Abstract

Rb–Sr and U–Pb data from the Borden complex of northern Ontario, a carbonatite associated with the Kapuskasing Structural Zone, indicate a mid-Proterozoic age. A 207Pb/206Pb age of 1872 ± 13 Ma is interpreted as the emplacement age of this body, grouping it with other ca. 1900 Ma complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A 206Pb–238U age of 1894 ± 29 Ma agrees with the Pb–Pb age but has a high mean square of weighted deviates (MSWD) of 42. A Rb–Sr apatite–carbonate–mica whole-rock isochron date of 1807 ± 13 Ma probably indicates later resetting of the Rb–Sr system.An εSr(T) value of −6.2 ± 0.5 (87Sr/86Sr = 0.70184 ± 0.00003) and an εNd(T) value of +2.8 ± 0.4 for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in the large-ion lithophile (LIL) elements. These closely resemble the ε values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 Ma plutons. The estimated initial 207Pb/204Pb and 206Pb/204Pb ratios from Borden calcites plot significantly below growth curves for average continental crust in isotope correlation diagrams, a pattern similar to those found in mid-ocean ridge basalts (MORB) and most ocean-island volcanic rocks, again suggesting a source depleted in LIL elements. The combined Nd and Sr, and probably Pb, data strongly favour a mantle origin for the Borden complex with little or no crustal contamination and support the model of Bell et al. that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.

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