新疆额尔齐斯韧性剪切带新生锆石及其年代学意义

新疆额尔齐斯韧性剪切带片理化的花岗岩类岩体中,锆石有新生热液锆石和残留原生岩浆锆石两种。微区分析数据在U-Pb谐和曲线上显示有原生岩浆结晶年龄和韧性剪切变形事件年龄,后者呈下交点模式分布。原生岩浆结晶年龄为329~447 Ma,来自北阿尔泰带志留纪。泥盆纪岩体及南阿尔泰带石炭纪花岗岩体原生岩浆锆石;韧性剪切变形事件年龄是视年龄,为295~305、267~286 Ma。由于额尔齐斯韧性剪切带内切穿韧性剪切带的未变形区域性岩墙群岩石SHRIMP锆石U-Pb年龄为277~286 Ma,所以变形岩石锆石谐和曲线反映的谐和年龄(295~305 Ma)才是韧性剪切带变形事件的主期年龄,下交点模式视年龄偏低。韧性剪切带新生锆石特征为晶棱清楚的简单(110)+(101)聚形,呈细长柱状,锥顶角为120°钝角,震荡环带较宽,阴极发光强度较弱,与原生岩浆锆石有明显不同。这些特征在新疆各韧性剪切带中重复出现,新疆石炭纪末—二叠纪初和志留纪末两期规模巨大的韧性剪切带在各构造带广泛发育,正确识别韧性剪切带热液锆石是走出侵入岩年龄认识误区的重要环节。

Neogenic Zircons in Ertix Ductile Shear Zone of Xinjiang and Their Geochronological Significance

Two kinds of zircons from schistosed granitoid rock in Ertix ductile shear zone of Xinjiang are neogenic hydrothermal and residual magmatic. According to the zircon U-Pb concordia diagram with microanalysis data, there are primary magma crystallization age and ductile shear deformed event age, and the latter is distributed on lower intersection in concordia diagram. The primary magma crystallization ages are 329-447 Ma, and the primary magmatic zircons are from Silurian and Devonian rocks of northern Altay belt and also from Carboniferous granites of southern Altay belt; the ductile shear deformed event ages are apparent with 295-305 Ma and 267-286 Ma. Due to the zircon from undeformed regional dike swarm cutting ductile shear zone in Ertix ductile shear zone, SHRIMP zircon U-Pb age is 277-286 Ma, so that the concordia age (295-305 Ma) from deformed zircon concordia diagram is the main age of deformed event of ductile shear zone, and the apparent age of lower intersection model is lower. The neogenic zircon of ductile shear zone is characterized by clear edge of crystal, simple combination (110)+(101), slim columnar, 120° cone apex angle, wide oscillatory zoning and weak cathodoluminescence (CL) intensity, which are significantly different from the primary magmatic zircon. These features are popular in various ductile shear zones of Xinjiang. Two huge ductile shear zones in Late Carboniferous-Early Permian and Late Silurian widely develop in various tectonic belts, so that identifying the hydrothermal zircon of ductile shear zone is important for improving the accuracy of intrusive rock ages.