Melt-rich segregations in the Skaergaard Marginal Border Series: Tearing of a vertical silicate mush

Humphreys, M. C. S. and Holness, M. B. (2010) Melt-rich segregations in the Skaergaard Marginal Border Series: Tearing of a vertical silicate mush. Lithos, 119 (3-4). pp. 181-192. DOI 10.1016/j.lithos.2010.06.006

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Abstract

The Marginal Border Series (MBS) of the Skaergaard Intrusion crystallised on the steeply dipping side-walls of the magma chamber. Melt-rich segregations, previously described as the [`]wavy pyroxene rock' are a key feature of the otherwise homogeneous outermost parts of the MBS. The lens-shaped segregations consistently strike parallel to the chamber wall and have a moderate dip towards it. The shape, size, grainsize and mineralogy of the segregations evolves systematically away from the chamber wall. The segregations become bigger, more widely spaced, chemically more evolved and more irregular in shape with increasing distance from the margin. The segregations were previously interpreted as a nucleation effect parallel to the thermal gradient. However, they dip toward the margin, not parallel to it. We offer a new interpretation: that the segregations represent tearing of poorly consolidated crystal mush during localised sagging of the vertical solidification front. The tears form during a process analogous to [`]hot tearing' of metal alloys, although the driving force for tearing is probably gravitational collapse of the vertical solidification front, or perhaps contemporaneous faulting during chamber filling. Small, regular tears formed in the thinner, more rapidly cooled outer parts of the MBS, while the larger, irregular tears formed in the coarser grained, inner parts. Movement of interstitial liquid fed the cracks after tearing, and the liquid crystallised as relatively evolved, coarse-grained segregations. The mineral chemistry and texture of the segregations suggests that the tearing occurs relatively early in the crystallisation history.

Item Type: Article
Uncontrolled Keywords: 2010AREP; IA61;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Lithos
Volume: 119
Page Range: pp. 181-192
Identification Number: 10.1016/j.lithos.2010.06.006
Depositing User: Sarah Humbert
Date Deposited: 21 Jul 2010 09:03
Last Modified: 23 Jul 2013 09:57
URI: http://eprints.esc.cam.ac.uk/id/eprint/1478

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