Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex
Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex
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DOI：
10.1016/j.epsl.2016.03.016
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发表时间：
2016-06-01
影响因子：
5.3
通讯作者：
Holtz, Francois
中科院分区：
文献类型：
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作者：
Fischer, Lennart A.;Wang, Meng;Holtz, Francois
布什维尔德杂岩(南非)是地球上最大的层状侵入体，在我们理解岩浆分异和成矿作用方面发挥了相当大的作用。在这项研究中，我们提供了布什维尔德上带辉长岩堆积物中以磷灰石为宿主的多相包裹体的新的地球化学数据。在高温(1060-1100摄氏度)下重新均一的包裹体在每个岩石样品中显示出一系列成分，从富铁(35wt.%FeOtot；28wt%SiO_2)到富硅(5wt.%FeOtot；65wt.%SiO_2)。这种趋势最好的解释是不混溶的过程，以及在沿着两液相场的双节点逐步冷却过程中，磷灰石晶体中对比的熔体被捕获。利用单一磷灰石颗粒中富硅和富铁不混溶熔体共存的现象，讨论了不混溶熔体相互分离的能力及其对矿物和块体堆积成分的影响。我们认为，不相容的液体没有完全分离，导致两种熔体的类似相结晶，但比例不同。然而，晶体泥中的部分分离以及富铁熔体和富硅熔体产生的不同相比例可能是从黑色(富Fe-Ti-P)辉长岩到白色(富含斜长石)辉长岩的循环演化的原因，这种循环演化在布什维尔德杂岩的上部地带常见，在那里垂直尺度为50至200米。(C)2016 Elsevier B.V.保留所有权利。
The Bushveld Complex (South Africa) is the largest layered intrusion on Earth and plays a considerable role in our understanding of magmatic differentiation and ore-forming processes. In this study, we present new geochemical data for apatite-hosted multiphase inclusions in gabbroic cumulates from the Bushveld Upper Zone. Inclusions re-homogenized at high-temperature (1060-1100 degrees C) display a range of compositions in each rock sample, from iron-rich (35 wt.% FeOtot; 28 wt.% SiO2) to silica-rich (5 wt.% FeOtot; 65 wt.% SiO2). This trend is best explained by an immiscible process and trapping of contrasted melts in apatite crystals during progressive cooling along the binodal of a two-liquid field. The coexistence of both Si-rich and Fe-rich immiscible melts in single apatite grains is used to discuss the ability of immiscible melts to segregate from each other, and the implications for mineral and bulk cumulate compositions. We argue that complete separation of immiscible liquids did not occur, resulting in crystallization of similar phases from both melts but in different proportions. However, partial segregation in a crystal mush and the production of contrasting phase proportions from the Fe-rich melt and the Si-rich melt can be responsible for the cyclic evolution from melanocratic (Fe-Ti-P-rich) to leucocratic (plagioclase-rich) gabbros which is commonly observed in the Upper Zone of the Bushveld Complex where it occurs at a vertical scale of 50 to 200 m. (C) 2016 Elsevier B.V. All rights reserved.