Linking in situ crystallisation 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 gravity driven 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.