Age and radiogenic isotopic systematics of the Borden carbonatite complex, Ontario, Canada.

Rb/Sr and U/Pb data from this complex, a carbonatite associated with the Kapuskasing structural zone, indicate a mid-Proterozoic age. A ²⁰⁷Pb/²⁰⁶Pb age of 1872 + or - 13 m.y. is interpreted as the emplacement age of this body, grouping it with other approx 1900 m.y. complexes that are the oldest known carbonatites associated with the Kapuskasing structure. A ²⁰⁶Pb/²³⁸U age of 1894 + or - 29 m.y. agrees with the Pb/Pb age. A Rb/Sr whole-rock isochron date of 1807 + or - 13 m.y. probably indicates later resetting of the Rb/Sr system. The epsilon_Sr(T) and epsilon_Nd(T) values for Borden indicate derivation of the Sr and Nd from a source with a time-integrated depletion in LIL elements. These closely resemble the epsilon values for Sr and Nd from the Cargill and Spanish River complexes, two other 1900 m.y. old plutons. The estimated Pb/Pb ratios from Borden calcites plot well below growth curves for average continental crust, a pattern similar to those found in MORB and most ocean-island volcanic rocks, again suggesting a source depleted in LIL. The combined data strongly favour a mantle origin for the Borden complex, with little or no crustal contamination, and support the hypothesis that many carbonatites intruded into the Canadian Shield were derived from an ancient, LIL-depleted subcontinental upper mantle.-P.Br.