Characteristics of Zircons from Volcanic Ash-derived Tonsteins in Late Permian Coalfields of Eastern Yunnan, China

The clayey coal-partings (tonsteins) of altered synsedimentary volcanic ash-fall origin are well developed in the Late- Permian coal-bearing formation of eastern Yunnan Province, an area of approximately 100, 000km~2. Because of their unique origin and stable lateral distribution over vast areas, great importance has been attached to the studies of tonsteins (i.e., unconventional claystones) . More than 20 layers (with a thickness in the range of 3-6 cm) of tonsteins were identified from the coal-bearing sequence 200-250 m thick. In order to distinguish tonsteins from the co-existing conventional claystones macroscopically and microscopically, to search for convergence of evidence on the origin of tonsteins from different approaches, to characteriae the individuality of tonsteins from different horizons, as well as to figure out the directions from which the source materials were derived, special techniques such as panned-concentrate separation, statistical and morphological analyses were performed on zircon minerals from claystones of two different origins and diageneses. The results demonstrate the fact that zircons of these two different origins are greatly different in their contents, morphological features and related parametric statistical patterns. The surficial features of these crystal solids also show apparent difference in origin. Tonsteins from the same area but different horizons are characterized by different zircon contents and different morphological paramieric statistical patterns. Tonsteins of the same horizon show close statistical associations in terms of zircon content and morphological parameters; there are continuous variations in statistical patterns in response to changes in lateral distribution of the same layer of tonstein. It was made possible to identify the different paths and directions in which the source materials of tonsteins (i.e., the volcanic ash-falls) or conventional claystones (i.e., the terrigenous clastic sediments) were derived and transported. Therefore, it is important to recognize the source materials and their nature and orgin by studying systematically the characteritics of zircons in various rocks (including tonsteins) along the profile of a coal-bearing formaiton, in order to provide the basic data as an aid in establishing the stratigraphic sequence, facilitating the more precise coal correlation in a mining area as well as improving the stratigraphic correlation on a reginal scale