不同成因锆石阴极发光及微量元素特征:以新疆阿尔泰地区花岗岩和伟晶岩为例

新疆阿勒泰可可托海地区出露大量花岗岩和伟晶岩脉,利用阴极发光显微照相(CL)、电子探针背散射(BSE)和激光剥蚀电感耦合等离子质谱技术(LA-ICP-MS),观察和分析岩石中锆石的内部结构、稀土元素及Th,U含量后结果表明:该区花岗岩锆石具振荡环带和强烈的阴极发光特征,Th/U比值较高(Th/U=0.16～0.99),轻稀土亏损、重稀土富集,具较大的Ce正异常,为典型岩浆成因锆石。伟晶岩(KP-08-11)锆石为热液锆石,不具振荡环带和阴极发光,具低的Th/U比值(0.01～0.13),强烈富集稀土元素,尤其是轻稀土元素较花岗岩锆石高一个数量级,Ce的正异常相对较低。伟晶岩(KP3-08-1)锆石为变质重结晶锆石,Th/U比值分布范围较广(0.01～0.78),强烈亏损稀土元素,稀土元素配分模式存在显著的"REE四分组效应"。微量元素特征表明,伟晶岩(KP-08-11)锆石可能结晶自富U贫Th的残余岩浆流体,而伟晶岩(KP3-08-1)的锆石经历了蜕晶质化和变质重结晶作用,但依然保持了共存伟晶岩熔体的微量元素特征。

CHARACTERISTICS OF ZIRCON CATHODOLUMINESCENCE AND TRACE ELEMENTS OF GRANITE AND PEGMATITE FROM ALTAI MOUNTAINS, NORTHWEST CHINA

The internal textures and trace elements abundances of zircons from granites and pegmatites in Koktokay region are determined by cathodoluminescence(CL),backscattered electron(BSE) and laser ablation inductively coupled plasma mass spectrometry(LA-ICPMS) respectively.Characteristics of oscillatory zoning,high Th/U ratios and chondrite-normalized REE pattern of a steeply-rising slope from the LREE to the HREE with the obviously positive Ce anomaly and negative Eu anomaly,indicate that the zircons in the granite are of magmatic genesis,while the zircons from pegmatite(KP-08-11) are resulted from hydrothermal activity.The zircons from pegmatite show internally textureless in CL images,and enrich in REE,especially rich in LREE relative to magmatic zircons from granites.For example,zircons from pegmatite(KP-08-11) have flatter light-REE patterns and smaller Ce positive anomalies,and lower Th/U ratios(Th/U= 0.01~0.13 vs 0.16~0.99 for magmatic zircons);zircons from pegmatite(KP-08-11) could be directly crystallized from zircon-saturated U-rich and Th-poor hydrothermal fluid.The zircons from pegmatite(KP3-08-1) are of metamorphic origin and recrystallization is developed in these zircon grains.It is considered that the zircons from pegmatite(KP3-08-1) inherited the coexisting melt REE pattern.