Partial melting of thickened lower crust in the intraplate setting: constraints from triassic postectonic baishandong granitic pluton in Eastern Tianshan

ABSTRACT

Thickened crust is of great significance for tectonic evolution and mineralization and generally occurs in collisional orogenic belts. Whether accretionary orogenic belts, such as the Central Asian Orogenic Belt (CAOB), are able to create remarkably thickened crusts and what the mechanism is, remain to be determined. In the southwestern of CAOB, the Eastern Tianshan with broad magmatism and integrated arc-continental system is an excellent area for understanding the crustal evolution. Here, we report geochronological, geochemical and Nd-Hf-O isotope data for Middle Triassic Baishandong granites in the Eastern Tianshan, which exhibit three stages: Group 1 (ca. 237 Ma) granodiorites, Group 2 (ca. 234 Ma) monzogranites, and Group 3 (ca. 229 Ma) highly fractionated I-type granites. The Group 1 and 2 granites exhibit high-K, calc-alkaline, and metaluminous characteristics, weak negative Eu anomalies, and low (K₂O+Na₂O)/CaO and FeO^T/MgO ratios with high Na₂O/K₂O, Sr/Y, and (La/Yb)_N values and low MgO, Y, and Yb contents, which are indicative of adakitic affinity. The Group 3 rocks have extremely high SiO₂ contents, notable Eu and Sr depletions, lower Mg# values, and the ‘tetrad effect’ of REEs, indicating that they are highly fractionated I-type granites. All the granites have similar depletion in Sr, Nd, Hf, and O isotope compositions (εNd(t) = 3.1–5.5, (⁸⁷Sr/⁸⁶Sr)i = 0.7036–0.7051, εHf(t) = 11.3–13.4, δ¹⁸O = 6.61–7.21) with young Nd, Hf second-stage model age (565–755 Ma, 425–541 Ma). It indicates that they are all the remelting of thickened juvenile lower crust that was initially derived from Neoproterozoic crust-mantle differentiation. Based on the discussion, we proposed that the granitic pluton intruded in an intraplate extensional setting, which indicates that thickened crust had existed before the Triassic and that the final amalgamation of the Paleo-Asian Ocean occurred in late Palaeozoic. Geochemical features indicate tectonic compression played a more important role in significant lower crust thickening.

KEYWORDS:

• Thickened crust
• triassic
• adakitic granite
• eastern tianshan
• CAOB

1. Introduction

The Central Asian Orogenic Belt (CAOB) (also referred to as the Central Asian Orogenic System) is one of the world<apos;>s largest accretionary tectonic belts, with considerable juvenile continental crustal growth in the Phanerozoic (Şengör 1993; Jahn et al. 2000; Windley et al. 2007; Kröner et al. 2014; Li et al. 2015). It has been suggested that juvenile crust accounts for over 50% of the CAOB (Wu et al. 2000; Windley et al. 2007; Du et al. 2020; Xiao et al. 2018; Ding et al. 2021(Figure 1(A)). The evolution of the CAOB involved multiple accretion and arc–continent collision events from the early Neoproterozoic to the Permian, resulting in the successive closure of the Paleo-Asian Ocean (Chung et al. 2003; Xiao et al. 2010, 2018; Ding et al. 2021). In contrast to the much-studied eastern part of the CAOB, relatively little is known about the Mesozoic petrogenesis of the batholiths in the western part, making it difficult to evaluate tectonic evolution and crustal growth.

Figure 1. (A) Simplified tectonic map showing the Central Asian Orogenic Belt (modified from Jahn et al. 2000); (B) Topographic map and terminology of the of North Xinjiang and adjacent area of the SW Central Asian Orogenic Belt (basemap from website http://www.ngdc.noaa.gov/mgg/global) showing the North Tianshan, Central Tianshan, South Tianshan and Southern Beishan from north to south; (C) Geological map showing the location of study area in Eastern Tianshan (Zhou et al. 2010; Zhang et al. 2011; Kröner et al. 2013; Breiter et al. 2014; Wang et al. 2017). Data sources for ages, εNd(t) and εHf(t) values of felsic intrusive rocks are presented in Table A6. Abbreviation: (Ad)- adakitic granite; (H)- highly fractionated granite; (I)- I-type granite; (Alk)- alkalic granite; (A)- A-type granite.

The Eastern Tianshan is located in the southwestern region of the CAOB (Figure 1(B)). This area containing several integrated tectonic units and more Triassic magmatism is very important to study the Early Mesozoic tectonic evolution of the southwestern CAOB (Figure 1(C)). Previous studies on crustal growth (Wang et al. 2008b), mantle–crust interactions (Lei et al. 2020) and the dynamic mechanism of tectonic setting (Zhang et al. 2017; Du et al. 2020) have been conducted in detail and have mainly focused on Palaeozoic granitoids and tectonic settings (Xiao et al. 2013; Ma et al. 2014; Zheng et al. 2018; Han et al. 2018b; Long et al. 2020), and little attention has been given to Triassic granites (Chen et al. 2019; Lei et al. 2020), which hinders the understanding of late-stage tectonic evolution of the Eastern Tianshan. As a result, there are still some problems under debate: (1) The Mesozoic tectonic setting and dynamic mechanism remains controversial (ZZhou et al. 2004; Windley et al. 2007; Wu et al. 2010; Qin et al. 2011; Mao et al. 2014; Zhang et al. 2017; Huang et al. 2018; Han and Zhao 2018a). (2) The controversies on the mechanisms of crustal thickening caused by crustal growth still exist, including tectonic thickening, underplating and intraplating, and mantle–crust interaction (Zhang et al. 2006; Mao et al. 2021).

Recent years, volume Triassic plutons have been discovered in the Eastern Tianshan, and are characterized by a variety of complex lithological associations, such as Baishandong Pluton (237–229 Ma; this study), Weiya alkali syenogranite, quartz diorite, monzogranite and quartz syenite (246–233 Ma; Zhang et al. 2011; Mao et al. 2015), Tianhu monzogranite and granodiorite (243–241 Ma; Zhao et al. 2017), Aqishan syenogranite and biotite monzogranite (ca. 230 Ma; Li et al. 2002; Wang et al. 2019). Among these Triassic plutons, the Baishandong Pluton has a unique lithologic association including adakitic and highly differentiated granites from early to late stages. In this paper, we present whole-rock geochemistry, Sr-Nd isotope data, zircon U-Pb ages and Hf-O isotope data to constrain the age and petrogenesis of the Baishandong granites. Building on these new data and previously published data on granitoids, we further discuss the Mesozoic tectonic setting and the possible mechanism of crustal thickening in the Eastern Tianshan.

2. Geological setting

The CAOB occupies a broad area between the Siberian, East European, and North China-Tarim Cratons (Figure 1(A)), which contains a wide range of microcontinents, arc-back arc systems, oceanic islands, ophiolites, and subduction-accretionary complexes (Şengör et al. 1993a; Heinhorst et al. 2000; Gao et al. 2004; Gu et al. 2006; Kröner et al. 2011, 2013). As part of the CAOB, Eastern Tianshan is characterized by the continuously development of orogenic processes from collision to intraplate regime, which occurs in the easternmost segment of the Tianshan Mountain Range in the southern CAOB (Xiao et al. 2004, 2010, 2015). It occupies a key position between the Central Asian Orogenic Belts to the west and east (Xiao et al. 2004) and is a very significant area for understanding the tectonic transition in the Indosinian (Figure 1(B)). It is mainly composed of the North Tianshan in the north and the South Tianshan in the south, with the Central Tianshan Massif situated between them (Shu et al. 2004). These three sections are divided by the Aqikekuduke-Weiya Fault and the Kawabulake Fault from north to south (Figure 1(C)). The North Tianshan mainly contains Carboniferous marine volcano-sedimentary rocks with limited Silurian, Devonian, Permian, and Meso-Cenozoic strata and magmatism (Cao et al. 2017). The Baishandong granites and other intrusions, such as the Kizil Tag intrusion (Li et al. 2006), are located in the eastern part of the North Tianshan that consist of greenchist-facies metamorphosed and ductile deformed Carboniferous volcaniclastic rocks, and ophiolitic fragments (Li et al. 2006aa). The Central Tianshan Massif is a unique tectonic unit due to its characteristic Precambrian crystalline basement and widespread Palaeozoic-Mesozoic plutonic rocks (Huang et al. 2019). The basement rocks are mainly Paleoproterozoic to Neoproterozoic in age and have experienced greenschist to amphibolite-facies metamorphism (He et al. 2014). It is considered to have been separated from the Tarim Craton during the southward subduction of the Tianshan Ocean along the Shaquanzi fault in the Early Palaeozoic (Hu 1990). The subduction and closure processes of the Junggar and/or the South Tianshan oceans or postcollisional extension induced the formation of the Palaeozoic intrusions (Xiao et al. 2004; Shi et al. 2014; Zhang et al. 2016). The South Tianshan resulted from the closure of the South Tianshan Ocean between the Central Tianshan Block and the Tarim Block (Wang et al. 2010). The Beishan terrane is a part of early Palaeozoic arcs (Song et al. 2013).

