Tectono-Thermal Events of Coal-Bearing Basin in the Northern North China Craton: Evidence from Zircon–Apatite Fission Tracks and Vitrinite Reflectance

Abstract

In order to further reveal the tectonic activity of the central and northern North China Craton (NCC) since late Paleozoic, the Datong coal-bearing basin was selected as the research object. The tectono-thermal events and uplifting cooling events of the basin were retrieved through zircon and apatite fission tracks and vitrinite reflectance measurements. The research shows that the Datong coal-bearing basin experienced three tectono-thermal events with ages of 245–207 Ma (middle–late Triassic), 179 ± 9 Ma (early Jurassic), and 140 Ma to 78 ± 11 Ma (middle–late Cretaceous), respectively. That just coincides with the lamprophyre activity, Kouquan fault activity, and Zuoyun basaltic andesite magmatic activity which surround the Datong coalfield. The basin also experienced three uplift events with the peak ages of 202 ± 18 Ma (late Triassic), 157 ± 7 Ma (late Jurassic), and 45 ± 3 Ma or 36 ± 3 Ma (middle Eocene), respectively. The Datong Permo-Carboniferous and Jurassic coal vitrinite reflectance proved that the average metamorphism temperature is 104–108 °C, even reaching 163–367 °C. The fission track results showed that the paleotemperature was even higher than 170–250 °C from 117 to 282 Ma and 80–120 °C from 20 to 68 Ma, in the Datong coal-bearing basin. The results show that the deep tectonic activities of the NCC were still active in the Mesozoic and even Cenozoic Paleogene.

1. Introduction

Fission-track is an effective thermochronological method which developed in the 1960s to obtain geochronological data. It can be used to determine geological ages by applying the decay rate of radioactive materials and tracking the quantities in a unit area produced during nuclear fission [1,2,3,4,5,6,7,8]. This technique has been widely used to analyze the uplift and cooling history of orogenic belts, to recover the thermal evolution and burial subsidence history of basins or provenance areas, to study the basin–mountain coupling relationship, fault activity time, hydrothermal metallogenic age, and other geological events. With the support of the research fruit, a new field of studying mineral-fission track annealing, using nuclear fission technology, has emerged [9,10,11,12,13,14,15,16]. Furthermore, the random huminite/vitrinite reflectance and apatite fission-track methods could be useful parameters for evaluating the paleotemperature of thermal and geodynamic evolution, and the burial history of coal-bearing sequences [17,18,19,20,21,22,23,24,25,26].

The Datong coalfield is located in the northern North China Craton (NCC) [27], between the west NCC and east NCC (Figure 1a,b). The Datong coalfield has a mining history of over 1500 years. It hosts Carboniferous–Permian and Jurassic coal-bearing strata.

In recent years, many researchers have studied the sedimentary, tectonic, and magmatic activities that have occurred since the late Paleozoic in the Datong area of the Shanxi Province [29,30,31,32,33]. They have confirmed that the Datong basin is still part of the NCC large sedimentary depression basin (i.e., the Ordos Basin) in the early–middle Triassic, although the NCC was subjected to the tectonic compression of the Yangtze and Siberian Plates in the late Triassic [34,35,36]. Because of the influence of the Siberian Plate, the northern Datong basin underwent a gradual uplift and denudation, which resulted in the late Paleozoic and Triassic strata becoming denuded in some areas [32,33,37,38]. During field explorations and coalfield drilling, numerous alkaline mafic–ultramafic lamprophyre sills and diabase dikes and a few carbonatite dikes were found in this basin, intruding the Taiyuan Formation or Shanxi Formation strata (Figure 1c and Figure 2) [39,40,41,42].

The aforementioned studies indicated multistage magmatic activity controlled by regional tectonic movements since the late Paleozoic, that has resulted in coal-bearing strata hiatus and magmatic intrusion. However, the specific characteristics and time intervals of these events are still unclear. Therefore, we used the fission track thermochronology of zircon and apatite to reconstruct the sedimentary–tectonic–magmatic events in this basin since the late Paleozoic. Furthermore, we attempted to elucidate the influences and restrictions of these events on the Carboniferous–Permian–Jurassic coalification and reveal the coalification evolution processes and coal metamorphism mechanisms. The results are significant for restoring the eastern boundary of the proto Triassic Ordos basin and helpful in understanding the surface response of the deep craton activity that was caused by the destruction of the NCC during the Mesozoic.

2. Geological Setting

As a part of the late Paleozoic giant coal-accumulating basin of the NCC, the Datong coal-bearing basin experienced weathering and denudation for up to 140Ma after the Ordovician, and was redeposited in the early late Carboniferous. The basement of the basin is Archaean Sanggan gneiss, and the Proterozoic, upper Ordovician, Silurian, Devonian, lower Carboniferous, Triassic, upper Jurassic, and upper Cretaceous strata are absent in the basin under the influence of multi-stage tectonic magmatic activities [43]. The lower Jurassic Yongdingzhuang Formation and the lower Cretaceous Zuoyun Formation are all in contact with the underlying strata in angular unconformity. The Permo-Carboniferous Taiyuan Formation, Shanxi Formation, and Jurassic Datong Formation are the main coal-bearing strata [44,45,46], in which, the Nos. 3, 5, and 8 (C8) coal seams are present in the Taiyuan Formation and the No. 4 (PS4) coal is in the Shanxi Formation; also, 15 other coal seams (Jd) in the Datong Formation (Figure 2). The dominant sedimentary environment changes from the coastal delta facies of the Taiyuan Formation to the river facies of the Shanxi Formation and the lake facies of the Datong Formation.

After Jurassic, the Datong Basin became the eastern margin of the wide and gentle folds of the Ordos Basin [47]. The basin boundary is marked by four tectonic features (Figure 1): the Hongtaoshan anticline (NW 50°) to the southeast; the Kouquan–Emaokou fault to the east; the Qingciyao reverse fault to the northeast; and the Maihutu–Weiyuanbao fault to the northwest. Only the gently plunging folds developed in the coalfield (Figure 1c).

