摘要：
红脊山构造混杂岩带位于羌塘地块的中西部,为古特提斯洋在该地区俯冲、碰撞形成的高压变质带,是羌塘中部低温高压变质带的重要组成部分.本文对红脊山混杂岩带内蓝片岩进行了系统的地球化学、锆石U-Pb定年及Sr-Nd同位素研究.结果显示,红脊山地区蓝片岩的原岩为碱性、亚碱性玄武岩,其中碱性玄武岩具有高TiO2 (2.86％～4.84％),属高Ti玄武岩,富集轻稀土元素[(La/Yb) N=11.42～20.05]和高场强元素,地球化学特征类似于OIB;而亚碱性玄武岩,具有低TiO2 (1.74％～1.81％),稀土总量较低(67.27×10 6～68.59×10-6)和轻稀土略微富集的特征[(La/Yb) N=2.49～2.81],与典型的E-MORB特征一致.Sr、Nd同位素组成:εNd(t)=-0.1～3.9,(87Sr/86 Sr)i=0.704812～0.708365,表明该地区基性岩浆来自亏损型地幔.锆石的Th/U比值为0.33～1.33,并具有典型岩浆振荡环带结构;获得两组206 Pb /238U年龄数据:其年龄加权平均值分别为288.3±1.9 Ma(n=15,MSWD=0.39)和304.2±2.3 Ma(n=14,MSWD=0.54),因此该两组年龄应代表蓝片岩原岩形成年龄,红脊山蓝片岩原岩形成时间相当于晚石炭世-早二叠世.结合区域地质事实和前人研究成果,红脊山基性原岩形成于大陆裂谷环境,其成因可能与地幔柱有关,与南羌塘二叠纪基性岩墙具有相同的构造背景及动力学机制.蓝片岩基性原岩年龄在红脊山乃至整个羌塘地区都鲜有报道,红脊山地区晚石炭世-早二叠世岩浆活动的厘定为精细刻画羌塘地区古特提斯构造演化过程提供了重要依据.%Hongjishan mélange belt is an important part of the late Triassic high-pressure metamorphic belt in central Qiangtang,which was fromed due to the collision between Paleo-tethys and Qiangtang Block.In this paper we present the results of geochemical and isotopic geochronological investigations on Hongjishan blueschists from this belt.Geochemical analyses show that the two rock series,alkaline and tholeiitic basalts,have grossly similar bulk compositions,but they can be subdivided into two groups based on their TiO2contents.These metamafic rocks are alkaline basalts and have high TiO2 (2.86％ ～ 4.84％),are enriched in light rare earth elements (LREEs) with (La/Yb)N values of 11.42 ～ 20.05 and high field strength elements (HFSEs),REE distribution patterns and trace element abundances are similar to ocean island basalt (OIB).The low TiO2 (1.74％ ～ 1.81％) rocks are tholeiitic and have low rare earth element abundances(67.27 × 10-6～ 68.59 × 10-6) and display light REE (LREE) slightly enriched patterns with (La/Yb)N values of 2.49 ～ 2.81,are more akin to enriched mid-ocean ridge basalts (E-MORB).Sr-Nd isotopic compositions of the protolith are εNd(t)=-0.1 ～ 3.9,(87Sr/86Sr)i=0.704812 ～ 0.708365,suggests that the basaltic protoliths of blueschists were derived from a depleted mantle source.LA-ICP-MS Zircon U-Pb dating of blueschists from Hongjishan mélange belt in order to determine the age and to discuss the geotectonic background and tectonic magmatic events that Hongjishan complex has experienced.Results show zircons from analyzed sample present available Th/U ratio between 0.33 ～1.33,and these zircons show oscillatory and sector-zoning,typical of magmatic origin.Zircons from the blueschist show U-Pb ages in the range 283 ～ 312 Ma,that there are two well defined group ages,i.e.weighted average 206Pb /238U ages are 288.3 ±-1.9 Ma (n=15,MSWD=0.39) and 304.2 ± 2.3 Ma (n=14,MSWD=0.54),thus the age probably represents the formation age of the protolith of blueschis.Therefore the formation time of the protolith of Hongjishan blueschists are equal to the late Carboniferousearly Permian.Coupled with the regional geological facts and results of previous studies,it can be concluded that basaltic protoliths formed in the rift valley setting,with a plume-related source,have the same tectonic setting and dynamic mechanism as the Permian mafic dyke swarms in southern Qiangtang.This paper is more inclined to believe the Hongjishan simultaneously appeared magmatic activity association with the South Qiangtang mafic dyke incidents.The blueschist mafic protolith zircon U-Pb ages are rarely reported not only in the Hongjishan but also in the whole Qiangtang region,therefore determination of the magmatic activity time provides crucial evidence for describing the Paleo-tethys tectonic evolution process precisely in the Qiangtang area.
