The relationship between E- and N-type magmas in the Baffin Bay Lavas

The picritic lavas of Baffin Island represent one of the most primitive Phanerozoic volcanic suites in the world with MgO contents ranging from 22 wt% (29 Mg, cation unit = Mg/100 cations) for olivine-rich lavas to 11 wt% (16 Mg) for olivine-poor lavas. Two magma types can be recognized on the basis of trace element and isotopic geochemistry. N-type magma, which dominates the high-MgO lavas, has depleted LREE patterns (La/Sm)_N0.6–0.7] typical of N-MORB, K/Ti<0.05, and ⁸⁷Sr/⁸⁶Sr <0.7032. E-type magma, which dominates the lower MgO lavas, has flat to slightly enriched LREE patterns (La/Sm)_N1.1–1.2] typical of E-MORB, K/Ti>0.5 and ⁸⁷Sr/⁸⁶Sr ranging between 0.7032–0.7039. These two magma types are, however, virtually indistinguishable in terms of major clements and many other trace elements. The E and N-type samples are intermixed throughout the volcanic succession, indicating that both types of magma erupted contemporaneously. Although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation, crystal fractionation cannot account for the difference in the LREE between E-type and N-type lavas. Crustal contamination involving a lower crust composition cannot reproduce the more magnesian E-type lavas.and can only repoduce the lowest (La/Sm)_N ratios of the E-type lavas if high degrees of assimilation (50%) have occurred. Partial melting models can reasonably account for the distinct (La/Sm)_N ratios and the similar Zr/Y values of the two magma types, but fail to reproduce the observed abundances or REE, Sr, Y and Zr. Compositionally different mantle sources are required to explain the two distinct magma types observed in Baffin Island. A model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of N and E-type magmas in the Baffin Bay picritic suite. The Baffin Island E-type lavas are less enriched in Sr, Y and Zr contents and have lower Zr/Y, for similar (La/Sm)_N than the other E-type lavas of the northern North Atlantic region.