Extensive alkaline mafic–ultramafic lamprophyre and carbonatite dikes intrude the Permo-Carboniferous coal strata (Figure 2 and Figure 3) [48]. The K–Ar and Rb–Sr geochronological ages of the lamprophyre biotite are 242 ± 53 Ma, 229 ± 11 Ma, and 224–198 Ma [38,48]. The intrusion age of the carbonatite dyke associated with the lamprophyre is 132.9 Ma [49], and the whole rock K–Ar age is 131–133 Ma. Additionally, a suite of early Cretaceous basaltic andesite covering an area of almost 3 km² is emplaced in the Zuoyun Formation nearby the Zuoyun–Jiugaoshan district (Figure 1c), along the western side of the basin [31].

3. Samples and Methods

The random vitrinite reflectance (%R_o) method was applied to quantify the maximum paleotemperature experienced by the Permo-Carboniferous and Jurassic stratum. The original No. 8 coal of the Taiyuan Formation (C8) and No. 4 coal of the Shanxi Formation (PS4) samples were collected from the subsurface. However, there are not many Jurassic coal resources left, and only a part of the Jurassic coal samples was collected from the underground of the Jinhuagong mine (JJ), Qinciyao mine (JQ), Sitai mine (JS), Xinzhouyao (JX), and Yanya mine (JY) (Figure 3). Every coal maceral section was made using a cool forming method after the whole-rock samples were crushed into 0.1 to 1.0 mm and then polished with white fused alumina powder and aluminum oxide polishing slurry, following the coal petrography procedure standard [50]. Then, the %R_o values of the coal were measured, using an Olympus BX51 microscope equipped with a Y-3 spectrophotometer. For measuring the maximum reflectance on a particulate sample, 50× oiled objectives were used with a peak transmittance in the range of 546 ± 5 nm and a half-peak transmittance band of less than 30 nm. More than 80 measurements per sample were measured. The maximum reflectance was taken after the microscope stage was rotated 360° (ISO 7404-5:2009) [51]. The maceral classifications and terminology applied in the current study are based on ISO 7404-4: 2017 and the ICCP System, 1994 [50,52].

The random vitrinite reflectance (%R_o) is an important index for coal rank determination and the thermal history analysis of sedimentary basins, and is not affected by later tectonic uplift and exhumation [21,22,53,54,55]. Its value increases with increasing temperature and burial depth. Thus, it can record maximum paleotemperature [56,57]. Therefore, the changing trends of the %R_o values under unconformity surfaces can be used to restore the tectonic uplift history and stratum denudation thickness [23,55,58,59,60,61,62]. Many methods use %R_o to study burial paleotemperatures, such as the Hood diagram [63], Karweil diagram [64], Barker formula [54,65,66,67], vitrinite reflectance gradient method, fitting calculation TTI method, and an improved model based on the TTI method [68,69,70,71,72,73,74].

Barker and Pawlewicz [54,65] proposed that calculating the paleotemperatures using the mean %R_o is relatively simple, and it is now commonly used (Equation (1)) [65]. The advantage of this equation is that considering the heating duration variables is not necessary, which is suitable for the geological evolution of organic matter with maximum burial temperatures T_peak between 25 °C and 325 °C and the %R_o of vitrinite between 0.2% and 4.0% [66]. Moreover, it has been proposed that, in the case of contact metamorphism, the accurate maximum paleotemperature could be obtained only when the coal seam is 0.3 times wider than the intrusive body or when the maximum paleotemperature is lower than 300 °C. In addition, Yao et al. [75] suggested that the paleotemperature value may be higher if this equation is used when the organic matter has achieved high maturity [75]:

T_peak = [ln(R_o) + 1.19]/0.078

(1)

In this paper, %R_o,max was used to replace %R_o (Equation (2)), where %R_o,max is the mean maximum reflectance of organic matter and T_peak is the maximum burial temperature:

T_peak = [ln(R_o,max) + 1.19]/0.078

(2)

For the fission track analyses, three groups of sandstone samples were collected from the Xiaoliaotangou and Sigou outcrop sections in northern Kouquangou of the Datong coal-bearing basin (Figure 2). The samples included medium–coarse-grained sandstone (K₂) (Figure 4a) from the base of the Taiyuan Formation (C₂t) of the upper Carboniferous, medium–fine-grained sandstone (K₃) (Figure 4b) from the bottom of the Shanxi Formation (P₁s) of the lower Permian, medium–coarse-grained sandstone (K₁₁) (Figure 4c) from the base of the Datong Formation of the middle Jurassic (J₂d), and compact basaltic andesite samples (Figure 4d) from the lower Cretaceous Zuoyun Formation (K₁z) in Jiugaoshan of Zuoyun county (Figure 1). The four group samples were collected from different field sites of each stratum to obtain reliable analysis results. The ZFT analysis results were obtained from three groups of sandstone samples. The AFT analysis results were obtained from the Datong Formation and Shanxi Formation sandstone samples and the basaltic andesite samples from the Zuoyun Formation.

The basic theory of fission tracks is founded on the annealing characteristics of mineral fission tracks. Fission tracks of natural minerals enriched in ²³⁸U can only be preserved below a certain critical temperature (closure temperature). Following the increase in temperature and heating time, the density and length of the tracks decrease and shorten, until they disappear completely, which is called annealing. The closure temperatures of the zircon fission track (ZFT) and the apatite fission track (AFT) are 210 ± 40 °C and 100 ± 20 °C, respectively [76]. These suggest that the temperature range of the zircon partial annealing zone is generally 170–250 °C, the temperature range of the apatite partial annealing zone is generally 80–120 °C, and the complete annealing temperatures of zircon and apatite are 250 °C and 120 °C, respectively. The fission-track ages of zircon and apatite record the time when the minerals cooled to below 210 °C and 100 °C, respectively, which are called the cooling ages [77,78]. Because the annealing processes of mineral fission tracks are only related to the annealing temperature and time and have no obvious relationship with annealing physical and chemical conditions, such as pressure, pH, and Eh, the annealing degree of the fission tracks can be regarded as a function of the mineral sealing temperature and time. The fission-track age measurement mainly adopted the direct testing method before the 1980s. More recently, it has used the zeta constant calibration method, which has a smaller measurement error.

