Acadiensis: Vol. 10, No. 2 Special issue on the historical geography of Maritime Canada (Fredericton: Department of History, University of New Brunswick, 1981. Pp. 192. $5·95)

Abstract

The Magdalen Islands are located in the Gulf of St. Lawrence, near the centre of the late Devonian to Carboniferous composite Maritimes Basin, which opened in response to extensional tectonism following continental collision during the Acadian orogeny. Composed essentially of cap rocks above salt diapirs, these islands expose some of the youngest mafic volcanic rocks in the Maritimes Basin. The Magdalen Islands volcanics are geochemically heterogeneous, tholeiitic to alkalic basalts. The alkalic basalts are generally more enriched in incompatible trace elements than the tholeiites, having higher Zr, Th, Ta, Hf, LREE, Ti and P abundances, average [La/Sm]CN of 3.0±0.8 and [Tb/Yb]CN of 1.6±0.2, and εNd values of +2.0 to +6.3 (at 330 Ma). In the group of tholeiites, average [La/Sm]CN is 1.6±0.9, average [Tb/Yb]CN 1.3±0.2, and εNd values range from −3.0 to +7.0 (at 330 Ma). The incompatible trace element signature of the least enriched tholeiites can be produced by different degrees of partial melting (≈7 to 15%) of a depleted normal mid-ocean ridge basalt (N-MORB) type to slightly enriched asthenospheric mantle source. The genesis of the alkalic basalts necessitates the involvement of a much more enriched, asthenospheric or lithospheric, mantle component having affinities with the source of HIMU-OIB; i.e., high εNdT, Ta/LREE and Nb/LREE values. Tholeiites enriched in incompatible trace elements display compositional shifts toward the alkalic basalts, suggesting various degrees of magma mixing between the two types of melts. Crustal contamination appears as a fairly limited process and may be invoked for only a few samples, explaining nevertheless the low εNd value of −3.0 obtained for one of the tholeiitic basalt. The available geochemical data cannot preclude the involvement of a deep mantle plume in the genesis of the Magdalen basalts. However, based on regional geological considerations (e.g., the relatively small volume of lavas and their dominantly alkalic character) we favor the alternative hypothesis that melting was caused by decompression melting and passive asthenospheric mantle upwelling associated with lithospheric extension, which in turn induced partial melting of the metasomatized subcontinental lower lithospheric mantle, generated small volumes of melts with some HIMU-OIB characteristics, and ultimately the alkalic basalts. The episode of metasomatism considered here is tentatively associated with an earlier phase of alkalic volcanism which occurred circa 550 Ma on the southeastern margin of the North American plate, and was related with the opening of Iapetus. One may suppose that alkalic melts were then trapped at the base of the lithospheric mantle, and that their isolation from the convecting asthenosphere for some 220 Ma led to the isotopic compositions of the most enriched tholeiites and alkalic basalts sampled on the Magdalen Islands.