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

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.