Syn-Tectonic Dolomite U-Pb Geochronology Constraining Intracontinental Deformation: A Case Study from the Gelouang Gold Deposit in the Qinling Orogen, China

: Determining absolute ages of orogenic faults is critical to understanding the deformation process in the upper crust, but obtaining age remains a problem due to the lack of readily available techniques. Carbonates occur as veins in faults in a range of geological settings, and thus it is a suitable mineral for U-Pb geochronology. Here, we apply the new approach of U-Pb dating on syn-tectonic dolomite veins from the Gelouang gold deposit in the western Qinling Orogen to unravel the absolute timing of the fault formation shedding new light on the regional upper crustal deformation archive. In situ LA-ICP-MS U-Pb dating of dolomite yielding a successful age of 115–112 Ma demonstrates that the dolomite precipitated coeval with tectonic events ascribed to the post-orogenic deformation phase in the Qinling Orogen. This event is possibly correlated with broader intracontinental processes and might be an inevitable response to the extensional deformation of the Qinling Orogen. The presented LA-ICP-MS dolomite U-Pb age successfully represents the age of a speciﬁc structure that encompasses the intracontinental process in the Qinling Orogen. Moreover, it demonstrates the utility of the method to decipher a response to complex deformation histories on a regional scale.


Introduction
Faulting and fracturing studies in the upper crust provide pivotal insights for understanding the mechanics of the pressure-dependent deformation process [1,2]. Moreover, faulting developed in the orogenic belt preserves an archive of how deformation controls landscape development, plate boundary interaction and the crustal deformation process. In addition to investigating the geometry, kinematics, and architecture characteristics, the timing of faults and how faults develop further through time is a prerequisite to establishing the framework of the tectonic evolution [3]. However, the absolute timing of fault slip and fracture formation remains a poorly constrained parameter, possibly due to the absence of syn-kinematic and authigenic minerals and readily available techniques. In general, carbonate is a common fault-hosted mineral that is suitable for U-Pb geochronology and is not fraught with closure temperature issues [4][5][6][7][8]. Recent in situ LA-ICP-MS U-Pb dating of accessory minerals, such as carbonate and monazite, has led to a proliferation of studies in constraining the period of fault [5,[9][10][11][12][13][14], diagenetic event [15][16][17][18], fluid flow [19][20][21], sedimentation [22,23], and mineralization of ore deposits [24][25][26][27][28][29][30][31][32] as well as magmatic-hydrothermal processes [33][34][35], demonstrating that this novel method is a powerful technique for determining the absolute timing of deformation in the upper crust.
The Qinling Orogen, which connects the Dabie Orogen to the east and the Qilian and Kunlun Orogens to the west, was formed by subduction and collision between the North Orogen showing faulting and ore deposits (modified after [45][46][47]).
The Gelouang gold deposit is located about 15 km west of Hezuo City in Gansu Province, and with an average grade of 2.18 g/t, approximately 27.7 t Au has been mined to the present day. The deposit is hosted in the metasedimentary rocks of the Triassic Gulangdi Formation and granitic rocks emplaced into the unit. Magmatism at Zaozigou is represented by sills and dikes of intermediate to felsic composition (Figures 1c and 2), which consist of porphyritic dacite, granodiorite, and porphyritic rhyolite (estimate ages of ca. 150 Ma, [64]). Two styles of mineralization have been recognized at Gelouang, which comprise early disseminated and stockwork ores. Ore-related alteration includes sericitization, sulfidation, silicification, and carbonatization of the wall rocks.  [45][46][47]).
The Gelouang gold deposit is located about 15 km west of Hezuo City in Gansu Province, and with an average grade of 2.18 g/t, approximately 27.7 t Au has been mined to the present day. The deposit is hosted in the metasedimentary rocks of the Triassic Gulangdi Formation and granitic rocks emplaced into the unit. Magmatism at Zaozigou is represented by sills and dikes of intermediate to felsic composition (Figures 1c and 2), which consist of porphyritic dacite, granodiorite, and porphyritic rhyolite (estimate ages of ca. 150 Ma, [64]). Two styles of mineralization have been recognized at Gelouang, which comprise early disseminated and stockwork ores. Ore-related alteration includes sericitization, sulfidation, silicification, and carbonatization of the wall rocks.

