西秦岭竹林沟新生代碱性玄武岩地球化学及其成因  

Geochemistry and petrogenesis of the Cenozoic alkaline basalt from the Zhulingou area,Western Qinling Mountain

作　　者：赖绍聪[1] 朱韧之 

机构地区：[1]西北大学大陆动力学国家重点实验室/地质学系,陕西西安710069

出　　处：《西北大学学报（自然科学版）》2012年第6期975-984,共10页Journal of Northwest University（Natural Science Edition）

基　　金：国家自然科学基金资助项目(41072052);国家自然科学重大计划基金资助项目(41190072)

摘　　要：目的研究出露在甘肃礼县竹林沟地区的新生代火山岩成因及其大陆动力学意义。方法运用系统的岩石学、地球化学和Sr-Nd-Pb同位素地球化学特征进行分析。结果竹林沟火山岩具有低SiO2(40.72%～42.47%)、高TiO2(3.32%～3.62%),全碱含量高(K2O+Na2O=2.59%～3.40%),以及Na2O>K2O,K2O/Na2O=0.68～0.75的特征,属于典型的碱性系列钠质苦橄玄武岩-碱玄岩类。火山岩高的Mg#值(63.0～69.8)表明其原生岩浆的属性,岩石富集轻稀土,(La/Yb)N=39.3～41.9,(Ce/Yb)N=27.8～29.8,Ba,Th,Nb,Ta等元素呈显著的富集状态,具有典型的板内火山岩特征,而K和Rb含量较低,呈相对亏损状态。岩石87Sr/86Sr(0.704 189～0.704 418),143Nd/144Nd(0.512 803～0.512 906),206Pb/204Pb(18.643 446～18.685 488),207Pb/204Pb(15.591 105～15.594 871),208Pb/204Pb(39.042 692～39.077 318)等同位素变化特征具有显著的混源属性,投影点位于EMI,EMII,BSE及PREMA等典型地幔储库的过渡部位,不同于单一地幔源局部熔融形成的玄武岩的同位素组成特征。结论结合西秦岭新生代的总体构造背景,竹林沟钠质苦橄玄武岩-碱玄岩可能起源于一个特殊的多源混合地幔的局部熔融作用。新生代期间,青藏、扬子及华北3大构造体系域在西秦岭—松潘地区的强烈汇聚拼贴作用导致的深部地幔混合和这套特殊的钠质碱性玄武岩的产出有密切的成因关系。Aim To trace the petrogenesis and its continental dynamic implications of the Cenozoic alkaline basalt from the Zhulingou area, West Qinling mountain. Methods Petrology, geochemistry, Sr-Nd-Pb isotopic research. Results The studied volcanic rocks show high SiO2 （40. 72 - 42. 47% ）, TiO2 （3.32 - 3.62% ）, K2O＋ Na2O （2. 59 -3.40% ）, and Na2O 〉 K2O, K2O/Na2O =0. 68 -0. 75, belonging to an association of the typical mantle generation sodic-alkaline basalt. The Mg# value of the rocks （63.0 -69.8） indicates the characteristics of primi- tive magma series. The rocks enriched in light rare earth elements, （La/Yb）N =39. 3 -41.9, （Ce/Yb）N =27. 8 - 29. 8, and in Ba, Th, Nb, Ta relative to a obviously depleted in K, Rb in the trace and rare earth element （REE） spider diagrams, they are similar to the geochemical signature of typical OIB-type alkaline basalt. The rocks exhibit ^87Sr/^86Sr =0. 704 189 -0. 704 418, ^143Nd/^144Nd =0. 512 803 - 0. 512 906, ^206Pb/^204pb = 18. 643 446 - 18.685 488, ^207Pb/^204Pb = 15.591 105 -15.594 871, ^208Pb/^204Pb =39. 042 692 -39. 077 318. The Sr-Nd-Pb isotopic composi-tions suggest that they are derived from a mixed mantle reservoir. Conclusion The Western Qinling tectonic region was controlled by Tibet, North China and Yangtze blocks since Cenozoic. Therefore, the region was in the stage of the substance converge from the lower mantle to upper crust, producing a mixed mantle reservoir in the studied are- a. Zhulingou basalts occurred in the specific tectonic background are a product of partial melting of a mixed-asthe- nospheric mantle reservoir in the Western Qinling tectonic node.

关 键 词：钠质碱性玄武岩 岩石成因 大陆动力学 新生代 西秦岭 

分 类 号：P588.12[天文地球—岩石学]