Diversity and dynamics of pyroclastic eruptions at Aso caldera indicated by thermoremanent magnetization

对分布于日本九州中部阿索破火山口北部和东部的阿索火山碎屑流沉积中的几个单元进行了古地磁研究，以评估其侵位温度。岩石样品的自然反射磁化的阻断温度被假定为它们在环境场中稳定的温度。结果表明，焊接件中的主要碎片大多带有单分量磁化，在560℃以下的热退磁过程中，这些碎片几乎不受阻碍。焊接凝灰岩的阿索-2A，2 R，3B，4A和4 B进行稳定的单组分磁化过程中从居里温度以上的冷却。古地磁测量的阿索-4 B显示一个数量级强于其他凝灰岩的磁化。破火山口北部具有挤出构造的阿索1凝灰岩具有二次流动的可能性。相反，阿索-4在破火山口东部具有类似的挤出结构，只有一个组件，这表明挤出部分的焊接凝灰岩是在阻塞温度以上侵位。阿索-2 R具有典型的二次流动结构，表明凝灰岩的侵位温度至少在500℃以上;阿索-2A和2B具有较大的倾斜角;阿索-3B具有向东倾斜的特征，这与前人报道的结果一致。

Paleomagnetic investigations were performed to evaluate emplacement temperatures of the several units from the Aso pyroclastic-flow deposits which are distributed north and east of Aso Caldera, central Kyushu, Japan. Blocking temperatures of natural remanent magnetizations of the rock specimens are assumed as temperatures at which they were settled in the ambient field. It is concluded that most of the essential fragments in welded parts carried one-component magnetizations which are almost unblocked in thermal demagnetization up to 560℃. Welded tuffs of the Aso-2A, 2R, 3B, 4A and 4B carried stable one-component magnetizations during cooling from above the Curie temperature. Palaeomagnetic measurements of the Aso-4B showed one order of magnitude stronger magnetization than those of the other tuffs. The Aso-1 tuff with squeeze-out structure in north of the caldera showed two components suggesting a possibility of secondary flowage. On the contrary, The Aso-4 with the similar squeeze-out structure in east of the caldera showed only one component suggesting that the squeeze-out part of the welded tuff was emplaced above blocking temperature. The Aso-2R with a typical secondary flowage texture indicated one component suggesting that the tuff was emplaced above at least 500℃.The other notable characteristics are that the Aso-2A and 2B showed larger angle on inclination and the Aso-3B indicates eastward declination as are consistent with the previously reported results.