东昆仑大格勒地区富铌橄榄岩矿物学、地球化学及Sr-Nd同位素特征

Mineralogy,Geochemistry,and Sr-Nd Isotopic Characteristics of the Niobium Rich Peridotite in the Dagele Area,East Kunlun

东昆仑大格勒地区碳酸岩-碱性杂岩富铌稀土矿化的发现,对青海省优质稀有稀土矿产的勘查及综合研究具有重要启示。本文以大格勒杂岩体中蛇纹石化富铌橄榄岩为研究对象,开展了矿物学、岩石地球化学和Sr-Nd同位素研究。矿物学分析显示,富铌橄榄岩中铌矿物主要为铌易解石(Nb_(2)O_(5)=50.33%~52.50%)、铌钙矿(Nb_(2)O_(5)=48.20%~50.27%)、褐铈铌矿(Nb_(2)O_(5)=42.80%~43.45%)、Stefanweissite矿(Nb_(2)O_(5)=33.14%~34.78%),次为钛铁矿(Nb_(2)O_(5)=3.45%~4.89%)。样品具低SiO_(2)(20.16%~29.20%)和K_(2)O+Na_(2)O(0.02%~2.47%)含量,高Mg O(22.88%~27.69%)含量和Mg^(#)值(74~82),稀土元素总量较高(∑REE=772~1807μg/g),稀土元素配分模式强烈富集LREE、亏损HREE,Eu异常不明显(δEu=0.87~1.17)。原始地幔标准化微量元素蛛网图显示,样品富集Nb、Ta、Th、U、Sr、Pb,Nb含量高,为981~1753μg/g,平均1378μg/g,可能与晚阶段碳酸岩侵入流体交代作用有关。样品的(87Sr/86Sr)i值为0.70378~0.70396,ε_(Nd)(t)值为-0.49~0.31,岩浆源区可能为EMⅠ型地幔端元。综合区域构造背景,大格勒岩体中富铌橄榄岩的形成可能与该地区拉张环境下岩石圈的强烈伸展和软流圈地幔的上涌相关。

The discovery of niobium rich rare earth mineralization in a carbonatite alkaline complex in the Dagele area of East Kunlun has important implications for the exploration and comprehensive research of high-quality rare earth mineral resources in Qinghai province.This article reports the mineralogical,geochemical,and stable isotopic compositions of the niobium rich rare earth mineralized peridotite in the Dagele complex.The results show that the niobium minerals in the niobium rich peridotite are mainly niobium-aeschynite(Nb_(2)O_(5)=50.33%-52.50%),fersmite(Nb_(2)O_(5)=48.20%-50.27%),brocenite(Nb_(2)O_(5)=42.80%-43.45%),Stefanweissite ore(Nb_(2)O_(5)=33.14%-34.78%),and ilmenite(Nb_(2)O_(5)=3.45%-4.89%).The peridotite samples have SiO_(2)ranging from 20.16%to 29.20%,Al_(2)O_(3)from 4.36%to 7.13%,FeO^(T)from 11.31%to 15.12%,MgO from 22.88%to 27.69%,Mg^(#)values from 74 to 82,P_(2)O_(5)from 6.72%to 12.57%,with low K_(2)O+Na_(2)O ranging from 0.02%to 2.47%.The peridotites are enriched in LREE and HSFE,showing steep right-declined REE patterns and obvious Nb,Ta,Th,U,Sr,Pb,and LREE enrichments on the primitive mantle normalized trace element spider diagrams.The peridotites have Nb contents varying from 981μg/g to 1753μg/g,with an average of 1378μg/g,possibly related to the late stage metasomatism.The(87Sr/86Sr)i(0.70378 to 0.70396)andε_(Nd)(t)values(-0.49 to 0.31)of the peridotite indicate a magma source of EM I type.Formation of the Nb-rich peridotite may be related to strong lithospheric extension and upwelling of the asthenosphere mantle under a regional extensional tectonic regime.