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Article

Structural Characteristics of E–W-Trending Shear Belts in the Northeastern Dabie Orogen, China: Evidence for Exhumation of High–Ultrahigh-Pressure Rocks

by
Yongsheng Wang
*,
Xu Zhang
and
Qiao Bai
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(12), 1205; https://doi.org/10.3390/min14121205
Submission received: 5 November 2024 / Revised: 23 November 2024 / Accepted: 24 November 2024 / Published: 26 November 2024
(This article belongs to the Special Issue Geochemistry and Geochronology of High-Grade Metamorphic Rocks)
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Abstract

:
The Dabie–Sulu Orogen hosts the largest area of ultrahigh-pressure (UHP) rocks in the world. There is still significant divergence regarding the exhumation process and mechanism of UHP rocks in the Dabie Orogen, which mainly resulted from the erosion of large volumes of rocks in the Orogen during the post-collisional stage. Based on detailed field investigations, this study discovered the occurrence of E–W-trending sinistral shear belts that developed on the northeastern Dabie Orogen. These shear belts formed under greenschist facies conditions and are characterized by steep foliation and gentle mineral lineation. E–W-trending shear belts developed in HP rocks with metamorphic ages ranging from 227 to 219 Ma and were cut by the older phase of ductile shear belts of the Tan-Lu Fault Zone, indicating that they were formed around 219–197 Ma. Based on a comprehensive analysis of existing data, it can be concluded that E–W-trending shear belts were formed during the exhumation process of HP–UHP rocks. When HP rocks returned to the shallow crust and the lower UHP rocks continued to move, stress concentration occurred in the HP rocks and further resulted in the formation of E–W-trending shear belts. The development of E–W-trending shear belts indicates that HP–UHP rocks had essentially returned to the shallow crust by the Late Triassic, marking the near completion of the exhumation process.

1. Introduction

The Dabie–Sulu Orogen in eastern China is the world’s largest area of ultrahigh-pressure (UHP) rocks and resulted from Triassic subduction of the South China Block (SCB) beneath the North China Block (NCB) [1,2,3]. UHP terranes worldwide can be divided into two types: large terranes that evolved slowly (over 10~30 Ma), and small terranes that evolved quickly (<10 Ma) [4]. The Dabie–Sulu Orogen is a representative of UHP rocks characterized by a large terrane and slow exhumation [4] and has been a key area for the exhumation of UHP rocks. Over the past 30 years, the exhumation of high-pressure (HP)–UHP rocks in the Dabie Orogen has led to several key findings; for example, the syn-collisional exhumation of the orogen was characterized by being multi-plate and multi-stage [5,6,7,8] and mainly occurred in the Late Triassic [9,10,11,12], UHP rocks were initially exposed on the surface in the Early Jurassic [13,14], and large-scale post-collisional uplift and erosion reshaped the tectonic framework of the orogen [15,16].
Previous results indicate that the syn-collisional exhumation may have continued until the Early Jurassic [7] and thus resulted in the formation of the Hefei Basin in the hinterland of the orogen [14,17,18]. However, large volumes of HP–UHP rocks have thus been removed far from the Dabie Orogen, which has led to competing theories about the orogen’s evolutionary history. Wang et al. [19] proposed that the orogen underwent large-scale eastward extrusion and clockwise rotation, while Faure et al. [8] suggested that the orogen was characterized by doming and migmatization in the Early Jurassic. In contrast, Li et al. [20] suggested that a tubular flow occurred from the Tongbai Orogen to the Dabie Orogen. Differing interpretations of the Early Jurassic paleo-stress field suggest it exhibits NW–SE [21] and near-S–N compression [22]. Therefore, research on the tectonic units formed in the Late Triassic is necessary to obtain a reasonable tectonic evolutionary history of the Dabie Orogen.
Based on recent field work, we found E–W-trending shear belts existed in the northeastern Dabie Orogen. These shear belts were developed in the HP eclogite unit and were cut by an older phase of the ductile shear belt of the Tan-Lu Fault Zone, indicating that they were formed in the Late Triassic. Therefore, they are excellent subjects for understanding the late-stage exhumation processes of HP–UHP rocks. In this study, we present new structural and geochronological data for these E–W-trending shear belts, which are helpful in understanding the exhumation history of HP–UHP rocks in the Dabie Orogen.

