The Origin of Olivine Basalts from Medvezhya Mount (Avachinsky Group of Volcanoes, Kamchatka): The Evidence for Assimilation of Sulfide-Bearing Cumulates

IF 1 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
D. P. Savelyev, N. V. Gorbach, M. V. Portnyagin, V. D. Shcherbakov
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Abstract

The role and conditions of liquid immiscibility or crystallization of sulfide phase during evolution of subduction-related magmas remains a debated topic, which bears relevance to the genesis of porphyry copper deposits and evolution of the continental crust. We studied rare volcanic rocks with inclusions of magmatic sulfides in olivine—the basalts of Medvezhya Mount in the Avachinsky group of volcanoes. The rocks belong to primitive (Mg# = 66 mol %) middle-K island-arc olivine basalts. Olivine with normal zoning predominates (~98%) among phenocrysts. The olivine compositions are typical for Kamchatka basalts, except for an unusual trend of increase of MnO content from 0.20 to 0.55 wt % and decrease of Fe/Mn from 60 to 35 with a change of olivine composition from Fo87.8 to Fo78.2. Olivine of this group contains numerous inclusions of spinel-group minerals varying in composition from chromium spinel to magnesian magnetite. Olivine phenocrysts with sulfide inclusions are characterized by the absence of or weak reverse zoning and reduced contents of Ca, Ni, Mn, Cr, and Al. The estimated crystallization temperatures are 1036–1241°C for olivine of the prevailing type and 1010–1062°C for sulfide-bearing olivine. The data suggest that crystallization of the main olivine population occurred under relatively shallow conditions and was accompanied by strong magma oxidation. On the contrary, the zoning pattern and compositional features of sulfide-bearing olivine suggest its xenogenic origin and the probable crystallization under deep-crustal conditions from low-temperature water-rich and/or low-Ca magmas. The results obtained confirm the possibility of saturation of oxidized island-arc magmas with sulfide phase at lower crustal conditions, but show that this process is rare and not typical for low-pressure crystallization stage.

Abstract Image

堪察加阿瓦钦斯基火山群Medvezhya山橄榄玄武岩的成因:含硫化物堆积同化的证据
在俯冲相关岩浆演化过程中,液态不混溶或硫化物相结晶的作用和条件一直是一个有争议的话题,这与斑岩铜矿床的成因和大陆地壳的演化有关。我们对阿瓦钦斯基火山群梅德韦日亚山玄武岩中含有岩浆硫化物包裹体的稀有火山岩进行了研究。岩石属原始(mg# = 66 mol %)中钾岛弧橄榄玄武岩。正常分带的橄榄石在斑晶中占主导地位(约98%)。除了MnO含量从0.20 wt %增加到0.55 wt %, Fe/Mn从60 wt %下降到35 wt %,橄榄石组成从Fo87.8变化到Fo78.2外,堪察加玄武岩的橄榄石组成是典型的。该组橄榄石含有大量尖晶石组矿物的包裹体,其组成从铬尖晶石到镁磁铁矿不等。含硫化物包裹体橄榄石斑晶的特征是不存在或弱逆带,Ca、Ni、Mn、Cr和Al含量降低。主要类型橄榄石的结晶温度为1036 ~ 1241℃,含硫化物橄榄石的结晶温度为1010 ~ 1062℃。资料表明,主要橄榄石群的结晶发生在相对较浅的条件下,并伴有强烈的岩浆氧化。相反,含硫化物橄榄石的分带格局和组成特征表明其异种成因,可能是低温富水和(或)低钙岩浆在深地壳条件下结晶形成的。研究结果证实了下地壳条件下氧化岛弧岩浆含硫化物相饱和的可能性,但这一过程较为少见,在低压结晶阶段并不典型。
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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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