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

加拿大维多利亚岛差异化镁铁基岩床中的晶体沉降、原位结晶、熔融迁移和岩浆补给

• 批准号: 1220374
• 负责人: H. Richard Naslund
• 金额: $ 17.06万
• 依托单位: SUNY at Binghamton
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-07-15 至 2017-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1220374&HistoricalAwards=false
• 关键词: Crystal; Settling; situ; Crystallization; Melt

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.

加拿大维多利亚岛差异化镁铁基岩床中的晶体沉降、原位结晶、熔融迁移和岩浆补给 智力优点。该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相关的镁铁质基岩中岩浆的演化。通过与加拿大地质调查局合作，项目负责人参与了这些基台的现场测绘，并通过其中一个基台获得了三个完整的钻芯。建议对该窗台进行详细研究，以破译在入侵期间和之后发挥作用的分化机制。岩床相对较薄（26 至 44 m 厚），横向范围广泛（走向长度为 36 km），并且对于如此薄的岩浆体而言差异显着（Mg#s 在 35 至 82 之间系统变化）。含有约 10% 橄榄石斑晶的上部和下部冷边缘突然让位于内部不含橄榄石的双基岩，这表明任何悬浮的斑晶都迅速沉降，或者以某种方式与基台边缘分离。事实上，靠近基部的地方存在着 5 至 8 m 厚的富含橄榄石的区域。整个过程中几乎恒定的橄榄石成分与橄榄石斑岩岩浆的注入以及随后橄榄石的整体沉降一致。橄榄石带上方的薄层（1 至 2 m 厚）含有丰富的自形扇形辉石。辉绿岩构成了窗台上部 80%，从底部向上，从顶部向下，逐渐变得贫镁和富铁，直至窗台中心附近的富含 Fe-Ti 元素的“三明治”区域。辉绿岩的变化趋势不能用橄榄石和辉石斑晶从初始岩浆中去除来解释，而一定是辉绿岩原位分异的结果。 Mg#？踢？在下辉绿岩中，可以水平追踪超过 10 公里，似乎代表了岩浆补给事件。尽管基台是元古代的，但初步数据的地球化学模型表明，主要元素和微量元素都没有发生显着改变。窗台的总体积成分与冷却边缘成分非常接近。建议对组成矿物进行岩相学和化学分析，以解释岩石结构，考虑到可能的后压实和晶糊中的熔体迁移。将检查 CSD 晶粒尺寸变化以确定存在的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]补给层上方和下方的矿物结构和成分变化，以及[3]在基台底部和顶部的辉绿岩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五年区域测绘计划合作进行的。维多利亚岛岩台的成分和构造环境与西伯利亚的岩台相似，西伯利亚的岩台含有 Noril'sk 型 Ni-Cu-PGE 矿化体；更好地了解它们的侵位和结晶历史可能会改进类似矿床的勘探策略。尽管这些薄基台代表了差异化岩浆房套件的一个极端成员，但它们可能为较大岩浆房中的初始结晶过程提供重要的见解，其中早期结晶历史的记录已被后来缓慢冷却过程中的重新平衡所覆盖。该项目还将为学生参与者提供极好的教育机会。