Rotation and Uplift of the Taoxi Dome and Its Implication for the Evolution of Wuyi Terranes in Cathaysia Block

: The Taoxi dome stands out as a rotated-circular structure on maps of the Wuyi terrane, south China. The origin and tectonic activity analysis of the Taoxi dome is a window to understand the tectonic framework and aggregation process of the Wuyi terranes in the Cathaysia block. Our field observations demonstrated that two major phases of deformation, counterclockwise rotation and dome uplift, were recorded in the Taoxi dome. Geochronological data indicated that the counterclockwise rotation was initiated at about 422 Ma as a result of a sinistral slide collision between the North Wuyi terrane and the South Wuyi terrane along the Nanping – Ninghua tectonic belt during the Caledonian. Zircon dating ages of the intrusions developed in the Taoxi dome suggested that the Taoxi dome had undergone at least two strong upheavals and the uplift of the Taoxi dome was active during the periods 229 – 222 Ma and 158 – 100 Ma. Geochemical data indicated that the intrusions developed in the Taoxi dome had characteristics of high silica (SiO 2 > 70 wt.%) and belonged to the A2-type granites, which suggest that the Taoxi dome was long governed by a tectonic setting of post-collisional or anorogenic magmatism since the Caledonian. The strong movement of the Caledonian rotation and coeval strike-slip movement in the Wuyi terranes suggests that a slide rotation may have been a major characteristic in response to the rifting process of micro-terranes during the late Paleozoic in the Cathaysia block.


Introduction
Rotation is an important component of describing deformation. Originally, horizontal layered rocks can become inclined strata and can be rotated as a result of faulting and folding processes. There are a number of reports of rotation at the plate and regional tectonic scales [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Tectonic rotation can provide much useful information for the evolution of the terranes. Understanding the rotating processes and related deformation in the geological record is of first-order significance in understanding the tectonic evolution of tectonic belts and the amalgamation of terranes.
The Wuyi terrane, developed as terrane in the Cathaysia block ( Figure 1), was one of the most intensively researched areas to understand the evolution of the Cathaysia block [16]. The Taoxi dome stands out as a rotated-circular structure. The origin and tectonic activity analysis of the Taoxi dome is a window to understand the tectonic framework and aggregation process of the micro-terranes in the Cathaysia block. Detailed investigations and analyses of the well-exposed rotated-circular structure are essential and crucial for understanding the origin of the Taoxi dome. However, the timing and mechanism of the formation of the Taoxi dome remain controversial. The Taoxi dome was suggested as the name that includes the annular structure or circular structure and the widespread development of ring fracture system and related metallogenic belt [17,18]. Previous research suggested that the Taoxi dome was a metamorphic core complex developed mainly during the Indosinian movement in an extensional setting [19,20]. Chen et al. (2010) [21] proposed that the Taoxi dome exhibits characteristics of a metamorphic core complex but mainly uplift as a result of the intrusion of the asthenosphere and magmatism during the Yanshanian. Huang et al. (2018) [22] proposed that the Taoxi dome experienced multistages of uplift from the Caledonian to the Yanshanian for the multi-stage magmatism. In this paper, we present structural, geochronological and geochemical data that show that the Taoxi dome has a counterclockwise rotation, and the rotated-circular structure is associated with the orogenesis within the Wuyi terranes during the Caledonian. Widespread deformation of the Precambrian basement and shear zones developed because of the counterclockwise rotation. Magmatism during the Indosinian and Yanshanian led to the main uplifting of the Taoxi dome, which accounts for the uplifting of the Precambrian basement in the South Wuyi terrane.

