Tectonic evolution of the Middle-Late Permian orogenic belt in the eastern part of the CAOB: Implications from the magmatism in the Changchun-Kaiyuan area

Abstract Various magmatisms during the subduction-collision process are crucial to reveal the long-term tectonic evolution of the eastern Central Asian Orogenic Belt. In this paper, we present major and trace elements of whole-rock, zircon U-Pb dating and Hf isotope of the Shanmen pluton. Results imply that the Shanmen pluton consists of quartz diorite and mylonitic granite, with zircon U-Pb ages of 263.7–259.6 Ma. The studied quartz diorite contains high Sr/Y (51.19–90.87) and (La/Yb)N (7.82–13.62) ratios, and belongs to adakitic rocks. Coupled with the positive εHf(t) values of +5.71 to +12.8 with no obvious Eu anomaly, we propose that quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge. In contrast, the mylonitic granite has lower MgO (0.28 wt% – 0.47 wt%) contents and positive εHf(t) values of +7.79 to +10.15, indicating an affinity with I-type granite originated by partial melting of the intermediate-basic lower crust. The geochemical characteristics and lithological assemblages, along with the Permian magmatic rocks in the Changchun-Kaiyuan area displaying arc rocks affinity, propose their formation is related to the southward subduction of the Paleo-Asian Ocean (PAO). Based on this study and previous evidence, we lean towards adopting a middle-late Permian slab break-off model, wherein the PAO did not close until the late Permian.


Introduction
The subduction zone plays a crucial role in the interaction of convergent plates resulting in various magmas and serving as a typical accretion orogenic system.A comprehensive understanding of the evolution of subduction zones, including its initiation and termination, as well as associated magmatic, metamorphic and tectonic processes, is essential for revealing crustal growth and circulation, palaeogeographic reconstruction and long-term evolution of the Earth's structure (Crameri et al. 2020;Soret et al. 2022).The Central Asian Orogenic Belt (CAOB) lies between the Siberian Craton to the north and the Tarim and North China Cratons (NCC) to the south (Şengör et al. 1993;Fig. 1a).It is the longest and most complex typical Phanerozoic accretionary orogenic belt on Earth, and it is composed of a wide range of tectonic units, including micro-continents, magma arc, ophiolites, relics of fore-arc and back-arc basins and subduction-accretion complexes (Şengör et al. 1993;Wilde et al. 2000;Xiao et al. 2003Xiao et al. , 2015;;Zhang et al. 2022).Typically, Solonker-Xar Moron-Changchun-Yanji Suture (SXCYS) was regarded to be a sign of the closure of the PAO (Wu et al. 2000(Wu et al. , 2007a(Wu et al. , 2011;;Xiao et al. 2003Xiao et al. , 2015;;Liu et al. 2021;Fig. 1b).
In Paleozoic, the North-east (NE) China, which is part of the eastern CAOB, underwent closure of the Paleo-Asian Ocean (PAO) and amalgamation of the NCC with several microcontinental massifs, from west to east, including the Erguna, Xing'an, Songliao-Xilinhot and Jiamusi blocks (Liu et al. 2017(Liu et al. , 2019;;Windley et al. 2007;Fig. 1b).Some researchers argued that it also consists of a curved Erguna-Jiamusi continent ribbon, early Paleozoic Xing'an-Zhangguancailing accretionary terranes and late Paleozoic Songliao accretionary terranes with some Precambrian micro-block relics in the core area of the orocline (Liu et al. 2021(Liu et al. , 2022(Liu et al. , 2023)).However, the tectonic evolution history in the eastern CAOB is still debated, and there is no consensus on the closure time of the PAO and its branches, which range from the Devonian (Xu et al. 2013;Zhao et al. 2016) to the Late Permian-Early Triassic (Jia et al. 2004;Jian et al. 2010;Cao et al. 2013;Xue, 2021).Furthermore, more tectonic models have been proposed to explain the tectonic affiliation of the eastern PAO during the Permian.These models include the continental rift model (Shao et al. 2015), continent-continent collision model (Zhang et al. 2007), post-orogenic extension model (Zhang et al. 2007;Zhao et al. 2008), slab break-off model (Yuan et al. 2016) and slab rollback model (Li et al. 2016(Li et al. , 2017)).
In this paper, we present zircon U-Pb dating, major and trace elements of whole-rock and zircon Hf isotope of the Shanmen pluton, combined with various data of Permian chronological and geochemical data in the Changchun-Kaiyuan area, to analyse the activity times, rock combination, tectonic environment and the relationship with the PAO.

