羌塘盆地两个湖地区闪长片麻岩锆石LA-ICP-MS U-Pb同位素定年及地球化学特征研究  

LA-ICP-MS U-Pb Zircon Dating of Dioritic Gneiss and Its Geochemical Characteristics in the Lianggehu Region,Qiangtang Basin

作　　者：李忠雄[1] 谢尚克[1] 何江林[1] 熊松 吴涛 邓奇[3] 

机构地区：[1]中国地质调查局成都地质调查中心,成都610081 [2]贵州省地质调查院,贵阳550004 [3]中国地质科学院,北京100037

出　　处：《地质学报》2014年第1期15-24,共10页Acta Geologica Sinica

基　　金：中国地质调查局青藏油气专项羌塘盆地及其外围重点盆地油气远景调查项目(资[2011]02-03-01);中国地质调查局青藏矿产专项羌塘盆地油气勘探目标优选与科探井井位预研究项目(资[2008]青藏15-01);国家自然科学基金项目(编号40972087)资助的成果

摘　　要：首次对羌塘盆地中央隆起带两个湖闪长片麻岩体进行系统的年代学与地球化学研究,以便确定其形成时代及成因。研究表明,两个湖闪长片麻岩体由闪长岩、辉长闪长岩组成,锆石呈长柱状自型—半自型晶,普遍发育有很窄的振荡生长环带,具有岩浆锆石的特征,三件闪长片麻岩中锆石的LA-ICP-MS U-Pb定年结果分别为220.48±0.49Ma、217.9±0.51Ma和219.98±0.49Ma,平均值为219.45±0.50Ma,表明岩体的形成时代为晚三叠世卡尼期。岩石的SiO2、MgO和Al2O3含量分别介于55.17%~58.61%、1.71%~2.74%和20.18%~21.43%之间,且以高Mg#值(0.38~0.53)和富Na(Na2O/K2O=1.17~7.38)为特征;强烈富集轻稀土元素和大离子亲石元素、明显亏损高场强元素,铕无异常或弱负异常,(87Sr/86Sr)t值和εNd(t)值分别变化于0.70868~0.71024和-3.4^-2.8之间,结合铅同位素特征,认为该岩体的岩浆源区应为遭受了强烈的陆壳物质改造的岩石圈地幔。Geochronology and geochemistry of dioritic gneiss in the Lianggehu region, Qiangtang basin was examined for the first time so as to determine its age and genesis. Results show that the dioritie gneiss is derived from a suite of diorite and gabbro-diorite. Zircons occur as long columnar euhedral and hypidiomorphic crystals with well-developed tight oscillating zonal structure in CL images, indicating magmatic origin. LA-ICP-MS U-Pb zircon dating of three dioritic gneiss samples yields 220.48±0.49 Ma, 217.9±0. 51 Ma and 219. 98±0. 49 Ma, respectively, with an average value of 219. 45±0. 50 Ma, indicating that dioritic gneiss formed in Triassic Carnian. The SiO2, MgO and Al2O3 contents of dioritic gneiss range from 55. 17% to 58. 61%, 1. 71% to 2. 74% and 20. 18% to 21. 43% respectively. The dioritic gneiss are characterized by high Mg^# value （0.38±0.53）, Na enrichment （Na20/K20= 1.17±7. 38）, strong LREE and LILE enrichment, obvious negative anomalies of HFSE, and no Eu anomaly. The isotopic （87Sr/a6Sr）, and Na（t） values of dioritic gneiss vary in range of 0. 70868 to 0. 71024 and--3.4 to -2.8 respectively. These characteristics combined with lead isotopic parameter indicate that the magmatic source of dioritic gneiss is derived from lithosphere mantle contaminated strongly by the continental crust.

关 键 词：羌塘盆地 闪长片麻岩 锆石U—Pb定年 地球化学 岩石成因 晚三叠世 

分 类 号：P597.3[天文地球—地球化学]