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Field, petrographical and geochemical characterization of a layered anorthosite sequence in the upper main zone of the Bushveld Complex

Julie E. Bourdeau, S. E. Zhang, B. Hayes and A. Logue
Field, petrographical and geochemical characterization of a layered anorthosite sequence in the upper main zone of the Bushveld Complex
South African Journal of Geology (July 2020) 123 (3): 277-296

Abstract

A sequence of eight poikilitic anorthosite layers (labeled 1 to 8), within the Upper Main Zone in the eastern lobe of the Bushveld Complex, are exposed along a road-cut, 5.3 km northeast of the town of Apel, Limpopo Province. The anorthosite layers are meter-scale in thickness (0.4 to 10 m), have sharp contacts and are defined on the size and shape of pyroxene oikocrysts they contain. The anorthosite sequence is bounded by typical Main Zone gabbronorites. Euhedral, zoned primocrystic laths of plagioclase (An62.5-80.6; 0.2 to 4 mm long) are morphologically identical throughout the anorthosite sequence and define a moderate to strong foliation that is typically aligned parallel to the plane of layering. Interstitial clinopyroxene and orthopyroxene typically occur as large (0.8 to 80 cm) oikocrysts enclosing numerous partly rounded plagioclase chadacrysts. Rarely, orthopyroxene appears as subophitic crystals enclosing few and significantly smaller (0.08 to 0.4 mm), equant plagioclase inclusions. Detailed plagioclase and pyroxene mineral compositions for layers 2 to 5 show minimal variations within layers (0.1 to 2.3 mol% An and 0.7 mol% Mg whereas compositional breaks occur between layers (0.5 to 3.8 mol% An and 1.3 mol% Mg In layers 2 to 5, the An-content of plagioclase cores and the Mgf both clinopyroxene and orthopyroxene crystals decrease by 2.5 mol%, 8.6 mol% and 13.0 mol% upwards, respectively. Bulk-rock incompatible trace element concentrations and patterns are similar for all analyzed anorthosite layers indicating that they are related to the same parental magma. However, bulk-rock major element oxides (e.g. Al2O3, TiO2, K2O) and atomic Mgecome more evolved upwards, consistent with magmatic differentiation. Based on the consistent plagioclase crystal morphologies and relatively constant chemistries within each anorthosite layer, we propose that each layer was formed by the intrusion of a plagioclase slurry. The upwards-evolving mineral chemistries, bulk-rock major element oxides and atomic Mguggests that each plagioclase slurry injection, that yielded an anorthosite layer, was derived from a slightly more fractionated parental magma prior to emplacement.


ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 123
Serial Issue: 3
Title: Field, petrographical and geochemical characterization of a layered anorthosite sequence in the upper main zone of the Bushveld Complex
Affiliation: University of the Witwatersrand, School of Geosciences, Wits, South Africa
Pages: 277-296
Published: 20200729
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 58
Accession Number: 2020-063338
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., strat. cols., 3 tables, geol. sketch maps
S25°30'00" - S25°30'00", E28°30'00" - E28°30'00"
Secondary Affiliation: PG Techno Wox, ZAF, South Africa
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202039
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