含氟流体助熔华南基底片麻岩的实验研究:对稀土成矿花岗岩成因的制约  

An Experimental Study on F-bearing Fluid-fluxing Partial Melting of South China’s Gneiss:Constraints on Formation of REE-mineralized Granites

作　　者：李嘉豪 丁兴[1,3] 刘军锋[2] 于津海[4] LI Jiahao;DING Xing;LIU Junfeng;YU Jinhai(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;School of Earth Science and Resources,Chang’an University,Xi’an 710064,Shaanxi,China;CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,Guangdong,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,Jiangsu,China)

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]长安大学地球科学与资源学院,陕西西安710064 [3]中国科学院深地科学卓越创新中心,广东广州510640 [4]南京大学地球科学与工程学院,江苏南京210023

出　　处：《大地构造与成矿学》2024年第2期213-231,共19页Geotectonica et Metallogenia

基　　金：广东省基础与应用基础研究重大项目“广东省稀土资源与环境风险”(2019B030302013);国家自然科学基金重点项目(91962221)联合资助。

摘　　要：为了明确含氟流体对花岗岩源区岩浆过程中稀土元素地球化学行为的影响,本文采用华南代表性的高稀土片麻岩和低稀土片麻岩作为初始物质,与~4%的流体(纯水、1.5 mol/L HF、1.0 mol/L NaF)在0.8 GPa、1000℃的条件下开展了流体助熔的部分熔融实验。实验结果表明,所有片麻岩的熔融程度达到40%以上,残余矿物组合主要为斜长石+石英+斜方辉石±单斜辉石+钛铁矿+磷灰石±黑云母。部分熔融产生的熔体的成分主要为花岗闪长质‒花岗质,总体具有高氟(0.11%~0.27%)、高水(0.38%~1.86%)和A2型花岗岩的特征;其中,高稀土片麻岩部分熔融形成的熔体为钾玄质和准铝质,而低稀土片麻岩熔融形成的熔体偏高钾钙碱性和强过铝质。在微量元素蛛网图上,两者均表现为富集K、Rb、Th、Ce、Sm、Y、Yb等元素,亏损Ba、Nb、Ta、Zr、Hf、Ti等元素;稀土元素配分曲线则表现为右倾、负Eu异常的特征。相对于原岩,含氟流体助熔会强化熔体中Si、K、Na、Rb、Sr、Ba、Th、U、LREE和F的富集,并增加熔体的La_(N)/Yb_(N)值。同时,实验也证实片麻岩部分熔融形成的熔体的稀土元素含量主要受原岩控制,高稀土片麻岩更易于熔出稀土元素初始富集的熔体;相对于原岩,各类流体助熔均会造成稀土元素在熔体中不同程度的富集,其中,含HF流体助熔最高可富集稀土46%~49%(近1.5倍富集)。我们因此认为在华南LREE型和HREE型成矿花岗岩形成过程中,含F流体发挥重要的作用。此外,本文也提出了一种新的A2型花岗岩形成机制,即含F流体助熔地壳基底岩石,这很好地解释了华南晚中生代大规模分布的与稀土、稀有金属及氟成矿相关的陆内A2型花岗岩的形成。To clarify the effect of fluorine-bearing fluids on the geochemical behaviors of rare earth elements(REE)during magmatic processes linked to the origin of granitic magmas,this paper conducted a series of piston-cylinder experiments on the fluid-fluxing partial melting of gneiss at 0.8 GPa and 1000℃using representative high-REE and low-REE gneiss powders and~4%solutions(i.e.,pure water,1.5 mol/L HF,and 1.0 mol/L NaF)as the starting materials.The experimental results show that the melting degree of gneiss for the experiments is more than 40%,and the residue minerals include plagioclase+quartz+clinopyroxene±orthopyroxene+ilmenite+apatite±biotite.All the melts originated from fluid-fluxing partial melting of gneiss are of granodiorite and granite in compositions,displaying high fluorine(0.11%–0.27%)and water(0.38%–1.86%)contents as well as analogous geochemical characteristics of A2-type granitoids.Specifically,the melts derived from melting of the high-REE gneiss are shoshonitic and metaluminous-weakly peraluminous,whereas those derived from the low-REE gneiss are high-K calc-alkaline and strongly peraluminous.All the melts also display positive anomalies of K,Rb,Th,Ce,Sm,Y,and Yb and negative anomalies of Ba,Nb,Ta,Zr,Hf,and Ti in the spider diagrams,whereas the REE patterns exhibit LREE-enriched features and slightly negative Eu anomalies.Compared with the protolith,addition of F-bearing fluids enhanced the enrichment of Si,K,Na,Rb,Sr,Ba,Th,U,LREE,and F in the melts and raise the A/CNK and La_(N)/Yb_(N)ratios of the melts.The experimental results also demonstrated that the REE contents in the melts were controlled by the protolith,which implies that the high-REE gneiss could more readily fuse out the melts with high REE contents.Besides,the partial melting with fluid assistance could cause REE enrichment in the melts to varying extents;for example,melts derived from HF-bearing fluid-fluxing partial melting have REE contents almost 1.5 times as high as that of the protolith.This paper thus highlights the sig

关 键 词：部分熔融 流体助熔 含氟流体 稀土元素 花岗岩 片麻岩 

分 类 号：P589.1[天文地球—岩石学]