Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills

Crystal settling and convection in the Shiant Isles Main Sill

“…Although the shape of the Fig. 1 Simplified geological map of the Shiant Isles, after Gibb and Henderson (2006). The Lower Sill is not of concern here, while the two components of the Main Sill described in this contribution are the picrodolerite/crinanite unit (PCU) and the Lower Picrite.…”

“…1) and comprises the bulk of the sill. It was formed from an olivine-phyric magma containing ~ 10 vol.% olivine phenocrysts, together with 1-2 wt% Cr-spinel and a small amount of plagioclase (Gibb and Henderson 2006), that was emplaced high in the picrite unit before the host rock was completely solidified, splitting the picrite into upper (~2 m thick) and lower (~22 m thick) leaves. Gibb and Henderson (1992, 1996 suggest that the olivine phenocrysts settled to the floor of the 135 m-thick unit to form a picrodolerite, leaving an essentially aphyric magma that crystallised to form the remainder (crinanite) of this unit.…”

“…GOP shows the stratigraphic distribution of the granular olivine picrodolerite (see text for details)), together with the stratigraphic variation of olivine mode (from Henderson et al 2000), bulk rock Cr content in Hole 1 (from Fig. 8 of Gibb and Henderson (2006). Location shown in Fig.…”

The geochemical effects of olivine slurry replenishment and dolostone assimilation in the plumbing system of the Franklin Large Igneous Province, Victoria Island, Arctic Canada

“…11. The geochemical effects of olivine slurry replenishment and mixing with a resident gabbroic mush described above have implications for other systems where slurry emplacement has been inferred (e.g., Gibb and Henderson (2006); Bédard et al (2007);Marsh (2013)). The isotopic constraints imply that the emplacement dynamics of these heterogeneous olivine slurry replenishment events are similar to the "braided sills" differentiation model of the Torres Del Paine Mafic Complex in Patagonia (Leuthold et al 2014).…”

“…It has also been proposed that the abundant phenocrysts seen in some basalts are remobilized cumulates that were flushed out of underlying feeder systems (Larrea et al 2012;Passmore et al 2012). Examples of crystal slurry emplacement have been documented in the Ferrar sills of Antarctica (Bédard et al 2007); the Shiant Isles sills in NW Scotland (Gibb and Henderson 2006); and the Franklin sills on Victoria Island (Hayes et al 2015). The emplacement of crystal slurries is also pertinent for the construction of large layered intrusions such as the Bushveld Complex (Mondal and Mathez 2007; Roelofse and Ashwal 2012;Wilson 2012).…”