The Jamestown Ophiolite Complex, Barberton mountain belt: a section through 3.5 Ga oceanic crust

The mafic to ultramafic rocks of the Barberton greenstone belt, South Africa, form a pseudostratigraphy comparable to that of Phanerozoic ophiolites. This Archaean complex, referred to here as the Jamestown Ophiolite Complex, consists of a high temperature tectono-metamorphic peridotite overlain by an intrusive extrusive igneous section, which in turn is capped by a chert-shale sequence. There is a complete range from komatiitic to tholeiitic compositions within single intrusive units. Crustal contamination and magma mixing is evident from field and geochemical data.

Pillow structures, ⁴⁰Ar/³⁹Ar ages and oxygen isotope analysis suggest that hydrothermal interaction with the Archaean ocean severely hydrated and chemically altered the entire simatic section during its formation. As a consequence, only a ‘ghost’ igneous geochemistry is preserved. This regional open-system alteration may have increased the MgO content of the igneous rocks by as much as 13%, and the most primitive liquids, from which the extrusive sequence evolved, were ‘picritic’ in character. Rocks with a komatiitic chemistry were derived during crystal accumulation from picritic-crystal mushes (predominantly olivine-clinopyroxene) and/or by metasomatism during one or more subsequent episodes of hydration-dehydration.

In contrast to Phanerozoic ophiolites, the Jamestown complex is relatively thin (≦3 km), which implies that locally at least the ca 3.5 Ga oceanic crust was also thin. This is consistent with the regionally extensive metasomatic alteration, and is compatible with theoretical and experimental models predicting higher Archaean heat transfer from the mantle concentrated within Archaean oceans.