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Growth of zircon and titanite during metamorphism in the granitoid-gneiss terrane south of the Barberton greenstone belt, South Africa

A. Dziggel^1,*,, R. A. Armstrong², G. Stevens³ and L. Nasdala⁴

¹ Economic Geology Research Institute, University of the Witwatersrand, Private Bag 3, PO Wits 2050, Johannesburg, South Africa
² Research School for Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
³ Department of Geology, University of Stellenbosch, Matieland, 7140, Stellenbosch, South Africa
⁴ Institut für Geowissenschaften – Mineralogie, Johannes Gutenberg-Universität, D-55099 Mainz, Germany

^* E-mail: adziggel{at}iml.rwth-aachen.de

SHRIMP U-Pb zircon and titanite dating have been used to constrain the timing of mid- to lower- crustal metamorphism (~650–700°C and 8–11 kbar) and syn-kinematic melting in the granitoid gneiss-dominated terrane south of the Barberton greenstone belt, South Africa. This study is concentrated on a clastic metasedimentary unit exposed in one of several greenstone remnants and a late-kinematic trondhjemite intrusive into spatially associated mixed gneisses. Locally, the clastic metasediments show extensive replacement of garnet and plagioclase by epidote and titanite. The titanites yield an upper intercept date of 3229±9 Ma, and provide a minimum age for the peak of metamorphism. Zircons separated from the same unit record a range of concordant and near-concordant ²⁰⁷Pb/²⁰⁶Pb dates between ~3560 and 3230 Ma, the youngest group yielding a weighted mean date of 3227±7 Ma. This range of dates is interpreted to be due to a combination of metamorphic recrystallization and high- temperature Pb-loss in originally detrital zircons during regional metamorphism. A minimum age for the timing of deformation is given by the emplacement age of 3229±5 Ma for the late-kinematic trondhjemite. Thus, geochronological data support the notion of a major metamorphic episode that coincided with the proposed short-lived terrane accretion event in the centre of the Barberton greenstone belt.

KEYWORDS: Barberton, SHRIMP U-Pb dating, zircon, titanite, basement, metamorphism