CRUST-MANTLE INTERACTION IN THE EVOLUTION OF THE ILIMAUSSAQ COMPLEX, SOUTH GREENLAND - ND ISOTOPIC STUDIES

Sm-Nd isotopic compositions were determined for the peralkaline Ilimau ssaq Complex of the Gardar Province of southern Greenland. The majorit y of the samples in the agpaitic and augite syenitic units have near c hondritic initial epsilon(Nd)(approximate to 0), whereas a few samples trend towards epsilon(Nd) values as low as -6 at the time of intrusio n (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-r ock granitoids, suggests that large-scale contamination took place onl y at the margins of the complex. The similarity of the Nd isotopic com positions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Kf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilat ion-fractional crystallization modeling of the isotopic compositions i ndicates that the Ilimaussaq magmas could have formed through fraction al crystallization of a basaltic melt while assimilating granitic crus t. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, w hich formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fra ctionation of a basaltic or augite syenite magma with concurrent assim ilation of Ketilidian crust. (C) 1997 Elsevier Science B.V.