Conference Agenda

The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dike stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats known from terrestrial and coastal-zone-facies sandstones of this unit. We differentiate three types of hydrothermal alteration: (1) Iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (2) silicification replaced matrix and most non-silica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (3) steam fracturing brecciated consolidated Moodies sandstone by closely spaced, randomly oriented fractures and quartz veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dikes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also – within the resolution of radiometric age data – contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archean microbial mats in a “thermal window” in the central BGB was controlled by the “just right” degree of heating and very early hydrothermal silicification.

“Cratonization” is a contentious term which may include the first-time access of surface systems to granite across large subaerial surfaces. Quartz- and feldspar-rich terrestrial and shallow-water sandstones of the Moodies Group of the BGB have at times been considered to represent such a signal (a view also held at times by the first author), documenting the age and mode of Kaapvaal craton stabilization.

Four lines of evidence, however, suggest an orogenic, intra-greenstone belt source for these up to 3.7 km thick sandstones, rather than enigmatic, now completely eroded extra-BGB granitic sources: (1) Texturally and mineralogically supermature Moodies sandstones can be traced up-paleocurrent into immature, feldspar-porphyry-rich conglomeratic debris flows and matrix-rich sandstones rimming the Onverwacht Anticline; (2) The up to 2.5 km thick rhyodacitic and coarse-grained volcaniclastic unit H6 of the Onverwacht Anticline, representing a stage of felsic volcanism in the upper Onverwacht Group at ~3445±4 Ma, and some units of the Schoongezicht Fm. (ca. 3227 Ma) show abundant microcline and some monocrystalline quartz; (3) Moodies detrital zircon ages suggest provenance from both units and generally lack ages representative of extra-GB sources; and (4) Moodies conglomerate in proximity to the suspected source area is commonly dominated by dacitic porphyry clasts, some of which contain abundant megaquartz, resembling granite. This clast type is subject to rapid mechanical breakdown even under clement weathering conditions and is therefore quickly eliminated with distance from the source. Moodies sandstones do therefore not reflect crustal stabilization of the Kaapvaal craton by buoyant, high-level, now eroded felsic plutons.

The rapidity of textural (sorting, rounding, clay removal) and mineralogical (Q-, F-content) maturation with increasing distance from the source area(s) within a few km may allow the modelling of Archean weathering conditions. Preliminary data indicate a high atmospheric P_CO2, possibly aided by strong tidal action, episodic and high rainfall, and elevated temperatures.