Timing and Tectonic Implications of the Development of the Orosirian Qianlishan Ductile Shear Zones in the Khondalite Belt, North China Craton

Abstract

Orogen-parallel ductile shear zones are conspicuous structures in the Khondalite Belt, but the timing of shearing remains poorly understood. Here, we present field-based structural and zircon U-Pb geochronological studies on the newly discovered Qianlishan ductile shear zones (QDSZ) in the Khondalite Belt. Our results show that the nearly E-W-trending QDSZ are characterized by steeply S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Two pre-kinematic intrusions yielded zircon crystallization ages of 2055 ± 17 Ma and 1947 ± 9 Ma, providing the maximum age limit for the QDSZ. Additionally, zircon overgrowth rims from three high-temperature mylonites gave metamorphic ages of 1902 ± 8 Ma, 1902 ± 26 Ma and 1884 ± 12 Ma, interpreted to record the timing of development of the QDSZ. Integrated with previous studies, we propose that the Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation led to the development of the QDSZ. Deformation events D1, D2 and D3 are considered to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. These events document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks.

1. Introduction

Shear zones are typically known as tabular high-strain zones that play a major role in accommodating tectonic strain, displacement and fluid migration in the lithosphere [1,2,3,4,5,6]. Such zones are important structures in orogeny and can occur at a variety of scales, which significantly influence the geometry and evolution of orogenic belts [4,7,8,9]. Mylonites are characteristic rock types of shear zones at middle to lower crustal levels, and they generally develop a range of ductile structures and fabrics, providing valuable information on the kinematic and tectonothermal processes during orogeny [10,11,12,13]. Therefore, the recognition and anatomy of ductile shear zones is a key requirement in geological investigations that attempt to unravel the deformation history and tectonic evolution of orogenic belts [4,14,15,16,17]. To achieve this goal, detailed field-based structural analysis integrated with geochronological studies on ductile shear zones is fundamentally necessary [8,12,13].

The Khondalite Belt has been widely regarded as an Orosirian continent-continent collisional orogen in the northwestern North China Craton (Figure 1) [18,19,20,21,22,23]. It extends nearly E-W for ~1000 km and is mainly composed of four high-grade metamorphic complexes, which from west to east are the Helanshan, Qianlishan, Wulashan-Daqingshan and Jining complexes [18,24]. Of particular interest is the widespread occurrence of NE to E-striking ductile shear zones in the Khondalite Belt, such as the Zongbieli, Xiashihao-Jiuguan and Xuwujia ductile shear zones of the Helanshan, Wulashan-Daqingshan and Jining complexes, respectively [24,25,26,27,28]. However, there is no evidence of orogen-parallel ductile shear zones in the Qianlishan Complex. Meanwhile, the formation and evolution of ductile shear zones in the Khondalite Belt have been poorly constrained, which inevitably hampers a better understanding of the polyphase deformation in the belt. Based on field investigations, we newly discovered nearly E-W-trending ductile shear zones in the Qianlishan Complex, named together as the Qianlishan ductile shear zones (QDSZ). In this study, we conducted field-based structural observations and LA-ICP-MS zircon U-Pb geochronology on the QDSZ. Combined with previous works, particularly the compilation of deformation-related geochronological data of the Khondalite Belt, these results shed new light on the deformation history and tectonic evolution of the Khondalite Belt.

2. Geological Setting

The North China Craton, one of the oldest cratons in the world, is proposed to have formed from the assembly of several Archean to Paleoproterozoic continental blocks along orogenic belts (Figure 1) [18,19,29,30,31]. In the western North China Craton, the Yinshan Block in the north was considered to collide with the Ordos Block in the south at ~1.95 Ga, resulting in the formation of the nearly E-W-trending Khondalite Belt [18,19,20,21,22,23,28,32,33,34,35,36,37,38,39,40,41]. This belt is dominated by polydeformed high-grade metasedimentary rocks (i.e., the khondalites), mainly including garnet-sillimanite gneisses, felsic paragneisses, quartzites, marbles and calc-silicate rocks [18,24,42]. Spatially associated with the khondalites are S-type granites, charnockites, minor TTG gneisses, intermediate-mafic magmatic rocks and mafic granulites [18,24,42]. Numerous geochronological studies reveal that the protolith of the khondalites was deposited at ca. 2.00–1.95 Ga, and then suffered regional upper-amphibolite to granulite-facies metamorphism at ca. 1.95–1.80 Ga [20,32,33,34,35,36,37,38,40,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. Of these, high-pressure pelitic granulites preserved similar clockwise P-T paths with post-peak near-isothermal decompression, and they indicate tectonic processes involving initial crustal thickening and subsequent rapid exhumation, related to the collisional orogeny between the Yinshan and Ordos blocks [37,50,52,53,58,59,60,61,62,63]. Noticeably, a series of orogen-parallel ductile shear zones commonly occurred in the Khondalite Belt, for example, the NE-SW to E-W-trending Zongbieli shear zones in the Helanshan Complex, the nearly E-W-trending Xiashihao-Jiuguan shear zone in the Wulashan-Daqingshan Complex and the NE(E)-SW(W)-trending Xuwujia shear zone in the Jining Complex [24,25,26,27,28,64,65].