2. Geological Setting

The Dabie Orogen forms the eastern part of the Qinling–Tongbai–Hong’an–Dabie orogen, central China [1,2,6,23]. It is separated from the Hong’an Orogen to the west by the NE–SW-trending Shangcheng–Macheng Fault (SMF). It is bounded by the Yangtze foreland fold-and-thrust belt to the south and the Hefei Basin to the north, respectively (Figure 1). The Dabie Orogen is divided, from north to south, into the North Huaiyang (NH) unit, North Dabie (ND) unit, HP–UHP eclogite unit, Susong Complex (SC) unit, and Zhangbaling Group [24,25].
The NH unit is located in the northern Dabie Orogen (Figure 1), mainly including the Foziling Group, Luzhenguan Group, and a few Carboniferous strata. The development of the Foziling Group is related to the Tethys oceanic subduction during the early Paleozoic [14,26,27], and the Luzhenguan Group is composed of orthogneisses formed by middle Neoproterozoic magmatic rocks [28]. Both underwent greenschist to low-amphibolite facies metamorphism, which may be related to the initial subduction of the SCB [3,8]. The ND unit consists mainly of migmatites, gray gneisses, plagioclase amphibolites, large volumes of Early Cretaceous granite, and a few mafic–ultramafic intrusions [12,15,16,29,30]. It is surrounded by normal faults such as the Xiaotian–Mozitan shear zone (XMSZ), the Wuhe–Shuihou Fault (WSF), and the SMF and belongs to the central area of the post-collisional uplift in the Dabie Orogen (Figure 1). The XMSZ, between the NH unit and the ND unit, formed as the Triassic continental subduction and exhumation channel was recognized in the Dabie Orogen [3,6,7,8]. The HP–UHP eclogite unit consists of a coesite-bearing UHP eclogite unit in the north and a quartz-bearing HP eclogite unit in the south [5]. Triassic metamorphic ages have been widely reported in HP–UHP eclogites and their gneiss wall rocks [7,10,31]. Previous studies have shown that the joining of the SCB and NCB began during the Permian [8,32], and the subducted SCB underwent HP–UHP metamorphism around 235–225 Ma [7,9,10,31]. Subsequently, the HP–UHP rocks were uplifted to the shallow crust from 218 to 206 Ma, mainly before 210 Ma [11]. The HP–UHP eclogite unit exhibits typical deformation with a top-to-the-NW sense of shear [6,8,33]. The SC unit is composed of metasediments and orthogneisses, which metamorphosed to amphibolite facies during the Triassic continent–continent collision, and their source rocks developed during the middle Neoproterozoic [34]. The Zhangbaling Group is located in the southern Dabie Orogen (Figure 1) and comprises metavolcanic rocks with protolith ages of 800~700 Ma [35].
The Tan-Lu Fault Zone is the eastern boundary of the Dabie Orogen and borders the Lower Yangtze region in the east (Figure 1). The Tan-Lu Fault Zone is the major fault belt within the East Asian continental margin, characterized by NE–SW to NNE–SSW trends and a length of 2400 km [36,37,38]. Regional-scale sinistral strike-slip ductile shear zones developed in HP–UHP rocks in the eastern Dabie Orogen. In the southern Dabie Orogen, the Tan-Lu Fault Zone developed brittle-ductile or brittle faults in the SC unit and Zhangbaling Group [35]. The evolution of the Tan-Lu Fault Zone on the eastern edge of the Dabie Orogen can be divided into two phases: the older phase of shear occurred with 40Ar-39Ar ages ranging from 197 to 182 Ma [37], and the younger phase of shear deformed during the late Mesozoic with a maximum 40Ar-39Ar age of ~150 Ma [38]. The older phase of ductile shear showing a NE–SW-trending is locally preserved within the younger shear belt with a NNE–SSW-trending in the eastern Dabie Orogen [37]. In the late Mesozoic, the East China continental margin began to exhibit an extension setting, and the Tan-Lu Fault Zone transformed from a sinistral ductile shear zone to a normal fault as early as 136 Ma [38]. The normal faulting of the Tan-Lu Fault Zone controlled the development of the Qianshan Basin in the east and conceals part of the sinistral ductile shear zones [38].

