摘要
内蒙古林西地区双井片岩北缘混合岩中的浅色体和暗色体锆石在阴极发光下均可观察到再生长边。浅色体中锆石的自形和生长环带显示其岩浆结晶锆石特征,LA-ICPMS分析结果显示其核部206Pb/238U加权平均年龄为270.2Ma±1.4Ma,边部年龄在250Ma±3Ma～266Ma±3Ma之间。暗色体中存在碎屑锆石和岩浆锆石,且显示不同程度的重结晶。碎屑锆石中除一颗锆石核部给出了3377Ma±7Ma的207Pb/206Pb年龄外,其余变化于2190Ma±8Ma～1804Ma±25Ma;岩浆锆石核部和边部的206Pb/238U加权平均年龄分别为262.6Ma±2.2Ma和248.3Ma±3.4Ma;发生明显重结晶作用的锆石拟合的不一致线与谐和线的上、下交点年龄分别为2112Ma±350Ma和256Ma±17Ma,与碎屑锆石和岩浆锆石的年龄分别对应。推测双井地区可能存在古元古代的结晶基底,在270.2Ma±1.4Ma发生岩浆活动,导致结晶基底的重熔以及混合岩化,随着岩浆的上升,温度和压力都不断降低,重熔的暗色体中在262.6Ma±2.2Ma结晶大量的锆石,由于岩浆的持续活动,这些先期结晶的锆石又受到不同程度的熔蚀并重结晶。暗色体被岩浆注入以后共同经历了一次热事件改造,形成了今天看到的双井地区混合岩的面貌。
Zircon U-Pb dating with LA-ICPMS method was used to determine the ages of melanosome and leucosome from the migmatite in the north Shuangjing schist in Linxi, Inner Mongolia. Almost all the zircons have light absorption rims in the CL image. The zircons in leucosome shows typical magmatic growth zone, the weighted average ^206pb/^238U age is 270. 2 Ma±1.4 Ma from the core, and the ^207TPb/^206pb age is 250 Ma±3 Ma～266 Ma±3 Ma from the rims respectively. Both detrital zircons and magmatic zircons exist in the melanosome with different grade of recrystallization. One grain of detrital zircon in the core yields a ^206pb/^238U age of 3 377 Ma±7 Ma, the other have an age of 2 190 Ma±8 Ma～1 804 Ma±25 Ma. The weighted average ^206Pb/^238U age of the magmatic zircons from the core and rim is 262.6 Ma±2.2 Ma～248.3 Ma±3.4 Ma. The recrystallized zircons constitute a discordia line with the upper intercepting age of 2 112 Ma±350 Ma and a lower intercepting age of 256 Ma±17 Ma,which are consistent with the ages yielded by detrital zircons and magmatic zircons in melanosome. The LA-ICPMS zircon U-Pb dating results of the melanosome and leucosome show that there may be paleoproterozoic crystal substance in Shuangjing area and a magma activity event occurred in 270.2 Ma±1.4 Ma. The magma intrusion resulted in the melt of upper stratum, then migmatization occurred. The temperature and pressure kept falling by the uprising of magma, which brought about magmatic zircons in 262. 6 Ma±2.2 Ma. The early formed zircons were melted and recrystallized duo to later continuous activity of magma. After the magma intrusion a thermal event affected the area and formed the geological characteristics of the migmatite.
出处
《矿物岩石》
EI
CAS
CSCD
北大核心
2008年第2期10-16,共7页
Mineralogy and Petrology
基金
中国地质调查局项目(1212010510507)
