Geochemistry and petrology of lavas in the upper onverwacht suite, Barberton Mountain Land, South Africa

摘要

The basis of this study comprises 540 geochemically analysed samples collected systematically from eighteen stratigraphie sections (2257 m in total length) through the submarine volcanic rocks of the tectonically separated Hooggenoeg, Kromberg and Mendon Complexes in the southwestern part of the Paleoarchean Barberton Greenstone Belt. The lavas are predominantly high- to lowMgO tholeiitic basalts but include minor komatiite and komatiitic basalt. They are non-deformed and preserve delicate igneous textures, but have been allochemically altered and regionally metamorphosed. Comparison of pillow cores and rims, samples from massive flows and interelement correlations demonstrate that Ti, Al, Cr, V, Nb, Ta, Zr, Hf, Y, Pb, Th and the REE were relatively immobile during alteration and hence preserve geochemical evidence bearing on the origin and tectonic setting of the lavas. Chondrite-normalized REE patterns are slightly LREE depleted in komatiite to slightly LREE enriched in basalts. MORBnormalized values of non-conservative elements (Cs, Ba, Pb, Th) are high relative to conservative elements (Ta, Nb, Zr, Hf, Y, HREE) in all of the volcanic rocks, particularly those of the Hooggenoeg Complex. Most of the samples exhibit enrichment of Cs and Ba, Pb anomalies and depletion in Nb and Ta, consistent with a subduction-related oceanic environment. With the exception of the lavas of the Hooggenoeg Complex, using primordial mantle (PM) values for normalisation generally subdues the enrichments of the non-conservative elements seen in MORB-normalised multi-element diagrams. However, negative Nb and Ta anomalies relative to La remain significant. High Ba/Th ratios indicate relatively shallow level enrichment of the magma source in large ion lithophile elements by aqueous fluids, whilst enhanced Th concentrations reflect deeper partial melting. Estimated subduction related contributions to Th vary in the ca. 2700 m thick section of the Hooggenoeg Complex and suggest changes in the depth to the subducting slab, which may relate to variation in the subduction angle in the course of ca. 10 million years. ε_(Nd(T)) values suggest earlier melt extraction and possible incorporation of older crustal material, probably subducted clastic sediments. Our inferred model for the formation of the Upper Onverwacht Suite, based on the lithological and structural development of the lavas and their geochemistry invokes eruption in intra-oceanic back-arc basins and volcanic island arcs. Magmas were generated by variable degrees of partial melting at different depths and temperatures of metasomatised mantle above subducting and dehydrating oceanic lithosphere, and were subsequently modified by fractional crystallization and hybridization. In terms of MORBand PM-normalised multi-element patterns, as well as Nd-isotope ratios, the volcanic rocks of the Onverwacht Suite are comparable with the west Pacific-Indonesian arc systems.