# Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt

Yazhou Miao, Jian Zhang, Karel Schulmann, Ondrej Lexa, Petr Jeřábek, Min

Sun, Yingde Jiang, Shuhui Zhang

► To cite this version:

Yazhou Miao, Jian Zhang, Karel Schulmann, Ondrej Lexa, Petr Jeřábek, et al. Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt. Journal of Structural Geology, 2023, 174, pp.104917. 10.1016/j.jsg.2023.104917. hal-04223445

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<figure><img src="image_1.png" /></figure>
1 Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai:

2 recording a Devonian-Permian tectonic regime switch in the southwestern

3 Central Asian Orogenic Belt

4

5 Yazhou Miao <sup>a, c</sup>, Jian Zhang <sup>a, b*</sup>, Karel Schulmann <sup>c, d*</sup>, Ondrej Lexa <sup>e</sup>, Petr Jeřábek <sup>e</sup>,

6 Min Sun <sup>b</sup>, Yingde Jiang <sup>f</sup>, Shuhui Zhang <sup>a</sup>

7

8 <sup>a</sup> School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou

9 510275, China

10 <sup>b</sup> Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong

11 Kong SAR

12 <sup>c</sup> Center for Lithospheric Research, Czech Geological Survey, 11821 Praha 1, Czech

13 Republic

14 <sup>d</sup> Institut Terre et Environnement de Strasbourg, UMR 7063 Université de Strasbourg

15 - CNRS, 1 Rue Blessig, 67084 Strasbourg, Cedex, France

16 <sup>e</sup> Institute of Petrology and Structural Geology, Faculty of Science, Charles

17 University in Prague, Albertov 6, 128 43 Prague 2, Czech Republic

18 <sup>f</sup> State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry,

19 Chinese Academy of Sciences, Guangzhou 510640, China

20

21 **Keywords:** Chinese Altai; Devonian and Permian orogens; EBSD inclusion trail

22 analysis; Foliation-intersection axis analysis
23 *Corresponding authors. Department of Earth Sciences

24 The University of Hong Kong

25 Hong Kong SAR

26 Email: jian@hku.hk

27 Tel: +852-39177840

28

29 Center for Lithospheric Research

30 Czech Geological Survey

31 Praha 1, Czech Republic

32 Email: karel.schulmann@geology.cz

33 Tel: +420 731 133 165

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45 Abstract

Recent studies revealed that the Chinese Altai may have experienced two major tectonothermal events in the Devonian and Permian but their mutual relationships remain poorly constrained. In this study, we carried out a comprehensive analysis of macro- and micro-structural features of the Ordovician schists that were subjected to Barrovian-type metamorphism in the Chonghuer area of the northwestern Chinese Altai. Field structural and microstructural analyses revealed that metamorphic foliation S1 was folded by the recumbent fold F2, forming the sub-horizontal foliation S2. The S2 fabric was later reworked by the D3 transpressional shearing along the narrow sillimanite-bearing zone to form the foliation S3. Monazite U-Pb ages obtained from the staurolite schist located close to the D3 shear zone, as well as the zircon U-Pb ages from both folded and unfolded pegmatites that cross-cut granite, indicate that the D3 deformation took place during the Permian period, approximately 257-250 Ma. The EBSD study of quartz in the matrix and staurolite porphyroblasts conducted along the NE-SW cross-section allowed us to: 1) constrain the medium-temperature D2 and high-temperature D3 regimes and, 2) reconstruct foliation-intersection axis (FIA) patterns both inside and outside the D3 deformation zone. The FIA between Devonian D1 and D2 preserved in staurolite porphyroblast was initially oriented in a N-S direction, which was ultimately reoriented to a NW-SE direction after undergoing the D3 shear deformation. This result is further reinforced by the numerical stochastic modelling, which quantifies the influence of the Permian HT sinistral transpression on the reworking of the Devonian FIA fabrics. Micro-structural
67 data of this study verifies the significant tectonic switch from the Devonian E-W
68 contraction to the Permian NNW-SSE transpression. Quantitative EBSD-based FIA
69 analysis proves to be an effective approach for the structural analysis of polycyclic
70 orogenic systems.

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## 1. Introduction

Since the discovery of relationships between deformation, metamorphism and growth of porphyroblasts (Zwart, 1960), the upgraded analysis of spiral-shaped inclusion trails was applied in several orogenic systems to depict their polycyclic and polyphase evolution (Kim and Bell, 2005; Skrzypek et al., 2011; Aerden et al., 2013; Sayab et al., 2021). Combined microstructural and in situ geochronological studies further allowed to constrain precisely P-T-t-D paths during an orogenic cycle (Bell, 2002; Soejono et al., 2021). The new concept of foliation intersection axis (FIA) was subsequently used as a proxy for multiple changes of orogeny-scale kinematics (Fay et al., 2008; Bell and Fay, 2016; Ruiz-Fuentes and Aerden, 2023). Differently oriented FIAs inclusion trails in garnet porphyroblasts successfully exhibit a general orogenic process switching from a sub-vertical to sub-horizontal orientations (Ruiz-Fuentes and Aerden, 2023). The FIA analysis as well as dating of porphyroblast growth are time-consuming approaches (Kim and Sanislav, 2017) and tectonic significance of resulting patterns is critically dependent on conceptual model of orogenesis. Therefore, the method was used only by few research groups during past decades and the interpretation of spiral inclusion trails became a matter of vigorous discussion (Vernon, 1989; Passchier et al., 1992; Ruiz-Fuentes and Aerden, 2023). However, the advent of the Electron Back Scattered Diffraction (EBSD) method has made FIA analysis more reliable and easier in examining the deformational features for modern structural geology (Skrzypek et al., 2011; Kim and Sanislav, 2017; Aerden and Ruiz-Fuentes, 2020).
In the last decades, a large amount of geological investigation revealed that the Chinese Altai underwent polycyclic tectonothermal evolution marked by a significant tectonic regime switch from Devonian to Permian (Broussolle et al., 2018, 2019; Jiang et al., 2019, 2021). The Permian event not only caused an anticlockwise rotation of Devonian fabrics but also resulted in regionally coaxial shortening, which makes it extremely difficult to distinguish the Devonian fabrics from the Permian ones (Jiang et al., 2015). It is because that Permian deformation and metamorphism intensity in the southeastern part of the Chinese Altai is very high and therefore the whole deformation record in the region is considered to be a result of the Permian event (Fig. 1; Jiang et al., 2019; Hu et al., 2020; Xu et al., 2021). However, in the northwestern part where the intensity of Permian reworking is weaker and most fabrics are considered to retain mainly the Devonian structural trends (Jiang et al., 2015; Zhang et al., 2015; Wang et al., 2021). Therefore, a synthetic structural and metamorphic analysis under a microscopic scale and FIA in porphyroblasts are of crucial significance to constrain this issue. Previous metamorphic data in the Chinese Altai have revealed that the growth of earliest Barrovian index minerals was affected by the Buchan-type metamorphism in the Devonian (Jiang et al., 2015; Broussolle et al., 2019). This Devonian orogenic cycle was succeeded and superimposed by the Permian tectonothermal event that was characterized by the new growth of Buchan-type index minerals (cordierite + andalusite; Liu et al., 2020a; Jiang et al., 2021). However, spatially and temporally structural relations between the two generations of metamorphic fabrics remain poorly constrained.
The Chonghuer area is a Barrovian metamorphic belt, where the nearly N-S
Devonian fabrics have been rotated into a Permian NW-SE orogenic trend but was
weakly reworked by the Permian Erqis shear zone (Fig. 1; Kong et al., 2022). It thus
represents the most promising area to reconstruct the structural overprinting
relationships between the Devonian and Permian events. With the aid of high-precise
U-Pb zircon and monazite dating, we reconstructed an integrated deformation-time
history of the area. By utilizing the EBSD analysis on quartz that forms the inclusion
trails inside the staurolite porphyroblasts and the quartz-bearing matrix, we were able
to determine the foliation-intersection-axes (FIA) and quantify the differences in
deformational temperature conditions between the Devonian and Permian events. We
further numerically modelled the complex transition from Devonian to Permian FIA
patterns, which allowed us to propose an updated model for polycyclic orogenic
evolution of the Chinese Altai.

