Hornblendite delineates zones of mass transfer through the lower crust

Archean eclogite-facies oceanic crust indicates modern-style plate tectonics

The onset time of plate tectonics is highly debated in the Earth sciences. A key indicator of modern-style plate tectonics, with deep subduction of oceanic plates, is the presence of eclogite (oceanic crust metamorphosed at high-pressure and low-temperature) in orogenic belts. Since no orogenic eclogites older than 2.1 billion y are currently documented, many scientists argue that modern plate tectonics started only 2.1 billion y ago (Ga). We document an Archean orogenic eclogite, providing robust evidence that subduction of oceanic crust reached to at least 65 to 70 km in depth at circa 2.5 Ga. This extends the known age of subduction-related eclogite-facies metamorphism back 400 My, showing that modern-style plate tectonics operated by the close of the Archean.

Establishing when modern-style plate tectonics with deep subduction began on Earth is one of the biggest questions in geosciences today. A lack of Archean age (>2.5 billion y ago [Ga]) eclogites or eclogite-facies crustal rocks (the high-pressure equivalent of basalt or gabbro) has led to an assertion that modern plate tectonics did not operate in the Archean. Here, we report eclogite-facies garnet clinopyroxenite associated with metagabbro in 2.52- to 2.53-billion-y-old ophiolitic mélange in the northern Central Orogenic Belt (COB) within the North China Craton. The garnet clinopyroxenites with normal mid-ocean ridge basalt (N-MORB) geochemical signatures are relicts of oceanic crust, recording peak eclogite-facies metamorphic assemblages indicating conditions of 792 to 890 °C/19.8 to 24.5 kbar, supported by abundant exsolution microstructures in garnet and clinopyroxene. Zircon U-Pb dating of the metagabbros and a granitic dike cross-cutting the metamorphic layering of the metagabbro constrain deformation and eclogite-facies metamorphism to >2.47 Ga. This finding implies that Archean oceanic crust was subducted to at least 65 to 70 km at the end of the Archean. Together with other asymmetric subduction records in the COB, it is inferred that modern-style plate tectonics evidenced by deep and asymmetric subduction along the circa 1,600-km-long orogen was operating at least by the end of the Archean era, when the planet was making a transition to the Proterozoic, witnessing the Great Oxidation Event, widespread emergence of continents, and development of crown node eukaryotic species on a more habitable planet.

Evaluating the importance of metamorphism in the foundering of continental crust

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

Cordillera Zealandia: A Mesozoic arc flare-up on the palaeo-Pacific Gondwana Margin

Two geochemically and temporally distinct components of the Mesozoic Zealandia Cordilleran arc indicate a shift from low to high Sr/Y whole rock ratios at c. 130 Ma. Recent mapping and a reappraisal of published Sr-Nd data combined with new in-situ zircon Hf isotope analyses supports a genetic relationship between the two arc components. A reappraisal of geophysical, geochemical and P-T estimates demonstrates a doubling in thickness of the arc to at least 80 km at c. 130 Ma. Contemporaneously, magmatic addition rates shifted from ~14 km³/my per km of arc to a flare-up involving ~100 km³/my per km of arc. Excursions in Sr-Nd-Hf isotopic ratios of flare-up rocks highlight the importance of crust-dominated sources. This pattern mimics Cordilleran arcs of the Americas and highlights the importance of processes occurring in the upper continental plates of subduction systems that are incompletely reconciled with secular models for continental crustal growth.