Incremental Construction of the Unit 10 Peridotite, Rum Eastern Layered Intrusion, NW Scotland

Hepworth, Luke N.; O’Driscoll, Brian; Gertisser, Ralf; Daly, J. Stephen; Emeleus, C. H.

doi:10.1093/petrology/egx008

Cited by 17 publications

(58 citation statements)

References 88 publications

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“…The increase in peridotite volume relative to allivalite discussed by Bédard and Sparks (1991) has also been observed here. The peridotites can be subdivided into the 'poikilitic peridotite' and 'well-layered peridotite,' respectively, similar to the subtypes found in Unit 10 (Palacz and Tait 1985;Hepworth et al 2017). These subtypes are not ubiquitous across the study area, with one subtype typically dominating the north and south, respectively, e.g.…”

Section: Field Relationshipsmentioning

confidence: 84%

“…This paradigm was first challenged by Bédard et al (1988) who established, based on field evidence, that the Unit 9 and 10 peridotites represented sill-like intrusions, later corroborated for Unit 9 by textural and geochemical evidence in the overlying allivalite (Holness 2005;Holness et al 2007;Leuthold et al 2014). A recent investigation of the Unit 10 peridotite by Hepworth et al (2017) supported an intrusive origin for this body, and showed that much of the peridotite was formed incrementally, via the intrusion of numerous thin sills (represented by harrisite layers). A similar interpretation was applied to the WLI (Hepworth et al 2018).…”

Section: Geological Backgroundmentioning

confidence: 99%

“…Layered intrusions offer an excellent opportunity to investigate the processes of magmatic differentiation and solidification within the upper crust, particularly regarding the formation of crystal mushes (Tepley and Davidson 2003;Holness 2005;Humphreys 2009;Tegner et al 2009;O'Driscoll et al 2007O'Driscoll et al , 2009aHumphreys and Holness 2010;Holness et al 2013Holness et al , 2015Holness et al , 2017aLeuthold et al 2014;Namur et al 2015;Hepworth et al 2017Hepworth et al , 2018. However, the mechanism by which layered intrusions form remains controversial and hotly debated (Latypov et al 2015;Marsh 2015;Latypov 2019;Scoates et al 2019).…”

Section: Introductionmentioning

confidence: 99%

“…It was speculated, based upon their investigation, that many (if not all) of the peridotites represented sills, echoing Harker's (1908) original interpretation. A recent investigation of Unit 10 provided further support for this argument, with emphasis on the role of small volume (incremental) replenishment (Hepworth et al 2017). Although the role of sill emplacement has been recognised in at least two of the macro-rhythmic units of the ELI, its wider applicability has yet to be established, with important implications for the formation of the layered intrusion.…”

Section: Introductionmentioning

confidence: 99%

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Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Hepworth

Kaufmann

Hecht

et al. 2020

Contrib Mineral Petrol

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The Rum Eastern Layered Intrusion (ELI; Scotland) is an open-system layered intrusion constructed of 16 macro-rhythmic units. Each of the macro-rhythmic units consists of a peridotite base and a troctolite (± gabbro) top, previously attributed to the fractional crystallisation of a single magma batch. This classic paradigm has been challenged, however, with evidence presented for the emplacement of peridotite sills in Units 9, 10, and 14, such as cross-cutting relationships, upward-oriented apophyses, and lateral discontinuities. To test whether the other major peridotites within the ELI represent sills, we have carried out new field, petrographic, and mineral chemical analyses of the peridotites in Units 7, 8 and 9. The peridotites display large-and small-scale cross-cutting relationships with the overlying troctolite, indicative of an intrusive relationship. The peridotites also show large-scale coalescence and lateral spatial discontinuities such that the ELI unit divisions become arbitrary. Harrisite layers and Cr-spinel seams found throughout Units 7, 8, and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma. Our observations allow for distinct subtypes of peridotite to be defined, separated by intrusive contacts, allowing for their relative chronology to be determined. Older, poikilitic peridotite, rich in clinopyroxene, is truncated by younger, well-layered peridotite, containing abundant harrisite layers. In addition to the new peridotite subtypes defined in this study, we find strong evidence for laterally oriented metasomatism within clinopyroxene-rich wehrlites at the top of the Unit 8 peridotite. The wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene-rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions. The observations presented in this study from the ELI cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre-existing feldspathic cumulate (gabbro or troctolite). The evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism, forming many of the unusual (often clinopyroxene-rich) lithologies that surround the sills. The broad-scale formation of the layered peridotites via incremental sill emplacement, suggested by the occurrence of upward-oriented apophyses, coalescence, and lateral discontinuity, could be applied to much larger ultramafic intrusions, which might have formed by similar mechanisms.

