Evidence for 3.3-billion-year-old oceanic crust in the Barberton greenstone belt, South Africa

Abstract

Recognition of oceanic crust in Archean greenstone belts has remained a controversial and unresolved issue, with implications for understanding early Earth geodynamics. In the search for early Archean oceanic crust, we present a multi-pronged approach to test for the presence of an ophiolite-type sequence preserved in the Paleoarchean Barberton greenstone belt (BGB) of South Africa. New field observations are combined with detrital U-Pb zircon geochronology and geochemistry on fresh drill-core material from the Kromberg type-section sequence of mafic-ultramafic rocks in the 3.56–3.33 Ga Onverwacht Group of the BGB. Trace element geochemistry indicates that the Kromberg metabasalts were derived from the primitive mantle. The e Nd values and Nd model ages of the metabasalts record a depleted Archean mantle source similar to CHUR (chondritic uniform reservoir) with no continental (tonalite-trondhjemitegranodiorite [TTG]) crustal contamination. U-Pb geochronology by laser ablation–inductively coupled plasma–mass spectrometry on detrital zircons from an uppermost chert unit indicate a homogeneous age distribution and a gabbroic source in the greenstone belt, in direct contrast to zircons from felsic conglomerates structurally underlying the Kromberg sequence. Collectively, the new data and field observations indicate that the 3.33 Ga Kromberg mafic-ultramafic sequence formed in a juvenile oceanic setting and represents a remnant of tectonically accreted oceanic crust. The identification of oceanic crust preserved as dismembered allochthons in the BGB suggests that horizontal plate tectonic processes were operating on the Archean Earth as early as Mesoarchean times.