新疆西准噶尔达拉布特蛇绿岩地幔橄榄岩成因

摘要

The peridotites in Darbute ophiolite consist mainly of harzburgite and minor dunite, lherzolite and dunite and exhibit strong alteration. The major elements of clinopyroxene, orthopyroxene, olivine and spinel of harzburgite show the characteristics of both abyssal peridotite and SSZ peridotite, which is identical to the features of the trace elements of clinopyroxene obtained from the LA-ICP-MS in-situ analysis, while lherzolite is similar to abyssal peridotite. Oxygen fugacities recorded by olivine-spinel of harzburgite equilibrium are+0.96 log units below and-0.14 log units above the FMQ buffer, overlapping the range from MOR to SSZ harzburgite, or from back arc basin peridotite to island arc peridotite. The plot of Ga-Ti-Fe3+# of spinels indicates that the origin of dunite was related to the interaction between peridotite and island arc tholeiite, and the harzburgite was probably the residue after the interaction between peridotite and MOR melt and the interaction between peridotite and hydrous melt at the SSZ environment. The values of∑REE of harzburgite and dunite are lower than those of C1 chondrite, and display the enriched LREE and the U-type REE pattern,which implies the fluid/melt-rock reaction. Taking into account the above data, the authors hold that the Darbute ophiolite was probably formed in the back arc spreading ridge and influenced by subducted fluid/melt.%达拉布特蛇绿岩中地幔橄榄岩的主体为方辉橄榄岩，含少量纯橄岩和二辉橄榄岩，岩石遭受强烈蚀变。方辉橄榄岩单斜辉石、斜方辉石、橄榄石和尖晶石的主量元素特征均显示从深海地幔橄榄岩向SSZ地幔橄榄岩过渡的特征，与斜方辉石原位LA-ICP-MS微量元素特征一致，二辉橄榄岩具有深海地幔岩的性质。采用尖晶石-橄榄石平衡氧逸度计算方法，得出方辉橄榄岩的Δlog (fo2) FMQ在-0.14至+0.96log FMQ之间，具有MOR地幔橄榄岩向SSZ地幔橄榄岩过渡的特点或弧后盆地至岛弧过渡的特征。尖晶石Ga-Ti-Fe3+#图解显示纯橄岩成因可能和地幔橄榄岩与岛弧拉斑玄武岩的反应有关，而方辉橄榄岩可能为地幔橄榄岩与MOR熔体反应以及SSZ环境中含水熔体反应后的残余。纯橄岩和方辉橄榄岩∑REE都低于球粒陨石，且具有LREE富集的U型稀土元素配分模式，暗示了岩石和流体/熔体之间的相互作用。综合以上研究表明，达拉布特蛇绿岩形成于弧后扩张脊并受俯冲流体/熔体影响。