How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300–291 Ma and 281–265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97–1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01–2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86Srinitial: 0.7039–0.7048, εNd(t): +5.7 to +6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/86Srinitial: 0.6978–0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai–Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai–East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.

Peraluminous granites, whose distinguishing feature is high alumina saturation index (ASI = molar Al2O3/(CaO + Na2O + K2O) > 1.0), widely occur within orogenic belts worldwide and provide essential clues for crustal rheological and thermal evolution (Sylvester, 1998). These granites are typically products of crustal anatexis (or partial melting) of metasedimentary or meta-igneous rocks (e.g., Clemens, 2003). Less commonly, they can also be derived from fractional crystallization from mafic and metaluminous (ASI < 1.0) magmas (Zen, 1986). Typical peraluminous granites, e.g., leucogranites, exist in numerous orogens worldwide, e.g., the Lachlan, Hercynian, and Himalayan orogenic belts (Sylvester, 1998), and the highly fractionated ones are often considered as an important sign of maturity of continental crust (e.g., Wu et al., 2017). Yet, the compositional characteristics of peraluminous granites are central to understanding the evolution and the basis for tectonic models in many orogenic belts (Weinberg, 2016, and references therein). The Cenozoic Himalayan leucogranite is the best case in point. For instance, the geochemical signature of middle Eocene leucogranites is consistent with partial melting related to crustal thickening and high-pressure conditions, while the late Miocene leucogranites exhibit geochemical characteristics typical for fluid-fluxed melting and decompression (Gao et al., 2017; Zeng et al., 2011). Consequently, these two generations of leucogranites were thought to be connected with the Eocene crustal thickening, and Miocene W-E crustal extension of the Himalayan crust, respectively (Le Fort et al., 1987; Lin et al., 2020, and references therein; Zeng et al., 2011).

The Central Asian Orogenic Belt (CAOB, also called the Altaids) represents the largest Phanerozoic accretionary system on Earth (Fig. 1A; Şengör et al., 1993,Windley et al., 2007; Xiao et al., 2015). The final amalgamation stage of the eastern tract of the CAOB was characterized by large-scale oroclinal bending, the collision of various crustal components and the development of large-scale transpressional zones associated with the closure of small remnant ocean basins (Edel et al., 2014; Laurent-Charvet et al., 2003; Li et al., 2015a; Xiao et al., 2018). The Chinese Altai orogenic belt occupies the hinterland part of the eastern CAOB in the early Paleozoic, and records a number of Permian geological phenomena associated with its interaction with the southerly Devonian–Carboniferous oceanic arc domain of East Junggar. One such phenomenon is the development of 300–280 Ma (ultra)high temperature–low pressure metamorphism (Tong et al., 2013) associated with crustal anatexis, which is expressed by narrow NE-trending tabular extrusion zones of partially molten rocks along the southern Chinese Altai (e.g., Broussolle et al., 2018). The southern Chinese Altai is also characterized by contemporaneous emplacement of mafic intrusions (e.g., Wan et al., 2013) and granite sheets parallel to Permian deformation zones (Jiang et al., 2019), documenting important magmatic activities in the region. Additionally, Permian structures along the southern Chinese Altai are exemplified by large thrust systems as well as tight upright folding associated with the development of penetrative sub-vertical schistosity (Briggs et al., 2007; Jiang et al., 2019; Li et al., 2015a, 2017). Besides, regional sinistral shear zones dated synchronously with the folding (Laurent-Charvet et al., 2003; Li et al., 2017) led these authors to propose that the southern Chinese Altai was affected by partitioned transpressional deformation resulting in the co-existence of pure-shear folding zones and simple-shear shear zones. These phenomena are collectively considered to be in response to the collision or the convergent-shortening process between the Chinese Altai and its southerly East Junggar arc domain (Jiang et al., 2019; Li et al., 2015a, 2017). Given that there are so far no signs (e.g., high-pressure metamorphic rocks) supporting a typical “collision” in-between the Chinese Altai and East Junggar, we hereby term their mutual interaction as a convergent-shortening process in this study. In this regard, the Chinese Altai orogenic belt may represent a crucial region for exploring the final convergent and shortening processes of the CAOB during the late Paleozoic.

While the Permian convergent-shortening process is generally regarded as one of the most striking tectonic events in the Chinese Altai, the understanding of its timescale and the related thermal evolution is still limited. Notably, recent studies revealed that a large number of syn-tectonic peraluminous granite dykes were emplaced along the deformation front of the southern Chinese Altai during convergence between the Chinese Altai and East Junggar arc (Jiang et al., 2019; Li et al., 2015a). These granite dykes can serve as an important petroprobe of the convergent process, which would therefore address the issue cited above. In the current study, detailed petrography, zircon and monazite U-Pb dating, and elemental and isotopic analyses were conducted on peraluminous granite dykes from the Fuyun and Kalasu-Aletai areas (Fig. 1B). The findings of this study combined with regional available data are used to better constrain magmatic and tectonic processes associated with convergence between the Chinese Altai orogenic belt and the East Junggar arc domain.

Regional Geological Framework

The CAOB is bounded by the Siberian, East European, Tarim, and North China cratons (Şengör et al., 1993; Şengör and Natal’in, 1996; Windley et al., 2007; Xiao et al., 2015). It was formed by a long-term amalgamation of various lithological units, including magmatic arcs, ophiolites, accretionary wedges, passive margins, and microcontinents, spanning mainly from Neoproterozoic to late Paleozoic (Windley et al., 2007; Xiao et al., 2015). These units were interpreted as orogenic components of a single “Kipchak” arc by Şengör et al. (1993). Alternatively, they were regarded as unrelated arc terrains and continental blocks similar to a western Pacific archipelago system (Kröner et al., 2007; Mossakovsky et al., 1993; Windley et al., 2007; Xiao et al., 2009). More recently, this giant accretionary system was roughly subdivided into three main accretionary collages based on their dissimilar structural relations, petrochemistry, geochronology, and paleomagnetic data, i.e., the Mongolian Collage System in the east, the Kazakhstan Collage System in the west, and the Tarim-North China Collage System in the south (Xiao et al., 2015).

The ~2500-km-long Ordovician volcanosedimentary unit known as the Altai accretionary wedge extending from eastern Kazakhstan, via Russia, through northwest China to southwestern Mongolia is an important element of the Mongolian Collage System (Figs. 1A and 1B; Jiang et al., 2017). The Altai accretionary wedge was reworked by Devono-Carboniferous deformation, metamorphism, and magmatism, forming the Altai orogenic belt. The Chinese segment of the orogen, the Chinese Altai, is separated from the Devono-Carboniferous East Junggar oceanic arc to the south by the NW-SE–trending Erqis fault (Fig. 1B; Li et al., 2015a; Windley et al., 2002).

Traditionally, the Chinese Altai was considered to consist of four tectonostratigraphic domains (also known as “terranes” by Windley et al., 2002) that are bounded by large-scale faults (Fig. 1B). From north to south, they were termed as the Northern Altai Domain, the Central Altai Domain, the Qiongkuer Domain, and the Southern Altai Domain. The Northern Altai Domain is mainly composed of low-grade Devonian to Carboniferous metasedimentary and metavolcanic rocks (Windley et al., 2002). The Central Altai Domain constitutes the major part of the Chinese Altai and is principally composed of Ordovician greywacke dominated turbidites and pyroclastic rocks of the Habahe Group that variably metamorphosed at greenschist- to amphibolite-facies conditions (Jiang et al., 2010; Long et al., 2007, 2008; Wang et al., 2014). The Qiongkuer Domain consists of the metamorphosed Habahe Group covered by Devonian metavolcanic rocks of the Kangbutiebao Formation and metasedimentary/volcanic sequence of the Altai Formation (Li et al., 2015b; Windley et al., 2002). Further south, the Southern Altai Domain, also termed as Erqis complex, consists of schist, paragneiss/orthogneiss, amphibolite, migmatite, and metachert (Li et al., 2015b; Windley et al., 2002). These fault-bounded domains were previously thought to have a specific structural, metamorphic, and magmatic evolution, which led the authors to define them as a range of suspect terranes (e.g., He et al., 1990; Windley et al., 2002). However, such a terrane subdivision, is speculative, because recent studies have documented that these “terranes” have strong mutual chemical affinities (Broussolle et al., 2019). The subdivision is also in contradiction to geophysical potential field data (gravimetry and magnetics) indicating that the proposed terrane boundaries do not match with any geophysical anomaly (Guy et al., 2020). Consequently, these domains were recently reinterpreted to represent different crustal levels heterogeneously affected by polyphase metamorphism and deformation (Broussolle et al., 2019; Guy et al., 2020; Jiang et al., 2019).