The Qianlishan Complex is located in the western segment of the Khondalite Belt and is unconformably covered by Mesoproterozoic sedimentary rocks of the Changcheng-Jixian System (Figure 1 and Figure 2) [18,21,24,65,66]. The complex predominantly comprises Paleoproterozoic granitoid plutons and the khondalites that have been known as the Qianlishan Group in this region (Figure 2) [20,21,24,39]. This group consists of the Chaganguole, Qianligou and Habuqigai formations, in which high-pressure pelitic granulites were found and characterized by a peak mineral assemblage composed of garnet + kyanite + K-feldspar + plagioclase + biotite + quartz, with corresponding P-T conditions at 11–15 kbar and 800–850 °C [37,60]. Available geochronological data of the Qianlishan Group demonstrate that U-Pb ages of detrital zircons dominantly range from 2.20 to 2.00 Ga, with a notable peak at ~2.03 Ga [20,37,67]. Metamorphic zircons yielded age groups of ~1.95 Ga and ~1.92 Ga, of which the former and latter were related to granulite-facies peak metamorphism and post-peak decompression processes, respectively [20,37,67]. Meanwhile, multiple magmatic pulses have been dated in the studied area at ca. 2.06 Ga, 1.95 Ga, 1.92 Ga and 1.88 Ga [20,21,39,68]. Additionally, previous structural investigations revealed two major phases of Paleoproterozoic deformation (D1–D2) in the Qianlishan Complex [21,24,39,69]. D1 mainly generated overturned to recumbent isoclinal folds (F1), transposition foliations/gneissosities (S1) and mineral lineations (L1) [21,69]. D2 produced tight to open doubly plunging upright folds (F2) [21,39]. Deformation ages of D1 and D2 were roughly constrained at 1976–1932 Ma and 1932–1899 Ma, respectively [21,39].

3. Samples and Methods

In this study, we conducted field-based (micro-)structural analysis to investigate the geometries, kinematics and temperature conditions of the QDSZ. Additionally, we collected four shear zone-related samples for zircon U-Pb geochronology, including three high-temperature mylonites (Samples 22QL03, 22QL13 and 22QL11) and a pre-kinematic granitoid pluton (Sample 20QL02). Zircons were extracted by magnetic and heavy liquid separation techniques. Subsequently, they were handpicked, mounted in epoxy resin, polished and photographed under reflected and transmitted light. The internal textures of these zircons were then studied through cathodoluminescence (CL) images. Zircon U-Pb analyses were performed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Guangzhou Tuoyan Analytical Technology Co., Ltd., Guangzhou, China. Detailed analytical methods and procedures are similar to those in [70]. The spot size and frequency of the laser were set to 30 µm and 6 Hz, respectively. Zircon 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. Zircon Plešovice was treated as an unknown sample to monitor the accuracy of acquired U-Pb data. The software ICPMSDataCal was used to conduct off-line inspection and integration of background and analytical signals, time-drift correction, and quantitative calibration for U-Pb dating and trace element analysis [71]. Concordia diagrams and age calculations were made using Isoplot/Ex_ver4.15 [72]. The uncertainties on weighted mean ages and intercept ages were quoted at the 95% confidence level (2σ). The zircon U-Pb data of this study are presented as Supplementary Material (Table S1).

4. Results

4.1. Structural Observations

The QDSZ mainly consist of a series of small narrow high-strain zones (e.g., Figure 3a,e and Figure 4), but there is no map view of the large-scale shear zones observed in the Qianlishan Complex. These zones dominantly strike E-W or SEE-NWW and are characterized by mylonitized rocks with varying deformation intensities. For example, a 1–2 m wide E-W-trending ductile shear zone was remarkably developed within a granitoid pluton, which suffered intensive shear deformation to become granitic mylonite (Figure 3). Regionally, the mylonitic foliations (Sm) in the QDSZ dip mostly to the S(SW) with steep to sub-vertical angles (Figure 3 and Figure 4). At some localities, moderately N(NE)-plunging stretching lineations (Lm) can be seen on the mylonitic foliation (Figure 4f). The appearance of kinematic indicators, including S-C fabrics, σ- and δ-type porphyroclasts and felsic aggregates (Figure 5), suggest a nearly top-to-N(NE) sense of shearing.

