阿武隈山地南部に産する角閃石カンラン岩の岩石学とジルコンU-Pb年代

抄録

Hornblende peridotite occurs in the Nishidohira metamorphic rocks in the southern Abukuma Mountains (Tagiri, 1971 Jour. Japan. Assoc. Min. Petrol. Econ. Geol.; Tanaka et al., 1982 Jour. Japan. Assoc. Min. Petrol. Econ. Geol.). This peridotite is characterized by large poikilitic hornblende grains with rounded olivine (± pyroxenes) grains (we call poikilitic hornblende peridotite hereafter). Poikilitic hornblende peridotite is frequently associated with granitic and metamorphic belts (e.g., Tiepolo et al., 2011 Jour. Petrol.; Itano et al., 2021 Lithos) and in the lower crust to upper mantle sequence of ophiolites (Ozawa, 1984 Jour. Geol. Soc. Japan; Ishiwatari 1985 Jour Petrol.) as a minor component. Poikilitic hornblende peridotite is also found as a sub-arc xenolith (Ishimaru et al., 2009 Jour. Mineral. Petrol. Sci.). The origin and tectonic implications of the poikilitic hornblende peridotite are unclear. The geochronological relationships of poikilitic hornblende peridotite and surrounding rocks provide essential information about their origin and tectonic implications. U-Pb zircon dating provides the timing of magmatic and metamorphic events. We report petrological characteristics and U-Pb zircon ages of the Nishidohira poikilitic hornblende peridotite. Poikilitic hornblende grains exhibit zoning in color and chemical composition, such as a dark core of high TiO₂-Al₂O₃ and a green margin of low TiO₂-Al₂O₃. The dark-colored core of hornblende contains numerous small ilmenite lamellae. Clinopyroxene often occurs in vermicular form with hornblende. Melt compositions calculated from hornblende and pyroxene core compositions based on melt-mineral partitioning (Shimizu et al., 2017 Geochim. Cosmochim. Act for amphibole, and Hart & Dunn, 1993 Contrib. Mineral. Petrol for clinopyroxene) are characteristic of continental arc magmas. The U-Pb zircon age for poikilitic hornblende peridotite is about 120 Ma, which is interpreted as a magmatic age. This period of arc magmatism is consistent with the U–Pb zircon age of gneissic granitic rocks in the studied area (Tagiri et al., 2011 Island Arc), suggesting the development of lower curst in prior to the widespread granitic plutonism in the southern end of the Abukuma Mountains at ca. 110–100 Ma (Takahashi et al., 2016 Island Arc).