Geological background
The Shanmen area in Jilin Province is located at the intersection of Daheishan Horst and SXCYS, bounded by the Shanmen Fault (Siping-Changchun-Dehui Fault) and Yilan-Yitong Fault, which belongs to the eastern part of the northern margin of the NCC (Fig. 2a).Owing to the alteration and destruction caused by magmatic activity during the Mesozoic, the study area has relatively limited remaining Palaeozoic stratigraphic formations.In the study area, the intrusive rocks primarily consist of Mesozoic granites and late Paleozoic intrusions (Fig. 2b).The Mesozoic granites mainly comprise Jurassic monzonitic granites and granodiorites.The late Paleozoic intrusive rocks are formed in the Middle Permian, and the lithology includes quartz diorite, syenite granite and granite.Initially, due to the lack of accurate isotope dating data, it was believed that the late Paleozoic intrusions were formed in the Ordovician.However, as the study progressed, 262 ± 2 Ma (Cao, 2013) and 264-260 Ma (this study) were obtained in the study area.In the southern part of the study area, a large area of 'Xia'ertai Group' is distributed, and the overall pattern is spread in a NE direction in a back-shaped pattern (Zhang, 2021).In addition, Mesozoic Cretaceous volcanic sedimentary strata and Cenozoic strata developed in the Songliao Basin (Fig. 2b).

3.a. Field relationships
The Shanmen pluton in this paper was discovered in the Shanmen Reservoir (124°28 0 13 0 0 E, 43°03 0 20 0 0 N), which is just ~20 km southeast of Siping.It is mainly composed of quartz diorite, syenite granite and granite.Field observation revealed that the left side of the pluton is a slip fault, with an occurrence of 284/85.Moreover, the mylonitic fine-grained granite intrusions can be observed in the form of veins within the quartz diorite, and both of them underwent metamorphic deformation (Fig. 3a, b).

4.a. Zircon U-Pb dating
The separation of zircon was performed in the Keda Rock Mineral Separation Company in Langfang City, Hebei Province.The samples were first crushed and then separated using gravitational and magnetic separation methods.Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon dating was carried out at the Key Laboratory of Mineral Resources Evaluation in NE Asia, Ministry of Natural Resources, Jilin University, Changchun, China.The correction for common Pb was made following the method of Andersen (2002).The data were processed using the ISOPLOT (Version 3.0) programme (Ludwig, 2003).

4.b. Major and trace elements analyses
Major and trace elements were analysed at the premises of ALS Chemex Co. Ltd. in Guangzhou.Major elements were measured by X-ray fluorescence spectrometry from prepared glass discs.Trace elements were instead analysed using ICP-MS after melting the samples at 1025 °C and digesting them using a HNO 3 þ HCL þ HF mixture.

4.c. In situ zircon Hf isotopic analyses
In situ zircon Hf isotopic analyses for sample (SM18-1) were undertaken using a Neptune multi-collector (MC) ICP-MS, equipped with a 193 nm ArF Excimer laser system at the Tianjin Institute of Geology and Mineral Resources in Tianjin, China.Details of the analytical procedures are described by Wu et al. (2006).Experiments of in situ Hf isotope ratio analysis for sample (SM21-1, SM21-2) were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China.Detailed instrument operating conditions and analysis methods can be referred to (Hu et al. 2012).
Seventeen zircons from sample SM18-1 give a range of 206 Pb/ 238 U ages from 267 to 260 Ma (Table 1) and yield a weighted mean age of 263.7 ± 2.7 Ma (MSWD = 0.3, n = 17).This weighted mean age is interpreted as the crystallisation age of the rock (Fig. 5a).
The 206 Pb/ 238 U ages from 23 analyses for the sample SM21-2 vary from 275 Ma to 256 Ma (Table 1), yielding a weighted mean age of 263.5 ± 1.9 Ma (MSWD = 1.2, n = 23; Fig. 5b), which is interpreted as the crystallisation age of the quartz diorite.

