东准噶尔石炭系火山岩的形成机制及其对准噶尔洋盆闭合时限的制约  

Genesis of Carboniferous volcanic rocks in the eastern Junggar:constraints on the closure of the Junggar Ocean.

作　　者：龙晓平 孙敏[2] 袁超[1] 肖文交[3] 陈汉林[4] 赵永久[1] 蔡克大[1] 李继亮[3] 

机构地区：[1]中国科学院广州地球化学研究所,广州510640 [2]香港大学地球科学系 [3]中国科学院地质与地球物理研究所岩石圈构造演化重点实验室,北京100029 [4]浙江大学地球科学系,杭州310027

出　　处：《岩石学报》2006年第1期31-40,共10页Acta Petrologica Sinica

基　　金：国家973项目(2001CB409801);香港RGC项目(HKU 7040/04P);自然科学基金(40572043);中科院领域前沿项目(GIGCX-03-01)资助成果。

摘　　要：扎河坝石炭系火山岩出露于东准噶尔扎河坝蛇绿岩带北侧的上石炭统巴塔玛依内山组地层内,主要为玄武岩及玄武安山岩,Mg~#介于64～73,中等的TiO_2(＜0．85%)和Al_2O_3(13．75%～16．88%)含量,相对低的CaO(4．51%～7．28%)和FeO^T(5．82%～7．71%)含量,显示初始岩浆特征。样品富Na(Na_2O/K_2O＞1),具有较高的全碱含量(K_2O+Na_2O)=4．4%～7．1%)。样品富集大离子亲石元素(LILE),相对亏损高场强元素(HFSE),具有较低的Nb含量(0．8×10^(-6)～2．6×10^(-6))和明显的Nb-Ta负异常,显示典型的消减带岩浆特征。样品显示微弱的轻稀土富集(La/Yb)_N=1．0～2．1,无明显的重稀土分馏(Gd/Yb)_N=1．1～1．3,无明显的铕异常(δEu=0．94～1．08)及铈异常(δCe=1．00～1．06)。此外,样品具有较高的(^(87)Sr/^(86)Sr)_i(0．705282～0．705420)和ε_(Nd)(t)值(+6．59～+7．58)。上述特征与样品低的Th及Pb含量(Th＜0．55×10^(-6),Pb＜3．52×10^(-6))和较高的Ce/Pb比值(4～79)可以排除陆壳混染,说明其形成于消减带环境。样品较高的Ce/Th(18～32),Ce/Pb,Ba/Rb(21～76)和Ba/Th(＞179)比值,ε_(Nd)(t)值接近亏损地幔,并缺乏明显的负铈异常,说明源区没有明显的消减沉积物加入。样品的微量元素特征及Sr-Nd同位素组成说明其形成于热液交代的洋壳俯冲脱水使上覆地幔楔部分熔融。而样品相对贫Pb的特征,可能与源于高温角闪岩相俯冲板片特殊的贫Pb流体有关。这套火山岩说明东准噶尔晚石炭世仍存在洋内的消减俯冲环境,准噶尔大洋的闭合应当发生在晚石炭世之后。Zhaheba Carboniferous volcanic rocks mainly composed of basalts and basaltic andesites occur within Upper Carboniferous Batamayi Group, north of Zhaheba ophiolite in the eastern Junggar. These rocks have relatively high Mg# （64 - 73 ） values, moderate TiO2 （ 〈0.85% ） , Al2O3 （ 13.75% - 16.88% ）, but low CaO （4.51% - 7.28% ） and FeO^T （5.82% - 7.71% ） contents, showing characteristics of primitive magmas. These rocks have relatively high Na2O/K2O ratios （ 〉 1 ） and alkali contents （ Na2O ＋ K2O） = 4.3% - 7.1% ）. They are enriched in LILEs but relatively depleted in HFSEs, with strong negative Nb-Ta anomalies, showing typical characteristics of subduction zone. These samples are slightly enriched in LREEs （ （La/Yb） N = 1.0 - 2.1 ） and possess weak Eu and Ce anomalies （0.94 - 1.08, 1.00 - 1.06, respectively）, without significant fractionations of HREEs （ GdN/YbN = 1.1 - 1.3 ）. Furthermore, they have high radioactive Sr （ ISr = 0. 705282 - 0. 705420） and low radioactive Nd （ εNd （t） = ＋ 6.59 - ＋ 7.58 ）. These characteristics, along with their low contents of Th （ 〈0.55 × 10^ -6 ） and Pb （ 〈 3.52 × 10^ -6 ） and high ratio of Ce/Pb （4 - 79）, preclude the possibility for involvement of continental crust, suggesting that they were most likely produced in a subductionrelated environment. Their high Ce/Th （ 18 - 32） , Ce/Pb, Ba/Rb （21 - 76） , Ba/Th （ 〉 179）, εNd （t） values and lack of obvious Ce anomalies indicate that there was no significant involvement of subducted sediment during their genesis. Moreover, their trace elements and Nd-Sr isotope compositions are consistent with their origination from a mantle wedge metasomatised by slab-derived fluid. Different from typical arc-related magmas, however, most Zhaheba samples show negative Pb anomalies and relatively high Ce/Pb ratios, which can probably be ascribed to Pb-poor fluid derived from an amphibolite-facies slab in high temperatures. These characteristics indica

关 键 词：东准噶尔 中亚造山带 俯冲消减 扎河坝 火山岩 地球化学 同位素 

分 类 号：P597.3[天文地球—地球化学]