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
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
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发表时间：
2001
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通讯作者：
Wu Fu
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作者：
Wu Fu
对马山杂岩南部露头的7个正片麻岩和副片麻岩样品进行的SHRIMP U-Pb锆石定年，为中国东北地区发生～500Ma晚期泛非高级变质事件提供了第一个确凿的证据。峰值麻粒岩相变质作用导致六毛局部深熔花岗岩的产生，这使得 206Pb/238U 年龄为（502±10）Ma（2σ）。正片麻岩和副片麻岩的变质年龄没有区别，这表明它们在变质作用之前在构造上是交错的。洗马山的副片麻岩含有碎屑锆石的证据，这些碎屑锆石在麻粒岩相变质作用期间没有再结晶。这些显示出从～550 Ma到1 900 Ma的207Pb/206Pb年龄一致到弱不一致的谱，表明烃源岩年龄为新元古代、中元古代和古元古代。六毛的变质片麻闪长岩还含有较古老的锆石，207Pb/206Pb 年龄为 546 Ma 至 1 460 Ma。这些被解释为代表火成岩烃源岩，形成于～ 1 400 Ma 之前，并受到 500 Ma 变质事件的不同影响。这表明六毛地区存在中元古代基底。然而，与之前的观点相反，没有同位素证据表明马山杂岩中存在太古宙岩石。考虑到马山杂岩和佳木斯地块在前寒武纪晚期到显生宙早期的位置，泛非晚期变质作用时代非常重要。根据现有证据，有人认为佳木斯地块很可能位于冈瓦纳大陆北缘的华北克拉通附近。
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.