Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology

The Dabie Shan contains two rare geologic features. One, the occurrence of ultra-high pressure metamorphic rocks (peak P > 2.7 Gpa = UHP) in the east-central part of this belt, and two, some of these UHP rocks are characterized by anomalously O-18-depleted oxygen isotopic compositions. Geochronological data are presented that constrain the age of the UHP metamorphism, the ages of protoliths, and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions. The age of UHP metamorphism in the eastern Dabie Shan is dated at 218.5 +/- 1.7 Ma using U/Pb in zircon separated from host gneisses. This age is identical within uncertainty with a less precise age using U/Pb in zircon separated from UHP eclogite (225.5 + 3/-6 Ma) and with previously published ages from UHP eclogites of the Dabie Shan. Map relationships strongly support an in-situ, as opposed to exotic, origin of the UHP eclogites in this region, The identical ages of UHP metamorphism in the gneisses and eclogites provides additional support for this interpretation. The age of the protolith gneisses is dated as 772.5 +/- 9.5 Ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the Qinling belt, The association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the Yangtze block at this time. These upper intercept ages may date the hydrothermal circulation of O-18-depleted meteoric water which gave rise to the anomalously O-18-depleted UHP protoliths of the Dabie Shan and Sulu region. The coincidence of these protolith ages with the early Sinian glaciation of South China is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time. The upper intercept age of the eclogite is imprecisely determined at 447 + 82/-79 Ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions. (C) 1997 Elsevier Science B.V.