[S-CG52] 海洋底地球科学

[SCG52-P13] New geochronological and geochemical data for the late Eocene alkalic volcanic activity at the Conrad Rise, southern Indian Ocean

*佐藤 暢¹、町田 嗣樹²、芦田 果奈²、仙田 量子³、石塚 治⁴、MEYZEN Christine⁵、BIZIMIS Michael⁶、佐藤 太一⁴、藤井 昌和^7,8、野木 義史^7,8 (1.専修大学経営学部、2.千葉工業大学次世代海洋資源研究センター、3.九州大学比較社会文化研究院、4.産業技術総合研究所地質調査総合センター、5.パドバ大学、6.サウスカロライナ大学、7.国立極地研究所、8.総合研究大学院大学複合科学研究科)

キーワード：コンラッドライズ、巨大火成岩岩石区、インド洋南部、プルーム、始新世

Large igneous provinces (LIPs) are mainly voluminous mafic magmatic events of intraplate affinity in continental and oceanic settings related to mantle plumes. Those distributed on the ocean floor are called “oceanic LIPs” and comprise oceanic plateau and ocean basin flood basalts. Oceanic LIPs are distributed in the Indian Ocean, most of which were formed during the Cretaceous. The Conrad Rise (CR) is one of the large igneous provinces in the Indian Ocean. The CR comprises three large seamounts (Ob, Lena, and Marion Dufresne from west to east) and a small unnamed seamount/knoll. Many previous studies have vaguely considered that the formation of the CR occurred during the Late Cretaceous because only a few direct studies have been performed in this area. Furthermore, geochronological studies have yet to be conducted. Here, we present new geochemical data, including Sr-Nd-Pb (-Hf) isotopes and ⁴⁰Ar-³⁹Ar data for the alkaline igneous rocks collected during the R/V Hakuho Maru cruises KH-10-7 in December 2010 and KH-19-1 in January 2019. Our results indicate that the main part of the CR (Ob and Lena seamounts) was formed at approximately 40 Ma in the intraplate setting. The compositions of the volcanic rocks from a small seamount north to the Ob seamount are equivalent to the Indian common plume component (the C component). Geochemical characteristics imply that the volcanic rocks from the CR could have formed by a mixture of the C component and the metasomatized subcontinental lithospheric mantle and/or the lower continental crust. The newly obtained ⁴⁰Ar-³⁹Ar ages denied the hypothesis that a stationary mantle plume is located beneath the CR. Therefore, the upwelling of the asthenosphere resulting from plate reorganization during the late Eocene or a small upwelling “blob,” which was recently proposed to explain seismic tomography in the Indian Ocean, might have yielded the CR lavas.