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A La-ICPMS zircon record of magmatic crystallization and compositional alteration in meta-igneous rocks of the eastern Kaapvaal Craton

R. Bolhar, A. Hofmann, C. M. Allen and R. Maas
A La-ICPMS zircon record of magmatic crystallization and compositional alteration in meta-igneous rocks of the eastern Kaapvaal Craton
South African Journal of Geology (July 2021) 124 (3): 761-782

Abstract

Archaean zircons from the Kaapvaal Craton, South Africa, were analyzed by Laser Ablation (LA)-ICP-MS to obtain a coupled record of U-Th-Pb isotope ratios and selected trace elements with the aim to develop insights into physico-chemical conditions during igneous zircon crystallization and subsequent compositional alteration. Four rock samples previously dated by SIMS U-Pb using zircon were selected: 3.56 Ga Ngwane Gneiss, 3.55 Ga Theespruit felsic metavolcanic, 3.50 Ga Steynsdorp Gneiss and 2.98 Ga Nhlangano Gneiss. LA-ICP-MS U-Pb zircon ages agree with published SIMS U-Pb ages within analytical uncertainty. Assessment of the magmatic crystallization histories was based on near-concordant grains, and discordant grains were used to examine post-igneous element mobilization and alteration. Time-resolved laser drilling experiments allowed distinction of concordant and discordant zircon domains, but also revealed systematic changes in REE+Ti geochemistry, U+Th content, discordance and metamictization. Th/U and Zr/Hf, coupled with REE patterns, effectively distinguish compositional zircon types that reflect variable degrees of igneous differentiation and melt compositions. Eu/Eu* values indicate significant feldspar fractionation in some magmas. Averaged crystallization temperatures of magmatic zircons, as derived from the Ti-in-zircon thermometer, define a narrow range of 650 to 750 degrees C for (near-)concordant grains, consistent with general constraints on temperatures at zircon saturation for felsic magmas, and testifying to a closed-system behavior of Ti (and other trace elements). Systematic deviations from primary igneous trace element signatures are strongly correlated with radiation damage. Specifically, Th/U and, to some extent, Zr/Hf decrease, and Ti increases with increasing U (+Th) content and isotopic disturbance (discordance).


ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 124
Serial Issue: 3
Title: A La-ICPMS zircon record of magmatic crystallization and compositional alteration in meta-igneous rocks of the eastern Kaapvaal Craton
Affiliation: University of the Witwatersrand, School of Geosciences, Johannesburg, South Africa
Pages: 761-782
Published: 20210723
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 120
Accession Number: 2021-059190
Categories: Igneous and metamorphic petrologyGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch maps
S27°00'00" - S18°00'00", E18°00'00" - E30°00'00"
Secondary Affiliation: Queensland University of Technology, AUS, AustraliaUniversity of Melbourne, AUS, Australia
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2022, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202141
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