利比里亚40Ar/39Ar和古地磁结果及西非地盾前寒武纪APW数据库

T.C. Onstott, J. Dorbor
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引用次数: 33

摘要

利比里亚宁巴县下角闪岩-麻粒岩相变质岩的全岩年龄为2.2 ~ 2.9 Ga RbSr,属于利比里亚太古宙省。我们报告了该地区角闪岩角闪石的2040 Ma4040Ar/39Ar高原年代,并认为这些岩石在Eburnean (~ 2.0 Ga)变质期也受到了严重的改造。邻近片岩中的黑云母和长石的40Ar/39Ar分析也表明存在1.5 Ga和0.6 Ga两个温和的热事件。对这些变质岩样品的古地磁分析揭示了磁化的三个组成部分。主要和最稳定的成分(273°E, 21°N)被认为是由于eburnea后隆起和~ 2.0 Ga的冷却而获得的,而两极位于235°E, 43°N和16°E, 36°N的两个不太稳定的成分可能分别与1.5 Ga和0.6 Ga的热脉冲有关。来自利比里亚南部的岩石单元也产生了两个次级磁化,一个在247°E, 37°N,另一个在104°E, 5°N,以及斜长石的1.5 Ga 40Ar/39Ar日期。将Eburnean ~ 2.0 Ga分量对应的古地磁极与西非已发表的古地磁数据进行比较,与之前对西非极移路径的解释不一致。我们的古地磁数据,当与来自卡拉哈里地盾的相当年龄的两极进行比较时,仍然表明自2.0 Ga以来,在卡拉哈里地盾和西非地盾之间发生了某种形式的位移。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
40Ar/39Ar and paleomagnetic results from Liberia and the Precambrian APW data base for the West African Shield

Lower amphibolite to granulite facies metamorphic rocks in Nimba County, Liberia have yielded 2.2–2.9 Ga RbSr whole rock ages, indicating that they are part of the Archean Liberian age province. We report a 2040 Ma4040Ar/39Ar plateau date on hornblende from an amphibolite in this region, and suggest that these rocks were also severelyreworked during the Eburnean (∼2.0 Ga) metamorphic episode. 40Ar/39Ar analyses of biotite and feldspars from neighboring schists also indicate the presence of two mild thermal events, at 1.5 Ga and 0.6 Ga. Paleomagnetic analyses of samples from these same metamorphic rocks reveal three components of magnetization. The predominant and most stable component (273°E, 21°N) is considered to have been acquired as a result of pos Eburnean uplift and cooling at ∼ 2.0 Ga, whereas the two less stable components with poles at 235°E, 43°N and 16°E, 36°N, probably correlate with the 1.5 Ga and 0.6 Ga thermal pulses, respectively. Rock units from southern Liberia also yield two secondary magnetizations, one at 247°E, 37°N and the other at 104°E, 5°N, and a 1.5 Ga 40Ar/39Ar date on plagioclase. Comparison of the paleomagnetic poles corresponding to the ∼2.0 Ga Eburnean component with published paleomagnetic data for West Africa is not consistent with prior interpretations of the polar wander path for West Africa. Our paleomagnetic data, when compared to poles of comparable age from the Kalahari Shield, still suggest that some form of displacement has occurred between the Kalahari and West African Shields since 2.0 Ga.

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