Geological Setting
It is generally believed that the Wuyi terranes rifted and moved away from the Cathaysia block on account of mantle magma upwelling since the Neoproterozoic [23][24][25][26][27][28]. The Wuyi terranes are composed of the North Wuyi terrane to the north and the South Wuyi terrane to the south, delineated by the Nanping-Ninghua tectonic belt (NNTB) with an approximately ENE-WSW strike ( Figure 1) [29][30][31][32][33][34]. The Taoxi dome is situated in the South Wuyi terrane and is bounded by the Nanping-Ninghua tectonic belt (NNTB) to the north ( Figure 1). The Wuping-Taoxi shear zone crosscuts the east part of the Taoxi dome in a SW-NE direction [35]. The Nanping-Ninghua tectonic belt was suggested to have been developed in an active continental margin setting with a strong deformation showing a sinistral sense of shear [30,[32][33][34].
The Taoxi dome appears on maps as a metamorphic core complex, consisting of granitic intrusions in the core, surrounded by the circular deformed Precambrian basement of epimetamorphic rocks and Paleozoic to Mesozoic cover sediments.
The granitic intrusions are composed of diachronous magmatic rocks during these orogenic episodes in Jingningian (mainly in the Neoproterozoic), Caledonian (mainly in the Early Paleozoic), Hercynian (mainly in the Late Paleozoic), Indosinian (mainly in the Early Mesozoic) and Yanshanian (mainly in the Late Mesozoic). The Jingningian intrusions are locally developed and deformed in the western part of the Taoxi dome. The Caledonian intrusions have characteristics of widespread deformation with small-scale sporadic distribution in the Taoxi dome. The Hercynian and Indosinian intrusions are relatively concentrated with no deformation at the core of the Taoxi dome. The Yanshanian intrusions have a wide emplacement at the southeast part of the Taoxi dome.
The Precambrian basement is mainly composed of rock associations of low-grade metamorphosed sand-argillaceous flysch [19,22]. These low-grade metamorphosed sandargillaceous rocks are mainly tectonic schist type in terms of deformation, including tectonic schist and tectonic gneiss. Circular ductile-brittle faults are common within the Precambrian basement at the surrounding part of the Taoxi dome.
Paleozoic to Mesozoic cover sediments were distributed around the Taoxi dome, of which the Early Paleozoic strata was suggested to be deposited in a back-arc basin setting [32]. Circular thrust and normal faults are common within the Paleozoic to Mesozoic cover sediments.

Field Observations and Microscopic Features
Our field observations and measurements were performed in the NE domain, the SW domain and the SE domain of the Taoxi dome, mostly in the Precambrian basement. The field characteristics of the Wuping-Taoxi shear zone were also investigated in detail.
Most parts of the Wuping-Taoxi shear zone can be observed in the field. The Wuping-Taoxi shear zone strikes NE at 20°-30°, dips toward the SE or NW at 30°-75° and extends along the eastern domain of the Taoxi dome for a distance of ~100 km, with a width of 2 to 10 km (Figure 1). The northern part of the shear zone developed in the Precambrian lowgrade metamorphosed sand-argillaceous rocks and intersected with the Nanping-Ninghua tectonic belt (NNTB) to the north. The middle part lies between the batholith of the Taoxi dome and the Caledonian intrusions, whereas the southern part extends into the Caledonian intrusions and is locally invaded by the Yanshanian intrusions. The Wuping-Taoxi shear zone is characterized by protomylonite, mylonite and locally ultramylonite in felsic granites, and tectonic schist and tectonic gneiss in the Precambrian low-grade metamorphosed sandargillaceous rocks. Foliations in the shear zone dip SE or NW at 30°-86° and are defined by aligned biotite and muscovite and by a serried mylonitic banding due to the flattening and alternation of quartzose, feldspathic and mica-rich porphyroclasts. Sub-horizontal mineralstretching lineations, defined by linear aggregates of biotite, muscovite and quartz ribbons, are always present (Figures 2-8). Various shear sense indicators, including S-C fabrics, rotated quartz-feldspar porphyroclasts and quartz veins all suggest a dextral sense of shear in the Wuping-Taoxi shear zone (Figures 3 and 8a).  The NE domain of the Taoxi dome is occupied by pelitic-aleuritic metamorphic rocks and are intruded by granitoids and felsic dikes (Figures 1 and 4). The geometric fabrics developed in the NE domain of the Taoxi dome show a NE-dipping shear plane at 14°-60°, and two groups of lineations are found locally on the ductile shear plane, of which one group downdips to the NE and another is NW-SE striking (Figures 2b and 4). The closer to the core of the Taoxi dome, the flatter are the ductile shear planes. The linear fabrics are generally sub-horizontal in the core, whereas they are transposed into parallelism with the shear plane to the edge of the Taoxi dome (Figure 5a