Sampling Description
Dolomite veins were taken from underground workings at the Gelouang gold deposit. The dolomite veins are characterized as syn-tectonic along the N-W trending normal fault and crosscut the orebodies and the wall rock Triassic slate ( Figure 2a). We sampled the exposed fresh grey to off-white dolomite veins (G28 and G29, Figure 3), which comprise a single phase with no shear displacement and a swarm of parallel contemporaneous dolomite veinlets in the vicinity. Sample G28 is from dolomite veins, which separate the mineralized slate from quartz veins (Figures 3a and 4), suggesting that the dolomite postdates slate and ore formation. At the center of the sample is a 5 mm thick dolomite vein, where euhedral rhomboid crystals form an elongate vein texture. Sample G29 is light grey with slickenfibres on the fractured fault surface (Figure 3b). Under the microscope, there is a ~200 μm calcite vein related to ore cross-cut by the dolomite vein ( Figure 4c). The blocky dolomite crystals are intergrown with the larger dolomite crystals, which, with sizes ranging from 100 μm to 3 mm, are very fine-grained and are all pointed in the same direction as the dolomite veins ( Figure 5). The elongated dolomite crystals deformed parallel or oblique to the fracture walls are observed in faults. Dated dolomite crystals are selected from the branches of the dolomite vein.

Sampling Description
Dolomite veins were taken from underground workings at the Gelouang gold deposit. The dolomite veins are characterized as syn-tectonic along the N-W trending normal fault and crosscut the orebodies and the wall rock Triassic slate ( Figure 2a). We sampled the exposed fresh grey to off-white dolomite veins (G28 and G29, Figure 3), which comprise a single phase with no shear displacement and a swarm of parallel contemporaneous dolomite veinlets in the vicinity. Sample G28 is from dolomite veins, which separate the mineralized slate from quartz veins (Figures 3a and 4), suggesting that the dolomite postdates slate and ore formation. At the center of the sample is a 5 mm thick dolomite vein, where euhedral rhomboid crystals form an elongate vein texture. Sample G29 is light grey with slickenfibres on the fractured fault surface (Figure 3b). Under the microscope, there is a~200 µm calcite vein related to ore cross-cut by the dolomite vein ( Figure 4c). The blocky dolomite crystals are intergrown with the larger dolomite crystals, which, with sizes ranging from 100 µm to 3 mm, are very fine-grained and are all pointed in the same direction as the dolomite veins ( Figure 5). The elongated dolomite crystals deformed parallel or oblique to the fracture walls are observed in faults. Dated dolomite crystals are selected from the branches of the dolomite vein.

Analtical Methods
All samples were cut into one-inch chips and in thin sections that were polished and examined using optical microscopy and in situ LA-ICP-MS U-Pb dating. Measurement of U-Pb ages from G28 was performed using laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) in the Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences. The instrumentation was an Agilent 7900 ICP-MS, coupled to a Resonetics RESOlution S-155 ArF Excimer laser source (λ = 193 nm) (Australian Scientific Instrument, Australia). Samples were ablated in the conditions with a fluence of 4 J/cm 2 , a beam diameter of 90 μm, and 8 Hz of ablation frequency. U-Pb dating of G29 was carried out with an Element XR sector field inductively coupled plasma-mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) coupled to a GeoLasPro 193 nm ArF Excimer laser ablation system (90 μm spot size) (CompexPro 102F, Coherent, Shibuya, Tokyo) at the State Key Laboratory of Ore Geochemistry, Institute of Geochemistry Chinese Academy of Sciences. Raw data from both samples were processed to calculate each element concentration offline using the ICPMS Data Cal 11.8 program. We standardized using NIST614 and the WC-1 calcite reference material for normalization [65]. The 207 Pb/ 206 Pb ratios were corrected for mass bias and the 206 Pb/ 238 U ratios for inter-element fraction by using NIST614 and DC. An additional correction has been applied on the 206 Pb/ 238 U to correct for difference in the fractionation due to the carbonate matrix [66]. This resulted in a lower intercept age of 23 WC-1 spot analyses of 254.1 ± 1.5 (MSWD = 1.5). According to the analyzed standard materials, accuracy and repeatability are assumed to be less than 2%. The U-Pb ages were plotted in the Tera-Wasserburg diagram and calculated at 2σ level by ISOPLOTR.

