吉林中部伊通地区放牛沟火山岩的形成时代及其地质意义

对伊通地区的放牛沟火山岩,以及后期侵入该火山岩的后庙岭花岗质侵入体进行了LA-ICP-MS锆石U-Pb年代学研究。3个样品中的锆石均呈自形-半自形晶,CL图像显示出明显的岩浆振荡生长环带,结合大多数锆石具有较高的Th/U比值(0.23~3.55),暗示了它们的岩浆成因。放牛沟火山岩由变玄武安山岩和变安山岩组成,其中变安山岩样品中锆石22个测点的~(206)Pb/~(238)U年龄加权平均值分为3组:420±4 Ma,402±3 Ma及280±1 Ma,其中280±1 Ma代表了安山岩的形成年龄;变玄武安山岩样品中锆石30个测点的~(206)Pb/~(238)U年龄加权平均值分为两组:401±1 Ma及279±1 Ma,后者代表了玄武安山岩的形成年龄;后庙岭花岗质侵入体中锆石18个测点的~(206)Pb/~(238)U年龄加权平均值为256±2 Ma。上述锆石U-Pb定年结果表明,放牛沟火山岩形成于早二叠世,而非前人认为的早古生代。对后庙岭侵入体的定年结果,进一步暗示放牛沟多金属硫铁矿床的成矿时代为二叠纪。

Geochronology and geological implications of Fangniugou volcanic rocks in Yitong area,central Jilin Province

LA-ICP-MS zircon U-Pb dating has been applied to Fangniugou volcanic rocks in Yitong area as well as Houmiaoling granitic pluton that intruded into the volcanic rocks. Zircons from three samples are euhedral to subhedral, revealing distinct oscillatory growth zoning in the CL images. Most of the zircons yield relatively high Th/U ratios of 0.23~3.55, implying their magmatic origins. The Fangniugou volcanic rocks consist of meta-basaltic andesite and meta-andesite. Weighted average 206Pb/238U ages of 22 zircon dating spots from the meta-andesite fall into three groups, i.e. 420±4 Ma, 402±3 Ma, and 280±1 Ma, of which the 280±1 Ma represents the formation age of the andesite. Weighted average 206Pb/238U ages of 30 zircons dating spots from the meta-basaltic andesite yield two clusters, i.e. 401±1 Ma and 279±1 Ma, of which the latter represents the formation age of the basaltic andesite. 206Pb/238U ages of 18 zircons dating spots from Houmiaoling granitic pluton yield weighted average value of 256±2 Ma. Above mentioned zircon U-Pb results demonstrate that the Fangniugou volcanic rocks formed during the Early Permian, rather than the Early Paleozoic as previously defined. Besides, dating results of the Houmiaoling pluton furtherly prove that mineralization of the Fangniugou polymetallic pyrite deposit occurred during Permian.