Zircon U-Pb Geochronology
For the purpose of constraining the protolith and deformation age of the Taoxi dome, one sample of metasandstone, one sample of syn-deformed veins, one sample of undeformed granite and one sample of mylonitic granite were collected for zircon U-Pb dating. In order to analyze the effects of magmatism on the Taoxi dome, 14 samples of the intrusive rocks, practically covering the Taoxi dome, were collected for zircon U-Pb dating. The sampling locations are shown on Figure 1, for the sample descriptions see below and dating results are given in Supplementary Table S1.

Analytical Methods
Zircon grains were extracted from the samples using conventional techniques, which include crushing, sieving, heavy liquid separation and hand picking. The LA-ICP-MS U-Pb zircon analyses were performed at Hefei University of Technology, Hefei, China. Zircon grains were mounted on the surface of epoxy resin and were polished down to expose their centers. The internal structures of the zircons were revealed by cathodoluminescence (CL) imaging, which were taken by the Langfang Yuneng Rock Mineral Sorting Technology Service Co. LTD, Langfang, China. The ICP-MS used was Agilent 7500a and the Laser ablation system was ComPex102 ArF, manufactured by Coherent Inc., Mass, Waltham, America. The laser beam spot diameter for zircon analysis was 32 μm, with a repetition frequency of 6 Hz. Nist610 glass was used as the trace elements external standard. Standard zircon 91500 was used as an external standard for isotope fractionation correction. More details on the analytical procedure were described in Yan et al. (2012) [37]. The data were processed using ICPMSDataCal 18.0 (v7.8) [38,39]. Correction for common Pb followed the method of Andersen (2002) [40]. Concordia diagrams were produced using ISOPLOT (ver.4.15) [41]. The ages used in the following analyses were mostly between 90% and 100% concordant (Supplementary Table S1).