Two major tectono-metamorphic cycles that extensively affected the Chinese Altai have so far been identified. The main cycle was connected with Silurian–Devonian suprasubduction dynamics characterized by a crustal thickening phase associated with burial (D1B), followed by a Middle Devonian extension phase and crustal melting (D1M), and a Late Devonian shortening phase (D2) (Jiang et al., 2019; Wang et al., 2021). In particular, the Middle Devonian extension led to widespread crustal anatexis, producing a large amount of magma that was emplaced at shallow crustal levels (Wang et al., 2021). Such a process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, and hence further facilitated the differentiation of the accretionary wedge into a stable and mature continental crust in the Chinese Altai (Huang et al., 2020; Jiang et al., 2016). The second and less extensive Early Permian event, termed as D3, affected mainly the southern Chinese Altai and showed a general decrease in deformation intensity from south to north (Broussolle et al., 2019). In the southernmost part, it produced a SSW-ward thrust system, and formed heterogeneous NW-SE–trending deformation zones associated with the intensive reworking of the former Devonian orogenic fabrics and generating an orogen-paralleled fabric (S3) (e.g., Broussolle et al., 2018), almost perpendicular to the Devonian structures (Qu and Zhang, 1994; Briggs et al., 2007). Further north, the Devonian structures are rotated to WNW-ESE–directed orientations and partly transposed by steep D3 steep fabric (Zhang et al., 2012; Broussolle et al., 2018; Jiang et al., 2019, 2021). The D3 event was dated between 290 and 260 Ma (Briggs et al., 2007; Li et al., 2017) and is commonly interpreted as an overall response to the nearly SSW-NNE–directed convergence between the Chinese Altai and the Junggar arc domain (Jiang et al., 2019; Li et al., 2016, 2017).

Accompanying with these tectono-metamorphic events, abundant granitoids, cumulating at ca. 400 Ma and ca. 280 Ma, were emplaced and occupy more than 40% of the mapping area of the current Chinese Altai (Wang et al., 2009a; Zou et al., 1988). The majority of mapped granitoids (more than 90%) in the Chinese Altai are the Devonian peraluminous granitoids of I- and S-type affinities that intruded across the whole Altai orogenic belt and were previously considered as subduction-related magmatism (e.g., Wang et al., 2006; Yuan et al., 2007; Yu et al., 2017; Zhang et al., 2019), whereas more recent findings suggested that they originated mainly from the magmatic recycling of the greywacke-dominated Ordovician Altai accretionary wedge, i.e., Habahe Group (Huang et al., 2020; Jiang et al., 2016). In contrast, Permian A- or S-type granitoids from various intrusions ranging from several meters to several kilometers are distributed exclusively along the southern Chinese Altai (Liu et al., 2018; Zhou et al., 2007). Mafic rocks are less widespread in the Chinese Altai. The Devonian (ca. 383–409 Ma) mafic-ultramafic rocks are mainly restricted in the central and southern Chinese Altai and usually have the form of stocks, dykes, or interlayered volcanic bands (Yu et al., 2020). The Permian (ca. 287–270 Ma) mafic-ultramafic intrusions/dykes are mostly gabbroic and sporadically distributed in the southern Chinese Altai and northern East Junggar, in particular close to the Erqis fault (e.g., Wan et al., 2013).

Apart from these above-cited felsic and mafic intrusions, numerous Permian granite and pegmatite dykes (or veins) were emplaced in the southern Chinese Altai. Recent structural analysis of the southern Chinese Altai revealed that emplacement and deformation of these Permian dyke swarms are closely linked to regional Permian deformation and folding (Jiang et al., 2019). It was well illustrated in previous studies that granite dykes were nearly undeformed when they were emplaced in tensional fractures at the convergent front of the Chinese Altai (i.e., the Erqis fault) and those, which were emplaced in the interior of the Chinese Altai farther to the north, were variably folded (Li et al., 2017, Broussolle et al., 2018; Jiang et al., 2019). Such structural discrepancies between the dykes in the above two areas were explained in a model where the D3 folding was already locked in the convergent front but actively growing in the interiors during the emplacement of the dykes (Jiang et al., 2019). These features were considered to be consistent with the northward development of folding in response to the Altai-Junggar collision (Jiang et al., 2019).

Geology of the Study Areas and Sample Descriptions

This study focuses on the Permian granite dyke swarms from the Fuyun and Kalasu-Aletai areas of the southern Chinese Altai (Fig. 1B). The Fuyun area is close to the Erqis fault, representing the convergent front, while the Kalasu-Aletai area in the interior of the Chinese Altai is 40–60 km north away from the Erqis fault. The structures of the dykes with respect to regional D3 evolution are further explored. Moreover, the geochemical and geochronological aspects of the dykes are additionally investigated in this study.

Host Rocks and their Metamorphic and Structural Characteristics

In the Fuyun area, the NW-SE–trending Erqis complex is separated from the southerly East Junggar domain by the Erqis fault (Figs. 1B and 2A). This complex consists of gneissic granitoid, schist, migmatite, amphibolite, and metachert, which has been interpreted as a late Paleozoic accretionary complex (Briggs et al., 2007; Li et al., 2015a). An Early Permian high-grade metamorphic event occurred in this area, which is recorded by metamorphic zircon U-Pb ages of 295–283 Ma in the migmatite (Li et al., 2015a; Zhang et al., 2012) and amphibolite (Chen et al., 2019). In addition, this region recorded a prominent Early Permian contractional deformation (designed as regional D3) exemplified by large SW-directed thrust systems and tight upright folding associated with the development of a penetrative S3 foliation (Briggs et al., 2007). In addition, several NW-SE–striking sinistral shear zones developed synchronously with the upright folding, and it was hence suggested that the convergence was oblique, resulting in the formation of folding domains and shear zones (Li et al., 2017).

The Kalasu area is ~30–50 km southeast of Aletai city, NW China (Fig. 2B) and mainly consists of metamorphosed Ordovician turbidite metasediments (i.e., Habahe Group), Early Devonian volcano-sedimentary rocks (i.e., Kangbutiebao Formation) and Middle Devonian marine clastic sedimentary sequences (i.e., Aletai Formation) (Broussolle et al., 2018). These Habahe Group metasediments were intruded by abundant Devonian granitoids that are commonly gneissified, and sparse Permian granitoids and gabbros (Broussolle et al., 2018). During the Early Permian, the whole package was affected by extensive metamorphism, locally reaching granulite-facies conditions (Broussolle et al., 2018). This metamorphic event is recorded by ca. 299–272 Ma U-Pb zircon age from metapelitic granulite (Tong et al., 2013; Wang et al., 2009b) and ca. 299 Ma zircon age from migmatite leucosomes (Broussolle et al., 2018). A detailed structural and geochronological study showed a vertical and tabular deformation zone in this area, along which Permian migmatites and granulites were extruded associated with upright folding during regional NE-SW shortening D3 (Broussolle et al., 2018).

Strata in the Aletai area (~6 km southwest of Aletai city; Figs. 2C2D) consist of the Ordovician Habahe Group, Early Devonian Kangbutiebao Formation volcano-sedimentary rocks, and Middle Devonian Aletai Formation marine clastic sedimentary sequences (Jiang et al., 2019). The region was affected by multi-phase tectonometamorphic events during the Devonian, which resulted in the juxtaposition of the upper amphibolite- to granulite-facies Devonian migmatite-granite complex along with the amphibolite-facies metamorphosed Habahe Group and up to the greenschist-facies Devonian volcano-clastic sedimentary succession, due to the Late Devonian D2 shortening (Jiang et al., 2019). The study area was further affected by regional D3 deformation characterized by an important NE-SW shortening, which formed a large number of NW-SE–trending upright F3 folds and sub-vertical penetrative S3 foliation (Jiang et al., 2019).

