AGE AND FORMATION OF ANORTHOSITIC ROCKS WITHIN THE BLAIR RIVER INLIER OF NORTHERN CAPE BRETON ISLAND, NOVA SCOTIA (CANADA)

Rocks from the Blair River inlier of Northern Cape Breton Island (Nova Scotia, Canada) have been correlated with either the Grenville basement of eastern Laurentia or the Avalonian basement. Additional in situ zircon U/Pb dates of spatially associated anorthositic lenses (424.8 ± 2.3 Ma) and a meta-gabbro (423.6 ± 2.4) from the Blair River inlier reveals Late Silurian emplacement ages. Their contemporaneity suggests they may be members of the same intrusive complex. The anorthositic rocks have high Eu/Eu* values (> 2.5) and bulk compositions similar to the mineral compositions of labradorite (An_50-70) and andesine (An_30-50). The meta-gabbro is compositionally similar to ocean-island basalt and does not seem to have been affected by crustal contamination (Nb/U > 24; Th/Nb_PM ≤ 1.1) although it was metamorphosed. The high Tb/Yb_N (1.8-1.9) ratios suggest the parental magma of the meta-gabbro was derived from a garnet-bearing peridotite. Fractional crystallization and mass balance calculations indicate that the anorthositic rocks can be derived by mineral accumulation from a mafic parental magma similar in composition to the meta-gabbro of this study. The Late Silurian ages indicate that the rocks were emplaced after the collision between greater Avalonia with the meta-gabbro composition indicating intrusion in a transitional post-collisional setting.