西南天山前陆盆地侏罗纪-白垩纪盆山格局——来自碎屑锆石年代学的证据

目前对天山地区,特别是天山南缘中生代盆山格局认识尚存分歧。本文着眼于侏罗纪-白垩纪这一盆山演化关键阶段,利用碎屑锆石LA-ICP-MS U-Pb定年法对西南天山前陆盆地康苏剖面中侏罗统杨叶组、下白垩统克孜勒苏群沉积开展物源分析。发现中侏罗统杨叶组碎屑锆石U-Pb年龄分布于369~2687 Ma间,基本分布在369~404 Ma(约占4%)、418~501 Ma(约占19%)和544~2687 Ma(约占77%)3个范围;下白垩统克孜勒苏群碎屑锆石U-Pb年龄分布于243~2820 Ma间,集中于253~414 Ma(约占35%)、423~489 Ma(约占27%)和668~2820 Ma(约占37%)3个范围。中侏罗统碎屑锆石年龄分布范围广,各年龄组分均较突显,反映中侏罗世西南天山前缘流域体系宽广,天山内各主要源区均得到沟通,物源范围广阔。下白垩统克孜勒苏群锆石年龄分布明显集中,反映早白垩世西南天山前缘源区范围有所缩小。西南天山前缘与库车前陆盆地的物源构成在中侏罗世存在一定差异,而在早白垩世呈现相似特征。包括西南天山前陆盆地在内的天山南缘或于早白垩世经历一期小规模构造反转,导致山-盆构造分异与抬升-剥蚀增强。

Jurassic-Cretaceous Basin-range Pattern in the Southwestern Tian Shan Foreland Basin: Evidence from Detrital U-Pb Zircon Geochronology

The Mesozoic basin-range pattern in the Tian Shan area is actively debated,especially on the southern flank of the Tian Shan.This important and contrasting hypothesis involves an intracontinental foreland basin,and a passive subsiding basin characterized by a wide source area,a weak basin-range differentiation,as well as a positive physiographic feature.This study focuses on the critical stage of basin-range relationship and reports detrital zircon U-Pb ages from the sandstones of Middle Jurassic Yangye Formation and Lower Cretaceous Group collected from the Kangsu section,aiming to constrain the Mesozoic provenance of the southwestern Tian Shan foreland basin.The U-Pb isotopic ages of zircons from these two units of clastic rocks were obtained by using in situ LA-ICP-MS.The U-Pb ages of detrital zircons from the Middle Jurassic series range from 369 Ma to 2687 Ma and can be divided into three main groups:(1) 369-404 Ma (accounting for 4％),(2) 418-501 Ma (accounting for 19％),(3) 544-2687 Ma (accounting for 77％),and the zircon ages from the Lower Cretaceous series range from 243 Ma to 2820 Ma and can be divided into three groups:(1) 253-414 Ma (accounting for 35％),(2) 423-489 Ma (accounting for 27％),(3) 668-2820 Ma (accounting for 37％).All age groups are prominent in the Middle Jurassic sample,suggesting a large-scale drainage system and wide source area characterized by established connection between the basin and main sources.The distribution of U-Pb ages of detrital zircons from the Lower Cretaceous sample has more groups and reflects a shrink age in coeval source area.The southern flank of southwestern Tian Shan and the Kuqa rejuvenated foreland basin have difference sediment provenance during the Middle Jurassic,whereas in the Early Cretaceous they have similar sedimentary provenance.The southwestern Tian Shan foreland basin,as well as the other areas of the south margin of the Tian Shan underwent a minor tectonic inversion and adjustment throughout the Early Cretaceous,which led to an intension in basin-range differentiation and corresponding uplift-exhumation,and a deconstruction of the link between sedimentary piedmont and older basements distributed in interior of the mountain.