Petrological, mineralogical and geochemical peculiarities of Archaean cratons

Abstract

The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength. A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C. Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al 2 O 3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter. We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.