Complexity of characterizing granitoids in high-grade terranes: An example from the Neoarchean Verbaard granitoid, Limpopo Complex, Southern Africa

Identifying the spatial distribution or extent of individual granitoids in Archean high-grade terranes can be complicated, with the effect of overprint events resulting in localized varieties. The case study presented here highlights this issue from the Limpopo Complex high-grade terrane in southern Africa. The Beit Bridge Complex segment preserves a variety of temporally and spatially associated Neoarchean granitoids. Of the different granitoids, the ~2.65–2.62 Ga biotite-bearing Verbaard granitoid is characterized by a conspicuous network of garnet-bearing leucocratic veins. Unlike these leucocratic veins with more or less straight line outline, garnet-bearing leucosomes with irregular outline are associated with surrounding migmatitic metapelites and have biotite-bearing melanocratic margins. Garnet from the Verbaard granitoid and migmatitic metapelite have comparable mineral chemical characteristics. The most common variety is characterized by low X_Ca ^Grt = 0.03–0.04 and the less common one has higher X_Ca ^Grt (0.05–0.06: granitoid; 0.08: migmatitic metapelite). No rim-core-rim zoning is observed in garnets from the host Verbaard granitoid and metapelite. However, garnets in the leucocratic veins and leucosomes exhibit distinct zoning with a decrease of X_Ca from 0.08 to 0.03. Phase equilibria modeling using PERPLE_X software reveals that such variation in grossular content of garnets reflects decompression from ~9.5 to ~6.5 kbar at 760–800 °C for both rocks. During decompression, melts were segregated trapping garnet including the high X_Ca variety. Both the Verbaard granitoid and migmatitic metapelite were affected by the segregated melts. Difference in composition between the modeled melts and leucosome/leucocratic vein is accounted in terms of different degree of a melt loss. Whole-rock geochemical characteristics of garnet- and leucocratic vein-free domains of the Verbaard granitoid indicate that they are similar to medium-pressure TTGs derived from the melting of metabasalts and are comparable to those of the Alldays granitoid. Thus, the Verbaard granitoid represents a local manifestation of the voluminous (widespread) Alldays granitoid magmatism in the specific (Verbaard) area.