The granitic magmatism included four stages: Late Devonian (390–370 Ma), Early Carboniferous (350–330 Ma), Late Carboniferous–Permian (320–250 Ma), and Early–Middle Triassic (250–230 Ma) (Zhou et al. 2010). These stages are consistent with pre-collision stage, primary collision stage, post-collision stage, and intraplate stage in the Eastern Tianshan, respectively (Gu et al. 2006; Zhu 2007; Zhou et al. 2008; Wang et al. 2009).

3. Triassic pluton and sample descriptions

The Baishandong granitic pluton is located about 15 km east of large-scale Baishan molybdenum deposit, and has an area of about 252 km². It lies within the Early Carboniferous Gandun Formation that is composed of deep-sea carbonaceous siliciclastic rocks. This granitic pluton can be divided into two suits, including the Xiaocaohu (Tx) and the Maanshanbei (Tm) suits according to previous studies (Liu et al. 2006), the Enhanced Thematic Mapper (ETM) images, and our detailed field mapping (Figure 2). The pluton is undeformed and cuts the regional tectonic lines, showing typical post-tectonic characters (Figure 2). The outcrops of the two suits are roughly elliptic and circular, especially the Maanshanbei (Tm) suits. Concentric zoning is a prominent feature of post-tectonic granitoid plutons (Holder 1979). The Xiaocaohu suit is mainly composed of three units (Tx¹, Tx², and Tx³), and the Maanshanbei circular suit are composed of two units (Tm¹ and Tm²). These granites can be classified into three groups based on the lithology. Group 1 consists of medium-coarse grained granodiorite (Tx¹ and Tm¹); Group 2 consists of medium grained biotite monzogranite (Tx² and Tm²); and Group 3 consists of fine-grained muscovite monzogranite (Tx³) from outside to inside according to the geochemical features, criteria, and methodology of the intrusive hierarchical units (Gao et al. 1991). In addition, based on the results of mineral maps by TIMA on the representive samples (Figure 3(A)), the Group 1 samples plot in the field of granodiorite and the Group 2 and 3 samples plot in the monzogranite field in the ternary QAP diagram (Figure 3(B)), which are consistent with above classifications via the specimens and microscopic observation. The emplacement boundaries can be clearly observed in the field and the ETM images (Figure 2(A)), which provide solid evidence from which to identify the chronologically sequence of the groups. Specifically, the Group 1 (Tx¹ and Tm¹) was intruded by the Tx² and Tm², and the Group 3 (Tx³) intruded into the Group 2 (Tx²). Several dioritic enlaves are also identified in the Group 1 and 2 (Figures 4(F,G)).

Figure 2. (A) ETM image and (B) Geological map of the Baishandong pluton, after XBGMR (1993) .

Figure 3. (A) Mineral maps of the Baishandong granitoids by TIMA; (B) A ternary QAP plot showing the relative modal proportions of quartz (Q), alkali feldspar (A), and plagioclase (P) to define the IUGS fields for the rocks (after Le Maitre, Defant et al. 2002; Chen et al. 2005) .

Figure 4. Representative photographs of the rocks of Baishandong pluton. (A)-(B) granodiorite; (C)-(D) biotite monzogranite; (E) muscovite monzogranite. (F)-(G) dioritic enclaves. (H) Porphyritic-like texture of granodiorite from Maanshanbei suit (Tm1); (I) Granitic texture of medium-fine-grained granodiorite from Xiaocaohu suit (Tx1); (J) Granitic texture of fine-grained monzonitic granite from Maanshanbei stock (Tm²); (K) Porphyritic-like texture of medium-fine-grained monzonitic granite from Xiaocaohu stock (Tx²); (L), (M) Granitic texture of medium-fine-grained monzonitic granite from Xiaocaohu stock (Tx³). Abbreviation: Qz = quartz; Pl = plagioclase; Kf = feldspar; Bt = biotite; Mu = muscovite; Hb = hornblende; Or = orthoclase; Ab = albite; Chl = Chlorite.

3.1 Group 1: medium-coarse grained granodiorites (Tx¹ and Tm¹)

The medium-coarse grained granodiorites are mainly composed of plagioclase (35 to 40 vol.%), orthoclase (20 to 30 vol.%), quartz (15 to 20 vol.%), hornblende (10 to 15 vol.%), and biotite (~5 vol.%). The main phenocryst minerals are feldspar, which are several centimetres in size (Figures 4(A,B)). The main accessory minerals include magnetite, zircon, apatite, and titanite. The subhedral plagioclase (1–2 mm) has been partially replaced by sericite and opaque. The K-feldspar (0.5–2 mm), which exhibit gridiron twining and twinning striations, locally replaced by plagioclase. The quartz and biotite are size of 0.5–2 mm and 1–2 mm in size, respectively. Symbiosis between the feldspar, amphibole, and biotite can be clearly observed in the microphotographs (Figures 4(H,I)).

3.2 Group 2: medium-grained biotite monzogranite (Tx² and Tm²)

The biotite monzogranite is greyish-white and medium-fine grained, has a granular structure (Figures 4(C,D)), and is composed of andesine or oligoclase (40 vol.%), perthite (25 vol.%), quartz (30 vol.%), and biotite (5 vol.%). Apatite, titanite and magnetite are present as accessory minerals. Concentric zoning in oligoclase is due to slow cooling of the crystal in a melt that changed composition as more crystals formed, producing well-developed oscillatory zoning. The secondary minerals in these samples, including sericite, chlorite, and titanite suggest that the granites are mildly-moderately altered and the rock-forming minerals, especially the oligoclase cores have been locally replaced by chlorite in the thin sections. The biotite produced chlorite with titanite precipitation (Figures 4(J,K)).

3.3 Group 3: fine grained muscovite monzogranite (Tx³)

The fine-grained muscovite monzogranite is intruded in the Tx². It is light red and inequigranular, has a granular texture (Figure 4(E)), and consists of albite (35 vol.%), orthoclase (25 vol.%), quartz (35 vol.%), and muscovite (5 vol.%), with minor apatite and zircon. Gridiron twining and sodic stripes can be seen in the K-feldspar. The minor kaoline and sericite in the feldspars indicate that the rocks have undergone mild alteration. The albite phenocrysts (2 mm) are slightly smaller than the anhedral quartz and microcline (>2 mm), and some of the minerals can be as large as 3–4 mm (Figure 4(L)). The muscovite is located between the quartz and feldspar, which is the result of late magmatic crystallization (Figure 4(M)).

4. Analyse methods

4.1 U-Pb Zircon age

Zircon grains were extracted by conventional heavy liquids and magnetic techniques, and then purified by hand picking under binocular microscope. Representative zircon grains were mounted in epoxy resin and polished down to expose the grain centre. The internal structure was examined using a Cathodoluminescence spectrometer (MonoCL3+, Gatan Company, England) attached to a JSM6510 scanning electron microscope at conditions of 15 kV and 120 nA at the Beijing GeoAnalysis Co., Ltd., Beijing.

Zircon LA-ICP-MS U-Pb dating and trace element analyses were conducted synchronously at the Mineral Laser Microprobe Analysis Laboratory (Milma Lab), China University of Geosciences, Beijing (CUGB), China. Laser sampling was performed using a NewWave 193UC excimer laser ablation system. The ablated material was transported by carrier gas into the plasma source of an Agilent 7900 ICP–MS. Detailed setting parameters for the instruments and experimental process are listed in (Chen et al. 2019).

Off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were performed using the ICPMS DataCal software (Tang et al. 2010). Common Pb corrections were calculated using ComPbCorr#3.17 and concordia diagrams, and ²⁰⁶Pb/²³⁸U weighted mean plots were made using Isoplot 3.0X.

