Zhangbaling Uplift is located in the east of Hefei Basin, which is bounded by the Tan-Lu fault zone. The uplift extends in the northeast direction, and Archean-Proterozoic Feidong Group and Neoproterozoic Zhangbaling Group are mainly exposed in the south. Only Neoproterozoic Zhangbaling Group is exposed in the north.

The metamorphic complex of Feidong Group belongs to high amphibolite facies and has different degrees of migmatization. Its protolith is an intrusive-volcanic-sedimentary rock series, which can be compared with Jiaonan Group/Donghai Group in Jiaonan orogenic belt and Dabie Group in Dabie orogenic belt. The contemporaneous zircon U-Pb age of biotite plagioclase gneiss at the bottom is Ma (Ge, Zhou Daozhi, 1993), reflecting that the source rock is Paleoproterozoic.

Zhangbaling Group is a set of shallow metamorphic rocks in greenschist facies, belonging to the metamorphic basement in the northern margin of Yangtze plate. During the collision between North China and Yangtze Plate in Indosinian period, it was involved in orogenic deformation and produced high-pressure metamorphic blueschist. The zircon U-Pb age of Zhangbaling Group is 1026Ma, and the spilite U-Pb age is 103 1Ma (according to the regional geology of Anhui Province, 1987). The 40Ar/39Ar age of polysilicate muscovite in the blue schist of Zhangbaling Group in Sanjie, Mingguang, Anhui Province is 245.65438±0.5ma (Li Shuguang et al., 1992), indicating that high-pressure metamorphism occurred in the synorogenic period.

(2) The strike-slip structural features of the Tan-Lu fault zone in the eastern margin of Hefei Basin.

The Tanlu fault zone is exposed in the Feidong Group in the southern section of Zhangbaling uplift belt, with a fault width of about 8km. Its strike-slip structure is characterized by several NE-trending and sinistral translational ductile shear zones composed of mylonite and ultramylonite, and generally consists of four shear zones, which are located in the west, middle and east of the uplift zone respectively. The ductile shear zone in the west and middle of the uplift zone tends to the west, with an inclination angle of about 60 ~ 80. However, the ductile shear zone on the east side of the uplift zone is mainly located between Feidong Group and Zhangbaling Group, and tends to the east with an inclination angle of 60 ~ 75. The nearly east-west old metamorphic foliation between shear zones is stretched and bent into NE-Nee direction.

In the Zhangbaling Group in the north of Zhangbaling uplift belt, the Tanlu fault zone is mainly characterized by brittle to brittle ductile left-lateral translation faults, which appear in the middle and both sides of the uplift belt. The larger fault in the central part is the NNE-SN-trending Guandian-Longwangjian sinistral translation fault, which mainly inclines to the west and locally to the east, with an inclination of about 70. The fault is mainly characterized by structural breccia and local brittle-ductile shear zone, and the associated structures near the fault indicate that the fault is a sinistral translation fault.

There are three Yanshanian granite bodies extending from north to south in the western edge of the northern section of Zhangbaling uplift belt, namely Guandian rock body, Wawuliu rock body and Wawuqian rock body, among which Wawuliu rock body develops a NE-trending left-lateral brittle-ductile shear zone.

Early Cretaceous andesite of Maotanchang Formation and andesite lava of Heishidu Formation are exposed in Longshan, Feidong County, eastern Hefei Basin. A series of sinistral translation faults with the occurrence of 290 ∠ 80 were found in the volcanic rocks of Heishidu Formation, with horizontal scratches on the fault plane and a series of breccias in the volcanic rocks.

(3) The translation age of the Tan-Lu fault zone.

The translation age of the Tan-Lu fault zone is related to such important issues as when the Tan-Lu fault zone began to intervene in the Hefei basin, when the Zhangbaling uplift began to appear in the eastern part of the basin, and when the deposits in the basin were related to the translation movement.

In order to accurately determine the translation age of the Tan-Lu fault zone, 40Ar/39Ar isotopic dating of strike-slip mylonite, ultramylonite and syn-strike-slip igneous rocks at the edge of the basin was carried out for the first time, and K-Ar isotopic dating of syn-strike-slip volcanic rocks in Longshan, east of the basin was carried out.

40Ar/39Ar isotopic dating of mylonite and ultramylonite 1.

40Ar/39Ar isotopic dating was carried out on the whole rock samples of 13 in the southern section of Zhangbaling uplift belt in the eastern margin of Hefei Basin (Figure 4-29) (Table 4-3).

Figure 4-29 Plane Distribution and Sampling Location of Isotope Age in Anhui Section of Tan-Lu Fault Zone (according to Lu, 2002)

1- basin; 2- metamorphic basement; 3- Mesozoic volcanic rocks; 4- Mesozoic rock mass; 5- Fault; 6- ductile shear zone; 7— sampling point; Ar represents 40Ar/39Ar age; K-k-ar age; Zircon U-u-Pb age

Table 4-3 40Ar/39Ar plateau age statistics of strike-slip mylonite samples in Anhui section of Tan-Lu fault zone

Note: The mass fraction is in brackets.

