Off-sequence plume magmatism near Ninetyeast Ridge in the Indian Ocean: evidence for extensive lateral flow of the Kerguelen plume

Abstract

One of the fundamental questions about Earth processes that is still outstanding is how far a moving plate can drag plume material at its base after its initial impact with the plume. The IODP-362 Expedition recovered igneous samples from drilled sites near Ninetyeast Ridge (NER) in the Indian Ocean. A detailed geochemical and isotopic (Sr–Nd) investigation of these samples found that the basement basalts of the core are typical N-MORB tholeiites, but the igneous sills intruding the upper layer of sediments are highly alkaline, with a composition equivalent to the Kerguelen plume magma. Based on biostratigraphy, the minimum age of the alkaline samples was calculated as c. 58 Ma, which is younger than the adjacent NER crust ( c. 82–78 Ma). The existence of such young plume magma amidst the older blocks of the NER is unusual and contrary to its southwards-younging age pattern. We propose that the fast-moving Indian plate dragged a considerable amount of Kerguelen plume material underneath the Indian Ocean lithosphere northwards during the Cretaceous–Paleocene, covering a longitudinal distance of c. 2220 km. The reactivation of deep fractures triggered decompression melting of the underlying plume material and emplaced this as magmatic sills and lava flows near the NER at c. 58 Ma.