羌塘中西部红脊山地区蓝片岩Sr-Nd同位素与锆石U-Pb年龄特征及其构造意义

发布时间：2018-09-06 12:23

【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,(~(87)Sr/~(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/238 U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。
[Abstract]:The Hongjishan tectonic melange belt is located in the central and western part of the Qiangtang block. It is a high-pressure metamorphic belt formed by the subduction and collision of the paleoTethys ocean and is an important part of the low-temperature and high-pressure metamorphic belt in the central Qiangtang area. In this paper, systematic geochemistry, zircon U-Pb dating and Sr-Nd isotopic studies of blueschist in the Hongjishan melange belt have been carried out. The results show that the original rocks of blueschist in Hongjishan area are alkaline and subalkaline basalts, among which alkaline basalts have high TiO2 (2.860.84%) and belong to high Ti basalts, which are rich in light rare earth elements [(La/Yb) 11.42 ~ 20.05] and high field strength elements. The geochemical characteristics are similar to those of OIB; and subalkaline basalts, with low TiO2 (1.741.81%), low rare earth contents (67.27 脳 10-6, 68.59 脳 10-6) and slight enrichment of light rare earths [(La/Yb) Nu 2.492.81], which are consistent with typical E-MORB characteristics. The composition of the isotopic composition of .SrND is 蔚 Nd (t) Nd (t) -0.1 ~ (3.9), (~ (87) Sr/~ (86) Sr) ihe 0.7048120.708365), indicating that the basic magma in this area is derived from the depleted mantle. The Th/U ratio of zircon is 0.33 / 1.33 and has a typical magmatic oscillatory ring structure, and two sets of age data of 206 Pb/238 U are obtained: the weighted average age of zircon is 288.3 卤1.9 Ma and 304.2 卤2.3 Ma, respectively. Therefore, the age of the two groups should represent the age of the blueschist protolith. The formation time of Hongji mountain blue schist is the same as that of late Carboniferous and early Permian. Combined with the regional geological facts and previous research results, the red ridge basic-proterite was formed in the continental rift environment, its genesis may be related to the mantle plume, and it has the same tectonic setting and dynamic mechanism as the Permian basic rock wall in South Qiangtang. The basic age of blueschist is rarely reported in Hongjieshan and even the whole Qiangtang area. The determination of late Carboniferous and early Permian magmatic activities in Hongjiishan provides an important basis for the detailed characterization of paleotectonics in Qiangtang area.
【作者单位】：西北大学大陆动力学国家重点实验室西北大学地质学系;陕西省地下水管理监测局;
【基金】：国家自然科学基金项目(编号:41072159,41190073,41421002)联合资助的成果
【分类号】：P597.3;P588.3

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