江西弋阳晚白垩世塘边组风成砂岩碎屑锆石U-Pb定年及其物源意义

Detrital zircon U-Pb dating of the Late Cretaceous aeolian sandstones from the Tangbian Formation in the Yiyang area of Jiangxi Province and its provenance significance

赣东北信江盆地晚白垩世圭峰群塘边组以发育大型高角度交错层理的风成砂岩为特征,总体为干旱气候条件下沙漠沉积环境的产物。该地区所采砂岩样品为紫红色中-细粒长石石英砂岩,碎屑颗粒主要包括石英、长石、云母和岩屑,多呈次圆状,分选性较好。利用碎屑锆石LA-ICP-MS测年法,对塘边组风成砂岩进行U-Pb同位素定年,分析物源。碎屑锆石颗粒大多数大于100μm,Th/U值为0.1～2.1,且大部分锆石内部具有清晰的振荡环带结构,表明多数为岩浆锆石。^(206)Pb/^(238)U年龄加权平均值为141.4±1.2Ma。锆石U-Pb年龄主要集中于5个年龄段,分别是137～170Ma、431～506Ma、798～1172Ma、1268～2493Ma、2501～3488Ma。这些主要年龄区间与中国东南地区已知的岩浆事件对应,分别为燕山运动、加里东运动、晋宁运动、吕梁运动。其中,燕山期酸性火山-侵入杂岩提供了主要物源。结合风成砂岩交错层理反映的古风向数据,认为弋阳塘边组碎屑物质主要来自盆地北西部的九岭地区及北东部地区。

The Tangbian Formation of the Late Cretaceous Guifeng Group in the Xinjiang Basin of northeastern Jiangxi Province is characterized by large-scale cross-bedded sandstones. They are generally products of a desert depositional environment under arid climate. The sandstone samples are purple red, fine-to medium-grained feldspar quartz sandstones. Sand particles are mainly composed of quartz, feldspar, mica and rock fragments, which are subangular to subrounded and well sorted. In order to analyze the source rocks of the Tangbian Formation, LA-ICP-MS zircon U-Pb analysis was performed. The zircon grains are mostly larger than 100μm. The results show that the Th/U ratios range from 0.1 to 2.1, and most of the zircons have a clear internal shock band structure, indicating that most of them were magmatic zircons. The mean age of 206 Pb/238 U is 141.4±1.2 Ma. The age of zircon U-Pb ages are mainly concentrated in five zones of 137~170 Ma, 431~506 Ma, 798~1172 Ma, 1268~2493 Ma, 2501~3488 Ma,corresponding to the major magmatic events in Southeast China, namely Yanshanian, Caledonian, Jinning, and Lvliang movements,with the Yanshanian volcanic-intrusive complex providing the dominant sediments. In combination with the paleo-wind directions from the aeolian sandstones, the sediments are interpreted to be largely from the northwestern Jiuling mountain area and the northeastern areas.