Study on the Pyroclastic Rocks of Late Paleozoic Coal Meausures, Eastern Hebei, China

The Late Palaeozoic coalfield of Jiyu, Chezhoushan and Kaiping areas of eastern Hebei, situates in the east of North- China Platform. In recent years, the authors have discovered 3- 7 layers of pyroclastic intercalations at the bottom of coal seam No 12, the top and the bottom of coal seam No.9 and No.8 and the bottom of coal seam No.6. These rocks mainly consist of breccia- bearing andesitic ignimbrite, breccia-bearing andesitic debris tuff, andesitic crystal-debris tuff, crystal tuff, tuffite, tuffaceous sandstone, tuffaceous siltstone and tuffaceous limestone etc. By analyzing grain-size and projecting the grain-size parameters into Fisher's (1982) δp-Mdp dispersed diagrams and Passaga's (1957) C-M diagram, we found two samples fell into volcanic-ash flow area, the rest samples all fell into volcanic-ash descesion area and the grain- sizes getting finer and finer from northwest (Jiyu coalfield) to southeast (Kaiping coalfield) . By the petrochemical analysis and conversion of petrochemical parameters, it is found that all of these pyroclastic rock samples fell into igneous rock area of Niggli's diagram and and esite area of Church's diagram. The determine result of oxygen isotope of the pyroclastic samples is basically similar to the study result made by Faure ( 1983) and the determination data made by Morrison ( 1985) on British Triassic volcanic rocks, it shows that the original materials came from eruption of andesite. K-Ar age dating of volcanic breccia and tuffaceous interstitial materials suggest that both of them were not formed by denudation, transportation and re-precipitation of ancient andesite, but formed in Permian period. According to the grain-size distribution feature and vertical as well as herizontal correltion of strata profile of these three coalfields, the volcanic activities can be divided into three eruption stages including seven eruption times, the transportation direction of the volcanic materials (ashes) was from northwest to southeast. In the first stage, the effusive eruption may be taken as the dominant form, the scattered area of volcanic ashes was small and merely covered Jiyu and Chezhoushan coal fields; In the second and the third stages, the explosive eruption may be taken as the dominat form, the covered area of volcanic ashes was wider and transported far to Kaiping coalfield. This study also provided an important basis for the division and correlation of strata and coal seams of this area as well as for the study of tectonic activity and volcano-catatrophic events of plate inner basin of North China in Late Palaeozoic era.