NATURE OF MANTLE SOURCE CONTRIBUTIONS AND THE ROLE OF CONTAMINATION AND IN-SITU CRYSTALLIZATION IN THE PETROGENESIS OF PROTEROZOIC MAFIC DYKES AND FLOOD BASALTS LABRADOR

Proterozoic tholeiitic dyke swarms share many compositional features w ith, and pose similar petrogenetic problems to, Phanerozoic continenta l flood basalts, but there are few extrusive equivalents of such swarm s. The Mesoproterozoic (1.27 Ga) Harp dyke swarm in Labrador is one wh ere possible extrusive equivalents exist in the Seal Lake group, but a re slightly displaced in space and time, and can probably be related b y models of progressive crustal extension. Here we try to evaluate the roles of crystal differentiation, in situ crystallisation, crustal as similation and the relative contributions of asthenosphere- and lithos phere-derived melts in the petrogenesis of the mafic magmas. Modelling of the major and trace element variations both within individual dyke s and between dykes, and within the lava sequence, does not suggest an important role for continental crust involvement. While in situ cryst allisation processes could account for some of the compositional varia tions, the most successful models invoke mixing or contamination of as thenospheric magmas with/by veined material in the lower lithosphere/u pper asthenosphere which carries the 'continental' characteristics. Th e results imply an important role for hydrous phases such as phlogopit e and hornblende in the sub-lithosphere mantle. Much of the low-MgO ch aracter of mafic dykes may result from significant removal of mafic ph ases during in situ crystallisation within the lithosphere.