埃达克岩与热液成矿过程中钾镁同位素分馏及其指示意义:以驱龙斑岩铜矿床为例  

Potassium and magnesium isotope fractionation during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks and its indicative significance:A case study of the Qulong porphyry copper deposit,southern Tibet

作　　者：字艳梅 田世洪 陈欣阳 侯增谦[6] 杨志明[6] 龚迎莉[7] 唐清雨 ZI Yanmei;TIAN Shihong;CHEN Xinyang;HOU Zengqian;YANG Zhiming;GONG Yingli;TANG Qingyu(State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,China;School of Earth Sciences,East China University of Technology,Nanchang 330013,China;SinoProbe Laboratory of Chinese Academy of Geological Sciences,Ministry of Natural Resources,Beijing 100094,China;Department of Earth and Space Sciences,University of Washington,Seattle,WA 98195,USA;International Center for Sedimentary Geochemistry and Biogeochemistry Research,Chengdu University of Technology,Chengdu 610059,China;Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;Department of Energy and Power Engineering,Tsinghua University,Beijing 100084,China)

机构地区：[1]东华理工大学核资源与环境国家重点实验室,江西南昌330013 [2]东华理工大学地球科学学院,江西南昌330013 [3]自然资源部深地科学与探测技术实验室,北京100094 [4]华盛顿大学地球与空间科学系,美国西雅图华盛顿98195 [5]成都理工大学沉积与生物地球化学国际研究中心,四川成都610059 [6]中国地质科学院地质研究所,北京100037 [7]清华大学能源与动力工程系,北京100084

出　　处：《地学前缘》2024年第3期150-169,共20页Earth Science Frontiers

基　　金：科学技术部国家重点研发计划“战略性矿产资源开发利用”专项“我国西部伟晶岩型锂等稀有金属成矿规律与勘查技术项目(2021YFC2901900)”“北喜马拉雅锂等稀有金属找矿预测与勘查示范课题(2021YFC2901903)”;中国铀业有限公司东华理工大学核资源与环境国家重点实验室联合创新基金项目(2022NRE-LH-05);自然资源部深地科学与探测技术实验室开放课题(SinoProbe Lab 202217);江西省“双千计划”创新领军人才长期项目(2020101003);江西省自然科学基金重点项目(20224ACB203011);东华理工大学高层次人才引进配套经费(1410000874)。

摘　　要：为研究后碰撞埃达克质岩岩浆分异和热液过程中的钾镁同位素分馏行为及其对斑岩铜矿床岩石成因的指示意义,本文对藏南驱龙斑岩铜矿床的一套埃达克质岩石开展了钾镁同位素分析。该套岩石包括闪长质包体、花岗闪长岩和花岗斑岩,后者是超大型斑岩铜矿床的成矿母岩。已有研究结果证实,这3套岩石很可能是由同一来源的不同程度的部分熔融和分离结晶作用形成的。研究结果表明,闪长质包体、花岗闪长岩和花岗斑岩的δ^(41)K组成分别为-0.38‰~-0.22‰、-0.43‰~-0.34‰和-0.59‰~-0.36‰。尽管3类岩石各自变化范围较小,但整体上δ^(41)K与K 2O和SiO_(2)含量呈现明显负相关趋势,表明全岩钾元素迁移和钾同位素发生了解耦,钾同位素产生了分馏。在闪长质包体花岗闪长岩演化期间,δ^(41)K与Sc和Y含量呈明显正相关关系,这指示了在该演化期间可能主要是角闪石分离结晶导致了钾同位素分馏。相比之下,花岗斑岩中具有明显的钾同位素组成变化,在花岗斑岩演化期间,δ^(41)K与Eu/Eu*和Sr含量呈正相关关系,表明在岩石演化晚期钾同位素的分馏可能主要跟斜长石分离结晶有关。然而,通过瑞利分馏模拟表明仅矿物分离结晶难以实现如此大的钾同位素分馏,花岗斑岩中的钾同位素分馏是岩浆热液共同作用的结果。值得注意的是,闪长质包体具有比地幔更重的钾同位素组成,导致这种现象的原因可能是复杂多样的:一方面,下地壳可能具有比地幔更重的钾同位素组成,岩浆混合机制下导致源区具有重钾同位素组成;另一方面,驱龙斑岩铜矿床的岩浆源区受到俯冲板片来源的富重钾流体/熔体所交代,导致其源区的钾同位素组成偏重。相比之下,这3类岩石的镁同位素组成比较类似(闪长质包体:-0.36‰~-0.19‰;花岗闪长岩:-0.28‰~-0.13‰;花岗斑岩:-0.44‰~-0.13‰),并且落在了地幔和下地壳�The study focused on investigating the behavior of potassium and magnesium isotopes during magmatic differentiation and hydrothermal processes in post-collisional adakitic rocks from the Qulong porphyry copper deposit in southern Tibet.The analyzed rocks included dioritic enclaves,granodiorites,and granite porphyries,with the latter containing a super-large copper deposit.The research confirmed that these rocks likely originated from different degrees of partial melting and fractional crystallization from the same source.Theδ^(41)K values for the dioritic enclaves,granodiorites,and granite porphyries ranged from-0.38‰to-0.22‰,-0.43‰to-0.34‰,and-0.59‰to-0.36‰,respectively.While the variation inδ^(41)K values among the rock types was small,there was a clear negative correlation betweenδ^(41)K and the concentrations of K 2O and SiO_(2),indicating potassium isotope fractionation during the migration of potassium in the rocks.The study suggested that the evolution of dioritic enclaves to granodiorites showed a positive correlation betweenδ^(41)K and Sc and Y,implying that the separation and crystallization of hornblende played a significant role in potassium isotope fractionation.In contrast,the composition of potassium isotopes varied significantly in granite porphyries.The evolution of granite porphyries displayed a positive correlation betweenδ^(41)K and Eu/Eu*and Sr,suggesting that potassium isotope fractionation in these rocks may be primarily related to the fractional crystallization of plagioclase during late-stage rock evolution.The study also indicated that the observed potassium isotope fractionation in granite porphyries likely resulted from the combined influence of magma and hydrothermal fluid,rather than solely from mineral separation and crystallization processes.The heavier potassium isotopic composition in dioritic enclaves compared to the mantle was attributed to potential factors such as the lower crust having a heavier K isotopic composition than the mantle,and metasomatism of the

关 键 词：驱龙斑岩铜矿床 埃达克质岩石 钾镁同位素 同位素分馏 岩浆热液流体出溶 

分 类 号：P597.2[天文地球—地球化学] P618.41[天文地球—地质学] P611