09:15 〜 09:30
[MGI32-07] Early Cretaceous syn-rift volcanic sequence on the Naturaliste Plateau, IODP Expedition 369 Site U1513
キーワード：Naturaliste Plateau, IODP Expedition 369, Site U1513, East Gondwana breakup
Drilling on the eastern flank of Naturaliste Plateau, offshore southwest Australia during the International Ocean Discovery Program Expedition 369 recovered a volcanic sequence at Site U1513. Up until this drilling expedition, no direct stratigraphic evidence corroborates the extensive volcanism that accompanied Early Cretaceous rifting between Australia-Antarctica and Greater India during breakup of Eastern Gondwana, which was inferred only from seismic reflection and dredging data. The volcanic sequence consists of several lava flow packages separated by volcaniclastic flow units. Multiple dolerite dikes intruded along steeply dipping faults. Paleomagnetic data obtained from the overlying sedimentary sequence indicate emplacement before chron M10Nn.1n, which could be as young as 130.9 Ma or as old as 134.2 Ma, depending on the time scale used. Age obtained from plagioclase separates from the topmost flow unit overlaps with the younger paleomagnetic age of 130 Ma, suggesting that deposition of the overlying sediments happened soon after. This interpretation agrees with subaerial to shallow water mode of emplacement inferred from petrological and textural evidence. Younger crystallization age of 123 Ma obtained from a dolerite dike and the 124-112 Ma alteration ages determined from the lower flows indicate that magmatism remained active during the Aptian (126.3–113.0Ma). Trace elements vary from light rare earth element (LREE) enriched transitional basalt to LREE-depleted tholeiitic basalt compositions. This is coupled with a change from isotopically more enriched to depleted compositions toward the upper flows, suggesting evolving mantle source. These observations possibly indicate thinning lithosphere from rifting to breakup of East Gondwana and increasing input from asthenospheric source.