Geochronological Results
Sample 20LH-YS was an epimetamorphic sedimentary rock, interpreted to be a metasandstone. Zircons from the sample were mostly rounded-prismatic in shape, with grain sizes ranging from 50 to 100 μm. Some of the zircon grains showed well-developed oscillatory-zoned relict cores and oscillatory-zoned rims, whereas others showed lightgrey planer-zoned mantles and dark homogeneous rims with oscillatory and sector zonation in the CL images (Figure 9a). Sixty LA-ICP MS spot analyses on these zircon grains were performed. The dating results showed that most zircon analyses were on or near the concordia curve (Figure 9a). The zircon spots had 206 Pb/ 238 U ages from 469 ± 14 Ma to 821 ± 23 Ma, with variable Th/U ratios from 0.22 to 2.14. The age distribution spectrum gave a single peak age at 731 Ma ( Figure 9b). Sample 20LH-MS was a K-feldspar vein, interpreted to be a syn-deformed vein crosscutting the epimetamorphic sedimentary rocks as seen in Figure 5e. Zircons from the sample were mostly long and prismatic with an irregular shape, with the grain size varying from 100 to 200 μm ( Figure 9c). The zircon grains showed well-developed bright oscillatory-zoned relict cores and dark oscillatory-sector zonation rims in the CL images. Thirty LA-ICP MS U-Pb analyses were carried out on this sample, mostly on the rims and partially on the cores. The age data can be grouped into two populations, of which most were plotted on or near the Concordia (Figure 9d). The oscillatory-zoned relict cores gave old 206 Pb/ 238 U ages older than 450 Ma with a relatively high Th/U ratio (>0.2), which are interpreted as the inherited zircon ages. The remaining analyses on the sector-zoned rims gave a range of 206 Pb/ 238 U ages from 337 ± 9 Ma to 444 ± 12 Ma with low Th/U ratios from 0.01 to 0.08, and most concordant analyses defined a weighted mean 206 Pb/ 238 U age of 422 ± 11 Ma (MSWD = 1.4) (Figure 9d). Based on the internal structure and the Th/U ratios of the zircon grains, we interpreted the age of 422 ± 11 Ma as the metamorphic-deformation age of the syn-deformed vein.
Sample CT20-ZXL-18 was an undeformed granite, crosscut by the Wuping-Taoxi shear zone (Figure 1). Zircons from the sample were mostly long prismatic with a euhedral-subhedral shape, with the grain size varying from 100 to 300 μm. Their CL images showed oscillatory-zoned cores and oscillatory-zoned rims with a dark CL luminosity (Figure 10a). Thirty spot analyses on thirty zircon grains were performed. Removing the slightly discordant spots, most zircon analyses were on or near the concordant curve (n = 27). Three analyses on the cores of the zircons had Th/U ratios ranging from 0.1 to 0.2 and yielded 206 Pb/ 238 U ages of 567 ± 16 Ma, 592 ± 15 Ma and 1011 ± 25 Ma, which were interpreted as the inherited zircon ages. The remaining analyses on the oscillatory-zoned rims gave a range of 206 Pb/ 238 U ages from 411 ± 11 Ma to 456 ± 12 Ma with Th/U ratios from 0.08 to 0.71, defining a weighted mean 206 Pb/ 238 U age of 432 ± 5 Ma (MSWD = 0.9), which was interpreted as the emplacement age of the undeformed granite (Figure 10b).
Sample LH-1002 was a mylonitic granite developed in the Wuping-Taoxi shear zone (Figure 1). Zircons from the sample were prismatic in shape and were characterized by a large number of euhedral crystals, with the grain size varying from 100 to 200 μm. Most of the zircon grains showed an oscillatory-zoned core and rim in the CL images ( Figure  10c). Twenty-five spot analyses were carried out on this sample, mostly on the rims. Removing one slightly discordant spot, most zircon analyses were on or near the concordia curve (n = 24). Two spots (spots 18 and 21) yielded 206 Pb/ 238 U ages of 495±5 Ma (Th/U ratio = 0.2) and 946±18 Ma (Th/U ratio = 0.15), respectively. The remaining analyses gave a range of 206 Pb/ 238 U ages from 411 ± 7 Ma to 430 ± 6 Ma with variable Th/U ratios from 0.04 to 0.42, defining a weighted mean 206 Pb/ 238 U age of 423±2 Ma (MSWD = 0.75) (Figure 10d), which was interpreted as the emplacement age of the deformed granite. The timing of the magmatic samples, widely developed in the Taoxi dome, were determined from the 206 Pb/ 238 U weighted mean age of the youngest group of the dated zircon grains. The youngest group of zircon grains had an igneous origin in all samples, as indicated by their idiomorphic to subhedral shapes, oscillatory zoning in CL images and high Th/U ratios (>0.2). The weighted mean ages for all the igneous zircons were interpreted to represent the timing of intrusion or the crystallization age for a particular sample. The dated zircon grains also included captured or inherited zircon that gave ages older than the youngest groups (Figures 11-13).   The two undeformed granitic samples, CT20-ZXL-3 (granite) and CT20-ZXL-22 (granite), considered to be Hercynian intrusions (Figure 1) [22], yielded 206 Pb/ 238 U weighted mean ages of 225 ± 3 Ma (Figure 11a) and 228 ± 4 Ma (Figure 11b), respectively. The two ages were interpreted to be intrusion ages. The Caledonian-Hercynian inherited zircon ages of 447 Ma to 290 Ma were also recorded in both of the two samples ( Figure  11).

Analytical Methods
The analysis of major elements and trace elements in the whole rock was completed at the National Research Center of Geoanalysis, China Geological Survey, Beijing, China. The major elements were analyzed with an X-ray fluorescence spectrometer (XRF), PW4400. The content of Fe was measured with an XRF to obtain the total Fe2O3 content, and the titration method was then used to determine the content of FeO. Finally, we obtained the content of Fe2O3 with the calculation formula of Fe2O3 = TFe2O3-FeO × 1.113. The whole-rock trace element content was conducted by plasma mass spectrometry (PE300Q).

