庐山地区星子群碎屑锆石SIMS U-Pb年龄及其地质意义

江西庐山地区出露的星子群为一套遭受角闪岩相变质的中深变质岩系,前人将其划分为古元古代地层,并将其与华南早期地壳的形成与演化和中生代变质核杂岩构造相联系。本文通过对星子群浅粒岩中碎屑锆石U-Pb SIMS年代学研究,获得了(834±4)Ma碎屑锆石年龄,代表了该套地层沉积时代的下限;再结合其上覆筲箕洼组(830±5)Ma的火山岩年龄,将星子群的地层时代限定于新元古代。这一研究结果表明庐山地区并未出露古元古代的变质基底;而且野外也并未找到典型大规模拆离断层的存在,因此前人厘定的中生代变质核杂岩构造也值得商榷。根据碎屑锆石U-Pb不一致线下交点年龄为(133±20)Ma,推断星子群的变质作用发生在中生代。另外,由星子群单一的碎屑锆石年龄、锆石形态特征及矿物组成特征推断,星子群是源区花岗质岩石快速抬升、风化剥蚀和搬运沉积的产物,暗示着强烈的地壳运动过程,可能与Rodinia超大陆的裂解有关。

SIMS U-Pb Dating of the Detrital Zircons From the Xingzi Group in Lushan Area and Its Geological Significance

The Xingzi Group exposed in the Lushan area,Jiangxi province,was previously considered to be Paleoproterozoic rock,which was applied to deduce the formation and evolution of the crust in South China and Lushan Mesozoic metamorphic core complex.In this paper,detrital zircons of the leptite from the Xingzi group were tested by SIMS technique,and the result of(834±4) Ma may represent the maximal formation age of the Xingzi group.Regarding the age of overlying Shaojiwa Formation(830±5)Ma,the Xingzi Group can be restricted in Neoproterozoic.The result which reveals that there is no Archaean-Paleoprotrozoic metamorphic basement exposed in Lushan area,and the stratigraphic missing on "denudation faults" by previous studies as well,invoke us to reevaluate the evidence of the so called "Lushan Metamorphic Core Complex".Based on lower intersection on the U-Pb Concordia diagram of the detrital zircons,the metamorphism of the strata might take place in Mesozoic.Integration of the ages and morphologic characters of detrital zircons and mineral compositions of the leptite suggest that the Xingzi group was derived from rapid accumulation due to rapid crustal uplift-erosion of granitoids.This process may be related to break up of the Rodinia supercontinent.