THE GROWTH OF CRATONS – A VIEW FROM THE LITHOSPHERIC MANTLE

The deep roots of continental lithosphere play a key role in the stability and preservation of ancient continental crust. Compressional thickening, or lateral accretion, has long been advocated as a mechanism for creating thick stable cratonic lithospheric keels. Geodynamic modelling (Wang et al., 2018) indicates that the collisional thickening of ~ 100km thick refractory, residual peridotitic mantle can create 200 km thick keels, stable over Gyr periods. The lateral forces required may come from increased convective vigor in Archean times.

The importance of lateral accretion to the craton formation and evolution process is clear from the Neoarchean to Palaeoproterozoic evolution of cratons and is critical to a long-overdue clarification of the term craton, from a timing perspective. It is clear from the complex crustal evolution of most cratons that they were amalgamated, from a mixture of ancient terranes, via lateral accretion during the Meso- to Neoarchean (Kaapvaal, Superior), to re-working (Rae) or assembly in the Palaeoproterozoic (Siberian, Amazonian). After ~1.8 Ga, the evolutionary history of cratons enters the stable phase, only punctuated by the intrusion of deeply derived basalt lithospheric and deep asthenospheric magmas such as lamproites and kimberlites. Hence the term craton should be applied to stable continental masses older than circa 1.8 Ga. New data from a variety of deep mantle keels indicates that significant lithospheric mantle was generated in the period between 2.0 to 1.8 Ga, prior to or during craton amalgamation.

The early part of craton assembly may be illustrated by the formation of the Zealandia continent with its crust comprising a series of arc terranes formed and accreted in Phanerozoic-Cenozoic times. This young crust is under-pinned by highly depleted lithospheric mantle characterized by very low bulk rock Al2O3and highly forsteritic olivine (~Fo 92) - chemically identical to cratonic lithospheric mantle(Scott et al.2019). Zealandia peridotites have a wide range in Re-depletion model ages that reach back into the Archean and are interpreted to reflect buoyant residual mantle formed through Gyr periods, swept together beneath Zealandia during collisional processes (Liu et al., 2015).

Scott etal. (2019) EPSL, 507, 175-1;H. Wang (2019) Tectonophysics, 746, 562-571.