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Section: Field Relationshipsmentioning

confidence: 84%

Section: Geological Backgroundmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Hepworth

Kaufmann

Hecht

et al. 2020

Contrib Mineral Petrol

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“…Effectively, it means that the Bushveld intrusion may have been built incrementally, as a sill complex. This is not a new idea, as it was suggested for the Rum layered intrusion as early as 1908 by Alfred Harker, and also by Jean Bédard and colleagues (1988), and most recently revisited by Hepworth et al (2017). If this exciting new avenue holds up to further scrutiny and investigation and multiple layered intrusions are shown to contain 'out-of-sequence' layering, it will be a profound development not just in layered intrusion research, but in igneous petrology and the geological sciences more generally.…”

Section: Current Contoversies and New Developmentsmentioning

confidence: 96%

Layered Intrusions: From Petrological Paradigms to Precious Metal Repositories

O’Driscoll

VanTongeren

2017

Elements

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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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Lateral variations in the Unit 7–8 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

Kaufmann

O’Driscoll

Hecht

2020

Contrib Mineral Petrol

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The Rum Layered Suite, NW Scotland, hosts Cr-spinel seams at the bases of peridotite–troctolite macro-rhythmic units in the eastern portion of the intrusion. Here, we present detailed field observations together with microstructural and mineral chemical analyses for the Unit 7–8 Cr-spinel seam and associated cumulates in the Eastern Layered Intrusion. Detailed mapping and sampling reveal significant lateral variations in the structural characteristics and mineral compositions of the Unit 7–8 boundary zone rocks. Although the Cr-spinel seam is laterally continuous over ~ 3 km, it is absent towards the centre and the margins of the intrusion. The compositional characteristics of Cr-spinel and plagioclase vary systematically along strike, exhibiting a chemical evolution towards more differentiated compositions with increasing distance from the main feeder conduit of the Rum intrusion; the Long Loch Fault. On the basis of our combined datasets, we propose that the upper part of the troctolite, the anorthosite layer underlying the Cr-spinel seam and the seam itself formed during a multi-stage magma replenishment event. The stages can be summarised as follows: (1) peridotite schlieren and anorthosite autoliths formed following melt infiltration and cumulate assimilation in the crystal mush of the Unit 7 troctolite. (2) The anorthosite layer then formed from the Unit 7 troctolite crystal mush by thermal erosion and dissolution due to infiltrating magma. (3) Subsequent dissolution of the anorthosite layer by new replenishing magma led to peritectic in situ crystallisation of the Unit 7–8 Cr-spinel seam, with (4) continued magma input eventually producing the overlying Unit 8 peridotite. In the central part of the Rum Layered Suite, the aforementioned assimilation of the troctolitic footwall formed the anorthosite layer. However, the absence of anorthosite in close proximity to the Long Loch Fault can be explained by enhanced thermochemical erosion close to the feeder zone, and its absence close to the margins of the intrusion, at maximum distance from the Long Loch Fault, may be due to cooling of the magma and loss of erosion potential. In line with other recent studies on PGE-bearing chromitites in layered intrusions, we highlight the importance of multi-stage intrusive magma replenishment to the formation of spatially coupled anorthosite and Cr-spinel seams, as well as the lateral mineral chemical variations observed in the Unit 7–8 boundary zone cumulates.

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Incremental Construction of the Unit 10 Peridotite, Rum Eastern Layered Intrusion, NW Scotland

Cited by 17 publications

References 88 publications

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Braided peridotite sills and metasomatism in the Rum Layered Suite, Scotland

Layered Intrusions: From Petrological Paradigms to Precious Metal Repositories

Lateral variations in the Unit 7–8 boundary zone of the Rum Eastern Layered Intrusion, NW Scotland: implications for the origin and timing of Cr-spinel seam formation

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