Magmatic rocks of the Mesoarchean 5-6 km thick Onverwacht Group, include the Theespruit (stratigraphically lowest), Komati, Hooggenoeg and Kromberg (stratigraphically highest) Formations. Stratigraphic logging and sampling of lavas have been carried out through the Hooggenoeg and Kromberg Formations (subsequently referred to HF and KF, respectively). 323 samples have been geochemically analysed from 11 profiles through the HF totalling 1689m of stratigraphic section from bottom to top, and 88 samples have been analysed from 3 profiles (totalling 280m) through the KF.
The pillow lavas (predominant) and massive lavas of the investigated sections are remarkably well preserved despite low grade metamorphism. Substantial alteration of the mafic lavas has, however, taken place during both spilitization and silicification, in particular beneath major chert horizons. In order to constrain element mobility a number of core-rim pairs of pillow lavas and several samples from individual massive lava flows have been analysed. These tests show that Ti, Al, Cr, Ni, Zr are least affected by alteration and hence characterization of the primary stratigraphic geochemical variations is largely based on these elements.
Low- to high-MgO metabasalts and komatiites are present in the HF and KF. The compatible elements Cr and Ni and the incompatible elements Zr and Ti define distinct stratigraphic groupings. In the HF the largest variations (from low-MgO basalts to komatiites) appear in the middle part of the volcanic sequence, whereas in the KF the lavas progressively change from low- to high-MgO metabasalts stratigraphically upward. The covariations between Cr, Ni and Zr, Ti show large scatter, indicating different magmatic processes, including both fractional crystallization and mixing of compositionally distinct magmas. This is in agreement with the physical features of pillow lavas in the uppermost part of the HF, in which magma mingling is clearly displayed. Multi-element, MORB-normalized diagrams of the metabasaltic lavas from various stratigraphic intervals in the HF and KF show flat, MORB-like patterns. The komatiitic samples, on the other hand, define patterns that are comparable to those of Phanerozoic boninites. Tentatively we propose that the magmas of the HF and KF originated from MORB-like to subduction-influenced mantle sources. The chemostratigraphy indicates eruption from magma chambers that evolved through time, and in which there was intermittent mixing of individual magma batches.