新疆且末—若羌地区黄绿色和田玉分析测试及特性表征

Characterization of Yellow-Green Hetian Jade in Qiemo—Ruoqiang,Xinjiang

新疆且末—若羌一带的和田玉,以带有黄绿色调为特征,针对该种和田玉的颜色成因和特性表征研究较少,影响了对这种和田玉的颜色和矿床成因的认识。本文利用电子探针(EPMA)、背散射电子图像(BSE)、X射线荧光光谱(XRF)、高分辨电感耦合等离子体质谱(HR-ICP-MS)分析技术对且末黄口料的矿物组成和化学成分进行测试,采用滴定法测试FeO含量,探究其颜色成因和成矿类型,并采用锆石SHRIMP U-Pb同位素定年约束其成矿时代。结果表明:该地区和田玉主要由磷灰石、方解石、白云石、透辉石、绿帘石、榍石和锆石组成。其稀土总量低(∑REE=2.61~19.1μg/g),稀土配分模式呈现出明显Eu负异常(δEu<0.05)、LREE右倾、HREE平坦,根据稀土配分模式和Cr、Ni元素含量,推测是一种镁质矽卡岩型和田玉。且末黄口料和田玉Fe^(3+)/Fe^(2+)平均值(0.16)与其他地区和田玉有重叠,但该比值稍高(0.07~0.26),而Fe^(3+)、Fe^(2+)含量与其他地区和田玉之间差异并不明显。因此,推测且末—若羌和田玉的颜色成因与Fe^(3+)/Fe^(2+)比值有关,与Fe^(3+)、Fe^(2+)含量并无明显关系;该地区和田玉中锆石SHRIMP U-Pb同位素定年获得的两个加权平均年龄为461.7±6.1Ma(MSWD=1.6)和498.1±4.6Ma(MSWD=1.16),代表了该地区黄绿色和田玉的形成时代,并可以进一步约束西昆仑的构造演化。

BACKGROUND:The Hetian nephrite belt in Xinjiang is the largest nephrite ore belt in the world,with a length of about 1300km.In addition to white,gray,black and brown nephrite,the Qiemo—Ruoqiang zone,Xinjiang also produces a yellow-green nephrite.There are few studies on the color origin and characterization of this kind of Hetian nephrite,which affects the understanding of the color and genesis of this kind of Hetian nephrite.OBJECTIVES:To understand the color genesis,genetic types and formation age of the yellow-green nephrite in Qiemo—Ruoqiang deposit.METHODS:Electron probe microanalysis(EPMA),backscattered electron(BSE)image,X-ray fluorescencespectrometry(XRF),and high resolution inductively coupled plasma-mass spectrometry(HR-ICP-MS)were used to investigate mineral and chemical composition.The FeO content was determined by titration method to explore its color origin and genetic type,and the zircon U-Pb isotope dating by sensitive high-resolution ion microprobe(SHRIMP)was used to constrain its mineralization age.RESULTS:Yellow-green nephrite in Qiemo—Ruoqiang was predominately composed of apatite,calcite,dolomite,diopside,epidote,sphene and zircon.The yellow-green nephrite whole rock had low total REE(∑REE=2.61-19.1μg/g)with obvious Eu negative anomaly(δEu<0.05),LREE right dipping,and HREE flat pattern.According to the distribution pattern of rare earth and the content of Cr and Ni,it was inferred that it was a magnesia skarn type Hetian nephrite.Compared with other nephrite in the world,the average value of Fe^(3+)/Fe^(2+)of the yellow-green nephrite in Qiemo—Ruoqiang(average=0.16)overlapped with nephrite in other regions.The ratio is slightly higher(0.07-0.26),while the content of Fe^(3+)and Fe^(2+)was not significantly different between the yellow-green nephrite and other color nephrite.The SHRIMP U-Pb dating of zircon in the yellow-green nephrite yielded ages of 461.7±6.1Ma(MSWD=1.6)and 498.1±4.6Ma(MSWD=1.16).CONCLUSIONS:The color genesis of the yellow-green nephrite in Qiemo is probably related to the ratio of Fe^(3+)/Fe^(2+),but not to the content of Fe^(3+) and Fe^(2+).The SHRIMP U-Pb dating results of zircon in yellow-green nephrite represent the formation age of the yellow-green nephrite.These new data provide geochronological constraints for the magmatism of Hetian nephrite belt and the tectonic evolution of the West Kunlun orogenic belt.