The Mashan Complex: SHRIMP U- Pb zircon evidence for a Late Pan- African metamorphic event in NE China and its implication for global continental reconstructions
Simon A Wilde1, WU Fu- yuan2, ZHANG Xing- zhou2 （ 1. School of Applied Geology, Curtin University of Technology, Perth 6845, Australian; 2. College of Earth Sciences, Jilin University, Changchun 130061, China）
SHRIMP U- Pb zircon dating of seven samples of ortho- and paragneisses obtained from the southern outcrop of the Mashan Complex provides the first conclusive evidence for a～ 500 Ma Late Pan- African high- grade metamorphic event in northeastern China. Peak granulite facies metamorphism resulted in the production of local anatectic granite at Liumao and this gives a 206Pb/238U age of (502± 10) Ma (2σ ). There is no distinction between the metamorphic age of the ortho- and paragneisses, indicating that they were tectonically interleaved prior to metamorphism. Paragneisses from Ximashan contain evidence of detrital zircons that were not recrystallised during the granulite facies metamorphism. These show a spectrum of concordant to weakly discordant 207Pb/206Pb ages from～ 550 Ma to 1 900 Ma, indicating source rocks of Neoproterozoic, Mesoproterozoic and Palaeoproterozoic age. A metamorphosed gneissic diorite from Liumao also contains older zircons, with 207Pb/206Pb ages from 546 Ma to 1 460 Ma. These are interpreted to represent an igneous source rock, emplaced～ 1 400 Ma ago and variously affected by the 500 Ma metamorphic event. This indicates the presence of Mesoproterozoic basement in the Liumao area. However, contrary to previous views, there is no isotopic evidence that Archaean rocks are present in the Mashan Complex. The Late Pan- African age of metamorphism is important when considering the location of the Mashan Complex and Jiamusi Massif during the late Precambrian to early Phanerozoic. On the available evidence, it is argued that the Jiamusi Massif was most likely located close to the North China Craton, on the northern margin of Gondwanaland.