The petrogenesis of highly fractionated gem-bearing pegmatites of Malawi: evidence from mica and tourmaline chemistry and finite step trace element modelling

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Tuhin Chakraborty, Steffen H. Büttner, Gelu Costin, Charles F. Kankuzi
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Abstract

Late-Pan-African granitic pegmatites in Malawi host gem mineralization (tourmaline, beryl/aquamarine/heliodor). We use major and trace element chemistry of mica and tourmaline as proxies to describe the geochemical characteristics and to analyze the evolution of the pegmatite-forming melts. Trace element contents and ratios of pegmatitic micas and tourmalines show characteristic fractionation trends. Mica from highly fractionated pegmatite typically shows high Rb, Cs, Zn, Nb, Ta, F, and Li concentrations but low Ni, Co, V, Ti and Sc concentrations. In their less fractionated counterparts, these compositional patterns are largely reversed. Exceptions in these element patterns are related to the presence or absence of other phases that may fractionate specific elements more strongly than mica. Tourmaline shows similar fractionation trends in major and trace elements. The observed patterns indicate fractional crystallization as the dominant process of melt evolution. A near exponential decrease of alkali element ratios, such as K/Rb and K/Cs, and an increase in Rb, Cs and Li in white mica from the less to the more strongly differentiated zones suggest Rayleigh fractional crystallization. The modelling of these element ratios shows that in different pegmatite bodies the least differentiated zone formed at a fractionation coefficient of F = 0.35–0.5. Zones of intermediate fractionation show F = 0.85–0.9. Gem mineralization is associated with the most highly fractionated pegmatites or pegmatite zones (F = ~ 0.99). These highly fractionated pegmatites show strong enrichment of Li, Rb and Cs in mica and tourmaline forming from melts rich in incompatible elements. The crystallization of gem phases depended on this highly enriched environment.

Abstract Image

马拉维高分选含宝石伟晶岩的岩石成因:来自云母和电气石化学和有限步微量元素模拟的证据
马拉维泛非洲晚期花岗质伟晶岩中有宝石矿化(电气石、绿柱石/海蓝宝石/日光石)。以云母和电气石的主微量元素化学为代表,描述了其地球化学特征,分析了伟晶岩形成熔体的演化过程。伟晶云母和电气石的微量元素含量和比例表现出特有的分选趋势。高分选辉晶岩云母的Rb、Cs、Zn、Nb、Ta、F和Li含量高,而Ni、Co、V、Ti和Sc含量低。在分馏程度较低的对应物中,这些组成模式在很大程度上是相反的。这些元素模式中的例外与其他相的存在或不存在有关,这些相可能比云母更强烈地分馏特定元素。电气石的主微量元素分选趋势相似。观察到的模式表明,分数结晶是熔体演化的主要过程。白云母中碱元素比值(如K/Rb和K/Cs)呈指数型下降,而Rb、Cs和Li从分异较弱的带向分异较强的带增加,表明存在瑞利分异结晶。这些元素比值的模拟表明,在不同的伟晶岩体中,分异最小带形成于分异系数F = 0.35 ~ 0.5。中间分馏区F = 0.85 ~ 0.9。宝石矿化与分异程度最高的伟晶岩或伟晶岩带有关(F = ~ 0.99)。这些高分异的辉晶岩在云母和电气石熔体中富集Li、Rb和Cs。宝石相的结晶依赖于这种高度富集的环境。
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来源期刊
Mineralium Deposita
Mineralium Deposita 地学-地球化学与地球物理
CiteScore
11.00
自引率
6.20%
发文量
61
审稿时长
6 months
期刊介绍: The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.
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