Emplacement-Deformation Relationships of Permian Granite Dykes

At the convergent front represented by the Fuyun area, a NNE-SSW–oriented sub-vertical granite dyke swarm crosscuts the high-grade Erqis complex including Permian migmatite S3 fabric (Figs. 2A and 3A). These dykes are commonly 1–2 m wide and several kilometers long, nearly undeformed and perpendicular to the regional subhorizontal WNW-ESE D3 stretching direction and F3 fold axes (Figs. 2A and 3A; Li et al., 2017; Zhang et al., 2012).

In the interior of the Chinese Altai represented by the Kalasu and Aletai areas, a large number of centimeter- to tens of meter-wide pegmatite and granite dykes are present. Here the dykes are emplaced at a high angle to the D3 stretching direction and are either undeformed or variably folded and internally deformed (Figs. 3B3G; Broussolle et al., 2018, Jiang et al., 2019), while some dykes and granite sheets are emplaced parallel to axial planes of the F3 folds which are still interconnected with variably folded dykes (Figs. 2B and 2C). These features suggest that the dykes in the region were emplaced during various stages of D3 deformation. Moreover, the dykes here express different geometric features between the rheologically weak schists and the more competent migmatite-granite complex. Dykes that intruded the migmatite-granite complex are almost unfolded or only gently folded by F3 (Figs. 3B3D). In contrast, the dykes intruding the Habahe Group schists are tightly folded by F3 and rotated into approximate parallelism with the regional NW-SE–trending F3 axial planes (Figs. 3E and 3F). Sometimes, boudins are developed in highly stretched F3 limbs (Fig. 3G). Restoration of the primary orientations of the dykes, intruding no matter the Habahe Group schists or the migmatite-granite complex, consistently show original NE-SW directions parallel to the principal D3 compressive stress. It is likely that the dykes were emplaced during variable stages of D3 deformation and the deformation was still ongoing after the magma emplacement and solidification (see sketches in Fig. 3H).

The dykes emplaced along the convergent front in the Fuyun area can be considered as intruded tensional “AC” fractures (Price and Cosgrove, 1990) after locking of D3 folds. Therefore, these dykes can be regarded as late tectonic with respect to late Paleozoic metamorphism and D3 folding in the Fuyun area. This notion is supported by published 286–277 Ma formation ages of these dykes, (Briggs et al., 2007; Gong et al., 2007) and 286–279 Ma 40Ar/39Ar cooling ages slightly postdating the late Paleozoic metamorphism in the region (Li et al., 2017). However, in the Kalasu and Aletai areas, the dykes were emplaced during active D3 folding and deformation. Here, most of the Permian granite dykes were originally NE-SW striking, i.e., orthogonal to the XY plane of the D3 strain ellipsoid, advocating that the granite dykes were emplaced along the syn-D3 tensional fractures. Even if most granite dykes were emplaced along the “AC” tensional fractures, their connection to axial-planar syntectonic sill-like intrusions indicates that they can be regarded as syn-D3 intrusions (Weinberg et al., 2013; Druguet, 2019). The formation ages of granite dykes in the Kalasu-Aletai area were dated at 280–273 Ma by zircon and monazite U-Pb geochronology (Jiang et al., 2019). Notably, recent fluid inclusions 40Ar/39Ar investigations on andalusite from syn-D3 quartz veins that are associated with the granite dykes in the Aletai area yielded consistent 282–277 Ma (Xiao et al., 2022), confirming the development of tensional fractures at least since the Early Permian. Contrasting characters of deformation of dykes in the Fuyun and Kalasu-Aletai regions is consistent with the gradual northward migration of D3 deformation from the deformation front toward the interior of the Chinese Altai, in response to the Chinese Altai–East Junggar convergence (Jiang et al., 2019).

Petrography

Eight samples were collected from different granite dykes from the Fuyun area (Fig. 2A; Table S11). They are massive and exhibit porphyritic texture, which can be categorized as granite porphyries. In general, these rocks are composed of 0.2–0.5 mm euhedral to subhedral phenocrysts (20–50 vol%) of feldspars, quartz, and biotite, associated with the <100 μm quartzfeldspathic matrix (Fig. 3I; Table S1).

Thirteen samples were collected from the granite dykes from the Kalasu-Aletai area and can be classified as two-mica granites and muscovite granites (Figs. 2B2D; Table S1), which are also termed as leucogranites by Hu et al. (2021). The two-mica granite dykes are massive, fine-grained (0.5–1.0 mm), and contain quartz (30–40 vol%), K-feldspar (20–40 vol%), plagioclase (20–35 vol%), muscovite (1–7 vol%), biotite (2–8 vol%), garnet (1–2 vol%), and tourmaline (3% vol%) (Fig. 3J; Table S1). The muscovite granite dykes are fine-grained (0.1–0.5 mm) and are composed of quartz (30–40 vol%), K-feldspar (25–35 vol%), plagioclase (20–25 vol%), muscovite (5–10 vol%), garnet (2–5 vol%), and tourmaline (3 vol%) (Figs. 3J3L; Table S1). Given these dykes were variably affected by the D3 folding, they often show a weak subsolidus solid-state microstructure.

U-Pb Zircon and Monazite Geochronology

Zircon and monazite grains were separated from whole-rock samples using standard heavy liquid and magnetic techniques and then purified by handpicking under a binocular microscope. These grains were then mounted in epoxy resin and polished. Zircon cathodoluminescence (CL) and monazite backscattered electron images (BSE) images were obtained using an analytical scanning electron microscope (JSM-IT100) connected to a GATAN MINICL system, installed at the Wuhan SampleSolution Analytical Technology Co., Ltd. (WSSAT), Wuhan, China.

U-Pb age dating of zircon/monazite was conducted by the laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) at WSSAT. The laser ablation system consists of a COMPexPro 102 ArF excimer laser (a wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. The spot size and frequency of the laser were set to 32 μm and 5 Hz for zircon U-Pb age dating, and 16 μm and 2 Hz for monazite U-Pb age dating. Each analysis incorporated a background acquisition of ~20–30 s followed by 50 s of data acquisition from the samples. Detailed operating conditions and analytical procedures are described in Zong et al. (2017). Note that zircon standard 91500 (1062.4 ± 0.4 Ma; Wiedenbeck et al., 1995) and monazite standard 44069 (424.9 ± 0.4 Ma; Aleinikoff et al., 2006) were used as primary reference materials to calibrate the zircon and monazite U-Pb isotopic data, respectively. In addition, zircon standards GJ1 (601.92 ± 0.7 Ma; Jackson et al., 2004) and Plešovice (337.13 ± 0.37 Ma; Sláma et al., 2008), as well as monazite standard Trebilcock (272 ± 4 Ma; Tomascak et al., 1996), were employed as secondary reference materials and treated as unknowns during zircon/monazite U-Pb analyses. Analyses on these secondary standards GJ1, Plešovice, and Trebilcock over the period of analyses gave weighted mean 206Pb/238U ages of 606.7 ± 5 Ma (n = 14, mean square weighted deviation [MSWD] 3.2), 340.1 ± 2 Ma (n = 11, MSWD = 1.2), and 274.3 ± 1.5 Ma (n = 8, MSWD = 0.45), respectively, which reproduce the recommended ages within uncertainty (Table S2; see footnote 1), confirming the accuracy of the analytical conditions and data reduction protocols. An Excel-based software ICPMSDataCal (Liu et al., 2008) was used to perform off-line selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003). The effect of common lead was corrected by employing the Tera-Wasserburg concordia diagram (Tera and Wasserburg, 1972), as well as the Age7Corr and AgeEr7Corr algorithms, integrated into Isoplot/Ex_ver3 (see details in Ludwig, 2003). Uncertainties on individual analysis are reported at the 95% confidence level (2s level).

Major- and Trace-Element Analyses

Whole-rock major elements were measured by X-ray fluorescence techniques using a Rigaku ZSX Primus II at WSSAT (a mixture of 0.6 g sample powder ground by using an agate mortar, 6.0 g lithium tetraborate and 0.3 g ammonium nitrate). The mixture was melted at 1150 °C for 14 min with a high-frequency bead sampler. Loss on ignition was calculated by the weight difference after ignition at 1000 °C. Using the Chinese National standards (GBW07104, GBW07111) to monitor instrument precision and assess data quality. The analytical uncertainties are generally less than 3%.

