知识与财富的链接
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(∑REE=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)_i=0.7051～0.7056,5_(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 uhrabasic cumulus rocks, the basic unit includes cumulus gabbros, basalts. The ultrabasic rocks characterized by low Al_2O_3, TiO_2, high Mg~#(Mg~# =0.88～0.92), enriched in Cr and Ni, low total rare earth elements (∑REE = 14.82 × 10~(-6) ～ 27.75 × 10~(-6)), and a reverse U-shaped chondrite-normalized REE pattern. Gabbros and basalts have lower Mg~# (0.70 ～0.79 and 0.51 ～ 0.66, respectively). The basic rocks display a tholeiitic evolution trend. The basahs can also be classified into two sub-groups. The Group 1 exhibit flat REE pattern, low Mg~# (Mg~# =0.44～0.46) and low total REE (∑REE =52.29 × 10~(-6) ～ 60.26 × 10~(-6)), whereas the Group 2 exhibit a LREE-riched REE pattern, relatively higher Mg~#(Mg~# = 0.54 ～ 0.68) and high total REE (∑REE = 62.13 × 10~(-6) ～101.87 × 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)_i ratios for ultrabasic unit are from 0.7051 to 0.7056 and ε_(Nd)(t) from 2.8 to 4.1; the basic unit has a range of (~(87)Sr/~(86)Sr)_i from 0.7050 to 0.7056 and ε_(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 Group 2 basalts, and finally Group 1 basalts were generated. These geochemical characteristics indicate that the Jiyidu ophiolite is subduction-related that may be formed during the intra-oceanic subduction of the Jinshajiang ocean.