Zircon standards GJ-1 and Plesovice were analysed as unknown samples that were inserted between 91,500 and the samples. We obtained weighted mean ²⁰⁶Pb/²³⁸U ages of 599.2 ± 4.6 Ma (2SD, n = 4) for GJ-1 and 336.8 ± 2.9 Ma (2SD, n = 4) for Plesovice, which are within error of recommended values. The results of ages can be seen in Table A2.

4.2 Major and trace elements

A suite of representative samples (n = 39) was sent to the AcmeLabs, Vancouver, Canada for analysis of major and trace elements. Samples were powdered in a mild-steel ring and puck mill at AcmeLabs. A 0.2 g aliquot of powder was mixed with 1.5 g of a mixed lithium metaborate – tetraborate flux and fused in a graphite crucible at 980°C for 30 min, and the resulting glass bead was dissolved in 5% HNO₃. Major elements were analysed by ICP-AES, and trace elements were analysed by ICP-MS. The sample data are presented in Table A3.

4.3 In situ zircon Hf-O isotope

Zircon oxygen isotopes were measured using the SHRIMP II e-MC instrument in the Beijing SHRIMP Center, Institute of geology, Chinese Academy of Geological Sciences, Beijing, China. Detailed analytical processes are similar to those described by (Gao et al. 2016). Oxygen isotopic measurements were made on the same zircon grains that had previously been analysed for U-Pb. Each ¹⁸O/¹⁶O analysis took approximately 6 minutes and the analytical procedures and conditions were similar to those described by (Wang et al. 2008a). The spots were about 30 μm in diameter. The reference material used for calibration of instrumental mass fractionation (IMF) was TEMORA 2 zircon (δ¹⁸O = 8.20‰; Gao et al. 2004).

In situ zircon Hf-isotopic analyses were performed at sites, or in the same age domains (identified used CL images), in the zircons used for U-Pb and O isotope analyses. Hf-isotopic analyses were undertaken using a New Wave UP213 laser-ablation microprobe, attached to a Neptune multi-collector ICP-MS at the institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. Lu-Hf isotopic measurements were made with ablation pits 44 μm in diameter, a repetition rate of 8 Hz, a laser beam energy density of 10 J/cm², and an ablation time of 26s. The detailed analytical procedures were similar to those described by (Li et al. 2007). Zircon GJ-1 was used as the reference standard during our routine analyses. The weighted mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of GJ-1 (0.282007 ± 25; 2σ) is similar to the commonly accepted weighted mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282013 ± 19 (2σ) reported for in-situ analysis by. The results are given in Table A4.

4.4 Whole-rock Sr-Nd isotope

The Sr-Nd isotopes were tested at Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China, Chinese Academy of Sciences. For Rb-Sr isotope analyses, Rb-Sr and light rare-earth elements were isolated on quartz columns by conventional ion-exchange chromatography with a 5-ml resin bed of AG 50 W-X12 (200–400 mesh). Nd and Sm were separated from other rare-earth elements on quartz columns using 1.7-ml Teflon powder coated with HDEHP, di (2-ethylhexyl) orthophosphoric acid, as cation exchange medium. Sr was loaded with a Ta–HF activator on pre-conditioned W filaments and was measured in single-filament mode. Nd was loaded as phosphate on pre-conditioned Re filaments and measurements were performed in a Re double filament configuration. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios are normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. In the LRIG, repeated measurements of Ames metal and the NBS987 Sr standard during the 2004/2005 period gave mean values of 0.512149 ± 0.000022 (n = 98) for the ¹⁴³Nd/¹⁴⁴Nd ratio and 0.710244 ± 0.000033 (n = 100) for the ⁸⁷Sr/⁸⁶Sr ratio. Results of Rb-Sr and Sm-Nd are given in Table A5. The external precision refers to the 2σ uncertainty based on replicate measurements on these standard solutions over 1 year. Total procedural blanks were <300 pg for Sr and <50 pg for Nd. ¹⁴³Nd/¹⁴⁴Nd = 0.51315 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.2137 in the calculation of the one-stage Nd mode age (Guan et al. 2012). The average continental crust value of ¹⁴⁷Sm/¹⁴⁴Nd is 0.118 in the calculation of two-stage Nd mode age (Allen et al. 1995).

5. Results

In total, 32 granitic samples were collected from the Baishandong Pluton for geochemical and chronological analyses.

5.1 Geochronology

The zircon U-Pb results for five samples for each unit from the Baishandong Pluton are presented in Table A1, A2. The concordia diagrams and representive CL images are shown in Figure 5. Most of the zircons are 50 to 230 μm, transparent, euhedral prismatic crystals. These zircons have length/width ratios of 2:1 to 3:1 and have magmatic oscillatory zoning in the CL images. The cores are also euhedral, with a few zircon grains containing ovoid cores. Most of these crystals are subhedral and have lengths ranging from 80 to 200 μm. The Tx¹ and Tm¹ granodiorites have weighted mean ²⁰⁶Pb/²³⁸U ages of 237.1 ± 1.5 Ma and 236.9 ± 1.4 Ma, which are represented by samples G186251-2.1 and G19720-1.1, respectively. Biotite monzogranite samples G186251-7.1 and G19718-1.1 from Tx² and Tm² have U-Pb ages of 234.8 ± 1.9 Ma and 234.2 ± 1.5 Ma, respectively, which are slightly younger (ca. 2–3 Ma) than those of Group 1. Sample G19720-11.1 from Tx³ has an U-Pb age of 228.7 ± 1.8 Ma.

Figure 5. (A)-(E) Representative zircon cathodoluminescence images of various granitoids from the Baishandong pluton. The yellow, blue and red circles and numbers on zircons show the positions and results for U-Pb, O and Lu-Hf isotope analysis, respectively. (F) The Chondrite-normalized REE patterns of zircons from the Baishandong pluton.

The U-Pb zircon dating produced weighted mean ²⁰⁶Pb/²³⁸U ages of 237 to 229 Ma, that is, a Middle-Late Triassic crystallization age. Group 1 rocks are the oldest units (Tm¹ and Tx¹), and Group 3 granites are the youngest units (Tx³). The age variations are consistent with the field observations and ETM images.

5.2 Whole-rock geochemistry

The whole-rock major and trace element data from the Middle-Late Triassic rocks are reported in Appendix A (Table A3). The three groups of granites exhibit different geochemical characteristics. Group 1 granodiorites have relatively high SiO₂ (64.32–69.70 wt.%) and K₂O (2.46–3.61 wt.%) contents, high Na₂O (3.71–4.39 wt.%), and CaO (2.89–4.28 wt.%). On the TAS diagram (Figure 6(A)), they plot in the granodiorite field, which are consistent with the petrological observations (Figure 3(B)). These granodiorites mostly belong to the high-K calc-alkaline series, with a few calc-alkaline samples (G186251-2.1 to G186251-2.3), and their A/CNK ratios (molar ratios of Al₂O₃/(CaO+Na₂O+K₂O)) of 0.90 to 0.98 indicate that they are all metaluminous (Figures 6(B,C)). Group 2 granites have higher SiO₂ (70.63–74.58 wt.%) and K₂O (3.41–4.42 wt.%) contents, stable Na₂O contents (3.71–4.59 wt.%), and lower CaO contents (1.22–2.66 wt.%), and they all plot within the high-K calc-alkaline series in the K₂O-SiO₂ diagram (Figure 6(B)). The values range from 0.93 to 1.04, plotting in the metaluminous field on the A/CNK vs. A/NK diagram (Figure 6(C)). Group 3 samples exhibit the highest SiO₂ (75.55–77.68 wt.%) and K₂O (3.73–4.52 wt.%) contents, relatively high Na₂O contents (4.17–4.36 wt.%), and the lowest CaO contents (0.14–0.65 wt.%). They are also high-K calc-alkaline and are metaluminous to weakly peraluminous on A/CNK vs. A/NK diagram (Figures 6(B,C)). The metal oxides strongly correlated with SiO₂ on the Harker diagrams, and the granites plot roughly along a straight line (Figures (6D-6I)).