Geochemical Results
The whole-rock geochemical data of 5 samples from the Caledonian intrusions, 12 samples from the Indosinian intrusions and 8 samples from the Yanshanian intrusions in the Taoxi dome are given in the Supplementary information (Table S2).

Timing of the Deformation in the Taoxi Dome
Our field observations demonstrated that two major phases of deformation were recorded by the kinematic indicators in the Taoxi dome. Movements along the Wuping-Taoxi shear zone were characterized by a strike-slip component with a dextral sense of shear, and along the NE-E domain of Taoxi dome it was sinistral with a dip-slip component. Along the SW domain of the Taoxi dome, it was sinistral, and along the SE domain of the Taoxi dome, it was dextral with a dip-slip component. Substantial strike-slip movement in the Taoxi dome was inferred to be a counterclockwise rotation (Figure 19a) and the dip-slip movement may have resulted in the uplift of the Taoxi dome. The counterclockwise rotation mainly developed in the basement rocks and the Caledonian intrusive rocks. Therefore, the timing of the counterclockwise rotation occurred after the emplacement age of the basement rocks and the intrusion of the Caledonian batholith. The basement rocks that developed in the Taoxi dome were well dated at sample 20LH-YS ( Figure 9) and gave zircon ages of 469-821 Ma, which showed a peak age at 731 Ma and constrained the provenance age of the basement rocks in the Taoxi dome. An undeformed but faulted granite in the Wuping-Taoxi shear zone, dated at sample CT20-ZXL-18, gave a date of 432 Ma. The above data indicated that the Taoxi dome experienced counterclockwise rotation after 432 Ma. A syn-deformed vein sample gave an age of 422 Ma, and one mylonitic granite sample in the Wuping-Taoxi shear zone gave a date of 423 Ma ( Figure 10). These data suggest an activation of the counterclockwise rotation initiated at about 422 Ma (Figure 19a).
Dip-slip movement and the widespread development of the intrusions in or around the Taoxi dome indicated that the Taoxi dome underwent strong uplift during magmatism. Undeformed granite in the Taoxi dome gave dates of 229-222 Ma and 158-100 Ma. These data suggest that the Taoxi dome underwent at least two strong upheavals and that the uplifting of the Taoxi dome was active during the period 229-222 Ma and 158-100 Ma (Figure 19b,c).

Origin and Tectonic Activity of the Taoxi Dome
The structures in the Taoxi dome showed a conspicuously counterclockwise rotation at about 422 Ma and it was bounded by the Nanping-Ninghua tectonic belt to the north (Figures 1 and 19). The Nanping-Ninghua tectonic belt was suggested to be a suture zone between the North Wuyi terrane and the South Wuyi terrane and was active during Caledonian orogeny times with a sinistral sense of shear, as proposed previously [34,48,49]. Similar geometrics and kinematics of shear zones were widely developed during the Cal-edonian in the Wuyi terranes [50][51][52][53][54]. The kinematic similarities and approximate deformation ages between the counterclockwise rotation of the Taoxi dome and the nearby boundary shear zones suggest their synchronous development and causal association.
High silica granites in general were suggested to possibly be extracted from an intermediate crystal mush into the shallow crust via the melt-crystal segregation process [55,56]. In this study, the Caledonian granites, Indosinian granites and Yanshanian granites that developed in the Taoxi dome showed geochemical characteristics of high silica (SiO2 > 70 wt.%) and belonged to the A2-type granites, with similar geochemical features and isotopic compositions. These features suggest that the high silica A2-type granites that developed in the Taoxi dome were possibly derived from a common magma chamber via melt-crystal segregation in a tectonic setting of post-collisional or anorogenic magmatism during various orogenic events [45].
Combined with our field observations, chronological data and geochemical data, we propose that the Taoxi dome may have originally developed as a result of the strike-slip movement between the North Wuyi terrane and the South Wuyi terrane along the Nanping-Ninghua tectonic belt during the Caledonian (Figure 20a). The sinistral slide movement between the Wuyi terranes was responsible for the widespread rotation and ductile shearing in the Taoxi dome and may give rise to a Caledonian magmatism at the core of the Taoxi dome, as recorded in the magmatic and inherited zircons (Figures 11 and  12).

