Geochemical characteristics of Proterozoic post-orogenic magmatism in the Nagssugtoqidian Mobil Belt of southeast Greenland

The final event in the Nagssugtoqidian (NAG) Mobile Belt of East Greenland is represented by the post-orogenic intrusion of plutonic bodies into an Archaean-Proterozoic polymetamorphic basement. The petrogenesis of plutonic rocks and dykes, which make up the Ikasaulak Intrusive Complex, is interpreted on the basis of their major- and trace-element geochemistry. The investigated intrusive complex consists of a lower ultramafic-mafic sequence and an upper-inter-mediate felsic sequence. A compositional gap exists at the 60-70% Si02 range. Silica-rich rocks, corresponding to a minimum-melt composition, are characterized by strong LREE enrichment and highly fractionated REE patterns, consistent with an origin by partial melting of a garnet-rich source, probably triggered by the intrusion of mantle-derived melts. Most primitive mafic rocks show enrichment in LREE and large-ion lithophile elements and depletion in Nb and Ta. Although these data are suggestive of magmas generated at destructive plate margins, there is no geological reason for assuming a subduction-related origin of the melts. Thus trace-element compositions appear to be a feature of the mantle source, probably retaining the geochemical signature of the crust-generating processes occurring during the previous NAG ocean subduction (1920-1840 Ma). Petrogenetic modelling of the major- and trace-element compositions indicates that fractional crystallization can only partially account for the compositional differences up to 60% Si02 and other mechanisms, such as concurrent assimilation and fluid activity, are involved. The present data suggest that the final event in the mobile belt was the stabilization of the sub-continental mantle, which occurred at least 200-300 Ma after the subduction-related crust-generating processes.