新疆西南天山高压-超高压变质带中磷灰石的地球化学研究

摘要

作为不同地质环境中普遍发育并能稳定存在的副矿物,磷灰石是卤素和REE、Sr、Y等微量元素的重要载体,对研究岩石成因、流体成分及演化等具有重要的指导意义.新疆西南天山高压-超高压变质带的榴辉岩及高压脉体中普遍发育磷灰石.磷灰石主要以矿物包体、粒间颗粒及脉体矿物的形式产出,本文主要以后两种粒径大于0.1mm的磷灰石为研究对象.电子探针分析结果表明,不同类型的磷灰石均为氟磷灰石,F含量变化范围为1.44％～3.56％,Cl含量则在0.09％以内变化,反映了与之平衡的流体盐度较低.LA-ICP-MS微区原位分析结果表明,磷灰石均相对富集Sr、Y和REE,亏损大离子亲石元素和高场强元素,稀土元素总量变化范围为10×10-6～660×10-6,变化较大.根据稀土元素总量和分馏情况,可以把样品中的磷灰石分为稀土元素总量较低的轻稀土亏损型、稀土元素总量适中的平坦型和稀土元素总量较高的中稀土富集型3类.自形大颗粒的磷灰石具有从核部到边部稀土元素含量明显降低的成分环带,可能反映了磷灰石的生长过程,并暗示了流体成分的变化.%Apatite is a ubiquitous accessory mineral in many geological settings and is stable over a wide range of pressure and temperature.With halogens,rare earth elements (REEs),strontium,yttrium and other minor elements,apatite can reflect the formation of rocks and change of fluid composition.Apatite is one of the most common accessory minerals in eclogites and high pressure veins from HP-UHP metamorphic belt in Chinese southwest Tianshan Mountains,Xinjiang.The occurrences of apatites include inclusions,intergranular grains and vein apatites.This study mainly focused on the apatites whose diameters are larger than 0.1 mm and which were formed at or after the stage of peak metamorphism.According to the results of major element analysis,all the apatites are fluorapatites.The concentration of fluorine ranges between 1.44 ％ and 3.56％,and that of chlorine is below 0.09 ％,which suggests that the salinity of the fluid in equilibrium with apatites is low.The results of LA-ICP-MS analysis suggest that all the apatites have relatively high concentrations of Sr,Y and REEs,low concentrations of large ion lithophile elements (LILEs,such as Rb and Ba) and high field-strength elements (HFSEs,such as Zr,Nb,Hf,Ta and Ti),and various values of total rare earth elements (∑REE) from 10 × 10-6 to 660 × 10 6.Based on the ∑REE and their differentiation,all the samples can be divided into three types:LREEs enrichment with low ∑ REE,flat pattern with medium ∑ REE and MREEs enrichment with high ∑REE.An euhedral elongated apatite displays a growth zoning with the concentrations of REEs gradually decreasing from core to rim,which may reflect the growing process of the apatite and change of fluid composition.