Linking In Situ Crystallization and Magma Replenishment via Sill Intrusion in the Rum Western Layered Intrusion, NW Scotland

Abstract

The construction of layered mafic-ultramafic intrusions has traditionally been attributed to gravitydriven accumulation, involving the mechanical settling of crystals onto the magma chamber floor, at the interface between the crystal mush at the base and overlying replenishing magma, such that the layered sequence of cumulates (i.e., the crystal mush) at the floor aggrades upwards. The Rum Western Layered Intrusion (WLI) is a ~250 m sequence of layered peridotite cumulates comprising the structurally lowest portion of the Rum Layered Suite (RLS). As such, it is taken to represent the oldest sequence in the RLS and has been assumed to young upwards. The WLI hosts the largest proportion of harrisite, a cumulate composed of skeletal olivine that formed by in situ crystallisation, in the Rum layered intrusion. Harrisite layers in the WLI ubiquitously exhibit extremely irregular upward-oriented apophyses, up to several metres high and metres across, alongside laterally extensive dome-like structures; features consistent with intrusive, sill-like emplacement of harrisite. The distribution and abundance of harrisite therefore points to chaotic sill-like emplacement of the magmas that produced at least half of the WLI cumulate. This probably occurred various ambient crystal mush temperatures and punctuated intervals during cumulate formation. The harrisite layers are associated with numerous Cr-spinel seams occurring along the tops, bases, and interiors of these layers, suggesting they formed in situ alongside harrisite sills within the crystal mush. Detailed quantitative textural and mineral chemical analysis of Cr-spinel seams support a simple in situ crystallisation process for their formation. It is suggested the Cr-spinel seams form within melt channels that develop along the same hot tears that allowed the harrisite parental melts to enter the crystal mush. The chemistry and texture of Cr-spinel is controlled by the volume of through-flow of melt through the melt channel. Where melt flux through channels was high, sulphide and platinumgroup minerals are more abundant, highlighting the key economic implications of this model for the platinum-group element enrichment of chromitite horizons in layered intrusions. We also highlight the role of infiltration metasomatism at multiple levels of the WLI, where porous percolation of interstitial melt and reactive liquid flow played a key role in cumulate formation, supporting the notion of layered intrusion growth by incremental sill emplacement.

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

“…Hepworth et al (2017) showed that harrisite layers found in the Unit 10 peridotite represent sills based largely on their topology. For example, they highlighted the presence of small-scale features such as upward-oriented apophyses, bifurcation, and lateral discontinuities, which must have formed within the crystal mush and not upon the magma chamber floor (see also Donaldson 1982;Hepworth et al 2018). The harrisite layers found throughout the welllayered peridotites in this study display the same features as those described in the Unit 10 peridotite ( Fig.…”

“…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). Despite the lack of layer continuity typical of sills in the ELI and WLI, intrusive peridotite is likely a major component of the CI (Volker and Upton 1990), contrary to the classic model (e.g.…”

“…1b). The WLI is composed of a thick sequence of layered peridotites which generally dip shallowly towards the east (Wadsworth 1961;Hepworth et al 2018), including the type locality for the skeletal-olivine textured peridotite 'harrisite' (Harker 1908;Wager et al 1960;Wadsworth 1961;Donaldson 1974). The ELI consists of 16 macro-rhythmic units, each consisting of a peridotite base, and a troctolite ± olivine gabbro top that dips shallowly towards the west (Brown 1956;Volker and Upton 1990;Fig.…”

“…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).…”

“…By recognising these additional subtypes, we can place tighter constraints on the relative chronology of complex sequences of crystal mushes, many layers of which may represent metasomatic reactions between a pre-existing lithology and younger sills (e.g. Hepworth et al 2018).…”