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147 [Fig. 1 here]

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2. Geological setting
---

2.1 Geology of the Chinese Altai

The Chinese Altai, in the northwestern border of China, forms a southern part of
large Mongolian Altai belt located between Mongolian Precambrian blocks to the NE
and the Junggar magmatic arcs to the south (Fig. 1a; Windley et al., 2007). The
Mongolian Altai belt constitutes mainly of early Paleozoic volcanic-sedimentary
rocks variably reworked by early to late Paleozoic metamorphic and magmatic events (Buriánek et al., 2022). Based on the difference in stratigraphy, petrology, and geochronology, from north to south, the Chinese Altai was subdivided into five fault-bounded terrains (Windley et al., 2002). However, most recent studies proposed that these different terrains were most likely developed on a single Cambro-Ordovician accretionary complex dominated by the Devonian and Permian magmatism and metamorphism. In this work, the Chinese Altai is not considered as an assemblage of distinct terranes, but as an orogenic domain, where accretionary wedge in the north was affected only by the early Paleozoic metamorphism (Fig. 1a; Broussolle et al., 2019). In contrast, its southern part was heterogeneously reworked by the Permian tectonothermal event (Broussolle et al., 2018; Guy et al., 2020).

The oldest known metasedimentary sequence in the Chinese Altai is the Ordovician Habahe Group that consists of thick turbidities and pyroclastic rocks metamorphosed into variable grades (BGMRX, 1993; Zhuang, 1994). It was traditionally considered to have been deposited at a passive continental margin along an unknown Precambrian micro-continent (Huijun et al., 2006; Wilhem et al., 2012), but recent petrological and geochemical data suggested that these metasedimentary rocks should have deposited in an active continental margin or a continental arc setting (Long et al., 2007, 2008; Jiang et al., 2016).

The Habahe group is unconformably covered by the early Devonian Kangbutiebao formation that consists mainly of volcanic rocks, middle Devonian volcanic-sedimentary Altai formation and middle-late Devonian Qiwendai formation,
177 consisting of sandstone and shale (BGMRX, 1993). The Devonian basaltic and
178 rhyolitic volcanism was interpreted to result from widespread extensional events
179 following the Ordovician-Silurian accretionary stage (Jiang et al., 2019; Wang et al.,
180 2021). The Devonian sequences generally experienced low-grade metamorphism or
181 were unmetamorphosed (Xu et al., 2021; Kong et al., 2022).

182

183 2.2. *Tectonic, metamorphic and magmatic evolution of the Chinese Altai*

184 Recent investigations from the central and western Chinese Altai indicate that the
185 region was affected by four distinct teconothermal events. The first event was related
186 to the development of early Paleozoic Barrovian metamorphism with biotite to
187 kyanite-sillimanite zones and downward increasing grade that was interpreted as a
188 result of crustal thickening (Jiang et al., 2021; Kong et al., 2022). The second event
189 was characterized by Buchan type metamorphism accompanied by voluminous I- and
190 S-type granitoid magmatism, mainly emplaced between 420-400 Ma (Sun et al., 2009;
191 Broussolle et al., 2019, 2020). Geochemical and isotopic data indicated that they were
192 derived mainly from the partial melting of young sediments under a syn-subduction
193 tectonic regime (Yuan et al., 2007; Sun et al., 2008, 2009; Cai et al., 2011; He et al.,
194 2015; Zhang et al., 2017). This event was associated with important lithospheric
195 thinning, crustal-scale extension and coeval deposition of Devonian Kangbutiebao
196 and Altai formations on the upper crust (Zhang et al., 2015; Jiang et al., 2016).
197 Subsequently, late Devonian upright N-S trending folds affected previously thinned
198 crust and resulted in the development of migmatite-magmatite domes surrounded by
199 synforms that consist of medium grade mostly Barrovian schists and weakly
200 metamorphosed Devonian sequences (Jiang et al., 2015, 2019; Wang et al., 2021;
201 Kong et al., 2022).

202 During the latest N-S Permian orogenic cycle, the Devonian domes and basins
203 rotated to E-W direction (Zhang et al., 2015; Jiang et al., 2019; Xu et al., 2021; Kong
204 et al., 2022). The Permian event is characterized by Buchan-type metamorphism that
205 overprinted the earlier rock assemblages (Li et al., 2014; Tong et al., 2014a; Liu et al.,
206 2020a). Ultra-high-temperature and lower-pressure granulite facies rock scatters in the
207 narrow NW-SE trending zones in the southern Chinese Altai (Tong et al., 2014a; Liu
208 et al., 2020b; Jiang et al., 2021). The Permian event also resulted in intrusions of
209 numerous A-type granite and gabbro plutons (Fig. 1a; Broussolle et al., 2019).

210 Geochemical and petrological features indicated that these metamorphic and igneous
211 rocks were mostly generated in a post-subduction intraplate setting (Tong et al., 2014b;
212 Lin et al., 2019). The Permian history was terminated by large-scale sinistral shearing
213 associated with the activity of the Erqis shear zone (Li et al., 2015, 2016; Zhang et al.,
214 2015), and timing of the sinistral shearing was constrained by the ⁴⁰Ar/³⁹Ar data at
215 283-265 Ma (Li et al., 2017; Hu et al., 2020). The Erqis shear zone is probably
216 connected with the coeval collision between the Chinese Altai with the Junggar arc
217 domain (Jiang et al., 2019).

218

219 [Fig. 2 here]

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221 2.3. Geology of the study area

222 Chonghuer area, located in the northwest Chinese Altai (Fig. 1b), comprises the
223 Devonian migmatite-granite dome to the south (Cai et al., 2011), Devonian
224 Kangbutiebao and Altai formations, and the Ordovician Habahe Group. According to
225 previous studies, the studied area represents a NW-SE trending large synform cored
226 by Devonian low-grade metamorphic basins and medium-grade metamorphic Habahe
227 group (Jiang et al., 2015; Kong et al., 2022). The studied section is characterized by
228 well-preserved Barrovian-type metamorphic zones (Jiang et al., 2015). The
229 metamorphic grade increases from southwest to northeast, from the biotite, garnet,
230 and staurolite zones to the kyanite-sillimanite zone adjacent to the intrusion of the
231 granitic pluton in the north (Figs. 1b, 2a). Large staurolite porphyroblasts with
232 macroscopically visible inclusion trails are developed in the Habahe group
233 metasediments in the central and northern part of the studied section. The study of the
234 internal fabric of these staurolite crystals with records of host rock fabric is the main
235 subject of this study.

