[M-GI31] 地球掘削科学

[MGI31-06] Igneous petrology of basaltic lavas and intrusions from the Agulhas Plateau: Preliminary results of IODP Expedition 392

*市山 祐司¹、森 光貴¹、石塚 治²、Jörg Geldmacher³、Peter Davidson³、Jia Liu⁴ (1.千葉大学、2.産業技術総合研究所、3.GEOMAR、4.浙江大学)

キーワード：アグラス海台、IODP第392次航海、巨大火成岩岩石区、ドレライト岩床

Basaltic lavas and intrusions (probably sills) were recovered from three sites (U1579, U1580, and U1582) in the Agulhas Plateau during IODP Expedition 392. Two basaltic intrusions were collected from Site U1579, and six basaltic intrusions with variable thicknesses from Site U1580 came out from 407 m to 528 m (depth below seafloor). Bubbles filled with secondary minerals are rich in upper marginal parts and decrease into internal parts in each intrusive. Pipe-like bubbles are also usually present in lower marginal parts. In each intrusive, marginal parts near the contact with sediments show volcanic textures, which consist of olivine, plagioclase, and clinopyroxene phenocrysts with the fine-grained groundmass of altered glasses and plagioclase laths. The olivine phenocrysts are completely altered, but plagioclase and clinopyroxene are well-preserved. While the grain size of crystals in each intrusive gradually increases into internal parts, the lithologies change from ophitic dolerite to granular gabbro. The mineral assemblage of dolerite and gabbro is olivine (altered), plagioclase, and clinopyroxene, and they crystallized in this order. Magnetite, apatite, and rare chromian spinel are included as accessory minerals. It is worth noting that the thickest intrusive (490-528 m) from Site U1580 shows different petrographic characteristics from the other intrusions. It sometimes contains dissolved plagioclase, interstitial quartz, and rare hydrous minerals (amphibole and biotite), and displays the earlier crystallization of clinopyroxene than plagioclase. These observations might imply the intrusion of H₂O-rich fractionated basaltic magma. Pillow lavas were successfully collected from Site U1582. The lavas are sparsely olivine- and plagioclase-phyric basalt with well-preserved glassy rinds at pillow margins. The olivine phenocrysts are completely altered.
The whole-rock geochemistry determined using an XRF spectrometer shows a tholeiitic trend in a FeO*/MgO-SiO₂ space. The oxide contents of the Site U1580 intrusions decrease in TiO₂, FeO*, Na₂O, K₂O, and P₂O₅ and increase in CaO from the top to the bottom. In particular, SiO₂ content and FeO*/MgO reach the highest values at about 510 m in the thickest intrusive of the U1580 intrusions. The whole-rock composition and petrographic characteristics of the thickest sill indicate that fractional crystallization effectively drove in the sill. The Zr/Y ratios of Site U1580 decrease into the bottom, indicating the gradual change in the fertility of magma composition. Compared with the two other sites, the samples from Site U1582 show the lowest Zr/Y ratios, and the ratios from Site U1580 are the highest and most various. As a whole, the whole rock composition of lavas and intrusions recovered from the Agulhas Plateau is consistent with those of MORB. In further investigation, we are planning to analyze the detailed geochemical compositions of rocks and minerals to determine the characteristics of the magmas that formed the Agulhas Plateau.