The thermal evolution of subduction zone lithosphere: Evidence from the chemical development of Mt. Ruapehu and surrounding vents, New Zealand.

Mt. Ruapehu is an andesite volcano located at the southwest termination of the Taupo Volcanic Zone (TVZ) in the North Island of New Zealand where the Pacific Plate subducts beneath the Australian Plate. Volcanism has migrated south with time as rifting from the Kermadec arc has propagated into the North Island lithosphere making Ruapehu the most recent addition to the TVZ. A detailed study of volcanic rocks and crustal xenoliths collected from Mt. Ruapehu and nearby satellite vents was conducted to establish how differentiation processes in arc magmas change with time in response to tectonic movements in the arc lithosphere. The thickness and composition of the arc crust in the southern TVZ directly controls the differentiation histories of magmas from Mt. Ruapehu and satellite vents. The crustal column beneath Ruapehu is composed of two main lithologies; a localised 5-10 km thick lower crustal meta-igneous granulite overlain by 20-25 km of meta-sediment. While fractional crystallisation is an important process in modifying the major and trace element concentrations, crustal assimilation plays a major role in modifying the isotopic composition of magmas from the southern TVZ. New trace element and isotope data for lavas and crustal xenoliths confirm a distinct change in assimilants between the oldest (Te Herenga - TH) and younger formations (Post Te Herenga - PTH) at Ruapehu. TH lavas are a product of lower crustal differentiation involving assimilation of meta-igneous granulite. PTH lavas display evidence for interaction with the same meta-igneous granulite followed by contamination with meta-sedimentary crust. This change in assimilant coincides with increased rates of extension at the southern tip of the Taupo Volcanic Zone (TVZ). We propose that the change in melt-crust interaction reflected in major geochemical changes from TH to PTH lavas is a response to adjustments in differentiation depth. Peripheral satellite vents near Ruapehu have produced some of the most primitive lavas in the southern TVZ with compositions resembling high-Mg andesite. This contrasts with the central-northern TVZ where the most primitive lavas are high-alumina basalt. The spatial distribution of these two magma series has developed as a result of the evolution of TVZ lithosphere. High-Mg andesites appear to be generated from depleted mantle during the very earliest stages of arc development when the lithosphere is thicker causing fractionation at relatively deep levels. High-alumina basalts however, represent a more mature phase of arc development and are derived from relatively fertile mantle following lithospheric thinning causing fractionation at relatively shallow levels. Finally, new osmium isotope data from meta-igneous xenoliths indicate the lower crust beneath Ruapehu is not formed of altered Jurassic oceanic crust. The relatively unradiogenic 187Os/188Os ratios of the xenoliths instead suggest they represent localised underplated material, which has only developed beneath the graben axis where the magma flux is greatest