5.b. Whole-rock geochemical compositions
Table 2 shows the results of analyses of trace and major elements of the representative samples.
Based on the spatial and temporal correlation between adakite and more abundant mafic rocks, a fractional crystallisation model is proposed (Macpherson et al. 2006;Jing et al. 2022a).However, the scarcity of mafic magmatic rocks and the variable La/Sm and Zr/Sm ratios also reveal that fractional crystallisation is not the primary mechanism, as shown in Fig. 10a, b.The lack of Eu anomalies of the Shanmen adakitic diorites indicates that fractional crystallisation is not the main genetic mechanism of the rocks (Jing et al. 2022a;Macpherson et al. 2006;Cao et al. 2013).Adakites derived from the partial melting of thickened lower crust typically exhibit low Cr, Ni and Mg # values (< 40).In contrast, the studied quartz diorite demonstrates high-Mg # values (44.93-55.55),and Cr (30 ppm -110 ppm) contents indicate that they cannot be formed by the partial melting of the thickened lower crust.In addition, the samples have a high Na 2 O/K 2 O ratio of 2.37-3.10,which is the characteristic of Na-rich and K-poor.This aligns with the characteristics that they are formed by the partial melting Tectonic evolution of the Middle-Late Permian orogenic belt of the subducted oceanic crust in an oceanic subduction zone, rather than by the delamination of the basaltic lower crust.(Fig. 9c, d; Defant & Drummond, 1990;Zhang et al. 2001;Wang et al. 2003Wang et al. , 2006)).
The interaction between slab melt and mantle wedge is also an important mechanism for the intermediate rocks with high Mg and Sr/Y ratios (Sen & Dunn, 1994;Kelemen, 1995;Rapp & Watson, 1995;Rapp et al. 1999;Wood and Turner, 2009;Jing et al. 2022a).In the MgO vs. SiO 2 and SiO 2 vs. FeO*/ MgO diagrams (Fig. 9e, f), the samples belong to low iron calc-alkaline (LF-CA) Magnesian Andesits, which are similar to the geochemical characteristics of magmatic rocks formed by the interaction of subducted slab and melt-mantle wedge (Deng et al. 2009).Furthermore, the zircon ε Hf (t) values of the quartz diorites recommend that the magma could have originated from metasomatized depleted lower mantle, further supporting this perspective.The above characteristics indicate that the quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge.