Whole-rock trace element analyses were conducted using Perkin-Elmer Sciex ELAN 6000 ICP-MS at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLaBIG, GIGCAS) and detailed procedures are the same as those described by Li et al. (2006). The United States Geological Survey (USGS) and Chinese National standards GSR-1, GSR-2, GSR3, SARM-4, SY4, AGV-2, W-2a, BHVO-2, and GSD-11 were selected for calibration of element concentrations of the analyzed samples. Analytical precision of the rare earth element (REE) and other incompatible element analyses is typically 1%–5%.

Whole-Rock Sr-Nd Isotopic Analysis

Whole-rock Sr and Nd isotope analyses were conducted by using a Neptune plus multicollector-ICP-MS at SKLaBIG, GIGCAS. Analytical procedures are identical to those described by Li et al. (2004). All measured 86Sr/88Sr and 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 and 86Sr/88Sr = 0.1194, respectively. The international standard samples, NBS987 and JNdi-1, were applied to evaluate instrument stability during the period of data collection. The measured values for the NBS 987 Sr standard and JNdi-1 Nd standard were 87Sr/86Sr = 0.710235 ± 0.000013 (n = 6, 2σ) and 143Nd/144Nd = 0.512121 ± 0.000007 (n = 5, 2σ), respectively. The USGS reference material BCR-2 was measured to monitor the accuracy of the analytical procedures, with the following results: 87Sr/86Sr = 0.705036 ± 0.000011 and 143Nd/144Nd = 0.512624 ± 0.000006, which are in agreement within analytical uncertainties with the recommended values given by Raczek et al. (2003).

Zircon and Monazite U-Pb Geochronology

LA-ICP-MS zircon and monazite U-Pb isotopic data are given in Tables S3 and S4 and shown in Figures 4 and 5 associated with their representative CL and BSE images.

The Fuyun Granite Dykes

Three zircon samples (18CA11, 18CA90, 18CA91) were collected from the granite dyke swarm that are straight dykes striking NNE-SSW (Figs. 2A and 3A). Zircon grains in granite porphyry sample 18CA11 are euhedral, prismatic, and range from 100 to 300 μm in length. All grains exhibit obvious oscillatory zoning with dark luminescence (Fig. 4A). These zircons show significantly variable Th and U concentrations (Th: 6-9386 ppm and U: 30-7448 ppm) with high Th/U ratios varying from 0.2 to 1.6. These features are compatible with magmatic zircons. The analyzed U-Pb isotopic data do not form a tight cluster on the 207Pb/235U versus 206Pb/238U concordia diagram (not shown) due to low values of concordance. Alternatively, all 32 analyses were performed on different zircon grains (uncorrected for common Pb) that constitute a linear array on the Tera-Wasserburg concordia diagram, giving a lower intercept age of 293 ± 2 Ma (MSWD = 1.9). In addition, the raw data were applied for common Pb correction, and a weighted mean 206Pb/238U age of 293 ± 2 Ma was obtained (MSWD = 1.9). These two ages are identical within analytical uncertainties, and thus can be considered as the best estimate of the crystallization age of the sample.

Zircon grains from granite porphyry sample 18CA90 have magmatic euhedral rhythmic oscillatory zoning with dark luminescence features on the CL images (Fig. 4B). They are prismatic with long axes ranging from 100 to 200 μm. The zircons in this sample also display very varied Th and U contents (Th: 5–7770 ppm and U: 34–3688 ppm) with high Th/U ratios (0.1–2.7). Thirty analyses on different zircon grains of this sample (uncorrected for common Pb) define a well-constrained linear array with lower intercept age of 300 ± 3 Ma (MSWD = 2.0). After the 207-based common Pb correction, the weighted mean average 206Pb/238U age is 303 ± 2 Ma (MSWD = 1.8), which is consistent with the lower intercept age within the analytical errors. Therefore, ca. 300 Ma can be taken as the crystallization age of the sampled granite.

Zircon grains from granite porphyry sample 18CA91 can be divided into two groups based on their brightness in CL images. The zircon grains with weak CL brightness have features similar to zircons from samples 18CA11 and 18CA90. This type of zircon is euhedral, prismatic, ranging from 100 to 200 μm in length, and has magmatic oscillatory zoning. These zircon grains have medium Th (252–3432 ppm) and U contents (521–3903 ppm) with high Th/U ratios (0.5–1.1). Eleven analyses were conducted on eleven zircons from this group, and yielded relatively younger 206Pb/238U ages ranging from 284 to 297 Ma, with a weighted mean 206Pb/238U age of 291 ± 3 Ma (MSWD = 4.4, Fig. 4C). The remaining zircon grains with high CL brightness are subhedral to euhedral with length ranging from 100 to 300 μm and have vague to clear magmatic oscillatory zoning. They have relatively low Th (42–316 ppm) and U (59–525 ppm) contents, but high Th/U ratios (0.5–1.0). Fifteen analyses from this type of zircon yielded relatively older 206Pb/238U ages ranging from 369 to 433 Ma (Fig. 4C). The younger age of 291 ± 3 Ma is therefore considered as the crystallization age of the rock whereas the second type of zircon with older ages are regarded as xenocrysts assimilated from wall-rock.

The Kalasu-Aletai Granite Dykes

One zircon sample (18CA83) was collected from a NE-trending, weakly deformed two-mica granite dyke. Zircons from this fine-grained two-mica granite sample are mainly euhedral, prismatic, and have a variable size varying from 100 to 250 μm in length. They have intermediate Th and U contents (228–3879 ppm, 371–5068 ppm, respectively) with Th/U values ranging from 0.19 to 1.31, typical of magmatic zircons. Most grains have typical magmatic oscillatory zoning but some exhibit rim-core structures. An inherited core gave an old 206Pb/238U age of 508 Ma (Fig. 5A). The other 25 measurements on different zircon rims yielded a 206Pb/238U age ranging from 265 to 294 Ma with a weighted mean 206Pb/238U age of 281 ± 3 Ma (MSWD = 6.6; Fig. 5A), which is interpreted as the crystallization age of the sampled granite.

In addition, monazite samples (18CA84, 18CA59, 18CA66) collected from three other granitic dykes were also selected for U-Pb dating. Sample 18CA84 was sampled from a gently folded, fine-grained muscovite granite dyke (Figs. 3C and 3K). Sample 18CA59 was sampled from a NE-trending, fine-grained two-mica granite dyke that crosscuts the S3 foliation at a high angle (Figs. 3D and 3J). Sample 18CA66 is also a fine-grained muscovite granite, but its host dyke is tightly folded by F3 associated with the development of an S3-parallel cleavage (Figs. 3F and 3L). Irrespective of their different petrographic features, monazite grains from these samples exhibit similar morphological features that are characterized by euhedral-subhedral shape with length ranging from 100 to 300 μm (Figs. 5B5D). In BSE images, the majority of monazite grains are homogeneous and some exhibit oscillatory zoning, while others show patchy zoning. The studied monazites exhibit relatively low P2O5, total REE contents and (La/Yb)N values, but high U, Th, and Y contents and Th/U ratios (Table S5; see footnote 1), suggesting magmatic origins (Qiu and Yang, 2011). Monazites from sample 18CA59 have relatively high Th (99317–152982 ppm), U (5475–11153 ppm), Y (20256–26067 ppm) contents and Th/U ratios of 10–20. Twenty-nine analyses from different monazite grains yielded 206Pb/238U ages ranging from 261 to 274 Ma with a weighted mean value of 267 ± 1 Ma (MSWD = 2.6; Fig. 5B). Likewise, monazites from sample 18CA84 contain high Th (46980–187320 ppm), U (3592–14498 ppm), Y (18792–37758 ppm) contents and Th/U ratios of 3–34. Twenty-one analyses were conducted on different monazite grains that yielded 206Pb/238U ages of 286–266 Ma and a weighted mean value of 275 ± 2 Ma (MSWD = 2.3; Fig. 5C). The resulting data of the remaining sample 18CA66 show relatively low Th (8578–55947 ppm), U (3946–27626 ppm), Y (8265–35867 ppm) contents, and Th/U ratios (1–10). Thirty analyses were performed on different monazite grains and yielded consistent 206Pb/238U ages ranging from 259 to 269 Ma. These data form a tight cluster of concordant points, giving a weighted mean 206Pb/238U age of 265 ± 1 Ma (MSWD = 1.6; Fig. 5D). These monazite U-Pb ages are collectively interpreted as the emplacement ages of the granite dykes.