3. Structural Features and Sample Descriptions

A narrow area of over 60 km long and about 5 km wide, known as the Tongcheng massif [33], developed mainly between the Tongcheng and Qianshan in the northeastern Dabie Orogen, and the ND unit was separated by a normal fault to the west (Figure 1). The Tongcheng massif consists of HP–UHP rocks, and the transition between the HP and UHP rocks is near the town of Yuantan [33].
NE–SW- to NNE–SSW-trending and E–W-trending ductile shear belts developed in the eastern Dabie Orogen. NE–SW- to NNE–SSW-trending shear belts widely developed in the HP–UHP eclogite unit [33,37,38], while E–W-trending shear belts only exist in the HP eclogite unit between Yuantan and Tongcheng (Figure 1). Gneisses in the Qianshan area dominantly dip at 20°~30° to the SE, S, and SW (Figure 1), and some exhibit gentle dipping with an angle of ~10° (Figure 2a). Ductile shear belts of the Tan-Lu Fault Zone are characterized by the widespread development of mylonite and ultramylonite [37,38], with steep mylonitic foliation and gentle mineral lineation (Figure 1d). The older phase of the NE–SW-trending ductile shear belt dominantly strikes N 50°~60° E, in contrast to a dominant strike of N 30°~45° E for the younger phase of the NNE–SSW-trending ductile shear belt.
E–W-trending ductile shear belts are preserved among NE–SW- to NNE–SSW-trending ductile shear belts, densely in the north and sparsely in the south (Figure 1c). The width of a single shear zone varies from a few meters to several tens of meters, which dominantly strikes N 85°~100° E (Figure 2b–f). These E–W-trending ductile shear belts are characterized by mylonite and ultramylonite, including chlorite-bearing ultramylonite (Figure 2b,e,f), granitic mylonite (Figure 2c), and marble mylonite (Figure 2d). The mylonitic foliation in the belts dips steeply and mineral stretching lineation plunges shallowly, with a dip angle ranging from 10° to 20°. In the Tongcheng area, the lineation plunges ~20° W. This plunge becomes shallower progressively southwards until horizontal or sub-horizontal and becomes ~5° E near the Yuantan area. Outcrop-scale kinematic indicators such as S-C fabrics and tailing of feldspar porphyroclasts all show a sinistral shear sense.
The mylonite within the E–W-trending shear belt can be divided into two main types: granitic mylonite and chlorite-bearing mylonite. The granite mylonite is mainly composed of quartz and plagioclase, and some rocks contain a large amount of K-feldspar, indicating that their protolith may be granitic gneisses or monzogranitic gneisses. The mineral grain of the chlorite-bearing mylonite is significantly fine, with a dark green color, indicating that their protolith should be basic gneisses. Along the striking direction of the shear belt, due to the varied rock types, these two types of mylonites are distributed in a cross pattern. In addition, a small amount of marble mylonite also developed within the shear belt (Figure 2).
Based on detailed microscopic observations, all of the feldspar in these rocks are broken, with grain sizes mostly less than 500 μm. The quartz is fine-grained due to dynamic recrystallization, with a grain size of 20~30 μm in general (Figure 3a,b), indicating that the rocks are mainly mylonite and ultramylonite. Mylonites are mainly composed of K-feldspar and/or plagioclase, quartz, and chlorite (Table 1); and a few rocks include a small amount of epidote (Figure 3c). Oriented broken feldspar and recrystallized quartz define the mylonitic foliation (Figure 3). Abundant chlorites are present in the ultramylonite, which are oriented along the mylonitic foliation (Figure 3c,d). Marble mylonite is mainly composed of calcite, with clear e-twinning (Figure 3f). The shear indicators, such as pressure shadow fabrics and S-C fabrics, all show a top-to-the-NW sense of shear.
Although almost all quartz grains are fine due to dynamic recrystallization, there are still a small number of larger quartz grains in rocks (Figure 3a,b). Most quartz samples in the mylonites exhibit irregular grain boundaries, with a few having straight grain boundaries (Figure 3b), which suggests that the recrystallization type of quartz is mainly bulging (BLG), with some subgrain rotation (SR) recrystallization. All of them indicate that these mylonite samples deformed under temperature conditions of 380–420 °C [39,40,41]. All samples contain newborn chlorite more or less along the mylonitic foliation (Figure 3c,d), indicating that the deformation condition of these rocks is greenschist facies. Combined with the generation of newborn chlorite and the quartz recrystallization of coexisting of BLG and SR, it can be inferred that the deformation temperature of the rock is probably ~400 °C (Table 1).

4. Quartz Crystallographic Preferred Orientation

Six mylonite samples were selected for quartz crystallographic preferred orientation (CPO) analysis. Oriented thin sections were cut perpendicular to the mylonitic foliation and parallel to the lineation (XZ plane in the finite strain ellipsoid) in the specimens. CPO data were collected using an Oxford NordlysMax2 electron backscatter diffraction (EBSD) instrument based on a FEI FEG-650 SEM at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, with a work step size of 15 μm; the analytical procedure was described by Zhang et al. [42]. Over 300 grains were analyzed from each sample. The quartz CPO data, obtained using lower-hemisphere, equal-area projection, are shown in Figure 4.
The quartz CPO in these samples mainly exhibits a monoclinic symmetry (Figure 4), reflecting the characteristics of plane strain. Monoclinic symmetry occurs generally in non-coaxial deformation and thus indicates that simple shear deformation dominates. The deflection of point maximum relative to the XY plane can indicate the shear direction [40]. Based on the quartz CPO data, the point maxima in all six samples significantly deviated to the left, indicating a top-to-the-W sense of shear.
The slip system of quartz mainly includes bottom, prism, and cylindrical sliding systems. Different sliding systems play a dominant role at different temperatures and exhibit different quartz c-axis fabric patterns [40,41]. In this study, the point maxima of samples NF37 and NF40 are distributed near the margin of the large circle, and NF37 shows an I-type girdle (Figure 4), indicating bottom <a> sliding dominance and thus a deformation temperature of ~400 °C [40,41]. The point maxima of samples NF41, NF41-2, NF58, and NF58-3 is mainly located in the middle of the large circle and closer to the large circle (Figure 4), indicating that both bottom and cylindrical <a> sliding occurred and thus a deformation temperature of slightly higher than 400 °C [40,41]. The point maximum of sample NF41-2 is partly located near the XY plane, which suggests that cylindrical <a> sliding dominates and may have resulted from early-stage high-temperature deformation during the exhumation of HP–UHP rocks. In summary, the quartz c-axis fabrics of the six samples indicate that deformation occurred at a temperature of ~400 °C.