236

237 [Fig. 3 here]

238

239 3. Macro- and Micro-structural features

240 3.1 *Structural pattern*

241 To determine the regional structural pattern and decipher the spatial
242 superimposition, we conducted detailed structural mapping throughout the Chonghuer
243 area (Fig. 2a). Three generations of deformation fabrics and their overprinting
244 relations can be depicted along the studied cross-section (Fig. 2b). In low-grade
245 Devonian rocks of the Kangbutiebao formation, the chlorite-muscovite bearing fabric
246 of unclear age is only partly transposing sedimentary bedding. It is either sub-
247 horizontal or gently dipping to the NE (Fig. 2). The adjacent part of the Habahe group
248 meta-sediments is metamorphosed in the biotite zone and the main structural element
249 is a ubiquitous schistosity S1 folded by recumbent close to isoclinal folds F2 (Figs. 3a,
250 b). Further towards the northeast in the garnet zone, the development of recumbent
251 isoclinal folds F2 is associated with axial planar cleavage S2 gently dipping to the NE
252 (Figs. 2b, 3c). S2 is locally reworked by open to tight upright folds F3 varying from
253 symmetrical to asymmetrical ones, which are sometimes associated with development
254 of a crenulation cleavage S3 (Fig. 3c). From SW to NE, S2 exhibits a gradual
255 transition from the crenulation cleavage in the garnet zone to penetrative foliation
256 mainly developed in the staurolite zone, all of which show a preferred NE-dipping
257 orientation at moderate to high angles (Figs. 2b, 3d). From the biotite to staurolite
258 zone, the fold axes of isoclinal F2 define a S- to SE-plunging lineation L2 (Fig. 2b).
259 Approaching the kyanite and sillimanite zones towards NE, the cleavage S3 develops
260 into steep NNE dipping penetrative fabric bearing mineral stretching lineation L3
261 with moderate to steep NE-E plunges (Fig. 2b). This zone thus overlapped with D3
262 high strain zone reworking regional S2 foliation with intersection axes parallel to
263 lineation L3 (Fig. 2b). The foliation in the high strain zone is therefore defined as a
264 composite foliation S2-3 (Fig. 3e). The boundary between the northeast granite and
265 the kyanite-sillimanite schist is also characterized by the presence of deformed granite
266 sheets striking parallel to the regional foliation (Fig. 3f). The granite shows
267 penetrative foliation that is parallel to the composite foliation S2-3 (Fig. 2a).

268

269 [Fig. 4 here]

270

271 An important feature of D3 high strain zone is the presence of numerous pegmatite
272 dykes that intrude both schist and deformed granite (Fig. 4). The pegmatite dykes
273 intruding the kyanite-sillimanite schists are folded by asymmetrical folds F3 with
274 axial planes parallel to host rock foliation S2-3 (Fig. 4), whereas those intruding the
275 granite do not show strong deformational fabrics but clearly crosscut the pre-existing
276 foliation at a high angle (Fig. 4f). The analysis of satellite images allowed to visualize
277 these relationships and shows that dykes intruding the granite are striking NNE-SSW
278 i.e., perpendicular to the main foliation of gneissic granite (Figs. 2b, 4a-d). On the
279 other hand, dykes intruding the kyanite-sillimanite zone are striking in the NW-SE
280 direction parallel to the main schistosity of the D3 high strain zone (Figs. 4a-e).

281

282 [Fig. 5 here]

283

284 3.2 *Microstructural features*

285 Distinct overprinting relations between the D1, D2 and D3 deformations, from
286 southwest to northeast, are well manifested by the microstructures in different
287 metamorphic zones (Fig. 5). In the Devonian sediments of the Kangbutiebao
288 formation, the metamorphic foliation is represented by a crenulation cleavage S2
289 reworking the S0 (Fig. 5a). Here, the cleavage S2 is defined by the alignment of
290 muscovite and chlorite (Fig. 5a). In the biotite zone of the Habahe group, the S1 is
291 intensely folded by F2 crenulations (Fig. 5b). The F2 fold axial planar cleavage S2 is
292 represented by newly formed metamorphic fabrics defined by alignment of muscovite
293 and recrystallized quartz aggregates (Fig. 5b). In the garnet zone, S1 is defined by the
294 internal inclusion trails of quartz within garnet and the alignment of biotite and
295 muscovite in the matrix (Fig. 5c). Recrystallized or deflected muscovite forms a new
296 cleavage S2 that is oriented at a high angle to S1 (Fig. 5c). In the staurolite zone, the
297 S1 is often completely transposed by S2 and the S1 is defined by inclusion trails in
298 garnet and staurolite (Fig. 5d). These inclusion trails are either parallel or at a high
299 angle to the external S2 (Fig. 5d). Here, the S2 is mainly formed by alignment of
300 biotite, muscovite and recrystallized quartz (Fig. 5d). In the D3 high strain zone and
301 the kyanite-sillimanite zone, the S1 is preserved only in kyanite and staurolite
302 porphyroblasts in the form of quartz inclusion trails (Fig. 5e). The S1 is either parallel
303 or oblique to S2 fabric in matrix which is represented by biotite bands and quartz
304 aggregates (Fig. 5e). In the sillimanite zone, the S1 is preserved as inclusion trails in
305 staurolite and kyanite and becomes oblique to the external foliation S2-3 that is
306 associated with the alignment of sillimanite and mica (Fig. 5f).

307 In the high strain domain of the granite that is adjacent to the D3 high strain zone,
308 the K-feldspar, plagioclase, and quartz exhibit dynamic recrystallization and form
309 polycrystalline aggregates surrounded by bands of biotite and muscovite (Fig. 5g).

310 Polycrystalline quartz is present in form of ribbons where individual grains show
311 either highly lobate or straight boundaries, which are characteristic of grain boundary
312 migration and post-deformation grain boundary reduction, respectively (Fig. 5g). By
313 contrast, in the low strain domains, plagioclase, quartz, and K-feldspar exhibit an
314 equigranular relationship and largely retain the magmatic texture (Fig. 5h; Passchier
315 and Trouw, 2005).

316

317 **4. Geochronology: U-Pb zircon and monazite**

318 **4.1 Methodology**

319 Zircon and monazite grains were extracted using standard heavy liquid and
320 magnetic techniques and then purified by hand picking under a binocular microscope.

321 About 100-250 representative zircon grains were then mounted in epoxy resin and
322 polished. The internal zircon and monazite structures were analyzed using a scanning
323 electron microscope ZEISS TM with the CL (Cathodoluminescence) System for high-
324 resolution imaging at Sun Yat-sen University. The U-Pb analysis was carried out at the
325 Wuhan Sample Solution Analytical Technology Co., Ltd with Agilent 7900
326 inductively coupled plasma mass spectrometer and Coherent 193 nm excimer laser
327 ablation system. The analytical data were reduced using GLITTER 4.0 software
328 (Macquarie University, Sydney, Australia). The weighted average ages and plotting of
329 Concordia diagrams were obtained using Isoplot 4.15 (Ludwig, 2012). The sample
330 age results are listed in **Supplementary Table 1**.
331

332 [Fig. 6 here]

333

334 4.2 Analytical results

335 4.2.1 Staurolite schist (17CHR16-1 and 18CHE18-9)

336 Samples 17CHR16-1 and 18CHE18-9 were collected from the staurolite schist
337 zone (Fig. 2a). In the field, the staurolite schist develop the pervasive S2 foliation
338 defined by the preferred orientation of biotite-rich layers and quartz-feldspar
339 aggregates. The S1 fabric was preserved in staurolite porphyroblast as inclusion trails
340 oriented at a high angle with respect to the S2 foliation (Fig. 6a).

341 Zircon grains extracted from Sample 17CHE16-1 are mostly prismatic in shape,
342 and range 100-200 µm in length with an aspect ratio of 2:1-4:1 (Fig. 7a). These zircon
343 grains show typical oscillatory zoning in their CL images and have Th/U ratios of
344 0.09-2.01 (Figs. 7a, 8a; Supplementary Table 1), suggesting their magmatic origin.

345 A total of sixty-one analyses were conducted on different grains and six of them
346 yielded a cluster with the youngest peak weight mean $^{206}$Pb/$^{238}$U ages of 453±2 Ma
347 (MSWD = 2.3; Fig. 8a). In our interpretation, this youngest peak age represents the
348 maximum sedimentary age of the rock that pre-dated the regional D1 deformation.

349 Sample 18CHE18-9 was selected for U-Pb monazite dating. Monazite grains from
350 the sample are mostly euhedral, 120-200 µm in length with an aspect ratio of 2:1-3:1
351 (Fig. 7a). Monazite BSE imaging shows homogeneous structures (Fig. 7a). Sixty
352 analyses form a tight cluster on the concordant diagram yielded with a weighted mean
gave $ ^{206}_{238}Pb $ age of 257±2 Ma (MSWD = 7.1; Fig. 8b), represents the metamorphic age of the staurolite schist indicating that the schist was influenced by the Permian tectonothermal event.