6.a.2. Mylonitic granite
The geochemical characteristics of the granite are consistent with the syenogranite found in the Shanmen region (Cao, 2013).The studied samples exhibit high SiO 2 (73.59 wt% -75.88 wt%), Al 2 O 3 (12.94wt% -13.90 wt%) and K 2 O (1.25 wt% -2.68 wt%), as well as low MgO contents.They also display low Mg # values and enrich in LREEs and LILEs and deplete in HREEs and HFSEs, which illustrates that our studied granites must have originated from crustal materials (Sun & McDonough, 1989;Rudnick & Gao, 2003).Moreover, the similarities in geochemical characteristics between these granites and I-type granites are further supported by the (Zr þ Nb þ Ce þ Y) vs. TFeO/MgO and Zr vs. 1000*Ga/Al diagrams (Whalen et al. 1987; Fig. 10c, d).The zircon εHf(t) values of the granite provide further evidence that the magma originated from the juvenile lower crust.The granite samples demonstrate depletion in Eu anomalies, which is consistent with the partial melting of source rocks with the plagioclase left as a residual mineral.The compelling depletion of Nb, Ta and Ti further confirms that rutile may be another residual mineral (Fig. 7d).Based on these indications, we propose that the granites originated from the partial melting of the intermediate-basic lower crust.
6.b.Tectonic implications 6.b.1.Diversified sources in the generation of the Permian magmatism In the eastern CAOB, the northern margin of NCC experienced the subduction of the PAO and the collision of related microcontinental blocks, resulting in widespread Late Paleozoic magmatism.In recent years, numerous Permian magmatic rocks have been discovered in the Changchun-Kaiyuan (Fig. 2a; Table 4).These Permian (ca.265-250 Ma) rocks mainly consist of high-K calc-alkaline intermediate rocks and granitic intrusions with a metaluminous to weak peraluminous affinity (Fig. 6).The intermediate rocks, including gabbro, gabbro diorite, monzodiorite, monzonite and quartz diorite, exhibit characteristics of arc magmatic rocks.They are enriched in LILE and LREE, while depleted in HFSE such as Nb, Ta, Ti and HREE (Fig. 7).The (2) partial melting of young and hot oceanic crust; (3) ridge subduction; and (4) slab break-off (Defant & Drummond, 1990;Sajona et al. 1993;Yogodzinski et al. 1995;Guivel et al. 1999;Calmus et al. 2003;Jian et al. 2010;Castillo, 2012).The CAOB is usually considered to have undergone prolonged subduction and accretion until the Early-Middle Triassic (Eizenhöfer et al. 2014;Eizenhöfer & Zhao, 2018;Li et al. 2022b;Xiao et al. 2003;Jing et al. 2020Jing et al. , 2021;;Huang et al. 2018;Wu et al. 2011).Therefore, the first and second assumptions are not suitable.The concept of slab windows was originally introduced by Dickinson and Snyder (1979), who associated them with the subduction of obliquely or orthogonally converging oceanic ridges and the process of transforming faults descending into oceanic trenches.The slab break-off can also lead to the slab windows.The upwelling of the asthenospheric mantle through slab windows induces decompression melting, generating mafic melts that interact with the lower crust, leading to the formation of extensive granite.Partial melting of the edge of the subducting slab produces distinctive rock assemblages (Yogodzinski et al. 1995).It is worth noting that the upwelling of the asthenosphere often triggers significant extension of the overlying lithosphere, which aligns with the tectonic environment conducive to the formation of Atype granite in the Permian.Recently, the presence of slab windows during the Permian has been proposed in the southwestern and southeastern parts of the CAOB (Windley et al. 2007;Yin et al. 2010).Numerous ca.250 Ma adakites, Nb-rich basalts and high-Mg andesites (HMAs) were reported in the Faku-Kaiyuan area,  Tectonic evolution of the Middle-Late Permian orogenic belt   Tectonic evolution of the Middle-Late Permian orogenic belt  Along the SXCYS, there is an east-west trending belt of Permian arc magmatic belt and Late Permian-Early Triassic high-Mg andesites (Yuan et al. 2016;Liu et al. 2012;Li et al. 2007;Shen et al. 2020;Fu et al. 2010).This belt roughly parallels the SXCYS.Although the subduction of the mid-ocean ridge parallel to the trench can also explain this belt, most of the mid-ocean ridges and subduction zones are oblique or orthogonal.Additionally, there was no regional metamorphism of high temperature and low pressure during the Late Permian to the Early-Middle Triassic in the study area.Based on the evidence, we propose that the formation of the Shanmen pluton can be attributed to the upwelling of hot asthenospheric, which is closely connected to the slab break-off mechanism (Fig. 11).Tectonic evolution of the Middle-Late Permian orogenic belt 3. The mylonitic granite is veined exposed in diorite, representing the product of partial melting of the intermediate-basic lower crust.4. The Shanmen pluton formed in an active continental margin setting, in response to southward subduction of the PAO, which is closely linked to the slab break-off mechanism.

Figure 1 .
Figure 1.(Colour online) (a) Simplified tectonic sketch map of the eastern Central Asian Orogenic Belt (modified after Sengör et al. 1993; Zhang et al. 2022); (b) Simplified tectonic sketch map of Northeast China (modified after Liu et al. 2017).

Figure 2 .
Figure 2. (Colour online) (a) Simplified regional geologic map of the Changchun-Kaiyuan showing the distribution of the Permian igneous rocks.All these reported age locations were presented in Table 4; (b) Geological sketch map of the shanmenzhen region, with the sample locations shown.SXCYS: Solonker-Xar Moron-Changchun -Yanji Suture.

Figure 4 .
Figure 4. (Colour online) Representative cathodoluminescence images of selected zircons of the quartz diorite and mylonitic granite.

Figure 5 .
Figure 5. (Colour online) (a, b, c) U-Pb concordia diagrams showing zircon ages obtained by LA-ICP-MS.The weighted mean age and MSWD are shown in each figure; (d) chondrite-normalised REE patterns for zircons from the quartz diorite and mylonitic granite.
-ICP-MS zircon U-Pb dating indicates the Shanmen pluton in the eastern part of the CAOB emplaced in the Middle-Late Permian (263-259 Ma). 2. The quartz diorite is the product of the interaction between different degrees of slab melt and the overlying mantle wedge.

Table 1 .
LA-ICPMS U-Pb zircon data for the Middle-Late Permain Shanmen pluton

Table 3 .
In situ zircon Hf isotopic compositions for the Middle-Late Permian Shanmen pluton

Table 4 .
Reported geochronological data for the Permian magmatic rocks in the Changchun-Kaiyuan area