Three tier transition of Neoarchean TTG-sanukitoid magmatism in the Beit Bridge Complex, Southern Africa
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Author list: Rajesh HM, Belyanin GA, Van Reenen DD
Publisher: Elsevier
Place: AMSTERDAM
Publication year: 2018
Journal: Lithos: Journal of Petrology, Mineralogy and Geochemistry (0024-4937)
Journal acronym: LITHOS
Volume number: 296
Start page: 431
End page: 451
Number of pages: 21
ISSN: 0024-4937
Languages: English-Great Britain (EN-GB)
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Abstract
Neoarchean TTG-sanukitoid associations of contrasting scales occur within the Belt Bridge Complex terrane of the Limpopo Complex in southern Africa. These include the smaller similar to 2.65-2.63 Ga Avoca granitoid and the voluminous similar to 2.73-2.64 Ga Alldays granitoid. This study characterizes the wide compositional spectrum preserved in these two granitoids. The elliptical Avoca pluton consists of a biotite-amphibole-orthopyroxene +/- clinopyroxene-bearing core that is dominantly trondhjemite with less dominant tonalite and granodiorite variants, and a thin amphibole-biotite-bearing granite rim, with local occurrence of two-pyroxene-bearing metabasite boudins. While both the core and rim rocks exhibit a linear fabric, the granite in addition preserves a penetrative foliation. Field relations of granite enclaves in the core rocks together with available ages indicate that the core rocks intruded the granite. The foliated biotite +/- amphibole-bearing Alldays granitoid contains inclusions of older supracrustals and rocks of the Messina layered intrusion, and is widely distributed. Compositionally, it include tonalites and granodiorites and to a lesser extent trondhjemites. Both the Avoca core and rim rocks are characterized by difference in mineral chemistry, with the mafic minerals Mg-rich in the TTG core, while they are Fe-rich in the granite and metabasite. In comparison, biotite is Mg-rich and amphibole is Fe-rich in the Alldays granitoid.Two groups of Alldays TTG can be delineated in terms of whole-rock geochemical characteristics, and are comparable to the low- to medium-pressure TTG groups delineated by Moyen (2011), while the Avoca TTG is similar to the high-pressure TTG group. The lowest silica samples from each group of granitoid have geochemical characteristics comparable to Archean sanukitoids, with those from the Avoca granitoid similar to low-Ti sanukitoids, and those from the Alldays granitoid similar to low-Ti and high-Ti sanukitoids. Separate petrogenetic models are suggested for different phases of the Avoca core, with the trondhjemite-tonalites considered as high-pressure melts of metabasalt, while the granodiorite with lower SiO2 content, higher K2O and MgO contents, and higher incompatible element contents, than the trondhjemite-tonalites, is a product of hybridization of earlier TTG melts and peridodite. Granite from the Avoca rim are low-pressure melts of pre-existing crustal lithologies. The two groups of Alldays TTG with lower Sr/Y ratios than the Avoca TTG are considered as low to medium-pressure melts of metabasalt, whose progressive interaction with peridotitic mantle at shallower angles account for the unique composition of Alldays low-Ti and high-Ti sanukitoids. Taken together with their spatial and temporal transition from southeastern (similar to 2.73-2.72 Ga; low-pressure TTG-low-Ti sanukitoid) to central (similar to 2.65-2.64 Ga; medium-pressure TTG-high-Ti sanukitoid) to northwestern (similar to 2.63 Ga; high-pressure sanukitoid) parts of the Beit Bridge Complex, the three tier transition of TTG-sanukitoid magmatism argues for the southern margin of the Belt Bridge Complex to represent an active arc in the Neoarchean. (C) 2017 Elsevier B.V. All rights reserved.
Keywords
Belt Bridge Complex, Ferroan calcic granite, High-Ti sanukitoid, Low-P, med-P, high-P TTG, Low-Ti sanukitoid, Neoarchean granitoid magmatism
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