利用基本的宝石学方法、紫外可见光分光光度计、红外光谱仪、激光拉曼探针和激光剥蚀等离子质谱技术(LA-ICP-MS)分析了黑龙江穆棱玄武岩风化冲积物中宝石级锆石的宝石学性质、谱学特征、包裹体特征、微量元素组成及U-Pb年龄.宝石学性质表明锆石为优质的、可直接加工利用的宝石原料;谱学特征显示锆石为未经历后期改造的高型岩浆锆石,且含有磷灰石、锆石和赤铁矿包裹体;LA-ICP-MS分析表明锆石具有明显的Ce正异常,富集重稀土元素[(Yb/Sm)N=21～63)]和HFSE元素,Th/U均大于1(1个数据例外);ΣREE较高,含量在1765 × 10-6～2 374 × 10-6,与基性锆石相似;明显的Ce异常和赤铁矿包裹体说明锆石形成于氧化环境;根据Ti-in-Zircon地质温度计计算,锆石结晶的最低温度为646～737℃;U-Pb年龄为9.39±0.4 Ma代表锆石的结晶年龄.基于本实验数据和前人研究成果,认为锆石巨晶形成于富集CO2的氧化环境并且与地慢的交代作用有关,是岩石圈减薄作用发展到一定阶段(衰退期)的产物.%Using classical gemological methods, UV-Vis-NIR, FTIR, LA-Raman spectra, cathode luminescence images, and LA-ICP-MS gemological properties, this paper systematically studied spectroscopic features, melt inclusions, trace elements and U-Pb radioactive ages of gem-quality zircon megacrysts found in alluvia of Tertiary alkali basalt deposits associated with Cenozoic basalts from Muling in Heilongjiang Province, which are distributed along the middle-northern section of Dunhua-Mishan fault, the northern branch of Tanlu fault between Xing-Meng orogenic belt and Pacific subduction zone.These normal zircons are generally colorless to reddish brown, transparent to semi transparent, and some of them are up to 17 mm long with euhedral to subhedral shapes which have clear surface melting features.Refractive indexes are higher than 1.78 and SG values vary from 4.7 to 4.9.The tetragonal prism and bipyramid constitute the most developed crystal forms.According to the classifica-tion of gem-quality of zircons, these samples are superior quality gemological material which can be used directly.The polarized UV-Vis absorption spectral analysis shows that the coloration mechanism of the zircon megacrysts are attributed to REE f-f and V4+ d-d charge transitions.Raman and IR spectral features confirm the viewpoint that these zircon megacrysts have undergone low extent of radiation damage and contain common melt inclusions of apatite, zircon and hematite.However, zircon megacrysts display obvious oscillatory zoning in CL images typical of igneous zircon, which suggests that they were crystallized from a melt or fluid surroundings.LA-ICP-MS analysis shows that they have distinct positive Ce anomalies and negative Eu anomalies, and are enriched in P, HREEs and HFSEs; most of Th/U ratios are higher than 1 with one exception, and (Yb/ Sm)N ratios are 21～63.∑REE concentrations vary from 1765 × 10-6 to 2374 × 10-6, and are much higher than those in kimberlitic zircons but similar to things of zircons from basic igneous rocks.In general, the shapes and trends of their REE patterns are similar to those of zircons from such areas of eastern China as Wenchang in Hainan Province, Mingxi in Fujian Province, and Changle in Shandong Province as well as to things of other zircon megacrysts associated with Cenozoic alkali basalts all over the world in spite of different absolute REE concentrations, thus suggesting similar origins.Pronounced positive Ce anomalies, negligible Eu anomalies and magnetite inclusions suggest crystallization from relatively oxidized conditions.Uptake of titanium in zircon is sensitive to temperature.In situ titanium analysis shows that model Ti-in-zircon temperatures at TiO2 and SiO2 activities = 1 range from 646 to 737℃.The weight mean of the remaining six points yields a U-Pb age of 9.39 ±0.4 Ma(MSWD=0.22) , which is interpreted as the crystallization age of these zircon megacrysts.Based on this study in combination with data obtained by other authors, the authors hold that Muling Cenozoic alkali basalt-related megacryst zircons might have originated from depleted metasomatized lithospheric mantle related to the oxidization conditions of CO2 fluid enrichment and resulted from retrogression of lithospheric thinning.
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