Implication for the Evolution of Wuyi Terranes
Previous research suggested that the tectonic setting between the South Wuyi terrane and the North Wuyi terrane has been an aborted rift valley since the Neoproterozoic [58][59][60]. However, Jiang Y. et al. (2019, 2020) [30,31] proposed that the South Wuyi terrane and the North Wuyi terrane had not formed a unified Wuyi Block before 713 Ma. Some studies proposed that the collision of the two terranes occurred in the Caledonian [48,49].
The geochemical features of the high silica A2-type granites that developed in the Taoxi dome, including the Caledonian granites, Indosinian granites and Yanshanian granites, indicated that the tectonic evolution of the Taoxi dome was long governed by a tectonic setting of post-collisional or anorogenic magmatism since the Caledonian, and suggest that the South Wuyi terrane and the North Wuyi terrane had accomplished the collision before the Caledonian.   [34] reported that widely Nanhua (756-720 Ma) island arc activity developed along the Nanping-Ninghua tectonic belt between the North Wuyi terrane and the South Wuyi terrane, which suggested that the South Wuyi terrane and the North Wuyi terrane had not collided during the Nanhua period. The detrital zircons in sample 20LH-YS have a peak age of 731 Ma, indicating that the provenance of the basement rocks in the Taoxi dome may have been derived from the Nanhua (756-720 Ma) island arc in the Nanping-Ninghua tectonic belt and that the South Wuyi terrane and the North Wuyi terrane must have been in close proximity to each other at that stage, which is possibly related to the compressional tectonic events during the Neoproterozoic (728-620 Ma) [61]. These features indicated that the aggregation time of the two terranes were constrained to the period between the Nanhua and Caledonian periods.
The counterclockwise rotation of the Taoxi dome indicated that the South Wuyi terrane showed an internal rotation deformation responding to the slide-shearing movement between the North Wuyi terrane and the South Wuyi terrane during the Caledonian, which is consistent with the widespread deformation at 460-420 Ma within the Wuyi terranes [16,53,55,[62][63][64]. The Caledonian deformation in the Wuyi terranes has long been interpreted as a result of the intracontinental orogeny in the Cathaysia block [25,26,53,65,66]. However, Lin et al. (2018) [16] proposed that multi-terrane accretion-collision and large-scale strike-slip motions may be common features in the Cathaysia block, and that large-scale post-collisional rifting possibly occurred at the margins of the Wuyi terranes in the late Paleozoic. The rotation of the Taoxi dome, related to the slide movement between the Wuyi terranes during the Caledonian, was similar to the strike-slip motion of the micro-terranes in the Cathaysia block [16,[67][68][69], indicating that the Caledonian slide-rotated rifting may have been one major deformation style in the tectonic evolution process in the Cathaysia block.

Conclusions
Our structural, geochronological and geochemical data for the Taoxi dome have led us to the following conclusions: (1) Two major phases of deformation were recorded by the geometric and kinematic indicators in the Taoxi dome. They constituted a counterclockwise rotation and dome uplifting of the Taoxi dome. (2) Zircon U-Pb dating of deformed rocks within the Taoxi dome constrained the timing of the counterclockwise rotation of the Taoxi dome to 422 Ma. Those of the intrusive rocks in the Taoxi dome showed features of A2-type granites and indicated that the uplifting of the Taoxi dome was active during the period 229-222 Ma and 158-100 Ma for the widespread magmatism at a post-collisional setting. (3) The Caledonian A2-type granites that developed in the Taoxi dome suggest that the South Wuyi terrane and the North Wuyi terrane had accomplished the collision before the Caledonian. The original development of the Taoxi dome may have been a result of regional strike-slip rifting between the North Wuyi terrane and the South Wuyi terrane along the Nanping-Ninghua tectonic belt during the late Paleozoic (~422 Ma), which suggests that a slide-rotated rifting may have been a major characteristic in the tectonic evolution of the micro-terranes in the Cathaysia block.