AR/AR GEOCHRONOLOGY OF ULTRAHIGH-PRESSURE METAMORPHISM IN CENTRAL CHINA

The Dabie Shan of China contain the largest areal exposure of ultrahigh-pressure regional metamorphic rock known on Earth. The thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation, preservation, and exhumation. Published ages of ultrahigh-pressure metamorphism and subsequent cooling range from Archean to Jurassic. By analyzing 21 Dabie hornblende, phengite, and biotite samples by the 40Ar/39Ar method, we find that (1) cooling from peak metamorphic temperatures to similar to 300 degrees C occurred between about 206 and 178 Ma; (2) widespread Cretaceous ages reflect reheating by a post-ultrahigh-pressure magmatic/extensional episode; (3) 40Ar/39Ar and K/Ar ages older than 230 Ma, and perhaps 210 Ma, are geologically meaningless due to the incorporation of excess 40Ar; and (4) Dabie ultrahigh-pressure metamorphic rocks are temporally related to blueschist-facies rocks farther west in the suture zone.