皖南伏岭岩体钾长石地球化学特征及其成因意义

伏岭岩体位于江南造山带东段,是皖南地区典型的高分异花岗复式岩体。文章对伏岭岩体花岗岩中钾长石进行了岩相学、矿物微区分析和微量元素模拟计算研究。钾长石分析表明,从Kfs-Ⅰ、Kfs-Ⅱ到Kfs-Ⅲ,钾长石 $ m(\mathrm{K})/m(\mathrm{Rb}) $逐渐降低,Rb和Cs的质量分数逐渐增加,Sr和Ba质量分数逐渐降低。利用钾长石成分(包括K、Rb、Cs)模拟岩浆结晶演化过程,Kfs-Ⅰ和Kfs-Ⅱ可由初始熔体经过0~90%的分离结晶后形成,表明分离结晶作用在伏岭岩体的演化过程中起重要控制作用,而演化程度最高的Kfs-Ⅲ需要初始熔体经过高于95%的分离结晶作用才能形成,而花岗质熔体很难发生如此高程度的分离结晶作用。当熔体因 $ H_{2}O $饱和发生流体出溶作用时,出溶流体将明显富集Pb。由于Ba具有低的流体、熔体分配系数,出溶流体在富集Pb的同时将亏损Ba,Kfs-Ⅲ具有极高的 $ \omega(\mathrm{Pb}) $,同时具有极低的 $ \omega(\mathrm{Ba}) $,表明其形成于流-熔体相互作用。因此,伏岭岩体的富集稀有金属元素特征不仅受控于岩浆演化,而且晚期的流体出溶作用可能是造成稀有金属元素极端富集的重要过程。

The Fuling pluton is located in the eastern section of the Jiangnan orogenic belt and is a typical highly differentiated granitic complex pluton in southern Anhui. In this paper, petrographic, mineral microbeam analysis (including electron microprobe and LA-ICP-MS) and trace element simulation calculations were carried out to study the K-feldspar in the granite of the Fuling pluton. The K-feldspar analysis shows that the K-feldspar $ m(\mathrm{K})/m(\mathrm{Rb}) $ ratio gradually decreases, the mass fractions of Rb and Cs gradually increase, and the mass fractions of Sr and Ba gradually decrease from Kfs-I, Kfs-II to Kfs-III. K-feldspar compositions (including K, Rb, and Cs) were used to simulate magma crystallization evolution, and the results show that Kfs-I and Kfs-II can be formed from the initial melt after 0-90% fractional crystallization, indicating that fractional crystallization plays an important controlling role in the evolution of the Fuling pluton, while the most evolved Kfs-III requires the initial melt to undergo fractional crystallization exceeding 95% to form, but the granitic melt is difficult to undergo such a high degree of fractional crystallization. When the melt is saturated with $ H_{2}O $ and the fluid exsolution occurs, the exsolution fluid will be significantly enriched in Pb. Since Ba has a low fluid-melt partition coefficient, the exsolution fluid will be enriched in Pb, while losing Ba, and the extremely high $ \omega(\mathrm{Pb}) $ and extremely low $ \omega(\mathrm{Ba}) $ of Kfs-III indicate that it was formed by fluid-melt interaction. Therefore, the enrichment of rare metal elements in the Fuling pluton is not solely controlled by magmatic evolution; the late fluid exsolution may be an important process causing the extreme enrichment of rare metal elements.