5. Zircon U–Pb Dating and Results

5.1. Analytical Methods

Zircons were separated via crushing and standard magnetic and heavy liquid separation at the Langfang Chengxin Geological Service Co., Ltd., Langfang, China and then handpicked under a binocular microscope. Then, 200 zircon grains from each sample were mounted on adhesive tape, enclosed in epoxy resin, and polished to around half of their original thickness. These zircons (Figure 5) were studied and imaged under cathodoluminescence (CL) using a JXA-8100 electron microprobe (JEOL Instruments, Tokyo, Japan) equipped with a CL3 + CL detector (Gatan, Pleasanton, CA, USA). Based on these images, the highest quality zircons were then selected for U–Pb dating.
High-resolution U–Th–Pb data were obtained for the zircons using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) at the Geological Laboratory of Hefei University of Technology, Hefei, China. The analyses were conducted using an Agilent 7500a ICP–MS (Agilent Technologies, Santa Clara, CA, USA) equipped with a 193 nm Geo-Las2005 laser. The analyses were performed using a laser beam with a diameter of 30 μm and a repetition rate of 7 Hz. Data acquisition for each analysis took 80 s (40 s on background; 40 s on signal). A standard zircon 91500 with a NIST SRM 610 silicate glass standard was used as an internal standard for calibration. Standard zircon GJ-1 and the Plešovice standard were used as the secondary standards to correct mass discrimination in the mass spectrometer and account for any elemental fractionation. The methodology employed here was the same as that described by Yan et al. [43], with uncertainties for individual analyses quoted at the 1σ level. Common Pb corrections were performed using the approach outlined by Andersen [44]. The resulting data were calculated using the ICP-MS-Data Cal 8.0 and Isoplot 3.14 [45] software packages and are shown in Table S1.

5.2. Analytical Results

In this study, six samples (NF38-1, NF38-3, NF41-1, NF41-4, NF43-3, NF58-2) were collected to perform zircon LA-ICP-MS U-Pb dating. Samples NF38-1 and NF38-3 are granitic mylonite collected from the same shear zone of NF37. NF41-1 is granitic mylonite and NF41-4 is chlorite-bearing ultramylonite, collected from the same shear zone as NF41 and NF41-2, respectively. Sample NF43-3 is a chlorite-bearing ultramylonite from the same shear zone as NF41-4. Sample NF58-2 is taken from the same rock as NF58-3. The petrological characteristics and mineral assemblages of these samples are similar to those of the samples used for EBSD analysis.
Zircons in granitic mylonites NF38-1, NF38-3, and NF41-1 were prisms in the size range of 100~200 μm (Figure 5). In the CL images, zircons in NF38-1 and NF38-3 exhibit core–rim textures, while NF41-1 does not exhibit zircon rims (Figure 5). The zircon cores of NF38-1, NF38-3, and NF41-1 are complete in shape and exhibit clear oscillatory zoning. Their high Th/U ratios of >0.1 (Table S1) indicate that these zircons are magmatic [46]. The rim of NF41-1 exhibits a planar texture and its low Th/U ratios of <0.1 (Table S1) indicate that these zircons are metamorphic [46]. Correspondingly, zircons in samples NF41-4, NF43-3, and NF58 appear as abraded and rounded shapes with grain sizes mostly around 100 μm. In the CL images, all three samples exhibit core–mantle textures with small relict cores or planar textures (Figure 5). The zircon core is significantly smaller and exhibits clear oscillatory zoning, with high Th/U ratios of >0.1 (Table S1) indicating that these zircons are magmatic [46]. The mantle exhibits planar textures, with low Th/U ratios of <0.1 (Table S1), indicating a metamorphic origin [46].
More than 20 ages with concordance values of 90%–100% were obtained from each sample (Table S1, Figure 6). The 206Pb/238U ages of sample NF38-1 (29 measurements) ranged from 240 Ma to 877 Ma, and clustered Neoproterozoic ages yielded a weighted mean age of 791 ± 41 Ma (MSWD = 0.5). The 206Pb/238U ages of sample NF38-3 (28 measurements) ranged from 212 Ma to 823 Ma, and clustered Neoproterozoic and Triassic ages yielded weighted mean ages of 775 ± 41 Ma (MSWD = 0.4) and 220.9 ± 6.6 Ma (MSWD = 1.9), respectively. The 206Pb/238U ages of sample NF41-1 (20 measurements) ranged from 567 Ma to 842 Ma, and clustered Neoproterozoic ages between 700 Ma and 900 Ma yielded a weighted mean age of 765 ± 23 Ma (MSWD = 2.1). The 206Pb/238U ages of sample NF41-4 (29 measurements) ranged from 212 Ma to 762 Ma, and clustered Neoproterozoic and Triassic ages yielded weighted mean ages of 747 ± 16 Ma (MSWD = 1.1) and 218.9 ± 2.7 (MSWD = 1.0), respectively. The 206Pb/238U ages of sample NF43-3 (39 measurements) ranged from 213 Ma to 251 Ma, yielding a weighted mean age of 224.1 ± 2.3 Ma (MSWD = 1.2). The 206Pb/238U ages of sample NF58-2 (28 measurements) ranged from 215 Ma to 726 Ma, and clustered Triassic ages yielded a weighted mean age of 226.5 ± 2.7 Ma (MSWD = 0.9). These Neoproterozoic ages are interpreted as the emplacement times of the gneiss, whereas the Triassic ages are regarded as metamorphic ages during syn-collisional exhumation. It is worth noting that four concentrated early Paleozoic ages were also measured in sample NF38-3, from 449 Ma to 494 Ma (Table S1), which is probably related to the Caledonian movement that occurred in the SCB.

