渣尔泰群、白云鄂博群碎屑锆石特征对源区地壳结构的启示

近年来，锆石的晶体形态、内部结构、年龄和Hf-O同位素分析已经成为盆地沉积物源区示踪的重要探针。渣尔泰群和白云鄂博群中碎屑锆石年龄主要集中在新太古代末期（～2.50 Ga）与古元古代末期（～1.90 Ga）两组，这两组碎屑锆石的相对丰度在层序上出现“振荡性”变化，这种变化特征与经典的源汇系统中构造—沉积响应的年代记录明显不一致。为研究渣尔泰群、白云鄂博群沉积层序中碎屑锆石年龄分布特征与源区构造演化之间的关系。本文系统总结了区域内大量相关的锆石U-Pb年龄数据，同时对渣尔泰群书记沟组和增隆昌组进行碎屑锆石LA-ICP-MS U-Pb年代学研究。研究表明这两组年龄与南部大青山—乌拉山造山带中基底岩石记录的构造热事件年龄相一致，同时根据锆石结构特征认为～2.50 Ga的锆石来源于造山带内新太古代岩浆—变质地体，～1.90 Ga的锆石来源于高级变质—深熔地体。大青山—乌拉山造山带与这套中-新元古代沉积地层构成源汇系统。源区造山带下地壳新太古代与古元古代变质岩系近水平互层叠置是大青山—乌拉山造山带下地壳结构的基本特征，岩性变化率在垂向上远大于水平方向，是控制盆地中碎屑锆石组合变化的直接原因。

The characteristics of detrital zircons from the Zhaertai Group and Bayan Obo Group and its implications for the crustal structure of the provenance area

Recently, study on crystal morphology, internal texture, chronology and Hf-O isotopes of detrital zircons has been widely used for tracing sediments source. In the lower Mid-Proterozoic sediments, all the previous geochronological data suggest that the detrital zircon U-Pb ages from the Zhaertai Group and the Bayan Obo Group are mainly concentrated in the Late Neoarchean （～2.50 Ga） and the Late Paleoproterozoic （～1.90 Ga）. Moreover, the relative abundance of these two groups of detrital zircons show “oscillatory” changes in sequence. This characteristic is obviously in conflict with the age records from the tectono-sedimentary response in the typical source-to-sink system. In order to better understanding the relationship between the age distribution characteristics of detrital zircons in the Zhaertai-Bayan Obo rift sedimentary sequence and the tectonic evolution of the provenance area, in the study, a large number of previous relevant zircon ages are systematically compiled. In addition, the detrital zircon LA-ICP-MS U-Pb ages from the Shujigou and Zenglongchang Formations from the Zhaertai Group are carried out. The two age groups are consistent with the age of tectonic events recorded in the basement rocks of the Daqingshan-Wulashan orogenic belt. Morever, the detrital zircon internal texture shows that the～2.50 Ga zircons are derived from the Neoarchean magma-metamorphic terrane in the orogenic belt, while, the～1.90 Ga zircons come from the Paleoproterozoic high-grade metamorphic terrane. All the facts present here suggest that the near-horizontal interbedding of Neoarchean and Paleoproterozoic metamorphic rock series is the basic feature of the crustal structure in the lower orogenic belt, which is the direct reason for controlling the changes in detrital zircon assemblage in the basin, and the near-horizontal structural characteristics of the lower crust of the orogenic belt in the source area lead to the lithologic change rate in the vertical direction being much higher than that in the horizontal direction.