In addition, it is worth noting that quartz grains in felsic and granitic mylonites generally occur as ribbons with straight boundaries (e.g., Figure 3c,f and Figure 4b,d,i), and they exhibit lobate to rectangular shapes in thin sections (Figure 3d and Figure 4c), which implies grain boundary migration recrystallization (GBM) dominates. Furthermore, these ribbons are alternated by fine-grained recrystallized feldspar-rich layers (Figure 3c,d and Figure 4c). In some cases, feldspar fish are locally observed (Figure 3d). The above-stated (micro-)structures indicate that the QDSZ developed under high-temperature deformation conditions, probably >650 °C [10,11,73]. This interpretation is also supported by the observation of a series of felsic segregates related to melting during the mylonitization, which commonly appear along the mylonitic foliation (e.g., Figure 4d,g and Figure 5c).

4.2. Zircon U-Pb Geochronology

4.2.1. Pre-Kinematic Granitoid Pluton (Sample 20QL02)

Sample 20QL02 was collected from a granitoid pluton in the northern Qianlishan Complex (Figure 2; 39°58′31.32″ N, 106°57′43.21″ E). This granitoid was cut by an E-W-trending ductile shear zone (Figure 3e,f), indicating that it was formed prior to the shear zone activity. The sample mainly consists of fine-grained K-feldspar, plagioclase, quartz and hornblende, and is characterized by the typical granular texture (Figure 3e). Zircons separated from the sample are dominantly euhedral, prismatic and 250–450 µm in size. CL images reveal that these zircons have concentrically oscillatory-zoned cores with high luminescence (Figure 6a), interpreted to be of igneous origin. In some cases, they are surrounded by dark structureless rims that are considered to result from metamorphic processes, but these rims are too narrow to be analyzed (Figure 6a). A total of six spots with Th/U ratio of 0.64–0.85 were analyzed (discordance degree ≤ 6.03%; Table S1). They yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2055 ± 17 Ma (n = 6, MSWD = 2.8; Figure 7a). Consistently, these spots also defined an upper intercept age of 2047 ± 17 Ma (n = 6, MSWD = 6.7).

4.2.2. Mylonitic Granitoid Pluton (Sample 22QL03)

This sample was taken from a mylonitic granitoid adjacent to Sample 20QL02 (Figure 2; 39°58′31.35″ N, 106°57′41.53″ E). Within the shear zone, the granitoid underwent intensive ductile deformation to become a granitic mylonite (Sample 22QL03; Figure 3a–c). Sample 22QL03 is featured by sub-vertical mylonitic foliation (Sm), defined by the preferential orientation of fine-grained feldspar-rich layers and quartz ribbons (Figure 3c,d). Zircons extracted from this sample are euhedral to subhedral, prismatic or stubby and 200–350 μm in size. In CL images, they generally displayed complex textures, most of which possessed bright patchy or oscillatory-zoned cores (Figure 6b), indicative of magmatic origin. Remarkably, these zircon cores mostly retained double overgrowth rims, including the inner and outer rims (Figure 6b). The inner rims were commonly dark, nebulous or chaotic, and surrounded by the structureless outer rims (Figure 6b). Both the inner and outer rims were interpreted to be of metamorphic origin. A total of twenty-two spots were analyzed, of which five spots (discordance degree ≤ 6.10%, Table S1) corresponded to zircon cores and defined an upper intercept age of 2044 ± 30 Ma (n = 5, MSWD = 4.3; Figure 7b). Six spots on the inner metamorphic rims plot along a discordia line with an upper intercept age of 1935 ± 18 Ma (n = 6, MSWD = 3.4; Figure 7b). The remaining eleven spots corresponded to the outer metamorphic overgrowth rims, of which the two most concordant data (discordance degree ≤ 1.35%, Table S1) gave apparent ²⁰⁷Pb/²⁰⁶Pb ages of 1885 ± 29 Ma and 1866 ± 26 Ma. These eleven spots defined an upper intercept age of 1884 ± 12 Ma (n = 11, MSWD = 3.7; Figure 7b).

