青藏高原新生代火车头山碱性及钙碱性两套火山岩的地球化学特征及其物源讨论  

Geochemistry and petrogenesis of the alkaline and caic-alkaline series Cenozoic volcanic rocks from Huochetou mountain,Tibetan plateau

作　　者：赖绍聪[1] 秦江锋[1] 李永飞[1] 隆平[1] 

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

出　　处：《岩石学报》2007年第4期709-718,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金(批准号:40572050;40272042);高等学校优秀青年教师教学科研奖励计划(批准号:教人司[2002]383号)联合资助

摘　　要：藏北羌塘火车头山新生代火山岩可区分为钙碱性及碱性两个不同的系列.钙碱性火山岩主要岩石组合为玄武岩-安山岩-英安岩,其SiO2介于49%～70%之间,Al2O3＞10%,Na2O/K2O＞1;其中玄武岩具平坦型稀土配分型式,LREE/HREE为1.3～1.8,（La/Yb）N为2.87～4.45,无明显铕异常,δEu为0.96～1.09;该套岩石的Mg#与SiO2相关关系以及La/Sm-La等亲岩浆元素与超亲岩浆元素协变关系表明,它们应为幔源岩浆经分离结晶演化的产物,其岩石组合类型以及低的Sm/Yb值（Sm/Yb=1.53～5.35）表明它们的原始岩浆应来源于岩石圈地幔尖晶石二辉橄榄岩的局部熔融.本区碱性火山岩为一套典型的钾质岩石系列,主要岩石组合类型为碱玄岩-碱玄质响岩-响岩,其SiO2介于44%～59%之间,Al2O3＞14%,Na2O/K2O介于0.47～1.51之间;岩石轻稀土强烈富集,LREE/HREE为13.20～15.76,（La/Yb）N=50.44～91.99;其岩石组合类型以及Mg^#与SiO2相关关系以及La/Sm-La协变关系同样表明它们为共源岩浆分离结晶演化的产物;然而,其较高的Sm/Yb值（Sm/Yb=2.63～13.98）表明它们并非地幔橄榄岩直接局部熔融的产物,岩石弱的负Eu异常（δEu=0.77～0.85）以及Th、U的强烈富集和Nb、Ta的相对亏损,又反映了原始岩浆中有显著的地壳物质的贡献;该套钾质碱性系列岩石在La/Co-Th/Co同分母协变图上呈直线型分布,而在La/Co-Sc/Th异分母协变图上呈显著的双曲线分布,从而表明其源区为二源混合型,是青藏高原特殊的壳幔混合层局部熔融的产物,这些特征是新生代青藏高原壳幔层圈物质交换的重要岩石学证据.The Cenozoic volcanic rocks in Huochetou mountain, North Qiangtang can be divided into two different series. The calc-alkaline series belongs to a typical basic-intermediate-silicic rock association. The main rock type is basalt-andesite-dacite. They show SiO2 =49%-70% , Al2O3 〉 10%, Na2O/K2O 〉 1. In addition, the basalt shows LREE/HREE = 1.3 - 1.8, （La/Yb） N = 2. 87 4.45, δEu = 0.96-1.09. It indicates that the basalt is just slightly enriched in light rare earth elements （ LREE ） and without remarkable Eu depletion. The Mg^#-SiO2 and La/Sm-La diagrams as well as the Sm/Yb（ Sm/Yb = 1.53 -5.35 ） indicate that this group of calc-alkaline volcanic rocks originated from the partial melting of the spinel-lherzolite at the upper part of the lithospheric mantle, Tibetan plateau. The alkaline volcanic rock association belongs to a typical potassium-rich shoshonitic series. The main rock type is tephrite- pollenite-phonolite. They exhibit SiO2 =44%-59%, Al2O3 〉 14%, Na2O/K2O =0.47 - 1.51, and LREE/HREE = 13.20-15.76, （La/Yb）N = 50.44-91.99, indicating distinct geochemical feature of strongly enrichment in light rare earth elements. On the Mg^#-SiO2 diagram, samples of the alkaline volcanic rock association plot between the mantle melt （basalts） and the amphiboliticeclogitic melt （ 1 - 3 GPa）, and accord with the crustal AFC evolutionary trend. The La/Sm-La diagram which exhibit the closely relationship between the magmatophile element and ultra-magmatophile element indicate that this group of volcanic rocks belongs to the fractional crystallization result of a basaltic comagrnatic system. The futures of the basic-intermediate-silicic rock association imply that this group of volcanic rocks should be originated from the partial melting of upper mantle. On the other hand, the distinctive geochemistry characteristics of high in Sm/Yb （Sm/Yb） = 2.63 -13.98, slight depletion in Eu and strong depletion in element Nb and Ta as well as strong enrichment in some lithophile elements （such as Th

关 键 词：地球化学 源区性质 地幔局部熔融 壳幔层圈物质交换 新生代火山岩 青藏高原 

分 类 号：P588.15[天文地球—岩石学] P594.1[天文地球—地质学]