Provenance of Garnet in the neoarchaen verbaard granitoid gneiss:complexity of characterizing granitoids in high-grade terranes

Neoarchean granitoids are commonly exposed along Archean high-grade metamorphic

terranes, where they occur as granitoid gneisses. A number of issues, including the effect of

overprint event(s), can complicate the characterization of these granitoids in Archean

terranes. A case study is presented here highlighting the complexity of characterizing

Neoarchean granitoid magmatism from the Limpopo Complex high-grade terrane in

southern Africa. Neoarchean granitoids occurring within the Central Zone of the Limpopo

Complex include the ~2.73-2.64 Ga biotite±amphibole-bearing Alldays granitoid, the ~2.68-

2.62 Ga garnet-bearing Singelele leucogranite, the ~2.65-2.63 Ga Avoca granitoid with

pyroxene-bearing and pyroxene-absent variants, the ~2.65-2.62 Ga biotite-bearing Verbaard

granitoid and the ~2.61-2.58 Ga Bulai granitoid with pyroxene-bearing and pyroxene-absent

variants. Of the different granitoids, the Singelele leucogranite and Verbaard granitoid

exhibit evidence for anatectic overprint. This study focuses on characterizing the nature of

overprint in the Verbaard granitoid. Exposures in the Verbaard area show a conspicuous

network of garnet-bearing leucocratic veins overprinting the light greyish biotite-bearing

Verbaard granitoid. These leucocratic veins, with more or less straight line outline, do not

have any biotite-bearing melanocratic margins, indicating that they formed elsewhere. On

the other hand, the garnet-bearing irregular outline leucosomes associated with surrounding

metapelites have biotite-bearing melanocratic margins indicating in situ formation by partial

melting. Detailed mineral chemical characteristics of garnet occurring in the metapelite,

associated leucosome, and the Verbaard granitoid, indicate comparable composition,

Abstract vii

suggesting common origin. It is argued that the garnet in leucocratic veins overprinting the

Verbaard granitoid represent relics from the metapelites, with the leucocratic veins

representing segregated melt away from the source region. Of the two types of metapelites

found in the study area – high-Al and low-Al metapelites – the distinction between the two is

suggested as an artifact of the extent of partial melting, with the high-Al metapelite

preserving more of the restitic domains, while the low-Al metapelite preserving more of the

leucosomic material. Using rim-core-rim garnet mineral chemical profiles, the leucosomes

are suggested as sites of preservation of different stages of garnet formation in the

associated metapelites.

Whole-rock geochemical characteristics of carefully sampled garnet-free domains of the

Verbaard granitoid indicate that they are similar to low-HREE Archean TTGs. Comparison

with experimental melts from a variety of source rocks indicates that the Verbaard granitoid

was likely derived from the melting of metabasalts. A comparative exercise with other

Neoarchean granitoids from the Central Zone indicates that the compositional

characteristics of the Verbaard granitoid are closely comparable to those of the Alldays

granitoid. It is suggested that the Verbaard granitoid represents the manifestation of the

Alldays granitoid magmatism in the Verbaard area.