Fenner Trend and the Role of Fractional Crystallization and Ferrobasaltic Magma Immiscibility in Granophyre Petrogenesis: the Case of the Mesoproterozoic Valaam Sill in the Ladoga Graben, Karelia

IF 1 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
A. A. Nosova, N. M. Lebedeva, A. A. Vozniak, L. V. Sazonova, I. A. Kondrashov, Y. O. Larionova, E. V. Kovalchuk
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引用次数: 0

Abstract

This paper presents the results of petrographical, mineralogical, geochemical, and isotope-geochemical studies of granophyres and host ferrogabbros, quartz ferromonzogabbros, quartz monzodiorites, and quartz monzonites in the Mesoproterozoic Valaam sill in the Ladoga Graben on the Karelian Craton. The sill is poorly layered: the ferrogabbros compose the lower part of the sill, the middle part consists of quartz gabbro-monzonites and quartz monzonites, while graphic leucogranites (granophyres) form a dense network of veins mainly in the upper part of the sill. Geochemical features of ferrogabbro, iron-rich composition of olivine and pyroxene, and low Ca composition of plagioclase indicate the evolution along the Fenner trend. The granophyres have petro- and geochemical characteristics of anorogenic alkaline granites, with negative Eu/Eu* = 0.15–0.49 and REE distribution patterns similar to those of granophyres of layered intrusions. All rocks of the sill have close Sr isotopic composition (87Sr/86Sr)T = 0.7043–0.7066, and εNd values ranging from −9.6 to −11.2. Model calculations show that fractional crystallization can lead the initial ferrogabbro melt into the immiscibility field. Ilmenite–magnetite–silicate microstructures have been identified in the ferrogabbro and ferromontzogabbro from the sill; similar microstructures in layered intrusions are considered as evidence for the immiscibility of Fe-rich and Si-rich liquids (Holness et al., 2011; Dong et al., 2013). The segregation of the high-silica melt may have occurred in an intermediate crustal chamber at around 350 MPa and 960oC; magma was supplied as crystalline mush at the sill emplacement level at around 70 MPa and acidic melt migrated through it. This melt underwent fractional crystallization and interacted with host minerals. At the level of sill emplacement, it crystallized under supercooling into granophyre aggregates. The example of the Valaam sill shows that when the Fenner fractionation reaches the final composition–-ferrogabbro, its further evolution with a conjugate decrease in SiO2 and Fe contents can be related to the incomplete separation and mixing of Fe-rich melts and immiscibly split felsic melt. Such a mechanism can be implemented for the formation of the mafic part of AMCG-type massifs.

Abstract Image

Abstract Image

芬纳趋势以及碎屑结晶和铁基性岩浆不溶性在花岗岩成岩过程中的作用:卡累利阿拉多加地块中新生代瓦拉岩山的案例
本文介绍了对卡累利阿克拉通拉多加地块中新生代瓦拉姆岩体中的花岗岩和寄主铁闪长岩、石英铁闪长岩、石英闪长岩和石英单长岩进行岩相学、矿物学、地球化学和同位素地球化学研究的结果。岩体的层理较差:铁闪长岩构成了岩体的下部,中部由石英辉长岩-单长岩和石英单长岩组成,而图形白云岩(花岗闪长岩)则主要在岩体的上部形成了密集的矿脉网络。铁闪长岩的地球化学特征、橄榄石和辉石的富铁成分以及斜长石的低钙成分显示了沿 Fenner 走向的演化过程。花岗岩具有成岩碱性花岗岩的岩石学和地球化学特征,负Eu/Eu* = 0.15-0.49,REE分布模式与层状侵入体的花岗岩相似。山体所有岩石的Sr同位素组成(87Sr/86Sr)T = 0.7043-0.7066,εNd值介于-9.6至-11.2之间。模型计算表明,部分结晶可将最初的铁闪长岩熔体带入不熔场。在来自山体的铁闪长岩和铁锰闪长岩中发现了钛铁矿-磁铁矿-硅酸盐微结构;层状侵入体中的类似微结构被认为是富铁液和富硅液不溶性的证据(Holness等人,2011年;Dong等人,2013年)。高硅熔体的偏析可能发生在 350 兆帕和 960 摄氏度左右的中间地壳室中;岩浆以结晶浆的形式供应到 70 兆帕左右的山体喷出层,酸性熔体通过它迁移。这种熔体经历了碎裂结晶,并与寄主矿物相互作用。在山体隆起层面,它在过冷作用下结晶成粒状集合体。瓦拉姆山体的例子表明,当芬纳分馏达到最终成分--铁闪长岩时,其进一步演变与二氧化硅和铁含量的共轭减少可能与富铁熔体和不熔融长熔体的不完全分离和混合有关。这种机制可用于形成AMCG型地块的岩浆部分。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Petrology
Petrology 地学-地球科学综合
CiteScore
2.40
自引率
20.00%
发文量
27
审稿时长
>12 weeks
期刊介绍: Petrology is a journal of magmatic, metamorphic, and experimental petrology, mineralogy, and geochemistry. The journal offers comprehensive information on all multidisciplinary aspects of theoretical, experimental, and applied petrology. By giving special consideration to studies on the petrography of different regions of the former Soviet Union, Petrology provides readers with a unique opportunity to refine their understanding of the geology of the vast territory of the Eurasian continent. The journal welcomes manuscripts from all countries in the English or Russian language.
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