6. Discussion

6.1. Development of E–W-Trending Sinistral Shear Zones

Large-scale dome-shaped uplift and extension occurred in the Dabie Orogen during the post-collisional stage, which rebuilt the tectonic framework of the orogen [15,16,47]. The question then arises of whether these E–W-trending ductile shear belts on the eastern Dabie Orogen were formed in situ or through dome-shaped uplift. Based on field observations, these E–W-trending ductile shear belts are exposed close to the NE–SW-trending ductile shear belts of the Tan-Lu Fault Zone. The Tan-Lu Fault Zone on the eastern margin of the Dabie Orogen exhibits typical characteristics of a strike-slip shear belt [37,38] and has not been affected by the post-collisional uplift. This indicates that the E–W-trending shear belts on the northeastern Dabie Orogen should have maintained their original characteristics and should not have been rebuilt by the post-collisional dome-shaped uplift. Moreover, these E–W-trending ductile shear belts terminate to the west at the boundary between the ND unit and the HP eclogite unit, indicating that their formation was not related to the post-collisional uplift.
These E–W-trending ductile shear belts were cut by the NE–SW-striking Tan-Lu Fault Zone, suggesting that they were formed earlier than the Tan-Lu Fault Zone. 40Ar-39Ar ages between 197 and 182 Ma were obtained from the older phase of shear belt of the Tan-Lu Fault Zone on the eastern margin of the Dabie Orogen [37], which were interpreted as cooling ages and further indicates that the older shear belt of the Tan-Lu Fault Zone should have developed earlier than 197 Ma. Therefore, the formation time of these E–W-trending ductile shear belts should be earlier than ~197 Ma. These E–W-trending ductile shear belts developed in the rocks of the HP eclogite unit which underwent typical Triassic retrograde metamorphism [7,10,11,31]. The dating results of this study also show that these mylonites exhibited metamorphic ages of 227~219 Ma (Figure 6), indicating that these E–W-trending ductile shear belts formed no earlier than 219 Ma.
It is worth noting that the quartz grain size in these mylonites is significantly smaller, and that all feldspar samples exhibit micro-fracturing, indicating a relatively low deformation temperature of the rock [39,40,41]. Therefore, these E–W-trending ductile shear belts should develop in the upper crust, and their deformation temperature of ~400 °C does not meet the temperature conditions required for zircon growth [48]. Ages of 227~219 Ma do not represent the formation time of these ductile shear belts, but rather the metamorphic ages of syn-collisional exhumation of HP–UHP rocks. Given that retrograde metamorphism occurred in the amphibolite facies environment, the formation time of the E–W-trending ductile shear zone should be significantly less than ~220 Ma.