Analtical Methods
All samples were cut into one-inch chips and in thin sections that were polished and examined using optical microscopy and in situ LA-ICP-MS U-Pb dating. Measurement of U-Pb ages from G28 was performed using laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) in the Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences. The instrumentation was an Agilent 7900 ICP-MS, coupled to a Resonetics RESOlution S-155 ArF Excimer laser source (λ = 193 nm) (Australian Scientific Instrument, Fyshwick, Australia). Samples were ablated in the conditions with a fluence of 4 J/cm 2 , a beam diameter of 90 µm, and 8 Hz of ablation frequency. U-Pb dating of G29 was carried out with an Element XR sector field inductively coupled plasma-mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) coupled to a GeoLasPro 193 nm ArF Excimer laser ablation system (90 µm spot size) (CompexPro 102F, Coherent, Shibuya, Tokyo) at the State Key Laboratory of Ore Geochemistry, Institute of Geochemistry Chinese Academy of Sciences. Raw data from both samples were processed to calculate each element concentration offline using the ICPMS Data Cal 11.8 program. We standardized using NIST614 and the WC-1 calcite reference material for normalization [65]. The 207 Pb/ 206 Pb ratios were corrected for mass bias and the 206 Pb/ 238 U ratios for inter-element fraction by using NIST614 and DC. An additional correction has been applied on the 206 Pb/ 238 U to correct for difference in the fractionation due to the carbonate matrix [66]. This resulted in a lower intercept age of 23 WC-1 spot analyses of 254.1 ± 1.5 (MSWD = 1.5). According to the analyzed standard materials, accuracy and repeatability are assumed to be less than 2%. The U-Pb ages were plotted in the Tera-Wasserburg diagram and calculated at 2σ level by ISOPLOTR.

Results
Field observations illustrate that the fault plane may be locally curved and irregular in detail with 10-cm-width, but generally dips to N and NNW following the regional northwest trend at a steep angle (Figure 2b, oriented 345 • /80 • (strike/dip)). The studied samples consist of dolomite vein precipitated in one fracture related to an extensional set, corresponding to NNW-SSE-trending normal faults. The approximate orientation of main stresses presumably indicates the NW-SE extension for the extensional set ( Figure 2). The fracture sets contain fibrous, elongated and blocky dolomite crystals ( Figure 5). They have been observed in north-west-trending veins and in parallel veinlets formed by the crack-seal mechanism in normal faults. Fibrous crystals show minimal growth competition (Figure 5a). Elongate dolomite crystals are perpendicular to the fracture walls (Figure 5c). In the faults, blocky crystals are also arranged in stepped sides, which are characterized by the crack-seal mechanism (Figure 5d).
We present U-Pb ages of dolomite from samples G28 and G29 on Tera-Wasserburg inverse concordia diagrams as 207 Pb/ 206 Pb and 238 U/ 206 Pb linear regression isochrons. Thirteen analyses from G28 and nineteen from G29 were carried out. Age data are presented in Figure 5 and listed in Table 1. The obtained U-Pb ages in this study are variable in terms of radiogenic Pb concentrations, the amount of scattering, and the datapoint uncertainties. We have used an objective criterion of age uncertainties less than 20% and an MSWD below 1.0 to screen for robust ages. The U and Pb concentrations in G28 vary ranging from 0.63 to 22.14 ppm (mean = 8.30 ppm, n = 13), and 0.93 to 2.75 ppm (mean = 1.93 ppm, n = 13), respectively. Such a distribution of U and Pb contents is similar to G29 with the range of 0.02-11.82 ppm (mean = 3.05, ppm, n = 19) and 0.37-3.

Results
Field observations illustrate that the fault plane may be locally curved and irregular in detail with 10-cm-width, but generally dips to N and NNW following the regional northwest trend at a steep angle (Figure 2b, oriented 345°/80°(strike/dip)). The studied samples consist of dolomite vein precipitated in one fracture related to an extensional set, corresponding to NNW-SSE-trending normal faults. The approximate orientation of main stresses presumably indicates the NW-SE extension for the extensional set ( Figure 2). The fracture sets contain fibrous, elongated and blocky dolomite crystals ( Figure 5). They have been observed in north-west-trending veins and in parallel veinlets formed by the crackseal mechanism in normal faults. Fibrous crystals show minimal growth competition (Figure 5a). Elongate dolomite crystals are perpendicular to the fracture walls (Figure 5c). In the faults, blocky crystals are also arranged in stepped sides, which are characterized by the crack-seal mechanism (Figure 5d).
We present U-Pb ages of dolomite from samples G28 and G29 on Tera-Wasserburg inverse concordia diagrams as 207 Pb/ 206 Pb and 238 U/ 206 Pb linear regression isochrons. Thirteen analyses from G28 and nineteen from G29 were carried out. Age data are presented in Figure 5 and listed in Table 1. The obtained U-Pb ages in this study are variable in terms of radiogenic Pb concentrations, the amount of scattering, and the datapoint uncertainties. We have used an objective criterion of age uncertainties less than 20% and an MSWD below 1.0 to screen for robust ages. The U and Pb concentrations in G28 vary ranging from 0.63 to 22.14 ppm (mean = 8.