Whole-Rock Major and Trace Elements

Major and trace elemental data of granite dyke samples from the Fuyun and Kalasu-Aletai areas are listed in Table S6 (see footnote 1) and shown in Figures 68. Previously published data by Gong et al. (2007), Zhang et al. (2012) and Hu et al. (2021) for granite dykes from these two regions are also integrated with the data of this study for comparison.

Granite samples from both the Fuyun and Kalasu-Aletai areas have high SiO2 (70.9–76.1 wt% and 71.9–81.2 wt%, respectively) and total alkalis (7.1–9.0 wt% and 4.6–9.9 wt%, respectively) contents (Fig. 6A). On the SiO2 versus K2O diagram (Fig. 6B), the Fuyun granites have relatively homogeneous K2O contents and fall in the field of high-K calc-alkaline series, whereas the Kalasu-Aletai granites have variable K2O contents and mainly plot in the field of middle- to high-K calc-alkaline series with a few in the fields of low-K tholeiitic and shoshonite series. The Fuyun granite samples show relatively low Mg# values (18–37) and most of them plot in the magnesian field, whereas the Kalasu-Aletai granite samples have much lower Mg# values (10–34) and straddle the ferroan and magnesian series on the FeOt/(FeOt + MgO) versus SiO2 diagram (Fig. 6C). The samples from the Fuyun area show weakly peraluminous composition (Fig. 6D) with the ASI values (ASI = molar Al2O3/(CaO + Na2O + K2O)) ranging from 0.97 to 1.13, with the majority below 1.1. In contrast, samples from the Kalasu-Aletai area show weakly to strongly peraluminous signature with ASI ranging from 1.01 to 2.17 (Fig. 6D). On Harker diagrams (Fig. 7), samples from the Fuyun area display similar trends, exemplified by decreasing Al2O3, Fe2O3, MgO, TiO2, CaO, and P2O5 contents and constant or weak increasing K2O and Na2O values with respect to increasing SiO2 contents. In contrast, the Kalasu-Aletai samples are usually scattered in the Harker diagrams without any obvious linear correlations between SiO2 and other major oxides (Fig. 7). In general, when compared with the Kalasu-Aletai granites, the Fuyun granite porphyries show higher CaO, Fe2O3, MgO, TiO2, Ba, Sr, and Zr but lower Na2O, P2O5, and Rb concentrations (Fig. 7).

On the chondrite-normalized REE diagram (Figs. 8A and 8C), granite samples from the Fuyun area display comparable patterns that are marked by enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) with (La/Yb)N ratios varying from 7.00 to 12.84. They commonly have negative Eu anomalies with Eu/Eu* ratios ranging from 0.33 to 0.95. In contrast, the granite samples from the Kalasu-Aletai area exhibit distinctive seagull-shaped flat REE patterns with (La/Yb)N ratios varying from 0.16 to 5.61 and variable Eu anomalies (Eu/Eu* = 0–0.79). Furthermore, the Kalasu-Aletai granite samples show an obvious “tetrad effect’’ (TE1,3 = 0.90–1.27), especially for three muscovite granite samples that have the highest TE1,3 ranging from 1.16 to 1.27. On the Primitive mantle-normalized multi-element diagrams, the Fuyun granites show enrichment in large ion lithophile elements (LILE, e.g., Rb, Th, U, Pb) but depletion in high field strength elements (HFSE, e.g., Nb, Ta, Ti) (Fig. 8B). The Kalasu-Aletai granite samples also show enrichment of LILE (e.g., Rb, Th, U, Pb) but strong depletion of Ba, Sr, Nb, Ti, and LREE (e.g., La, Ce). Notably, Ta exhibits a positive anomaly because of the strong negative La anomaly (Fig. 8D).

Whole-Rock Nd and Sr Isotopic Compositions

Three granite porphyries from Fuyun and eight samples of two-mica granite and muscovite granite from Kalasu-Aletai were selected for whole-rock Sr-Nd isotopic analysis, and their Sr-Nd isotopic compositions are listed in Table S7 (see footnote 1). The initial 87Sr/86Sr ratios, εNd(t) values, and Nd model ages were calculated back to ca. 265 Ma, representing the youngest crystallization age of the studied dykes. The Fuyun granites have constant εNd(t) values of +5.7 to +6.1 and a narrow range of (87Sr/86Sr)i ratios varying from 0.7039 to 0.7048 (Table S7). In contrast, the Kalasu-Aletai granites show a variable εNd(t) values of -7.6 to +3.0 and a wide range of (87Sr/86Sr)i ratios ranging from 0.6978 to 0.7183 (Table S7). The corresponding two-stage Nd model ages (TDM-2) of granites from Fuyun and Kalasu-Aletai areas are 0.54–0.57 Ga and 0.79–1.65 Ga, respectively.

Late Paleozoic Peraluminous Granitic Magmatism in the Southern Chinese Altai

Permian peraluminous granitoids are prevailing along the southern Chinese Altai (e.g., Tong et al., 2014), however, their temporal and spatial relationships with respect to regional tectonic events have so far been poorly investigated. Many Permian granitoids form isolated circular to elliptic bodies varying from hundreds of meters up to a few kilometers. Their seemingly un-deformed map-view features (i.e., circular shape), led many to consider them as posttectonic intrusions (e.g., Liu et al., 2018; Tong et al., 2014). However, based on microstructural analysis, recent studies revealed that significant magmatic fabrics parallel the main regional D3 deformation trend developed in Permian granites from the Kalasu-Aletai areas (e.g., Broussolle et al., 2018; Jiang et al., 2019), suggesting syntectonic emplacement. A new study on another semi-circular Permian granite pluton shows weak or absent deformation of the main granite body, however, the contemporary granite dykes intruding the host rocks are strongly folded and foliated during regional D3 shortening (Xu et al., 2021). Besides, the granite bodies could also be protected from their strong hornfels carapace and stay unaffected when the incompetent wallrocks were strongly deformed, as Lehmann et al. (2010) demonstrated on the Mongolian GobiTienshan pluton. Taken together, it is evident that the Permian granites occurring along the southern Chinese Altai were affected by regional D3 deformation.

In this study, peraluminous granite dykes from the Fuyun area gave emplacement ages ranging from 300 ± 3 Ma to 291 ± 3 Ma (Fig. 4). These ages are slightly older than the published zircon U-Pb ages of similar dykes in the area, which are in the range of 286–277 Ma (Briggs et al., 2007; Gong et al., 2007), except for one relatively younger age of 252 ± 2 Ma (Zhang et al., 2012). It is therefore apparent that the majorities of the granite dykes in the Fuyun area have emplacement ages of 300–280 Ma. For Kalasu-Aletai granite samples, one zircon U-Pb age of 281 ± 3 Ma (Fig. 5A) and three monazite U-Pb ages of 265 ± 1 Ma, 275 ± 2 Ma, and 267 ± 1 Ma (Figs. 5B5D) were obtained. Together with previously published monazite ages (279–267 Ma, Hu et al., 2021; Jiang et al., 2019) of similar granite dykes in the region, the current data constrain the formation ages of the dykes at 279–265 Ma. These monazite U-Pb ages are broadly overlapping in time with available zircon U-Pb ages (280–270 Ma, Hu et al., 2021; Jiang et al., 2019) from similar granite dykes of the region, but are systematically younger than the ages (300–280 Ma) of granite dykes in the Fuyun area to the south. In other words, a northward younging trend exists in the emplacement of granite dykes along the southern Chinese Altai. Given that these dykes are considered to be emplaced into the tensional fractures that formed in response to regional D3 NNE-SSW and/or NE-SW shortening and the principal compressive stress (e.g., Jiang et al., 2019), the northward younging trend of the ages further supports a northward propagation of deformation together with magmatic activity spanning more than 30 m.y.

