The Heerenveen Batholith, Barberton Mountain Land, South Africa: Mesoarchaean, Potassic, Felsic Magmas Formed by Melting of an Ancient Subduction Complex

Abstract

The Heerenveen batholith exemplifies an aerially extensive group of � 3·1 Ga felsic potassic batholiths that mark the final magmatic event associated with the cratonization of the central^eastern Kaapvaal Craton, and the transition from the earlier sodic tonalite^ trondhjemite^granodiorite (TTG) to the more recent style of monzogranitic and granitic magmatism. As is typical of these bodies, it is geochemically and mineralogically heterogeneous, sheeted in structure, and was assembled incrementally, with eight main units in four assembly stages. Apart from a metaluminous quartz monzonite, all the Heerenveen rocks are peraluminous.There are few identifiable geochemical lineages among the rocks of the batholith. The geochemical heterogeneity is largely due to protolith internal heterogeneity, tapping of multiple sources, pulsed magma extraction related to sequential occurrence of discrete melting reactions, and varying degrees of entrainment of peritectic minerals formed during partial melting of the crustal protoliths, together with varying structural controls on magma extraction and ascent, which allowed distinct magma sources to be tapped, at different periods during batholith assembly. Fractional crystallization, magma mixing and wall-rock assimilation were not significant processes. TTG rocks are not suitable source materials for any of the Heerenveen magmas; nor are highly aluminous metasediments. For magmatic rocks with SiO2570 wt %, the likely source rocks were intermediate to mafic igneous rocks, with variable degrees of K-enrichment. For those rocks with SiO2472 wt %, the sources were K-enriched intermediate to felsic rocks. It seems likely that the partial melting reactions that produced the Heerenveen magmas occurred at moderate temperatures and that hornblende was present in the residues. This suggests that initial melting may have occurred in the presence of an H2O-rich fluid. There is no evidence that the genesis of the potassic granitic magmas involved either recycling of TTG crust or the presence of aluminous metasediments in the deep crust. The K-enrichment in the protoliths was most probably due to their ultimate derivation from mantle rocks metasomatized by fluids evolved in subduction-like processes. The monzonite in the batholith may have been derived directly by partial melting of this metasomatized mantle.