The fission-track samples were analyzed at the State Key Laboratory of Geological Process and Resources, China University of Geosciences (Beijing, China). The Track-key [79] software was used to treat the data. After being observed under a Lecia DM4P microscope, the grains were crushed to 60 mesh, the samples were randomly selected and dried, and the grains were purified, using conventional magnetic separation and gravity separation to separate their zircon and apatite mineral grains. The ZFT testing process is as follows. First, zircon grains were placed on polytetrafluoroethylene propylene plastic slices, made into thin slices, and polished to reveal the inner surface of mineral grains. Second, the thin slices were etched with a high-temperature melt of KOH + NaOH for 20–35 h, to reveal spontaneous tracks at 210 °C. Subsequently, a low-uranium muscovite slice was used as an external detector to cover the optical slice, which contacts closely with the inner surface of the grains and is irradiated by thermal neutrons along with a CN2 standard uranium glass. Finally, the induced tracks were revealed by etching for 20 min in 40% hydrofluoric acid (HF) at 25 °C [10]. On another note, the AFT testing process is as follows. First, the apatite grains were placed on a glass slice, dripped with epoxy resin, and were ground and polished to expose the inner surface of the mineral. Second, the spontaneous track was revealed by etching 7% HNO₃ at 25 °C for 30 s. Then, a low-uranium muscovite external detector was incorporated into the reactor for irradiation. After that, the induced tracks were revealed by 40% HF etching at 25 °C for 20 s. The neutron flux was calibrated using a CN5 uranium glass [10]. The age values were calculated, according to the N constant method recommended by the International Union of Geological Sciences and the standard fission-track age equation. The detailed experimental procedures and principles can be referred to in the literature [10,80].

In this study, the zeta constants of apatite and zircon are 392 ± 18.7 and 102.1 ± 3.8, respectively [81]. The results of the sample analysis were tested, using Chi-square statistics, and P(χ²) was the test probability. P(χ²) > 5% usually indicates that the ages of single mineral granules belong to the same age group, reflecting the same thermal history activity; thus, the pooled age is used. P(χ²) < 5% suggests that a sample has not passed the test, indicating that the ages of the samples may not be of a single age group; thus, the central age is applied. In addition, the pooled age is still applied if the single grain age of the sample is 0 Ma [10].

4. Results

4.1. Random Vitrinite Reflectance and Paleotemperature

Considering the characteristics of coal metamorphism in the Datong coalfield, the %R_o,max of the Permo-Carboniferous coal seam is below 0.96, suggesting that Barker’s formula for calculating paleotemperatures is suitable in this study. The %R_o,max data were selected from the test results of the No. 8 coal of the Taiyuan Formation (C8 coal, 76 normal coal samples, and 12 contact metamorphism coal samples, a total of 88 coal samples) and the No. 4 coal of the Shanxi Formation (PS4 coal, 35 normal coal samples, and 11 contact metamorphism coal samples, a total of 46 coal samples). Figure 3 shows the sampling location.

The %R_o,max of the C8 normal coal is 0.57%–0.96% (mean = 0.74%) (

Table 1. The maximum vitrinite reflectance (%R_o,max) and the paleotemperature (T_peak/°C).

No.	C8 Ro,max	Tpeak	No.	C8 Ro,max	Tpeak	No.	C8 Ro,max	Tpeak	No.	C8 Ro,max	Tpeak	No.	PS4 Ro,max	Tpeak	No.	PS4 Ro,max	Tpeak
1	0.96	147	20	0.82	127	39	0.73	112	58	0.67	101	1	0.83	128	20	0.7	106
2	0.96	147	21	0.82	127	40	0.72	110	59	0.67	100	2	0.81	125	21	0.69	105
3	0.94	144	22	0.82	126	41	0.71	109	60	0.66	99	3	0.81	125	22	0.69	105.
4	0.93	143	23	0.8	124	42	0.71	108	61	0.65	98	4	0.8	124	23	0.67	101
5	0.88	135	24	0.8	124	43	0.7	107	62	0.65	97	5	0.79	121	24	0.67	101
6	0.86	134	25	0.79	123	44	0.7	107	63	0.64	96	6	0.78	120	25	0.67	101
7	0.86	133	26	0.79	123	45	0.7	107	64	0.64	96	7	0.76	117	26	0.67	101
8	0.86	132	27	0.79	123	46	0.69	106	65	0.64	95	8	0.75	115	27	0.67	101
9	0.85	132	28	0.79	122	47	0.69	105	66	0.64	94	9	0.75	115	28	0.67	101
10	0.85	131	29	0.79	122	48	0.69	105	67	0.63	94	10	0.75	115	29	0.66	99
11	0.85	131	30	0.78	120	49	0.69	105	68	0.63	94	11	0.75	115	30	0.65	98
12	0.85	131	31	0.78	120	50	0.69	105	69	0.63	93	12	0.75	115	31	0.64	95
13	0.85	131	32	0.78	120	51	0.69	105	70	0.63	93	13	0.74	114	32	0.64	95
14	0.84	130	33	0.78	120	52	0.69	104	71	0.62	92	14	0.74	113	33	0.62	92
15	0.84	129	34	0.77	119	53	0.68	103	72	0.61	90	15	0.73	112	34	0.61	89
16	0.83	128	35	0.77	119	54	0.68	103	73	0.61	90	16	0.73	112	35	0.59	84
17	0.83	128	36	0.77	119	55	0.68	102	74	0.61	89	17	0.72	110	—	—	—
18	0.83	128	37	0.75	116	56	0.67	101	75	0.59	85	18	0.72	110	—	—	—
19	0.82	127	38	0.73	113	57	0.67	101	76	0.57	80	19	0.71	108	—	—	—

), with a T_peak of 80–147 °C (mean = 114 °C). The %R_o,max of the PS4 normal coal is 0.59%–0.83% (mean = 0.71%) (

Table 2. The contact metamorphism coal vitrinite reflectance (%R_o,max) and the calculated paleotemperature (T_peak/°C).

