40Ar/39Ar Constraints on the tectonic history and architecture of the ultrahigh-pressure Sulu orogen

摘要

New 40Ar/39Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino-Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar 40Ar/39Ar ages. Subsequent unroofing at rates of 5-25 km Myr-1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40Ar/39Ar ages in the 219-210 Ma range, implying cooling at crustal depths at rates of 50-200 degrees C Myr-1. Ages in the 210-170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166-149 Ma plutonism was followed by cooling at rates of c. 15 degrees C Myr-1, suggesting relatively deep crustal conditions, whereas Cretaceous 129-118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr-1, suggesting relatively shallow crustal depths.