Figure 6. Discrimination diagrams for the granites from Baishandong pluton. (A) TAS classification diagram of the granitoids from Baishandong pluton, after Middlemost (Brown 1994); (B) K₂O-SiO₂ diagram after (Zhou et al. 2010; Zhang et al. 2011; Kröner et al. 2013; Breiter et al. 2014; Wang et al. 2017). The shadow area represents pre-Mesozoic slab melting adakites in Eastern Tianshan (Mao et al. 2021); (C) A/NK-A/CNK diagram after (Masuda and Akagi 1989). Hark variation diagrams showing the major and trace element variations of the Triassic Baishandong pluton in the Eastern Tianshan; (D) TiO₂, (E) Al₂O₃, (F) FeOT, (G) MgO (Martin et al. 2005), (H) Mg#, Reference fields are from (Gu et al. 2006; Kröner et al. 2011), (I) P₂O₅, respectively.

On the chondrite-normalized rare earth element (REE) diagrams, the Group 1 granodiorites exhibit moderately fractionated REE patterns, with (La/Yb)_N ratios ranging from 10.71 to 24.45 and low total rare earth element contents (∑REE = 63.22–123.75 ppm). The δEu values (0.78–1.02) indicate that the granodiorites have small negative Eu anomalies, which suggest the lack of plagioclase fractionation (Figure 7(A)). Because the distribution coefficient of Eu element in plagioclase is much higher than that of other minerals, and the absence of plagioclase in the source is the main reason for the positive/slightly negative Eu anomaly. The (La/Yb)_N ratios (7.78–23.46) and ∑REE contents (48.43–107.43) of Group 2 rocks are similar to those of Group 1. However, the Group 2 rocks have more notable negative Eu anomalies (δEu = 0.52–0.89), indicating enhanced fractional crystallization (Figure 7(C)). The Group 3 muscovite monzogranites exhibit flater ‘tetrad effect’ patterns and have the lowest (La/Yb)_N ratios (2.49–5.19, except one sample with 7.05). The extremely low ∑REE (24.02–34.68) and δEu (0.03–0.39) values are quite distinct from those of the other two groups (Figure 7(E)).

Figure 7. The Chondrite normalized REE patterns and N-MORB normalized trace element diagrams for Triassic Baishandong pluton in the Eastern Tianshan. The chondrite values and N-MORB values are from (allen et al. 1995) and Sun et al., 1989.

Figure 8. The whole-rock geochemical discriminative diagrams of the Baishandong Pluton. (A) FeOT/MgO, and (B) (K2O+Na2O)/CaO vs. (Zr+Nb+Ce+Y) classification diagrams (Barnes et al. 1996), FG: Fractionated felsic granites; OGT: unfractionated. (C) Sr/Y vs. Y diagram from (Defant and Drummond 1990; Barbarin 1999); (D) (La/Yb)N vs. YbN; (E) K/Rb vs. K/Ba; (F) Zr/Hf vs. ?Eu in which the common range of granites is reported by (Bau 1996; Jahn et al. 2001); (G)-(J) SiO2 vs. Dy/Yb, Sm/Nd, Sr/Y, and Zr/Hf diagrams; (K) La vs. La/Sm diagram; (L) Nb/U vs. Ce/Pb diagram. Trace elements are plotted in ppm, Chondrite values are from (Masuda and Akagi 1989) .

On the primitive mantle-normalized spider diagrams, the Group 1 granodiorites are enriched in light REEs (LREEs) and large ion lithophile elements (LILEs, such as K, Rb, and Sr) and are depleted in high field strength elements (Ta, Nb, P, and Ti) (Figure 7(B)). The Group 2 monzogranites are also enriched in LILEs (Rb, K, and Sr) with more obvious depletions in Ta, Nb, P, and Ti (Figure 7(D)). The Group 3 muscovite monzogranites from the Xiaocaohu pluton are all enriched in Rb, Th, U and LREEs, moderately depleted in Ta and Nb, and strongly depleted in Ba, Sr, Eu, P and Ti (Figure 7(F)).

5.3 Whole-rock Sr-Nd isotopes

The initial ⁸⁷Sr/⁸⁶Sr ratios and εNd(t) values of the granitic samples are calculated using the U-Pb zircon ages. The Rb-Sr and Sm-Nd isotopic compositions of 10 granitoids are presented in Appendix A (Table A4). The calculated εNd(t) values and initial (⁸⁷Sr/⁸⁶Sr)i ratios range from 3.1 to 5.5 and from 0.7036 to 0.7051, respectively, which are close to the ranges of the depleted mantle (Figure 10(C)). These homogeneous values indicate that the magmatism was mainly derived from a depleted mantle-derived source. These isotopic characteristics are quite similar to most granitoids in the CAOB (Chang et al. 2018; Ding et al. 2021). The Sm-Nd model ages (T_DM) exhibit a restricted range (502–745 Ma), and the two-stage model ages T_DM2 range from 755 to 565 Ma. These ages fall within the range of the 800 to 500 Ma juvenile granitoids in the CAOB (Heinhorst et al. 2000; Jahn et al. 2000, 2001; Wu et al. 2002; Ding et al. 2021). The low (⁸⁷Sr/⁸⁶Sr)i and positive εNd(t) values are indicative of a significant contribution from a mantle component or mantle-derived materials.

5.4 In-situ zircon Hf-O isotopes

The results of zircon Hf-O isotopic analyses are presented in Appendix B (Table A5) and Table 1, and are plotted on the zircon δ¹⁸O vs. εHf(t) diagram in Figure 10(D).

The 48 magmatic zircons from granodiorite in first unit of Xiaocaohu (Tx¹, G186251-2.1), with a weighted mean ²⁰⁶Pb/²³⁸U age of ~237 Ma, have εHf(t) values of +10.1 to +14.7 and T_DMC model ages of 0.33 Ga to 0.62 Ga. The εHf(t) versus crystallization age plot shows that the Hf isotopic results are consistent with those from the whole-rock Nd isotopes (Figure 9).

Figure 9. Correlations between Hf isotopic compositions and U-Pb zircon ages of from the Baishandong pluton in the Eastern Tianshan. CHUR = chondritic uniform reservoir; DM = depleted mantle, Data of the Neoproterozoic granites from Chang et al. (2018) .

Figure 10. (A) εNd(t) vs. SiO₂ diagram for the granitoids of the Baishandong pluton; (B) Whole-rock δ¹⁸O and initial ⁸⁷Sr/⁸⁶Sr correlation diagram (after James, 1981), Whole-rock δ¹⁸O ratios were calculated based on the formula proposed by Valley et al. (1998); (C) Whole-rock εNd(t) vs. (⁸⁷Sr/⁸⁶Sr)i after (Zindler et al. 19862007b) for the Baishandong pluton; (D) Zircon δ¹⁸O vs. εHf(t). The εHf(t) values of depleted mantle, the SCLM beneath the NCC, mixed mantle, and ancient crust are +16.2, ﹣8 (Guan et al. 2012),+6, and﹣16, respectively. The ?18O of ancient crust and mantle are +9.5 ± 0.5‰ (average of 2.5 Ga and 1.8 Ga crust) and + 5.3 ± 0.3‰ (Brown 1994) .

The 16 Hf isotopic analyses were obtained from granodiorite sample G19720-1.1. The zircons with the same weighted mean ²⁰⁶Pb/²³⁸U age of ~237 Ma have εHf(t) values of +9.7 to +13.4 and T_DMC model ages of 0.41 Ga to 0.65 Ga.

The 40 zircons with a weighted mean ²⁰⁶Pb/²³⁸U age of ~235 Ma from biotite monzogranite sample G186251-7.1 from the Middle Triassic Tm² have εHf(t) values of +7.5 to +15.7 and T_DMC model ages of 0.26 Ga to 0.79 Ga. The eleven zircons from the synchronous sample G19718-1.1 of the Tx² have εHf(t) values of +9.5 to +14.3 and T_DMC model ages of 0.35 Ga to 0.66 Ga. The points 1, 13, and 15 are excluded due to the meaningless model ages which are even younger than their emplacement ages. The point 3 with T_DMC model ages of 1.12 Ga may indicate the evolvement of Mesoproterozoic materials.

Magamatic zircons from muscovite granite sample G19720-11.1 from the unit Tx³ with a weighted mean ²⁰⁶Pb/²³⁸U age of ~229 Ma have εHf(t) values of +9.7 to +15.0 and T_DMC model ages of 0.30 Ga to 0.64 Ga.

Corresponding to the εHf(t) values, all the granites have relatively homogeneous O isotope compositions that fall within a similar range of 5.90 ‰to 7.99 ‰, which are relatively higher than the mantle value 5.3 ± 0.3‰ (Valley et al. 1998) and lower than that of the ancient crust (9.5 ± 0.5‰) in general (Taylor 1978). Most of the analysis spots for a single sample have a narrow range of δ¹⁸O values (<1.5‰), except the biotite monzogranite in the second unit of Maanshanbei (10.64‰).

