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

Abstract

Significance 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.

Reply to Zou et al.: Neoarchean eclogite-facies oceanic crust in the North China Craton

“…Low-sodic clinopyroxene is typically caused by 1) low protolith Na 2 O, indicated by bulk compositions (table S1 in ref. 2 ), similar to some normal mid-ocean ridge basaltic gabbro from the modern seafloor ( 9 ); or 2) dehydroxylation, and/or breakdown of clinopyroxene during exhumation, reported from other UHP eclogites ( 10 , 11 ).…”

“…However, Zou et al. ( 1 )’s claims concerning our geobarometry and petrography ( 2 ) are demonstrably misleading.…”

No evidence for Archean eclogite-facies metamorphism

“…Furthermore, the petrographic features presented in Ning et al. ( 1 ) are inconsistent with eclogite-facies metamorphism. Neither garnet exsolution from clinopyroxene nor rutile exsolution from garnet emphasized by the authors is compelling evidence for eclogite-facies metamorphism.…”

“…Ning et al. ( 1 ) report Archean eclogite-facies metamorphic conditions (792 °C to 890 °C, 19.8 kbar to 24.5 kbar) whereby they argue that deep subduction and thus modern-style plate tectonics have operated since at least the Late Archean. However, the conclusion is compromised by flawed geobarometric estimates and petrography that is inconsistent with eclogite-facies metamorphism.…”

“…( B ) Comparison between experimental calibration range of the Grt-Cpx geobarometer ( 2 ) and Al iv contents of metamorphic Cpx in Ning et al. ( 1 ) (after ref. 2 ).…”