Geochemical constraints on the development of Archaean greenstone belts: New insights from the Stolzburg Complex, Barberton Greenstone Belt, South Africa
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2021
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Abstract
Ultramafic-mafic layered complexes are an important but poorly-studied component of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton greenstone belt, at least 27 such complexes occur intimately with the supracrustal succession. The petrogenesis of one of these layered bodies, namely Stolzburg Complex (SC), is explored together with its relationship to the surrounding Barberton volcanic succession.
In contrast to published models for the development of Barberton layered complexes (namely subvolcanic sills, ponded lavas, and alpine-type tectonites), the present work is most consistent with the emplacement as sheeted sills into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards each magmatic unit in the complexes as a discrete sill. Using major and trace element geochemistry, the Lower and Upper divisions of the Stolzburg Complex are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks.
On the basis of certain geochemical indicators (the low (La/Yb)PM ~ 1.47 and high Zr/Th ~ 135), the SC is shown to be free of crustal contamination, consistent with development in an oceanic setting. The chemistry of the magmatic source appears to be similar to that of the Primitive Mantle, except that it was enriched in LREE. Furthermore, trace element systematics (lack of heavy REE depletion, chondritic Zr/Y and Al2O3/TiO2 ratios) suggest that the parental magma was generated through partial melting of a mantle source with no garnet in the restite. In contrast to the general belief that Al-undepleted melts were generated in the spinel stability field, it is proposed that the SC parental magma was produced through very high degree partial melting of an undepleted mantle source, where garnet was completely consumed. It is speculated that an unknown high pressure phase remained in the restite, consequently retaining Nb and Ta in the residual solid. None of the modern geodynamic environments appear to provide a suitable framework to explain the geochemical aspects of the SC. Consequently, a preferred model is presented, whereby SC parental magmas formed as impact-generated melts. The development of Archaean greenstone belts is, correspondingly, attributed to bolide impact events in the early Earth. Accordingly, the origin of cratonic roots and TTG suites are ascribed to the above processes and not endogenic processes as proposed in previously published studies.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in the Faculty of Science, School of Geosciences, University of the Witwatersrand, 2021