滇西龙陵-瑞丽低钠流纹岩年代学、岩石地球化学和锆石Hf同位素组成及其地质意义  

Geochronology,geochemistry and zircon Hf isotope of the low Na rhyolite at Longling-Ruili belt,and its geological implications

作　　者：韦诚[1,2] 戚学祥 沈辉[3] 吉风宝[4] 任玉峰 刘旭峰 WEI Cheng;QI XueXiang;SHEN Hui;JI FengBao;REN YuFeng;LIU XuFeng(Key Laboratory of DeepEarth Dynamics,Ministry of Natural Resources,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China;Yunnan Institute of Geology&Mineral Resources Exploration,Kunming 650051,China;Tibet Museum of Natural Science,Lhasa 850000,China)

机构地区：[1]自然资然部深部动力学重点实验室,中国地质科学院地质研究所,北京100037 [2]北京大学地球与空间科学学院,北京100871 [3]云南省地质矿产勘查院,昆明650051 [4]西藏自然科学博物馆,拉萨850000

出　　处：《岩石学报》2020年第10期3117-3136,共20页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(91755101);中国地质调查局项目(DD20190060)联合资助。

摘　　要：龙陵-瑞丽早白垩世流纹岩带断续延伸约100km,其形成的构造背景是揭示怒江洋演化及腾冲与保山地块关系的重要证据。笔者在野外调研的基础上,对本区流纹岩开展了岩石学、地球化学、锆石U-Pb定年和Hf同位素等方面的研究。结果表明流纹岩具有高硅贫钠、高钾钙碱性和高分异特征;锆石LA-ICP-MS U-Pb年龄分布在129~123Ma区间,代表流纹岩的喷发时代。流纹岩锆石ε_(Hf)(t)值分布于-12.3^-6.3之间,对应的地壳模式年龄为1591~1826Ma;其微量元素蛛网图、稀土元素配分模式图和Nb/Ta比值、Th/U比值等与大陆中上地壳相似;岩石中无斜长石斑晶、Sr含量极低以及在Al_2O_3/(MgO+FeO^T)]-[CaO/(MgO+FeO^T)]和Rb/Ba-Rb/Sr图解上,样品都分布在变泥质岩部分熔融区;表明流纹岩岩浆来源于古老地壳物质部分熔融的产物,其低钠是源区中斜长石含量低或斜长石未参与部分熔融造成的。流纹岩轻稀土元素和大离子亲石元素(Th、U和K)富集,Nb、Ta、Ti、P和Sr强烈负异常;在构造环境判别图解上样品全部落在活动大陆边缘区;在空间上,这些流纹岩呈带状分布,构成俯冲增生杂岩带的一部分,并与同时代侵入岩紧密相邻,成因上类似于区域内同期S型花岗岩。这些特征表明研究区内流纹岩与高黎贡构造带内同时代花岗岩类相似,形成于俯冲的大陆边缘弧环境,即幔源岩浆底侵至下地壳,这一过程提供了足够的热源使壳源物质(可能为富云母、贫斜长石的变沉积岩类)发生部分熔融生成酸性岩浆,它们在经历了高度的结晶分异后喷出地表形成低钠流纹岩,并在弧前或弧间盆地内被浅海相碎屑沉积物覆盖。The Longling-Ruili Early Cretaceous rhyolite belt extended intermittently about 100 km,and its tectonic setting can provide key evidence to show the subduction of the Nujiang Ocean and collision between Tengchong and Baoshan blocks.Based on field work,petrological and geochemical studies together with zircon U-Pb dating and Hf isotopic analysis of the rhyolites have been carried out.The results show that the rhyolites are characterized by high-Si and low-Na,high-K calc-alkaline and highly differentiation index(DI).The zircon LA-ICP-MS U-Pb ages range from 129 Ma to 123 Ma,representing the eruption ages of the rhyolites.The zirconεHf(t)values range from-12.3 to-6.3 with tDMC of 1591~1966 Ma.Trace element spider diagram,REE patterns,and Nb/Ta and Th/U values of the rhyolites are similar to those of the middle and upper continent crust.There’s no plagioclase phenocrysts in the rock,and Sr content is extremely low,also,all samples fall in the metapolitical partial melting field on the[Al2O3/(MgO+FeOT)]-[CaO/(MgO+FeOT)]and Rb/Ba-Rb/Sr diagrams.These indicate that the rhyolitic magma was derived from partial melting of ancient crustal material.The low Na content was resulted from partial melting of plagioclase-poor rocks or the situation that plagioclase did not melt during the partial melting.With high-K calc-alkaline,enrichment in light REE and large ionic lithophile elements(Th,U and K),and obvious negative anomalies of Nb,Ta,Ti,P and Sr,all samples fall in the active continental margin field on the tectonic discrimination diagram.As a part of subduction and accretionary complexes,the spatially banded rhyolites are closed to the contemporaneous intrusive rocks,their geochemical characteristics are consistent with those of the coeval S-type granites in Gaoligong orogen.All above demonstrates that the rhyolites were formed in an active continental margin related with subduction of Nujiang oceanic crust,i.e.,the rhyolitic magma was formed by the partial melting of crustal material(mica-rich and plagioclase-poor meta

关 键 词：锆石U-PB定年 Lu-Hf同位素组成 低钠流纹岩 增生杂岩带 滇西 

分 类 号：P588.141[天文地球—岩石学] P597.3[天文地球—地质学]