青藏高原西南部赛利普超钾质火山岩富集地幔源区和岩石成因：锆石U-Pb年代学和Hf同位素制约  

Enriched mantle source and petrogenesis of Sailipu ultrapotassic rocks in southwestern Tibetan Plateau:constraints from zircon U-Pb geochronology and Hf isotopic compositions.

作　　者：孙晨光[1] 赵志丹[1] 莫宣学[1] 朱弟成[1] 董国臣[1] 周肃[1] 陈海红[2] 谢烈文[3] 杨岳衡[3] 孙金凤[3] 于枫[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室中国地质大学地球科学与资源学院,北京100083 [2]中国地质大学地质过程与矿产资源国家重点实验室,武汉430074 [3]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2008年第2期249-264,共16页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2002CB412603);国家自然科学基金(40473020;40503005;40572048;40672044);中国地调局综合研究项目;高等学校学科创新引智计划(B07011);中科院广州地化所同位素开放实验室基金;教育部留学回国人员科研启动基金资助项目资助

摘　　要：西藏拉萨地块西南部赛利普钾质-超钾质火山岩为一套含地幔包体的粗面安山岩,高K_2O,MgO、Cr、Ni含量,K_2O/ Na_2O比值和Mg~#,为地幔低度部分熔融的原始岩浆,或经橄榄石、单斜辉石或Fe-Ti氧化物分离结晶。岩石强烈富集大离子亲石元素和轻稀土元素、亏损高场强元素Nb、Ta、Ti,富集放射性成因Sr、Pb和Nd同位素,指示岩浆源区为富集地幔。采用LA-ICP-MS测定赛利普钾质-超钾质火山岩三件样品的18颗新生岩浆锆石U-Pb年龄为15.8～19.2Ma,其中钾质岩石样品SL0628中11个点的加权平均值为17.7±0.3Ma,与他人获得的^(40)Ar/^(39)Ar年龄一致。三件样品中新生岩浆锆石的ε^(Hf)(t)变化范围为-7.6～3.9,平均地壳模式年龄(t_(DMC)=0.86～1.59Ga)变化较大,除两个分析点显示亏损特征外,总体显示富集特征,表明岩石源于富集源区,但有少量亏损地幔物质加入。三件样品共获得49颗继承锆石的U-Pb年龄介于20～1907Ma,其Hf同位素组成(ε_(Hf)(t)=-25.9～5.3)和平均地壳模式年龄(t_(DMC)=0.79～4.08Ga)变化较大;其中的37颗年龄较小的继承锆石(20～110Ma)指示地幔源区可能受到四期明显的岩浆改造事件(62.2～64.0Ma,43.3～55.1Ma,29.5～37.7Ma和20.1～27.4Ma)和两个岩浆活动间歇期(70～90Ma和37.7～43.3Ma)。在拉萨地块首次发现29.5～37.7Ma的岩浆活动,并发现与林子宗火山岩同期的、Hf同位素富集的岩浆活动(62.2～64.0Ma,ε_(Hf)(t)=-21.2～3.0)。三件样品中49颗继承锆石的Hf同位素研究表明源区富集组分可能源自拉萨地块古老地壳基底和俯冲的印度大陆地壳。赛利普钾质-超钾质岩石形成可能是印度大陆地壳前缘撕裂和分段俯冲的结果。The Sailipu potassic-uhrapotassic volcanic rocks in western Lhasa block, Tibetan plateau, are trachyandesite with mantlederived xenoliths. The high contents of K2O, MgO, Cr, Ni, high K2O/Na2O ratio and Mg^# of the rocks suggest a primitive magma of mantle low-degree partial melting, or some fractionation of olivines, clinopyroxenes and Fe-Ti oxides. The rocks also show extremely enrichment of LILE, LREE, significant negative Nb, Ta and Ti anomalies, enrichment in radiogenic Sr, Pb and Nd, those of which suggest that an enriched mantle source region should have been responsible for the Sailipu magmatism. LA-ICP-MS has been carried out on dating 18 new zircons from Sailipu potassic-uhrapotassic rocks, and gives U-Pb ages between 15.8Ma and 19.2Ma. The weighted mean of 11 U- Pb analyses for potassic sample SL0628 is 17.7 ± 0.3Ma, in agreement with previous ^40Ar/^39Ar data. These zircons have εHf（t） values from -7.6 to 3.9, yielding a wide range of crustal Hf model ages （tDMC = 0. 86 - 1.59Ga）, generally showing enriched characteristics except two analyses. This indicates that they were derived from an enriched source with a little depleted mantle materials involved. 49 inherited zircons, obtained from three samples, have U-Pb ages from 20Ma to 1907Ma, wide ranges of Hf isotope （εHf（t） = -25.9- 5.3） and crustal Hf model ages （tDMC = 0. 79 -4.08Ga）. Among the above 49 zircons, 37 inherited ones, with younger U-Pb ages between 20 to 110Ma, suggest the mantle source may have experienced four stages of magmatism （62.2 -64.0Ma,43.3 -55.1Ma,29.5 -37.7Ma and 20.1 -27.4Ma） and two magmatic gaps （70 -90Ma and 37.7 -43.3Ma）. We also identify a significant, previously unknown stage of magmatism between 29.5Ma and 37.7Ma in Lhasa block, and find a magmatism stage with ages at the end of Linzizong volcanic rocks but enriched Hf isotope （62.2 -64.0Ma, εHf （t） = -21.2 - 3.0）. The above Hf analyses of 49 inherited zircons suggest enriched compositions in mantle source were derived from Lhasa

关 键 词：超钾质岩石 锆石U—Pb年龄 HF同位素 地幔源区 赛利普 青藏高原 

分 类 号：P597.3[天文地球—地球化学]