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冈底斯中段碱长花岗岩锆石U-Pb-Hf同位素特征及地质意义

ISSN号：1000-3657
期刊名称：中国地质
时间：2015.10.15
页码：1202-1213
分类：P597.1[天文地球—地球化学;天文地球—地质学] P597.3[天文地球—地球化学;天文地球—地质学]
作者机构：[1]中国地质科学院地质研究所大陆构造与动力学国家重点实验室,北京100037
相关基金：国家自然科学基金创新群体（40921001）; 国土资源公益性基金项目（201211093）; 中国地质调查局项目（12120115026801）; 大陆构造与动力学国家重点实验室资助基金（Z1301-a12）联合资助
相关项目：青藏高原大陆动力学及资源环境效应

关键词：锆石U-PB测年, LU-HF同位素, 碱长花岗岩, 冈底斯岩浆带, zircon U-Pb dating, Lu-Hf isotopes, alkali-feldspar granite, Gangdise magmatic belt

中文摘要：

本文对冈底斯岩浆带中段谢通门塔玛地区碱长花岗岩进行了原位LA-ICP-MS锆石U-Pb定年和LA-MCICP-MS锆石Lu-Hf同位素分析。研究结果表明,采自该岩体不同部位的两件锆石U-Pb同位素加权平均年龄分别为（40.02±0.39）Ma和（40.65±0.32）Ma,具有几乎一致的地质年龄,即碱长花岗岩侵位结晶年龄为40 Ma左右。LA-MCICP-MS锆石Lu-Hf同位素研究显示,176Hf/177Hf比值在0.282633~0.282878,平均值为0.282765,计算所得的εHf（t）值介于-4.08~4.15,平均值为0.28,峰值在-1~＋1之间;TDMC模式年龄在822~1373 Ma,平均值为1075 Ma,峰值年龄为1000~1200 Ma。其次,样品的εHf（t）值具有正负相间的特点,εHf（t）也相对较小,为典型的壳幔混染型,岩浆源区主要以古老地壳的熔融为主。综合研究表明,谢通门塔玛地区碱长花岗岩主要是由新特斯洋板片的断离,致使软流圈地幔上涌,引起拉萨地体地壳物质的熔融、再循环而形成的,在这个过程中有部分幔源物质的加入。

英文摘要：

In this paper, the authors conducted zircon LA-ICP-MS U-Pb dating and Lu-Hf analysis of the alkali-feldspar granite in Tama of Xietongmen region along the middle Gangdise magmatic belt. The results suggest that the weighted mean ages of two samples are（40.02±0.39）Ma and（40.65±0.32）Ma for the pluton, respectively. In essence, the two ages are of the same geological significance in that they both represent the crystallization age or emplacement age of the alkali- feldspar granite. Zircon Lu- Hf analytical data reveal that values of176Hf/177 Hf vary from 0.282633 to 0.282878（averaging 0.282765）, and the values of εHf（t）rangefrom-4.08 to 4.15（0.28 on average）, with the peak values of εHf（t）varying from-1 to 1. TDMCmodel ages are from 822 Ma to 1373Ma（averaging 1075 Ma） with peak model ages being ca.1000-1200 Ma. The obvious feature is that the values of the εHf（t）show alternating positive and negative values, corresponding to a small value and indicating the crustal- mantle contamination process,which resulted from partial melting of the ancient crust. Combined with geological setting and study results, the authors consider that the alkali- feldspar granite might have generated from the slab break off of the Neo- Tethys oceanic crust, which led to upwelling of the asthenosphere and further induced the partial melting and recycling of the ancient crust. During the process, the mantle materials played a vital role in forming the alkali-feldspar granite in the study area.