The Skaergaard trough layering: sedimentation in a convecting magma chamber

Z Vukmanovic,J C Ø Andersen,M B Holness,K Monks

doi:10.1007/s00410-018-1466-1

Z Vukmanovic, J C Ø Andersen + Show 2 more

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https://doi.org/10.1007/s00410-018-1466-1

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Abstract

The upper parts of the floor cumulates of the Skaergaard Intrusion, East Greenland, contain abundant features known as troughs. The troughs are gently plunging synformal structures comprising stacks of crescentic modally graded layers with a sharply defined mafic base that grades upward into plagioclase-rich material. The origin of the troughs and layering is contentious, attributed variously to deposition of mineral grains by magmatic currents descending from the nearby walls, or to in situ development by localised recrystallisation during gravitationally-driven compaction. They are characterised by outcrop-scale features such as mineral lineations parallel to the trough axis, evidence of erosion and layer truncation associated with migration of the trough axis, and disruption of layering by syn-magmatic slumping. A detailed microstructural study of the modal trough layers, using electron backscatter diffraction together with geochemical mapping, demonstrates that these rocks do not record evidence for deformation by either dislocation creep or dissolution–reprecipitation. Instead, the troughs are characterised by the alignment of euhedral plagioclase crystals with unmodified primary igneous compositional zoning. We argue that the lineations and foliations are, therefore, a consequence of grain alignment during magmatic flow. Post-accumulation amplification of the modal layering occurred as a result of differential migration of an unmixed immiscible interstitial liquid, with upwards migration of the Si-rich conjugate into the plagioclase-rich upper part of the layers, whereas the Fe-rich immiscible conjugate remained in the mafic base. Both field and microstructure evidence support the origin of the troughs as the sites of repeated deposition from crystal-rich currents descending from the nearby chamber walls.

Highlights

The many possible mechanisms by which rhythmic layering forms in mafic intrusions are not well-understood, despite a wealth of publications on the subject
We address the specific example of trough layering in the Skaergaard intrusion of East Greenland; a contentious example of rhythmic layering that has generated a dichotomous set of interpretations in the literature since their first description by Wager and Deer (1939)
We build on the detailed observational work of Wager and Deer (1939) [with further details summarised by Wager and Brown (1968)] and the many studies Neil Irvine has published on the subject (Irvine and Stoeser 1978; Irvine 1983; Irvine et al 1998), taking advantage of the relatively recent application of electron backscatter diffraction (EBSD) to igneous rocks (e.g., Morales et al 2011; Satsukawa et al 2013) to generate a detailed picture of grain-scale fabrics that can be tied to field observations

Summary

Introduction

The many possible mechanisms by which rhythmic layering forms in mafic intrusions are not well-understood, despite a wealth of publications on the subject (recently reviewed by Namur et al 2015). Consensus on the mode of formation of any particular type of rhythmic layering derives from the fact that many studies do not involve the synthesis of detailed observations from outcrop- down to the grain-scale. In this contribution, we address the specific example of trough layering in the Skaergaard intrusion of East Greenland; a contentious example of rhythmic layering that has generated a dichotomous set of interpretations in the literature since their first description by Wager and Deer (1939). By building a comprehensive picture of the evidence preserved in the rocks themselves, we argue

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