Archaean basalts record evidence of lithospheric extension prior to cratonisation

Marthe Klöcking , Karol Czarnota, Ian H. Campbell, Hugh Smithies, David C. Champion, D. Rhodri Davies

The dynamics of Earth’s early mantle remain enigmatic. A wide range of tectonic settings have been proposed for the Archaean eon prior to cratonisation, a time from which few samples are preserved in the geological record. Here, we reexamine this topic by estimating temperatures and depths of melt generation in the late Archaean mantle using a new geochemical data compilation of mafic igneous rocks from the Yilgarn craton, Australia. We combine these results with stratigraphic and geodynamic constraints to better resolve Archaean upper mantle dynamics. The igneous data compilation was screened to identify samples most representative of melting conditions in the convecting mantle and to minimise the effects of crystal fractionation and assimilation of crustal or cumulate material. The dataset predominantly comprises tholeiitic basalts in the well-studied Kalgoorlie terrane that lie at the base of the stratigraphic sequence beneath komatiites, later mafic to felsic volcanic sequences, and the granites that make up the bulk of the Yilgarn cratonic crust. The screened data display depleted MORB-like rare earth element patterns with no evidence of a garnet signature. Forward and inverse modelling of these compositions, assuming a partially depleted peridotite mantle source, predicts melting at depths as shallow as ~40 km and mantle potential temperatures elevated by ~110-270 °C compared to present-day ambient mantle. These results are consistent with decompression melting under near-ambient Archaean mantle conditions. Lithospheric extension and the calculated temperature excess could be caused by non-adiabatic mantle flow ahead of a rising plume and/or melting of the plume head itself.