The Coupling Relationship Of Coal Metamorphism And Sedimentary-Tectonic Magmatic Activity For Datong Double Period Coal-Bearing Basin

As one of the unique double period coal-bearing basins in China, Datong coal basin has attracted attention from scholars all over the world for its special characteristics, such as basin evolution, sedimentation, coal formation and metamorphism etc. This study used the Datong coal-bearing basin located in northern Shanxi province as the sample, applying many theories and technical methods to research the coupling relationship between coal metamorphic evolution and sedimentary-tectonic magmatic activity. These included lithology,sedimentology, geotectonics, coalfield geology, fission track chronology, vitrinite reflectance and sedimentary ratio analysis. By these methods, the coal metamorphism factors, such as paleogeothermal, effective time etc., were obtained in order to study the coal class, vitrinite reflectance, coal seams subsidence-burial history and the paleogeothermal field characteristic to the Carboniferous-Permian and Jurassic coal. From the analysis of sedimentary-tectonic magmatic activity events the characteristics of the coal and the process of coal metamorphism were obtained.Following are the main results and some new knowledge.1.The gas coal was the dominant type of main Carboniferous-Permian coal seam in Datong double period coal basin followed by long flame coal, long flame gas coal, gas-1/3coking coal and 1/3 coking coal. Also formed was some meager lean coal, anthracite coal and natural coke in a local mine. The maximum vitrinite reflectance(Ro, max) of normal coal is 0.57~ 0.96 %, even up to 5.36 % for some individual coal seams. The weak caking coal was the dominant type for the main Jurassic coal seams with the Ro, max of 0.51~0.94 %. The Jurassic coal had the same low rank as the Carboniferous-Permian coal.2. The maximum sedimentary and erosion thickness of the original strata in the Datong coal-bearing basin are 2162 m and 1583 m respectively which date from Upper Paleozoic to the Yongdingzhuang Fm of lower Jurassic. The lowest Triassic strata ever deposited in the Datong region was at the early Mesozoic with the thickness of 705 ~ 1374 m. The maximumsedimentary and erosion thickness of the original strata are 868 m and 353 m respectively which date from the Yongdingzhuang Fm of lower Jurassic to the Zuoyun Fm of lower Cretaceous. The maximum sedimentary and erosion thickness of the original strata are 1269 m and 704 m respectively which date from Zuoyun Fm of lower Cretaceous to the Neogene overburden.3. Triple tectonic thermal events and triple uplift cooling events have occurred in Datong region since late Paleozoic. The age values of the triple tectonic thermal events were207.45-245.39 Ma, 179 Ma, 136.16-78±11 Ma, which correspond to the middle-late Triassic lamprophyre magma activity, the Kouquan fault activity which was sustained in Early- Late Jurassic and the early Cretaceous basaltic-andesite and diabase magma activity events respectively. The peak age of triple uplift-cooling events were 202 Ma, 157 Ma, 45±3-36±3Ma, which corresponds to the uplifting events that occurred in the Datong region in late Triassic, late Jurassic and Paleogene Eocene respectively. The analysis of the results show that the maximum buried depth of Carboniferous-Permian coal seams was up to 2100.00 m before the first strong uplifting which occurred at late Triassic. Furthermore, it was 962.0 m and1878.15 m respectively before the second strong uplifting event in Late Jurassic and the third in Eocene. The maximum buried depth of the Jurassic coal seam was 829.27 m and 1745.67 m respectively before the second event strongly uplifted in Late Jurassic and third strongly uplifted in Eocene.4. The common maximum paleo-geotemperature(Tmax) and the abnormal Tmax, which ever existed during the process of metamorphism for No.8 coal of Taiyuan Fm from Datong Coal Field were 80.29-146.69 ℃ and 255.30-362.01℃ respectively. For No.4 coal of Shanxi Fm these were 83.83-128.19 ℃ and 235.30-366.97 ℃. The basic average paleo-geotemperature gradient value was 3.72-4.78 ℃/100 m, and it once reached to7.74 ℃/100 m due to the influence of local magmatism. The Tmax was 66-144.26 ℃ for Jurassic coal with a paleo-geotemperature gradient higher than 4.4 ℃/100 m which was the average value for Northern China at the time.5. The Carboniferous-Permian coal seam mainly experienced the abnormal thermal metamorphism, which made the Ro, max had reached up to the 0.5 ~ 0.7 % and the coal type became the flame coal, before the strong uplift of late Triassic. Middle-late Triassic widespread lamprophyre magma intrusion activities lead to contact coal metamorphism occurring in local regions, such as Dianwan, Yaerya etc, This caused Ro, max of individual coal seams to read up to 1.92 ~ 5.36 % and formed meager-lean coal, anthracite coal and natural coke. Magmatic thermal metamorphism took place in some coal seams near the lamprophyre body where the Ro, max of normal coal was as high as 0.6- 0.8 % and formed flame coal andgas coal were also present. Because of the insufficient coal buried depth and paleo-geothermal,there was no further metamorphism of Carboniferous-Permian coal before the second uplift in late Jurassic, while the Jurassic coal seam only turned into the primary coal metamorphism stage.The Early Cretaceous paleo-geothermal gradient in the Datong region showed higher average values than that in Northern China of 4.4 ℃ / 100 m, which was affected by the basaltic andesite and the hypabyssal phase of diabase magmatic activity. The second coal metamorphism to occur was Carboniferous- Permian coal before the third uplift in Eocene with the influence of a bury depth of 1878 m and a higher paleo-geothermal field. As a result,the Ro, max read up to 0.8 ~ 0.9 % and the coal class changed to gas coal and 1/3 coking coal type as well as eventually attaining the current degree of coal metamorphism. In the same stage, the Jurassic coal also finished its abnormal thermal metamorphism under the double influence of buried depth of 1745 m and a higher paleo-geothermal field. This resulted in Ro,maxvalues of up to 0.61~0.93 % and a change in the coal class to weak caking coal.The comprehensive research shows that the coal metamorphism evolution process always maintained a good coupling relationship with the sedimentary-tectonic magmatic activity events that have occurred since the late Paleozoic period in Datong double period coalfield.