Paleozoic metamorphism in the Qinling orogen, Tongbai Mountains, central China

The Qinling orogen (Fig. 1) extends for over 2000 km across central China, separating the Archean and Proterozoic Sino-Korean block in the north from the Proterozoic Yangtze block in the south. Although the orogen has been recognized as a continental collision zone for some time, the nature, age, and duration of the collision have been the subject of considerable debate (see review in Hacker et al., 1996). The debate has intensified since the recognition of coesite-bearing ultrahigh-pressure rocks in the Dabie region of the South Qinling orogenic belt, which imply subduction of crustal rocks to depths on the order of 100 km. (Wang et al., 1989; Okay et al., 1989). Recent radiometric data suggest that these ultrahigh-pressure rocks formed during Triassic continental subduction (Li et al., 1993; Hacker and Wang, 1995; Ames et al., 1996). However, few reliable radiometric data are available for the North Qinling orogenic belt, and no evidence of Triassic metamorphism has been reported, which is surprising, considering that an orogenic event as significant as ultrahigh-pressure metamorphism was occurring at that time in the Dabie Mountains. Kröner et al. (1993) reported early Paleozoic dates from single zircons in the Tongbai Mountains and Zhai et al. (1995) suggested that the collision zone contains evidence for multiple orogenies. Xue et al. (1996a, 1996b) reported 207Pb/206Pb single-zircon dating in the Qinling Mountains and suggested that a 440 Ma (Ordovician–Silurian) intraoceanic arc built on 480 Ma (Ordovician) basement was emplaced onto the southern margin of the Sino-Korean block, and stitched together by 400 Ma postcollisional intrusions. These proposals raise some interesting questions. Why is the high-pressure metamorphism Triassic rather than Devonian, and what happened in the intervening 150 m.y.? We have studied the North Qinling orogenic belt in the northern Tongbai Mountains and have determined 40Ar/39Ar hornblende ages at several key localities. Our results indicate that the main metamorphic event in the North Qinling belt occurred during the Silurian or earliest Devonian and that a Carboniferous metamorphism affected units separating the North and South Qinling belts. Our data do not show significant regional Mesozoic metamorphism in the North Qinling orogenic belt.