No.	C8 Borehole	Ro,max	Tpeak(°C)	PS4 Borehole	Ro,max	Tpeak(°C)	No.	C8 Borehole	Ro,max	Tpeak(°C)	PS4 Borehole	Ro,max	Tpeak(°C)
1	303	5.16	362	W907	5.36	367.	7	705	3.70	320	W7	2.34	261
2	2105	4.96	357	W901	4.88	355	8	1907	3.60	316	2504	2.16	251
3	8404	4.70	350	W603	4.58	347	9	2106	3.57	315	W5	1.92	236
4	2203	4.41	342	W3	4.23	3367	10	8401	3.49	312	W1006	1.15	170
5	1706	4.13	334	W13	3.85	324	11	2306	3.00	293	W14	1.09	163
6	8518	3.75	321	W506	3.21	301	12	T901	2.24	255	—	—	—

), with a T_peak of 83–128 °C (mean = 108 °C). On the basis of the %R_o,max of the 12 contact metamorphism coal samples from the C8 and 11 samples from the PS4, the highest paleotemperatures recorded were approximately 255–362 °C (

Table 3. The coal vitrinite reflectance (%R_o,max) and the calculated paleotemperature T_peak(°C) for Jurassic Datong Formation.

Coal seam	Ro,max	Tpeak	Coal seam	Ro,max	Tpeak	Coal seam	Ro,max	Tpeak	Coal seam	Ro,max	Tpeak
J2	0.78	120	J7	0.71	108	J11	0.69	105	J14	0.76	117
J2	0.65	97	J7	0.67	101	J11	0.73	112	J14	0.75	115
J2	0.63	93	J7	0.75	115	J11	0.70	107	J14	0.61	89
J2	0.66	99	J8	0.61	89	J11	0.67	101	J14	0.78	120
J2	0.75	115	J8	0.60	87	J11	0.69	105	J14	0.75	115
J2	0.69	105	J8	0.74	114	J11	0.73	112	J14	0.87	134
J2	0.70	107	J8	0.78	120	J11	0.76	117	J14	0.72	110
J2	0.70	107	J9	0.67	101	J12	0.74	114	J14	0.80	124
J3	0.69	105	J9	0.74	114	J12	0.76	117	J14	0.80	124
J3	0.75	115	J9	0.55	76	J12	0.70	107	J14	0.70	107
J3	0.75	115	J9	0.65	97	J12	0.70	107	J14	0.64	95
J3	0.55	76	J9	0.70	107	J12	0.72	110	J14	0.87	134
J3	0.75	115	J10	0.64	95	J12	0.65	97	J15	0.89	137
J3	0.64	95	J10	0.65	97	J12	0.69	105	J10	0.81 *	125
J3	0.63	93	J10	0.72	110	J12	0.80	124	JJ2	0.813	126
J3	0.75	115	J10	0.74	114	J12	0.76	117	JJ3	0.851	136
J4	0.54	73	J10	0.72	110	J12	0.79	122	JJ7	0.872	135
J5	0.51	66	J10	0.60	87	J12	0.77	119	JS12	0.666	100
J5	0.62	91	J10	0.72	110	J12	0.85	131	JS14	0.734	113
J5	0.61	89	J10	0.72	110	J12	0.76	117	JX11	0.587	84
J5	0.52	69	J11	0.78	120	J12	0.70	107	JX15	0.746	115
J7	0.72	110	J11	0.74	114	J14	0.93	143	JQ7	0.684	104
J7	0.59	85	J11	0.64	95	J14	0.72	110	JQ11	0.739	114
J7	0.69	105	J11	0.74	114	J14	0.63	93	JY14	0.577	82
J7	0.94	144	J11	0.65	97	J14	0.68	103	—	—	—
J7	0.69	105	J11	0.67	101	J14	0.76	117	—	—	—

Note: 0.81 *—data from Bai and Li, 2001; J2~J14-No. 2~No.14 coal from Datong Fm.; JJ2-No. 2 coal from Jinhuagong mine; JQ7-No. 7 coal from Qinciyao mine; JX11-No. 11 coal from Xinzhouyao; JS12-No. 12 coal from Sitai mine, JY14-No. 14 coal from Yanya mine. For the sampling location, see Figure 3.

) in C8 and 163–367 °C in PS4. The highest T_peak could cause the zircons to anneal and produce fission tracks, with the zircons even annealing completely until the fission tracks disappear. This indicates that the temperature could be higher than the annealing temperature of zircon.

We thoroughly collected the %R_o,max of the mining coal in the Jurassic Datong Formation, including 10 subsurface samples, one literature sample data [82], and 91 measurements from the exploratory geology report (

Table 4. The test results of fission track ages and lengths for the samples of Datong Coal-bearing basin.

