北武夷上水桥萤石矿区黑云母石英二长岩成因:来自年代学及地球化学的制约

Petrogenesis of the Biotite Quartz Monzonite from Shangshuiqiao Fluorite Deposit,North Wuyi Area:Constraints from Geochronology and Geochemistry

上水桥萤石矿是北武夷地区近些年来新探明的一处极具开采前景的中型萤石矿床,目前开展的研究工作极为薄弱。该项工作以矿区赋矿岩体黑云母石英二长岩为研究对象,对其展开系统的岩相学、锆石U-Pb年代学和全岩地球化学分析。锆石U-Pb定年结果表明,上水桥黑云母石英二长岩成岩年龄为235~236 Ma,为印支期岩浆活动的产物。全岩地球化学分析结果显示,该岩体具有富碱、贫磷钛的特征,具有较高的σ值(2.59~3.61)和偏低的A/CNK值(0.96~1.16),属钾玄质、准铝质—弱过铝质岩石;微量及稀土元素方面,岩石相对富集大离子亲石元素(Rb、Th、U、K)和LREE,亏损Ba、Sr、Nb、Ta、P、Ti、HREE等元素,同时整体具有较高的稀土含量(ΣREE=356×10^(-6)~448×10^(-6))和弱负铕异常(Eu/Eu*=0.61~0.76)。此外,岩石整体具有较高的Zr+Nb+Ce+Y含量(381×10^(-6)~546×10^(-6))和Ga/Al比值(10^(4)×Ga/Al=2.60~3.07),以及较低的Rb/Sr(0.31~0.82)、Rb/Ba(0.11~0.18)和TFeO/MgO比值(2.02~2.22),属后造山型(PA型)铝质A型花岗岩。综合地球化学投图判别结果及区域岩浆活动、沉积作用等特征,文章认为上水桥矿区黑云母石英二长岩可能为多板块汇聚模式下陆内造山作用末期后造山伸展的构造背景下的产物,成矿阶段岩石中黑云母的水解及斜长石的绢云母化作用可能为萤石成矿提供F和Ca来源。

Shangshuiqiao fluorite ore is a newly discovered medium fluorite deposit with great exploitation prospect in northern Wuyi area in recent years.However,little research has been carried out on this deposit.In this paper,petrographic,zircon U-Pb chronological and whole-rock geochemical analyses have been carried out on the biotite quartz monzonite from the the Shangshuiqiao fluorite deposit.The zircon LA-ICP-MS U-Pb ages show that the biotite quartz monzonite was formed at 235~236 Ma,which is the product of Indosinian magmatism.According to the geochemical analyses,the biotite quartz monzonite samples are characterized by high alkali(Na2O+K2O),Mg and Fe contents and low P and Ti contents.They have highσvalues(2.59 and 3.61)and low A/CNK ratios(0.96 to 1.16),belonging to the typical high-K series with metaluminous to weakly peraluminous.In terms of trace and rare earth elements,the biotite quartz monzonite is enriched in Rb,Th,U,K and light rare earth elements(LREEs),but depleted in Ba,Sr,Nd,Ta,P,Ti and heavy rare earth elements(HREEs),as well as high rare earth elements(ΣREE varying from 356×10^(-6)to 448×10^(-6))and weak negative Eu anomalies(Eu/Eu*between 0.61 and 0.76).Besides,the biotite quartz monzonite displays high Zr+Nb+Ce+Y content(varying from 381×10^(-6)to 546×10^(-6))and Ga/Al ratio(10^(4)×Ga/Al between 2.60 and 3.07),with lower Rb/Sr(0.31 to 0.82),Rb/Ba(0.11 to 0.18)and TFeO/MgO ratios(2.02 to 2.22),belong to post-orogenic(PA type)aluminous A-type granites.In addition,based on the results of geochemical mapping and characteristics of regional magmatism and sedimentation,we suggest that the biotite quartz monzonite from Shangshuiqiao fluorite deposit may be formed in the postorogenic extensional tectonic environment at the end of intracontinental orogeny under the background of multi-plate convergence.During the mineralization stage,the biotite hydrolysis and sericitization of plagioclase in biotite quartz monzonite may provide F and Ca sources for fluorite mineralization.