青藏高原安多岛弧型蛇绿岩地球化学及成因

Geochemistry and genesis of the island-arc ophiolite in Anduo area, Tibetan plateau.

安多蛇绿岩位于西藏安多县城北侧、班公错-怒江缝合带中段。该蛇绿岩块呈近东西向展布,长约25km,宽约5km,主要由低钾拉斑玄武岩和辉长岩组成。高精度ICP-MS分析结果表明,玄武岩和辉长岩稀土总量较低,均具有亏损型稀土配分型式,∑REE:29×10^(-6)～44×10^(-6),∑LREE/∑HREE=0.90～1.06,(La/Yb)_N=0.29～0.41,(Ce/Yb)_N=0.42～0.60。表明其源于N-MORB型亏损地幔源区。然而,相对于典型的大洋中脊玄武岩(N-MORB)而言,其Nb和Ta,尤其是Nh含量明显偏低(Nb=0.6×10^(-6)～3.13×10^(-6),平均1.19×10^(-6);Ta=0.072×10^(-6)～0.253×10^(-6),平均0.105×10^(-6)),在N-MORB标准化痕量元素配分图上具显著的Nb谷。表明安多玄武岩+辉长岩组合既非典型的洋中脊成因,又与岛弧型火山岩有一定区别,它们很可能形成于边缘海(弧后)盆地环境,由于消减带之上的地幔对流导致新洋壳的产生而形成,是特提斯大洋岩石圈在俯冲过程中引发弧后次级扩张的产物。

There are two important ophiolite belts in Tibet. One is the well-known Yarlung Zangbo River ophiolite belt, and the other is called the Bangongcuo-Nujiang River ophiolite belt, located about 200similar to300 km north of the former one and is basically parallel to it. Anduo island-are ophiolite is located at the middle part of Bangongcuo-Nujiang River suture zone, distributed to the north of Anduo County. The ophiolite which stretches from west to east more than 25 km long and about 5 km wide consists of low-potassium tholeiite and gabbro. Based on the intensive field investigation and ICP-MS systematic studies of petrology and geochemistry, this paper discusses about the ophiolite-type and its source region features. The ICP-MS analyses result indicates that both of the low-potassium tholeiite and gabbro of Anduo ophiolite exhibit low Sigma REE and light rare earth element depletion, Sigma REE=29x10(-6)similar to44x10(-6), Sigma LREE/Sigma HREE=0.90similar to1.06, (La/Yb)(N)=0.29similar to0.41, (Ce/Yb)(N)=0.42similar to0.60, quite similar to those of the typical N-MORB-type basalt. And their trace element ratios of Th/Yb, Ta/Yb, Ti/Y, Ti/Zr, Nb/Zr/Y, Ti/Zr/Y show that they were originated from a depleted asthenosphere mantle with distinctive geochemical features of N-MORB-type basalt. On the other hand, this group of basalt and gabbro exhibit Nb, Ta (especially for the Nb) depletion in the curves of N-MORB normalized trace element distribution patterns (for the basalt and gabbro collected from the studies area, Nb=0.6x10(-6)similar to3.13x10(-6), mean value is 1.19x10(-6); Ta=0.072x10(-6)similar to0.253x10(-6), mean value is 0.105x10(-6)), which is quite different both from the typical mature mid-ocean ridge basalt and the typical island-arc volcanic rock associations. Therefore, this group of rocks not only shows N-MORB-type basalt features but also exhibits island-arc basalt characteristics. And the Anduo ophiolite should belong to island-arc ophiolite. It was formed in the marginal basin (back-arc basin) tectonic setting, resulted from the back-arc mantle convection and back-arc oceanic crust formation which was a spreading center near the subduction zone.