4.2.3. Mylonitic Granitic Dyke (Sample 22QL13)

Sample 22QL13 was collected from a mylonitic granitic dyke of the southern Qianlishan Complex (Figure 2; 39°41′13.44″ N, 107°00′01.50″ E). The sample was composed of strongly aligned grains of K-feldspar, plagioclase and quartz (Figure 4e). Most zircons from this sample were euhedral to subhedral, prismatic and 50–200 µm in size. CL images show that they are characterized by typical core-rim textures with bright and oscillatory-zoned cores (Figure 6c), evidently of magmatic origin. These cores are generally surrounded by dark, nebulous or structureless overgrowth rims, interpreted to be of metamorphic origin (Figure 6c). Fourteen spots were analyzed in this sample, and five spots on magmatic cores gave an upper intercept age of 1947 ± 9 Ma (n = 5, MSWD = 1.9; Figure 7c). The remaining nine spots were performed on metamorphic overgrowth rims, of which the most concordant spot yielded a ²⁰⁷Pb/²⁰⁶Pb age of 1899 ± 42 Ma (discordance degree = 1.84%, Figure 7c; Table S1). These nine spots defined an upper intercept age of 1902 ± 8 Ma (n = 9, MSWD = 2.0; Figure 7c).

4.2.4. Felsic Mylonite (Sample 22QL11)

Sample 22QL11 is a felsic mylonite of the southern Qianlishan Complex (Figure 2; 39°41′17.53″ N, 109°00′00.38″ E). Metasedimentary rocks of the Habuqigai Formation were subjected to ductile shear deformation and transformed into mylonites (Figure 4g,h). The mylonitic foliation (Sm) is characterized by the preferred alignment of feldspar-rich layers and quartz ribbons with straight boundaries (Figure 4i). Zircon grains from this sample are mainly subhedral to anhedral, stubby and 100–250 µm in size. CL images reveal that these zircons mostly possess patchy or oscillatory-zoned cores surrounded by nebulous overgrowth rims (Figure 6d). The cores and rims are regarded to be of magmatic and metamorphic origin, respectively. A total of eight spots were analyzed in this sample, of which three spots correspond to inherited cores and yielded concordant ²⁰⁷Pb/²⁰⁶Pb ages of 2289 ± 12 Ma, 2026 ± 12 Ma and 2016 ± 25 Ma (discordance degree ≤ 7.43%, Figure 7d; Table S1). The remaining five spots were analyzed on metamorphic overgrowth rims and defined an upper intercept age of 1902 ± 26 Ma (n = 5, MSWD = 10.8; Figure 7d).

5. Discussion

5.1. Timing of the Development of the Qianlishan Ductile Shear Zones (QDSZ)

Combined with field-based structural observations, geochronological data from shear zone-related intrusions and mylonites can help us to constrain the deformation age of the QDSZ. In the northern Qianlishan Complex, a small-scale ductile shear zone reworked a pre-kinematic granitoid (Figure 2 and Figure 3). Magmatic zircons from this rock (Sample 20QL02, Figure 3e,f) yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2055 ± 17 Ma (Figure 6a and Figure 7a). It is interpreted as the crystallization age of the granitoid and provides the maximum age limit for the shear zone (Figure 8a). Zircons from the granitic mylonite (Sample 22QL03, Figure 3a,c) within the shear zone mostly possess magmatic cores surrounded by both inner and outer metamorphic overgrowth rims (Figure 6b). The zircon cores gave a crystallization age of 2044 ± 30 Ma (Figure 7b), consistent with that obtained from Sample 20QL02. On the one hand, the inner rims yielded a metamorphic age of 1935 ± 18 Ma (Figure 7b). A similar metamorphic age of 1948 ± 26 Ma was obtained from a gneissic granitoid in another outcrop of the same pre-kinematic pluton (Figure 2) [20,21,39]. These metamorphic ages of 1948–1935 Ma are largely coherent with those of ~1950 Ma from high-pressure granulites of the Qianlishan Group (Figure 8a) [20,37,67], indicating that the granitoid also experienced regional metamorphism. On the other hand, the outer overgrowth rims gave a metamorphic age of 1884 ± 12 Ma (Figure 7b). This age was interpreted to most likely record the timing of the shear zone activity in the northern Qianlishan Complex.

