Sm/Nd, Rb/Sr, and Ar-40/Ar-39 isotopic systematics of the ultrahigh-pressure metamorphic rocks in the Dabie-Sulu belt, central China: A retrospectiveview

Because of a complicated metamorphic history, the isotopic systematics of t he ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu belt, east China, appear to be rather different from what were expected. Depending on the degree of retrograde metamorphism and on the retentivity of isotopes, t he radiogenic isotopic systematics in the UHP metamorphic rocks yielded a w ide range of radiometric ages. Some of these ages are geologically meaningf ul, but others may not be. In some fine-grained UHP metamorphic rocks, Sm/N d isotopic systematics appear to be in equilibrium among the UHP phases, sh owing the best estimate for the age of peak metamorphism at 226 +/- 3 Ma. O n the other hand, retrograde overprinting often makes the interpretation of isotopic data more difficult. It is common to find that the Sm/Nd and Rb/S r isotopic systematics among the UI-IP phases and retrograde phases are not in equilibrium, Regression of isotopic data involving both UHP and retrogr ade minerals in isotopic correlation diagrams often yields geologically mea ningless ages. Although Ar-40/Ar-39 dating of UHP metamorphic rocks has bee n reported not to he very helpful in establishing the thermal history becau se of the presence of excess argon, a good correlation between excess argon and rock type in the Dabie-Sulu belt would provide a criterion in identify ing the possible sources of excess argon. By taking all the possible effect s into consideration, a T-t path with two rapid cooling stages for UHP meta morphic rocks from the Dabie-Sulu belt can be postulated. An initial rapid cooling stage in the period from 226 to 219 Ma may have resulted from rapid exhumation of UHP metamorphic rocks immediately after the peak metamorphis m, The second rapid cooling stage, from 450 degrees C to 300 degrees C, may have been caused by the exhumation of the entire terrane, including UHP me tamorphic units and their host gneisses, during the period from 180 to 167 Ma.