Ulf Söderlund

Baddeleyite (ZrO2) is today recognized as the key mineral for dating the emplacement of mantle-derived igneous rocks, primarily gabbro and dolerite dykes and sills. The comment by Michard and Gasquet calls for a discussion on the significance of baddeleyite U-Pb TIMS dates of two dykes in the Bas-Draa inlier, dated at 1381 +/- 8 and 1384 +/- 6 Ma, presented in El Bahat et al. (2013-in this issue). They suggest that the U-Pb systematics were "seriously perturbed in the Bas-Draa region" with the main argument being a lack of age matches between the ca. 1385 Ma dykes in Bas-Draa and the 2040, 1650 and 885 Ma dykes in adjacent inliers to the east. To infer an older age for these dykes, as suggested by Michard and Gasquet, and explain the ca. 1385 Ma TIMS results would require two separate events of perturbation, in the Mesoproterozoic and a subsequent Variscan event. However, such an alternative is improbable from basic theoretical reasoning using U-Pb isotopic systematics and our current understanding of the behaviour of baddeleyite in metamorphic systems. In addition, we present new LA-ICPMS U-Pb age data of baddeleyite from one of the dated dykes from Bas-Draa. The new LA-ICPMS U-Pb age of 1416 +/- 7 Ma is slightly but significantly older than our TIMS 1384 +/- 6 Ma upper intercept age, nevertheless confirming this dyke could not correspond to any of the other mafic intrusions dated in the inliers to the east. The IA-ICPMS result also contributes to a deeper understanding on the effects of secondary processes in baddeleyite in general, and suggests a modified methodological approach for samples with complex geological histories. (C) 2013 Elsevier B.V. All rights reserved.