In the southern Qianlishan Complex, a pre-kinematic granitic dyke was mylonitized (Sample 22QL13, Figure 4e). Zircons from this mylonite show typical core–rim textures (Figure 6c), of which the magmatic cores gave a ²⁰⁷Pb/²⁰⁶Pb age of 1947 ± 9 Ma (Figure 7c), regarded as the crystallization age of the dyke. Similarly, a mylonitized pre-kinematic granitic dyke was dated at 1936 ± 28 Ma (Figure 4a,b) [21,69]. These crystallization ages of 1947–1936 Ma indicate that the mylonitization must have occurred at some time after ~1936 Ma (Figure 8a). In this way, zircon overgrowth rims from Sample 22QL13 yielded a metamorphic age of 1902 ± 8 Ma (Figure 7c), considered to probably reflect the timing of the shear zone activity. This age is in good agreement with the dating results of Sample 22QL11 (Figure 4h,i), a felsic mylonite whose zircon overgrowth rims gave a metamorphic age of 1902 ± 26 Ma (Figure 6d and Figure 7d). It is also worth noting that these ages of ~1902 Ma are comparable to that of 1884 ± 12 Ma obtained from the granitic mylonite (Sample 22QL03) of the northern Qianlishan Complex. Additionally, an Rb-Sr mineral isochron age of 1821 ± 32 Ma was obtained from a deformed metapelite of the southern Qianlishan Complex [74]. Shen et al. (1988) also reported a similar biotite ⁴⁰Ar/³⁹Ar plateau age of 1839 ± 10 Ma and interpreted that these ages of 1839–1821 Ma record the timing of a late-stage tectonothermal event after the granulite-facies peak metamorphism [75], probably related to the ductile shear zone activity. Therefore, we regard that the development of the QDSZ started at ca. 1902–1884 Ma and continued to ca. 1839–1821 Ma (Figure 8a).

5.2. Three Phases of Orosirian Deformation (D1–D3) in the Qianlishan Complex

Previous structural investigations show that the Qianlishan Complex underwent at least two major phases of deformation (D1–D2) in the late Paleoproterozoic (Figure 8b) [21,24,39,69]. The D1 deformation generated small-scale NWW-trending overturned to recumbent isoclinal F1 folds, originally sub-horizontal penetrative S1 transposition foliations/gneissosities and NNE-SSW-oriented L1 mineral lineations [21]. Based on our previous geochronological results of pre-D1 and post-D1 leucocratic dykes (

Table 1. Summary of available deformation-related geochronological data for the Khondalite Belt.

