[S-CG52] Ocean Floor Geoscience

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

*Hiroshi Sato¹, Shiki Machida², Kana Ashida², Ryoko Senda³, Osamu Ishizuka⁴, Christine MEYZEN⁵, Michael BIZIMIS⁶, Taichi Sato⁴, Masakazu Fujii^7,8, Yoshifumi Nogi^7,8 (1.School of Business Administration, Senshu University, 2.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 3.Faculty of Social and Cultural Studies, Kyushu University, 4.Geological Survey of Japan, AIST, 5.Università degli Studi di Padova, 6.University of South Carolina, 7.National Institute of Polar Research, 8.School of Multidisciplinary Sciences, The Graduate University for Advanced Studies, SOKENDAI)

Keywords:Conrad Rise, Large Igneous Province, Southern Indian Ocean, plume, Eocene

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.