The Harp olivine diabase dikes, of Neohelikian age, form an east-northeast trending swarm that cuts rocks of the Harp Lake Complex in central Labrador. The petrography, and rock and mineral chemistry of the dikes indicate that they are transitional in character between tholeiitic and alkalic compositions. The major element chemistry of the dikes is similar to basaltic magmas from other comparable continental settings and in particular closely resembles basalts and diabase sills of the Neohelikian Seal Lake Group.Correlation of the Seal Lake – Harp dikes magmatism is suggested with two other groups of hypabyssal intrusions of olivine gabbro east and southeast of the Seal Lake synclinorium (Michael gabbros and diabase dikes in the Mealy Mountains complex). All of this basic magmatism may have been related to a Neohelikian zone of continental rifting or incipient rifting. Intrusion and extrusion of basic magma under conditions indicative of crustal extension closely follows, or is associated with, uplift and erosion of anorogenic anorthosite–'granite' complexes in other places in the world and is inferred to be a consequence of a continuing evolving process of mantle–crust interactions; in Labrador, the process began in the Paleohelikian with intrusion of major anorthosite–adamellite complexes.

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