6.2. Implications for the Exhumation of the HP–UHP Rocks

The continent–continent collision between the SCB and NCB was characterized by the oblique subduction of the SCB beneath the NCB [1,2,49,50]. The HP–UHP rocks in the Dabie Orogen also exhibit oblique exhumation with a top-to-the-NW sense of shear [6,8,33,51,52]. A large number of high-precision zircon U–Pb aging studies have shown that the HP–UHP rocks were exhumed to the shallow crust at ~210 Ma [10,11]. The exhumation time of the HP–UHP rocks gradually decreases from south to north (Figure 7) [51], which is consistent with the exhumation polarity of towards-the-SE and top-to-the-NW sense of shear [8,11,33,51,52]. It is worth noting that the retrograde metamorphic ages of the Bixiling UHP eclogite and the Huangwei HP eclogite, exposed in the south and north flanks of the ND unit, respectively, are both ~210 Ma (Figure 7b), which is a younger age for exhumation to the shallow crust of the HP–UHP rocks. When the subducted rocks in the Dabie Orogen were exhumed to the shallow crust, they exhibited a stacked state of different units, from bottom to top, including the ND unit, UHP eclogite unit, HP eclogite unit, SC unit and Zhangbaling Group. The NH unit was only distributed in the northern flank of the orogen. Large-scale uplift caused the exposure of the ND unit on the surface and the erosion of its overlying units during the Early Cretaceous and rebuilt the distribution state of the HP–UHP unit (Figure 7).
Given that the ND unit was also covered with UHP rocks, the Bixiling UHP eclogite should be located in the middle of the UHP eclogite unit. The Huangwei HP eclogite is close to the XMSZ as a subduction channel [3] and belongs to the younger HP rocks that returned to the shallow crust. Therefore, their consistent ages and different spatial distribution indicate that the exhumation of the UHP rocks was slightly earlier than that of the HP rocks [51]. It is likely that UHP rocks with an age of less than 210 Ma are present in the deeper parts near the subduction channel. The exhumation process of HP–UHP rocks in the Dabie Orogen should have therefore continued until after 210 Ma. Strong evidence for this is that the northwest margin of the Yangtze Plate moved continuously towards the NW during the Late Triassic [50].
HP–UHP rocks typically exhibit a top-to-the-NW sense of shear [5,6,8,33,52]. As HP rocks returned to the shallow crust, and UHP rocks continued to move towards the SE, strain concentration occurred in the overlying HP rocks and thus caused the formation of E–W-trending sinistral ductile shear belts (Figure 8). Generally, there is an acute angle between the direction of compressive stress and the trending of the strike-slip shear belt. However, there is an example of strike-slip shear belts perpendicular to compressive stress, in the Tibetan Plateau [53,54]. The development of these strike-slip shear belts in the Tibetan Plateau were interpreted as closely relating to the escape of plateau material towards the E or SE [53,54]. The subducted rocks in the Dabie Orogen returned to the shallow crust towards the SE, which may have yielded a mechanism similar to escape tectonics, resulting in strain concentration in the HP eclogite unit and the development of E–W-trending shear belts (Figure 8).
These E–W-trending shear belts are characterized by being dense in the north and sparse in the south, indicating that the greatest level of strain concentration occurred near the subduction channel, and gradually decreased away from the channel. Given that these E–W-trending ductile shear belts were cut by the older phase of the Tan-Lu Fault Zone ductile shear belt, it can be inferred that the formation of these shear belts should be before 197 Ma. It is worth noting that both the older and younger ductile shear belts of the Tan-Lu Fault Zone developed on the gneisses of the Dabie Orogen [37,38], and deformation occurred under greenschist facies conditions, indicating that the formation of these ductile shear belts of the Tan-Lu Fault Zone probably occurred after the subducted rocks returned to the shallow crust. The movement of the Tan-Lu Fault Zone might therefore not affect the exhumation of rocks in the orogen and is thus not shown in the model (Figure 8).
The HP rocks widely distributed in the southern part of the Dabie Orogen are characterized by shear deformation with a top-to-the-NW sense of shear [6,8,28,52], and there is no clear development of E–W-trending shear belts. The retrograde metamorphic age of the Huangzhen HP eclogite, in the southern Dabie Orogen, is ~230 Ma [51], which is significantly higher than that of the Huangwei HP eclogite near the subduction channel. Therefore, the absence of E–W-trending shear belts in the HP eclogite unit may be related to the earlier exhumation of the HP rocks preserved in the southern orogen. Although E–W-trending ductile shear belts were developed in HP rocks, they just appeared in the shallow crust of greenschist facies and were insufficient to drive the HP–UHP rocks towards to the SE. Therefore, the subducted rocks in the Dabie Orogen were mainly exhumed to the shallow crust containing multiple plates; this does not support the flow of orogenic material towards the SE.

7. Conclusions

(1)
The E–W-trending ductile shear belt is a sinistral shear belt developed under greenschist facies conditions at ~400 °C. This shear belt developed on the gneisses of the HP eclogite unit and was cut by the older phase of ductile shear belts of the Tan-Lu Fault Zone. It is inferred that it was formed around 219~197 Ma, and its movement is attributed to syn-collisional exhumation.
(2)
The development of these E–W-trending ductile shear belts supports the viewpoint that the syn-collisional exhumation of the orogen is characterized by a multi-plate structure.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14121205/s1, Table S1: Zircon LA-ICP-MS U-Pb ages for the mylonites in the northeastern Dabie Orogen.

