| Summary: | Plumasite-type corundum in ultramafic rocks from the Nattivit area (Isertoq Terrane), in South-East Greenland formed in the Paleoproterozoic Nagssugtoqidian Orogen during convergence and continent collision between Rae Craton and the North Atlantic Craton. The continental crust in Isertoq Terrane, consist of 3.0-2.9 Ga (εNd TDM model) mafic to ultramafic rock and tonalitetrondhjemite-granodiorite sheets with intrusion ages of U/Pb zircon 2841±11 Ma to 2651±7 Ma. The lherzolite and dunite record an incomplete resetting in their isotopes at 2390±70 Ma (Pb-Pb errorchron age), which is interpreted as a time with continental rifting, basaltic volcanism, metasomatism and formation of oceanic floors in the area prior to the convergence started ~ 2.0 Ga. The geochemical data for the mafic to ultramafic rocks in Isertoq Terrane suggest a continental arc tectonic setting. Peak metamorphism in Isertoq Terrane occurred at 1891±9 to 1871±9 Ma at 700-740°C and 7-9 kbar. Pegmatite intrusion occurred in three episodes between 1855±16 and 1735±27 Ma. The second generation (G2) pegmatite triggered corundum formation at 1843±4Ma (U/Pb zircon age) or 1859±19 Ma (Rb/Sr isochron age) at the contact with ultramafic rocks, at estimated P-T conditions close to peak metamorphsm at 680-700° and 7-9 kbar, during the collisional phase. The G2 pegmatites are peraluminous, calc-alkaline and have a granitic to granodioritic composition and classify as the Muscovite Class granitic pegmatite. The Rb-Sr, Sm-Nd, Pb and oxygen isotopes indicate the pegmatite are derived from partial melts from a lower crust mafic granulite or subducted plate. A typical setting for plumasite-type corundum appears to be in the upper plate of a convergent geotectonic setting. The desilication of the pegmatite and metasomatism of ultramafic rocks causes an increase in Ca, Mg, K, Mn, Al and Fe towards the centre of the metasomatic reaction zone, where there additionally is observed an increase in Cr, Ni, Mg, Fe, Sc, Co, V, Zn, Ti, and Mn, derived from the ultramafic ...
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