苏格兰当巴（Dunbar）下石炭统的古地磁研究——用烘烤效应解决岩石剩磁年龄问题

苏格兰当巴（Dunbar）下石炭统含钙砂岩组12个采样点的热退磁结果,显示一致的稳定磁化方向:偏角D=196°,倾角 I=-45°,α₉₅=4.3。侵入其中的一条20m厚的石炭系岩脉,其6个采样点的热退磁结果显示不同的磁化方向:D=195°,I=15°,α₉₅=10.7。 本文研究烘烤效应,论证了上述两个磁化方向均为原生。这与现存的对不列颠群岛该时期古地磁方向的看法矛盾。其原因可能是:1.以往的数据解释不可靠,特别是重磁化问题;2.构造原因;3.持续相当时间的异常地磁场。 对比纬度相似的太平洋夏威夷群岛熔岩流的“点记录”,本文针对当巴提出一种磁化模式:各有关古地磁磁化成分可能均产生于同一地质时代. 火成岩的尖晶石相方向,最可能为原生.它在古地磁分析中应给以较大权重.

PALAEOMAGNETIC STUDIES OF CARBONIFEROUS ROCKS IN DUNBAR,SCOTLAND——AN APPLICATION OF BAKING TEST TO THE PROBLEM OF MAGNETISATION AGE

Thermal demagnetisation of 12 sites sampled in the Lower Carboniferous Calci-ferous Sandstone Series, exposed at Dunbar, Scotland, shows consistent stable magne-tisation with a mean direction of 196° （Dec.）, -45°（Inc.） (a₉₅ = 4.3). Thermal de-magnetisation of 6 sites from a 20m Carboniferous dyke has a radically different ma-gnetisation of 195° （Dec.）, 15° （Inc.）(a₉₅ = 10.7). The Baking effects are studied. This allows the original directions to be argued forboth the sediments and the dyke, but they differ from that expected conventionally forrocks of this age. The reason for this could be （1） unreliability of the interpretationof previous data, particularly remagnetisation; （2） tectonic effects; or （3） persistentfield anomalies. A comparison is made between a series of spot readings obtained fromHawaiian lava flows, which are on the latitude similar to the palaeolatitude of thesediments, Dunbar. A model of magnetisation acquisition is thus implied: All therelevant components have originally been acquired in the same age: Carboniferous. The spinel phase direction of igneous rocks is most likely to be the original one, andthus should be given more weight in palaeomagnetic analysis.