北祁连东段红土堡基性火山岩锆石LA-ICP-MS U-Pb年代学及其地质意义

A LA-ICP-MS U-Pb Chronological Study of Zircons from Hongtubu Basic Volcanic Rocks and Its Geological Significance in the East Segment of North Qilian Orogenic Belt

为精确限定红土堡基性火山岩的形成时代,结合阴极发光分析,分别对北祁连东段红土堡枕状玄武岩和辉绿岩墙进行了LA-ICP-MS单颗粒锆石微区U-Pb同位素测年。红土堡枕状玄武岩获得(443.4±1.7)Ma和(1 782±34)Ma两组年龄值;同时,获得红土堡基性火山岩中辉绿岩墙的年龄为(385.7±7.9)Ma,并含有(1 697±6)-(2 692±4)Ma的捕晶锆石年龄信息。认为红土堡基性火山岩的形成时代为晚奥陶世,与其北的陈家河中酸性火山岩的形成时代大体一致,推断红土堡基性火山岩的原始岩浆不同程度遭受了陇山岩群的混染。这一新资料,对进一步研究北祁连造山带东段大地构造格局、构造演化以及北秦岭—北祁连衔接关系具有重要意义。

In order to limit accurate formation time of Hongtubu basic volcanic rocks in the east segment of North Qilian orogenic belt, LA-ICP-MS U-Pb isotope dating technique, combined with cathodoluminescence image （CL） study, was separately used to determine the zircons from pillow basalt and diabase dyke. Two main populations of zircons from Hongtubu pillow basalt were obtained, giving an average ^206Pb/^238U ages of（443.4 ± 1.7）Ma and an upper-intercept ^206Pb/^238U-^207pb/^235U age of（ 1 782± 34）Ma. Meanwhile, LA-ICP-MS U-Pb analyses for zircon grains from the diabase dyke within Hongtubu basalts yielded a mean age of （385.7 ± 7.9） Ma, and a group of apparent ^207Pb/^206Pb ages from（ 1 697 ± 6）Ma to（2 692 ±4）Ma. With the relevant data synthesized, Hongtubu basic volcanic rocks were considered to form in the Late Ordovician, corresponding to the age of Chenjiahe mediumacid volcanic rocks which locate in the north of Hongtubu basic volcanic rocks. It was deduced that primitive magma of the basic volcanic rocks was contaminated in different degree by Longshan rock group. These new data have important significance for further research on the tectonic formwork in the east segment of the north Qilian orogenic belt, its tectonic evolution, and the connected relation of North Qilian and North Qinling orogenic belt.