新疆拜城县波孜果尔碱性花岗岩体中角闪石与黑云母地球化学特征及其对成岩成矿的记录

角闪石和黑云母是含稀有稀土金属矿物的高分异碱性花岗岩中常见的暗色矿物。角闪石和黑云母对不同元素(尤其是Nb、Ta、Zr、REE等稀有稀土金属元素)的相容性特点使它们的主微量元素特征成为岩浆演化的"日志",对构建含矿岩浆的成岩成矿模型具有一定意义。本文针对新疆拜城波孜果尔碱性花岗岩体中的角闪石和黑云母,利用电子探针(EPMA)和激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)等分析技术,开展精细的矿物学研究,综合厘定Nb、Ta等元素在岩浆结晶过程中的富集和分配过程。波孜果尔岩体中的角闪石均为亚铁钠闪石,总体表现为较高的Na和Fe含量,而相对贫Ca(0.15 apfu)、Mg(0.06 apfu)和Ti(0.07 apfu),成分的均一性以及轻稀土元素的亏损特征暗示了角闪石较晚结晶且角闪石结晶前大量富轻稀土元素矿物结晶分离。根据共生关系黑云母可分为A、B两类,它们显示出不同的化学成分。A类黑云母具有相对较高的F含量(A类为3.81%～4.82%,B类为1.68%～3.65%)和SiO_2含量(A类为41.5%～43.3%,B类为35.9%～38.5%),相对较低的TFeO含量(A类为28.0%～31.4%,B类为32.2%～35.7%)和Al_2O_3含量(A类为6.45%～7.62%,B类为9.70%～11.00%)。黑云母与角闪石中氟的含量变化指示了氟在结晶过程中的富集、饱和过程以及氟饱和引起的稀有稀土金属矿物大量结晶现象。由于在主要矿物中的不相容性,成矿元素Nb、Ta在熔体中含量逐渐上升,但主要矿物及大部分副矿物中均高于全岩的Nb/Ta值产生了"Ta丢失"现象。结合前人研究,本文认为气热相带出与锆石的结晶分离两种机制共同作用导致Ta从熔体中分离。

Chemical characteristics of amphibole and biotite from the Boziguo'er alkaline granitic pluton in Baicheng County, Xinjiang, and their implications for petrogenesis and mineralization

Amphibole and biotite are dark-colored minerals commonly found in highly differentiated alkaline granite containing REE-bearing and rare metal-bearing minerals. The different compatibilities of amphibole and biotite for different elements (especially rare metals and rare earth metals such as Nb, Ta, Zr and REE) make their characteristics of major and trace elements become the "log" of magma evolution, which has a significance in the reconstruction of the petrogenesis and mineralization process of ore-bearing magma. In order to determine the enrichment and distribution process of elements such as Nb and Ta during the magmatic crystallization, the authors studied mineral chemistry of amphibole and biotite in the Boziguo''er alkaline granite pluton in Baicheng, Xinjiang, by means of electron microprobe (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The amphibole in the pluton is arfvedsonite, which is generally characterized by high Na and Fe and relatively poor Ca (<0.15 apfu), Mg (<0.06 apfu) and Ti (<0.07 apfu). The uniformity of the composition and the deficit of LREE suggest that the amphibole was crystallized at the late stage and a large number of LREE-rich minerals were crystallized before amphibole. According to the association relationship, the biotite is divided into two types(type-A and type-B)in this paper, which have different chemical compositions. Compared with type-B, type-A has a higher content of F (3.81%~4.82% in type-A and 1.68%~3.65% in type-B) and SiO₂ (41.5%~43.3% in type-A and 35.9%~38.5% in type-B), and a lower content of TFeO (28.0%~31.4% in type-A and 32.2%~35.7% in type-B) and Al₂O₃ (6.45%~7.62% in type-A and 9.70%~11.00% in type-B). The different values of fluorine in biotite and amphibole indicate the enrichment and saturation process of fluorine in the crystallization process and a large number of REE-bearing and rare metal-bearing minerals crystallized on account of fluorine saturation. Due to the incompatibility in the main minerals, the ore-forming elements Nb and Ta should be gradually enriched in the melt, whereas the Nb/Ta ratios of the main minerals and most of the secondary minerals are higher than those of the bulk-rock, resulting in "Ta-missing". Combined with previous studies, it is believed that the combination of the volatile saturation and the crystallization of zircon caused the separation of Ta from the melt.