Silurian-Devonian and Jurassic Thermal Events in the Or dos Basin, China: Indications from K-Ar Dating on Illites

In the Ordos basin, two distinct thermal events of different ages have been identified for the first time by means of K–Ar dating combined with illite crystallinity analysis. For the Late Triassic and Late Permian samples, the K–Ar ages of the < 0.2 µm fractions (159-173 Ma) refleet an unitization age related to the Yanshanian movement and indicate a short thermal event in the Middle Jurassic; the K–Ar ages of the < 2 µm fractions (210-308 Ma) are interpreted as mixed ages of detrital material and authigenic illites. The K–Ar ages of both < 0.2 µm and < 2 µm fractions of a Middle Cambrian sample (368 Ma and 419 Ma) correspond to the ages of the metamorphism and earliest granite intrusion in the northern Caledonian Qinling fold zone (380-420 Ma) and show a thermal event during Silurian–Devonian time.