Palaeomagnetism of the Lappajärvi impact structure, western Finland
Palaeomagnetism of the Lappajärvi impact structure, western Finland
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芬兰西部 Lappajärvi 撞击构造的古地磁学
DOI：
10.1016/0040-1951(92)90160-8
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发表时间：
1992
期刊：
影响因子：
2.9
通讯作者：
F. Pipping
中科院分区：
文献类型：
--
作者：
L. Pesonen;N. Marcos;F. Pipping
介绍了芬兰西部拉帕耶尔维陨石撞击构造岩石的古地磁和岩石磁学结果。定向样品是从火山口中央岛屿的熔融岩石和火山口边缘的古元古代（Svecofennian）目标岩石中收集的。熔融岩石在退磁过程中的自然还原磁化强度一般较弱，处于亚稳定状态。由磁黄铁矿携带的特征NRM分量产生古磁极（纬度55.4°N，经度152.6°E，dp= 6.2°，dm= 8.7/dg，6个地点），表明撞击的年龄约为195 Ma。从撞击熔融岩中钻取的深岩心的结果显示出类似的古地磁方向，但倾角略浅。碰撞的古地磁年龄与40 Ar-3939 Ar年龄77 Ma不同。假设40 Ar-39 Ar年龄是正确的，那么古地磁年龄和放射性年龄之间的差异最好用熔融层在撞击后（和冷却后）的倾斜来解释。倾斜（15°）可能是由于熔体中沿沿着接缝的等静压反弹。撞击剩磁也被隔离在边缘的一些目标岩石中。这种定义不清的叠印可能是叠加在1.9 Ga古老的Svecofennian方向上的轻微冲击反射磁化。这种解释是支持的观察，NRM强度增强的标本具有影响叠印。古地磁结果与早期基于地球物理、形态学、矿物学和地球化学数据的解释一致，即拉帕耶尔维结构是一个复杂的陨石撞击遗址，年龄不到200 Ma。
Palaeomagnetic and rock magnetic results are presented for rocks from the Lappajärvi meteorite impact structure, western Finland. Oriented samples were collected from the melt rocks in the central island of the crater and of the Palaeoproterozoic (Svecofennian) target rocks at the crater rim. The natural remanent magnitization (NRM) of the melt rocks is generally weak and metastable during demagnetization. The characteristic NRM component, carried by pyrrhotite, yields a palaeomagnetic pole (latitude 55.4°N, longitude 152.6°E,dp= 6.2°,dm= 8.7/dg, 6 sites) suggesting an age of ~ 195 Ma for the impact. The results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations. The palaeomagnetic age differs from the40Ar-3939Ar age of 77 Ma for the impact. Assuming that the40Ar-39Ar age is correct, the discrepancy between palaeomagnetic and radiometric ages is best explained by post-impact (and post-cooling) tilting of the melt layer. The tilting (15°) may be due to isostatic rebounds along joints in the melt. Impact remanence has also been isolated in a few target rocks at the rim. This poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1.9 Ga old Svecofennian direction. This interpretation is supported by the observation that the NRM intensity is enhanced in specimens having the impact overprint. The palaeomagnetic results are consistent with earlier interpretations based on geophysical, morphological, mineralogical and geochemical data that the Lappajärvi structure is a complex meteorite impact site less than 200 Ma old.