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 rock. In the reaction zone in the lherzolite tschermakite and biotite are formed first in centre of the metasomatic reaction zone, and are followed by plagioclase, hercynite, dolomite, and corundum. The corundum samples studied contain more Fe than Cr, and corundum hosted in the lherzolite have a higher Ga content than the corundum hosted in dunite.
The thermodynamic fluid composition modelling with the Perple_X suite of programs for a metasomatic reaction zone in lherzolite, with plumasite-type corundum, show that the pegmatite fluid have pH of 0.7 units above neutral and thus is alkaline. The pegmatite had a maximum X(CO2) of 0.1, and raised the oxygen fugacity level in the lherzolite from NiNiO-3 to NiNiO-2.
The fluid in equilibrium with the metasomatic mineral assemblage of enstatite, anthophyllite, actinolite, phlogopite/biotite, chlorite, dolomite, magnesio-chromite, ilmenite, and pyrrhotite contains Fe and Al as major cations. Beside the Fe and Al cations in the fluid, components of K, Si and Ca were also present, and Na, Mg and Cr aqueous components as minor ligands. The pegmatite fluid was also F-rich, where the fluid F and Cl content is estimated to 121-161 ppm from
F contents in apatite and biotite, and Cl is between 0.5 and 5 mass% depending on the partitioning coefficient (D) for apatite-fluid chosen. (Hydr)oxide and carbonate species dominate the fluid, and cation concentrations are not sensitive to the Cl-content of the fluid. The combination of reduced and alkaline conditions results in high Al mobility, and this is enhanced by the elevated F-content in the fluid. Saturation in corundum in the lherzolite is therefore proposed to be a result of a combination of active Al addition from the lherzolite and residual enrichment of Al from the pegmatite vein, and the Fe- instead of Cr-enrichment in the corundum a direct result of high Fe and low Cr mobility in the metasomatic fluid.