Author Contributions

Methodology, X.Z. and Q.B.; software, X.Z.; investigation, Y.W., X.Z. and Q.B.; resources, Y.W.; data curation, X.Z. and Q.B.; writing – original draft, Y.W. and X.Z.; writing—review & editing, Y.W.; project administration, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the National Natural Science Foundation of China (grant number 42272240).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We gratefully acknowledge Wang Fangyue of the Geological Laboratory, Hefei University of Technology and Zhang Bo of the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, for their assistance with the U–Pb zircon LA–ICP–MS and EBSD analyses, respectively.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geological sketch of the northeastern Dabie Orogen. (a) The location of the Central China Orogenic Belt. (b) A simplified tectonic framework of the Dabie Orogen (modified after Xu et al. [25]). (c) Detailed structural map of the Tongcheng massif. (d) Lower-hemisphere, equal-area stereograms of poles to the mylonitic foliation and plunges of mineral elongation lineation of the E–W-trending and NE–SW-trending shear belts in the northeastern Dabie Orogen. (e) Cross-sections showing tectonic framework of the Dabie Orogen (Section line in (b)). TLF: Tan-Lu Fault Zone; SMF: Shangcheng–Macheng Fault; XMSZ: Xiaotian–Mozitan shear zone; WSF: Wuhe–Shuihou Fault; HMF: Hualiangting–Mituo Fault; TMF: Taihu–Mamiao Fault; XSF: Xishui Fault; XGF: Xiangfan–Guangji Fault.
Figure 1. Geological sketch of the northeastern Dabie Orogen. (a) The location of the Central China Orogenic Belt. (b) A simplified tectonic framework of the Dabie Orogen (modified after Xu et al. [25]). (c) Detailed structural map of the Tongcheng massif. (d) Lower-hemisphere, equal-area stereograms of poles to the mylonitic foliation and plunges of mineral elongation lineation of the E–W-trending and NE–SW-trending shear belts in the northeastern Dabie Orogen. (e) Cross-sections showing tectonic framework of the Dabie Orogen (Section line in (b)). TLF: Tan-Lu Fault Zone; SMF: Shangcheng–Macheng Fault; XMSZ: Xiaotian–Mozitan shear zone; WSF: Wuhe–Shuihou Fault; HMF: Hualiangting–Mituo Fault; TMF: Taihu–Mamiao Fault; XSF: Xishui Fault; XGF: Xiangfan–Guangji Fault.
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Figure 2. Field photos of gneisses and mylonites in the northeastern Dabie Orogen. (a) Development of brittle NE–SW-trending faults in gneisses with flat-lying foliation; (b) steeply chlorite-bearing ultramylonite and (c) granitic mylonite; (d) S-C structures in marble mylonite indicates sinistral shear; (e,f) development of E–W-trending shear zone in gneisses with flat-lying foliation.
Figure 2. Field photos of gneisses and mylonites in the northeastern Dabie Orogen. (a) Development of brittle NE–SW-trending faults in gneisses with flat-lying foliation; (b) steeply chlorite-bearing ultramylonite and (c) granitic mylonite; (d) S-C structures in marble mylonite indicates sinistral shear; (e,f) development of E–W-trending shear zone in gneisses with flat-lying foliation.
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Figure 3. Micrographs of mylonites from the E–W-trending shear belts in the northeastern Dabie Orogen. (a) Broken feldspar and recrystallized quartz; (b) fine-grain recrystallized quartz with a few larger quartz grains; (c) crossed polarizers and (d) single polarizers micrographs of ultramylonite, with banded chlorite, sericite and a few epidotes, with sinistral shear; (e) σ-type feldspar porphyroclasts in the ultramylonite indicating sinistral shear; (f) e-twin of calcite in the marble mylonite. Quartz within rectangular frame for EBSD testing. Qz: quartz; Fsp: feldspar; Pl: plagioclase; Ep: epidote.
Figure 3. Micrographs of mylonites from the E–W-trending shear belts in the northeastern Dabie Orogen. (a) Broken feldspar and recrystallized quartz; (b) fine-grain recrystallized quartz with a few larger quartz grains; (c) crossed polarizers and (d) single polarizers micrographs of ultramylonite, with banded chlorite, sericite and a few epidotes, with sinistral shear; (e) σ-type feldspar porphyroclasts in the ultramylonite indicating sinistral shear; (f) e-twin of calcite in the marble mylonite. Quartz within rectangular frame for EBSD testing. Qz: quartz; Fsp: feldspar; Pl: plagioclase; Ep: epidote.
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Figure 4. Quartz CPO pattern of mylonites from E–W-trending shear belts in the northeastern Dabie Orogen. Lower-hemisphere, equal-area projection. n: measured grain numbers. X and Z are principal axes of finite strain. Thin sections are parallel with XZ plane.
Figure 4. Quartz CPO pattern of mylonites from E–W-trending shear belts in the northeastern Dabie Orogen. Lower-hemisphere, equal-area projection. n: measured grain numbers. X and Z are principal axes of finite strain. Thin sections are parallel with XZ plane.
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Figure 5. Cathodoluminescence images of a selection of the dated zircons from mylonites in the eastern Dabie Orogen. Red circles indicate analytical sites.
Figure 5. Cathodoluminescence images of a selection of the dated zircons from mylonites in the eastern Dabie Orogen. Red circles indicate analytical sites.
