滇西吉义独蛇绿混杂岩的岩石地球化学特征、成因和构造环境探讨  

Petrological and geochemical characteristics of the Jiyidu ophiolitic melange,West Yunnan:Constraints for its tectonic implications.

作　　者：何琦[1] 肖龙[2] 魏启荣[3] 倪平泽[1] 

机构地区：[1]中国地质大学研究生院,武汉430074 [2]中国地质大学地球科学学院,武汉430074 [3]中国地质大学资源学院,武汉430074

出　　处：《岩石学报》2009年第12期3229-3240,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40572031)资助成果

摘　　要：滇西吉义独蛇绿混杂岩位于金沙江缝合带的南端,岩石组合出露较齐全,包括堆晶橄榄岩、堆晶辉石岩、堆晶辉长岩以及玄武岩等,它们呈构造岩片的形式产出并与外来岩块组成蛇绿混杂岩。堆晶橄榄岩和辉石岩具低Al_2O_3,低TiO_2,而高Mg~#值(Mg~#=0.88～0.92),富集Cr和Ni,稀土总量偏低(∑REE=14.82×10^(-6)～27.75×10^(-6)),倒"U"型的稀土元素分布特征。堆晶辉长岩和玄武岩的Mg~#值较低,分别为0.70～0.79和0.51～0.66,具拉斑系列的演化趋势。玄武岩可以细分为2组:第一组玄武岩以平坦型稀土配分模式,低Mg~#(Mg~#=0.44～0.46),低稀土总量(∑REE=52.29×10^(-6)～60.26×10^(-6))为特征;第二组玄武岩则为LREE弱富集型的稀土配分模式,其Mg~#较高(Mg~#=0.54～0.68),稀土总量也较高(ΣREE=62.13×10^(-6)～101.87×10^(-6))。在原始地幔标准化的微量元素配分图解中,两组玄武岩均相对富集大离子亲石元素而亏损Nb、Ta和Ti,与岛弧岩浆岩类似,明显不同于N-MORB。岩石的Sr-Nd同位素组成较为均一和稳定,堆晶橄榄岩和辉石岩的(^(87)Sr/^(86)Sr);=0.7051～0.7056,ε_(Nd)(t)=2.8～4.1,玄武岩的(^(87)Sr/^(86)Sr)_i=0.7050～0.7056,ε_(Nd)(t)=5.1～5.8,且显示出原始地幔的同位素组成特征,暗示这些岩石为同源岩浆分异演化而成的岩浆产物。岩浆演化的主要方式为分离结晶作用,受地壳混染不明显。岩浆结晶形成岩石的顺序为:堆晶橄榄岩→堆晶辉石岩→堆晶辉长岩→玄武岩2组→玄武岩1组。岩石地球化学特征表明,吉义独蛇绿岩的形成与俯冲作用有关,且形成于金沙江洋内俯冲的消减环境。The Jiyidu ophiolitic melange in the southern end of the Jinshajiang suture zone, west Yunnan Province, consist mainly of basic and ultrabasic rocks and limestones showing characteristics of a typical variably disrupted ophiolitic suite. The peridotites and pyroxenites are ultrabasic cumulus rocks, the basic unit includes cumulus gabbros, basalts. The ultrabasic rocks characterized by low Al(2)O(3), TiO(2), high Mg(#)( Mg(#) =0.88 similar to 0. 92), enriched in Cr and Ni, low total rare earth elements (Sigma REE =14. 82 x 10(-6) similar to 27.75 x 10(-6)), and a reverse U-shaped chondrite-normalized REE pattern. Gabbros and basalts have lower Mg(#) (0. 70 similar to 0. 79 and 0. 51 similar to 0. 66, respectively). The basic rocks display a tholeiitic evolution trend. The basalts can also be classified into two sub-groups. The Group 1 exhibit flat REE pattern, low Mg(#) ( Mg(#) = 0.44 similar to 0. 46) and low total REE (Sigma REE = 52. 29 x 10(-6) similar to 60. 26 x 10(-6)), whereas the Group 2 exhibit a LREE-riched REE pattern, relatively higher Mg(#)( Mg(#) = 0. 54 -0. 68) and high total REE (Sigma REE = 62. 13 x 10(-6) similar to 101. 87 x 10(-6)). In the primitive mantle normalized trace elements patterns, all basalts from the ophiolites are enriched in light ion lithophile elements and depleted in Nb, Ta and Ti, which similar to island arc rocks and are distincted from the N-MORB. Their Sr-Nd isotope composition are relatively homogeneous, in which ((87)Sr/(86)Sr); ratios for ultrabasic unit are from 0.7051 to 0. 7056 and epsilon(Nd) ( t) from 2. 8 to 4. 1; the basic unit has a range of ((87)Sr/(86)Sr), from 0. 7050 to 0. 7056 and epsilon(Nd) (t) from 5. 1 to 5. 8. The Sr-Nd isotopic features suggest that all the rocks may be derived from a common parental magma and the mantle source close to primitive mantle reservoir. The fractional crystallization is the dominated magma process and the fractionation order is from peridotites to pyroxenites and then gabbros to Croup 2 basalts, and finally Group 1 basalts

关 键 词：西吉 蛇绿混杂岩 岩石地球化学特征 成因 构造环境 ophiolitic MELANGE geochemical characteristics 玄武岩 堆晶 REE fractional crystallization 稀土总量 稀土配分模式 辉石岩 橄榄岩 rare earth elements Jinshajiang suture zone 岩浆分异演化 basic unit 辉长岩 

分 类 号：P58[天文地球—岩石学]