四川北川地区中泥盆统养马坝组碎屑锆石U-Pb年代学特征及其构造意义

扬子板块与冈瓦纳大陆在中泥盆世是否存在亲缘性一直有较大的分歧,两者的古地理位置关系对扬子板块的构造演化具有重要意义。扬子板块西北缘四川北川地区中泥盆统养马坝组的碎屑锆石U-Pb定年结果表明:碎屑锆石年龄谱主要集中在1 000～900、850～720、650～500 Ma等3个区间,与冈瓦纳大陆以及下伏地层志留系碎屑锆石年龄谱进行比较,发现它们具有相似的碎屑锆石特征,均显示出典型的冈瓦纳特征。结合古地磁数据、养马坝组沉积环境演化特征、岩相古地理特征、大地构造演化特征以及前人研究成果等信息,综合分析了研究区养马坝组陆源碎屑的可能来源,认为其碎屑物质可能来自于下伏地层志留系沉积岩的沉积再旋回,中泥盆世扬子板块与冈瓦纳大陆不具有亲缘性。

Detrital Zircon U-Pb Dating of Middle Devonian Yangmaba Formation in Beichuan Area of Sichuan,China and Its Tectonic Significance

Whether Yangtze block and Gondwana continent are related to each other during Middle Devonian, the paleo-geographical relationship between the two is of great significance to the tectonic evolution of Yangtze block. Based on the detrital zircon U-Pb chronology of Middle Devonian Yangmaba Formation in Beichuan area of Sichuan on the northwestern margin of Yangtze block, the results show that the age spectrum of detrital zircon is mainly concentrated in three intervals of 1 000-900, 850-720 and 650-500 Ma. Compared with Gondwana continent and the underlying Silurian detrital zircon age spectrum, it is found that they have similar detrital zircon characteristics, all showing typical Gondwana characteristics. Combined with the paleomagnetic data, the evolution characteristics of sedimentary environment of Yangmaba Formation, the evolution characteristics of lithofacies and paleogeography, the characteristics of tectonic evolution, and the results of previous studies, the possible sources of terrigenous clastics in Yangmaba Formation are comprehensively analyzed. The detrital material in Yangmaba Formation may come from the sedimentary recycle of the underlying Silurian sedimentary rocks, and Yangtze block is not related to Gondwana continent during Middle Devonian.