356

[Fig. 7 here]

358

4.2.2 Gneissic granite (18CHE08-2)

Sample18CHE08-2 was collected from the granite pluton in the northeast part of the Chonghuer area, which develops the pervasive S2-3 foliation (Fig. 6b). Zircon grains show sub-euhedral crystal shape and are 50-150 µm in length. CL images show oscillatory zoning texture and indicate a magmatic origin (Fig. 7b). Th/U ratios vary from 0.35-1.84 (Fig. 8c; Supplementary Table 1). Twenty-five analyses were conducted, and nine of the analyses yielded a $ ^{206}_{238}Pb $ weighted mean age of 392±6 Ma (MSWD = 4.1; Fig. 8c). This age represents the crystallization age of the intrusion, and indicating the pluton was emplaced during the D2 deformation.

368

[Fig. 8 here]

370

4.2.3 Granitic pegmatite dykes (18CHE19-2 and 18CHE11-2)

Sample 18CHE19-2 was collected from folded pegmatite intruding the granite in a pluton bearing strong S2-3 foliation (Fig. 6c). Zircon grains from the sample are subhedral and have a length of 100-120 µm. A total of twenty-four analyzed spots
yielded $^{206}$Pb/$^{238}$U ages ranging from 432 to 246 Ma (Supplementary Table 1). In general, these grains can be divided into two sub-groups based on their internal structure. The first sub-group shows oscillatory zoning textures and Th/U ratios of 0.27-0.75, consistent with an igneous origin. Fourteen analyses on the first sub-group grains yielded a $^{206}$Pb/$^{238}$U weighted mean age of 408±5 Ma (MSWD=4.6; Fig. 8d).

The second sub-group of zircons is featured by the sector zoning and Th/U ratio from 0.01 to 0.36 indicating their metamorphic origin (Figs. 7c, 8d; Supplementary Table 1). Seven analyzed spots yielded $^{206}$Pb/$^{238}$U ages ranging from 308 to 246 Ma, and two analyses on the second sub-group grains yielded a $^{206}$Pb/$^{238}$U weighted mean age of 253±3 Ma (MSWD=10.8; Fig. 8d).

Sample 18CHE11-2 was collected from one undeformed pegmatite which cut through the S2-3 foliation (Fig. 6d). Nineteen zircons from this sample were analyzed for U-Pb isotopic compositions. These analyses grains were subdivided into two distinct groups, which is similar to the Sample 18CHE19-2. One group analyzed grains are generally transparent, euhedral, and prismatic, with clear oscillatory zoning and high Th/U ratios varies from 0.38-0.74 (Fig. 7d; Supplementary Table 1), indicating a magmatic origin. Thirteen analyses on the first sub-group grains yielded a $^{206}$Pb/$^{238}$U weighted mean age of 409±5 Ma (MSWD=3.7; Fig. 8e). The second sub-group is featured by the sector zoning texture and Th/U ratio varies from 0.00 to 0.06 (Figs. 7d, 8e; Supplementary Table 1), indicating a metamorphic origin. Three analyses on the second sub-group grains yielded a $^{206}$Pb/$^{238}$U weighted mean age of 250±3 Ma (MSWD=1.14; Fig. 8e).
The difference in textures and age of the two types of zircon may indicate either two magmatic pulses or contamination of pegmatite melt by crustal rocks at the time of intrusion. These hypotheses are in contradiction with the large age span between both zircon populations and the large amount of older zircon in both samples.

Therefore, the Devonian ages are interpreted as the timing of formation of the Devonian lower crust (Jiang et al., 2015, 2016; Broussolle et al., 2018; Kong et al., 2023) as testified by age of granite intruded by pegmatite dykes (Sample 18CHE08-2), large amounts of Devonian granite plutons intruding the Chinese Altai crust (Yuan et al., 2007; Sun et al., 2009; Cai et al., 2011) and widespread Devonian anatexis in the region (Jiang et al., 2015, 2019). This lower crust was remolten from late Carboniferous and remobilized during Permian HT event leading to emplacement of pegmatite dyke swarm in the middle and upper crust (Xu et al., 2021; Shu et al., 2022).

## 5. Staurolite fabric

As staurolite represents a characteristic mineral of amphibolite facies regional metamorphism, its growth is roughly restricted to a field of medium P-T conditions (Bell and Kim, 2004; Mezger, 2010; Bell and Fay, 2016). Due to their ability to form euhedral poikiloblastic porphyroblasts that can retain inclusion trails, staurolite can provide information for reconstructions of past and changing deformation environments (Frieman et al., 2013). As mentioned above, the Chonghuer area has experienced three phases of deformation, and porphyroblast-matrix relations are important tools for delineating the tectonothermal history of the area.
419

420 [Fig. 9 here]

421

422 5.1 Relationships of staurolite fabric to D1, D2, and D3 deformation

423 To identify the porphyroblast growth and its relation to deformation, we selected three representative oriented samples that clearly preserved the overprinting relationships between the D2 and D3 deformations observed in the field (18CHE17-2, 18CHE23-1, 18CHE20-1). Due to the variable degrees of superimposition between the D2 and D3 deformations, these three selected samples largely represent the different fabric relations along the SW-NE cross-section (Fig. 2b). The samples were cut and polished with respect to the XYZ axes of D3 finite strain ellipsoids. Based on both macroscopic and microscopic observations, staurolite grains show strong shape preferred orientation in the XZ section and weak to absent in the YZ section (Figs. 5d-f, 9).

432 5d-f, 9).
433 Sample 18CHE17-2 (Fig. 9a) was collected from the central part of the studied section (Fig. 2b). In the YZ section of the D2 fabric, the S1 foliation is observed as inclusion trails in porphyroblasts thereby representing internal foliation Si (short black dash line in the mineral) that is continuous with respect to external S1 foliation. S1 is folded by F2 folds and locally completely transposed by S2 axial planar cleavage. S2 is further folded by open F3 chevron folds. Open F2 folds are locally sealed by growing porphyroblasts which indicates that staurolite growth post-dated D1 but grew before or at the early stages of D2.
441 Sample 18CHE23-1 was taken at the SW margin of the D3 high strain zone (Fig. 2b). Here, the S1 foliation was intensely folded by F2 folds leading to the development of axial-planar cleavage S2 that is locally overprinted by cleavage S3 (Fig. 9b). The staurolite porphyroblasts are extensively rotated within fine-grained matrix. Their internal fabric Si mostly does not show continuity with external S2. Rootless isoclinal F2 folds are locally preserved.

447 Sample 18CHE20-1 was collected from the internal part of the D3 high strain zone (Fig. 2b). Here, the staurolite porphyroblasts are extensively rotated with respect to the external S2-3 foliation (Fig. 9c). The internal fine-grained fabric shows lack of continuity with surrounding fabrics. In some places, domains with relict fine-grained S1 fabric are attached to staurolite porphyroblasts and show continuity with Si inclusion trails. These regions show gentle bending by F2 folds and the local presence of S2 axial planar cleavage. However, most of the external fabric is formed by coarse grained and well annealed microstructure, where biotite and quartz show either weak or strong shape preferred orientation. These zones of variable shape preferred orientation are truncated by narrow bands of fibrous sillimanite, which defines the new foliation S3. Altogether, the sample is defined by relics of S1 and F2 folds but is dominated by D3 high temperature microstructure.