Mineral	Sample	Strata	Strata Age/Ma	ρs/(105/cm2) (Ns)	Ρi/(105/cm2) (Ni)	Ρd/(105/cm2) (-)	P(χ2)/%	Central Age/Ma (±1σ)	Pooled Age /Ma (±1σ)	Length /µm(n)	Grain (N)
apatite	KKA	K1z	100.5 ~145.0	2.736 (78)	12.383 (353)	18.079 (8116)	98.9	78 ± 11	78 ± 11		12
	KJA	J2d	168.3 ± 1.3~174.1 ± 1.0	1.834 (330)	14.826 (2667)	18.52 (8116)	98.1	45 ± 3	45 ± 3	12.1 ± 2.2 (100)	28
	KPA	P1s	283.5 ± 0.6~298.9 ± 0.15	1.674 (412)	16.741 (4119)	18.3 (8116)	100	36 ± 3	36 ± 3	12.0 ± 2.2 (84)	28
zircon	KJZ	J2d	168.3 ± 1.3~174.1 ± 1.0	120.788 (5723)	50.882 (2408)	13.134 (10030)	61.6	157 ± 7	157 ± 7		24
	KPZ	P1s	283.5 ± 0.6~298.9 ± 0.15	114.329 (7586)	36.758 (2439)	13.478 (10030)	0	202 ± 18	211 ± 10		27
	KCZ	C3t	298.9 ± 0.15~307.0 ± 0.1	78.346 (5545)	28.626 (2026)	12.905 (10030)	0.6	179 ± 9	178 ± 8		29

Note: ρ_s—spontaneous fission-track density in minerals; ρ_i—recorded mineral induction fission density by mica external detector; ρ_d—induction fission density by standard U glass monitor; (Ns), (Ni) and (-)—the fission quantity, respectively; P(χ²) is Chi-sq inspection probability; N is the amount of actual mineral grain; n is the actual number of the fission-track branches.

). As can be inferred from Table 4, the %R_o,max of the Datong Formation coal ranges from 0.51% to 0.94%, with an average value of 0.71%. According to Barker’s formula for calculating T_peak values, the maximum values recorded in the Datong Formation coal ranged from 66–144 °C (mean = 108 °C). The mean %R_{o, max} of the Datong Formation coal is slightly lower than that of C8, but similar to that of PS4. The similar %R_o,max of the Permo-Carboniferous and Jurassic coal seams in the Datong coalfield indicated that they experienced similar maximum paleotemperatures.

4.2. Zircon Fission Tracks (ZFT)

The samples from the Taiyuan Formation (KCZ) and the Shanxi Formation (KPZ) were analyzed, using Zircon Fission Tracks. All of the fission-track ages were lower than the stratigraphic age, indicating that the rocks experienced a higher paleotemperature and zircon had undergone annealing (Figure 5). Although some of the ZFT ages from the Datong Formation (KJZ) sandstone were older than the stratigraphic age, they can be used to analyze tectonic thermal events or to predict provenance.

The P(χ²) test of the KJZ zircon samples was greater than 5%, indicating that they underwent the same thermal event. Therefore, the pooled age of the samples is presented as 157 ± 7 Ma (193–128 Ma) (Table 1, No. 3, and No. 6 in Figure 5a). According to the age analysis (Figure 5a), a total of 24 zircon detrital grains could be measured in the Datong Formation, with 20 grains younger than the host strata, with ages ranging from 128 to 174 Ma (No. 19 in Figure 5a), and four zircon grains older than the host strata, with ages ranging from 177 (No. 19 in Figure 5a) to 193 Ma.

The P(χ²) tests for the KPZ and KCZ were less than 5% (Table 4), suggesting that these zircon grains have undergone more than two tectonic thermal events. Therefore, the central ages of the samples are presented as 202 ± 18 Ma (279–118 Ma) for the KPZ (Figure 5b) and 179 ± 9 Ma (282–117 Ma) for the KCZ (Figure 5c), with the approximate peak ages being 180–190 Ma (Figure 5f). Twenty-seven zircon detrital grains could be measured in the KPZ, with three particle fission-track ages greater than their host strata ages (836, 946, and 1028 Ma) (No. 27, No. 26, and No. 25 in Figure 5b). The KCZ fission-track result shows that their ages are less than those of the host strata, indicating that the samples were partially or completely annealed and that the strata paleotemperature was even higher than 180 °C [83].

According to the fission-track age diagram (Figure 5a–c), the heating ended at approximately 118–117 Ma. One zircon grain heating age from the KPZ is 279 Ma (No. 24 in Figure 5b), which is the earliest time from when the host stratum started experiencing high temperatures. Starting from 245 Ma (No. 20), the grains’ heating age frequency increased, with those of seven grains ranging from 245 to 207 Ma (No. 17) with the final cooling age of 118 Ma (No. 5). Conversely, the earliest thermal age in KCZ is about 282–277 Ma (No. 14 and No. 15 in Figure 5c); the grains’ heating age frequency increased from 232 Ma (No. 2), with five particle ages ranging from 232 to 209 Ma (No. 26) with the final cooling age of 117 Ma (No. 28). The age range of the zircon grains from the Shanxi Formation is similar to that of the grains from the Taiyuan Formation. The heating age of the zircon grains is substantially younger than that of the host strata, potentially reflecting the heating history of the former. The common heating age ranges of the ZFTs from the Shanxi and Taiyuan Formations indicate strong tectonic thermal events in the Datong coal-bearing basin during the middle to late Triassic.

Using fission-track analysis and the BINOMFIT [84] simulation software, an age-frequency distribution diagram of the ZFT is depicted (Figure 5d–f). We analyzed the uplift cooling history and tectonic thermal events of the Datong basin combined with the distribution and occurrence characteristics and sedimentary evolution history of its strata. The zircon ages of the three groups’ sandstone samples show that the primary age of the KCZ ranges from 166 (No. 27) to 232 Ma (No. 2), with a peak age of approximately 180 Ma, a weak peak age of approximately 210 Ma (Figure 5f), and another peak age of 151–133 Ma (No. 4 and 6) (Figure 5a–c). The main cooling age of the KPZ ranges from 162 (No. 2) to 245 Ma (No. 20), with an obvious peak age at approximately 180 Ma, a weak peak age of 210 Ma (Figure 5e), and another peak age at 140–128 Ma (No. 4 and 10) (Figure 5b–e).