Except for the important Early Permian zircon populations, both Fuyun and Kalasu-Aletai granite dykes contain abundant older zircons, which are considered as xenocrysts that are probably inherited from the source rocks. Notably, the xenocrystic zircons from the Fuyun area have a significant Devonian–Carboniferous age population, whereas xenocrystic zircons from the Kalasu-Aletai area have relatively older ages, i.e., Early Cambrian (Figs. 4C and 5A), implying different sources. It is noteworthy that the Early Cambrian zircon population is the most common age of the detrital zircons in the Habahe Group metasedimentary rocks of the Chinese Altai (Long et al., 2007, 2010; Jiang et al., 2011; Sun et al., 2008). In contrast, the source of the Devonian–Carboniferous xenocrystic zircon is rather obscure, since rocks of such ages are exposed in both the southern Chinese Altai and the East Junggar arc domain.

Petrogenetic Considerations: Fractionated I-type versus Highly Fractionated S-type Granites

The Fuyun Granite Dykes

All the Fuyun granites show similar structural features, coherent whole-rock major and trace element patterns, implying a common genetic origin. On the diagram of Zr versus 10000*Ga/Al ratio (Whalen et al., 1987), all Fuyun granite samples plot into the field of I-, S-type granite, and show an affinity with fractionated I/S-type granite (Fig. 9A). On the diagram of (Na2O + K2O)/CaO versus Zr + Nb + Ce + Y values (Whalen et al., 1987), these samples straddle the fields of fractionated granite and unfractionated granite (Fig. 9B). Such transitional geochemical features may suggest some degree of fractional crystallization. The Fuyun granite dykes were previously thought to belong to A-type granite based on their relatively high K2O and Na2O contents (Gong et al., 2007). However, the low abundances of Zr + Nb + Ce + Y (120–281 ppm), low 10000* Ga/Al ratios (<2.5), and the lack of alkaline melanocratic minerals preclude typical A-type geochemical affinities. It is generally accepted that A-type granite is characterized by high temperature (800–900 °C or more) (Huang et al., 2019, and references therein). Whole-rock zirconium saturation thermometry is often used as an approximate estimate of the magmatic temperature (Watson and Harrison, 1983). In the case of Fuyun granite samples, the zirconium saturation temperature estimation (TZr) yields a range of 719–803 °C with an average of 774 °C (Table S6), which is obviously lower than that of typical A-type granites.

The Fuyun granites show metaluminous to weakly peraluminous compositions (ASI = 0.97–1.13, mostly below 1.1; Fig. 6D), which distinguishes them from typical S-type granites that have ASI values commonly higher than 1.1 (Chappell and White, 1992). Alternatively, the P2O5 contents in these granites are relatively low (mostly <0.1 wt%) and negatively correlated with SiO2 contents, in agreement with typical I-type granites (e.g., Chappell and White, 1992; Li et al., 2007) (Fig. 7). Moreover, these samples exhibit pronounced positive correlations on Th versus Rb and Y versus Rb diagrams (Figs. 10A and 10B), which resemble the typical I-Type granite (Chappell and White, 1992). This is further supported by the plotting of data on the diagram of Zr versus Ba contents (Fig. 10C), where they exhibit cloudlike or fanning distribution patterns that also favor an I-type affinity (Clemens, 2018). Based on these features, it is proposed here that the Fuyun granites can be assigned as typical fractionated I-type granites.

Crystal fractionation played an important role in the formation of these granites, as is further attested by their clear trends in the Harker diagrams (Fig. 7). The negative correlation of Fe2O3, MgO, and TiO2 versus SiO2 contents indicates the crystallization of mafic minerals, most likely biotite. The negative correlations of Al2O3 and CaO versus SiO2 contents suggest plagioclase fractionation, which is also consistent with the negative Eu, Sr anomalies in the spider diagram (Fig. 8B). Consistently lower but steadily decreasing P2O5 content with respect to increasing SiO2 content implies apatite separation during magma evolution (Fig. 7). Moreover, Fe-Ti oxides and/or biotite fractionation is expressed by negative Ti anomalies on the spider diagram (Fig. 8B).

The Kalasu-Aletai Granite Dykes

The Kalasu-Aletai granite dykes contain abundant Al-rich minerals (e.g., garnet, muscovite, and tourmaline) and shows weakly to strongly peraluminous compositions (ASI = 1.01-2.17), which are assigned as typical S-type granites (Chappell and White, 1992). This is also supported by their large variations of P2O5 contents, which are different from those of I-type granites, and nearly all of them plot into the field of common Lachlan Fold Belt S-type granites (Fig. 7; Chappell, 1999). Moreover, these granite dykes show a high degree of correlation between the Ba and Zr contents (Fig. 10C), which is considered as a common characteristic of S-type granites and can be interpreted as a response to progressive breakdown of micas in the source (Clemens, 2018). In addition, nearly all granite samples from the Kalasu-Aletai area plot in the field of fractionated granites on the Zr versus 10000*Ga/Al and (Na2O + K2O)/CaO versus Zr + Nb + Ce + Y diagrams (Fig. 9). Their large variations of Rb/Sr, Zr/Hf, and Y/Ho ratios (0.3–240.8, 9.2–30.3, 27.3–37.1, respectively, Table S6) correspond to high fractionated S-type granites around the world (e.g., Breiter, 2012), advocating that the Kalasu-Aletai granites represent typical S-type granites.

The strongly negative Sr and Eu anomalies of the granites (Figs. 8C and 8D) indicate that plagioclase fractionation is extensive. These granites have high Rb/Sr ratios (up to 240.8), which also points to a high degree of fractionation of plagioclase (Fig. 11A). Besides, K-feldspar is another major fractionation phase, as exemplified by the pronounced depletion of Ba (Fig. 8D) and good linear trends on the covariation diagrams of Ba versus Sr and Ba/Sr versus Sr (Fig. 12). The negative Zr anomalies with lower Zr abundances of these granites may indicate zircon fractionation that would lower the Zr/Hf ratios. Furthermore, the marked negative Ti anomalies of these samples may be related to the fractionation of Fe-Ti oxides (ilmenite) and/or biotite.

Notably, the Kalasu-Aletai granites also exhibit an obvious REE tetrad effect (Fig. 8C) with TE1,3 of 0.90–1.27, which is implausible to be produced merely by mineral fractionation (Irber, 1999). Many studies have emphasized that the tetrad effect commonly results from melt-fluid interaction (Irber, 1999; Liu and Zhang, 2015; Chen et al., 2018). This is supported by the field observations that the granite dykes are spatially and temporally associated with emplacement of a large amount of pegmatite and quartz dykes (Jiang et al., 2019) and often contain typical volatile-rich minerals, such as tourmaline. This is also in agreement with the lower zirconium saturation temperature (TZr mostly 650–720 °C) of these samples, which attested to melt-fluid interaction (Johannes and Holtz, 2012). Notably, lower zirconium saturation temperatures of the Kalasu-Aletai granite dykes are also consistent with their highly evolved geochemical signatures, similar to those described in the Himalayan leucogranites (Liu et al., 2014, 2016).

Magma Sources

The Fuyun Granite Dykes: Partial Melting of Juvenile Junggar Arc Components

In general, I-type granites are considered to be generated either by partial melting of mafic to intermediate igneous rocks or by extensive fractionation of mantle-derived magma (Chappell et al., 2012; Li et al., 2007). The latter mechanism is obviously not the case for the Fuyun granite dyke swarm, because extensive fractional crystallization of mantle-derived mafic/ultramafic magma usually produces dioritic melt (Chen and Jahn, 2004) and leaves behind abundant mafic-ultramafic cumulates (Gao et al., 2016). However, these phenomena have so far not been observed in the Fuyun area. Additionally, the relatively narrow range of SiO2 contents (70.9–76.1 wt%) for the granite dykes does not support an extensive fractional crystallization process, precluding a model of extensive fractional crystallization of mantle-derived mafic/ultramafic magma. Alternatively, partial melting of mafic to intermediate igneous rocks can be considered as a potential mechanism to generate the granite dykes in the Fuyun area. Granite dykes from the Fuyun area have relatively low Rb/Sr and Rb/Ba ratios and are thus chemically comparable with partial melts derived from clay-poor and plagioclase-rich sources, such as basaltic rocks or greywackes (Fig. 11A). In the discrimination diagrams of Patiño Douce (1999) and Rollinson (2015), the Fuyun granite dykes are mostly plotted in the field of amphibolite-derived melt and greywacke-derived melt (Figs. 11B11D). It has been documented that partial melts from dehydration melting of amphibolitic/basaltic rocks would have Mg# values below 40, regardless of what degree the partial melting experienced (Rapp and Watson, 1995). Moreover, partial melting of low-K basaltic rocks would form melts with low K2O contents and K2O/Na2O (<1) ratios (Rapp and Watson, 1995), whereas partial melts from medium- to high-K mafic to intermediate rocks were expected to have more felsic, high K2O contents and K2O/Na2O (>1) ratios (Sisson et al., 2005). In this study, the Fuyun granite dykes have low Mg# values (18–37) but high K2O (3.7–5.4) and Na2O (3.1–4.2) contents, and slightly high K2O/Na2O ratios (1.03–1.73). These features are consistent with partial melts derived from medium- to high-K mafic to intermediate sources. It should be noted that the Dulate arc in East Junggar is mainly composed of mafic to intermediate, transitional to high-K calc-alkaline volcanic rocks (Fig. 6B), which may serve as the possible source rocks for the Fuyun granite dykes.

