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

Abstract

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 XCaGrt = 0.03–0.04 and the less common one has higher XCaGrt (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 XCa 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 XCa 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.