In contrast with the KCZ and KPZ, the KJZ only shows one peak age at approximately 150–131 Ma (Figure 5d, No. 20, No. 23) and no peak age at 180 Ma. However, the KJZ began to experience thermal interactions after 193–182 Ma, according to the radar diagram (No. 4 and 10) (Figure 5b–e). Combining the analog curve and the radar diagram (Figure 5) shows that the KCZ, KPZ, and KJZ samples experienced significant thermal events at approximately 180 Ma (193–177 Ma) and 151–140 Ma. Other thermal events occurred at approximately 210 Ma and from 245–232 Ma in the KCZ and KPZ. Additionally, some of the grains retained the age information of detrital provenance in the KPZ, with a peak age of approximately 800 Ma.

The above results indicate that the KCZ and KPZ underwent at least three annealing processes, with one significant thermal event at approximately 180 Ma (179 ± 9 Ma), and two less distinct thermal events at approximately 210 Ma (202 ± 18 Ma) and 151–140 Ma. In contrast, the KJZ underwent the last annealing process at approximately 150 Ma (157 ± 7 Ma). Because the thermal age of the KJZ begins at approximately 193–182 Ma, which is older than the sedimentation age of the Datong Formation, its source may have undergone the same thermal events that the KCZ and KPZ underwent. Although the lacustrine conditions in the Datong Basin were common during the precursor peat accumulation, clastic influx into the depositional environment seems to be possible. Therefore, further provenance analysis should be completed in the future.

4.3. Apatite Fission Tracks (AFT)

In this study, we applied 80–120 °C as the temperature of the AFT annealing zone [78,85]. The AFT results show that the P(χ²) test is significantly greater than 5% (Table 4), manifesting similar thermal events. Therefore, the pooled ages of the KJA and KPA samples are 45 ± 3 Ma and 36 ± 3 Ma, respectively, which are considered valid (the Paleogene Eocene). The AFT ages are much younger than those of the host strata. The samples underwent annealing and subsequent uplift cooling, which could be used for the uplift activity analysis. In addition, the apparent age and track length of the apatite from the Datong Formation (KJA) and Shanxi Formation (KPA) samples are comparable and are proven reliable. The average track lengths of the KJA and KPA are 12.1 ± 2.2 µm and 12.0 ± 2.2 µm, respectively, revealing a single peak pattern (Figure 6d–f), with more moderate track lengths and a wider track distribution. These correspond to the P(χ²) values of 98.1% and 100%, respectively, and can represent a single thermal event. These results suggest that the maximum temperatures of the KJA and KPA samples were over 120 °C and that the fission track of the apatite underwent complete annealing, with a long residence time in the annealing zone.

According to the fission-track annealing theory, when the sedimentary diagenetic time is experienced earlier, the burial depth will be deeper with the higher maximum paleotemperature, and the annealing age will also be younger [38]. The age range of the KJA is 68–21 Ma (Figure 6a,d), whereas that of the KPA is 49–20 Ma (Figure 6b,e). The minimum thermal age of a single particle is the same in both the KJA and KPA. In contrast, the maximum age decreased considerably with greater depths, suggesting that the increase in burial depth and the temperatures in later periods of diagenesis may be the primary reasons for AFT annealing. However, the annealing ages of the different strata were completed simultaneously from 21 (No. 10 in Figure 6a) to 20 Ma (No. 21 in Figure 6b), indicating that both of the strata had undergone the same tectonic activity (uplifted to the near surface) during this time. The annealing age difference of the apatite at the start in the KPA and KJA is nearly 19 Ma, the pooled age difference is 9 Ma, and the annealing age difference at the end is only about 1 Ma, indicating that the uplift rate and the stratum burial depth changed during this period.

The basaltic andesite (KKA) from the Zuoyun Formation could not be used for the thermal history simulation because it contained fewer grains (Figure 6c,f), and displayed no statistical fission-track length. However, its corresponding P(χ²) value is 98.9%, indicating that its age has reference significance. The age of the KKA ranges (Figure 6c) from 140 (No. 8) to 39 Ma (No. 11), with central and pooled ages of 78 ± 11 Ma. This suggests that the apatite grains annealed or completely annealed from the late Jurassic–early Cretaceous to the Eocene. The paleotemperature during this stage should be higher than the lowest limit of the annealing temperatures (80 °C), indicating that the tectono-thermal events occurred in the late Cretaceous, which is nearly the same age as that of the eruption of the Zhuozi basalt in Liangcheng, Inner Mongolia [86]. This means that the paleotemperature of the coal seams in the Jurassic Datong Formation and the Carboniferous–Permian Shanxi and Taiyuan Formations, which underlie the Cretaceous basaltic andesite, was at least 80 °C.

The AFT age of the KKA indicates that a tectonic thermal event may have occurred in the western coal-bearing basin during the late Cretaceous at 78 ± 11 Ma. The basin uplifted and eroded the Cretaceous strata. The AFT ages of KPA and KJA indicate that a tectonic thermal event occurred in the eastern coal-bearing basin at approximately 45–36 Ma during the Eocene. This age is close to the formation time of the Sanggan River fault depression and the Shanxi Graben system in northern Shanxi Province.

5. Discussion

According to the ZFT and AFT results, the following events were recorded: (1) a tectono-thermal event at 245–207 Ma; (2) a slight cooling event with a peak age of 210 Ma; (3) a thermal event with a peak age of 180 Ma; (4) a cooling event range from the late Jurassic to early Cretaceous (157 ± 7 Ma), during which the basin was uplifted and the earlier Cretaceous strata eroded; (5) a tectono-thermal event occurring in the western coal-bearing basin at 140–78 ± 11 Ma; and (6) an uplift event in the eastern basin at 45–36 Ma, indicated by the KPA and KJA samples’ AFT age data. In addition, the T_peak values that were calculated using the vitrinite reflectance equation surpassed the ZFT and AFT annealing temperatures since the late Paleozoic. The following section discusses how these events are related to regional tectonic activity.

