Insights on Archean continental geodynamics from gravity modelling of granite–greenstone terranes

Abstract

Abstract This work investigates continental geodynamics, in the Middle and Late Archean, by way of gravity models of cratonic regions that range in age from ca. 3.4 to ca. 2.7 Ga. Selected regions of three major cratons are compared: the Pilbara craton, Australia (3.4–3.2 Ga), the Yilgarn craton, Australia (2.7 Ga) and the Abitibi Subprovince (2.7 Ga), which is part of the Superior Province, Canada. The modelled regions include large tonalite–granodiorite batholiths hosted within greenstones. Each region is modelled using several 2-D gravity profiles to obtain a 3-D picture of the first order structure of the upper to middle crust. In the 3.4 Ga Pilbara craton, some modelled batholiths have well defined roots that extend to depths greater than 10 km whereas others have roots extending to less than 6 km depth. The greenstones form keels, some extending to 10 km depth. The models for the 2.7 Ga Yilgarn craton and the Abitibi Subprovince show no major differences between the two younger terranes. In them, batholiths share a commonly thin shape with shallow roots that are not as well defined as those of the Pilbara batholiths. The rarity of deep roots and a thickness of 5–6 km are characteristics of the ca. 2.7 Ga batholiths. The surrounding greenstones form keels of up to 10 km depth, as in the modelled regions of the older Pilbara craton. Comparison of the results from the older and younger cratons suggests the following. (1) The presence (Middle Archean) or quasi-absence (Late Archean) of deep batholith roots may indicate changes in continental geodynamics and in the processes of cratonic formation and batholith assembly from 3.4 to 2.7 Ga. The differences in batholith shapes may be interpreted as the result of an evolution in the processes involved in crustal differentiation from a crustal-scale diapirism-type model prior to 2.8 Ga to a more plate tectonic-like model after 2.8 Ga. At least, the model results suggest that if crustal diapirism were an important process in the Archean, it may have been limited to the period preceding 2.8 Ga. (2) Greenstones in the Middle and Late Archean terranes form deep keels, which are consistent with the crustal diapirism model in the case of the older greenstones, and may be explained by crustal folding in the younger terranes.