Dissecting a granulite-facies migmatite: Leucosome – melanosome – bulk rock relationships and the role of garnet (Namaqua Metamorphic Province, South Africa)

A geochemical-mineralogical study carried out on a migmatite from the granulite zone of the Bushmanland Domain in the Grenville-cycle Namaqua-Natal Belt in southern Africa reveals an anatectic process largely controlled by biotite dehydration melting that resulted in leucosome compositions which are less siliceous and which have a higher maficity compared to the melanosomes. The rocks show an unusual internal S-L structure with a dominantly linear, parallel arrangement of isolated leucosome patches within a melanosome matrix. Although the leucosome proportion is high (between 40 and 60 vol%), there is very little local interconnectivity between adjacent leucosome patches, with no clear evidence for either larger-scale melt accumulation, or for melt escape channels along or across the rocks’ structural anisotropy. Geochemical evidence indeed points to anatexis without significant melt loss, whereby the resultant leucosome and melanosome segregations combine in the bulk rock to a composition that corresponds to a pre-anatectic metapelitic protolith. The leucosomes show no evidence for the involvement of externally-derived melts.

Garnet plays a critical role for the major and trace element distribution between leucosome and melanosome, as it is concentrated in the former, lowering the Si content relative to the bulk rock and increasing FeO, MnO, Sc and HREE. The high concentration of peritectic garnet within the newly formed melt would also have increased its density markedly. In conjunction with the particular (constrictional deformation-related) distribution of unconnected leucosomes in these migmatites, this may well have impeded effective melt loss from the source rock, despite the high melt proportion.