GEOCHEMISTRY OF POST-ACADIAN, CARBONIFEROUS CONTINENTAL INTRAPLATE BASALTS FROM THE MARITIMES BASIN, MAGDALEN ISLANDS, QUEBEC, CANADA

The Magdalen Islands are located in the Gulf of St. Lawrence, near the centre of the late Devonian to Carboniferous composite Maritimes Basi n, which opened in response to extensional tectonism following contine ntal collision during the Acadian orogeny. Composed essentially of cap rocks above salt diapirs, these islands expose some of the youngest m afic volcanic rocks in the Maritimes Basin. The Magdalen Islands volca nics are geochemically heterogeneous, tholeiitic to alkalic basalts. T he alkalic basalts are generally more enriched in incompatible trace e lements than the tholeiites, having higher Zr, Th, Ta, Hf, LREE, Ti an d P abundances, average [La/Sm](CN) of 3.0 +/- 0.8 and [Tb/Yb](CN) of 1.6 +/- 0.2, and epsilon(Nd) 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 epsilon(Nd) values range from -3.0 to +7.0 (at 330 Ma). The incompatible trace element signature of the least en riched tholeiites can be produced by different degrees of partial melt ing (approximate to 7 to 15%) of a depleted normal mid-ocean ridge bas alt (N-MORB) type to slightly enriched asthenospheric mantle source. T he genesis of the alkalic basalts necessitates the involvement of a mu ch more enriched, asthenospheric or lithospheric, mantle component hav ing affinities with the source of HIMU-OIB; i.e., high epsilon(NdT), T a/LREE and Nb/LREE values. Tholeiites enriched in incompatible trace e lements display compositional shifts toward the alkalic basalts, sugge sting various degrees of magma mixing between the two types of melts. Crustal contamination appears as a fairly limited process and may be i nvoked for only a few samples, explaining nevertheless the low epsilon (Nd) value of -3.0 obtained for one of the tholeiitic basalt. The avai lable geochemical data cannot preclude the involvement of a deep mantl e plume in the genesis of the Magdalen basalts. However, based on regi onal geological considerations (e.g., the relatively small volume of l avas and their dominantly alkalic character) we favor the alternative hypothesis that melting was caused by decompression melting and passiv e asthenospheric mantle upwelling associated with lithospheric extensi on, which in turn induced partial melting of the metasomatized subcont inental lower Lithospheric mantle, generated small volumes of melts wi th 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 relat ed with the opening of Iapetus. One may suppose that alkalic melts wer e 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 bas alts sampled on the Magdalen Islands. (C) 1998 Elsevier Science B.V. A ll rights reserved.