The Detrital Zircon Signature of Pre-Plate Tectonic Earth and a 3.2 Ga Transition

Limited preservation of the rock record of early Earth limits our ability to unravel two important questions: when did plate tectonics start and what tectonic regime preceded it? Some suggest initiation of plate tectonics in the time period from Hadean to Neoproterozoic and pre-plate tectonic modes ranging from mobile-lid to stagnant-lid (with or without episodic tectonism). To test such models, we use detrital zircon geochronology to help constrain the tectonic settings of Archean basins. Previously, detrital zircon age patterns have been applied to constrain the transportability of magmatic sources to the depocenters as well as the local resurfacing rate within a plate tectonic context. In this work we use numerical simulations and the geologic record to predict detrital zircon age patterns of basins formed in possible pre-plate tectonic environments. These predictions are then compared with detrital zircon age records for a global compilation of Archean basins, including new data from six metasedimentary rocks from the Pilbara supracrustal belt (Western Australia) that were deposited between 3470 Ma and 2950 Ma. Results from Pilbara samples show that the youngest 80% of the zircons in ~3470 Ma and ~3430 Ma sediments crystallized less than ~70 and ~90 Myr before the time of deposition, whereas for the ~3290-2970, ~3240-3180 and ~2950 Ma units, this age span increases to ~230, 270 and 460 Myr. Global Archean detrital zircons yield similar trends. For sediments older than 3400 Ma, the youngest 80% of the zircons in each sediment crystallized averaged less than ~60 Myr before the time of depositional, whereas for those deposited at 3400-3200 Ma and 3200-2800 Ma, this increases to ~220 and ~290 Myr, respectively. These data are consistent with: 1) progressively slowing resurfacing rates, and/or; 2) more diverse basin types with an increasing contribution of relatively old zircons from Eoarchean to Mesoarchean. Our results are only consistent with plate tectonic depositional settings after 3240-3180 Ma. Before this period, the detrital zircon age records are consistent with non-plate tectonic cooling mechanisms. Consequently, 3240-3180 Ma may record the transition from a pre-plate tectonic mode to plate tectonics.