Nd and Sr isotope systematics of the Oka complex,Quebec, and their bearing on the evolution of the sub-continental upper mantle

A detailed isotopic study of minerals and whole rocks from the Cretaceous Oka complex, Quebec, Canada, shows a very small variation in initial Nd and Sr isotopic compositions. Assuming an age of 109 Ma for the complex, apatite, calcite, garnet, melilite, monticellite, olivine and pyroxene and whole rocks yield a range for initial ⁸⁷Sr/⁸⁶Sr of 0.70323–0.70333; and for initial ¹⁴³Nd/¹⁴⁴Nd of 0.51271–0.51284 (_SR(T)= –14.8 to –16.2; _Nd(T)=+4.1 to +6.6). The negative _SR and positive _Nd indicate derivation of the Nd and Sr from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. This agrees with data from other Canadian carbonatites and confirms that a large part of the Canadian Shield is underlain by a source region depleted in the LIL elements. The new data from Oka suggest that the depleted source may have remained coupled to the continental crust until recent time.