Nd and Pb isotopic constraints on the origin of the Purtuniq ophiolite and Early Proterozoic Cape Smith Belt, northern Québec, Canada

Abstract

We report Pb isotopic compositions for 35 samples and Sm-Nd isotopic data for 44 volcanic rocks from the Purtuniq ophiolite, the continental Povungnituk and oceanic Chukotat groups in the Cape Smith Belt. Pb-Pb whole-rock dating of the sheeted dike complex and cumulate rocks of the ophiolite, which are preserved in separate thrust sheets, yield identical isochron ages of 1.97 ± 0.04 and 1.97 ± 0.11 Ga, respectively, supporting a contemporaneous origin. Combined data for layered cumulates, sheets dikes and pillow basalts constrain the age of the ophiolite to 1.98 ± 0.03 Ga ( μ 1 = 8.0) in good agreement with a previously reported 1.998-Ga U-Pb zircon age. In contrast, Pb isotopic data of the ∼1.96−1.92-Ga continental Povungnituk and related transitional continental-oceanic Chukotat Group basalts yield young dates of 1.89 and 1.72 Ga, respectively, which may record tectonothermal events related to continental collision. Sm-Nd isotopic data for the ophiolite indicate highly heterogeneous sources ( ϵ Nd( t) + 1.0to + 6.1) and form a mixing array corresponding to an anomalously old age of ∼2.2 Ga. A similar Sm-Nd isotopic array with an age of ∼2.3 Ga is displayed by the data for LREE-enriched continental Povungnituk volcanic rocks and the associated, more depleted, oceanic basalts of the Chukotat Group. Magma contamination by Late Archean basement can be ruled out and we attribute the excessively old Sm-Nd ages to melting and mixing of two distinct mantle sources. The dominant magma source with ϵ Nd( t)-values of + 3 to + 5 is similar to the depleted Early Proterozoic mantle. A subordinate mantle component displays ϵ Nd( t)-values as low as + 1 and enrichment in LREE relative to the depleted Early Proterozoic mantle. It may have resulted from a relatively young enrichment event that produced a source with high Nd/Sm and moderately radiogenic Nd at 2 Ga. At the other extreme, it may reflect a much older event with little source enrichment in LREE. In this case the strong LREE enrichment in some volcanics would be due to LREE fractionation during melting, and it can be assumed that the isotopically evolved component had been stored outside the depleted mantle for a long time, in order to avoid its destruction by convection. The isotopic evidence is compatible with a magma origin in a plum pudding type mantle or a heterogeneous highly depleted mantle plume. The isotopic heterogeneity in the ophiolite suggests an origin at a slowly spreading ridge in an immature ocean basin.