那拉提山北缘寒武纪玄武岩的元素地球化学特征及构造意义  

Elemental geochemistry and tectonic significance of Cambrian basahs from the northern side of the Nalati Mountain.

作　　者：钱青[1] 徐守礼[2] 何国琦[3] Reiner KLEMD 熊贤明[5] 龙灵利[1] 高俊[1] 

机构地区：[1]中国科学院地质与地球物理研究所矿产资源重点实验室,北京100029 [2]江苏石油勘探局,扬州225009 [3]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871 [4]维尔茨堡大学矿物研究所,德国维尔茨堡97074 [5]贵州大学资源与环境工程学院,贵阳550003

出　　处：《岩石学报》2007年第7期1708-1720,共13页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2007CB411302;2001CB409803);国家自然科学基金项目(40472047;40672153)资助.

摘　　要：我国西南天山地区那拉提北缘断裂北侧出露寒武纪(516.3±7.4Ma)橄榄拉斑玄武岩,其 Al_2O_3(12.71%～14.14%)、P_2O_5(0.09%～0.21%)含量较低,TiO_2(1.39%～2.35%)、MgO(5.35%～8.14%)含量中等,全 Fe_2O_3(11.74%～16.03%)含量较高,球粒陨石标准化稀土分布型式平坦,(La/Sm)_N 和(Y/Nb)_N 比值接近1,为过渡型洋脊玄武岩(T-MORB);并且 Th/Ta 比值(0.8～1.5)低,高场强元素(Nb,Ta,Zr,Hf,P)没有明显亏损,缺乏陆壳混染或俯冲带流体活动的印迹。部分 T-MORB 的 Fe、Ti 含量较高(FeO^T>12%,TiO_2>2%,FeO^T/MgO>1.75),Mg~#值(0.40—0.49)和 Cr(53×10^(-6)～110×10^(-6))含量较低,属于 Fe-Ti 玄武岩(ferrobasalt),应为岩浆依 Fenner 趋势发生较高程度结晶分离演化的产物,很可能形成于洋中脊的扩展型裂谷环境。我们认为,那拉提北缘寒武纪 T-MORB 和 Fe-Ti 玄武岩可能与境外吉尔吉斯斯坦境内北天山南缘的早古生代 Terskey 蛇绿岩带相连,为 Terskey 蛇绿岩的残片;寒武纪我国境内发育 Terskey 洋。那拉提北缘断裂可能与尼古拉耶夫线相连,代表西南天山地区 Terskey 洋闭合形成的一条早古生代缝合带。This paper reports the major and trace elemental compositions of Cambrian （516.3±7.4Ma） tholeiitic basalts from the northern side of the North Nalati fault （NNF） in the Chinese southwestern Tianshan Mountains. The basalts are typical transitional （T-） type mid-ocean ridge basalts （ MORB）, characterized with low Al2O3 （ 12.71% - 14.14% ） and P205 （0.09% - 0. 21% ）, moderate TiO2（1.39% -2.35% ） and MgO （5.35% -8.14% ） and high total Fe （11.74% - 16.03% as Fe2O3） contents, as well as flat chondrite-normalized rare earth element （REE） patterns. （La/Sm）N and （Y/Nb）N ratios are close to unit, also substantiating a T-MORB affinity. Crustal contamination and subduction fluid activity are not evidenced by the low Th/Ta ratios （0.8 - 1.5 ） and lack of depletion of high field strength elements （ Nb, Ta, Zr, Hf and P）. Some T-MORB samples have relatively enriched iron and titanium and lower Mg^# （0. 40 -0. 49） and Cr （53× 10^-6 - 110 × 10^-6） values, and can be classified as Fe-Ti basalts （ferrobasalts） , which are defined as having FeO^T 〉 12%, TiO2 〉2% and FeO^T/MgO 〉 1.75. The Fe-Ti basalts were most likely produced from the MORB-type magma through relatively high degree of Fenner trend differentiation at a propagating spreading centre of mid-ocean ridge. The T-MORB and Fe-Ti basalts are believed to be counterparts of the Terskey ophiolite belt along the southern margin of Kyrgyzstan Northern Tianshan, and consequently fragments of the Terskey ophiolites. We propose that （ 1 ） the Terskey ocean developed in the Chinese southwestern Tianshan region in Cambrian; （2） the NNF is the eastern extension of the Nikolaev Line, and represents an early Paleozoic suture zone formed by closure of the Terskey ocean.

关 键 词：西南天山 寒武纪 Fe-Ti玄武岩 Terskey洋 那拉提北缘断裂 尼古拉耶夫线 

分 类 号：P588.145[天文地球—岩石学]