西南天山特克斯科桑溶洞火山岩的锆石U-Pb年代学研究

Zircon U-Pb age for Kesang Rondong volcanic rocks,Southwest Tianshan Mts.,Tekes,Xinjiang

新疆特克斯县乌孙山科桑溶洞火山-沉积岩剖面由变质岩、火山-沉积岩(中酸性火山熔岩、火山碎屑岩、生物碎屑灰岩)和侵入其中的花岗质岩石组成,花岗岩中保留着英安岩-安山岩顶垂体。夹在火山岩地层之间的厚层生物碎屑灰岩(其中发育溶洞)发生褶皱变形。在剖面多处见到逆冲构造,断裂带局部塑性变形。采自剖面下部英安岩样品的锆石232Th/238U比值为0.4～0.6,U-Pb谐和年龄368.3±1.7Ma(MSWD=1.5,n=15)。剖面上部流纹岩中锆石的232Th/238U比值为0.8～2.2,U-Pb谐和年龄为356.4±1.7Ma(MSWD=1.3,n=19)。大约12Ma的时差说明存在两次火山喷发事件,在这两次火山喷发的间歇形成了巨厚生物碎屑灰岩(>1000m)。这种间歇性的火山喷发在西天山地区普遍发育,形成了规模巨大的滨海相火山-沉积盆地。剖面下部的英安岩与剖面上部的流纹岩的锆石具有不同的微量元素组成,预示着其岩浆源区性质的差异。

This paper deals with the geology of volcanic rocks and the measured U-Pb ages of dacite and rhyolite in Kesang Rongdong section near Tekes Town(southwestern Tianshan),consisting of metamorphic rocks,volcano-sedimentary rocks(including bioclastic limestone),and the intruded granite.Some dacite-andesite blocks preserved in granite as pendant.Karst cave(Kesang Rondong) developed in bioclastic limestone inter-layered in volcanic-sedimentary units.Mylonitized volcanic rocks and muscovitequartz schist formed locally in the trust faults.Zircons separated from dacite in the lower part of the section gives an average U-Pb concordia age of 368.3 ± 1.7Ma(MSWD = 1.5,n = 15) with 232Th/238U ratios of 0.4 ～ 0.6.Zircons separated from rhyolite sample in the upper part of the section gives an average U-Pb concordia age of 356.4 ± 1.7Ma(MSWD = 1.3,n = 19) with 232Th/238U ratios of 0.8 ～ 2.2.This age difference(～ 12Ma) between dacite and rhyolite samples in the volcano-sedimentary section implies that there were two episodes of volcano eruption.Bioclastic limestone of littoral facies(> 1000m) formed after the first episode of volcano eruption.Intermittent volcanic eruptions were common in western Tianshan during the Late Paleozoic,and formed intermittent volcanosedimentary basins.Based on the different geochemical characteristcs of the measured zircons separated from dacite and rhyolite with different ages,the magma corresponding to these two lava samples in the Kesang Rongdong section must have different origin.