the Dushan pluton
3.3.1. Lithological Units and Bulk Architecture
3.3.2. Fabric and Structure of the Dushan Pluton
3.4. Microscopic Observation
3.5. New Dating Results
3.6. Measurement of Anisotropy of Magnetic Susceptibility
3.6.1. Sampling and Measurements
3.6.2. Magnetic Susceptibility Carrier
3.6.3. Anisotropy Degree and Shape Parameter
3.6.4. Fabric Pattern and Comparison with Mesoscopic Fabric
3.7. Gravity Modeling
3.8. Discussion
3.8.1. A continuum of fabric variation from magmatic to solid-state conditions in the Dushan pluton
3.8.2. Emplacement mode
3.8.3. Tectonic implication
Chapter 4. Emplacement of the Late Triassic granitic Wangtufang pluton 
4.1. Introduction
4.2. Geological setting
4.3. Structural observations
4.3.1. Field observations
4.3.2. Microscopic observations
4.4. Geochronology and geochemistry
4.4.1. Zircon U–Pb geochronology
4.4.2. Major and trace elements analyses
4.4.3. In-situ zircon Hf isotopes
4.5. Anisotropy of magnetic susceptibility
4.5.1. Magnetic mineralogy
4.5.2. AMS results
4.6. Gravity modeling
4.6.1. Residual Bouguer anomaly and 2D modeling
4.6.2. Results
4.7. Discussion
4.7.1. Magma sources of the Wangtufang pluton
4.7.2. Emplacement model
4.7.3. Relationships between the pluton emplacement and regional tectonics
Chapter 5. Emplacement of the Jurassic Jianchang-Jiumen pluton
5.1. Introduction
5.2. Geological Setting
5.2.1. Tectonic setting
5.2.2. Jurassic Geological Overview of the NCB
5.2.3. Jianchang-Jiumen pluton
5.3. Structural Observations
5.3.1. Field observations
5.3.2. Microscopic Observations
5.4. SIMS zircon U-Pb Dating
5.5. Anisotropy of magnetic susceptibility (AMS)
5.5.1. Sampling and Measurements
5.5.2. Magnetic Mineralogy
5.5.3. Fabric Patterns of the Jianchang-Jiumen pluton
5.6. Gravity Modeling
5.6.1. Residual Bouguer anomaly map
5.6.2. 2D gravity modeling
5.7. Discussion
5.7.1. Significance of Magnetic Fabrics
5.7.2. Emplacement Modes and Their Bearings on the Regional Tectonics
Chapter 6. Early Mesozoic tectonic framework of the northern North China Block and geodynamic mechanism
6.1. Early Mesozoic tectonic framework of the northern NCB
6.1.1. Early-Middle Triassic N-S compression in the northern NCB
6.1.2. Late Triassic NE-SW extension in the northern NCB
6.1.3. Early Jurassic-early Middle Jurassic N-S extension in the western NCB and NW-SE extension in the eastern NCB
6.1.4. Late Middle Jurassic N-S compression
6.1.5. Early Late Jurassic NE-SW extension in the eastern NCB
6.1.6. Late Late Jurassic-Early Cretaceous NW-SE large-scale compression and extension
6.2. Discussion of geodynamic mechanism
Chapter 7. Conclusions and Perspectives
7.1. Conclusions
7.2. Perspectives
References

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