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December 2017 - Volume 13, Number 6

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Layered Intrusions

Brian O'Driscoll and Jill A. VonTongeren Guest Editors

Table of Contents

Overview

For more than 50 years, layered mafic-ultramafic intrusions have served as natural laboratories for studying magma chamber processes and magmatic differentiation. Further enhancing our understanding of layered intrusion formation will yield valuable insights into the solidification of magmatic systems, the formation of precious metal deposits, and the timescales over which substantial crust-forming episodes occur. This issue of Elements offers an overview of the current state-of-the art in the petrology of layered intrusions and a looks forward to the future challenges in the field.

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2018 Topics

Thematic Articles

Layered Intrusions: From Petrological Paradigms to Precious Metal Repositories

By and

Layered mafic–ultramafic intrusions have occupied a position of central importance in the field of igneous petrology for almost a century. In addition to underpinning petrological paradigms such as cumulus theory, some layered intrusions are exceptionally enriched in base and precious metals, including the platinum-group elements. Technological advances are driving the current and future state-of-the-art in the study of layered intrusions and, looking forward, it is clear that these bodies will continue to inspire and challenge our understanding of magmatic systems and magma solidification for many years to come.

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The Skaergaard Intrusion of East Greenland: Paradigms, Problems and New Perspectives

By , and

The Skaergaard Intrusion of East Greenland is the quintessential example of low-pressure closed-system fractionation of basaltic magma. Field evidence of extensive layering and associated quasi-sedimentary structures, and the resultant ‘cumulate’ paradigm of crystal settling in magma chambers, has led to many long-standing controversies. Of particular significance is the lack of consensus about the microstructural record and the mechanisms by which interstitial liquid is expelled from solidifying crystal mushy zones. Skaergaard remains a cradle for new insights into igneous processes, with recent work highlighting the importance of separation of immiscible liquids on magma evolution.

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Metasomatic Chromitite Seams in the Bushveld and Rum Layered Intrusions

By and

Millimeter–centimeter thick layers of chromite-rich rock (chromitites) are rare, but ubiquitous, features of the Bushveld (South Africa) and Rum (Scotland) layered intrusions. Despite their meager dimensions, the chromitites provide insight into processes that modify igneous layering and, in the Bushveld, the formation of the platinum-group element–rich Merensky Reef. The Merensky Reef chromitites represent reaction zones formed in a compositional gradient between hydrous silicate melt and a crystalline cumulate assemblage, analogous to reaction zones in metamorphic systems. At Rum, the chromitites formed at the melting front between newly injected magma and the magma chamber floor, an analogous process but one driven by thermal, rather than chemical, energy.

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Plagioclase Zonation: An Archive of Trapped Liquid and Crustal Contamination

By , and

Many cumulates in layered intrusions contain plagioclase crystals that are compositionally zoned in terms of their major elements, and, less commonly, in their 87Sr/86Sr isotopic ratios. Major-element zoning in plagioclase is best explained by trapped liquid in the pore spaces between cumulus crystals, which is a result of the complex interplay between the rate of crystal growth and the cooling rate. Isotopic zoning in feldspars likely reflects crystal growth in a magma that is becoming, or has become, isotopically contaminated through wall rock partial melting and assimilation processes. Mineral-scale isotopic zoning, such as detected in plagioclase, can be used to infer the cooling rates of layered intrusions.

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Quantitative Textural Insights into the Formation of Gabbro in Mafic Intrusions

By and

Rock textures provide a key to deciphering the physical processes by which gabbro forms in mafic intrusions. Developments in both direct optical and crystallographic methods, as well as indirect magnetic fabric measurements, promise significant advances in understanding gabbroic textures. Here, we illustrate how bulk magnetic fabric data, particularly from intrusions with sparse silicate-hosted magnetite, may be used to extend direct crystallographic observations from thin sections. We also present a scheme for characterizing crystallographic foliation and lineation and use this to suggest that the strength of gabbro plagioclase foliations and lineations varies significantly with geodynamic environment.

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Layered Mafic–Ultramafic Intrusions of Fennoscandia: Europe’s Treasure Chest of Magmatic Metal Deposits

By and

Northeastern Fennoscandia hosts a rich diversity of mafic–ultramafic intrusions of variable shape and size, emplaced in different tectonic regimes over a period spanning ~600 million years (between 1.88 Ga and 2.5 Ga). Several of the bodies contain world-class ore deposits, notably the Kemi chromium deposit and the Pechenga nickel deposits. Other deposits include nickel and copper at Kevitsa, Kotalahti and Sakatti; vanadium at Koillismaa; and platinum-group elements at Portimo and Penikat. These deposits constitute important resources that could shield Europe from potential future supply shortages of these key industrial metals.

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