广西云开地区元古代不整合事件的确定及其构造意义

通过详细的地质填图,在云开地区原定为“云开群”的变质岩系中识别出古老的不整合事件,表现为中浅变质岩系不整合覆盖在中深变质岩系之上,二者在岩石组合、原岩沉积环境以及变形变质特征上均存在较大的区别,同位素年龄及变质作用P-T-t轨迹的研究证明上覆中浅变质岩系明显晚于下伏中深变质岩系形成。利用SHRIMP法测得下伏岩系石榴辉石岩锆石的谐和207Pb/206Pb年龄为(1894.4±17.36)M a和(1846.8±58.58)Ma,并获得3个分析结果的207Pb/206Pb加权平均年龄(1817±36)Ma,其形成时代应为古元古代。上覆岩系中一些微古植物化石保存完好,微古植物化石组合的时代为长城纪—蓟县纪,部分为青白口纪;获得其变质基性火山岩锆石SHRIMP法同位素年龄为(1462±28)Ma,表明上覆岩系的时代应为中—新元古代。分析结果表明,二者之间存在一明显的巨大灾变事件,其应为古元古代末吕梁期造山作用形成的产物。笔者把下伏岩系从“云开群”中分离出来,并命名为“天堂山岩群”。

Confirm of Proterozoic Unconformity Event in the Yunkai Area of Guangxi and Its Tectonic Significance

Through the 1∶250,000 regional geological survey of the Yulin sheets of the southeastern Guangxi,this paper has distinguished an archaic unconformity event from metamorphic series that had been named Yunkai Group formerly,showing that the middle-shallow metamorphic series unconformity cover on the middle-deep metamorphic series.They have great distinction in the composition of rocks,sedimentary circumstance of originality rocks and deformation and metamorphism.The study of isotope chronology and P-T-t path of metamorphism indicates that the middle-shallow metamorphic series were formed posterior to the middle-deep metamorphic series.The ages of garnet-pyroxenite of the middle-deep metamorphic series were(1,894.4±17.36)Ma,(1,846.8±58.58)Ma and three zircon dating gave a concordia 207Pb/206Pb age of(1,817±36)Ma using the SHRIMP zircon U-Pb method,indicating that it is the product of Paleoproterozoic.Some microfossils of the middle-shallow metamorphic series were well conserved,the ages of microfossils assemble are Changcheng period,Jixian period or Qingbaikou period.The age of the metabasic volcanic rocks of the middleshallow metamorphic series is(1,462±28)Ma using the SHRIMP zircon U-Pb method,indicating that it is the product of Meso-Neoproterozoic.It can be concluded that an obvious tremendous disaster event has occurred between the middle-shallow metamorphic series and the middle-deep metamorphic series,indicating that they are the products of Lüliang period of last Paleoproterozoic collision orogenics.The middle-deep metamorphic series were separated from "Yunkai Group"and named "Tiantangshan Group Complex".