5.1. Tectonic–Magmatic Activity

As mentioned previously, the Datong coal-bearing basin experienced three tectonic–magmatic thermal events since the late Paleozoic [87]. The first event was related to lamprophyre magmatic activity, the second was linked to the Kouquan–Emaokou fault zone activity, and the third was related to a basaltic andesite magmatic activity.

Previous studies indicated that numerous alkaline basic–ultrabasic lamprophyre sills and carbonatite dikes intruded the Permo-Carboniferous coal-bearing strata in the Datong basin, even densely affecting the coal quality (Figure 2 and Figure 7b–f) [88]. The magmatic activity occurred at approximately 229 ± 11 Ma and cooled at approximately 224–119 Ma [38,48,49]. The results of the ZFT analysis show that the peak age of the first cooling interval was approximately 210 Ma, whereas 117–118 Ma was the earliest time that zircon could have reached the annealing temperature (120 °C). These sequences of heating–cooling stages coinciding with a large-scale lamprophyre magmatism indicate that the first tectono-thermal event since the late Paleozoic in the Datong area was the lamprophyre magmatism in the middle–late Triassic. According to the radar diagram of the ZFT (Figure 5a–c), the annealing temperatures were approached at approximately 245–232 Ma, which may be the earliest time recorded for magma initiation. It is believed that the magma was the product of the partial melting of the lithospheric mantle [48]. The researcher proposed that a deep fault formed in the early Indosinian period was the magma’s intrusion channel. The ZFT record of the Ningwu Basin sample [46] has a front peak age of 210 Ma, which reflects the Indosinian tectono-thermal events and coincides with the lamprophyre activity in the Datong basin. This means that the lamprophyre magmatism started at 245 Ma, and another event followed, with a peak age of 210 Ma. The latter event simultaneously developed in the Ningwu Basin. This inference indicates that the thermal event occurred earlier in the Datong basin than in the Ningwu basin, explains why the C8 and Jurassic coal have similar %R_o, and proves that the Triassic stratum were deposited around Datong.

During the Yanshan Movement Period, strong tectonic activity began in the Kouquan of Datong in the Early Jurassic, then forming the Kouquan–Emaokou overthrust fault belt [45]. The most obvious cooling age in the ZFT analysis was approximately 180 Ma, which coincides with the strong activity of the Kouquan–Emaokou fault, sufficiently proving that a tectono-thermal event had occurred in the Datong area since the Early Jurassic. The relative active time was from 193 to 173 Ma (No. 2 and 6 in Figure 5b), with a peak age of 179 ± 9 Ma, and persistent activity was observed from the early Jurassic to the middle–late Jurassic. Meanwhile, a strong intracontinental polydirectional compression was consistently recorded in the middle and late Jurassic (174–145 Ma) in the western Shanxi and the entire tectonic belt boundary of the Ordos basin [89]. Furthermore, the age of the pyroclastic rock in the lower Jurassic Yongdingzhuang Formation (J₁y) is 179.2 ± 0.79 Ma [90], whereas the age of the tuffaceous carbonate rocks at the top of the middle Jurassic Yungang Formation (J₂yg) is 160.6 ± 0.55 Ma [29,90]. These represent the initiation time of the Jurassic tectonic or magmatic activity (Yanshan Movement). These cooling ages record the tectono-thermal event of the reactivated Kouquan fault belt and are also consistent with the regional tectonic background, where a large-scale fault belt exists from Lishi along Ningwu, Huairen, and Datong to Tianzhen and Zhangjiakou region (Figure 7a).

The AFT age of the basaltic andesite reflects that thermal magmatic activity occurred around the early Cretaceous, beginning at 140 Ma and with a peak cooling age of 78 ± 11 Ma. The basaltic andesite is commonly distributed in the Jiugaoshan area of Zuoyun county along the western margin of the Datong basin, and occurs as stratiform or stratiform-like in the Cretaceous Zuoyun Formation. Consequently, the AFT of the Datong Formation and Shanxi Formation scarcely recorded this event. It is known that the basaltic andesite and the Bainvyangpan Formation volcanic rocks in Liangcheng–Zhuozi county (Figure 7), Inner Mongolia, are the volcanism result of the same period [86], characterized as 126.9-139.1 Ma in age from the K–Ar isotopic age data. The stratigraphic sequences and chronological data of the basaltic andesite indicate that its magmatic active age is the late Mesozoic (Cretaceous) of the Yanshanian, suggesting that magmatic activity remained active in the north–central part of the NCC. The latest zircon U–Pb isotope-sensitive high-resolution ion microscopy and laser-ablation inductively coupled plasma mass spectrometry dating results of the Zijinshan alkaline complex in the western Shanxi flexural fold belt are 125–138 Ma [91,92,93]. The intrusion age of the Huyanshan alkaline monzonite in the northwest Jiaocheng county, Jinzhong City, is 114–127 Ma [94]. The isotopic age of the quartz monzonite that intruded the middle Jurassic strata in the Nangufeng drilling in Qixian county (Figure 7) is 141 Ma [86], coinciding with that of the Datong basaltic andesite. The paleotemperature was restored using vitrinite reflectance, and fission-track analysis also showed that a tectono-thermal event occurred in the Ordos Basin and Qinshui Basin of the Shanxi Province from the late Jurassic to the early Cretaceous (150–140 Ma) [95,96].

The frequent tectonic magmatic activity in the study area during Mesozoic is related to the regional lithosphere thinning. This is also an important manifestation of the tectonic system transformation, from the extensional tectonic to the contractional tectonics of the Central and western NCC since late Jurassic to early Cretaceous. The AFT age of the KKA is 140 Ma in the Datong basin, indicating that it responded to this magmation during the early Cretaceous, but this ended in the late Cretaceous at 66 Ma (Figure 6c, No. 2).

