Tectonics

Geochronology and isotopic character of ultrahigh-pressure metamorphism with implications for collision of the Sino-Korean and Yangtze cratons, central China

Authors

  • Leslie Ames,

  • Zhou Gaozhi,

  • Xiong Baocheng


Abstract

The collision zone between the Sino-Korean and Yangtze cratons in central China is marked by subduction-related metamorphic rocks composed of crustal protoliths. These range in metamorphic grade from ultrahigh-pressure eclogite to low-greenschist facies. The paragenesis of the ultrahigh-pressure rocks (coesite and diamond-bearing) indicates subduction to depths greater than ∼120 km as a result of continental collision. Despite extreme metamorphic conditions, zircons have not undergone Pb loss but rather illustrate two periods of growth: crystallization between 700 and 800 Ma and ultrahigh-pressure metamorphism at 218 ± 2.5 Ma. Previous studies have predicted a time-transgressive collision, but age data presented in this study indicate that the collision was approximately coeval along the length of the suture. U-Pb zircon ages of these ultrahigh-pressure metamorphic rocks are 218.4 ± 2.5 Ma and 217.1 ± 8.7 Ma for the Dabie Mountains and Shandong peninsula, respectively. The similar ages imply that the Tan-Lu fault, which offsets the ultrahigh-pressure metamorphic rocks, is a secondary feature not related to collision or subduction. Whole rock isotopic data show that both Rb-Sr and U-Pb were open systems during metamorphism but that Sm-Nd probably remained closed. Nd initial isotopes display crustal signatures, indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth. Sm-Nd data in conjunction with U-Pb zircon dating indicate that the ultrahigh-pressure rocks are not typical Yangtze craton basement (2.9 Ga) but that they originally crystallized in a rift environment between ∼700 and 800 Ma. Discrepancies between 40Ar/39Ar plateaus, Sm-Nd isochrons, and U-Pb zircon ages render calculation of a P-T-t path unrealistic.

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