Primitive olivine melanephelinite dykes from the Orkney Islands, Scotland

Abstract

Two suites of Permian dykes in the Orkney Islands consist of a predominantly ENE trending camptonite suite and a mainly N-to NE-trending suite formerly referred to as monchiquitic. Some of the silica-poor and more magnesium dykes in the latter suite can be more precisely defined as olivine melanephelinites. Within the parochial context of the British Isles, such compositions appear to be unique. The magmas are deduced to have been primitive, small-scale (? < 0.2%) partial melt products of the asthenospheric mantle that have experienced relatively little modification in transit through the lithosphere.

The tectonomagmatic evolution of Scotland

“…of the monchiquites (see section 7.5). Upton et al (1992) suggested that primitive olivine melanephelinite dykes from the Orkney Islands were formed by very small-scale melting (<0•2%) of asthenospheric mantle, extra interest being given to this occurrence by the fact that it is the only known example of this lithology in the UK.…”

Carboniferous and Permian magmatism in Scotland

“…Among the most primitive melts reaching near-surface levels (with > 12 wt% MgO) were the Late-Permian olivine melanephelinite dykes in Orkney (Upton et al 1992). In a general review of the geochemistry of the alkaline basic dykes in the Highlands and Islands, Baxter (1987) concluded that the magmas resulted from small degrees (0.5-2%) of partial melting of a chemically heterogeneous garnet lherzolite mantle source.…”

Helium isotope signature of lithospheric mantle xenoliths from the Permo-Carboniferous magmatic province in Scotland — no evidence for a lower-mantle plume

“…The 1 mwide xenolith-bearing dyke sampled at Rinnibar, South Ronaldsey is one of a series of monchiquite dykes intruded in the area during the Permo-Carboniferous. The dykes are primarily volatile-rich alkali olivine basalts with kaersutite _+ biotite (Upton et al 1992). The Rinnibar dyke is composed largely of xenoliths, predominantly of spinel lherzolite, but also of harzburgites, wehrlites and clinopyroxenites with little interstitial host basalt (Bonadiman et al 2002).…”