6. Discussion

6.1 Age of emplacement

According to the above study, the Baishandong pluton has two suits with five units and can be classified into three groups based on lithology, geochronology, geochemistry and isotopes. Previous studies on the Baishandong pluton focused on the geochemical features without systematic dating for each group. Liu et al. (2006) has reported a 237.8 ± 4.3 Ma U-Pb zircon age of the Maanshanbei adakitic pluton (Tm¹), which is very consistent with our new ages of the Group 1 (237.1 ± 1.5 Ma, 236.9 ± 1.4 Ma, Figure 5). New data demostrate that ages of Group 2 (234.8 ± 1.9 Ma, 234.2 ± 1.5 Ma) were slightly later than that of Group 1, and Group 3 (228.7 ± 1.3 Ma) formed approximately 6 Ma later than Group 2. The geochronological results are consistent with the field observations and ETM images (Figure 2). Accordingly, the Baishandong Pluton formed in the middle Triassic.

Previous studies indicate that significant coeval magmatic and tectonic reactivation was much stronger in the eastern Tianshan (Zhang et al. 2017). A series of Triassic plutons were discovered in the Kangguer-Huangshan ductile shear zone located in the southern part of the Jueluotage Belt, which were represented by Baishandong (237 ~ 229 Ma), Baishan (246 ~ 227 Ma), Shuangchagou (252 ~ 247 Ma), Yamansu (231 ~ 227 Ma), Donggebi (236 ~ 227 Ma) and Aqishan (251 Ma) from east to the west (Figure 1(C)). The Tianhu pluton (250 ~ 210 Ma), Weiya diorite porphyrite and alkaline granite (248 ~ 236 Ma) (Zhang et al. 2005; Mao et al. 2015), Xingxingxia granites (246 ~ 223 Ma) (Lei et al. 2013, 2020), Yamansunan (250 ~ 245 Ma) (Zhao et al. 2018) and Xiaobaishitou biotite granite (252 ~ 241 Ma) were emplaced in the Central Tianshan Massif. The typical plutons in the Beishan terrane include the Dahuoluo (240 ~ 238 Ma), Huaniushan (221 ~ 217 Ma), and Dongdaquan (225 ~ 221 Ma) plutons, which exhibit I-type/highly fractionated I-type granites with features of A-type granites (Li et al. 2013b) (Figure 1(C)).

In addition, many gold and molybdenite ore deposits formed in the Triassic and have a close spatial-temporal association with tectono-magmatic processes. For instance, the Baishan molybdenum deposit (ca. 225 Ma, Wang et al. 2020a), the Donggebi molybdenum deposit (234 Ma, Han et al. 2018b), and the Xiaobaishitou W-Mo deposit (245 Ma, Deng et al. 2017). All these situations indicate that the Triassic was also an important period for significant magmatism and mineralization, which was the crucial stage for understanding magmatism and tectonic evolution.

6.2 Petrogenesis

6.2.1 Types of the granitoids

All granites have low degrees of postmagmatic chemical alteration, with loss on ignition values of 0.02–1.50 wt.% and SiO₂/Al₂O₃ ratios of 4.03–6.21 (mean = 5.00), which indicate that their petrochemical signatures can reasonably reflect the nature of the Baishandong granites (Nesbitt and Young 1982; Sylvester 1998; Barbarin 1999). As an accessory mineral, apatite can provide a reliable criterion for distinguishing between I-type and S-type magmas because its solubility changes with SiO₂ content (Chappell and White 1992; Brown 1994). In metaluminous magmas, apatite solubility decreases with decreasing of temperature and increasing SiO₂ content, which indicates that in I-type and A-type granites, P₂O₅ and SiO₂ are negatively correlated. The P₂O₅ contents of the metaluminous Baishandong Pluton range from 0.02 to 0.24 wt.%, and they decrease with increasing SiO₂, which is consistent with I-type granites instead of S-type granites (Figure (6I)). The low Sr isotopic ratios (average = 0.7041) also clearly indicate that the Baishandong granites are not S-type (Table 1). All granites have lower Zr+Nb+Ce+Y values (54.40–273.16 ppm) and FeO^T/MgO (2.70–17.62) and (Na₂O+K₂O)/CaO (1.63–28.21) ratios, which are also clearly distinct from those of A-type granites (Figures 8(A,B)).

Based on their geochemical compositions, the Group 1 granodiorites have high silica contents (SiO₂ > 63 wt.%); high Al₂O₃ contents (average = 15.58, >15 wt.%); more Na₂O than K₂O (Na₂O/K₂O = 1.03–1.69), low MgO contents (average = 1.76, <3 wt.%); low HREE, Y, and Yb (Y < 18 ppm, Yb < 1.9 ppm) contents; high Sr contents (average = 618.26, >400 ppm), Sr/Y ratios (average = 59.96, >40), and (La/Yb)_N (average = 14.84); negligibly negative δEu anomalies (0.78–1.02) implying lack of fractionation of feldspar or feldspar residue in the source; depleted HFSE; and (⁸⁷Sr/⁸⁷Sr)i values of less than 0.7040 (Table A3, A4), which are all typical of an adakite composition (Defant and Drummond 1990). Except for having higher silica contents (70.63–74.58 wt.%), the Group 2 monzogranites exhibit systematically lower major and trace element contents and ratios, including Al₂O₃ (average = 14.29 wt.%), Na₂O/K₂O ratios (average = 1.06), MgO (average = 0.57 wt.%), Sr (average = 340.38 ppm), La (average = 17.93 ppm), Y (average = 9.55 ppm), Yb (average = 1.01 ppm), Sr/Y (23.87–74.03), and (La/Yb)_N (7.78–23.46). The δEu values (0.52–0.89) are slightly lower than those of Group 1 (Figures 7(C)). The lower Al₂O₃, Na₂O, Sr, and Yb contents of the Group 2 granites are not consistent with adakities, but the Sr contents are still higher than those of normal granites. These high-Sr granites could also indicate that source regions and petrogenesis of these granites have characteristics similar to those of adakitic rocks (Liu et al. 2002). In addition, all granites of Group 1 and Group 2 have high Sr/Y and La/Yb ratios that plot in the adakite fields (Figures 8(C,D)). Accordingly, these Group 1 and 2 granites belong to adakitic granites.

The Group 3 muscovite monzogranites mainly intruded into the centre of the Xiaocaohu suit with clear boundaries. This is similar to many plutons in eastern Tianshan and southern China (Chen et al. 2014), which are characterized by extremely high SiO₂ contents (75.55–77.68 wt.%), higher K₂O (3.73–4.52 wt.%), and lower FeOT (0.59–0.84 wt.%), MgO (0.04–0.09 wt.%), CaO (0.14–0.65 wt.%), Zr (24.20–41.00 ppm), and total REE (24.02–34.68 ppm) contents than the Group 1 and Group 2 granites (Table A3). As was previously elaborated upon, the Group 3 granites are high-K, calc-alkaline, muscovite monzogranites, which are I-type granites, not S-type or A-type granites. Furthermore, the following evidence proves that they are highly fractionated I-type granites. (1) The formation of muscovite was the result of strong fractional crystallization of hornblende and plagioclase (Figure 4(M)), which caused the magmas to evolve towards an enriched aluminous composition (Chen et al. 2014). This is consistent with the values of A/CNK (1.02–1.11, Figure 6(C)). The fact that these granites have the lowest MgO contents suggests fractionation of mafic minerals, and petrographic evidence indicates possible biotite fractionation. (2) The striking Ba, Eu, Sr, P, and Ti depletions indicate that copious crystallization of plagioclase, apatite, and Ti-Fe oxides took place during the formation of these granites (Figures 7(E,F)) (Janousek et al. 2004; Zhu et al. 2009). Breiter et al. (2014) proposed that highly fractionated granites generally possess low whole-rock Zr/Hf ratios (<25) and high K/Ba ratios. They also plot far from the field of ‘the common range’ of granites in the K/Ba vs. K/Rb and Zr/Hf vs. δEu diagrams (Figures 8(E, F)). As proposed by Whalen et al. (1987), in some of the FeOT/MgO and (Na₂O+K₂O)/CaO vs. (Zr+Nb+Ce+Y) discrimination diagrams (Figures 8(A,B)), most of these granites plot within the highly fractionated I-type field. (3) Compared to the Group 1 and 2 granites, the Group 3 granites show the lowest ∑REE and (La/Yb)_N values and with most pronounced negative Eu anomalies, implying strong fractional crystallization of plagioclase. Importantly, the ‘tetrad effect’ was observed in the REE patterns of the Group 3 granites which are characteristic of all highly fractionated granites (Whalen et al. 1987; Masuda and Akagi 1989; Bau 1996; Monecke et al. 2011). (4) Trace element ratios have the advantage of measuring the degree of differentiation, such as the Sm/Nd, Dy/Yb, Sr/Y, and Zr/Hf ratios (Guo et al. 2014; Deng et al. 2017). During the process of differentiation, the Sm/Nd and Dy/Yb ratios increased, while the Sr/Y and Zr/Hf ratios decreased (Figure 8(G)-8 J). The Rb/Sr ratios increase from 0.33 in Groups 1 and Group 2 to 5.78 in Group 3, indicating feldspar fractionation because Sr is compatible and Rb is incompatible. Consequently, the elevated Rb/Sr ratio of the Group 3 samples indicates highly fractionated magmas. (5) The zircon saturation temperatures of the grantie reach 652°C–682°C (Table A3; Watson and Harrison 1983), which is similar to those of highly fractionated I-type granites of the world (Wang et al. 2017) but lower than those of typical A-type granites (Chappell and White 1992). Therefore, Group 1 granodiorites and Group 2 biotite monzogranites both belong to adakitic granites and Group 3 muscovite monzogranites are mainly highly fractionated I-type granites.

