锆石形貌与成分特征对藏南洛扎地区二云母花岗岩岩浆‒热液演化过程的指示  

Internal Morphology and Compositional Characteristics of Zircons from the Two-mica Granites in the Luozha Area,Southern Xizang:Insights into the Magmatic-hydrothermal Evolution of the Himalayan Leucogranites

作　　者：蒙安平 戴国庆 张娜 刘志超 MENG Anping;DAI Guoqing;ZHANG Na;LIU Zhichao(School of Earth Science and Engineering,Sun Yat-sen University,Zhuhai 519082,Guangdong,China)

机构地区：[1]中山大学地球科学与工程学院,广东珠海519082

出　　处：《大地构造与成矿学》2024年第3期529-546,共18页Geotectonica et Metallogenia

基　　金：第二次青藏高原综合科学考察研究项目(2019QZKK0802);国家自然科学基金项目(41888101)联合资助。

摘　　要：喜马拉雅淡色花岗岩的高分异成因和稀有金属成矿潜力是近年来特提斯构造域研究的热点问题,但目前关于这些花岗岩,特别是对具有较低演化程度的二云母花岗岩的岩浆‒热液演化过程的认识较为薄弱,严重制约了对喜马拉雅淡色花岗岩成岩、成矿机制的全面理解。广泛存在于花岗质岩石中的锆石可以有效记录岩浆演化过程的物理化学信息,已成为追溯岩浆‒热液演化历史的有效工具。为此,本文对藏南洛扎地区早中新世(约20~18 Ma)和中中新世(约12 Ma)两个期次二云母花岗岩中的锆石进行了系统的形貌特征观察和化学成分分析。洛扎二云母花岗岩中的锆石以富含继承核和具有围绕其生长的后期增生边(与淡色花岗岩岩浆作用有关)为特征。根据形貌特征,岩石中的新生锆石可分为两类:第一类自形程度较好,具有清晰的振荡环带,属于岩浆锆石;第二类自形较差,含较多包裹体,阴极发光图像下呈斑驳状,与典型的热液锆石特征一致。这两类锆石均具有较高的HfO_(2)和极高的U含量,结合其他微量元素成分(Nb/Ta、Y/Ho、REE等)特征,指示它们形成于分异程度较高的熔体,其中第二类锆石形成于富卤素流体活动的环境,表明岩浆体系在演化过程中发生了流体相的出溶。两类锆石在HfO_(2)、UO_(2)、P_(2)O_(5)含量和δEu值等化学组成上没有明显差异,表明流体出溶过程并没有伴随着显著的结晶分异作用。同时,两类锆石给出了相似的结晶温度,暗示流体出溶作用发生在岩浆等温减压过程。根据锆石Ce-U-Ti氧逸度计估算结果,两个期次二云母花岗岩的岩浆体系均具有极低的氧逸度(ΔFMQ<–1),并在岩浆向热液状态转化的过程中岩浆体系的氧逸度有一定程度的上升,该特征与其他典型的稀有金属成矿花岗岩具有相似性。The genesis and rare-metal metallogenic potential of highly evolved Himalayan leucogranites have been hot topics in the study of the Tethyan tectonic domain in recent years.Nevertheless,the magmatic evolution history of these granites,especially the magmatic-hydrothermal transition processes,remains unclear,hindering our comprehensive understanding of their petrogenesis and mineralization mechanisms.Zircon is a common accessory mineral in granitoid rocks,and the morphology and chemical composition of zircon can be used to trace the evolutionary history of magmatic systems.In this study,systematic morphological observations and element compositional analyses were conducted on zircons from the Early and Middle Miocene two-mica granites(ca.20-18 Ma and ca.12 Ma)in the Luozha area,Southern Xizang.The zircons have distinct restitic cores overgrown by magmatic rims(igneous origin).Igneous zircons can be divided into two types based on their morphological features.TypeⅠzircons,which often occur as euhedral crystals,show clear oscillatory zoning in cathodoluminescence(CL)images.TypeⅡzircons typically exhibit subeuhedral to anhedral morphology and are characterized by mottled or spongy internal textures with abundant mineral inclusions,consistent with typical hydrothermal zircon features.The high HfO_(2)and U contents in the two zircon types indicate that they crystallized from highly differentiated melts,which is supported by the Nb/Ta,Y/Ho,and REE compositions.Furthermore,the high Y/Ho ratio indicates that some zircons were formed in magmas with halogen-rich fluid.There is no significant differences in the HfO_(2),UO_(2),and P_(2)O_(5)contents andδEu values of the two zircon types,indicating that the fluid exsolution was not accompanied by significant crystal fractionation.Additionally,the two types of zircons have similar crystallization temperatures,suggesting that fluid exsolution occurred in the process of isothermal decompression of the magma.According to the Ce-U-Ti zircon oxygen fugacity estimation,the two m

关 键 词：喜马拉雅淡色花岗岩 二云母花岗岩 锆石 岩浆‒热液演化 氧逸度 

分 类 号：P581[天文地球—岩石学] P595[天文地球—地质学]