The 40Ar/39Ar plateau age of the whole rock sample D- 1 of strike-slip mylonite in the Tan-Lu fault zone is/kloc-0 120.48±0.75Ma；; The 40Ar/39Ar plateau age of No.4-2 sample is118.75 0.45 Ma, and the two plateau ages are consistent within the error range, indicating the Early Cretaceous (Figure 4-3 1). The 40Ar/39Ar plateau age of the whole phyllite (No.13-2) is/kloc-0 120.96±0.66ma, which also belongs to the Early Cretaceous. The 40Ar/39Ar plateau ages of mylonite are128.35 0.64ma (No.8-12),130.61.88ma (No.8-3-2) and 65438, respectively. In particular, the ultramylonite (No.8-12) and mylonite (No.8-3-2) at the same sampling point give consistent age values within the error range (128.35 0.64 Ma and 130.438+0).

40Ar/39Ar of the whole rock sample gives basically the same plateau age in the middle release zone of 70% ~ 80% (Table 4-3). The older apparent age appears in the high and low temperature release region of the sample (generally less than 10%), which reflects the possible influence of excessive argon. In order to check the influence of excessive argon on the test results, the 40ar/36ar/36ar isochronous processing was carried out (Table 4-4), and all the data points of seven samples were fitted to the isochronous time domain with good correlation coefficient r, and the initial value of 40ar/36ar was between 300.9 4.67 and 308.0 6.44, which was different from the atmospheric value (295. It can be seen that the influence of excessive argon on the plateau age in the central region can be ignored, and its value is credible. This also reflects that during the formation of mylonite in the deep strike-slip ductile shear zone in Anhui section of the Tan-Lu fault zone, the 40Ar/39Ar isotope system was rebalanced and was not disturbed by other tectonic thermal events, thus recording the cooling age of strike-slip deformation of the fault zone.

Table 4-4 plateau age, isochron age, correlation coefficient (r) and initial value of 40Ar/39Ar of mylonite samples.

The test results show that the cooling age of ductile strike-slip deformation in Anhui section of the Tan-Lu fault zone is 120 ~ 132 Ma, which belongs to the early Early Cretaceous, reflecting that the large-scale sinistral strike-slip of the Tan-Lu fault zone should occur in the early Early Cretaceous.

2. 40Ar/39Ar isotopic dating of intrusive rocks during strike-slip period.

Guandian pluton, Wawu Ada pluton and Wawu Cave pluton (Figure 4-29) intrude along the Tan-Lu fault zone, and their long axes all extend along the Tan-Lu fault zone, which is syntectonic intrusive granite during the strike-slip process of the Tan-Lu fault zone.

The zircon U-Pb age of Guandian granite is1281ma (Li Xueming et al., 1985). In order to further determine the strike-slip time of the Tan-Lu fault zone and the emplacement ages of the Wawuliu rock mass and the Wawuxue rock mass, 40Ar/39Ar isotopic dating of biotite was carried out for these two rock masses. The 40Ar/39Ar plateau ages of biotite in Wawuliu and Wawu caves are/kloc-0 127.87±0.46ma and/kloc-0 120.00±0.50ma respectively (Table 4-5, Figure 4-3 1), which belong to the Early Cretaceous. Both samples have given ideal platform spectra, and the platform age, isochron age, correlation coefficient and the initial value of 40Ar/39Ar in the 80% intermediate release zone are consistent (Figure 4-30, Table 4-5), which shows that the platform ages of the two samples are reliable and are not affected by excessive argon. Therefore, Liang Ping's age shows that the emplacement time of these two rock masses is early Cretaceous, thus verifying that the strike-slip age of the Tanlu fault zone is early Cretaceous. At the same time, it also shows that the emplacement time of Guandian rock mass and Wawu Ada body (128Ma) is slightly earlier than that of Wawu Cave rock mass (120Ma), that is, the emplacement of rock mass has the law of migration from north to south.

Fig. 4-30 40Ar/39Ar age spectrum of strike-slip mylonite in Anhui section of Tan-Lu fault zone.

40Ar/39Ar isotopic dating results of biotite from Wawuliu pluton and Wawuxue pluton.

Fig. 4- 40Ar/39Ar age spectrum of strike-slip granite biotite in Zhangbaling uplift zone of Tanlu fault zone.

3. K-Ar isotopic dating of volcanic rocks in strike-slip period.

Only andesite of Maotanchang Formation is exposed in Longshan, Feidong County, the eastern margin of the basin (Figure 4-29), which can be compared with Maotanchang Formation in the northern margin of Dabie Mountain. The whole rock K-Ar isotopic age of this volcanic rock is119.2 23 Ma, so it is determined that the eruption time of this volcanic rock belongs to the Early Cretaceous, not the Late Jurassic as previously thought. The eruption age of this volcanic rock is consistent with the deformation age of strike-slip mylonite in the Tanlu fault zone (120Ma). Obviously, the strike-slip movement of the Tan-Lu fault zone induced volcanic activity, and the rock mass was cut by the left-lateral translation fault of the fault zone, so it was a volcanic eruption in the same strike-slip period.