6.2.2 Sources regions

The term adakite was first introduced by Defant and Drummond (1990) to describe intermediate- to high-silica igneous arc rocks. They have been proposed to be the direct melts of modern subducted oceanic basalts (Barnes et al. 1996; Defant et al. 2002; Martin et al. 2005; Wang et al. 2007a, 2007b; Tang et al. 2010). Subsequently, alternative sources related to continental settings were reported (Xu et al. 2002; Li et al. 2007), which include the following aspects: partial melting of thickened or delaminated lower continental crust (Atherton and Petford 1993; Barnes et al. 1996, 1996; Chung et al. 2003; Gao et al. 2004a; Condie 2005; Wang et al. 2007a, 2007b; Xu et al. 2007; Ma et al. 2015), crustal assimilation and fractional crystallization from parental basaltic magmas (Barbarin 1999; Macpherson et al. 2006), and magma mixing between basaltic and crust-derived felsic magmas (Li et al. 2007, 2007; Wang et al. 2008a; Kröner et al. 2013). Here, we believe that the magma sources for Group 1 and Group 2 adakitic granites were mainly melts of thickened lower crust.

Fractional crystallization from basaltic magmas cannot account for the adakitic rocks in the Baishandong area. The ratios of Dy/Yb and Sr/Y would increase with increasing SiO₂ content for the garnets involved in the crystallization process. However, Baishandong adakitic rocks display decreasing trends that differ from the increasing trend of Group 3 highly fractionated rocks (Figures 8(G,I)). The diagram of La/Sm‐La suggests a partial melting trend instead of a fractional crystallization trend for the adakitic granites in the Baishandong area (Figure 8(K)). These Baishandong granites exhibit negligible Eu anomalies (Figures 7(A,C)), indicating that fractional crystallization of plagioclase was not significant for their formation. Accordingly, fractional crystallization was not responsible for the Baishandong adakitic rocks.

The primary sources of granitic magmatism were derived from the lower-middle parts of normal/thickened crust, especially the crust–mantle interaction zone (Brown 1994). It was inevitable that crust–mantle interactions occurred during the process of magmatism, which was supported by the dioritic enclaves identified in Groups 1 and 2 (Figure 4(F,G)). Baishandong adakitic rocks exhibit low MgO contents, Mg# values, and HREE components but high SiO₂ and Sr contents, which are distinct from the source of effective magma mixing between felsic and mafic magmas. These features indicate limited mixing in the source region.

Moreover, partial melting of delaminated lower crust and subducted oceanic crust will elevate MgO, Cr and Ni contents through the involved mantle peridotite. Mg# (26–40), Na₂O (3.71–4.59 wt.%), and Na₂O/K₂O (0.84–1.69) values from Groups 1 and 2 adakitic granites are all notably lower than those of adakites derived from melting of a subducted slab (Na₂O = 4.88 wt.%, Na₂O/K₂O = 2.5–2.65, Mg# >40; Defant and Drummond 1990; Sajona et al. 2000). These granites have high K₂O and low A/NK [=molar Al₂O₃/(Na₂O + K₂O)], most of which are higher than those of the pre-Mesozoic adakites derived by slab melting in the Eastern Tianshan (Figure 6(B); Mao et al. 2021). In addition, the Nb/U and Ce/Pb ratios of adakitic granites in the Baishandong area were distinct from those of the ocean ridges but resembled the crustal values (Figure 8(L)). Thus, the partial melting of a thickened lower crust is the most reasonable mechanism for the Group 1 and 2 adakitic granites in the Baishandong area.

The Baishandong adakitic granites exhibit high εHf(t) values of 7.5–18.8 (12.0 on average). These values plot between the depleted mantle evolution line and the crustal evolution region and are close to the former. Notably, some values of Group 2 granites plot above the line (Figure 9). In the process of partial melting of lower crust, the contribution of supracrustal materials was inevitable. As shown in Figure 10(A), the εNd(t) values of the Baishandong granitoids slightly fluctuate with increasing SiO₂ contents, suggesting that the source of these samples are likely to suffer supracrustal materials assimilation (Feng and Zheng 2021). The calculated whole-rock δ¹⁸O and ⁸⁶Sr/⁸⁷Sr ratios plot along the convex-upward mixing curve (Figure 10(B)), which mean contamination of mantle-derived basaltic magma through assimilation of or isotope exchange with continental supracrustal materials (James et al., 1981; Feng and Zheng 2021). Although the proportions of supracrustal materials assimilation seem like to be 20%–50%, depleted initial ratios and young model ages of the isotopes (Sr, Nd and Hf), and the absence of inherited zircons exclude the possibility of large-scale assimilation. It means that Baishandong granites almost inherited the features of the initial source.

The Group 1 and Group 2 adakitic granites have relatively high SiO₂ contents and low MgO and total FeO contents. Furthermore, the adakitic granites have Zr/Hf (on average 37.65 and 31.43), Nb/Ta (on average 12.46 and 8.33), and Rb/Sr (on average 0.15 and 0.42) ratios similar to the average crustal values (33, 12–13, and 0.32, respectively), which indicate crustal sources for the Baishandong pluton (Barth et al. 2000). However, these two groups possess low (⁸⁷Sr/⁸⁶Sr)i values (0.7036–0.7042) and high εNd(t) (+3.13 to +5.48) values, indicating mantle-derived sources (Figure 10(A)). The samples have remarkable positive zircon εHf(t) values (+7.47 to +18.78) and homogeneous δ¹⁸O values (5.90 to 7.99 ‰), representing a depleted mantle reservoir (Figure 10(D)). In other words, these rocks have major crustal and trace geochemical features but also have isotopic characteristics of depleted mantle. However, mantle peridotite is unable to evolve into granite directly, and it must go through the basaltic crust stage (Wyllie et al. 1984; Barth et al. 2000; Sisson et al. 2005). Previous studies suggest that partial melting of the juvenile lower crust can produce felsic melts with relatively depleted Sr-Nd-Hf isotope compositions (Zheng et al. 2015). According to the discussion above, the most reasonable source for the Group 1 and Group 2 adakitic granites would be partial melting of lower crustal material derived from depleted mantle. The Sr-Nd isotopic simulation also supports this point (Figure 10(C)). These εHf(t) values of Baishandong granites are similar to those of Neoproterozoic granites (Chang et al. 2018). Based on the zircon Hf model ages (T_DMC = 511–541 Ma) and the whole-rock Nd model ages (T_DM2 = 565–755 Ma) (Table 1), the juvenile lower crust derived from the Neoproterozoic depleted mantle with weak crust–mantle interaction is the main source for Baishandong granites, which is largely compatible with the general scenario for many of the Phanerozoic granitoids in the CAOB (Hong et al. 2004; Wang et al. 2009).

