加纳东南部RbSr和KAr地球年代学

E.K. Agyei , J.E.J.M. van Landewijk , Richard Lee Armstrong , J.E. Harakal , K.L. Scott
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引用次数: 18

摘要

加纳南部西非克拉通海岸角花岗岩杂岩的全岩RbSr测年为2051±45 Ma。矿物年代测定:角闪石(KAr)为~ 20006年,黑云母(RbSr)为1974±28 Ma,黑云母(KAr)为~ 1900 Ma。少数黑云母日期较年轻。全岩87Sr/86Sr初始比值较低,为0.7015±0.0002,表明花岗岩形成于2050 Ma之前。较年轻的矿物年代是由隆起、侵蚀和埃布尼亚克拉通化事件后的冷却造成的。由于泛非造山运动,加纳东南部达美岩变质岩的黑云母Rb年代学为330-542 Ma,大部分为508-542 Ma。角闪石KAr年代稍早,在561-611 Ma。来自Teshie、Numgua和Tema Beach的片麻岩是一个至少2841±468 Ma的旧地下室的一部分,初始比值为0.7041±0.0014。Ho和Dzigakope的片麻岩似乎也属于前eburnian时期,RbSr的年代分别为2176±44 Ma和约2800 Ma,但这还没有得到很好的证实。9个达美岩片麻岩全岩样品分布在515±16 Ma等时线附近,初始比值为0.7056±0.0002。金坪侵入碳酸岩的全岩RbSr、黑云母RbSr和黑云母KAr年代值分别为593±250、545±11和572±15 Ma,而斜长石KAr和Rb年代值分别为665±20和975±167 Ma。怀疑与地质体无关的异种晶体中存在过量的Ar和放射性成因Sr。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
RbSr and KAr geochronometry of southeastern Ghana

The Cape Coast granite complex of the West African Craton in southern Ghana has been dated as 2051 ±45 Ma by whole rock RbSr;. Mineral dates are ∼ 20006 for hornblende by KAr, 1974 ± 28 Ma for biotite by RbSr and ∼ 1900 Ma for biotite by KAr. A few biotite dates are somewhat younger. The low whole-rock 87Sr/86Sr initial ratio, 0.7015 ± 0.0002, indicates that the granites were created shortly before 2050 Ma. The successively younger mineral dates result from uplift, erosion, and cooling following the Eburnian cratonization event.

Dahomeyan metamorphic rocks of southeastern Ghana give biotite RbAr dates of 330–542 Ma, mostly 508–542 Ma, due to the Pan-African orogeny. Hornblende KAr dates are somewhat older, at 561–611 Ma.

Gneisses from Teshie, Numgua, and Tema Beach are parts of a reworked older basement at least 2841 ± 468 Ma old with an initial ratio of 0.7041 ± 0.0014. Gneisses from Ho and Dzigakope also appear to be pre-Eburnian with RbSr dates of 2176 ± 44 Ma and about 2800 Ma, respectively, but this is not as well established. Nine whole-rock samples of Dahomeyan gneiss scatter about an isochron of 515 ± 16 Ma with an initial ratio of 0.7056 ± 0.0002.

An intrusive carbonatite from Kpong gives concordant whole-rock RbSr, biotite RbSr, and biotite KAr dates of 593 ± 250, 545 ± 11 and 572 ± 15 Ma, respectively, but it also gives discordant and geologically contradictory plagioclase KAr and Rb dates of 665 ± 20 and 975 ± 167 Ma, respectively. Excess Ar and radiogenic Sr in xenocrysts unrelated to the groundmass are suspected.

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