PARENTAL MELTS OF MELILITOLITE AND ORIGIN OF ALKALINE CARBONATITE - EVIDENCE FROM CRYSTALLIZED MELT INCLUSIONS, GARDINER COMPLEX

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt i nclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3 ) natrocarbonatite. The melilitite inclusion (1) homogenisation temper ature of 1060 degrees C is similar to Liquidus temperatures of experim entally investigated natural melilitites. The compositions are peralka line, low in MgO (ca. 5 wt%), Ni and Cr, and they are low-pressure fra ctionates of more magnesian larnite-normative ultramafic lamprophyre-t ype melts of primary mantle origin. Low-alkali carbonatite composition s (2) homogenise at 1060-1030 degrees C and are compositionally simila r to immiscible calcite carbonatite dykes derived from the melilitolit e magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 9 00 degrees C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are relat ed to the calcite carbonatite dykes, are very Zr-rich and agpaitic (mo lecular Na2O + K2O/Al2O3 > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indic ate unmixing of carbonatite compositions (ca. 10% alkalies) from evolv ing melilitite melt and continued fractionation of melilitite to nephe linite. It is suggested that the natrocarbonatite compositions represe nt degassed supercritical high temperature fluid formed in a cooling b ody of strongly larnite-normative nephelinite or evolved melilitite. T he Gardiner complex and similar melilitolite and carbonatite-bearing u ltramafic alkaline complexes are believed to represent subvolcanic com plexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbo natites of Oldoinyo Lengai.