島根県津和野地域の舞鶴帯から古原生代18.5億年花崗岩質岩体の発見とその意義

島根県津和野地域の舞鶴帯北縁部から古原生代の花崗岩類と正片麻岩を見出した．この岩体は苦鉄質岩類を伴い，ペルム系の堆積岩類に挟まれた幅数百mの狭長な地帯にブロックの集合体として産する．ジルコンのU-Pb年代は，正片麻岩が1836±17Ma，トーナル岩-石英閃緑岩が1853±14Ma，花崗閃緑岩が415.2±2.5Maであった．古原生代のジルコンは2.8-2.1Gaの古いコアを持ち，北中国地塊由来と考えられる．デボン紀のものは年代から舞鶴-大江地域の舞鶴帯北帯西部岩体に対比され，岩相構成と産状から古原生代岩体も舞鶴帯北帯構成要素と考えられる．本岩体は，古い地質体ほど構造的上位に位置するという西南日本内帯のパイルナップ構造の一般則から外れ，ジュラ紀の付加体・変成岩と接する舞鶴帯北縁部に産することから，舞鶴帯北縁に沿う横ずれ断層によって異地性ブロックとして定置した可能性がある．
We report a newly discovered Paleoproterozoic granitoid at the northern margin of the Maizuru Terrane (Tsuwano area, southwestern Shimane Prefecture, SW Japan). The granitoid is associated with metadolerite and metagabbro, and appears as lenticular blocks within a narrow (150-300 m wide) zone. Orthogneiss and tonalite-quartz diorite from the granitoid yield zircon U‒Pb ages of 1836±17 Ma and 1853 ± 14 Ma, respectively. Younger U‒Pb ages of 415.2±2.5 Ma were obtained from a granodioritic lithology. The Paleoproterozoic zircon grains commonly contain inherited older (2800‒2100 Ma) cores. The Paleoproterozoic age of the granitoid body supports the suggestion that this part of the Maizuru Terrane includes exotic blocks that originated from the North China Craton. The younger (Early Devonian) granodiorite is possibly related to igneous rocks of similar age from the Maizuru‒Oe area (Northern Zone of the Maizuru Terrane). Based on the lithology and structural position of the Paleoproterozoic rocks, it is suggested that they are part of the Northern Zone. The recognition of a narrow zone of Paleoproterozoic rocks embedded in Permian strata is in contrast to the nappe structure in SW Japan, which typically contains older units at higher structural positions. We therefore suggest that the Paleoproterozoic blocks were displaced by strike-slip faulting along the northern boundary of the Maizuru Terrane.
本研究には，文部科学省の2013-～2015年度科学研究費補助金（課題番号 : 25400486）を使用した．