ORIGINAL PAPER Formation of corundum megacrysts during H 2 O-saturated incongruent melting of feldspar: P–T pseudosection-based modelling from the Skattøra migmatite complex, North Norwegian Caledonides Ka ˚re Kullerud • Pritam Nasipuri • Erling J. K. Ravna • Rune S. Selbekk Received: 4 February 2010 / Accepted: 3 May 2012 / Published online: 17 May 2012 Ó Springer-Verlag 2012 Abstract Corundum megacryst-bearing rocks associated with the high-pressure migmatites of the Skattøra migma- tite complex (SMC) belonging to the Nakkedal Nappe Complex, North Norwegian Caledonides, display a classi- cal example of incongruent melting of plagioclase under water-saturated conditions. Petrography and micro-textures suggest that several centimetre long corundum megacrysts formed from the silicate melt along with amphibole (pargasite) and plagioclase (X An * 0.47). The corundum- bearing leucosomes are rich in biotite compared to the other mafic units of SMC. Locally, margarite occurs in coronas around corundum megacrysts. Geochemically, the corundum-bearing rocks are enriched in Al, K, Rb and Ba and depleted in Fe, Mg and Ca compared to the leuco- gabbroic host rock. A P–T pseudosection of the leuco- gabbro indicates that feldspar breakdown and corundum formation occurred at temperatures [ 850 °C and pressure [ 1.2 GPa. The calculated equilibrium P–T of the corun- dum-bearing rock corresponds to 750–825 °C and 0.9–1.1 GPa. The P–T pseudosection of margarite indi- cates that margarite formed after cooling and decompres- sion to P–T conditions corresponding to 600 °C at 0.5 GPa. Based on geochemical and mineral chemical analysis coupled with thermodynamic modelling, we suggest that formation of corundum occurred as a result of high-pres- sure incongruent melting of plagioclase in the presence of a K-, Rb- and Ba-rich external fluid. It is also suggested that the external fluid transported out portions of Ca, Fe and Mg, resulting in an increase of the peraluminousity of the melt and promoting further growth of corundum. Keywords Corundum megacrysts  Plagioclase incongruent melting  High-pressure melting  Pseudosection  Skattøra migmatite complex  North Norwegian Caledonides Introduction Corundum megacrysts have received considerable attention in the geological literature, principally because of the importance of the mineral for gem production. Corundum is a common mineral in a large variety of metamorphic rocks of aluminium-rich compositions. Dependent on the chemistry of its surroundings, corundum is stable at P and T conditions ranging from amphibolite to granulite facies. Since aluminium commonly behaves as a highly immobile element under sub-solidus conditions (e.g. Newton and Manning 2008; and references therein), only small grains (1 mm) of corundum usually form during metamor- phism. However, the formation of corundum megacrysts during metasomatic processes is known from several Communicated by C. Ballhaus. Electronic supplementary material The online version of this article (doi:10.1007/s00410-012-0765-1) contains supplementary material, which is available to authorized users. K. Kullerud (&)  P. Nasipuri  E. J. K. Ravna Department of Geology, University of Tromsø, 9037 Tromsø, Norway e-mail: kare.kullerud@uit.no P. Nasipuri e-mail: pritam.nasipuri@uit.no E. J. K. Ravna e-mail: erling.ravna@uit.no R. S. Selbekk Natural Historic Museum, Box 11722, Blindern, 0318 Oslo, Norway e-mail: r.s.selbekk@nhm.uio.no 123 Contrib Mineral Petrol (2012) 164:627–641 DOI 10.1007/s00410-012-0765-1