Structural parameters of Cr-bearing spinels and pleonaste from the Cuillin Igneous Complex (Isle of Skye, Scotland): Implications for metamorphic and cooling history

The Outer Layered Suite of the Cuillin Igneous Complex (Isle of Skye, NW Scotland) comprises a Peridotite Series and a younger Allivalite Series (the latter comprising troctolites, eucrites and gabbros). Close to the junction between the Peridotite and the Allivalite Series (but wholly contained within the latter), an ultramafic breccia unit containing abundant peridotite xenoliths crops out. In the Peridotite Series, reddish-brown Cr-bearing spinels are present as disseminated crystals in the peridotite and also as chromitite seams, while in the peridotite xenoliths of the breccia unit, green pleonaste occurs in both of these modes of textural occurrence. Optical absorption spectroscopy reveals that the colour difference between the two spinel phases is related mainly to variable Al, Cr and Fe contents, while crystal structural analysis shows that the cooling rate calculated utilizing the oxygen positional parameter is comparable for all samples. The intracrystalline closure temperature for the Cr-spinel in the Peridotite Series is different for the disseminated and seam textural occurrences of the spinels, while the temperatures yielded by pleonaste in the peridotite xenoliths are the same for both textural occurrences. Our dataset suggests that the pleonaste in the peridotite xenoliths has been heated and equilibrated under subsolidus conditions, probably during breccia formation. During this heating, homogenization of the closure temperatures of pleonaste spinels occurred.