哀牢山新元古代斜长角闪岩的形成时代、地球化学特征及其大地构造意义  

Geochronological and Geochemical Characteristics of the Neoproterozoic Amphibolite from Ailaoshan Zone,Western Yunnan and its Tectonic Implications

作　　者：蔡永丰[1,2,3] 王岳军[2,4] 刘汇川[1,3] 马莉燕[5] 邢晓婉[1,3] 刘雷[6] 

机构地区：[1]中国科学院,广州地球化学研究所,同位素地球化学国家重点实验室,广东广州510640 [2]中山大学地球科学系,广东广州510275 [3]中国科学院大学,北京100049 [4]广东省地质过程与矿产资源探查重点实验室,广东广州510275 [5]桂林理工大学地球科学学院,广西桂林541004 [6]中国建筑材料工业地质勘查中心宁夏总队,宁夏银川750021

出　　处：《大地构造与成矿学》2014年第1期168-180,共13页Geotectonica et Metallogenia

基　　金：国家自然科学基金项目(批准号:41190073,40825009,40830319)联合资助

摘　　要：哀牢山带内零星发育一些变基性岩,如斜长角闪岩、变辉长岩和变辉绿岩等,它们主要呈似层状、透镜状、豌豆状或碎片状分布于哀牢山群中。这些变基性岩对研究扬子板块西南缘的大地构造属性具有重要意义。目前关于它们的形成时代、岩石成因及其大地构造意义尚不明确。本文对采自哀牢山群中的斜长角闪岩进行了系统的地球化学分析和锆石 U-Pb 年代学测试。岩石地球化学分析表明,斜长角闪岩的 SiO2含量为46.85%～49.03%,具有较高的 FeOt（11.56%～13.09%）、MgO（6.83%～11.57%）和TiO2（1.71%～2.22%）含量,样品的全碱（K2O＋Na2O）含量为1.61%～2.26%, Na2O/K2O比值为1.88～9.25。样品相对富集轻稀土元素,轻、重稀土元素分馏程度相对较强（（La/Yb）N=6.55～7.52）,稀土总量为112～130μg/g。样品的Sr、P呈负异常,无明显Eu异常（δEu=0.97～1.03）；Nb、Ta亏损不明显, Nb含量变化于14.28～18.43μg/g,与世界上一些典型的富Nb玄武岩类似。定年结果显示斜长角闪岩的形成年龄为814±12 Ma,表明哀牢山带在新元古代时期（～814 Ma）存在基性岩浆活动。综合分析认为该富Nb岩石形成于弧后盆地环境,初步认为其岩石成因与新元古代俯冲体制下导致的弧后拉张作用有关。Some small-sized outcrops of metabasic rocks （e.g., amphibolite, metagabbro and metadiabase） occur mainly as lens, pods and fragments in the Ailaoshan Complex along the Ailaoshan zone, which were previously mapped as Proterozoic basement. These rocks provide possibility for better understanding the tectonic setting of the SW margin of the Yangtze Block in South China, however, their formation time, petrogenesis and tectonic implications are poorly documented so far. The petrological, geochemical and zircon U-Pb geochronological data for the plagioclase amphibolites in the Ailaoshan Complex are reported in this paper. Geochemical analyses show that the amphibolites have SiO2 contents ranging from 46.85% to 49.03% and with relatively high FeOt （11.56%～13.09%）, MgO （6.83%～11.57%） and TiO2 contents （1.71%～2.22%）, K2O＋Na2O vary from 1.61%to 2.26%with Na2O/K2O ratios of 1.88～9.25. The rocks are characterized by enriched LREE （（La/Yb）N=6.55～7.52） and Nb （Nb=14.28～18.43 μg/g）. On primitive mantle-normalized trace element diagrams, the rocks show no obvious Nb and Ta anomalies and significant Sr-P depletion, displaying geochemical characteristics similar to those of the Yunkai （South China） back-arc basin basalt. Zircon U-Pb dating of the amphibolite vields a mean age of 814±12 Ma. In conjunction with other geological observations, it is proposed that there was an Early Neoproterozoic （～814 Ma） arc-back-arc setting along the Ailaoshan zone in the SW margin of the Yangtze Block.

关 键 词：锆石U-PB定年 新元古代 斜长角闪岩 弧后盆地 哀牢山 

分 类 号：P595[天文地球—地球化学] P597[天文地球—地质学]