缅甸翡翠中锆石的LA-ICP-MS微量元素U-Pb年龄:对其成因的制约  

Trace Elements and U-Pb Ages of Zircons from Myanmar Jadeite-Jade by LA-ICP-MS:Constraints for Its Genesis

作　　者：蔡诗诗 张恩 CAI Shi-shi;ZHANG En(School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China;Guangdong Provincial Key Laboratory of Geological Processes and Mineral Resources Survey, Guangzhou 510275, China)

机构地区：[1]中山大学地球科学与工程学院,广东广州510275 [2]广东省地质过程与矿产资源探查重点实验室,广东广州510275

出　　处：《光谱学与光谱分析》2018年第6期1896-1903,共8页Spectroscopy and Spectral Analysis

基　　金：国家自然科学基金项目(41273054)资助

摘　　要：翡翠产自与俯冲作用相关的HP/LT变质区域,对俯冲作用的研究具有重要意义。锆石常用来确定岩石的年龄,可以用于翡翠成因和形成时代的研究。缅甸是世界上优质翡翠最主要的产地,锆石是缅甸翡翠中常见的副矿物。选择含锆石较多的缅甸翡翠原石,对晶形较好的大颗粒锆石运用阴极发光(CL)图像观察锆石中颜色的分布和韵律环带形态的变化,并根据CL图像特征进一步采用激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)对锆石的微量元素和U-Pb年龄进行测定。结果表明具不同颜色和韵律环带形态的锆石,其微量元素分布有一定的差异,但总体却表现为:锆石的标准化球粒陨石稀土配分显示轻稀土元素(LREE)亏损,重稀土元素(HREE)富集,有明显的Ce(Ce/Ce~*=19.2~74.2)正异常和微弱的Eu(Eu/Eu~*=0.49~0.72)负异常。稀土元素总含量(∑REE)高(810~3 984μg·g^(-1)),(Yb/Sm)_N值为23.1~195.1(大部分值小于100);Th/U比值为0.28~1.18。研究锆石属于岩浆锆石,形成于俯冲带上覆地幔楔的镁铁质玄武岩浆之中,这类岩浆是陆下地幔橄榄岩受到俯冲过程中释放的流体作用部分熔融形成的。Ti温度计显示锆石的形成温度为762~923℃。锆石的^(206)Pb/^(238 )U年龄为(157.3±1.3)Ma,这一年龄为锆石的形成时间。CL图像显示锆石中颜色的分布和韵律环带形态的变化与锆石的形成时间无关,是锆石中微量元素差异分布造成的,反映了锆石形成过程中岩浆熔体成分的动态演化。Jadeite-jade originates in subduciton-related HP/LT metamorphic zone, thus study of it is of great importantance for shedding lights in subduction research. Zircon is often used to determine the age of rocks, and it’s usually applied to the investigation of genesis and formation time of jadeite-jade. Myanmar is the most primary origin of high quality jadeite-jade, of which zircon is a common accessory mineral. To explore the genesis and formation time of Myanmar jadeite-jade, raw jadeite-jade rock with enriched zircon inclusions were studied. In addition, Cathodoluminescence （CL） images were obtained to characterize distribution of colors and diversity of oscillating zones from well-crystallized zircons. Moreover, trace elements composition and U-Pb ages of these zircons were further determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry （LA-ICP-MS） according to characteristic CL images. The results showed that zircons with different colors and oscillating zones shapes have different trace elements distributions. The overall performance was that： chondrite-normalized REE pattern of zircons displays depleted LREE, enriched HREE, with edivdent Ce（Ce/Ce＊=19.2-74.2） positive anomalies and slight Eu（Eu/Eu＊=0.49-0.72） negative anomalie. High total REE （∑REE=810～3 984 μg·g-1）, （Yb/Sm）N values ranged from 23.1 to 195.1 （most of them less than 100）, and Th/U ratios 0.28 to 1.18. These zircons can be classified into magmatic zircons, originated from mafic basalt magma in the upper mantle wedge of subduction zone. This type of magma was formed by partial melting of subcontinental peridotite which reacted with fluids released from the subduction process. Moreover, Ti thermometer reveals that the forming temperatures of those zircons ranged from 762 to 923 ℃. The mean 206Pb/238U age is （157.3±1.3） Ma, which represents the formation time of these zircons. Distribution of colors and diversity of oscillating zones in CL images are irrelevant to forming ages o

关 键 词：缅甸翡翠 锆石 CL图像 微量元素 U-PB同位素年龄 

分 类 号：P578[天文地球—矿物学]