No. 1	Sample	Rock Type and Structural Feature	Age (Ma)	Type 2	n	MSWD	Interpretation	References
Qianlishan Complex
1	-	Deformed metapelite	1821 ± 32	MIA	-	-	D3-related process	[74]
2	N80-48	Deformed metapelite	1839 ± 10	Bt/PA	-	-	D3-related process	[75]
3	22QL03	Granitic mylonite	1884 ± 12	M/UI	11	3.7	Syn-D3 metamorphism	This study
			1935 ± 18	M/UI	6	3.4	Syn-D1 metamorphism
			2044 ± 30	I/UI	5	4.2	Pre-D3 intrusion
4	20QL02	Granitoid cut by DSZ *	2055 ± 17	I/WM	6	2.8	Pre-D3 intrusion	This study
5	22QL13	Mylonitized granitic dyke	1902 ± 8	M/UI	9	2.0	Syn-D3 metamorphism	This study
			1947 ± 9	I/UI	5	1.9	Pre-D3 intrusion
6	22QL11	Felsic mylonite	1902 ± 26	M/UI	5	10.8	Syn-D3 metamorphism	This study
			2289–2016	D/SG	3	-	Inheritance
7	QL32-1	Mylonitized granitic dyke	1936 ± 28	I/UI	12	6.2	Pre-D3 intrusion	[21]
8	QL27-1	F2-cutting undeformed pegmatite dyke	1854 ± 68	I/UI	10	4.2	Post-D2 intrusion	[21]
			2118–1976	X/SG	10	-	Inheritance
9	QL12	F2-cutting undeformed pegmatite dyke	1899 ± 6	I/UI	9	1.9	Post-D2 intrusion	[39]
10	QL14	S1-parallel and F2-folded leucocratic dyke	1932 ± 24	I/UI	18	6.5	Pre-D2 intrusion	[39]
11	QL08-2	S1-cutting leucogranitic dyke	1909 ± 27	I/UI	14	9.5	Post-D1 intrusion	[21]
			2089–1940	X/SG	9	-	Inheritance
12	QL24-1	S1-cutting leucocratic dyke	1932 ± 47	I/UI	12	16	Post-D1 intrusion	[21]
			2082–1977	X/SG	6	-	Inheritance
13	QL01-5	Isoclinally F1-folded leucogranitic dyke	1940 ± 33	M/UI	5	3.4	Syn-D1 metamorphism	[21]
			1976 ± 21	I/UI	7	6.1	Pre-D1 intrusion
			2689–1997	X/SG	13	-	Inheritance
14	QL19	S1-foliated granitoid	1948 ± 26	M/UI	12	2.2	Syn-D1 metamorphism	[39]
			2011 ± 12	I/WM	8	1.2	Pre-D1 intrusion
15	QL20	S1-foliated granitoid	1954 ± 18	M/UI	6	1.17	Syn-D1 metamorphism	[39]
			2053 ± 14	I/WM	13	3.2	Pre-D1 intrusion
16	QL05	S1-foliated granitoid	1956 ± 27	M/UI	7	5.2	Syn-D1 metamorphism	[39]
			2056 ± 17	I/WM	6	1.9	Pre-D1 intrusion
17	QL03	S1-foliated granitoid	1966 ± 25	M/UI	17	8.0	Syn-D1 metamorphism	[39]
			2016 ± 24	I/WM	5	3.4	Pre-D1 intrusion
Helanshan Complex
18	HL07	Granitic mylonite	1866 ± 12	M/UI	33	1.6	Syn-D3 metamorphism	[65]
			1951 ± 5	I/WM	6	0.22	Pre-D3 intrusion
19	HL14	S1-foliated granitic dyke	1867 ± 21	M/UI	13	0.48	Syn-D3 metamorphism	[28]
			1954 ± 13	I/UI	24	0.1	Syn-D1 intrusion
			2049 ± 32	X/WM	3	0.02	Inheritance
20	HL12	S1-cutting and F2-folded leucocratic dyke	1911 ± 43	I/UI	11	0.73	Syn-D2 intrusion	[28]
21	HL09	Isoclinally F1-folded leucocratic dyke	1949 ± 8	I/UI	15	0.45	Syn-D1 intrusion	[28]
22	HL31	Isoclinally F1-folded metasandstone	1951 ± 9	M/UI	22	0.03	Syn-D1 metamorphism	[28]
			2173–2018	D/SG	22	-	Inheritance
23	HL13	S1-foliated granitoid	1954 ± 14	M/UI	14	0.06	Syn-D1 metamorphism	[28]
			2032 ± 34	I/WM	6	3.9	Pre-D1 intrusion
24	HL18	S1-foliated leucocratic dyke	1954 ± 13	M/UI	21	0.08	Syn-D1 metamorphism	[28]
			2039 ± 17	I/WM	7	0.66	Pre-D1 intrusion
Wulashan-Daqingshan Complex
25	NOR24-2	Granitic mylonite	1814 ± 13	Bt/PA	-	-	D3 deformation	[27]
26	NOR8-3	Mylonitic felsic gneiss	1819 ± 14	Bt/WM	-	-	D3 deformation	[27]
27	NOR14-1	Mylonitic garnet-biotite gneiss	1885 ± 20	Bt/PA	-	-	D3 deformation	[27]
28	-	Granite cutting DSZ	1819 ± 3	I/SG	1	-	Post-D3 intrusion	[64]
29	NM0922	Undeformed coarse-grained syenogranite	1822 ± 17	I/WM	12	0.82	Post-D3 intrusion	[54]
30	NM0919	Deformed syenogranite with augen structure in DSZ	1853 ± 6	M/SG	1	-	Syn-D3 metamorphism	[54]
			1964 ± 4	I/WM	13	1.00	Pre-D3 intrusion
31	NOR114-2	Sm-parallel granitic dyke in DSZ	1858 ± 21	I/WM	13	0.22	Syn-D3 intrusion	[27]
32	NM1112	Strongly Sm-foliated syenogranite vein in DSZ	1866 ± 23	M/UI	6	0.97	Syn-D3 metamorphism	[54]
			2112 ± 13	I/UI	6	0.55	Pre-D3 intrusion
33	NM0915	Lm-lineated metagabbro dyke in DSZ	1884 ± 11	M/WM	5	1.3	Syn-D3 metamorphism	[47]
			1951 ± 9	I/WM	9	1.2	Pre-D3 intrusion
34	NM1119	Strongly Sm-foliated charnockitic gneiss in the DSZ	1896 ± 17	M/WM	4	0.38	Syn-D3 metamorphism	[54]
			1987 ± 19	I/WM	5	0.80	Pre-D3 intrusion
35	NM0814-2	Lm-lineated meta-gabbro dyke in DSZ	1906 ± 13	M/WM	6	1.0	Syn-D3 metamorphism	[47]
			1968 ± 8	I/WM	20	1.3	Pre-D3 intrusion
36	NM1004	S1-cutting undeformed pegmatite vein	1841 ± 9	I/WM	14	1.5	Post-D1 intrusion	[76]
37	16BT13-1	S1-cutting granitic vein	1874 ± 17	I/WM	8	0.12	Post-D1 intrusion	[77]
38	NM0615	S1-foliated syenogranite	1952 ± 6	I/WM	4	1.07	Syn-D1 intrusion	[54]
39	NM0920	Metagabbro dyke with a weak S1 foliation	1960 ± 7	I/WM	8	1.2	Syn-D1 intrusion	[47]
40	NM0614-2	Strongly S1-foliated syenogranite	1939 ± 17	M/WM	8	0.27	Syn-D1 metamorphism	[54]
			2047 ± 25	I/SG	1	-	Pre-D1 intrusion
Jining Complex
41	D06JN023	Mylonitic metapelite in DSZ	1866 ± 22	M/WM	24	0.26	Syn-D3 metamorphism	[78]
42	D06JN034	Granite cut by DSZ	1957 ± 19	I/WM	30	0.78	Pre-D3 intrusion	[78]
43	13AZS14	Pegmatite dyke cutting S1 foliation	1918 ± 3	I/WM	19	1.1	Post-D1 intrusion	[56]
44	13AZS12	S1-foliated metaleucogranite	1950 ± 26	I/UI	15	2.5	Syn-D1 intrusion	[56]
45	13AZS11	S1-foliated metaleucogranite	1954 ± 18	I/UI	10	1.5	Syn-D1 intrusion	[56]

