独居石矿物地球化学对南秦岭宁陕地区Be-Nb伟晶岩成因的指示

Geochemistry of monazites from Be-Nb rare-element pegmatites in the Ningshan district,South Qinling Orogen:Implications for pegmatite genesis

近年来,在秦岭造山带宁陕伟晶岩区发现了数条Be-Nb稀有金属伟晶岩矿脉。本文通过扫描电镜和电子探针分析了Be-Nb伟晶岩中独居石的矿物学特征和矿物成分。研究结果表明,伟晶岩脉中的独居石可分为两类:Ⅰ类独居石主要分布在石英、长石等造岩矿物内部,在背散射图像中多数Ⅰ类独居石内部均匀,部分可见振荡环带,具有较高的Th O2含量(7.0%~13.9%,平均10.5%);Ⅱ类独居石单矿物在背散射图像下同样显示内部均匀,但亮度明显低于Ⅰ类独居石,且Th O2含量较低(0.9%~4.4%,平均2.2%)。这指示了Ⅰ类独居石的岩浆成因和Ⅱ类独居石的热液成因。岩浆成因独居石具有较高的Th O2含量与独居石中磷钙钍石的类质同象替代密切相关。岩浆成因独居石的U-Pb测年结果为200.8±2.1 Ma,代表伟晶岩的成岩年龄,该年龄结果接近宁陕岩基中的二长花岗岩。岩浆和热液成因独居石的εNd值的范围为4.6~3.0,这与二长花岗岩的εNd值范围基本一致,说明区内Be-Nb伟晶岩与上述二长花岗岩的同源性。综上所述,本研究认为宁陕地区Be-Nb伟晶岩的形成是宁陕岩基中二长花岗岩同源岩浆在早阶段结晶分异演化后的残留熔浆结晶所致。

In recent years,several Be-Nb rare-element pegmatite dikes have been discovered in the Ningshan district of the Qinling orogenic belt through geological exploration.This study used scanning electron microscopy and electron probe microanalysis to investigate the mineralogical characteristics and mineral composition of monazite in Be-Nb pegmatite.The results indicate that monazite can be divided into two types.The first type mainly occurs in the rock-forming minerals such as quartz and feldspar.Most of the first type monazite exhibit internally homogeneous backscattered electron(BSE)images,while a small portion may show oscillatory zoning.The first type monazite has a higher ThO_(2) content(7.0%~13.9%,averaging 10.5%).In contrast,the second type monazite also exhibits internally homogeneous BSE images,but with noticeably lower brightness compared to the first type.The second type monazite has a lower ThO_(2) content(0.9%~4.4%,averaging 2.2%).This indicates that the first type of monazite has a magmatic origin,while the second type has a hydrothermal origin.Higher concentrations of ThO_(2) in the primary magmatic monazite are associated with an increase in the brabantite molecule substitution.The UPb dating of the magmatic monazite yielded an age of 200.8±2.1 Ma,representing the age of the pegmatite emplacement,which is close to that of the monzonite granite in the Ningshan batholith.The ε_(Nd) values of both the magmatic and hydrothermal monazite range from 4.6 to 3.0,which is consistent with theεNd value range of the monzonite granite.This indicates the genetic relationship between the Be-Nb pegmatite and the monzonite granite.Based on various lines of evidence,this study concludes that the formation of Be-Nb rare-element pegmatites in the Ningshan district is the result of the residual magma crystallization of the co-genetic monzonite granite magma differentiation in the early stage in the Ningshan batholith.