K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics
K-Ar Age and Chemistry of Phengite from the Sanbagawa Schists in the Kanto Mountains, Central Japan, and their Implication for Exhumation Tectonics
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DOI：
10.1016/s1342-937x(05)70917-5
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发表时间：
2002-10
影响因子：
6.1
通讯作者：
A. Miyashita;T. Itaya
中科院分区：
文献类型：
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作者：
A. Miyashita;T. Itaya
本文对日本中部关东山三八川变质带泥质片岩中的多硅白云母进行了电子探针分析和K-Ar年龄测定。多硅白云母产于关东山脉的三坝川泥质片岩中，通常是细粒晶体的集合体。它在与相对坚硬的石榴子石和钠长石变斑晶相邻的区域中具有极细的晶粒。这表明，在伴随着寄主片岩折返的韧性变形过程中，多硅白云母发生了变形导致的粒度减小。电子探针分析表明，多硅白云母的化学成分在薄片尺度上是不均匀的，这表明它是在退变质过程中形成的限制平衡域。韧性变形促进了退变化学反应。多硅白云母的K-Ar年龄从南部单元（82 Ma）到北方单元（58 Ma）逐渐变年轻。年龄范围与四国中部相似。较老的片岩产于四国中部的高级变质带和关东山脉的低级变质带。四国中部的热结构是倒置的，因此最高等级带出现在表观地层序列的上部或中部。相比之下，关东山脉有一个正常的热结构：较高级别的带是在较低的部分，明显的地层序列。在距离800 km的四国中部三八川带西侧和关东山脉三八川带东端，由于折返过程中不同的构造特征，形成了两种截然不同的年龄-温度-构造关系。也就是说，在四国中部的西部三八川带比关东山脉的东部三八川带在折返过程中经历了更长的韧性变形。
EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan. Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation. K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains.