冈底斯新近纪钾质火山作用：消减沉积物折返的地球化学与Sr-Nd-Pb同位素证据  

The Eocene potassic volcanism in the Gangdese:Geochemical and Sr-Nd-Pb isotopic evidences for recycling of subducted sediments.

作　　者：魏瑞华[1] 高永丰[1] 侯增谦[2] 孟祥金[3] 杨竹森[3] 

机构地区：[1]石家庄经济学院,石家庄市050031 [2]中国地质科学院地质研究所,北京100037 [3]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2008年第2期359-367,共9页Acta Petrologica Sinica

基　　金：国家自然科学基金(40672042);国家重大基础研究计划(2002CB412600);国家科技支撑计划项目(2006BAB01A04)资助课题

摘　　要：在冈底斯岩浆岩带的羊八井地区,林子宗火山岩系上部出现小规模的粗面质火山岩。岩石学与地球化学研究表明粗面岩与时空密切相关的大体积安山质-英安质-流纹质火山岩属于不同的火山岩系:碱性钾质系列与钙碱性系列。详细的地球化学研究证明林子宗晚期小体积钾质岩具有独立的岩浆源区,而非早期钙碱性系统低压岩浆过程演化的产物。粗面质熔岩SiO_2含量为62.91%～64.63%,具有高K_2O(7.52%～8.05%)、Al_2O_3(16.64%～17.35%)、低TiO_2(0.59%～0.68%)与MgO(0.15%～0.77%)。钾质熔岩富集LILE与LREE,亏损HFSE,具有高Rb/Sr(1.1～2.3)、Th/La(0.59～0.80)、Th/U(6.3～10.9)和低Ce/Pb(4.1～6.2)与Ba/Th(4.3～14.5)比值,其同位素组成变化范围分别是^(87)Sr/^(86)Sr_(i)=0.7068～0.7075、^(143)Nd/^(144)Nd_(i)=0.51241～0.51252、^(206)Pb/^(204)Pb=18.87～18.95、^(207)Pb/^(204)Pb=15.63～15.70和^(208)Pb/^(204)Pb=39.24～39.68。粗面岩的地球化学与同位素特征表明冈底斯新近纪钾质火山岩来源于角闪岩相的消减沉积物熔融。因此,羊八井新近纪钾质火山熔岩提供了印度-亚洲大陆碰撞早期消减沉积物折返的证据。In the Yangbajing area in south Tibet, small-volume trachytic lavas occur in the upper succession of Linzizong calcalkaline volcanic suites. The mineralogical and geochemical characteristics of the trachytes and the voluminous andesitic to rhyolitic rocks reveal that they belong to two magma series： calc-alkaline and potassic alkaline. The geochemical and Sr-Nd-Pb isotopic characteristics indicate that low-pressure evolutionary processe of the large voluminous calc-alkaline volcanic rocks was not responsible for the petrogenesis of the younger potassic rocks, and the Eocene potassic lavas in the Gangdese belt have a distinct magma source from those of the calc-alkaline volcanic suites. The trachytes have high K2O （7.52% - 8.05% ） and Al2O3 （ 16.64% - 17.35% ）, low TiO2 （0.59% - 0.68% ） and MgO （0.15% - 0.77% ） with moderate SiO2 contents （62.91% - 64.63% ）. The alkaline potassic lavas are characterised by high LILE and LREE contents, and also have high Rb/Sr （ 1. 1 - 2. 3 ）, Th/La （ 0.59 - 0. 80 ） and Th/U （ 6.3 - 10.9 ） , low Ce/Pb （4.1 - 6.2 ） and Ba/Th （4.3 - 14.5 ） ratios. The Yangbajing potassic rocks have high radiogenic ^87Sr/^86Sr（i） ratios （0. 7068 -0. 7075 ） and low ^143Nd/^144Nd（i） （0. 51241 - 0.51252）, high ^206Pb/^204Pb （ 18.87 - 18.95 ）, ^207pb/^204Pb （15.63- 15.70） and ^208spb/^204Pb （39.24 -39.68）. The combined geochemical and Sr-Nd-Pb isotopic characteristics could thus suggest that the Eocene alkaline potassic rocks in the southern Lhasa block were derived from melting of metasediments. Identification of the Eocene potassic lavas and the delineation of their separate source provide reliable evidence for recycling of subducted sediments during the early stage of the India-Asia collision.

关 键 词：钾质火山作用 消减沉积物 源区多样性 藏南地区 

分 类 号：P588.143[天文地球—岩石学]