ABSTRACT

LA-ICP-MS U–Pb and Hf isotope analysis was conducted on zircons from the metamorphic unit of Omo Formation and a granodorite body of Tokunoshima in the Amami Islands, Southwest Japan. Zircons from dioritic gneiss (S9, S46) and pelitic schist (S99) yielded Paleoproterozoic upper intercept and Early Jurassic lower intercept ages. The positive ε_Hf(t) values of the Paleoproterozoic zircons (between +12.7 and 0.8) and the negative ε_Hf(t) values of the Jurassic zircons (−19.6 and −22.8) indicate Early Jurassic reworking of the older continental crust. The results suggest that the Omo Formation is an eastern equivalent of Cathaysia of the South China block. Zircons from granodiorite (S62) yielded a ²⁰⁶Pb/²³⁸U mean age of 61.3 ± 1.0 Ma with positive ε_Hf(t) values of +12.0 and +12.6. The ε_Hf(t) values indicate juvenile magmatic activity, which is correlated with that in the San-in batholith belt of Southwest Japan. Dioritic gneiss (S30) and psammitic schist (P91) yielded zircon U–Pb ages comparable to that of granodiorite (S62) with positive ε_Hf(t) values (from +6.4 to +10.4). This suggests that the intrusion of the granodiorite induced the crystallization of zircon grains in the host rocks.

摘要

对来自日本西南部奄美群岛大毛组变质单元和德之岛花岗闪石体的锆石进行了 LA-ICP-MS U-Pb 和 Hf 同位素分析。闪长片麻岩（S9、S46）和泥质片岩（S99）的锆石产生了古元古代上截距和早侏罗世下截距年龄。古元古代锆石的正 ε _Hf (t) 值（在 +12.7 和 0.8 之间）和侏罗纪锆石的负 ε _{Hf (t) 值（-19.6 和 -22.8）表明早侏罗世旧大陆地壳的改造。}结果表明，Omo 组是华南地块华夏的东部等同物。来自花岗闪长岩 (S62) 的锆石产生²⁰⁶ Pb/ ²³⁸U 平均年龄为 61.3 ± 1.0 Ma，正 ε _Hf (t) 值为 +12.0 和 +12.6。ε _Hf (t) 值表示幼年岩浆活动，这与日本西南部山阴基岩带的活动相关。闪长片麻岩 (S30) 和砂砾片岩 (P91) 产生的锆石 U-Pb 年龄与花岗闪长岩 (S62) 相当，具有正 ε _Hf (t) 值（从 +6.4 到 +10.4）。这表明花岗闪长岩的侵入引起了主岩中锆石晶粒的结晶。