The Group 3 muscovite granites are highly fractionated I-type granites that have much less capacity to reflect the characteristics of the magma source via trace elements (Deng et al. 2017). However, isotopes of the Group 3 muscovite granites could provide robust evidence. These Middle Triassic muscovite granites have depleted whole-rock Sr-Nd isotope compositions (εNd(t) = 4.18 to 4.40; (⁸⁷Sr/⁸⁶Sr)i ratios = 0.7041 to 0.7051) and young Nd model ages (T_DM2 = 664 Ma to 648 Ma), depleted εHf(t) values (+9.7 to +15.0) and young Hf model ages (T_DMC = 301 Ma to 637 Ma) (Table A5). The muscovite granites also have homogeneous δ¹⁸O compositions (average = 6.61 ‰), which emphasize the significant role of the mantle component and limited crustal contamination. These isotopic features are similar to those of Group 1 and Group 2 granites, which indicate a high proportion of juvenile crust materials. It is concluded that the Group 3 muscovite monzogranites were generated from remelting of thickened juvenile lower crust as the Groups 1 and 2 biotite monzogranites. Then, the melts experienced strong fractionation of plagioclase, biotite, apatite, and titanite due to fluids from amphibole dehydration (Rapp and Watson 1995), which are presented by the LREE rich chondrite-normalized REE patterns in Figure 5(F). Accordingly, thickened juvenile lower crust derived from the Neoproterozoic mantle materials was the main source for the Baishandong pluton.

6.2.3 Petrogenesis

Partial melting and fractional crystallization are the main mechanisms leading to the composition change of the granite (Wyllie et al. 1984; Grove et al. 2003; Lee and Bachmann 2014; Gao et al. 2016; Deng et al. 2017). Based on the discussions above, the adakitic and highly fractionated granites of the Baishandong pluton may have different mechanisms.

The Groups 1 and 2 adakitic granites have distinct petrographic features, e.g. the Group 1 granites contain more hornblende and biotite and have larger phenocrysts, while the Group 2 granites exhibit more uniform granularities and have more calcium-rich plagioclase. However, they are both high potassium calc-alkaline series granites and have coherent trace element trends and consistent temperature ranges. Most importantly, these two groups of granites have consistent isotopic compositions indicating that they were derived from similar sources. The Group 3 highly fractionated I-type granites have mineral assemblages that are different from those of the Group 1 and Group 2 granites, including a fine equigranular texture, abundant muscovite and albite, an REE tetrad effect, and the strongest Ba, Eu, Sr, P, and Ti depletions, negative Eu anomalies implying strong plagioclase fractionation. However, it should be noted that the Group 3 granites also have depleted isotopic compositions similar to the Group 1 and Group 2 granites. These consistent isotopic compositions point to a juvenile crustal source derived from Neoproterozoic mantle materials. Coeval Nb-enriched basalt and high-Mg andesite have not yet been discovered, suggesting that no significant mantle materials were involved. Previous studies have shown that the branch of the Paleo-Asian Ocean in eastern Tianshan had already closed (Xiao et al. 2015). The formation of Middle Triassic adakitic magma was not related to the process of subduction of the oceanic plate. The Baishandong granitic emplacement was related to post-orogenic lithospheric extension, under which asthenospheric upwelling triggered the melting of a hydrous mantle source, and likely promoted mantle-derived magma underplating at the base of the lower crust and induced partial melting of the thickened lower crust. The magma chamber formed at a higher level due to the decreasing pressure during the extension process after ca. 6 Ma. The Group 3 highly fractionated I-type granites formed through fractional crystallization of plagioclase, biotite, apatite, and titanite (Figure 12).

6.3 Implications for tectonic setting

Zircon U-Pb dating of the granitoids in the Baishandong region yields ages of 237–229 Ma, indicating that the Baishandong pluton was emplaced during the Middle Triassic period. However, the tectonic framework of the Eastern Tianshan during this period is still poorly understood. Several tectonic setting models have been proposed for the Eastern Tianshan, including the transition from subduction-related margin accretion to postsubduction continental collision (Xiao et al. 2009b) or from syncollisional compression to postcollisional extension (Zhang et al. 2014; Wang et al. 2015; Deng et al. 2017; Wu et al., 2017a; Chen et al. 2018; Du et al. 2020). Xiao et al. (2015) proposed that the entire Chinese Tianshan orogenic belt was finally assembled from the end of the Permian to the middle Triassic. Han and Zhao (2018a) believed that amalgamantion between the Central Tianshan block and the North Tianshan tectonic units was a pre-Permian (310–300 Ma) event based on the following evidence: absence of ophiolite or (U)HP metamorphism younger than 300 Ma; latest Carboniferous to Permian large-scale dextral strike-slip faulting; Early Permian A2-type and bimodal magmatic rocks; early Permian rocks in the North Tianshan containing abundant Precambrian detritus from the Central Tianshan. The zircons’ εHf(t) values of granitoids indicated the subduction setting of the Northeastern Tianshan shifted to a post-collisional background during the Early Permian (Du et al. 2020). The studies on the Turpan Basin further indicate Late Permian to Early Triassic extension in Eastern Tianshan (Allen et al. 1995). We believe that the tectonic transition from collision to extension in the Eastern Tianshan took place in the late Permian. It also supported by the stages of magmatism (Ding et al. 2021). Geochronological data of the magmatic rocks suggest that magmatic activities were waning from the early to middle Permian and ceased at 270–250 Ma (Figure 11(A)), implying that the continental collision related to the Paleo-Asian Ocean tectonic domain has come to the end in the late Permian period. Therefore, the tectonic evolution of the Eastern Tianshan had acted as a holistic plate since the Early Triassic, that is, intracontinental environment.

Figure 11. (A) Histograms of ages of granitoids in the Eastern Tiansha. Locations of the five areas are shown in Figure 1B; (B)-(C) Plot of Sm/Yb and Dy/Yb ratios versus rock ages from the North Tianshan and Central Tianshan. Arrows in diagram B highlight the different episodes of crustal thickening. The dashed horizontal line in the Dy/Yb diagram marks the boundary between garnet-dominated (>2) and amphibole-dominated (<2) compositions. Low values of the Dy/Yb ratio indicative of amphibole involvement, show a significant increase at ca. 240 Ma and afterwards, whereas higher values of the Sm/Yb ratio, which is indicative of garnet involvement, have notably increased during the Later Permian and Early Triassic.

Although the Baishandong pluton exhibits subduction or syncollisional geochemical characteristics similar to those of the adjacent Baishan granite, previous studies suggested that the subduction or syncollisional geochemical characteristics of the Baishandong granite were most likely inherited from late Palaeozoic crustal sources (Li et al. 2019; Mao et al. 2021). Robust evidence distinctly indicates that the Eastern Tianshan had been in postcollisional extension stage since the Triassic (Xiao and Kusky 2009a; Xiao et al. 2009b; Pirajno 2013; Zhang et al. 2016; Du et al. 2020). (1) The absence of collision-related regional dynamic metamorphism since the early Permian and the Triassic ophiolitic complex rules out the possibility of vast oceans existing at this time, indicating an intracontinental environment (Wang et al. 2014). (2) The coeval posttectonic plutons usually are undeformed, euhedral to subhedral shapes and concentric oscillatory zoning structures. They clearly cut through the late Palaeozoic volcanic-sedimentary strata and ENE-trending regional faults. These topographic features indicate that the emplacements occurred in an extensional setting after the closure of the ocean and collision in late Permian. (3) The Baiganhu, Donggebi, and Weiya plutons belong to A-type granites or alkalic granites, which are commonly related to extensional settings (Chappell and White 1992). (4) Triassic mafic and felsic magmas, such as Weiya hornblende gabbro and syenogranite (Feng and Zheng 2021) and Jingerquan gabbro and granitoids (Du et al. 2020), form the bimodal igneous series, indicating a Triassic post-collisional extensional setting in the eastern Tianshan. (5) Group 3 fine-grained muscovite monzogranite in the Baishandong pluton belongs to highly fractionated granites that are mainly associated with postorogenic events and large extensional structures (Cao et al. 2017). The Baishandong pluton was the result of the Middle Triassic magmatism under the intracontinental extensional setting.