¹ Numbers also indicate the dated samples and references in Figure 9. ² Bt, I, M, X and D indicate biotite, igneous, metamorphic, xenocrystic and detrital zircon, respectively. MIA, PA, UI, WM and SG denote mineral isochron age, plateau age, upper intercept age, weighted mean age and single grain age, respectively. Except for single grain age, the remaining ages are presented at the 2σ level. * Abbreviations: DSZ, ductile shear zone.

), D1 was regarded to roughly happen in the period between ~1976 Ma and ~1932 Ma (Figure 8) [21,39,69]. The extensive zircon U-Pb data also demonstrate that the Paleoproterozoic granitoids and the Qianlishan Group were together subjected to syn-D1 granulite-facies peak metamorphism at ~1950 Ma (Table 1; Figure 8) [20,21,37,39,67,68,69].

Subsequently, the D2 deformation mainly produced NW(W)-SE(E)-trending tight to open doubly plunging upright F2 folds that have variably deflected the previous D1 structures (Figure 8b) [21,69]. It is considered that D2 coevally developed with post-peak decompression processes [21,69]. The crystallization ages of pre-D2 and post-D2 leucocratic dykes indicate that D2 approximately took place at some time between ~1932 Ma and ~1899 Ma (Table 1; Figure 8) [21,39]. Notably, the superposition of F2 antiforms on S1 foliations gave rise to a series of gneiss domes, particularly in the northern Qianlishan Complex [21,39]. Qiao et al. (2023) proposed that these well-preserved domal structures are among the most representative Paleoproterozoic gneiss domes of the Khondalite Belt, which were less affected by post-D2 deformation [39].

In this study, nearly E-W-striking ductile shear zones were discovered in the Qianlishan Complex (e.g., Figure 3 and Figure 4). As discussed above, available geochronological data show that they occurred between ~1902 Ma and ~1821 Ma (Table 1; Figure 6, Figure 7 and Figure 8). Here, we attribute the QDSZ to the D3 deformation that affected the studied area. Within these high-strain zones, the khondalites and pre-kinematic intrusions were variably subjected to mylonitization and shear deformation (Figure 3, Figure 4 and Figure 5). Our (micro-)structural observations reveal that the QDSZ were probably developed under high-temperature (T > 650 °C) deformation conditions (e.g., Figure 3c,d and Figure 4b,d,i). These shear zones are characterized by steep to sub-vertical S(SW)-dipping mylonitic foliations and mainly exhibit a top-to- N(NE) sense of shearing (Figure 3, Figure 4 and Figure 5). In summary, the Qianlishan Complex records three phases of Orosirian deformation, namely D1 (1.97–1.93 Ga), D2 (1.93–1.90 Ga) and D3 (1.90–1.82 Ga) (Figure 8) [21,39,69], respectively.

