Journal of Petrology Advance Access originally published online on January 4, 2008
Journal of Petrology 2008 49(4):697-715; doi:10.1093/petrology/egm080
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Boninites and Adakites from the Northern Termination of the Tonga Trench: Implications for Adakite Petrogenesis
1Arc Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
2School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia
3Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, USA
RECEIVED MARCH 15, 2007; ACCEPTED NOVEMBER 19, 2007
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Abstract |
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Adakitic rocks were recovered by dredging from the northern termination of the Tonga Trench during the 1984 voyage of the R.V. Natsushima and the 1996 voyage of the R.V. Melville. These contain magmatic zircons that have been dated at 2·5 Ma by U–Pb methods, indicating that they are contemporaneous with boninite magmatism previously described from this area. This is the first time adakites and high-Ca boninites have been reported from the same active tectonic setting. The Tonga adakites are classified as high-SiO2 adakites, and are compositionally consistent with an origin as partial melts of subducted Pacific oceanic crust and sediment. Geochemical modelling suggests that the adakites are not involved in the petrogenesis of the Tongan high-Ca boninites. However, the recovery of adakites and boninites from the termination of the northern Tonga Trench suggests that both magma types are related to the unique tectonic setting of this region, where a transition from steep subduction to a transform fault plate boundary has created a slab window with an associated slab edge. Boninites are generated as a result of hot Samoan plume mantle moving through the slab window and subsequently being fluxed by H2O-rich fluids from the subducting Pacific oceanic crust. The Tonga adakites, in contrast, result from the direct melting of the slab edge as a result of the juxtaposition of the subducting slab against hot mantle derived from the Samoan plume.
KEY WORDS: adakites; boninites; Tonga; slab melting; Samoa; slab edge
*Corresponding author. E-mail: trevor.falloon{at}utas.edu.au
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