Associations and Formation Conditions of a Body of Melilite Leucite Clinopyroxenite (Purtovino, Vologda Oblast, Russia): an Alkaline–Ultrabasic Paralava

IF 1 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
A. Y. Barkov, A. A. Nikiforov, R. F. Martin, V. N. Korolyuk, S. A. Silyanov, B. M. Lobastov
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Abstract

A novel petrogenetic scheme is discussed for the formation of a melilite leucite clinopyroxenite body from an alkaline–ultrabasic paralava in the Purtovino area. Its protolith was likely a mixture of Upper Permian sedimentary rocks (aleurolite, marl, among others). Degassing, evaporation, and thermal (contact) metamorphism have significantly influenced the petrogenesis to produce a wide diversity of species present in mineral associations. The crystallization of paralava in a shallow setting was accompanied by an intense degassing and vesiculation of the melt, causing locally high porosity in the rock. An elevated degree of oxidation of the initial melt and progressive rise of fO2 were likely related to the H2 loss during the vesiculation and dissociation of H2O. Consequently, ferrian magnesiochromite (Mchr) and chromian spinel (Fe3+-enriched) were the early phases to crystallize; they were followed by members of the magnesioferrite–magnetite series. In situ melting of quartz-bearing and carbonate–clay rocks led to the development of domains of peralkaline felsic glass that surround partially resorbed quartz grains. Numerous grains of wollastonite and rare larnite formed during contact pyrometamorphism. The alkalis increased progressively during crystallization, with a notable enrichment in Na (up to 0.30 apfu) in the åkermanite–gehlenite series. The formation of leucite following melilite is indicated. Euhedral grains of Cpx display concentric cryptic zonation, with a zone of extreme Mg enrichment due to a local deficit in Fe2+. As consequences of the continuing rise in fO2, esseneite crystallized in the rim of zoned clinopyroxene. Two schemes of coupled substitution account for the composition of Cpx grains analyzed in various textural relationships: Mg2+ + Si4+ → (Fe3+ + Al3+) and (Ti4+ + Al3+) + (Na + K)+ → 2Mg2+ + Si4+. The pre-existing grains of olivine (associated with Mchr) were likely replaced completely by sepiolite–palygorskite associated with brownmillerite and its probable Fe3+-dominant counterpart, srebrodolskite. The investigated layer of alkaline microclinopyroxenite is unique in the Russian Plate, and a search is thus required to recognize other pyrogenic products. Also, further research is required to evaluate the contents and volumes of coal (or other sources of hydrocarbons) that could cause spontaneous and long-lasting combustion to form the considerable volume of paralava recognized in the Purtovino area.

Abstract Image

Abstract Image

俄罗斯沃洛格达州普尔托维诺)美徕石褐铁矿体的关联和形成条件:碱性-超基性副熔岩
本文讨论了从普尔托维诺地区的碱性-超基性副熔岩中形成美拉特白云石clinopyroxenite岩体的一种新的岩石学方案。其原岩很可能是上二叠统沉积岩(白云石、泥灰岩等)的混合物。脱气、蒸发和热(接触)变质作用对岩石成因产生了重大影响,从而产生了多种多样的矿物组合。帕拉瓦岩在浅层环境中结晶时,伴随着强烈的脱气和熔体的气泡化,导致岩石局部孔隙率较高。初始熔体的氧化程度升高和 fO2 的逐步上升可能与气泡过程中 H2 的损失和 H2O 的解离有关。因此,铁性菱镁铬铁矿(Mchr)和铬尖晶石(富含 Fe3+)是较早出现的结晶相;随后出现的是菱镁铁-磁铁矿系列。含石英岩和碳酸盐粘土岩的原位熔化导致了围着部分被吸收的石英颗粒的碱性长石玻璃域的形成。在接触热变质过程中,形成了大量硅灰石和稀有的拉氏石晶粒。在结晶过程中,碱性物质逐渐增加,在芒硝-绿帘石系列中,Na的含量明显增加(高达0.30apfu)。这表明白云母是在黑云母之后形成的。Cpx的八面体晶粒显示出同心的隐伏分带,由于局部的Fe2+缺乏,导致Mg极度富集区。由于 fO2 的持续上升,霰石在带状clinopyxene 的边缘结晶。两种耦合置换方案解释了在不同质地关系中分析的霞石晶粒的组成:Mg2+ + Si4+ → (Fe3+ + Al3+) 和 (Ti4+ + Al3+) + (Na + K)+ → 2Mg2+ + Si4+。先前存在的橄榄石(与 Mchr 有关)晶粒很可能完全被与褐铁矿有关的霞石-辉绿岩及其可能以 Fe3+ 为主的对应物--褐铁矿所取代。所调查的碱性微闪长岩层在俄罗斯板块中是独一无二的,因此需要寻找其他热成产物。此外,还需要进行进一步研究,以评估煤炭(或其他碳氢化合物来源)的含量和体积,这些物质可能会导致自燃和持久燃烧,从而形成在普尔托维诺地区发现的大量副熔岩。
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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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