Accordingly, the Baishandong pluton formed in an intracontinental extensional environment instead of tectonic transition settings from syn-collisional compression to post-collisional extension, indicating that the final amalgamation of the southern Paleo-Asian Ocean occurred prior to the middle Triassic. The activities of the Qiugemingtashi-Huangshan and Xingxingxia zones indicate ca. 300–280 Ma nappe shearing and 263–243 Ma strike-slip shear deformation, respectively (Chen et al. 2005; Wang et al. 2010). The ca. 270–260 Ma tectonic interval time is consistent with the ca. 270–250 Ma quiet period of magmatic activities (Figure 11(A), Table A6). Furthermore, this period was still under an oceanic crust subduction tectonic setting, which was evidenced by the 261 Ma Longdong adakitic quartz diorite related to oceanic plate subduction (Li et al. 2007). The extensional setting represented by the Weiya and Baiganhu Alkalic granites started in ca. 250 Ma. Accordingly, the tectonic setting in Eastern Tianshan belongs to a subduction and compressional environment in the Palaeozoic and turned into an intracontinental extensional setting in the Triassic. Our result further supports the point that a geodynamic transition from the Paleo-Asian ocean subduction–collision system to the Paleo-Tethys ocean regime took place in late Palaeozoic.

6.4 Mechanism of crustal thickening

Two types of adakites have been identified in the Tianshan area (Zhao et al. 2006): (1) Devonian-early Carboniferous (>320 Ma) subduction-type adakitic granites, representing subduction of oceanic slabs, such as the Liuyuan biotite granite (ca.424 Ma; Mao 2009), Kezirkalasay granodiorite (ca. 375 Ma; Si et al. 2020), and the Yulin adakitic granite (ca. 327 Ma; Chang et al. 2018); (2) Triassic adakitic granites associated with basaltic magma underplating, which were represented by the Shuangchagou (ca. 252–247 Ma; Ding et al. 2021), Baijiantan, Hancaohu, Jianquan (ca. 243–235 Ma; Mao et al. 2021), Baishan (ca. 246–227 Ma; Wang et al. 2020a), and Baishandong (ca. 237–229 Ma). The source of early adakitic granites are characterized by calc-alkaline magma with enriched Al element and Na₂O content, which differ from the Triassic adakitic granites. These Triassic adakitic rocks mainly derived from the melting of thickened basaltic lower crust with enriched K element, Na₂O+K₂O, and low Al₂O₃ content. In the Figure 6(B), the two types of adakitic granites located in the different fields. The pre-Meozoic slab melting adakites distribute in the areas of Calc-alkaline and Tholeiitic while Triassic adakites represented by Baishandong pluton mainly centre on the High-K calc-alkaline area. Subduction-related adakitic granites usually have high MgO, Cr contents, and Mg# values, which were caused by the strongly magma mixing with mantle peridotite when primitive adakitic magma from slab melting went through the mantle wedge (Figure 6(H)). It is the typical features of adakites derived from subduction oceanic plate melting (Barbarin 1999; Defant et al. 2002). On the contrary, the lower Mg and higher SiO₂ content of the Triassic adakitic granites indicate that the adakitic magma was not mixed by mantle peridotite, and the thickened basaltic lower crust is the most reasonable source (Wang et al. 2007b). Shu et al. (2004) and Xiao et al. (1992) proposed that the continental crust in the Eastern Tianshan underwent significant shortening and thickening from the latest Permian to the Triassic. The identification of the nonsubduction-type Triassic Baishandong adakitic granites supports this point by their petrogenesis (Figure 12). It can be understood that these Triassic adakitic rocks indicate a pre-existing thickened lower crust in the Eastern Tianshan (Gao et al. 2016; Du et al. 2020; Ding et al. 2021; Ding et al., 2021) and provide constraints on the mechanism of crustal thickening.

Figure 12. Schematic diagram showing the generation of the Baishandong pluton in the Eastern Tianshan. Thickness of continental crust and lithospheric mantle are not to scale.

In addition to petrology, geochemistry provides valid evidence for understanding major crustal thickening. Mao (2009) underline that the maximum Dy/Yb and Sm/Yb ratios are significant parameters for crustal thickness, which track the presence of the high-pressure minerals amphibole and garnet in the lower crust. The marked increases in maximum Dy/Yb and Sm/Yb ratios displayed by magmatic rocks reflect successive changes in magma genesis during crustal thickening. Based on the above knowledge, we checked the crust of the North Tianshan and Central Tianshan using Dy/Yb and Sm/Yb ratios. Normal crustal thickness had not changed until ca. 250 Ma (Figures 11(B, C)). Rapidly increasing Dy/Yb and Sm/Yb ratios of Early-Middle Triassic magmatic rocks indicate that the sources of these rocks were from remarkable thickened crust (Figures 11(B,C)). However, the magmatic expression of a trace element signatures of further crustal thickening was delayed, which may be due to the thermal relaxation time in the centre to deep crust after crustal thickening and must be overcome before the garnet signature by assimilation is formed (Mao 2009). As we know, the extensional setting was not able to result in significant crustal thickening. It is reasonable that the adakitic geochemical features of Triassic granites would be inherited from the events of crustal thickening caused by collision and intracontinental compression before the Triassic. In order to provide the source for Triassic granites, rapid and significant crustal thickening caused by tectonic shortening of the middle-lower crust most likely occurred in the late Palaeozoic which was also the transition stage.

Hf isotopes and Sr-Nd isotope compositions (Wang et al. 2009) indicate continuous addition of mantle-derived materials to the lower crust during the late Palaeozoic in the Eastern Tianshan (Figure 9). Han et al. (2006) proposed that continental thickening was the result of underplating and differentiation of mantle-derived magmas during the postcollision or postaccretion regime. Du et al. 2020 supported this hypothesis by comparing the Sr and Nd isotopic compositions of the Palaeozoic and Mesozoic granitoids, which emphasized that new underplating by mantle-derived mafic magma led to vertical continental growth. However, the ratios of Dy/Yb and Sm/Yb argue that the crustal thickness did not significantly change in the Palaeozoic subduction period (>270 Ma) (Figures 11(B,C)). The results suggest that the crustal thickness was balanced dynamically in subduction stage and plate collisions might be the key role to destroy this balance in the transition stage. Mao et al. (2021) also suggests crustal thickening in Eastern Tianshan is the result of tectonic thickening. It is concluded that tectonic compression caused by the closure of the Paleo-Asian Ocean in the late Palaeozoic contributed to rapid thickening of the juvenile lower crust. The mantle-derived materials also had been continuously added by mantle–crust interactions since the Neoproterozoic in the Eastern Tianshan. In the Triassic extensional setting, underplating and differentiation of mantle-derived magmas provided extra heat that accelerated the partial melting of thickened lower crust to form adakitic magmas. Accordingly, long-term crust–mantle interactions were able to replace the composition and changed the nature of lower crust, while subduction-collisional events contributed to significant crustal thickening in the short term.

7. Conclusions

Based on new whole-rock major and trace element, Sr-Nd isotope compositions, and zircon Hf-O data and U-Pb zircon ages for the Triassic Baishandong pluton in the Eastern Tianshan, we reached the following conclusions.

(1) The Baishandong pluton is composed of Xiaochaohu and Maanshanbei suits, which can be classified into three groups. Group 1, 2, and 3 granitoids were emplaced ca. 237 Ma, ca. 234 Ma, and ca. 229 Ma, respectively, and they are high-K, calc-alkaline, adakitic granites (Group 1 and 2), and highly fractionated I-type granites (Group 3).

(2) The Baishandong granites were formed by partial melting of juvenile thickened lower crust derived from Neoproterozoic crust-mantle differentiation and formed in intraplate extensional environment, indicating that the final amalgamation of the Paleo-Asian Ocean occurred prior to the Triassic.

(3) The Baishandong pluton indicates the existence of thickened lower crust before the Triassic in the Eastern Tianshan. Tectonic compression, which is different from mantle-crust interactions, is an important mechanism for producing thickened lower crust.

Highlights

• The postectonic Baishandong pluton can be classified into three groups which are composed of adakitic granites and highly fractionated granite.

• The similar Sr-Nd-Hf-O isotopes indicate that these granites were all derived from partial melting of thickened juvenile lower crust.

• They formed in an intraplate extensional setting that indicate that the existence of thickened lower crust before the Triassic in the Eastern Tianshan.

Acknowledgments

We thank the Editor and the two reviewers for their constructive comments that have greatly improved the manuscript. This work was supported by the National Key Research and Development Program of China (grant numbers 2018YFC0603702 and 2016YFC0600106); the Geological Survey Program of China (grant number DD20190685). We thank AJE (https://secure.aje.com) for its linguistic assistance during the preparation of this manuscript

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Key Research and Development Program of China [2018YFC0603702,2016YFC0600106]; Geological Survey Program of China [DD20190685].