秦岭南缘勉略构造带小碥河-长坝地区变质地层碎屑锆石U-Pb年龄及地质意义

南秦岭勉略构造带小碥河-长坝地区发育一套时代有争议的低角闪岩相变质沉积地层。由于该变质沉积地层受到了强烈变质和变形作用的改造,且缺少精确的年代学证据,在以往的地质研究和地质图中将其形成时代划归为古元古代、早古生代、志留纪-三叠纪、泥盆纪、泥盆纪-石炭纪等。笔者以该区的变质碎屑岩为研究对象,进行LA-ICP-MS锆石U-Pb年代学研究,完成锆石测点一个样品共计80个,获得小碥河-长坝地区的变质地层主要碎屑锆石年龄为850~658Ma,最小年龄加权平均值为(576.5±12.4)Ma。结合前人资料,认为小碥河-长坝地区变质地层的沉积时代主体应为早古生代;物源主要来自勉略构造带内部火山岩、碧口微地块及扬子地块西北缘新元古代岩浆岩;其沉积事件对应了勉略构造带及扬子地块西北缘地区早古生代伸展裂陷沉积过程。

Detrital Zircon U-Pb Age and Geological Significance of Metamorphic Strata from Xiaobianhe-Changba Area in the Mianlue Tectonic Belt of Southern Qinling

A set of low-amphibolite facies metasedimentary stratum with the age undetermined is exposed in the Xiaobianhe-Changba area,Mianlue tectonic belt of the South Qinling orogeny.The formation of the era was classified as Paleoproterozoic,Early Paleozoic,Silurian-Triassic,Devonian etc,due to the long-standing controversy over its formation and the lack of accurate chronological evidence.In this paper,the study of LA-ICP-MS zircon U-Pb chronology of metamorphic clastic rocks is carried out on the metamorphic clastic rocks in this area,and a sample of zircon measuring points is completed with a total of 80 points.The metamorphism is obtained in the Xiaobianhe-Changba area,in which the minimum age-weighted average of metamorphic stratigraphic units in Xiaobianhe-Changba area is 576.5+12.4 Ma,and the age of main clastic zircon ages is between 850 Ma and 658 Ma.Based on the previous data,it is considered that the sedimentary age of metamorphic stratigraphic units should be Early Paleozoic in Xiaobianhe-Changba area;the provenances mainly come from the volcanic rocks in the Mianlue tectonic belt,the Neoproterozoic magmatic rocks in Bikou block in the northwestern margin of the Yangtze block;their sedimentary events correspond to the Early Paleozoic sedimentary process of the extensional rifting in Mianlue tectonic belt and the northwestern margin of the Yangtze block.