459 In summary, staurolite porphyroblasts of the Chonghuer area recorded all three main deformation stages. They grew after D1 and possibly before or at the early stages of D2. During D3 the porphyroblasts experienced rigid body rotation in the viscous matrix at high temperature, and behaved as active kinematic markers.
463

464 [Fig. 10 here]

465

466 5.2 Kinematic analysis of D3 deformation

467 In the high strain zone, staurolites show variably asymmetrical relationships to the
468 penetrative S2-3 foliation (Fig. 10a). On the XZ plane (Fig. 10b), the staurolite
469 porphyroblasts show various angles between the S<sub>i</sub> and S<sub>e</sub> trails (S<sub>i</sub>-S<sub>e</sub> intersecting
470 angle - α angle Fig. 10e), all of which exhibit a uniform sinistral sense of shear (Fig.
471 10b). On the XY plane, the long axis of the staurolite porphyroblasts and micas show
472 a strong crystal preferred orientation and define the L3 lineation that plunges to NEE
473 at moderate angles (Fig. 10c). On the YZ plane, the staurolite porphyroblasts
474 experienced intense shear deformation and show sub-parallel to S2-3 foliation (Fig.
475 10d). The internal S<sub>i</sub> foliation shows variable angles with respect to the external S2-3
476 foliation.

477 To investigate the relationship between staurolite porphyroblasts and matrix, we
478 counted 182 staurolite porphyroblasts and performed a statistical analysis to
479 determine the correlation between the α angle and the aspect ratio between the long
480 and short axis (Fig. 10f; Mezger, 2010). The correlation between progressive
481 deformation and porphyroblasts rotation can be explained using either the Jeffery
482 (Jeffery, 1922) or the March (March 1932) model. While the Jeffery model proposes
483 that porphyroblasts rotate continuously with respect to velocity gradients, the March
484 model suggests that porphyroblasts follow the deformation of the surrounding matrix
in a passive manner (Ježek et al., 1996). Instead of orbiting through the shear plane as predicted by Jeffery, most porphyroblasts rotate towards the shear plane and remain in that position, regardless of their aspect ratios and initial orientations (Ježek et al., 1996). This is consistent with our observation that most staurolite porphyroblasts with high aspect ratios tend to align with their long axis close to the schistosity plane (Fig. 10f; Supplementary Table 1). This aligns with analogue and numerical models proposed for the particles with an axial ratio higher than 2 (e.g., Passchier, 1987; Arbaret et al., 2001; Passchier and Trouw, 2005). Therefore, the behavior of most studied porphyroblasts can be considered as “March” type.

494

[Fig. 11 here]

496

5.3 Foliation intersection axis constrained by EBSD data

The YZ sections of D2 and D3 deformation were used to determine the Foliation Intersection Axis (FIA) for the three above-described representative samples (18CHE17-2, 18CHE23-1, and 18CHE20-1 in Figs. 9a-c). FIAs were determined as misorientation axes reflecting rotation of Si in staurolite porphyroblasts, i.e. of relict S1 foliation-lineation pairs revealed by quartz inclusion trails. The orientation of S1 foliation was reconstructed using the trace of inclusion trails and Y axis of the D1 deformation ellipsoid (Figs. 11a-c) determined from quartz crystallographic preferred orientation (CPO). Such determination of the Y axis is possible upon the assumption that quartz grains in the inclusion trails formed by dynamic recrystallization and
507 activation of prismatic <a> slip that is typical for P-T conditions of staurolite
508 formation (e.g., Stipp et al., 2002).

509 EBSD mapping covering several staurolite grains in each sample (red rectangles
510 in Fig. 9) was applied to obtain quartz CPOs from staurolite inclusion trails. The
511 EBSD data were collected at the Institute of Petrology and Structural Geology in
512 Prague and processed and plotted using the MTEX MATLAB™ toolbox (Hielscher
513 and Schaeben, 2008). The observed c-axis CPOs typically show point maxima that
514 coincide with the orientation trend of S1 inclusion trails (cf. EBSD maps and pole
515 figures in Figs. 11a-c). The S1 foliation plane in each staurolite (Figs. 9a-c) was then
516 constructed using the trace of inclusion trails and the eigenvector of the largest
517 eigenvalue of the quartz c-axis distribution (great circles in Fig. 11a-c). The S1
518 foliation and eigenvector define the coordinate system attached to each staurolite
519 grain. It should be noted that both S1 foliation and eigenvector are axial features, so
520 there are two equivalent vector representations. Therefore, 4 possible coordinate
521 systems could be defined for each grain.

522 The resulting FIAs were determined as rotation axes with minimum rotation
523 angles between the coordinate systems attached to individual staurolite grain in the
524 sample. The FIAs plotted with respect to the YZ sections of D3 for the three analyzed
525 samples showed relatively low scatter and point distribution in Sample 18CHE17-2,
526 girdle distribution in Sample 18CHE23-1 and relatively large scatter in Sample
527 18CHE20-1 (Fig. 11a-c). As discussed later, such distributions are compatible with
528 weak D3 deformation affecting Sample 18CHE17-2 and strong D3 affecting Samples
529 18CHE23-1 and 18CHE20-1, respectively (Fig. 9).

530

531 [Fig. 12 here]

532

533 5.4 Deformation temperatures deduced from quartz c-axis patterns for S2 and S3 fabrics

535 CPO of recrystallized quartz in matrix of the S2 and S3 fabrics was analyzed in
536 order to compare their deformational conditions. The EBSD maps covering S2 matrix
537 in Sample 18CHE23-1 and S3 matrix in Sample 18CHE20-1 have been produced
538 (blue rectangles in Fig. 9). The obtained quartz CPOs from the S2 and S3 matrix were
539 plotted separately with respect to the XZ plane (Fig. 12). As shown in Fig. 12a, the
540 quartz CPOs of the S2 matrix exhibited a concentration of c-axes at the center of the
541 pole figure (close to Y-axis), while the maximum distribution of a-axes was
542 concentrated at the periphery of the pole figure (close to X-axis). In contrast, the
543 quartz CPOs of the S3 matrix displayed the maximum concentration of c-axes (close
544 to X-axis) and the maximum distribution of poles to m-planes close to Z axis (Fig.
545 12b). Such CPO patterns are indicative of prismatic <a> slip activity in the S2 matrix
546 and prismatic <c> slip activity in the S3 matrix (e.g. Schmid and Casey, 1986). The
547 activity of prismatic <a> and <c> slips in quartz are typically corresponding to the D2
548 amphibolite and D3 granulite facies conditions (e.g., Law, 2014). The P-T conditions
549 of this study are in agreement with the stability of staurolite during the D2
550 deformational phase and sillimanite during the D3 deformational phase, respectively.
551

552 [Fig. 13 here]

553

554 **6. Discussion**

555 **6.1 D1 and D2 deformational history of the Chonghuer area**

556 The studied section recorded three deformational and metamorphic events that can
557 be correlated with those recently described in the Mongolian and Chinese Altai (Jiang
558 et al., 2019; Soejono et al., 2021; Kong et al., 2022). The earliest metamorphic fabric
559 was determined only in the Habahe group metasediments and is represented by a
560 schistosity folded by recumbent isoclinal folds similar in geometry. Both macro- and
561 micro-structural observations show transposition of this early foliation S1 by new
562 sub-horizontal fabrics in higher metamorphic grades (Figs. 3 and 5). The staurolite
563 porphyroblasts overgrew the S1 passively, indicating their possible inter-tectonic
564 growth between D1 and D2 (Fig. 9; straight inclusion trails). However, several
565 staurolite porphyroblasts overgrew gently folded S1 inclusion trails suggesting that
566 they also formed at the onset of D2 folding. In our interpretation, the sub-vertical S1
567 foliation was originated from the horizontal compression during the D1 crustal
568 thickening, which is in alignment with the findings of Soejono et al. (2021). This
569 implies that while the S1 fabric is sub-vertically oriented, the associated Barrovian
570 metamorphic zone is positioned perpendicular to the S1 fabric and is sub-horizontally
571 aligned (Fig. 13a). Such a relationship between deformation and Barrovian-type
572 metamorphic zonation is a prevalent phenomenon in the thickening of the upper plate,
573 which is different from the lower plate metamorphic scenario of the Himalayan-type
574 orogens (Peřestý et al., 2016).

