扬子克拉通北缘元古宙基底同位素地质年代学和地壳增生历史：Ⅰ．后河群和西乡群

ISOTOPIC GEOCHRONOLOGY AND CRUSTAL GROWTH OF PROTEROZOIC BASEMENT ALONG THE NORTHERN MARGIN OF YANGTZE CRATON:Ⅰ.HOUHE GROUP AND XIXIANG GROUP

多种同位素年代学方法综合研究表明，扬子克拉通北缘中段基底岩系后河群和西乡群变质地层的原岩形成时代并非早期推测或报道的新元古代．后河群斜长角闪岩原岩的形成时代应为古元古代，并有太古宙岩石存在的可能．后河群混合岩化变质作用发生于２１４１Ｍａ．西乡群底部白勉峡组属古元古代，而中上部孙家河组为中元古代末期．

It is important to study the Proterozoic magmatic rocks in the Yangtze Craton,which constitute the major Precambrian basement,although the earliest continental crust in th earea was formed in Neoarchean.The Tiechuanshan Formation is one of the most important volcanic formation along the northern margin of the Yangtze Craton,and is located in the Beiba area.This formation is studied in this paper by the isotopic geochemistry of Rb-Sr and Sm-Nd isotopes.The volcanic suite was originated from some highly depleted reservoir(ε Nd (t) ) of+7.53±0.10) in Mesoproterozoic of about(1668±65)Ma and has theT DM ((1.68±0.10) Ga) close to its isochron age.From the Rb-Sr isotopic system,the Tiechuanshan Formation was subjected to serious tectonothermal influence in the early Neoproterozoic (the Jinningian Period).The timing of the tectonothermal event is contemporaneous with regional scale magmatic activities characterized by the intrusion of a series of basic-ultrabasic,acidic and alkaline magmatic rocks.The magmatic rocks formed during the Jinningian Period in the area have the T DM values close to the formation time of the Tiechuanshan volcanic basement and were derived from less depleted mantle sources as indicated by the low ε Nd (t) values.Importantly,in the adjacent Xixiang area,the Baimianxia Formation of the Xixiang Group has not only almost the same age,T DM andε Nd (t) ,but also the similar Rb-Sr isotopic system to the above tectonothermal event.It is assumed that the Baimianxia Formation has been intruded by the Jinningianbasic-ultrabasic,acidic and alkaline magmatic rocks with the similar characteristics to those of Jinningian rocks in the Beiba area.These results give new evidence to support a tectonic movement with a mechanism similar to the modern plate tectonics during the Jinningian period along the northern margin of the Yangtze Craton.