锆石La-ICP-MS U-Pb与白云母~(40)Ar/~(39)Ar年代学及其对中国东南部早燕山事件的制约

起始于中、晚侏罗世之交的燕山运动使中国东南部上三叠统至中侏罗统发生强烈的褶皱和逆冲变形,形成北北东向构造带,并诱发广泛的深熔作用和岩浆活动。迄今为止,对该事件的形成时代尚缺乏精确的年代学制约。基于武夷山地区的野外调查,选取挤压活动前形成的正长岩和挤压过程中形成的片麻状花岗岩进行锆石La-ICP-MSU-Pb定年,并对逆冲剪切过程中形成的云母片岩进行40Ar/39Ar年代学测试分析,结合区域上后构造期的伸展型花岗岩对早燕山事件的形成时代进行制约。结果表明,形成于伸展构造背景的福建政和县铁山正长岩,其结晶年龄为(169.3±1.6)Ma,提供了早燕山事件的下限;形成于挤压环境的粤北平远县八尺片麻状花岗岩,其结晶年龄为(165.4±1.2)Ma,指示早燕山事件的主变形时代;同变形期的闽西长汀县濯田云母片岩,其40Ar/39Ar坪年龄为(162±2)Ma,代表逆冲剪切的冷却年龄,可视作早燕山事件的上限。结合区域上侵入北北东向褶皱的花岗岩年龄,认为早燕山挤压事件发生在(165±4)Ma,其动力作用与古太平洋板块向中国东南大陆之下俯冲有关。

Geochronology of Zircon La-ICP-MS U-Pb and Muscovite~(40)Ar/~(39)Ar: Constrains to Early Yanshanian Event in Southeast China

The Mid-Late Jurassic Yanshan movement in China induced the intense folds and thrust deformation in Upper Triassic to Middle Jurassic layers, forming the NNE-trending tectonic belts in Southeast China. At the same time, it initiated widespread anatectic melting and magma activity. However, there is a lack of high precise geochronology to constrain its timing as yet. Based on detailed field investigation, syenite and granitic gneiss are selected to be dated by zircon La-ICP-MS U-Pb method, and muscovite ~(40)Ar/~(39)Ar method. It′s found that these two granitoid plutons were formed in pre-collision and collision tectonic environment respectively. Syenite in Tieshan, Zhenghe County, formed in extensional environment, gives an age of (169.3±1.6) Ma and the lower bound of Early Yanshanian event. While Bachi granitic gneiss in Pingyuan, northern Guangdong Province, formed in compressional environment, yields an age of (165.4±1.2)Ma. This indicates the main timing of deformation. In contrast, ~(40)Ar/~(39)Ar plateau age of syn-deformation muscovite in Changting, western Fujian Province, is (162±2)Ma. This is interpreted as cooling age of the thrust shear, supplying the upper bound of Early Yanshanian event. With the combination of intruding ages of regional granitoid into NNE-striking folds, it can be concluded that Early Yanshanian event should have occurred in (165±4)Ma, which is caused by subduction of paleo-Pacific plate underneath southeast China.