Magma generation and formation of continental crust in the Izu arc

Sammanfattning: The Izu oceanic arc is produced by the subduction of the Pacific Sea Plate beneath the Philippine Sea Plate. The proto-Izu oceanic arc was initially formed at ca. 50 Ma, then it was separated into the Kyushu-Palau and Izu arc by the extension of the Shikoku basin from ca. 25 Ma. After the formation of the Shikoku basin, volcanism initiated in the Izu rear-arc at ca. 17 Ma. Nowadays, arc volcanism has built up a thick crust observed by seismic velocities that extends from the Izu fore-arc to rear-arc and the upper crust has felsic compositions. The bimodal mafic-felsic volcanism in the Izu arc-front displays depleted light rare earth elements whereas in the Izu rear-arc the volcanic rocks display enriched light rare earth elements. International Ocean Discovery Program (IODP) Site U1437 is the first scientific drill site in the Izu rear-arc, which drilled 1.8 km into the volcaniclastic successions between the Manzi and Enpo seamount chains. Site U1437 is divided into seven stratigraphic units that range in age from 0 to ca. 16 Ma. This thesis aims to investigate the magma generation and the formation of continental crust in the Izu rear-arc using boron (δ11B) and oxygen (δ18O) isotopes. The felsic melt inclusions from Unit II and Unit IV show that felsic volcanic rocks are formed by partial melting of the pre-existing altered and fresh oceanic crust. Meanwhile, fractional crystallization of mafic magmas affects the diverse compositions of the oceanic crust protolith. Therefore, partial melting of oceanic crust and fractional crystallization of mantle-derived mafic magmas both influence the formation of felsic continental crust in the Izu rear-arc. These findings indicate that extensional settings, like the Izu rear-arc, should be considered as potential locations for the formation of continental crust in the Phanerozoic, Archean and even in the Hadean.The basaltic melt inclusions in Unit VII show that the mafic volcanic rocks in the Izu rear-arc are not derived from the mantle wedge that is metasomatized by slab-derived fluids. Instead, primary magmas in the Izu rear-arc are formed by mélange diapirs ascending into the mantle wedge. This could explain the across arc mantle heterogeneity observed in the Izu arc, with the arc front volcanic rocks are associated with slab-derived fluids and the Izu rear-arc compositions are influenced by mélange. Mélange compositions generated in the rear-arc may be recycled into the upper mantle forming the upper mantle heterogeneity observed in MORB globally.

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