In situ fractional crystallization of a mafic pluton: Microanalytical study of a Palaeogene gabbronorite plug in East Greenland

Spot analysis techniques have been used to unravel crystallization processes in a Palaeogene coarse-grained, olivine-bearing, gabbronoritic plug from East Greenland. Whole-rock analyses of the gabbronorite and its chilled margin are identical to within analytical error, yet the gabbronorite is coarse-grained and has large, strongly zoned pyroxene, olivine and plagioclase crystals. The textures of both the gabbronorite and its chilled margin suggest high crystal growth rates. Electron- and ion-microprobe and LA-ICP-MS analyses of clinopyroxene crystals reveal a complex zonation pattern, with a large core segment dominated by oscillatory zoning, and strong normal zoning towards the rim. The general zoning pattern of Fe-Mg and Sr, Ti, Cr, Ce, Y, and Zr in clinopyroxene and orthopyroxene can be explained in terms of fractional crystallization of the four-phase assemblage plagioclase-clinopyroxene-orthopyroxene-olivine, with phase proportions similar to the mode of the gabbronorite. The oscillatory zoning of clinopyroxene is most likely a result of the development of transient chemical boundary layers between the fast-growing clinopyroxene crystal and the bulk melt reservoir. Calculated melt compositions in equilibrium with clinopyroxene cores from the gabbronorite are close to the composition of the chilled margin for most elements, thus lending support to this approach for estimating the composition of melts parental to plutonic rocks, in which post-cumulus equilibration with migrating interstitial melts is limited.