The 40Ar/39Ar isotopic ages of strike-slip mylonite and strike-slip granite biotite in the Tanlu fault zone are basically consistent with the K-Ar ages of strike-slip volcanic rocks, indicating the early Early Cretaceous. This undoubtedly shows that the large-scale translation of the Tan-Lu fault zone occurred in the early Early Cretaceous. Therefore, the Tan-Lu fault zone has been involved in Hefei Basin since the Early Cretaceous, becoming the eastern boundary of the basin, and the Zhangbaling uplift formed by its large-scale translation has also appeared in the eastern part of Hefei Basin since the Early Cretaceous. There was no Tan-Lu fault zone before the late Jurassic, so the Hefei basin in Jurassic extended eastward, which may be connected with Laiyang basin where the Tan-Lu fault zone staggered to the left. After the Early Cretaceous, it became two independent basins and experienced different structural and sedimentary evolution.

(4) The prototype of Hefei Basin from Early Cretaceous to Early Late Cretaceous.

The early Cretaceous-early Late Cretaceous sedimentary areas in Hefei Basin mainly remain in Da Qiao, Feidong and Shucheng areas in the east of the basin. The thickest parts of Zhuxiang Formation (K 1z) and Xiangdaopu Formation (K2x) in Da Qiao fault depression are located in the east of the fault depression, and they are distributed in NNE along the Tanlu fault. The maximum apparent thickness revealed by drilling exceeds 1000m, and the maximum thickness interpreted by seismic data can reach 4000m, showing the characteristics of being thick in the east and thin in the west.

The Tan-Lu fault zone became the eastern boundary of Hefei Basin in the early Cretaceous, and only crossed the Tan-Lu fault zone eastward in Zhangguang area of Dingyuan County. Deep lacustrine deposits were found in Zhuxiang Formation (K 1z) in Gucheng area, southeast of Da Qiao fault depression, which contained dark mudstone. Zhuxiang Formation (K 1z) in Well Heqian 8 and Well Heqian 9 is mainly composed of dark marl without marginal facies, reflecting that the sedimentary center is located near the fault zone. Seismic profile shows that Zhuxiang Formation (K 1z) has no obvious thickening phenomenon near the Tan-Lu fault zone.

Zhuxiang Formation (K 1z) and Xiangdaopu Formation (K2x) are exposed without drilling holes in the east of Shucheng fault depression, and the remaining thickness of seismic interpretation is about 2000m. It is estimated that Zhuxiang Formation (K 1z) and Xiangdaopu Formation (K2x) in Feidong area also have similar fillings, that is, they are controlled by the Tanlu fault and extend in the northeast direction.

To sum up, during the sedimentary period of Zhuxiang Formation (K 1z), affected by the translation of the Tan-Lu fault zone, the eastern Hefei Basin was a strike-slip flexural basin formed on one side of the fault (Figure 4-32).

Figure 4-32 Model Diagram of Early Cretaceous Strike-Slip Flexural Basin in East Hefei Basin

According to the interpretation of seismic data, Zhuxiang Formation (K 1z) in Huoqiu fault uplift is relatively thin, but according to the HF2000-384 seismic line, Zhuxiang Formation (K 1z) tends to thicken westward, and the current Lower Cretaceous is characterized by stripping and loss in the west of Da Qiao fault depression, so it is inferred that the Hefu Basin in the Early Cretaceous has the characteristics of wide basin deposition.

During the sedimentary period of Xiangdaopu Formation (K2x) in Late Cretaceous, the Tanlu fault was characterized by strike-slip normal fault. According to the outcrop data of Well He Qian 9 and Well He Qian 10, alluvial fan deposits appear on the west side of the Tan-Lu fault zone, and alluvial fan groups are formed along the west side of the Tan-Lu fault zone. Therefore, Zhangbaling Uplift in the sedimentary period of Xiangdaopu Formation (K2x) became one of the provenances of Hefei Basin. At this time, Hefei basin is still a wide basin, and Zhangba Longling was formed due to the strike-slip uplift of the Tanlu fault. When Xiangdaopu Formation (K2x) in the east of the basin was deposited, this area was a faulted basin. Today's preservation conditions are the result of the late Yanshanian tectonic movement.

The large-scale translational movement of the Tan-Lu fault zone induced magmatic activity dominated by intermediate acidity. In addition to the emplacement and volcanic eruption of the upper early Cretaceous granite in Zhangbaling Uplift, there are also volcanic eruptions in Zhuxiang Formation (K 1z) in the basin, such as andesite in Longshan area of Feidong County and basalt in Heqian 9 underground, which reflects the active tectonic background at that time.

In a word, the early Cretaceous Hefei basin was still dominated by foreland basin, and the eastern part of the basin was superimposed with the characteristics of flexural basin. The eastern part of the basin was a faulted basin in the early Late Cretaceous.