Petrogenesis of subduction-related lavas from the southern Tonga arc

Abstract

We present new whole-rock geochemical data for volcanic rocks dredged from active submarine volcanoes TA15, TA16, and TA19 that mark the southern part of the Tonga Arc near Ata island. The lava samples show a wide range of compositions from basalt to dacite within the low-K tholeiitic series, and geochemical data indicate fractional crystallization of olivine, orthopyroxene, clinopyroxene, and plagioclase. The origin of magmas in the southern Tonga Arc can be explained by the mixing of three components: a depleted mantle wedge, a melt derived from the partial melting of subducted pelagic sediments (PS) and sediments of the subducted Louisville Seamount Chain (LSC), and a fluid produced during subduction by the dehydration of altered oceanic crust (AOC). Our geochemical modeling suggests that mafic magmas in the study area can be generated by 10%–25% partial melting of a depleted mantle that had been metasomatized by ~1% AOC fluids as well as ~1% melts derived from a 40:60 mixture of LSC and PS sedimentary components. On the other hand, volcanoes V, U, and Monowai, located farther south near the present LSC–Tonga Trench intersection, can be generated by 15%–25% partial melting of a depleted mantle that had been metasomatized by ~0.5% AOC fluids as well as ~1.5% sediment-derived melts (a 90:10 mixture of LSC and PS). These results are consistent with previous proposals that the influence of the LSC-derived components was stronger in the southerly volcanoes V, U, and Monowai than in the volcanoes near Ata. © 2019 Elsevier Ltd