秦岭南缘勉略带构造属性及晚古生代地质背景: 来自碎屑锆石U-Pb年代学的制约

勉略构造带作为秦岭造山带内重要的构造边界，关于其构造属性及晚古生代以来的地质背景，一直是学术界争论的焦点。碎屑锆石U-Pb年代学在限定地层单元的最大沉积年龄、研究区域构造岩浆事件及约束构造地质背景等方面行之有效。基于此，通过对勉略带内五郎坪北侧两河口变沉积地层和侵入其中的变形花岗岩脉体进行LA-ICP-MS锆石U-Pb年代学研究。获得2件变形花岗岩脉的结晶年龄均为406±1Ma。碎屑锆石主年龄谱分别为422~456Ma和558~826Ma，峰值年龄为441Ma和771Ma、813Ma，次级年龄谱分别为942~1495Ma和1658~2981Ma，峰值年龄不明显。依据最小一组碎屑锆石的峰值年龄（441Ma），和侵入其中的变形花岗岩脉（406±0.6Ma），限定该变沉积地层形成时代为406~441Ma（S1-D1）。碎屑锆石年龄谱显示该套变沉积地层物质来源较为复杂，其中秦岭造山带及扬子板块北缘早古生代、新元古代岩浆岩为其提供了74%±的物源，古老变质基底为其提供了26%±的物源。通过与区域上已有资料对比，认为勉略构造带内晚古生代沉积地层形成环境与邻区大致相同，且本次所获得的变沉积岩碎屑锆石年龄谱也与邻区泥盆系相似。综合认为，勉略构造带与邻区在晚古生代应属同一构造环境，晚古生代"勉略海盆"应当包括整个南秦岭。

Tectonic attributes and Late Paleozoic geological background of Mian-Lue belt in the southern margin of Qinling: Constraints from U-Pb geochronology of zircon

Mian-Lue tectonic belt is an important tectonic boundary in the Qinling orogenic belt, and its structural attributes and geological backgrounds since the Late Paleozoic have been the focus of debate in academic circles. The U-Pb chronology of detrital zircons is effective in defining the maximum sedimentary age of stratigraphic units, studying regional tectono-magmatic events, and constructing tectonic and geological settings. Based on this, the LA-ICP-MS zircon U-Pb geochronology study was carried out on the meta-sedimentary rocks and granite intrusions within the Lianghekou on the north side of Wulangping in Mian-Lue belt. We obtained 406±0.6Ma crystallization ages of two granite veins in the meta-sedimentary rocks, indicating that their formation age should not be later than 406Ma. Detrital zircon U-Pb age spectrum can be explicitly divided into Early Paleozoic and Neoproterozoic two main age spectrum and the Paleoproterozoic, Mesoproterozoic, Early Neoproterozoic secondary age spectrum. The main age spectra were within ranges of 422~456Ma and 558~844Ma, with three peak ages of 441Ma, 771Ma and 813Ma, respectively. The secondary age spectra were 942~1495Ma and 1658~2981Ma respectively, without any obvious peak age. Based on the peak age of the smallest group of detrital zircons (441Ma), and the age of the deformation granite dike (406Ma), we infer that the metasedimentary strata were formed at 406~441Ma (S₁-D₁). Combined with geological data in the study area and its adjacent areas, the meta-sedimentary rocks on the north side of Wulangping in Mian-Lue belt has a multiple supplying sources from different time periods, namely, Early Paleozoic, Neoproterozoic, Mesoproterozoic, Palaeoproterozoic and Archean. Furthermore, the Early Paleozoic magmatic rocks and Neoproterozoic magmatic rocks in Qinling orogenic belt and the northern margin of the Yangtze provide about 74%±of sediment source, while the old metamorphic basement provides only about 26%±of sedimentary debris for meta-sedimentary rocks. In addition, by comparing with the existing data in the region, this study suggests that the Late Paleozoic sedimentary strata in the Mian-Lue tectonic belt were formed in roughly the same environment as the adjacent areas, and the age spectra of the detrital zircons from the meta-sedimentary rocks obtained in this study are similar to those of the Devonian strata in the adjacent area. It is concluded that Mian-Lue tectonic belt and its adjacent areas should belong to the same tectonic environment in Late Paleozoic, and "Mian-Lue sea basin" in Late Paleozoic should include the south Qinling Mountains.