| Popis: |
Two distinct groups of granites can be recognized among the late plutons of the Barberton Mountain Land. Discrimination between the two groups is provided by geochemical and mineralogical parameters which are considered to reflect the source material from which the magmas were derived. A high-Ca suite has geochemical parameters consistent with derivation from an igneous source (I-type), and an accessory mineral assemblage comprising zircon, apatite, allanite and sphene. A low-Ca suite contains distinctly different accessory minerals, comprising zircon, apatite, zirconosilicates, CaTh-phosphates, very rare xenotime and early monazite, and has a chemical signature consistent with derivation from a metasedimentary precursor (S-type). Major and trace element trends on Harker diagrams support the existence of two distinct granite suites. The I-type suite exhibits inverse relationships between SiO2 and CaO, TiO2, Fe2O3, MgO and P2O5, while Al2O3, K2O and Na2O remain relatively constant. These characteristics are consistent with magma formation from an intermediate parent composition, and in equilibrium with a hornblendite restite. The S-type granites formed by vapour-absent melting of siliceous metapelite and exhibit inverse correlations between SiO2 and Al2O3, Na2O and K2O, with the other major elements remaining relatively unchanged. This interpretation of restite unmixing is not unambiguous since RbSr trends could be viewed as reflecting crystal fractionation and partial melting processes. High field strength elements tend to reflect the character of accessory mineral phases and are not amenable to modelling in terms of crystal-melt equilibria. The recognition of possible S- and I-type suites among the late granite plutons of the Barberton Mountain Land supports recent models which propose plate tectonic analogues in the region. The two granite suites are distributed along two subparallel linear arrays which suggests scenarios invoking subduction or hot-spot related magmatism, with the I-types forming in the time span 2740-2690 Ma along a western array. The putative S-types occurring entirely along an easterly belt were emplaced at 3074 and 2820 Ma. The presence of S-type plutons as old as 3074 Ma implies 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. |