575 The D2 was associated with recumbent folding and the development of sub-
576 horizontal S2 fabric that superimposed the S1 sub-vertical fabric (Fig. 13b). The onset
577 of this event is also recorded by the overgrowth of folded D1 in some staurolite
578 porphyroblasts (Figs. 3a, b). The development of the horizontal S2 fabric and
579 recumbent F2 folds is most likely attributed to the horizontal flow of the lower crust,
580 which were also presented in the adjacent Tarlang massif (Zhang et al., 2015) and
581 Mongolian Altai (Hanžl et al., 2016). Micro-structural observations revealed that the
582 S2 horizontal fabric was associated with the growth of sillimanite that replaced either
583 staurolite (this study), or cordierite and andalusite (e.g., Mongolian Altai). However,
584 order of Barrovian metamorphic zones did not change i.e., they still indicated an
585 increase of metamorphic grade with the depth (Fig. 13b). Such occurrences are often
586 interpreted as evidence of crustal extension, as reported by Jiang et al. (2021),
587 Soejono et al. (2021), and Kong et al. (2022). The crustal thinning further led to the
588 formation of extensional core complexes characterized by the placement of
589 migmatite-magmatite core in juxtaposition with the Devonian weakly metamorphosed
590 cover (Fig. 13c; Kong et al., 2022). In summary, the studied Chonghuer section
591 displays solid evidence of both D1 thickening and D2 extensional fabrics, as reflected
592 by the preservation of Barrovian metamorphic zones and superimposed deformation
593 fabrics on both macro- and micro-scales.

594
595 6.2 The origin of syncline structure of the Chonghuer area and the D3 high strain
596 zone

597 Kong et al. (2022) demonstrated that the extensional fabric S2 was refolded by
598 kilometer-scale upright antiforms dominated by migmatite-magmatite lower crust.
599 These antiforms were surrounded by wider synforms that were formed by medium-
600 grade Habahe group rocks and weakly to unmetamorphosed Devonian sediments (Fig.
601 13d). Of particular importance is that this deformation was responsible for the
602 reworking of partially molten lower crust within the core of antiforms, resulting in the
603 new vertical migmatitic fabrics (Fig. 13d). On the other hand, no vertical
604 transposition was observed in the synforms (Jiang et al., 2019; Wang et al., 2021;
605 Kong et al., 2022). The orientations of Late Devonian upright folds were originally N-
606 S and rotated to NW-SE during the Permian orogeny (Fig. 13e; Wang et al., 2021).
607 The latter event was responsible for the massive shortening of the rotated Devonian
608 fabric and extrusion of hot Permian lower crust along tabular NW-SE trending zones,
609 as observed in the Kalasu and Altai regions (Broussolle et al., 2018, 2019). As a
610 weakly superimposed area by the Permian event, the Chonghuer area maintained a
611 gradual transition from the S2 fabric to the D3 transpression zone (Fig. 13e). This
612 transition developed along the edge of the gneissified granite to the north, where the
613 steepened S2 becomes parallel to the D3 fabric (Fig. 13f). It is likely that the synform
614 had already formed during the D2 extension, but it was amplified by the subsequent
615 shortening. However, the timing of the synform's shortening, whether it took place
616 during the late Devonian or Permian events, is crucial but has not been well
617 established in the previous studies.

618 This problem could be solved by structural and geochronological evaluation of
619 deformed pegmatite dykes that intruded the gneissified granite and adjacent kyanite-
620 sillimanite schists in the north of the Chonguer area (Figs. 2, 4). The pegmatite dykes
621 that intruded the gneissified granite are either undeformed and perpendicular to the
gneissosity within the pluton, or they are folded at the southern margin of the pluton
622 (Figs. 2, 4). In contrast, the pegmatite dykes that intruded the D3 transpression zone
623 are parallel to the strike of the S2-3 foliation (Fig. 4). These observations indicate that
624 pegmatite dykes intruded the granitic pluton after its gneissification, whereas at the
625 southern margin of the pluton, the same dykes were either reworked or completely
626 parallelized to the weak kyanite-sillimanite schist S2-3 foliation of the D3 high strain
627 zone (Fig. 13f). Therefore, it suggests that the dykes that intruded the interior of the
628 granite were not affected by the D3 deformation that developed along the southern
629 margin.

630 It is important to note that Wang et al. (2021) and Kong et al. (2022) have obtained
631 a late Devonian age for the pegmatite dykes that are perpendicular to the axis of the
632 migmatite-magmatite domes in the west and northeast Chinese Altai. However, in
633 southeast Chinese Altai, the NNE-SSW trending pegmatite dyke swarms intruded
634 both Devonian migmatite-magmatite domes and adjacent synforms. These pegmatite
635 dyke swarms were dated as Permian by Jiang et al. (2019) and Shu et al. (2022).
636 These authors suggested that the Permian dykes retained their NNE-SSW orientation;
637 while in weak Barrovian schists, they were completely transposed and aligned parallel
638
639 to NW-SE trend of the Permian deformational fabric (Fig. 13f).

640 Based on all of the above observations, it can be inferred that the gneissification of
641 the northern granite in the Chonghuer area occurred either before or during Devonian
642 doming. However, it is still challenging to unequivocally determine whether the D3
643 deformation at the southern edge of the granite developed during the Devonian or
644 Permian periods. It thus requires high-precise geochronological data to constrain this
645 issue.

646

647 *6.3 Geochronological constrains of the two main deformation cycles*

648 The zircon U-Pb dating of the northerly granite pluton yields an age of ca. 392 Ma
649 (Sample 18CHE08-2; Figs. 2, 8c), which can be interpreted as the age of
650 emplacement. In addition, another age cluster of ca. 498 Ma and 794 Ma (Sample
651 18CHE08-2; Fig. 8c) represents the inherited zircons that are characteristic of the
652 partially molten Cambrian-Ordovician Habahe group that were dated at ca. 453 Ma
653 (Samples 17CHR16-1 and 18CHE18-9; Figs. 8a, b). The central part of the granite
654 pluton displays steeply oriented solid-state fabrics, which is comparable to those in
655 the adjacent southern dome (e.g., Tarlang pluton), where these sub-vertical fabrics
656 were intruded by the 370-360 Ma granite sills (Wang et al., 2021), indicating that the
657 doming was formed in the Devonian.

658 On the other hand, some pegmatite dykes (Samples 18CHE19-2 and 18CHE11-2)
659 exhibit U-Pb zircon ages of ca. 253-250 Ma, indicating that they were emplaced in the
660 Permian (Figs. 8d, e). These pegmatite dykes remain weakly deformed or undeformed
in the core of the foliated Devonian granite dome but are intensively folded and parallelized to the S2-3 foliation of the D3 high strain zone (Fig. 13f). It suggests that the D3 high strain zone represents an important ductile deformational event that occurred in Permian but only developed along the southern edge of the Devonian granite dome (Fig. 2a). The S2-3 foliation is characterized by presence of sillimanite that formed the shear bands (Figs. 5g, f). It indicates that the growth of sillimanite corresponds to the significant D3 high-temperature shearing deformation that overprinted the Devonian sub-vertical Barrovian-type fabric, as observed in the east part of Chinese Altai (Wang et al., 2014; Jiang et al., 2019). The monazite U-Pb data from the staurolite schist (Sample 18CHE18-9) indicate the presence of metamorphic age of ca. 257 Ma, which is the same age as that of the pegmatite dykes preserved in the D3 high strain zone (Samples 18CHE19-2; Fig. 8d). In summary, the S1-S2 foliations were formed during the early Paleozoic orogenic cycle when staurolite porphyroblasts started to grow after D1 and at the beginning of D2 (as shown in Figs. 9a, 13a) and were further folded by the subsequent D2 deformation (Figs. 9b, 13d). During the Permian tectonothermal event, the staurolite porphyroblasts experienced rotation within a simple shear zone that mainly developed along the southern border of the Devonian granite (Figs. 13f).

