[S-MP24] 変形岩・変成岩とテクトニクス

[SMP24-P20] カナダ、スレーブ州の始生代アカスタ片麻岩における古原生代広域加熱事件

佐藤 恵、兵藤 博信²、杉浦 慶³、辻森 樹³、*板谷 徹丸^1,4 (1.蒜山地質年代学研究所・地質技術センター、2.岡山理科大学、3.東北大学理学研究科、4.地球年代学ネットワーク)

キーワード：K–Ar系年代、黒雲母、角閃石、アカスタ片麻岩、ウオップメイ造山帯、カナダスレーブ州

The Slave Province of Canada is an Archean granite-supracrustal terrane in the northwestern corner of the Canadian Shield. It is bounded by the Thelon-Taltson Orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay Orogen (1.9 to 1.8 Ga) to the west. The Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located near the gneisses, were systematically collected for K-Ar and laser step heating ⁴⁰Ar /³⁹Ar single crystal analyses of biotite and amphibole. The K-Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The ⁴⁰Ar/³⁹Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages are within the reported U-Pb zircon age range of the Wopmay Orogen. The K-Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The ⁴⁰Ar/³⁹Ar analyses of biotite crystals from the Acasta gneisses show a wide range of ages compared to the Wopmay samples. The three samples yield plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K-Ar amphibole ages range from 1737 ± 27 Ma to 1952 ± 27 Ma, with the exception of a younger sample (1685 ± 25 Ma), giving a wider range of ages compared to the biotite ages. The ⁴⁰Ar/³⁹Ar analyses of the amphibole crystals show the different age relations. The two samples yield plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Two crystals from a mafic sample were analyzed. One crystal shows a consistent age relation between the plateau age (1847 ± 15 Ma) and the total gas age (1827 ± 14 Ma). Another crystal shows no plateau age spectra and has an apparent age of 2000 Ma in the middle temperature fractions. Crystals from several other samples also have fractions older than 2000 Ma. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny, but not completely reset. These observations suggest that the K-Ar system ages of biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the late Paleoproterozoic. The heat source that formed the Hepburn batholith north of the Acasta gneisses is thought to be asthenospheric extrusion into the wedge mantle, a process probably triggered by subduction rollback. The Hepburn plutons extend in a north-south direction. It is likely that asthenospheric intrusion occurred in a strip along this direction. This intrusion also occurred beneath the Acasta gneiss, which exists on the extension of the N-S trending Hepburn plutons, and had a thermal effect on the Acasta gneiss, rejuvenating the K-Ar system ages of biotite and amphibole.