Recently, the research group led by Professor Timothy Kusky, director of the Central for Global Tectonics of our university, published a paper "Archean eclogite-facies oceanic crust indicates modern-style plate tectonics" in the prestigious top-rated journal PNAS(Proceedings of the National Academy of Sciences of the United States of America). The authors identified residual oceanic crust of eclogite-facies formed at the end of Neoarchean (~2.5 Ga) in the Zunhua segment of the Central Orogenic Belt of the North China Craton. This is the oldest known orogenic eclogite-facies oceanic crust remnant on Earth, which provides a key petrological constraint for the operation of modern plate tectonics at least by the Neoarchean (2.5 Ga). The first author of the paper is Wenbin Ning, a doctoral student of the School of Earth Sciences. The corresponding authors are Professor Timothy Kusky and Lu Wang of the School of Earth Sciences and the State Key Laboratory of Geological Processes and Mineral Resources. The co-author is Bo Huang, an Associate Researcher of the Badong National Observation and Research Station of Geohazards, Hubei Province.
The plate tectonic theory is one of the most important achievements in Earth Science, which connects Earth's surface and interior systems through subduction of lithospheric plates and magmatism along mid-ocean ridges, and is an important mechanism for heat transfer from Earth's interior. The prevailing view about plate tectonics is that, with the continuous cooling of the mantle, the thickness and strength of the lithosphere changed, resulting in the transition of the plate tectonic style, from early-style plate tectonics to modern-style plate tectonics. Eclogites and blueschists are high-pressure/low-temperature metamorphic rocks that are considered to be diagnostic products of plate tectonic-related subduction of oceanic plates at convergent boundaries in the modern plate tectonic mosaic. However, a lack of Archean age eclogites or eclogite-facies crustal rocks has led to an assertion that modern plate tectonics did not operate in the Archean.
Through detailed structural field mapping, metamorphic petrology, geochemistry and isotopic dating, Professor Timothy Kusky’s team has identified eclogite-facies oceanic crustal remnants within Neoarchean Zunhua-Shangying ophiolitic mélange (2.50–2.52 Ga) of the Central Orogenic Belt. This oceanic crustal remnant consists of mafic-ultramafic blocks in dominantly metasedimentary matrix, showing typical characteristics of mélanges of all ages, such as "blocks-in-matrix" structures. Geochemical analyses reveal that the metagabbro blocks and the enclosed garnet clinopyroxenite with geochemical affinities of N-MORB (normal mid-ocean ridge basalts) are metamorphosed oceanic gabbro. The peak metamorphic P-T conditions of the garnet clinopyroxenite are 792–890°C/1.98–2.45 GPa, reaching eclogite-facies. Geochronological data indicate that this metamorphism occurred before 2.47 Ga. The authors then conclude that the garnet clinopyroxenite and metagabbro blocks in the Zunhua-Shangying ophiolite mélange represent the relicts of the subducted Archean oceanic crust, which have subducted to a depth of 65–70 km, followed by the breakup of the oceanic crust and differential exhumation in the subduction channel to form the ophiolite mélange.
This study documents the oldest known orogenic eclogites derived from oceanic crust on Earth, pushing back the previously accepted time of eclogite-facies metamorphism by 400 Ma, and suggests that oceanic crust subducted to a depth of at least 65–70 km by the end of the Neoarchean, signifying the operation of modern plate tectonics at least since then.
Article information:
Ning, W. B., Kusky, M.T., Wang, L., Huang, B. Archean eclogite-facies oceanic crust indicates modern-style plate tectonics. Proceedings of the National Academy of Sciences of the United States of America119, https://doi.org/10.1073/pnas.2117529119(2022).
Links to other news reports about this discovery:
PNAS online news release: https://www.pnas.org/doi/10.1073/pnas.2117529119
Center for Global Tectonics: https://cgtch.cug.edu.cn/en/
