Zircon U-Pb ages of plutonic rocks in the southern Abukuma Mountains: Implications for Cretaceous geotectonic evolution of the Abukuma Belt

Abstract

Plutonic rocks in the southern Abukuma Mountains include gabbro and diorite, fine-grained diorite, hornblende–biotite granodiorite (Ishikawa, Samegawa, main part of Miyamoto and Tabito, Kamikimita and Irishiken Plutons), biotite granodiorite (the main part of Hanawa Pluton and the Torisone Pluton), medium- to coarse-grained biotite granodiorite and leucogranite, based on the lithologies and geological relations. Zircon U–Pb ages of gabbroic rocks are 112.4 ±1.0 Ma (hornblende gabbro, Miyamoto Pluton), 109.0 ±1.1 Ma (hornblende gabbro, the Hanawa Pluton), 102.7 ±0.8 Ma (gabbronorite, Tabito Pluton) and 101.0 ±0.6 Ma (fine-grained diorite). As for the hornblende–biotite granodiorite, zircon U–Pb ages are 104.2 ±0.7 Ma (Ishikawa Pluton), 112.6 ±1.0 Ma (Tabito Pluton), 105.2 ±0.8 Ma (Kamikimita Pluton) and 105.3±0.8 Ma (Irishiken Pluton). Also for the medium- to fine-grained biotite granodiorite, zircon U–Pb ages are 106.5±0.9 Ma (Miyamoto Pluton), 105.1 ±1.0 Ma (Hanawa Pluton) and the medium- to coarse-grained biotite granodiorite has zircon U–Pb age of 104.5 ±0.8 Ma. In the case of the leucogranite, U–Pb age of zircon is 100.6 ±0.9 Ma. These data indicate that the intrusion ages of gabbroic rocks and surrounding granitic rocks ranges from 113 to 101 Ma. Furthermore, K–Ar ages of biotite and or hornblende in the same rock samples were dated. Accordingly, it is clear that these rocks cooled down rapidly to 300 °C (Ar blocking temperature of biotite for K–Ar system) after their intrusion. These chronological data suggest that the Abukuma plutonic rocks in the southern Abukuma Mountains region uplifted rapidly around 107 to 100 Ma after their intrusion. 南部阿武隈山地に分布する深成岩類は,斑れい岩及び閃緑岩,細粒閃緑岩,角閃石黒雲母花崗閃緑岩,黒雲母花崗閃緑岩,中-粗粒黒雲母花崗閃緑岩及び優白質花崗岩からなる.斑れい岩類のジルコンU–Pb年代は,112 my(角閃石斑れい岩,宮本岩体),109 my(角閃石斑れい岩,塙岩体),103 my(斑れいノーライト,田人岩体)及び101 my(細粒閃緑岩)であった.角閃石黒雲母花崗閃緑岩のジルコンU–Pb年代は,104 my(石川岩体),113 my (田人岩体),105 my(上君田岩体, 入四間岩体)であった.中-細粒黒雲母花崗閃緑岩のジルコンU–Pb年代は,107 my(宮本岩体),105 my(塙岩体)で,中-粗粒黒雲母花崗閃緑岩のジルコンU–Pb年代は105 myであった.また,優白質花崗岩のU–Pb年代は101 myであった.同一試料中の黒雲母及び角閃石のK–Ar年代を測定した結果,これらの深成岩類は,113–101 myに貫入定置し,その後,急速に上昇したことが示唆された.