Longlin-Ruili subduction-accretionary complex belt in the southeastern Gaoligong orogen, and its relationship with the evolution of the Meso-Tethyan Ocean

Abstract

龙陵-瑞丽增生杂岩带的构造属性对确定高黎贡构造带作为腾冲-保山地块的边界及班怒带南向延伸十分关键。本文在地质填图的基础上，通过岩石学、矿物学、岩石地球化学、同位素年代学和同位素示踪等方法，查明混杂岩带主要由蛇纹石化橄榄岩、玄武岩/辉长岩、硅质岩、碳酸盐岩、含放射虫层状硅质岩和锰结核的深海沉积岩等岩块呈规模不等的团块状、透镜状分布于浊积岩基质中，具有典型的俯冲增生杂岩岩石组合特征。蛇纹石化橄榄岩原岩由方辉橄榄岩和少量纯橄岩组成，具有轻稀土轻微富集、Mg^#值高（88~92），铬尖晶石Cr^#、Mg^#值分别在60~70和20~26区间，橄榄石Fo值为90~95。在铬尖晶石Cr^#-Mg^#指数图解和橄榄石Mg^#-铬尖晶石Cr^#图解上样品都落在弧前SSZ型橄榄岩区，说明研究区内的地幔橄榄岩是经历了高度部分熔融（>30%）和熔体抽离后的残留相，形成于弧前构造背景。玄武岩和辉长岩地球化学特征类似，具有富钛（TiO₂>2.26%）、高Mg^#值特征（49~57），其稀土配分模式和微量元素蛛网图、构造环境判别图解、ε_Nd（t）值（+2.2~+5.1）、以及岩石中含少量富钛角闪石和黑云母等，表明它们属于洋岛/海山型基性岩类，其岩浆来源于富集地幔。橄榄岩中脉状辉石岩锆石U-Pb年龄为183~185Ma，浊积岩中杂砂岩最小碎屑锆石U-Pb年龄为212~241Ma。此外，增生杂岩带中存在含早白垩世流纹岩/凝灰岩夹层的弧前/弧间沉积，并被晚白垩世陆相沉积岩不整合覆盖，这些特点说明俯冲增生杂岩带形成于晚三叠-早白垩世。研究区内混杂岩带的构造属性和时代与班怒带（拉萨-南羌塘地块间）和缅甸境内密支那蛇绿混杂岩带完全一致，是中特提斯洋演化的产物。中生代，高黎贡东南缘混杂岩带北连班怒带、南东连密支那蛇绿混杂岩带，随后，在新生代印度板块向北俯冲过程中，密支那蛇绿混杂岩带被Sagaing断裂带分支——八莫断裂带右行走滑位移到现今位置。;The tectonic setting of the Longlin-Ruili mélange belt is a key to identify the Gaoligong tectonic zone as the suture zone between Tengchong and Baoshan blocks, and the southward extension of the Bangonghu-Nujiang suture. Here we present new geological, petrochemical, mineral chemical, geochronological and isotopic data for the Longlin-Ruili mélange belt in the southeastern Gaoligong orogen. The slices inside this mélange belt include serpentinization peridotite, basalt/gabbro, chert,carbonite and pelagic sedimentary rock with radiolarian bearing layered siliceous rocks and manganese nodules distributed in the turbidites, showing typical rock assemblage of a subduction-accretionary complex. The peridotite consists of the harzburgite and a small amount of dunite. It is characterized by the slightly enriched LREE, high values of Mg^# (88~92), high values of Fo (90~95) for olivine, and Cr^# and Mg^# values for chrome-spinel are 60~70 and 20~26, respectively. The Mg^# (spinel) vs. Cr^# (spinel) and Mg^# (olivine) vs. Cr^# (spinel) diagrams show the peridotite is the residual phases after high degree partial melting (>30%) and extraction of melt in supra-subduction environment. The basalt and gabbro are oceanic island or seamount type whose mafic magmas are derived from a metasomatized mantle, with features of the enriched TiO₂(>2.26%) and high values of Mg^# (49~57), and their REE patterns and trace element spider lines are consistent with that of OIB, with low ε_Nd(t) (+2.2 to +5.1). Furthermore, a small amount of Ti-enriched biotite and hornblende occurred in mafic rocks. The U-Pb dating of the zircons from pyroxenites within harzburgite yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 183~185Ma. The U-Pb dating of the detrital zircons from greywackes in turbidite yielded the youngest group ²⁰⁶Pb/²³⁸U ages of 212~241Ma. There are forearc and inter-arc sedimentary rocks with Early Cretaceous rhyolite or tuff in mélange belt. The Upper Cretaceous continental sedimentary rocks unconformably onlapped on the mélange zone. All these features suggest that this mélange belt was formed in a supra-subduction zone during Late Triassic to Early Cretaceous. The tectonic setting and age of the Longlin-Ruili mélange belt are consistent with that of the Bangonghu-Nujiang and Myitkyina ophiolitic mélange belts which are related with the subduction of the Meso-Tethyan Ocean. As a result, the Longlin-Ruili mélange belt should be connected with Bangonghu-Nujiang suture zone in the north, while the Myitkyina ophiolitic mélange belts in the southeast during Mesozoic. After that, the Myitkyina ophiolitic mélange belt was displaced to present position by Sagaing dextral slip-shearing in Cenozoic.