Palaeomagnetism and Potassium-Argon Ages of Volcanic Rocks of Ngorongoro Caldera, Tanzania
Palaeomagnetism and Potassium-Argon Ages of Volcanic Rocks of Ngorongoro Caldera, Tanzania
复制标题
坦桑尼亚恩戈罗恩戈罗火山口火山岩的古地磁和钾-氩时代
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发表时间：
1971
期刊：
影响因子：
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通讯作者：
R. L. Hay
中科院分区：
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作者：
C. S. Grommé;T. Reilly;A. Mussett;R. L. Hay
总结
在坦桑尼亚恩戈罗恩戈罗火山口西南壁的 21 个熔岩流和 1 个凝灰岩中测量了自然剩磁 (NRM)。最下面的三个熔岩是正常磁化的，接下来的两个熔岩具有中间方向，其余的则相反；钾-氩测年将反转置于 2.45 My，对应于高斯松山纪元边界。交变场退磁可以有效消除普遍存在的闪电产生的二次磁化，但部分热退磁则不然。熔岩的居里温度分为两组：250°-350°C 和 570°-585°C，可能分别对应于未氧化和氧化的钛磁铁矿。熔岩中的 NRM 方向出现在几个不同的、地层连续的组中，表明熔岩组间歇性喷发。 NRM 组与其他参数（例如居里温度）之间没有发现对应关系。与太平洋盆地低纬度的晚第四纪熔岩相比，平均 NRM 方向的离散度较大，但相对于非洲非偶极地磁场的比例可能并不大。平均古地磁极与地理极偏移10.5°，但由于离散度较大，差异并不显着。
Summary
Natural remanent magnetization (NRM) has been measured in 21 lava flows and 1 tuff in the south-west wall of Ngorongoro caldera, Tanzania. The lowest three lavas are normally magnetized, the next two have intermediate directions, and the remainder are reversed; potassium-argon dating places the reversal at 2.45 My, corresponding to the Gauss Matuyama epoch boundary. Alternating-field demagnetization was effective in removing the ubiquitous lightning-produced secondary magnetizations, but partial thermal demagnetization was not. Curie temperatures of the lavas fell into two groups: 250°-350°C and 570°-585°C, probably corresponding to unoxidized and oxidized titanomagnetites respectively. NRM directions in the lavas occurred in several distinct and stratigraphically continuous groups, suggesting intermittent eruption of groups of lavas. No correspondence was found between NRM groups and other parameters, such as Curie temperatures. The dispersion of mean NRM directions is large in comparison with late Quaternary lavas from low latitudes in the Pacific basin, but probably is not large relative to the proportion of the geomagnetic field in Africa that is non-dipolar. The mean palaeomagnetic pole is offset 10.5° from the geographic pole, but because of the large dispersion, the difference is not considered significant.