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Figure 6. Concordia diagrams and weighted mean ages of zircons from mylonites in the E–W-trending shear belts in the northeastern Dabie Orogen.
Figure 6. Concordia diagrams and weighted mean ages of zircons from mylonites in the E–W-trending shear belts in the northeastern Dabie Orogen.
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Figure 7. Schematic diagram of tectonic evolution during syn-collisional exhumation and post-collisional uplift. (a) During the Late Triassic to the Early Jurassic, the subducted SCB was exhumed to the shallow crust and resulted in the formation of a fold-and-thrust belt in the northern Yangtze Block and the Hefei foreland basin in the southern NCB. (b) Post-collisional uplift caused rocks overlying the ND unit to be eroded and rebuilt the tectonic framework of the Dabie Orogen. The diamond represents the outcrop location of HP–UHP rocks, and the numbers in parentheses are the ages when these HP–UHP rocks returned to the shallow crust. The dashed lines show the eroded part of units in the Dabie Orogen. Age data are from Liu et al. [10], Leech et al. [11], Ayers et al. [31], and the references therein. Please refer to the manuscript for abbreviated names.
Figure 7. Schematic diagram of tectonic evolution during syn-collisional exhumation and post-collisional uplift. (a) During the Late Triassic to the Early Jurassic, the subducted SCB was exhumed to the shallow crust and resulted in the formation of a fold-and-thrust belt in the northern Yangtze Block and the Hefei foreland basin in the southern NCB. (b) Post-collisional uplift caused rocks overlying the ND unit to be eroded and rebuilt the tectonic framework of the Dabie Orogen. The diamond represents the outcrop location of HP–UHP rocks, and the numbers in parentheses are the ages when these HP–UHP rocks returned to the shallow crust. The dashed lines show the eroded part of units in the Dabie Orogen. Age data are from Liu et al. [10], Leech et al. [11], Ayers et al. [31], and the references therein. Please refer to the manuscript for abbreviated names.
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Figure 8. Schematic model of the formation of E–W-trending shear belts in the northeastern Dabie Orogen during the Late Triassic. To highlight the relationship between the formation of E–W-trending shear belts and the exhumation process of HP–UHP rocks, we neglected other units within the orogen in this figure. Please refer to the manuscript for abbreviated names.
Figure 8. Schematic model of the formation of E–W-trending shear belts in the northeastern Dabie Orogen during the Late Triassic. To highlight the relationship between the formation of E–W-trending shear belts and the exhumation process of HP–UHP rocks, we neglected other units within the orogen in this figure. Please refer to the manuscript for abbreviated names.
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Table 1. Microscopic characteristics of mylonites for EBSD analysis from the E–W-trending shear belt in the northeastern Dabie Orogen.
Table 1. Microscopic characteristics of mylonites for EBSD analysis from the E–W-trending shear belt in the northeastern Dabie Orogen.
SampleLocationRockMineral AssemblageFeldspar
Deformation		Quartz
Recrystallization		Deformation
Temperature
NF3731°01′59.6″ N
116°54′26.6″ E		Granitic myloniteP (20%): Fsp
M (80%): Fsp + Qz + Chl		Micro-fracturingBLG + SR~400 °C
NF4031°01′53.8″ N
116°54′6.7″ E		Granitic myloniteP (15%): Fsp
M (85%): Fsp + Qz + Chl		Micro-fracturingBLG + SR~400 °C
NF4130°58′32.9″ N
116°49′31.1″ E		Granitic myloniteP (15%): Fsp
M (85%): Fsp + Qz + Chl		Micro-fracturingBLG + SR~400 °C
NF41-230°58′32.9″ N
116°49′31.1″ E		Chlorite-bearing ultramyloniteP (5%): Fsp
M (95%): Fsp + Qz + Chl + Ser + Ep		Micro-fracturingBLG + SR~400 °C
NF5830°50′29.6″ N
116°39′44.6″ E		Granitic myloniteP (25%): Fsp
M (75%): Fsp + Qz + Chl		Micro-fracturingBLG + SR~400 °C
NF58-330°50′29.6″ N
116°39′44.6″ E		Chlorite-bearing ultramyloniteP (5%): Fsp
M (95%): Fsp + Qz + Chl + Ser		Micro-fracturingBLG + SR~400 °C
P: porphyroblasts; M: matrix; BLG: bulging; SR: subgrain rotation; Chl: chlorite; Ser: sericite; Qz: quartz; Fsp: feldspar; Ep: epidote.
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Wang, Y.; Zhang, X.; Bai, Q. Structural Characteristics of E–W-Trending Shear Belts in the Northeastern Dabie Orogen, China: Evidence for Exhumation of High–Ultrahigh-Pressure Rocks. Minerals 2024, 14, 1205. https://doi.org/10.3390/min14121205

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Wang Y, Zhang X, Bai Q. Structural Characteristics of E–W-Trending Shear Belts in the Northeastern Dabie Orogen, China: Evidence for Exhumation of High–Ultrahigh-Pressure Rocks. Minerals. 2024; 14(12):1205. https://doi.org/10.3390/min14121205

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Wang, Yongsheng, Xu Zhang, and Qiao Bai. 2024. "Structural Characteristics of E–W-Trending Shear Belts in the Northeastern Dabie Orogen, China: Evidence for Exhumation of High–Ultrahigh-Pressure Rocks" Minerals 14, no. 12: 1205. https://doi.org/10.3390/min14121205

APA Style

Wang, Y., Zhang, X., & Bai, Q. (2024). Structural Characteristics of E–W-Trending Shear Belts in the Northeastern Dabie Orogen, China: Evidence for Exhumation of High–Ultrahigh-Pressure Rocks. Minerals, 14(12), 1205. https://doi.org/10.3390/min14121205

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