Boninitic blueschists record subduction initiation and subsequent accretion of an arc–forearc in the northeast Proto-Tethys Ocean

Dong Fu1*, Bo Huang1, Tim E. Johnson2, Simon A. Wilde2, Fred Jourdan2, Ali Polat1,3, Brian F. Windley1,4, Zhaochu Hu1 and Timothy Kusky1*

1 State Key Laboratory of Geological Processes and Mineral Resources, Three Gorges Research Center for Geohazards, Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

2 School of Earth and Planetary Sciences, Curtin University, Perth 6845, Australia

3 Department of Earth and Environmental Sciences, University of Windsor, Ontario N9B 3P4, Canada

4 School of Geography, Geology and the Environment, The University of Leicester, Leicester LE1 7RH, UK

ABSTRACT

Subduction of oceanic lithosphere is a diagnostic characteristic of plate tectonics. However, the geodynamic processes from initiation to termination of subduction zones remain enigmatic mainly due to the scarcity of appropriate rock records. We report the first discovery of early Paleozoic boninitic blueschists and associated greenschists from the eastern Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which have geochemical affinities that are typical of forearc boninites and island arc basalts, respectively. The boninitic protoliths of the blueschists record intra-oceanic subduction initiation at ca. 492–488 Ma in the eastern North Qilian arc/forearc–backarc system, whereas peak blueschist facies metamorphism reflects subsequent subduction of the arc/forearc complex to high pressure at ca. 455 Ma. These relations therefore record the life circle of an intra-oceanic subduction zone within the northeastern Proto-Tethys Ocean. The geodynamic evolution provides an early Paleozoic analogue of the early development of the Izu–Bonin–Mariana arc and its later subduction beneath the extant Japanese arc margin. This finding highlights the important role of subduction of former upper plate island arc/forearcs in reducing the likelihood of preservation of initial subduction-related rock records in ancient orogenic belts.

 Fu et al.-2021-Geology-boninitic blueschist in eastern North Qilian.pdf