5.3. Implications for the Deformation History of the Khondalite Belt

Integrated with previous studies, structural analysis and dating results of the Qianlishan Complex can provide important constraints on the geochronological framework of the deformation history of the Khondalite Belt. As shown in Table 1 and Figure 9, we compiled available deformation-related geochronological data from forty-five samples from the Khondalite Belt. Similar to the Qianlishan Complex, other high-grade complexes in the Khondalite Belt experienced three phases of deformation (D1–D3) in the late Paleoproterozoic [21,24,26,27,28,39]. As mentioned earlier, it is considered that D1 structures of the Qianlishan Complex were formed at ca. 1976–1932 Ma [21,39]. This is consistent with structural observations in the Helanshan Complex, where two syn-D1 leucocratic dykes that have been isoclinally F1-folded or S1-foliated were dated at 1954 ± 13 Ma and 1949 ± 8 Ma [28]. Similar ~1950 Ma syn-D1 leucocratic dykes were also reported in the Wulashan-Daqingshan and Jining complexes [47,54,56]. Additionally, the D1 deformation is inferred to be simultaneously accompanied by high-pressure granulite-facies peak metamorphism [21,22,28,69]. Regionally, high-pressure granulites of the Helanshan, Qianlishan, Wulashan-Daqingshan and Jining complexes yielded metamorphic zircon ages of 1963–1946 Ma, 1966–1941 Ma, 1965–1943 Ma and 1957–1945 Ma, respectively [20,21,22,33,35,37,47,54,65,78]. Subsequently, the timing of the D2 deformation in the Qianlishan Complex was constrained at ca. 1932–1899 Ma [21,39,69]. This interpretation is further supported by the crystallization age of a 1911 ± 43 Ma syn-D2 leucocratic dyke that truncated S1 foliations and was deflected by F2 folds in the Helanshan Complex [28].

Later, a series of orogen-parallel ductile shear zones appeared throughout the Khondalite Belt [24,26,27,28,64,65,77]. Of these, the D3 deformation led to the development of ductile shear zones in the Qianlishan Complex at ca. 1902–1821 Ma (Figure 8). Comparably, five mylonites from the Wulashan-Daqingshan Complex were dated and their zircon overgrowth rims yielded metamorphic ages ranging from 1906 ± 13 Ma to 1853 ± 6 Ma (Table 1; Figure 9) [47,54]. These ages are coherent with our inference that the widespread shear zone activity of the Khodnalite Belt probably started at ~1900 Ma. Similar metamorphic overgrowth rims with ages of 1866 ± 12 Ma and 1866 ± 22 Ma were obtained in high-temperature mylonites from the Helanshan and Jining complexes [65,78]. Remarkably, Gong et al. (2014) reported three biotite ⁴⁰Ar/³⁹Ar ages of 1885 ± 20 Ma, 1819 ± 14 Ma and 1814 ± 13 Ma on mylonites from the Wulashan-Daqingshan Complex and speculated that they corresponded to the ductile shearing event [27]. Notably, a biotite ⁴⁰Ar/³⁹Ar age of 1839 ± 10 Ma was also obtained in the Qianlishan Complex [75] and interpreted to be related to the D3 tectonothermal event. Additionally, the E-W-trending ductile shear zones of the Wulashan-Daqingshan Complex were cut by two post-kinematic undeformed granitic intrusions with crystallization ages of 1822 ± 17 Ma and 1819 ± 14 Ma [54,64], providing the minimum age limit for the shear deformation. Thus, the D3-related orogen-parallel shear zones of the Khondalite Belt are interpreted to have approximately developed in the period of 1.90–1.82 Ga (Figure 9). Considering all the data together, we propose that the Khondalite Belt experienced a prolonged orogenic history (>100 Myr) and three phases of Orosirian deformation (D1–D3) at ca. 1.97–1.82 Ga, related to the NNE-SSW-directed collision between the Yinshan and Ordos blocks [18,19,21,22,23].

6. Conclusions

In this study, we report for the first time the nearly E-W-trending ductile shear zones in the Qianlishan Complex of the Khondalite Belt. They are characterized by steep to sub-vertical S(SW)-dipping mylonitic foliations and mainly display a top-to-N(NE) sense of shearing. Microstructures indicate that these shear zones probably developed under high-temperature (T > 650 °C) deformation conditions. Geochronological data reveal that the development of the Qianlishan ductile shear zones approximately started at ca. 1902–1884 Ma and continued to ca. 1839–1821 Ma. The Qianlishan Complex suffered three phases of Orosirian deformation (D1–D3), of which the D3 deformation gave rise to the aforementioned orogen-parallel ductile shear zones. Deformation events D1, D2 and D3 are regarded to have occurred at ca. 1.97–1.93 Ga, 1.93–1.90 Ga and 1.90–1.82 Ga, respectively. The polyphase deformation (D1–D3) document that the Khondalite Belt underwent a protracted (>100 Myr) orogenic history in response to the collision between the Yinshan and Ordos blocks.