K-Ar and Rb-Sr dating of authigenic illite-smectite in Late Permian coal measures, Queensland, Australia: implication for thermal history

K-Ar and Rb-Sr isotopic studies were carried out on authigenic illitic clay minerals in Late Permian coal measures from the Bowen Basin (Australia), i n order to determine the timing of maximum paleotemperatures, which were re sponsible for coal maturation and coal seam gas generation. The results ind icate two major thermal events affected the Bowen Basin region, at 205-215 and 140-155 Ma. The narrow range of age data from different size fractions, lithologies and stratigraphic depths indicates episodic, short-lived therm al events, rather than gradual temperature increase due to progressive buri al as previously believed. The earlier, thermal event in the latest Triassi c postdates maximum burial of the Bowen Basin strata, which occurred during the Middle to Late Triassic. The ages of 205-215 Ma correspond with the pu blished evidence for regional Late Triassic extension in eastern Australia. The younger (140-155 Ma), thermal phase during the Late Jurassic-Early Cre taceous is related to the initial rifting and associated widespread igneous activities prior to break-up of Gondwana. The K-Ar dates in conjunction wi th vitrinite reflectance data as paleotemperature indicators indicate that the younger thermal event occurred at lower temperatures than the earlier o ne, except in the shallow part of the southern Bowen Basin. This event is r ecorded exclusively in less illitic, R = 0 illite-smectite (I-S) mixed-laye r samples mainly in the shallow part of the southern Bowen Basin (Baralaba Coal Measures). (C) 2001 Elsevier Science B.V. All rights reserved.