The Fuyun granite dykes are characterized by depleted and homogeneous Sr-Nd isotopic signatures (εNd (t) = +5.7 to +6.1; ISr = 0.7039–0.7048), suggesting that these granites were derived from an isotopically less evolved crustal source. Their young two-stage Nd model ages (TDM-2 = 0.54–0.57 Ga) suggest a juvenile rather than an older Precambrian magma source. These isotopic features resemble that of the neighboring eastern Junggar rocks but are distinct from those of the Chinese Altai rocks (Fig. 13), which may imply that the Junggar arc-type basement would be the best source candidate for the Fuyun granite dykes.

The hosting rocks of the Fuyun granite dykes are considered as the Erqis accretionary complex that was incorporated into the southern margin of Chinese Altai during the late Paleozoic. The presence of mafic and dense East Junggar basement underneath the Chinese Altai was also imaged via potential field geophysical analysis (Guy et al., 2020). Combined with structural analysis, these authors claimed that the underthrusting of East Junggar basement beneath the Chinese Altai probably had started at least since the Early Permian. Furthermore, some amphibolites within the Erqis accretionary complex were recently interpreted as the metamorphic counterparts of oceanic island arc fragments, in particularly the Dulate arc, of the East Junggar arc domain, based on the close geochemical similarities between the amphibolites and East Junggar arc components (Chen et al., 2019). This assertion further supports arc-affinity trace element patterns (i.e., enrichment in LILE, LREE and depletion of HFSE) of the Fuyun granite dyke swarm (Fig. 8B), which is comparable with the geochemical signatures of the East Junggar arc rocks. Indeed, the Dulate arc of East Junggar mainly consists of a Devonian to Carboniferous volcanic-sedimentary sequence containing abundant high-K basaltic-andesitic rocks (Fig. 6B). All that indicates that partial melting of these high-K rocks could form the geochemical characteristics observed in the Fuyun granite dykes. This is also advocated by the fact that the Fuyun granite dykes preserved Devonian-Carboniferous xenocrystic zircons (Fig. 4C) that represent the common zircon population of the East Junggar arc domain. Therefore, the Fuyun granite dyke swarm is interpreted to have formed from partial melting of juvenile volcanogenic components of the Dulate arc of the East Junggar arc domain.

The Kalasu-Aletai Granites Dykes: Partial Melting of Heterogeneous Habahe Group of the Chinese Altai

It is generally accepted that peraluminous, silica-rich S-type granites derived mainly from partial melting of metasedimentary protoliths (Sylvester, 1998), even though fractional crystallization of amphibole (Chappell et al., 2012) and partial melting of basaltic to andesitic rocks under water-saturated conditions (Rapp and Watson, 1995; Sisson et al., 2005; Sylvester, 1998) have also been considered as possible mechanisms. Note that the latter two processes would generate melts with a high abundance of Sr (Zen, 1986). This is not the case with the Kalasu-Aletai granite dykes, because the dykes have relatively low Sr contents (1.65–93.30 ppm). The high Rb contents (up to 397.10 ppm) and Rb/Sr ratios (up to 240.81) of these dykes suggest a micaceous source. Thus, partial melting of metasedimentary protoliths could be the most possible mechanism to form the studied Kalasu-Aletai granites.

The Kalasu-Aletai granite dykes display higher Rb/Sr and Rb/Ba ratios (Fig. 11A), comparable with partial melts from a clay-rich source (Sylvester, 1998). This notion is consistent with the findings that the Kalasu-Aletai granite dykes are compositionally similar to felsic pelite-derived melts, but distinct from the melts produced by amphibolite and greywacke (Figs. 11B11D). In particular, Kalasu-Aletai granite dykes are compositionally analogous with typical peraluminous leucogranites around the world, which have been commonly supposed to derive from a felsic pelitic source (Fig. 11; Patiño Douce, 1999).

The εNd(t) values of the Kalasu-Aletai granites range from -7.6 to +3.0 (mostly <-0.2), which are comparable with the bulk early–middle Paleozoic peraluminous granitoids as well as the Habahe Group in the Chinese Altai (Fig. 13). Likewise, the corresponding two-stage Nd model ages (TDM-2) of the Kalasu-Aletai granites vary from 0.79 to 1.65 Ga (Table S7) that are matching well with those of terrigenous components of the Habahe Group (0.9–1.6 Ga) and the Silurian–Devonian granitoids (0.8–1.5 Ga) (Huang et al., 2020). Recent studies have suggested that the bulk peraluminous early–middle Paleozoic granitoids were most likely derived from the Habahe Group (Jiang et al., 2016). The close similarities of the isotopic signatures between the Kalasu-Aletai granites and the Habahe Group suggest that they could also originate from the Habahe Group rocks.

Notably, the geochemical data for the Kalasu-Aletai granite samples are scattered and cloud-like on the Harker diagrams (Fig. 7), implying that these rocks were not derived from fractional crystallization of the same parental magma. Their variable Sr and Nd isotopic compositions (Fig. 13) and wide ranges of Rb, Sr, Ba, and Eu/Eu* values also imply that they evolved from heterogeneous sources (Villaros et al., 2009), a typical geochemical feature of the Habahe Group rocks (Huang et al., 2020; Jiang et al., 2016). In summary, the Kalasu-Aletai granite dykes were probably derived from the magmatic recycling of the Habahe Group.

Tectonic Implications

It has been proposed that the convergent evolution between the Chinese Altai and Junggar arc domain was associated with the closure of the Erqis Ocean (also known as the Ob-Zaisan Ocean, Li et al., 2017). This idea is advocated by a recent comparison on detrital zircon U-Pb age patterns of the southern Chinese Altai and East Junggar, which showed dissimilar detrital age spectra between them during the Late Carboniferous, implying that these two systems were still separated by an oceanic basin at this time (Li et al., 2017). On the basis of geochronology of the youngest (313 Ma, Cai et al., 2012) subduction-related igneous rocks along the southern Chinese Altai, it is believed that the southern Chinese Altai was still facing northward-directed subduction of oceanic lithosphere until the Late Carboniferous. This is also compatible with ca. 307–299 Ma crystallization ages of the stitching plutons intruding both sides of the western part of the Erqis fault zone in East Kazakhstan (Kuibida et al., 2009). These data indicate convergence between the Altai and Junggar systems should have occurred prior to 300 Ma (Han et al., 2010). In contrast, others have considered the Junggar as a continental block. In this context, the East Junggar arc domain was built through southward subduction of the Paleo-Asian Ocean underneath the Junggar (e.g., Xu et al., 2013). In this case, the convergence between the Chinese Altai and East Junggar was completed via southward-directed subduction, or by an opposite double-subduction system as discussed in Xiao et al. (2015). While different subduction polarities were proposed, the recent geophysical analysis demonstrated that the East Junggar basement was underthrusted beneath the Chinese Altai (Guy et al., 2020, 2021), attesting that subduction of oceanic crust preceding the convergence between the Chinese Altai and East Junggar was evidently northward subducted.

In this study, the oldest granite dyke from the Fuyun area is dated at ca. 300 Ma. It represents the earliest syn-tectonic granite dyke structurally associated with the compressive stress related to the convergent-shortening process along the southern Chinese Altai. As cited above such granite dykes were most likely derived from Junggar arc rocks, it is therefore suggested that the underthrusting of the East Junggar basement beneath the southern Chinese Altai started prior to 300 Ma. The extensive emplacement of 300–265 Ma syn-D3 granite dykes associated with significant anatexis of the deep crust could therefore reflect the main stage of the convergence and massive deformation of the Chinese Altai.

