Ilmenite-magnetite geothermometry in trondhjemites from the Scourian complex of NW Scotland

Abstract

THE crystallization history of four trondhjemite samples from the Scourian complex, NW Scotland, has been investigated using composite ilmenitemagnetite grains. A variety of compositions are present both as largeand small-scale exsolution lamellae, which can be used to unravel the complex cooling history of these rocks. The samples were collected near Upper Badcall, Sutherland, where intrusive trondhjemite sheets I-2 m thick cut banded gabbro. The trondhjemites have a complex history that includes four stages: magmatic intrusions, granulite facies metamorphism, hydration and retrogression to amphibolite facies, and slow cooling with uplift. Ilmenite-magnetite grains in samples HR. 49, 53, 86 display a complex exsolution pattern (fig. IA). An original titanomagnetite exsolved into large-scale (up to 5o pm wide) ilmenite-magnetite lamellae from which have subsequently exsolved small-scale lamellae (c.4 pm wide) parallel to the earlier lamellae. The ilmenite-magnetite pairs form subhedral