湘东北幕阜山西北部黑(二)云母二长花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

Zircon U-Pb Ages,Geochemical Characteristics and Geological Significance of Biotite Monzogranite and Two-Mica Monzogranite in the Northwestern Mufushan of Northeast Hunan Province

近年来,湘东北幕阜山地区发现多个花岗伟晶岩型锂铌钽铍等大型稀有金属矿床,这些稀有金属矿与二云母二长花岗岩、伟晶岩具有密切成因关系。本次研究对幕阜山复式岩体西北部的黑(二)云母二长花岗岩开展了LA-ICP-MS锆石U-Pb定年和岩石地球化学研究。分析结果显示,该区花岗岩的主量元素富硅(w(SiO_(2))=71.99%~76.23%)、富铝(w(Al_(2)O_(3))=13.37%~14.86%),属于弱-强过铝质(A/CNK=1.08~1.14)、中高钾钙碱性(w(K_(2)O)=2.89%~5.27%,σ=1.67~2.38)S型(S-I型)花岗岩。两个样品的锆石U-Pb加权平均年龄分别为137.5±1.2 Ma和134.7±1.6 Ma,表明其形成于白垩纪早期。幕阜山西北部浅色花岗岩为典型壳源成因的高分异花岗岩,普遍相对富集Rb、Th、U、La、Ce,相对亏损Ba、Nb、Sr、Yb。此外,岩石具有显著的Li正异常(39.6×10^(-6)~97.8×10^(-6)),球粒陨石标准值标准化的稀土元素(REE)配分曲线图呈高分异花岗岩常见的富轻稀土(LREE/HREE=8.16~22.53)右倾特征,暗示其演化程度高,具有较大的锂成矿潜力。结合前人研究,本文认为该区浅色花岗岩母岩浆可能形成于冷家溪群地层或同时代地层的贫黏土变质砂岩部分熔融,其成因构造背景处于碰撞挤压向拉张转换的过渡期。

In the Mufushan region of northeastern Hunan Province,a significant discovery has been made in recent years,with the unearthing of several large-scale rare metal deposits,particularly those of the granitic pegmatite-type,rich in lithium(Li),niobium(Nb),tantalum(Ta),and beryllium(Be).These deposits exhibit a pronounced metallogenic association with the two-mica monzogranite and associated pegmatite formations.This study delves deeper into the petrogenesis and metallogenic potential of these monzogranites by employing LA-ICP-MS zircon U-Pb dating and whole-rock geochemical analyses on samples collected from the northwestern part of the Mufushan massif.The analysis results show that these monzogranites in this area are rich in silicon(w(SiO_(2))=71.99%~76.23%)and aluminum(w(Al_(2)O_(3))=13.37%~14.86%),belonging to weak-to-strong peraluminous(A/CNK=1.08~1.14),medium-to-high K(w(K_(2)O)=2.89%~5.27%,σ=1.67~2.38),calc-alkaline S-type(S-I type)granites,suggesting a crustal origin and a possible role of crustal melting in their formation.The zircon U-Pb weighted average ages of the two samples were determined to be 137.5±1.2 Ma and 134.7±1.6 Ma,respectively,indicating that they were formed in the Early Cretaceous.This temporal framework is crucial for understanding the tectonic and magmatic evolution of the region.These leucogranites in the northwestern part of the Mufushan massif are highly differentiated granites of typical crust-derived origin.They exhibit a relative enrichment in elements such as rubidium(Rb),thorium(Th),uranium(U),lanthanum(La),and cerium(Ce),which are typically associated with the presence of accessory minerals that can host these elements.Conversely,they show relative depletion in elements like barium(Ba),niobium(Nb),strontium(Sr),and ytterbium(Yb),which could be indicative of fractionation processes during their petrogenesis.A noteworthy observation is the significant positive lithium anomalies in these granites,with concentrations ranging from 39.6×10^(-6)to 97.8×10^(-6).This,coupled with their chondrite-normalized rare earth element(REE)patterns that display a clear light rare earth element(LREE)enrichment(LREE/HREE=8.16~22.53)and a right-leaning feature,is characteristic of highly evolved,fractionated granites.Integrated the results of this study with previous research,it is inferred that the leucogranite-hosting magmas may have originated from the partial melting of clay-poor Lengjiaxi Group sediments or coeval metamorphic sandstone.The tectonic setting of this magma generation is likely to have been during a critical transition period from collisional extrusion to a tensile regime,reflecting the dynamic nature of the Earth's crust during the Early Cretaceous in this region.This study thus contributes to a more nuanced understanding of the complex interplay between magmatism,tectonics,and the potential for economically significant mineral deposits in the Mufushan area.