北秦岭官坡地区稀有金属伟晶岩锡石年代学、岩石地球化学特征及地质意义

Cassiterite U-Pb Age,Geochemistry and Their Geological Significances of Rare Metal Pegmatites in Guanpo Area,North Qinling,China

北秦岭官坡-丹凤地区是我国重要的伟晶岩型铀和稀有金属成矿区,目前官坡地区稀有金属伟晶岩的形成时代和成因机制尚不清楚.通过对该地区花岗伟晶岩开展锡石U-Pb年代学、锆石Hf同位素地球化学及全岩主微量元素地球化学研究,分析了伟晶岩成因并探讨秦岭造山带古生代造山作用的时限.锡石LA-MC-ICP-MS U-Pb测年首次确定官坡地区南阳山稀有金属伟晶岩的成岩成矿时代为420±2 Ma,形成于末志留世,表明此时北秦岭造山带处于造山作用晚期或造山后相对稳定的构造背景.花岗伟晶岩锆石εHf(420 Ma)值为-7.0~-7.8,二阶段Hf模式年龄为2.45~2.52 Ga,表明其源区为古老的地壳物质.花岗伟晶岩铝饱和指数大于1.10,具有极低的稀土总量(ΣREE=0.43×10^(-6)~23.14×10^(-6)),变化范围较大的LaN/YbN值(0.7~22.3)和δEu值(0.09~1.84).岩石富集大离子亲石元素Rb、K和高场强元素Nb、Ta、P、Hf,亏损Ba、Nd、Zr、Ti,具有低的Nb/Ta比值(平均值2.66),稀土元素四分组效应明显,这些特征指示岩浆经历了高度结晶分异作用.同位素和岩石地球化学研究表明,官坡地区花岗金属伟晶岩与其南侧灰池子岩体和大毛沟岩体不具岩浆演化关系,稀有金属伟晶岩为过铝质花岗岩浆高程度结晶分异演化的产物.

The Guanpo-Danfeng area in the North Qinling Belt is the most important pegmatite-type uranium metallogenic and an important pegmatite-type rare metal metallogenic region in China.However,the formation age and the genetic mechanism of the rare metal pegmatites in this area still remain unclear.This paper presents LA-MC-ICP-MS cassiterite U-Pb age,Hf isotopic and geochemical data of granitic pegmatites in the Guanpo area to determine the formation age,the petrogenesis and tectonic setting of the rare metal pegmatites.Cassiterite LA-MC-ICP-MS dating of Nanyangshan rare metal pegmatites yields concordant age of420±2 Ma,representing the the formation age of the end of the Silurian,which indicated North Qinling was in late or postorogenic tectonic setting.The pegmaties exhibit a restricted range of negative zircon εHf(t) values of-7.0 to-7.8,with TDM2age of 2.45 to 2.52 Ga,suggesting that the magma dominantly derived from ancient crust.The pegmatites are characterized by high A/CNK(A/CNK1.1),with extremely low rare earth content(ΣREE=0.43×10^(-6)~23.14×10^(-6)) and widely variable LaN/YbN ratios(0.7~22.3) and δEu(0.09~1.84).The enrichment in large ion lithophile elements(Rb and K) and high field strength elements(Nb,Ta,P and Hf),depletion in Ba,Nd,Zr,and Ti,low Nb/Ta ratios(average 2.66) and tetrad effects of rare earth elements comprehensively indicated that the magma has experienced high degree crystallization differentiation.The isotopic and geochemical data confirm that the rare metal pegmatite in the Guanpo area formed from high degree crystallization differentiation of peraluminous granitic magma and the genetic connection between the pegmatites and Huichizi and Damaogou granites is absent.