Early Cretaceous Dongbo ophiolite in the western part of the Yarlung Zangbo Suture Zone, Tibet: Constraints on multi-stages from ultra-slow spreading influenced by a mantle plume to intra-oceanic subduction

Abstract

雅鲁藏布江缝合带西段出露东波和普兰等多个大型的超镁铁岩体，其成因还存在争议。为此我们近年来对东波蛇绿岩开展了地表填图并实施了一口1002m的科学钻探DSD-1。研究显示具有厚幔极薄壳特征的东波蛇绿岩记录了多期岩浆事件：（1）~137Ma的OIB型玄武岩和138~144Ma的OIB型辉绿岩和辉长岩，以及普遍出露熔融程度达30%以上的亏损型方辉橄榄岩，反映了早白垩世初期古洋盆受到地幔柱活动影响；（2）129Ma的薄壳均质辉长岩（I型）、128Ma的辉长岩脉（Ⅱ型）和科钻岩芯中厚18m辉绿岩（Ⅲ型）的地球化学成分均与西南印度洋洋脊玄武岩类似，它们是在慢速-超慢速扩张脊附近大洋核杂岩侵位过程中形成的；（3）地表辉绿岩脉（Ⅳ型）的SIMS和SHRIMP锆石U-Pb年龄为121~123Ma，全岩地球化学具有异常低的REE、HFSE含量及明显的Th、Nb、Zr、Hf负异常，类似于Albanide-Hellenide造山带蛇绿岩中的无壳源物质混染的中钛玄武岩，指示辉绿岩脉形成于洋内初始俯冲环境。总的来说，东波蛇绿岩记录了早白垩世地幔柱影响的超慢速扩张和初始洋内俯冲过程。;The genesis of several large ultramafic massifs outcropped in the western part of the Yarlung Zangbo Suture Zone, such as the Dongbo and Purang massifs, are still controversial. Geological mapping and an 1,002m-length scientific drilling (DSD-1) are carried out on the Dongbo ophiolite in recent years to solve this problem. Our study indicates that the Dongbo ophiolite is characterized by a thick mantle and very thin crust structure which recorded multi-stage magmatic events. The evolution of the major events of this massif can be diveided into three stages as follows: (1) The ~137Ma OIB-like basalt, 138~144Ma OIB-like dolerite and gabbro dikes, and their wall-rock depleted peridotite with a melting degree of more than 30% reflect the influence of mantle plume activity on the ancient ocean basin in the Early Cretaceous; (2) The 129Ma isotropic gabbro (type I), 128Ma gabbro dike (type II) and 18m thick dolerite (type Ⅲ) in the DSD-1drill core have similar geochemical compositions to the Southwest Indian ocean ridge basalts, which were produced during the emplacement of the ocean core complex near the slow-ultraslow spreading ridge; (3) The dolerite dikes (type Ⅳ) dated by SIMS and SHRIMP zircon U-Pb with ages of 121~123Ma have abnormally low contents of rare earth elements and high field-strength elements, and displayed obvious negative Th, Nb, Zr and Hf anomalies in their whole rock geochemical compositions. These features are similar to those of medium-titanium basalts without the addition of subduction components in the Albanide-Hellenide ophiolites, indicating that these dolerite dikes were formed in an initial intra-ocean subduction environment. In a word, the Dongbo ophiolite records an ultra-slow spreading influenced by a mantle plume and initial intra-ocean subduction process during the Early Cretaceous.