## 6.4. A kinematic and numerical model of FIA evolution

As mentioned above, the D3 shearing deformation intensively overprinted and reworked the schists and pegmatite dykes to make them parallel to the S2-3 foliation
683 (Fig. 4e). Our restoration of orogenic fabrics in the Chonghuer area further indicates
684 that the originally sub-horizontal extensional S2 fabric was possibly steepened already
685 during late Devonian shortening (Fig. 13). However, the deformation effect of this
686 event hardly to be depicted in the field.

687 An important finding of our study is that the quartz textures associated with the
688 Devonian S2 fabric are characterized by a dominant activity of prism <a> slip system
689 (Fig. 12a). This result is consistent with the growth of staurolite porphyroblast at
690 metamorphic temperature exceeding 500°C, as reported by Jiang et al., 2015). By
691 contrast, the quartz textures associated with the D3 Permian event exhibit a dominant
692 activity of prism <c> slip system (Fig. 12b). This result is characteristic for a much
693 higher metamorphic temperature condition that is consistent with the presence of
694 sillimanite growing parallel to the S3 foliation (Fig. 5f). Such a high metamorphic
695 temperature also caused a considerable thermal weakening and reduction in the
696 viscosity of pelitic schists. In conclusion, it is plausible to suggest that the staurolite
697 porphyroblasts could rotate freely within a slowly moving and viscous matrix during
698 the D3 event. This is a necessary requirement for the application of both Jeffery and
699 March models.

700

701 [Fig. 14 here]

702

703 In order to identify the nature of deformation that leads to the re-orientation of the
704 staurolite porphyroblasts, we created a Monte Carlo model of porphyroblast rotation
705 (Fig. 14). The model was specifically designed to reflect an inhomogeneous strain
706 field that developed due to active and passive folding (supplementary materials at
707 https://github.com/ondrolexa/miaoetal). In this study, we utilized a nonlinear
708 deformation gradient to develop folds with a parabolic shape. These folds were
709 parametrized by the bulk flattening and extension along the axial plane. We also
710 created an amplification factor to simulate the nonlinear growth of active folds (Fig.
711 14). In order to model the rotation of the staurolite porphyroblasts that were assigned
712 internal coordinates, we made them randomly distributed along folded surface and
713 then used the March model to monitor the individual rotation of each porphyroblast.
714 For each pair of reoriented staurolites, the mutual rotation axis was calculated as the
715 minimum rotation within their internal coordinate systems (Fig. 14).

716

717 [Fig. 15 here]

718

719 In our modelling, we simulated the distribution of FIAs resulting from D2 and their
720 subsequent redistribution during the D3 event. The FIAs that were originally
721 determined from the inclusion trails of the staurolite porphyroblasts with respect to
722 YZ plane of D3 finite strain ellipsoid (Figs. 11a-c), were firstly reoriented to the
723 geographic reference frame (Fig. 15). In Sample 18CHE17-2, there are not clear
724 signals of intense D3 overprint, and thus the data revealed a relatively minor scatter
725 and a predominantly N-S orientation of FIAs (Fig. 15a). In contrast, two Samples
726 18CHE20-1 and 18CHE23-1 that located towards northeast in the D3 high strain zone,
demonstrated a high scatter with SSE-plunging and SEE-NWW trending FIAs, respectively (Figs. 15b, c). The simulations of D3 deformation applied to the initial D2-related NNE-SSW trending distribution of FIAs (blue rectangle; Fig. 15d) covered pure shear, transpression and simple shear geometries with shortening axis 217/30, shear plane 37/60 and shear direction 119/14 (Fig. 15e). The modelled distribution of FIAs is similar to the FIAs distribution in Samples 18CHE20-1 for pure shear dominated (green rectangle) less intense D3 and 18CHE23-1 for simple shear dominated transpression (yellow rectangle) of moderate intensity D3, respectively (Fig. 15f). However, we note that the observed distribution of FIAs is not consistent with simple shear dominated D3 (Fig. 15f).

In summary, staurolite inclusion trail FIA analysis performed in the Chonghuer area proves to be a highly effective technique for structural analysis of polycyclic orogenic terrains. Our results revealed that the fabric variations between different deformation and metamorphic zones may have resulted from two independent tectonic cycles. During these tectonic cycles, the growth and rotation of porphyroblasts were essentially controlled by the contrasting kinematic frames. The application of March model on simulating FIA patterns provides additional evidence supporting the validity of FIA analysis introduced by Australian structural geology school (Bell, 2002; Fay et al., 2008; Bell and Fay, 2016). Our simulation results further enable the discrimination among the pure, simple shear and transpression overprints, which provides a more precise constraint for larger scale tectonic models.
749 **7. Conclusions**

750 (1) The Barrovian metamorphic series, which have affected the Ordovician ca.
751 453 Ma metasediments, display a progressive increase in metamorphic grade from the
752 biotite zone in the southwest to the kyanite-sillimanite zone in the northeast
753 Chonghuer area. Our macro- and microscopic structural analysis revealed that the
754 sub-horizontal metamorphic S2 fabric was formed through the recumbent F2 that
755 refolded the earlier steep S1 Barrovian metamorphic fabric. The intensity of S2
756 increases towards northeast, reaching the edge of the gneissified granite pluton where
757 it was transposed into the NW-SE trend by the D3 sinistral transpressional
758 deformation.

759 (2) EBSD analysis of the S2 and S3 fabrics revealed that the medium-
760 temperature condition was involved in the D2 deformation, while the D3 deformation
761 was associated with the high-temperature condition. The combination of EBSD and
762 foliation-intersection-axis (FIA) pattern provides compelling evidence for the tectonic
763 regime switch from the initial NE-SW-trending D2 to the NW-SE-trending D3
764 patterns. EBSD based FIA analysis proves to be an effective approach for
765 investigating the polycyclic orogenic systems.

766 (3) New zircon and monazite U-Pb dating results indicated that the regional D2
767 deformation developed in the Middle to Late Devonian (392-370 Ma), while the D3
768 deformation took place in the Late Permian (257-250 Ma). These findings further
769 confirm the significant tectonic regime switch from the Devonian to Permian in the
770 Chonghuer area and a wider area of the Chinese Altai.
771

772 Acknowledgment

773 We thank the journal editor Prof. Jianhua Li and two anonymous reviewers for their

774 insightful comments, which help us greatly improve the manuscript. This study was

775 financially supported by the National Natural Science Foundation of China

776 [42025204]; Grant Agency of the Czech Republic (GAČR EXPRO grant GX19-

777 27682X); Guangdong Province Introduced Innovative R&D Team of Geological

778 Processes and Natural Disasters around the South China Sea [2016ZT06N331], and

779 International Partnership Program of Chinese Academy of Sciences (No.

780 132744KYSB20190039), and the Third Xinjiang Scientific Expedition Program

781 (2022xjkk1301). The supplementary material about simple Monte Carlo model is

782 shown at https://github.com/ondrolexa/miaoetal.

783

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1020

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1023

1024

**Figure captions**

1024

1025 **Fig. 1** (a) Simplified geological map of the Chinese Altai (revised after Broussolle et
al., 2019). (b) Geological map of the Chonghuer area.
1026

1027

1028 **Fig. 2.** (a) Geological map for the Barrovian-type domain showing the main structural
features and metamorphic zones. (b) Interpretative cross-section for the Barrovian-
type domain showing three generations of deformation fabrics and their overprinting
relations.
1029
1030
1031

1032

1033 **Fig. 3** (a and b) S1 preserved within the biotite zone, which was folded by recumbent
close to isoclinal folds F2. (c) S2 is locally reworked by open to tight upright folds F3
1034
1035 and is sometimes associated with development of crenulation cleavage S3. (d)
1036 Penetrability S2 foliation preferred NE-dipping orientation at moderate to high angles.
1037 (e) D3 high strain zone reworking regional S2 foliation as S2-3 foliation in the kyanite
1038 and sillimanite zones. (f) Granite and the kyanite-sillimanite schist sheets were
1039 striking parallel to the regional foliation.
1040

1040

1041 **Fig. 4** (a, b, c and d) Satellite images allow to visualize these relationships and show
that pegmatite dykes intruding the granite and the kyanite-sillimanite zones are
1042
perpendicular to each other. The location of the satellite images is shown as the black
rectangles in Fig. 2a. (e) The pegmatite dykes intruding the kyanite-sillimanite schists
are entirely parallel to host rock schistosity. (f) The pegmatite dykes crosscut the pre-
existing foliation and granite at a high angle without internal deformation.

