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

Journal article

Authors/Editors

Rajesh, Hariharan

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Publication Details

Author list: Rajesh HM, Safonov OG, Basupi TO, Belyanin GA, Tsunogae T

Publisher: Elsevier

Place: AMSTERDAM

Publication year: 2018

Journal: Lithos: Journal of Petrology, Mineralogy and Geochemistry (0024-4937)

Journal acronym: LITHOS

Volume number: 318

Start page: 399

End page: 418

Number of pages: 20

ISSN: 0024-4937

Languages: English-Great Britain (EN-GB)

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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 similar to 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 PER-PLE_X software reveals that such variation in grossular content of garnets reflects decompression from similar to 9.5 to similar to 6.5 kbar at 760-800 degrees 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. (C) 2018 Elsevier B.V. All rights reserved.

Keywords

Anatectic overprint, Archean high-grade terrane, Biotite-bearing granitoid, Garnet-bearing leucocratic vein, Migmatitic metapelite, Phase equilibria modeling

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