EGU2020-10410
https://doi.org/10.5194/egusphere-egu2020-10410
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Cold avalanche, “super subduction”, mantle overturn, followed by buoyant subduction of an oceanic plateau and the formation of TTG´s during the Eocene in Viti Levu, Fiji islands

Holger Sommer1, Alfred Kröner2, Dorrit E. Jacob3, Xiao-chao Che2, Jean Wong4, and Hangqiang Xie2
Holger Sommer et al.
  • 1Department of Geology, The University of Namibia, Southern Campus, Windhoek, Namibia, e-mail: info@holgersommer.de
  • 2Beijing SHRIMP Centre, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, 100037, China
  • 3Australian Research Council Centre of Excellence for Core to Crust Fluid Systems and Department of Earth and Environmental Sciences, Macquarie University, North Ryde, NSW 2109, Sydney, Australia
  • 4Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China

Tonalite, Trondhjemite, Granodiorite (TTG) rocks in Viti Levu, Fiji islands formed through hydrous melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek. During mid Miocene time, magmatic underplating took place and a Qtz-diorite unit was formed out of the gabbro under granulite-facies conditions. The investigated TTG´s occur as stocks and veins within the older gabbroic unit of the Yavuna Pluton.

Zircon ages show the parental gabbro to be ~47.5 Ma in age, whereas the TTG´s, which can be subdivided into a tonalite and a Qtz-diorite suite, are ~37.1 Ma and ~16.5 Ma, old respectively. The average d18O value of ~4.8 in zircon selected from the parental gabbro and the tonalite suggest a very homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples showing lower d18O values, and these are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep depleted mantle source. eHf in zircon values of ~13 in the analyzed TTG´s are interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations suggest that the tonalite formed by melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ~770 °C and a pressure of ~3.8 kbar, whereas the Qtz-diorite formed at a temperature up to ~900 °C at very shallow depth close to the Earth’s surface caused by the emplacement of a magmatic underplate during the mid Miocene. Our investigation provides new evidence for episodic growth of continental crust < 0.1 Ga in the South Pacific region.

How to cite: Sommer, H., Kröner, A., Jacob, D. E., Che, X., Wong, J., and Xie, H.: Cold avalanche, “super subduction”, mantle overturn, followed by buoyant subduction of an oceanic plateau and the formation of TTG´s during the Eocene in Viti Levu, Fiji islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10410, https://doi.org/10.5194/egusphere-egu2020-10410, 2020

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