1047

1048 **Fig. 5** Field photomicrographs. Kangbutiebao formation: (a) The metamorphic
foliation is represented by a crenulation cleavage S2 reworking the S0 bedding.
1049
1050 Habahe group: (b) The S1 is intensely folded by F2 crenulations in the biotite zone. (c)
1051 Recrystallized or deflected muscovite forms a new cleavage S2 that is oriented at a
1052 high angle to S1 in the garnet zone. (d) The S1 is often completely transposed by S2
1053 in the staurolite zone. (e) The S1 is either parallel or oblique to S2 fabric in matrix in
1054 the kyanite zone. (f) External foliation is deflected towards shear bands associated
1055 with sillimanite and assigned as S2-3 foliation in the sillimanite zone. Granite: (g)
1056 Mylonitic penetrative S2-3 foliation defined by preferred orientation of mica-rich
1057 layers and the quartz-feldspar aggregates. (h) The matrix shows equi-granular grains
1058 of plagioclase, quartz and k-feldspa.

1059

1060 **Fig. 6** Representative photos of the selected samples for U-Pb zircon and monazite
dating.

1062

1063 **Fig. 7** The CL images of zircons and monazites from samples.

1064
1065 **Fig. 8** Concordia diagrams of LA-ICP-MS U-Pb zircon and monazite analytical
1066 results and graphic showing the relationship between Th/U ratio and age.

1067
1068 **Fig. 9** Deformation characteristic photomicrographs of the staurolite schists (a-c).
1069 Sample location is shown in the cross section (Fig. 2b). Measurements of foliation
1070 and lineation are included in the figure. The red rectangles indicating location of
1071 EBSD data are present in Fig. 11 and the blue rectangles are present in Fig. 12.

1072
1073 **Fig. 10** Macro-characteristic photographs of the staurolite schists in the high strain
1074 zone (a-d). (e) The parameter for state the rotation character of the staurolite
1075 porphyroblast. (f) The diagram shows intersecting angle α between Se and Si foliation
1076 is mainly connected with the ratio of Ll: Ls.

1077
1078 **Fig. 11** The foliation intersection axes FIAs derived from quartz inclusion trails in
1079 staurolite and plotted with respect to the YZ sections (equal area, lower hemisphere).
1080 Internal foliation Si corresponds to relics of S1 determined by quartz c-axes CPO
1081 maximum and the trace of inclusion trails in the staurolite porphyroblasts. Location of
1082 EBSD maps is shown in the thin section scans (red rectangles in Fig. 9). The colored
1083 maps show quartz grains reconstructed from EBSD maps; the colors of individual
1084 grains follow the inverse pole figure scheme in the inset where quartz crystallographic
1085 orientation is colored with respect to the axis perpendicular to the analyzed thin
1086 section (e.g., red grains have their c-axis perpendicular to the thin section). The quartz
1087 c-axis pole figures (equal area, upper hemisphere) (a-c) show CPOs with multiples of
1088 uniform distribution of the orientation distribution function calculated by Fourier
1089 method with de la Vallée Poussin kernel and a half width of 10°. The orientation
1090 distribution function is based on one orientation per one grain data (grain fabric). CPO
1091 maxima (black squares in c-axis pole figures) correspond to orientation of the
1092 eigenvector associated with the highest eigenvalue calculated from the
1093 crystallographic orientation data. (d-f) show the reconstructed FIAs with respect to the
1094 analyzed thin sections.

1095

1096 **Fig. 12** Maps of quartz grains reconstructed from EBSD maps. The location of
1097 analyzed EBSD maps is indicated by blue rectangles in Fig. 9. The colors of
1098 individual quartz grains follow the inverse pole figure scheme in the inset where
1099 quartz crystallographic orientation is colored with respect to the axis perpendicular to
1100 the analyzed thin section (e.g., red grains have their c-axis perpendicular to the thin
1101 section). Quartz c-axis fabric pole figures (upper hemisphere, equal area) for quartz
1102 bearing matrix from staurolite schist plotted with respect to XZ section. The
1103 determination of CPO followed the same procedure described in caption of Fig. 11.

1104

1105 **Fig. 13** Tectono-metamorphic evolutionary cartoon of the Chonghuer area partly
1106 based on tectonic, geochronology and metamorphic studies of Wang et al. (2021),
1107 Kong et al. (2022) and Jiang et al. (2015) from adjacent regions. (a) Horizontal
1108 Barrovian-type metamorphic zones developed in vertical S1 fabric. (b) Transposition
1109 of S1 metamorphic fabric by D2 recumbent folding. Note that the order of
1110 metamorphic zones remains unchanged. Migmatite zone in the depth and Devonian
1111 basin at the top of sequence indicated. (c) Scheme of mid-Devonian extensional D2
1112 core complexes evolution after Kong et al. (2022). (d) Amplification of extensional
1113 core complexes during late Devonian shortening – might be absent in the studied
1114 region. (e and f) Permian rotation and sinistral transpression zone developed at the
1115 southern border of the Devonian granite in the D3 event.

1116

1117 **Fig. 14** Principles of the method to obtain FIA axes of rotated staurolite
1118 porphyroblasts. (a) Growth of poikilitic staurolites with enclosed quartz fabric. The
1119 local coordinate system is defined according to orientation of foliation and single c-
1120 axes maxima. (b) Calculation of rotation matrix Rm representing minimum rotation
1121 between two local coordinate systems. FIA is calculated as rotation axis defined by
1122 rotation matrix Rm

1123

1124 **Fig. 15** Modelling of superposed deformation D3 based on distribution of FIAs: (a-c)
1125 FIAs from Figure 11 rotated into geographic reference frame, (d) initial distribution of
1126 FIAs related to D2, (e) D3 reference frame used in the modelling (xyz – principal
1127 stretching axes, shear plane and direction), (f) modelling of FIA distribution after D3
1128 deformation in pure shear, transpression and simple shear geometries. Density
1129 contours are multiples of uniform distribution.

1130
<figure><img src="image_2.png" /><figcaption>Fig. 1_R1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2_R1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4_R1</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6</figcaption></figure>
<table><caption>Fig. 7</caption><thead><tr><th></th><th>Zircon (17CHR16-1)</th><th>Monazite (18CHE18-9)</th></tr></thead><tbody><tr><td>(a) 17CHR16-1<br>18CHE18-9<br>Staurolite schist<br>100 µm</td><td>453±5<br>452±5<br>458±5</td><td>257±2<br>255±2<br>261±2</td></tr><tr><td>(b) 18CHE08-2<br>Gneissic granite<br>100 µm</td><td>784±6<br>798±10<br>504±4</td><td>489±7<br>391±4<br>396±3</td></tr><tr><td>(c) 18CHE19-2<br>Granitic pegmatite<br>100 µm</td><td>419±4<br>414±4<br>407±4</td><td>308±4<br>257±2<br>246±3</td></tr><tr><td>(d) 18CHE11-2<br>Granitic pegmatite<br>100 µm</td><td>415±5<br>405±4<br>406±4</td><td>253±3<br>250±2<br>247±2</td></tr></tbody></table>
<figure><img src="image_8.png" /><figcaption>Fig. 8_R1</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 9_R1</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 10_R1</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 11_R1</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 12_R1</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 13_R2</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 14</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 15</figcaption></figure>