5.2. Uplift Cooling Events

Since the late Paleozoic, there have been three major uplift events in the Datong basin (Figure 8), represented by: (1) the absence of Triassic strata with an unconformity between the early Jurassic Yongdingzhuang Formation and its underlying Carboniferous–Permian rocks; (2) the absence of the Tianchihe Formation of the late Jurassic with an unconformity separating the early Cretaceous Zuoyun Formation or Quaternary from the Jurassic rocks; and (3) the absence of late Cretaceous and Paleogene rocks marked by an unconformity between the Neogene and underlying older deposits. Ren et al. [97] also thought that the southeastern Ordos basin experienced two denudation stages in the late Triassic–early Jurassic and the late Cretaceous [96].

Because of the tectonic–magmatic activity during the Mesozoic, the ZFT did not clearly show this uplift cooling event, which may be related to the large-scale intrusion of lamprophyres in the extensional tectonic setting. The partial uplift of the coal-bearing basin led to the erosion of the early Triassic strata. The uplift event could have lasted until the deposition of the Yongdingzhuang Formation in the early Jurassic. Considering that the Datong coalfield is located in the northeast Ordos basin and the eastern part of the north Lvliang Mountains, Liu et al. proposed that the Datong and Ordos coal-bearing basins should be considered as one large basin separated during the Lvliang Mountain uplift in the Triassic and Jurassic [81]. However, the thickness of the Triassic strata can reach more than a thousand meters in Ningwu, but only several hundred meters around the Datong area. On the basis of the ZFT analysis, the Datong area uplifted after a local magmatic activity at 202 ± 18 Ma.

Figure 6 shows that the age range of the AFT of KJA is 68–21 Ma, with a peak age of 45 ± 3 Ma. The age range of the KPA AFT is 20–49 Ma, with a peak age of 36 ± 3 Ma. The KJA and KPA apatite age data prove that an uplift cooling event occurred in the Datong area during the Eocene, with a time interval of 45 ± 3 Ma to 36 ± 3 Ma. The Eocene uplift cooling event is reflected specifically in the unconformity contact between the Neogene and underlying strata around the Datong basin. This result also demonstrates that a third uplift cooling event occurred around Datong since the late Paleozoic, characterized by a strong uplifting and rifting in the west and east Kouquan–Emaokou Fault zones, respectively (Figure 9). Subsequently, the Sanggan River downfaulted basin began to form, and its strong uplift occurred from 45 ± 3 Ma to 36 ± 3 Ma.

As proposed by Zhao [46], the AFT ages in the Ningwu–Jingle syncline are 62 ± 4 Ma and 70 ± 6 Ma, respectively. These ages are consistent with the transpression of Shanxi during the late Jurassic, when the NNE-trending anticline and syncline developed. A large-scale uplift had taken place in the Ordos basin and adjacent areas since the late Cretaceous, considering the regional sedimentary burial history. However, the early uplift (in the late Cretaceous) had manifested slowly until 70–62 Ma before the strata entered the partial annealing zone [46]. The ages of the two samples collected from the Liulin area are consistent with the cooling age of 23 Ma recorded by the AFT in the Lou No. 1 borehole from the western Shanxi Province [99], the Qian No. 1 borehole, and the Zhenchuan No. 1 borehole from the eastern slope of northern Shanxi Province. Because of the far-reaching impact of the tectonism that occurred at 22 ± 2 Ma in central and eastern Asia [100,101], the Ordos basin and its adjacent areas were also generally uplifted and denuded. The graben basins, such as the Hetao, Yinchuan, and Weihe basins, were eroded and accompanied by sedimentary discontinuity. The eastern Bohai Bay basins and the southwestern Qinghai–Tibet Plateau also recorded this obvious uplift and denudation event. Therefore, the fission-track ages of KPA and KZA likely record this tectonic uplift event that occurred in the western Shanxi fold belt and Lvliangshan area in the early Miocene. These events happened during the distinctive geological time limit of the unconformity formation since the late Paleozoic in the Datong coalfield.

6. Conclusions

The ZFT and AFT results of the Datong basin’s coal-bearing strata show that the zircon grains were annealed at 282–117 Ma, the apatite grains were annealed from 68 to 20 Ma. The high-temperature duration for the Datong Formation was at least 47 Ma, compared with at least 29 Ma for the Shanxi Formation. Moreover, on the basis of the vitrinite reflectance analysis, the Taiyuan Formation coal seam may undergo paleotemperatures higher than the zircon complete annealing temperature of 250 °C.

The AFT and ZFT data show that the Datong basin which lies in the northern NCC experienced three thermal events and three uplift events since the late Paleozoic. The age values of the three thermal events are 245–207 Ma, 179 ± 9 Ma, and 140 Ma to 78 ± 11 Ma, respectively. These coincide with the lamprophyre activity, Kouquan fault activity, and Zuoyun basaltic andesite magmatic activity in the Datong coalfield and surrounding areas, respectively. The peak ages of the three uplift cooling events are 210 Ma (202 ± 18 Ma), 157 ± 7 Ma, and 45 ± 3 Ma or 36 ± 3 Ma, corresponding to the late Triassic, late Jurassic, and Paleogene Eocene, respectively.

According to the combined local tectonic, sedimentary evolution, and vitrinite reflectance data, the ZFT and AFT ages recorded the absent strata events of the Triassic, upper Jurassic, upper Cretaceous, and Paleogene, also corresponding to the igneous intrusion during the middle Triassic and early Cretaceous and fault action during the early–middle Jurassic. The beginning age (245 Ma) recorded by ZFT and AFT is approximately equal to the magmatic dating age of 229 ± 11 Ma, with a cooling time limit of approximately 224–119 Ma [48]. These prove that the thermal events began at 245–232 Ma around the northern NCC and that the crust underwent uplift cooling at 202 ± 18 Ma in Datong. The results show that the deep tectonic activities of the NCC were still active in the Mesozoic and even the Cenozoic Paleogene.