Interpretations of Dolomite U-Pb Ages
Carbonate precipitation within fault zones along individual fractures and fault planes is almost ubiquitous and easily recognizable in the field in the forms of slickenfibers, fault gouge, cement and veins [68,69]. Therefore, distinguishing pre-, syn-, and post-kinematic carbonate is key to constraining the timing of fault slip through the carbonate chronometer. The tectonic link could be supported by using careful field observation and petrographic analyses. Veins are intimately related to fracture mechanics, for most veins from by growth of minerals into space are created by fractures [69,70]. Several field occurrences of carbonate precipitation along the fault plane are commonly used to directly infer the timing of fault slip and associated with fracture opening. These include slickenfibres and carbonate filling veins that occurred in extensional jogs, opening-mode fracture with no displacement (single phase), and as multi-phase of sub-parallel contemporaneous carbonates, vuggy crystals growth, and en echelon fractures [71]. Simultaneously, linking mineral precipitation to fault kinematics and associated fracturing processes depends on its origin and morphology. Roberts and Holdsworth (2022) reviewed what examples of carbonate mineralization can more confidently infer the connection than others and how the mechanism can more reliably link to fault kinematics from "crack-seal-slip" [72], "crack-seal" to "crack-fill" types [73]. Ramsay (1980) introduced the term "crack-seal" mechanism, which is appropriate in the context, suggesting the veins, especially those with elongate to fibrous crystals, have been interpreted in terms of this mechanism [74]. Generally, veins exhibit a wide range of internal structures ascribe to various shapes of vein-filling carbonates precipitation and their growth direction [70,74,75]. As crystals grow side by side in the same direction, the crystals become elongated in the growth direction. If crystals grow into an open space, on growing, nucleation of new crystals suppresses the elongate shape, and more equant grains fill the vein to produce a blocky texture ( Figure 5). Fibrous crystals sometimes develop with extreme length/width ratios, and their boundaries are typically smooth. In previous literature, fibrous growth is sometimes attributed to the opening of a vein in small increments, which can only be formed if growth competition is inhibited, while blocky shapes are attributed to growing into an open or fast vein opening [70,76]. In this sense, dolomite showing elongated blocky textures indicate they are syn-kinematic, while blocky textures provide evidence of the precipitation after vein opening or at lower rates than vein opening. Despite this, they are all formed by a crack-seal mechanism, and stepped sides indicate syn-kinematic growth. Such veins, especially crystals precipitation features, indicate the studied dolomite veins are syn-kinematic and occurred synchronously with movement along the fault plane [77][78][79][80]. Another key observation linking measured dolomite ages with specific fault motion is that the dolomite precipitation has not been altered or recrystallized, therefore, the explicitly the U-Pb isotopic system has not been reset [81]. The disposition of dolomites in the Gelouang deposit indicates that their growth occurred from both fracture walls to the vein centre and from one vein wall or another vein wall. These crack-seal dolomites fill along the fault direction and record information on growth which is unlikely to have formed during the phases of post-faulting fluid flow and dolomite reprecipitation. In addition, the data points fit within the error of the concordia line and provide a robust estimate of lower intercept ages, suggesting the absence of Pb diffusion. In this case, structural observations suggest the dolomite precipitated synchronous with movement along the fault plane as a consequence of fracture opening. We thus interpret the ages as the precipitation of dolomite veins coeval with the fault formation. The N-W dolomite veins within the fault zone cut Early Triassic slate and orebodies in the Gelouang gold deposit, indicating that the fault formation was later than the host rocks formation and mineralization. The obtained datapoints from dolomite overlap with each other within 2-sigma uncertainties, suggesting that they represent the timing of dolomite formation. The two samples display isochron ages of 112 ± 4 Ma and 115 ± 4 Ma, indicating that the dolomite veins precipitated in the Early Cretaceous.