Combined with regional available data, our findings allow us to propose a two-stage convergent evolution between the Chinese Altai and the Junggar arc domain as illustrated in Figure 14. The first initial stage (Fig. 14A) is characterized by the stacking of the Junggar arc components beneath the southern margin of the Chinese Altai. This event could follow the Carboniferous subduction of a remnant oceanic basin. Li et al. (2016) claimed that an important Late Carboniferous to Early Permian thickening phase affected the Chinese Altai crust, which might be connected with the initial convergent scenario proposed here. During this stage, the Junggar arc rocks could be emplaced beneath the crust and above the Moho of the Chinese Altai in the form of a viscous channel (e.g., Beaumont et al., 2006) or crustal relaminant of Maierova et al. (2018). This stage should postdate the youngest subduction-related igneous rocks in the southern Chinese Altai, i.e., ca. 313 Ma (Cai et al., 2012) and predate the oldest granite dykes that originated by melting of underthrusted Junggar arc rocks, i.e., ca. 300 Ma as shown in this study. The second stage is the advanced convergent-shortening stage (Fig. 14B). This stage is featured by the melting of the deeply buried Junggar rocks and extrusion of molten rocks beneath the Chinese Altai and forming high-grade deformation tabular zones (Broussolle et al., 2018; Jiang et al., 2019). Jiang et al. (2019) proposed a northward progressive folding starting from the deformation front in the south and propagating northwards into the interior of the Chinese Altai. This assertion is supported by progressive northward propagation of tensional fractures filled by syn-tectonic granite magmas. The prolonged and progressive emplacement of syn-tectonic granite dykes from ca. 300 to ca. 270 Ma witnessed continuous long-lived and massive shortening across the southern Chinese Altai.

Even the studied granite dykes witnessed the Chinese Altai–East Junggar convergence, their geochemical characters do not exhibit typical high-pressure signatures. This is consistent with the fact that the magmatism is coeval with regional low-pressure/high-temperature metamorphism rather than high-pressure metamorphism (e.g., Tong et al., 2013; Wang et al., 2009b), indicating that the convergence did not cause significant crustal thickening, distinct from typical collisional orogeny. The remaining question is what underlying mechanism caused high-temperature metamorphism and melting during the Chinese Altai–East Junggar convergence. The occurrence of mafic igneous rocks as well as (ultra)high-temperature metamorphism are both considered as products of a mantle plume (e.g., Tong et al., 2013; Zhang et al., 2012) or postcollisional extension (Tong et al., 2014). However, the Permian gabbros commonly contain magmatic hornblende with low crystallization temperatures (715–826 °C), excluding a mantle plume source that is typically characterized by an abnormally high melting temperature (Wan et al., 2013). Furthermore, distributions of the related high-temperature metamorphic rocks, as well as mafic intrusions, are restricted in a narrow linear belt rather than having circumcentric or radial patterns (Ernst and Buchan, 2001). These features are apparently at odds with a mantle-plume model. In addition, the fact that high-temperature metamorphic rocks are highly folded by D3 shortening (Broussolle et al., 2018) and hence do not support a post-collisional scenario, Li et al. (2015a) proposed tectonic switching characterized by shortening followed by extension and renewed shortening during the convergence between the Chinese Altai and East Junggar. A similar tectonic switching model was indeed proposed to explain the extensive crustal anatexis associated with significant input of juvenile mantle materials into the crust during a regional Devonian tectonometamorphic event (Jiang et al., 2016; Li et al., 2019), which is analogous to the tectonic scenario that explained large-scale magmatism in circum-Pacific orogens (e.g., Collins, 2002). However, such a tectonic switching model may not be sufficient to explain the high-temperature metamorphism and melting process associated with the Permian convergence of the region cited above, because the high-temperature metamorphism and associated gabbroic magmatism are restricted to a narrow deformation zone, implying localized heat advection. It is noteworthy that the narrow deformation zone is marked also by important gravity and magnetic highs (Guy et al., 2020). Such a relationship among magmatism, metamorphism, and the geophysical signal was reported from late Proterozoic vertical and tabular shear zones in southern Madagascar, resulting from the final collision between East and West Gondwana (Martelat et al., 2000, 2012). These authors proposed that elevation of hot mantle cusps at the bottom of shear zones was responsible for massive and localized heat advection leading to melting of the crust in the interior of deformation zones. Similarly, the origin of Permian deformation zones in the Chinese Altai can be interpreted as analogs of the Madagascar Proterozoic examples (Martelat et al., 2014, 2020). In this regard, the development of vertical deformation zones during the Chinese Altai–East Junggar convergence probably caused a localized thermal and mechanical perturbation of the weak mantle lithosphere beneath the Chinese Altai. The arrival of mantle-derived mafic magmas via the vertical deformation zones would cause partial melting of both proximal Junggar and remote Chinese Altai components and form granite magmas eventually emplaced in the syn-tectonic tensional fractures along the southern Chinese Altai.

Geochronological and geochemical investigations on Permian syn-tectonic granite dykes from the southern Chinese Altai allowed us to constrain the duration and processes of the convergence between the Chinese Altai and Junggar arc domain. The principal conclusions are:

  1. Granite dykes were emplaced within syn-tectonic tensional fractures in the southern Chinese Altai in response to Chinese Altai–East Junggar convergence and have formation ages of 300–265 Ma, with an important northward younging trend.

  2. The granite dykes emplaced in the deformation fronts are fractionated I-type granites, originated from partial melting of the juvenile materials from the southernly East Junggar arc domain. In contrast, granite dykes emplaced farther north are highly differentiated S-type granites and probably derived from partial melting of heterogeneous Ordovician wedge sediments of the Chinese Altai.

  3. The Chinese Altai–East Junggar convergence can be divided into two main stages: an early East Junggar underthrusting stage (313–300 Ma) and an advanced stage of massive shortening of the southern part of the Chinese Altai (300–265 Ma). The initial stage is characterized by the stacking of underthrusting Junggar arc rocks beneath the southern margin of the Chinese Altai while the advanced stage probably caused a localized thermal and mechanical perturbation of weak mantle lithosphere beneath the Chinese Altai. This event was responsible for the partial melting of pre-existing Junggar and Chinese Altai components and the formation of granite magmas syn-tectonically emplaced within the tensional fractures orthogonal to the East Junggar-Chinese Altai deformation front.

1Supplemental Material. Table S1: Summaries of sample locations and petrographic characteristics of the studied granite dykes from the southern Chinese Altai. Table S2: LA-ICP-MS U-Pb geochronological analyses of zircon standards. Table S3: LA-ICP-MS zircon/monazite U-Pb data of the granite dykes from the southern Chinese Altai. Table S4: LA-ICP-MS monazite U-Pb data of the granite dykes from the southern Chinese Altai. Table S5: Monazite composition of the studied granite dykes from the southern Chinese Altai. Table S6: Major and trace element compositions and calculated parameters of the granite dykes from the southern Chinese Altai. Table S7: Whole-rock Sr and Nd isotope data for granite dykes from the southern Chinese Altai. Please visit https://doi.org/10.1130/GSAB.S.20272671 to access the supplemental material, and contact editing@geosociety.org with any questions.
Science Editor: Wenjiao Xiao
Associate Editor: Yongjiang Liu

This study was supported by the National Natural Science Foundation of China (42022017, 42021002), the International Partnership Program of the Chinese Academy of Sciences (132744KYSB20190039), and the China Scholarship Council (CSC) (202004910489). The Czech Science Foundation (Grant Agency of the Czech Republic grant EXPRO 19-27682X) to K. Schulmann is also acknowledged. Pengfei Li is thanked for his suggestions on our manuscript. We appreciate the assistance of Kang Xu and Jun Ning with field work, Ming Xiao, Yuan Fang, and Zhitai Li with lab analyses. Dr. Stephen Collett is thanked for reading the manuscript. We are indebted to Jérémie Lehmann and two anonymous reviewers for the constructive reviews and valuable comments that improved the manuscript.

Gold Open Access: This paper is published under the terms of the CC-BY license.