Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)

Leuthold, J. and Blundy, J. D. and Holness, M. B. and Sides, R. (2014) Successive episodes of reactive liquid flow through a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland). Contributions to Mineralogy and Petrology, 168 (1). pp. 1-27. ISSN 0010-7999, 1432-0967 DOI 10.1007/s00410-014-1021-7

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Abstract

We present a detailed microstructural and geochemical study of reactive liquid flow in Unit 9 of the Rum Eastern Layered Intrusion, Scotland. Unit 9 comprises an underlying lens-like body of peridotite overlain by a sequence of troctolite and gabbro (termed allivalite), with some local and minor anorthosite. The troctolite is separated from the overlying gabbro by a distinct, sub-horizontal, undulose horizon (the ‘major wavy horizon’). Higher in the stratigraphy is another, similar, horizon (the ‘minor wavy horizon’) that separates relatively clinopyroxene-poor gabbro from an overlying gabbro. To the north of the peridotite lens, both troctolite and gabbro grade into poikilitic gabbro. Clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro. The poikilitic gabbros contain multiple generations of clinopyroxene, with Cr-rich (~1.1 wt% Cr2O3) anhedral cores with moderate REE concentrations (core1) overgrown by an anhedral REE-depleted second generation with moderate Cr (~0.7 wt% Cr2O3) (core2). These composite cores are rimmed by Cr-poor (~0.2 wt% Cr2O3) and REE-poor to -moderate clinopyroxene. We interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine-phyric picrite to form the discontinuous lenses that comprise the Unit 9 peridotite. Loss of clinopyroxene-saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene-poor gabbro. The spatial extent of clinopyroxene loss is marked by the minor wavy horizon. A second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite, with the major wavy horizon marking the extent of melting during this episode. The poikilitic gabbro formed from clinopyroxene-saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route. This process, called reactive liquid flow, is potentially important in open magma chambers.

Item Type:	Article
Uncontrolled Keywords:	2014AREP; IA68,
Subjects:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title:	Contributions to Mineralogy and Petrology
Volume:	168
Page Range:	pp. 1-27
Identification Number:	10.1007/s00410-014-1021-7
Depositing User:	Sarah Humbert
Date Deposited:	25 Aug 2014 15:43
Last Modified:	07 Sep 2014 01:35
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3109

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