大兴安岭南段那斯嘎吐云英岩型铍矿化的形成时代和成因探讨:独居石年代学和地球化学证据

Formation age and genesis of the Nasigatu greisen-type beryllium mineralization in the southern Great Xing an Range:Monazite chronological and geochemical evidence

本文报道了大兴安岭南段林西地区新发现的那斯嘎吐云英岩型铍矿点的独居石U-Pb年龄、主微量元素及Nd同位素组成,并与大兴安岭南段同期的经棚高分异碱长花岗岩中的独居石地球化学数据进行对比,以探讨云英岩型铍矿化的形成时代与成因。那斯嘎吐云英岩主要呈带状赋存于早白垩世(~140Ma)碱长花岗岩岩体顶部,矿物组成包括石英、白云母、萤石、独居石以及锡石-绿柱石等铍-锡稀有金属矿物,其全岩Be含量可达397×10^(-6)~13908×10^(-6)。那斯嘎吐富铍云英岩-云英岩化花岗岩与经棚贫铍碱长花岗岩中的独居石均具有较高的ThO_(2)(平均值为3.01%~6.88%)和较低的UO_(2)含量(<0.55%),并呈现出富集轻稀土元素、亏损重稀土元素和强烈Eu负异常(δEu<0.08)的地球化学特征。与经棚岩浆成因独居石(TE_(1,3)=0.95~1.00)相比,那斯嘎吐云英岩化相关的独居石具有明显高的TE_(1,3)值(0.98~1.26)。岩相学和矿物地球化学研究结果表明,那斯嘎吐独居石形成于岩浆-热液过渡阶段,起源于高演化花岗质岩浆的富钠钙氟流体在其形成过程起到了关键作用。独居石LA-ICP-MS U-Pb同位素定年结果显示,那斯嘎吐云英岩型铍矿化形成于早白垩世(~139Ma),与赋矿碱长花岗岩时代一致。那斯嘎吐独居石的ε_(Nd)(t)值为-0.29~0.95,与经棚独居石的ε_(Nd)(t)值(-2.38~-1.68)和大兴安岭南段早白垩世高分异花岗岩的全岩ε_(Nd)(t)值(-2.51~2.11)基本相当。独居石年龄和Nd同位素结果指示,那斯嘎吐富铍云英岩的形成与早白垩世花岗质岩浆高度分异晚期熔-流体相互作用密切相关,源区主要由新生和古老地壳物质共同组成。综合区域地质资料,查明早白垩世是大兴安岭南段Sn-W-Li-Be等稀有金属成矿作用的一个重要时期,其形成于古太平洋板块回卷诱发的岩石圈伸展环境。那斯嘎吐云英岩型铍矿的发现表明大兴安岭南段具有铍成矿的良好前景,林西地区是未来铍矿产勘查的重点靶区,值得在今后研究过程中予以充分关注。

This paper presents monazite U-Pb ages,major and trace elements and Nd isotope compositions for the newly discovered Nasigatu greisen-type beryllium deposit from the Linxi region of the southern Great Xing an Range(SGXR).These data,compared with monazite geochemistry of the coeval Jingpeng highly fractionated alkali-feldspar granite in the SGXR,allow us to explore the formation time and genesis of the greisen-type beryllium mineralization.The Nasigatu greisens occur as strips within the roof of the Early Cretaceous(~140Ma)alkali-feldspar granite pluton,with a mineral assemblage of quartz,muscovite,fluorite,monazite and rare metal minerals including cassiterite and beryl.The whole rock beryllium contents of the beryl-bearing greisens can reach 397×10^(-6)~13908×10^(-6).The monazites from the Jingpeng beryllium-depleted alkali-feldspar granite and the Nasigatu beryllium-riched greisen and greisenization granite all have relatively high ThO_(2)(averaged at 3.01%~6.88%)and low UO_(2)(<0.55%)contents,with geochemical features of LREEs-enrichments,HREEs-depletions and strong negative Eu anomalies(δEu<0.08).Compared with the Jingpeng magmatic monazites(TE_(1,3)=0.95~1.00),the Nasigatu greisenization-related monazites have obviously higher TE_(1,3) values of 0.98~1.26.Petrology and geochemistry results indicate the Nasigatu monazites formed in a magmatic-hydrothermal transition stage,and the Na-,Ca-,F-riched fluids derived from the highly evolved granitic magma play a key role in the generation of the Nasigatu monazites.LA-ICP-MS U-Pb dating results of the monazite reveal that the Nasigatu greisen-type beryllium deposit was formed during the Early Cretaceous with the age of~139Ma,which is consistent with the ages of the ore-related alkali-feldspar granite.The Nasigatu monazite ε_(Nd)(t)values range from-0.29 to 0.95,mostly overlapping with the Jingpeng monazite ε_(Nd)(t)values(-2.38~-1.68)and the whole rock ε_(Nd)(t)values(-2.51~2.11)of the Early Cretaceous highly fractionated granites from the SGXR.Monazite ages and Nd isotopes suggest that the Nasigatu beryllium-riched greisens were formed closely associated with the melt-fluid interaction during the late stage of high differentiation processes of Early Cretaceous granitic magmas,and their sources mainly consist of both juvenile and old crustal materials.Combined with the regional geological information,it is determined that the Early Cretaceous is an important period of rare metal(e.g.,Sn-W-Li-Be)mineralization within the SGXR,and this mineralization peak occurred in a lithosphere extensional environment induced by the roll-back of the Paleo-Pacific Plate.The discovery of the Nasigatu greisen-type beryllium deposit shows that the SGXR has good prospects for beryllium mineralization,and the Linxi area is a key target area for future beryllium mineral exploration,which deserves full attention in the future research.