CONTRASTING LOW-CA AND HIGH-CA GRANITES IN THE ARCHEAN BARBERTON MOUNTAIN LAND, SOUTHERN AFRICA

Two distinct groups of granites can be recognized among the late pluto ns of the Barberton Mountain Land. Discrimination between the two grou ps is provided by geochemical and mineralogical parameters which are c onsidered to reflect the source material from which the magmas were de rived. A high-Ca suite has geochemical parameters consistent with deri vation from an igneous source (I-type), and an accessory mineral assem blage comprising zircon, apatite, allanite and sphene. A low-Ca suite contains distinctly different accessory minerals, comprising zircon, a patite, zirconosilicates, Ca-Th-phosphates, very rare xenotime and ear ly monazite, and has a chemical signature consistent with derivation f rom a metasedimentary precursor (S-type). Major and trace element tren ds on Harker diagrams support the existence of two distinct granite su ites. The 1-type suite exhibits inverse relationships between SiO2 and CaO, TiO2, Fe2O3, MgO and P2O5, while A12O3, K2O and Na2O remain rela tively constant. These characteristics are consistent with magma forma tion from an intermediate parent composition, and in equilibrium with a hornblendite restite. The S-type granites formed by vapour-absent me lting of siliceous metapelite and exhibit inverse correlations between SiO2 and A12O3, Na2O and K2O, with the other major elements remaining relatively unchanged. This interpretation of restite unmixing is not unambiguous since Rb-Sr trends could be viewed as reflecting crystal f ractionation and partial melting processes. High field strength elemen ts tend to reflect the character of accessory mineral phases and are n ot amenable to modelling in terms of crystal-melt equilibria. The reco gnition of possible S- and I-type suites among the late granite pluton s of the Barberton Mountain Land supports recent models which propose plate tectonic analogues in the region. The two granite suites are dis tributed along two subparallel linear arrays which suggests scenarios invoking subduction or hot-spot related magmatism, with the 1-types fo rming in the time span 2740-2690 Ma along a western array. The putativ e S-types occurring entirely along an easterly belt were emplaced at 3 074 and 2820 Ma. The presence of S-type plutons as old as 3074 Ma impl ies that sediments (such as the Moodies and Fig Tree Groups, or their equivalents) had been buried to depths of at least 15 km, and that by this time at least in the Barberton area, granites had evolved beyond the TTG mode that typifies many early Archaean shield areas.