image: The left part shows the >3230 Ma TTGs formed by melting of intracrustal mafic rocks with no involvement of supracrustal sediments or low-temperature altered basalts. Compressional tectonics took place at ca. 3230 Ma and allowed recycling of supracrustal materials in deep crust due to crustal thickening or proto-subduction. The subsequent melting of supracrustal materials and mafic rocks within a partly thickened lithosphere generated the ≤3230 Ma TTGs with systematically elevated oxygen isotopes. view more
Credit: ©Science China Press
On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth’s surface to its interior, but how far this mechanism dates back in the Earth’s history is uncertain.
The Kaapvaal Craton and specifically the Barberton granitoid-greenstone terrain (BGGT) provide a robust rock-based benchmark for early Earth studies because of exceptional preservation of early Archean TTG and greenstone rocks. The authors examined O isotopes in zircon grains from tonalite–trondhjemite–granodiorite (TTG), the major felsic lithology of the Archean continental crust, to evaluate the recycling of supracrustal materials in the Archean.
Zircon δ18O values of the TTG rocks in the Barberton area show an overall increase in terms of ranges and averages at ca. 3230 Ma, close to the Paleoarchean-Mesoarchean boundary. The >3230 Ma TTGs have zircon δ18O values mostly in the range of mantle zircon with averages from +5.07‰ to +6.02‰, whereas the TTGs of ≤3230 Ma have mildly elevated zircon δ18O values with the largest value close to ~8‰ (averages ranging from 5.95‰ to 7.08‰).
These findings from the BGGT TTGs provide the first robust evidence for significant crustal recycling to depths of >40 km from ca. 3230 Ma in ancient continents. Because 18O enrichment is a diagnostic signature of low temperature water-rock interaction on the Earth’s surface, the nearly synchronous increase in the maximum zircon d18O points to the onset of widespread reworking of supracrustal materials at crustal depths of >40 km at the Paleoarchean-Mesoarchean boundary.
The authors also proposed that plate tectonics was operating through warm subduction for lithospheric thickening along converging plate boundaries. As soon as the mafic oceanic crust was transformed to garnet granulite through regional metamorphism at moderate thermal gradients, gravitational delamination (or proto-subduction) and large thrust faults would take place along previously thickened boundaries. This may induce the partial melting of the low-T altered mafic oceanic crust to form high-δ18O zircon in the resultant TTGs. Such proto-subduction may be short-lived and intermittent due to frequent slab breakoff in the hotter Archean mantle.
See the article:
The onset of deep recycling of supracrustal materials at the Paleo-Mesoarchean boundary. Xiao-Lei Wang et al. National Science Review, https://doi.org/10.1093/nsr/nwab136
Journal
National Science Review