Implications for Intracontinental Orogen
In light of the above discussion, the ca. 114 Ma U-Pb ages from the fault date brittle deformations in the Qinling Orogen. These U-Pb ages provide new absolute chronological markers for dating the tectonic events in the Qinling Orogen. The fault is assumed to correlate with the extensional collapse and was interpreted as the consequence of the intracontinental process in the Early Cretaceous. This tectonic evolution history of the Qinling Orogen has been well documented by lines of geological, geophysical, geochemical, and geochronological records on magmatic events from the Early Jurassic to Paleogene [37][38][39]41,46,52], indicating the diverse units of the Qinling Orogen witnessed complex deformation.
Following the collision in the Triassic, the Qinling Orogen underwent tectonic transition, and the regime changed from compressional deformation to extensional rifting from the Early Jurassic-Early Cretaceous to Late Cretaceous-Paleogene. This intracontinental process is indicated by Mesozoic strata formations with unconformities or angular unconformities near contacts between different tectonic units [38,82]. In the western Qinling Orogen, the Jurassic and Cretaceous strata are also separated by an angular unconformity in places [83]. Similarly, the north margin of the South China Block contains the pre-Cretaceous strata incorporated into the foreland fold-thrust belt, illustrating that the Qinling Orogen evolved into a compressional tectonic setting [37,40]. These southwards fold-thrust deformations were triggered by overthrusts of the South Qinling Terrane [40,42,53,54,82]. After the intensive compression events, the Qinling Orogen evolved into an extensional regime, forming extensive sinistral strike-slip shearing and sinistral-slip-related echelon sedimentary basins along previous faults [42]. Sun et al. (2022) highlighted the decompression and exhumation phase with mylonites and a ductile shear zone occurring at 119 Ma (Amphibolite Ar-Ar) in the Shagou shear zone [44]. In addition, the 126 to 90 Ma intracontinental deformation phase thermotectonic evolution as inferred from the western and eastern Qinling Orogen is correlated using apatite fission-track data [41,84], which are similar to the deformation and sedimentation investigated in Qinling area [85]. Furthermore, voluminous mafic and felsic magmatism and large-scale Mo-Au-Ag polymetallic mineralization are extensively distributed in the Qinling Orogen [46,86,87]. Their geology and geochemistry suggest an intracontinental extensional setting during the Late Mesozoic [58,88]. For example, a large number of nearly vertical mafic dikes intruded into the western and central part of the Qinling Orogen and showed the ages of ca. 114 Ma (Zircon U-Pb) with a strike roughly parallel to the orogen. Sporadic Late Cretaceous mafic magmatic rocks have been reported in the south margin of the North China Block [89], such as Huanglongpu diabase (129 Ma, Zircon U-Pb), Tianqiaogou diorite (122 Ma, Zircon U-Pb), Funiushan lamprophyre (117 Ma, Zircon U-Pb, [90]), and Niangniangshan granitoids (~123 Ma, Zircon U-Pb, [89]). Ca. 110 Ma felsic dyke intrusion is identified from the Laojunshan region in eastern Qinling Oregon, and shows tectonic regime transformation from compression to extension [88,91].
Taken together, these data mostly record the intracontinental tectonic events and the relative age of deformation and syn-deformational deposition in the eastern part of the Qinling Orogen, while scarce data were available from the western Qinling Orogen. This intracontinental regime is evident from the ages of dolomite veins in the Xiahe-Hezuo district, which crosscut the deformed strata and were emplaced in association with the fault and the intrusions. Synthesizing the above regional works, the dolomite U-Pb ages are close to the period of crustal deformation in the Early Cretaceous, exhibiting an inevitable response to the tectonic evolution occurring within the West Qinling Orogen. The orientations of the analysed normal faults fit with the observed main normal faults related to north-south trend graben/half-graben tectonics in eastern Qinling Orogen. These ages constrain the timing of the brittle deformations of the Qinling Orogen during the Early Cretaceous. This tectonic event appears to be correlated with the intracontinental extensional regime of the Qinling Orogen.

Conclusions
(1) Dolomite from veins along the fault plane was dated with the U-Pb system, yielding ages of 112 ± 4 Ma and 115 ± 4 Ma, which we interpret as reflecting syn-deformational precipitation of the dolomite. (2) The new geochronological finding constrains the post-orogenic phase of faulting correlated with intracontinental extensional regime in the Qinling Orogen during the Early Cretaceous. This event exhibits an inevitable response to the tectonic evolution on a regional scale.

Data Availability Statement:
All the data is presented in the paper.