The Sr isotopic composition of early Jurassic mafic rocks of Atlantic Canada: Implications for assimilation and injection mechanisms affecting mafic dykes

Abstract

Early Mesozoic rift-related mafic rocks of Atlantic Canada (Caraquet, Minister Island, Shelburne and Avalon dykes and North Mountain Basalt) show high initial 87Sr/ 86Sr ratios (0.705–0.709) suggesting derivation from a LILE-enriched subcontinental mantle or assimilation of continental crust during ascent. Despite wide geographic distribution, Rb, Sr and Sr isotopic data for each body tend to overlap on variation diagrams, implying a comagmatic origin and/or similar petrogenetic processes. Modelling of these data supports assimilation of average continental crust by a fractionally crystallizing normal MORB magma. Rb/Sr ratios show a slightly positive or negative correlation with 87Sr/ 86Sr (depending on igneous body). Apparently assimilation involved early absorption of a Rb-rich partial melt with progressive assimilation of residual material enriched in radiogenic Sr. This Rb/Sr 87Sr/ 86Sr relationship is difficult to explain with a LILE-enriched subcontinental mantle model. Chilled contact samples generally display lower 87Sr/ 86Sr ratios than dyke interiors, implying that early pulses of magma through dyke systems were less evolved than later pulses. Assimilation occurred prior to injection or at deep crustal levels. Once in the dyke fracture system magma was quenched against cool upper-crustal rocks preventing further assimilation.