Abstract

Two sets of Proterozoic (between 1290 and 1042 Ma) alkali to transitional basaltic dikes occur sparsely in an area of at least 5000 km2 within the Nain anorthosite complex. Both are compositionally similar in terms of major elements, but the set that trends roughly north–south (HP dikes) contains much higher concentrations of incompatible elements than the more abundant dikes that trend roughly east-northeast–west-southwest (LP dikes). Both types have low normative clinopyroxene relative to olivine—a characteristic shared with widespread troctolitic rocks in the Nain complex. Low Mg numbers and low Ni contents suggest that the magmas producing these dikes have undergone extensive fractional crystallization. The low normative Di content of these dikes suggests that this fractional crystallization occurred at depths equivalent to at least 10 kbar (1000 MPa).High equilibration temperatures for coexisting Fe–Ti oxides in the matrices of these dikes indicate that the dikes cooled rapidly and were emplaced near the surface. The exposed level of the Nain anorthosite complex (emplaced at depths of 8–10 km) indicates that the anorthosites were uplifted and eroded relatively soon after their emplacement, i.e., prior to dike emplacement.

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