Genesis of Fe-Ti oxide-bearing ultramafic intrusions in the Duluth Complex, Minnesota, USA

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Journal of Petrology Pub Date : 2024-03-25 DOI:10.1093/petrology/egae031
Jackie M Kleinsasser, Adam C Simon, Dean Peterson, Amartya Kattemalavadi, Ian R Goan, Tobias Keller, George J Hudak, Kaitlin Koshurba
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Abstract

The Duluth Complex is a large mafic intrusive system located in northeastern Minnesota emplaced as part of the 1.1 Ga Midcontinent Rift. Several Fe-Ti oxide-bearing ultramafic intrusions are hosted along the Western Margin of the Duluth Complex, and are discordant bodies present in a variety of geometries, hosted in multiple rock types, and dominated by peridotite, pyroxenite, and semi-massive to massive Fe-Ti oxide rock types. Their origin has been debated, and here we present geochemical evidence and modeling that supports a purely magmatic origin for the Titac and Longnose Fe-Ti oxide-bearing ultramafic intrusions. Ilmenite and titanomagnetite textures indicate a protracted cooling process, and δ34S values of sulfides reveal little assimilation of the footwall Virginia Formation, a fine-grained pelitic unit that contains sulfide-rich bands. We model the crystallization of a hypothetical parental magma composition to the host intrusion of Longnose using Rhyolite-MELTS and demonstrate that the accumulation of Fe-Ti oxides in the discordant intrusions cannot be explained by density-driven segregation of crystallized Fe-Ti oxides. Instead, we show that the development of silicate liquid immiscibility, occurring by the unmixing of the silicate melt into conjugate Si- and Fe-rich melts, can result in the effective segregation and transportation of the Fe-rich melt. The Fe-rich melt is ~2 orders of magnitude less viscous than the Si-rich melt, allowing the Fe-rich melt to be more effectively segregated and transported in the mush regime (crystallinities > 50%). This suggests that viscosity, in addition to density, plays a significant role in forming the discordant Fe-Ti oxide-bearing ultramafic intrusions. We propose a genetic model that could also be responsible for the Fe-Ti oxide-rich layers or bands that are hosted within the igneous stratigraphy of mafic intrusions of the Duluth Complex.
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美国明尼苏达州德卢斯综合体含铁钛氧化物超基性侵入体的成因
德卢斯复合体是位于明尼苏达州东北部的大型岩浆侵入系统,是 1.1 Ga 中大陆裂谷的一部分。沿德卢斯复合体西缘有几处含铁钛氧化物的超基性侵入体,这些侵入体呈不和谐体,几何形态各异,赋存于多种岩石类型中,以橄榄岩、辉绿岩和半块状至块状铁钛氧化物岩石类型为主。它们的成因一直存在争议,在此,我们提出地球化学证据和模型,支持Titac和Longnose含铁钛氧化物超基性侵入体的纯岩浆成因。钛铁矿和榍石的纹理显示了一个漫长的冷却过程,而硫化物的δ34S值则显示了脚墙弗吉尼亚地层(包含富硫化物带的细粒辉长岩单元)的同化程度很低。我们使用 Rhyolite-MELTS 模拟了 Longnose 主侵入体的假定母岩浆成分的结晶过程,并证明不和谐侵入体中铁钛氧化物的累积不能用结晶铁钛氧化物的密度驱动偏析来解释。相反,我们证明了硅酸盐熔体不混合成共轭的富硅和富铁熔体所导致的硅酸盐液不溶解性的发展,可导致富铁熔体的有效偏析和运移。富Fe熔体的粘度比富Si熔体低约2个数量级,这使得富Fe熔体能够在蘑菇云状态(结晶度> 50%)下更有效地分离和运输。这表明,在形成不和谐的含铁钛氧化物超基性侵入体的过程中,除了密度之外,粘度也起着重要作用。我们提出了一个遗传模型,该模型也可能是德卢斯岩群黑云母侵入体火成岩地层中富含铁-钛氧化物层或带的成因。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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