Crystal Settling, In-situ Crystallization, Melt Migration, and Magma Recharge in a Differentiated Mafic Sill, Victoria Island, Canada

加拿大维多利亚岛分异基性岩床中的晶体沉降、原地结晶、熔体迁移和岩浆再充。本项目的重点是与加拿大维多利亚岛暴露的元古代富兰克林火成岩事件有关的镁铁质岩床中岩浆的演化。通过与加拿大地质调查局的合作，PI参与了这些岩床的实地测绘，并通过其中一个岩床获得了三个完整的岩心。详细的研究，这个窗台提出破译的分化机制，已运作期间和之后的侵位。岩床相对较薄（26至44米厚），横向广泛（36公里的走向长度），并显着分化为这样一个薄的岩浆体（Mg#s变化系统从35至82）。上部和下部冷却边缘约10%的橄榄石斑晶突然让位于内部的无橄榄石辉绿岩，这表明任何悬浮斑晶迅速沉降，或以某种方式从岩床边缘分离。事实上，在岩床底部附近有一个5至8米厚的富橄榄石区。几乎恒定的橄榄石成分与橄榄石斑岩岩浆的注入和随后的橄榄石沉降是一致的。橄榄石带上方的薄层（1至2米厚）含有丰富的自形辉石。构成岩床上部80%的硅质岩，从基底向上，从顶板向下，逐渐贫镁富铁，直到岩床中心附近的富铁-钛排除元素的“三明治”区。闪长岩的变化趋势不能用橄榄石和普通辉石斑晶从初始岩浆中的去除来解释，而必须是闪长岩就地分异的结果。一个Mg#？踢？在较低的地壳中，可以水平追踪超过10公里，似乎代表了一次岩浆补给事件。虽然岩床是元古代的，但初步数据的地球化学建模表明，主要元素和微量元素都没有发生显著变化。窗台的总体积组成非常接近地匹配冷却边缘组成。岩石学和化学分析的组成矿物的建议解释岩石质地方面可能的侵位后压实和熔体迁移的晶体糊状物。将检查CSD粒度变化，以确定主要矿物的成核和生长历史。拟议研究的目标是评估：[1]富含橄榄石的基底带的起源、压实和再平衡，[2]补给层位上方和下方的矿物结构和成分变化，[3]在岩床底部和顶部的地壳变动中记录的原位分异趋势的起源，以及压实在这两个序列中的作用。更广泛的影响。该项目是与加拿大地质调查局的一个五年期区域测绘方案合作开展的。维多利亚岛岩床在成分和构造背景上与西伯利亚的诺里尔斯克型Ni-Cu-PGE矿化岩床相似;更好地了解其侵位和结晶历史可能会改善类似矿床的勘探策略。虽然这些薄的门槛代表一个极端的分化岩浆房套件的端员，他们可能会提供关键的见解在较大的岩浆房的早期结晶历史的记录已被后来的再平衡过程中缓慢冷却的初始结晶过程。该项目还将为学生参与者提供良好的教育机会。

Crystal Settling, In-situ Crystallization, Melt Migration, and Magma Recharge in a Differentiated Mafic Sill, Victoria Island, Canada Intellectual Merit. This project focuses on the evolution of magmas emplaced in mafic sills associated with the Proterozoic Franklin Igneous event as exposed on Victoria Island, Canada. Through cooperation with the Canadian Geological Survey, the PI has been involved in field mapping of these sills and has access to three complete drill cores through one of the sills. Detailed studies of this sill are proposed to decipher the differentiation mechanisms that have operated during and after emplacement of the intrusion. The sill is relatively thin (26 to 44 m thick), laterally extensive (36 kms of strike length), and remarkably differentiated for such a thin magma body (Mg#s vary systematically from 35 to 82). Upper and lower chilled margins with ~10% olivine phenocrysts give way abruptly to olivine-free diabasic rocks in the interior, suggesting that any suspended phenocrysts settled out rapidly, or were segregated in some way from the sill margins. Indeed an olivine-rich zone 5 to 8 m thick occurs near the base of the sills. Near-constant olivine composition throughout isconsistent with injection of an olivine-porphrytic magma, followed by en-masse settling of olivine. A thin (1 to 2 m thick) layer above the olivine zone contains abundant, euhedral, sector-zoned augite. Diabase, which makes up the upper 80% of the sill, becomes progressively Mg-poor and Fe-rich upward from the base and downward from the roof to an Fe-Ti-excluded element rich "sandwich" zone near the center of the sill. The variation trend in the diabase cannot be explained by the removal of olivine and augite phenocrysts from the initial magma, and must have resulted from in-situ differentiation of the diabase. A Mg# ?kick? in the lower diabase can be traced horizontally for over 10 kms, and appears to represent a magma recharge event. Although the sill is Proterozoic, geochemical modeling of preliminary data, suggests that neither major nor trace elements have been significantly altered. The summed bulk composition of the sill very closely matches a chilled margin composition. Petrography and chemical analyses of the constituent minerals is proposed to interpret rock textures with regards to possible post-emplacement compaction and melt migration in the crystal mush. CSD grain size variations will be examined to determine the nucleation and growth history of the major minerals present. Goals of the proposed study are to evaluate: [1] the origin of, and compaction and re-equilibration within, the olivine-rich basal zone, [2] mineral textural and compositional variations above and below the recharge horizon, and [3] the origin of in-situ differentiation trends recorded in the diabase at the floor and roof of the sill, and the role of compaction in these two sequences. Broader Impacts. This project is being carried out in collaboration with a five-year regional mapping program of the Geological Survey of Canada. The Victoria Island sills are similar in composition and tectonic setting to sills in Siberia that host Noril'sk-type Ni-Cu-PGE mineralization; a better understanding of their emplacement and crystallization history may lead to improved exploration strategies for similar deposits. Although these thin sills represent one extreme end-member of the differentiated magma chamber suite, they may provide critical insights into initial crystallization processes in larger magma chambers where the record of the early crystallization history has been overwritten by later re-equilibration during slow cooling. This project will also provide excellent educational opportunities for the student participants.