Reply to Comment on "U-Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas-Draa inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton" by Andre Michard and Dominique Gasquet
Reply to Comment on "U-Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas-Draa inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton" by Andre Michard and Dominique Gasquet
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DOI：
10.1016/j.lithos.2013.04.003
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发表时间：
2013-08
期刊：
影响因子：
3.5
通讯作者：
U. Söderlund;M. Ibáñez-Mejia;Abdelhakim El Bahat;R. Ernst;M. Ikenne;A. Soulaimani;N. Youbi;B. Cousens;M. E. Janati;A. Hafid
中科院分区：
文献类型：
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作者：
U. Söderlund;M. Ibáñez-Mejia;Abdelhakim El Bahat;R. Ernst;M. Ikenne;A. Soulaimani;N. Youbi;B. Cousens;M. E. Janati;A. Hafid
斜锆石（ZrO 2）是现今公认的确定幔源火成岩（主要是辉长岩和粗玄岩脉和岩床）侵位年龄的关键矿物。Michard和Gasquet的评论呼吁讨论Bas-Drâa内层中两个岩脉的斜锆石U-Pb TIMS年龄的意义，年龄为1381±8和1384± 6 Ma，在El Bahat等人（2013年-在本期）中提出。他们认为U-Pb系统学“在Bas-Drâa地区受到严重干扰”，主要论点是ca之间缺乏年龄匹配。在Bas-Drâa的1385 Ma的堤坝和在东部相邻的inliers的2040，1650和885 Ma的堤坝。为了推断这些堤坝的年龄较大，建议由米哈德和加斯奎特，并解释约。1385 Ma的TIMS结果需要两个独立的扰动事件，在中元古代和随后的华力西事件。然而，这样的替代是不可能的，从基本的理论推理，使用U-Pb同位素系统学和我们目前的理解的行为斜锆石在变质系统。此外，我们提出了新的LA-ICPMS U-Pb年龄数据的斜锆石从一个过时的墙从Bas-Drâa。新的LA-ICPMS U-Pb年龄为1416± 7 Ma，比我们的TIMS上截距年龄1384± 6 Ma稍老，但明显更老，但证实该岩墙不能与东部内围区的任何其他镁铁质侵入体相对应。LA-ICPMS的结果也有助于更深入地了解斜锆石中的次生过程的影响，并提出了一个修改后的方法与复杂的地质历史的样品。
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-Drâa inlier, dated at 1381±8 and 1384±6Ma, presented in El Bahat et al. (2013-in this issue). They suggest that the U–Pb systematics were “seriously perturbed in the Bas-Drâa region” with the main argument being a lack of age matches between the ca. 1385Ma dykes in Bas-Drâa and the 2040, 1650 and 885Ma 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. 1385Ma 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-Drâa. The new LA-ICPMS U–Pb age of 1416±7Ma is slightly but significantly older than our TIMS 1384±6Ma 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 LA-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.