Geochemical evolution of Monowai volcanic center: New insights into the northern Kermadec arc subduction system, SW Pacific

We present trace element and Sr-Nd-Pb isotope data on volcanic rocks recovered from the submarine Monowai volcanic center, which marks the midpoint of the similar to 2500 km long Tonga-Kermadec arc. The center consists of a large (12 x 9 km) partly hydrothermally active caldera and a 12 km diameter similar to 1500 m high volcanically and hydrothermally active stratovolcano. The stratovolcano lavas are tholeiitic basalts, which show variable evidence for plagioclase (+/-pyroxene) accumulation. The caldera lavas range from basalt to andesite, with the compositional variation being consistent with fractional crystallization as the dominant process. The mafic Monowai magmas were generated by relatively high degrees (12%-20%) of partial melting of a previously depleted MORB-type spinel-peridotitic mantle, metasomatized by slab-derived fluids. Strongly fluid mobile Sr-87/Sr-86 and Pb-207/Pb-204 ratios of the Monowai basaltic lavas are similar to those from the Putoto, Raoul, and Macauley volcanic centers 200-400 km to the south, suggesting derivation largely from subducted sediment. In contrast, variably fluid immobile Nd-143/Nd-144 ratios suggest an isotopically heterogeneous mantle along this segment of the arc. Higher Pb-206/Pb-204 in Monowai lavas imply some influence from the nearby subducting Louisville seamounts in melt generation. The formation of one of the Earth's largest submarine mafic calderas can best be explained through drainage of a single magma reservoir and subsequent collapse caused by trench-perpendicular extension, probably via southward progressive rifting of the northern Havre Trough.