Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

Abstract

ABSTRACT An attempt is made to confirm the existence of the two discrete UHP and HP metamorphic belts in the central orogenic belt in China. Detailed geological mapping and structural and petrological analyses of the Kanfenggou ( ) and Xiangfanggou ( ) slices exposed in the eastern Qinling ( ) orogen indicate that they experienced ultrahigh pressure and high pressure metamorphism, respectively. The former, situated in northern Qinling, contains a large volume of fine–grained coesite and quartz pseudomorphs after coesite- and microdiamond-bearing eclogite lenses, whereas the latter, located in southern Qinling, preserves the relicts of a high pressure metamorphic mineral assemblage. Based on extensive fieldwork together with compilations at the scale of the orogenic belt, and a comparison of Pb isotopic compositions between the UHP metamorphic rocks from Kanfenggou slice and the Dabie UHP metamorphic belt, we propose that there are at least two discrete ultrahigh pressure metamorphic belts with different ages and tectonic evolution within the central orogenic belt in China. The first is the South Altun-North Qaidam-North Qinling ultrahigh pressure metamorphic belt of Early Paleozoic age (∼500–400 Ma). The Kanfenggou ultrahigh pressure slab is located at its eastern segment. The second is the well constrained Dabie ( )-Sulu ( ) ultrahigh/high pressure metamorphic belt of Triassic age (∼250–220 Ma). The Xiangfanggou high pressure metamorphic slab is a westward extension of the Dabie-Sulu ultrahigh/high pressure metamorphic belt. The Pb isotopic compositions of the UHP metamorphic rocks from Kanfenggou UHP fragment in East Qinling are different from those of the UHP rocks in Dabie UHP metamorphic belt, but are consistent with those of the rocks from the Qinling rock group and Erlangping ( ) rock group. The East Qinling UHP metamorphic belt does not appear to link with the Dabie-Sulu UHP metamorphic belt. These two ultrahigh metamorphic belts are separated by the suture-like Shangnan ( ) Danfeng ( ) fault system or Guishan ( )-Meishan ( ) fault, and a series of fault-bounded tectonic slices with different ages and rock assemblages. The Early Paleozoic ultrahigh metamorphic belt towards the east cannot be linked to the Triassic Dabie-Sulu ultra-high/high pressure metamorphic belt, and so does not form a huge ultrahigh pressure metamorphic belt extending more than 4 000 km in the central orogenic belt. In addition, the South Altun-North Qaidam-North Qinling ultrahigh metamorphic belt probably represents an intercontinental deep subduction/collision belt between the Sino-Korean and Yangtze cratons, occurring during the Early Paleozoic. The Dabie-Sulu ultrahigh/high pressure belt may have been formed by an intracontinental deep subduction/collision process occurring in the northern part of the Yangtze craton during the Triassic. It is suggested that the two phase crustal subduction/collision processes, with an interval of ∼200–300 Ma, responsible for the formation of the two ultrahigh/high pressure metamorphic belts occurred along spatially different belts within the central orogenic belt, China.