滇西新生代超钾质煌斑岩的元素和Sr-Nd同位素特征及其对岩石圈地幔组成的制约  

Geochemistry and Sr-Nd isotopes of Cenozoic ultrapotassic lamprophyres in western Yunnan: Constraints on the composition of sub-continental lithospheric mantle

作　　者：李献华[1] 周汉文[1] 韦刚健[1] 刘颖[1] 钟孙霖[2] 罗清华[2] 李寄嵎[2] 

机构地区：[1]中国科学院广州地球化学研究所,广东广州510640 [2]台湾大学地球科学系

出　　处：《地球化学》2002年第1期26-34,共9页Geochimica

基　　金：国家杰出青年科学基金(49725309);中国科学院知识创新工程重要方向项目(KZCX2-102);国家攀登计划预选项目(95-预-25)资助

摘　　要：系统分析了滇西早第三纪晚期煌斑岩的主元素、微量元素和Sr-Nd同位素组成。绝大多数煌斑岩属超钾质系列岩石(K2O/Na2O=2.1～5.2),少数为钾玄质系列(K2O/Na2O=1.6～1.7)。这些超钾质煌斑岩富集大离子亲石元素、轻稀土元素和Pb,亏损高场强元素,具有高的初始87Sr/86Sr比值(0.70624～0.70924)和负的εNd(t)值(-1.7～-4.6),类似于与俯冲环境有关的高K/Ti-低Ti钾质系列岩石。这些超钾质煌斑岩母岩浆来源于含金云母的交代地幔,经历了不同程度的结晶分异和地壳物质的同化混染。与藏北钾玄质系列岩石相比,滇西超钾质煌斑岩的Th/U比较低、Rb/Sr比较高,而且Nd模式年龄系统偏低0.1～0.4Ga,表明青藏高原北部和东南部岩石圈地幔组成存在区域性的差异。Geochemical and Sr Nd isotopic data are reported for the late Paleocene lamprophyres in western Yunnan. These lamprophyres are mostly ultrapotassic in composition with K2O/Na2O being 2.1 to 5.2, except for a few of samples with shoshonitic affinity showing slightly lower K2O/Na2O of 1.6 to 1.7. They are characterized by enrichment in large ion lithophile elements, light rare earth elements and Pb, depletion in high field strength elements, high initial 87Sr/86Sr ratios (0.706 24 to 0.709 24), and negative εNd(t) values (-1.7 to -4.6), resembling those of high K/Ti and low Ti potassic magmas formed in subduction related settings. The ultrapotassic magmas were generated by partial melting of a metasomatized, phlogopite bearing mantle source, followed by crystal fractionation accompanying varying degrees of crustal assimilation. Compared to the shoshonitic series rocks from northern part (< 15 Ma) of the Qinghai Tibet Plateau, the ultrapotassic lamprophyres in western Yunnan (40～30 Ma) display lower Th/U, higher Rb/Sr and younger (0.1～0.3 Ga) Nd model ages, indicating heterogeneity of sub continental lithosphere mantle beneath southeastern and northern parts of the Qinghai Tibet plateau.

关 键 词：新生代 煌斑岩 地球化学 SR-ND同位素 云南 青藏高原 岩石圈 锶 钕 

分 类 号：P588.13[天文地球—岩石学] P597[天文地球—地质学]