Trace element geochemistry of Archean granitic rocks from the Barberton region, South Africa

Abstract

Archean granitic rocks from the Barberton region in South Africa fall into several geochemical groups. Tonalite diapirs (∼3.3 b.y.) are characterized by low LIL-element contents and element ratios (K 2O/Na 2O, Rb/Sr, Ba/Sr). Dalmein-type plutons (2.9–3.3 b.y.) are composed principally of granodiorite and the Lochiel batholith (∼3.0 b.y.) of quartz monzonite. These granitic rocks are characterized by intermediate LIL-element contents and element ratios and small negative Eu anomalies. Mpageni- and Sicunusa-type plutons and the Pongola batholith (2.6–2.8 b.y.) are characterized by granite and quartz monzonite with high LIL-element contents and element ratios. The Mpageni type has moderate negative Eu anomalies and Sr contents and high Ba contents while the Sinunusa type and Pongola have large negative Eu anomalies and, in the case of the Sicunusa type, low Sr contents. Fractional crystallization and partial melting models for the origin of these granitic rocks have been tested using element distributions. The tonalite is best accounted for by about 10% melting of eclogite and the Dalmein type and Lochiel by 50% melting of siliceous, garnet-bearing granulite. The Mpageni and Sicunusa types and the Pongola can be produced by 70–80% fractional crystallization of a Dalmein-type magma at shallow depths. A mantle plume model is proposed for the Barberton region. It has the advantages of including both moderate and high geothermal gradients and accounting for the observed episodisity of magmatism.