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Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth

Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth
Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth
U-Pb ion-microprobe (SHRIMP) work on zircon populations from 13 Zimbabwean Archaean felsic rocks are presented and interpreted. Samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves. The data demonstrate a Late Archaean volcanicity spanning 250 Ma which began at least 2900 Ma ago and ended at 2650 Ma. The
intrusion of extensive granitoid sills of the Chilimanzi suite at c. 2.6 Ga marks the widespread stabilization of the craton. Based on the new zircon data and a re-evaluation of published mapping, a new stratigraphic subdivision is presented for the Late (< 2.9 Ga) Archaean of Zimbabwe. A feature of the stratigraphic model is the cyclicity of magmatism which begins with ultramafic-mafic rocks, progresses through felsic volcanism and ends with a granitoid event. These cycles are repeated at least three or four times in the 250 Ma time span. An important characteristic of the felsic volcanic rocks is that the bulk of the material examined contains inherited, xenocrystic zircons whose ages range from 1000 Ma to 20 Ma older than the host rocks. The oldest xenocrystic zircons (c. 3.6 Ga) are restricted to volcanic rocks which erupted through the Tokwe segment; itself the only known > 3.3 Ga fragment of Archaean crust in Zimbabwe. These data suggest that the Early Archaean crust is restricted to the Tokwe segment in the south of the country. Since even the oldest of the felsic volcanics (2.90 Ga, Lower Belingwean) have zircons which are 50 Ma older, it is suggested that remnants of earlier cyclic greenstone--granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of Zimbabwe were developed on sialic crust.
0305-8719
109-126
Wilson, J.F.
88530122-5a73-45d7-8dc5-ef6f32897e77
Nesbitt, R.W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Fanning, C.M.
e0e22ef4-2218-464c-a64b-715840a0c191
Wilson, J.F.
88530122-5a73-45d7-8dc5-ef6f32897e77
Nesbitt, R.W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Fanning, C.M.
e0e22ef4-2218-464c-a64b-715840a0c191

Wilson, J.F., Nesbitt, R.W. and Fanning, C.M. (1995) Zircon geochronology of Archaean felsic sequences in the Zimbabwe craton: a revision of greenstone stratigraphy and a model for crustal growth. Geological Society London Special Publications, 95 (1), 109-126. (doi:10.1144/GSL.SP.1995.095.01.07).

Record type: Article

Abstract

U-Pb ion-microprobe (SHRIMP) work on zircon populations from 13 Zimbabwean Archaean felsic rocks are presented and interpreted. Samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves. The data demonstrate a Late Archaean volcanicity spanning 250 Ma which began at least 2900 Ma ago and ended at 2650 Ma. The
intrusion of extensive granitoid sills of the Chilimanzi suite at c. 2.6 Ga marks the widespread stabilization of the craton. Based on the new zircon data and a re-evaluation of published mapping, a new stratigraphic subdivision is presented for the Late (< 2.9 Ga) Archaean of Zimbabwe. A feature of the stratigraphic model is the cyclicity of magmatism which begins with ultramafic-mafic rocks, progresses through felsic volcanism and ends with a granitoid event. These cycles are repeated at least three or four times in the 250 Ma time span. An important characteristic of the felsic volcanic rocks is that the bulk of the material examined contains inherited, xenocrystic zircons whose ages range from 1000 Ma to 20 Ma older than the host rocks. The oldest xenocrystic zircons (c. 3.6 Ga) are restricted to volcanic rocks which erupted through the Tokwe segment; itself the only known > 3.3 Ga fragment of Archaean crust in Zimbabwe. These data suggest that the Early Archaean crust is restricted to the Tokwe segment in the south of the country. Since even the oldest of the felsic volcanics (2.90 Ga, Lower Belingwean) have zircons which are 50 Ma older, it is suggested that remnants of earlier cyclic greenstone--granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of Zimbabwe were developed on sialic crust.

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Published date: 1995
Organisations: Geochemistry

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Local EPrints ID: 361648
URI: https://eprints.soton.ac.uk/id/eprint/361648
ISSN: 0305-8719
PURE UUID: 19fc85ff-a075-4ce5-a3ea-61a2c0e24dd1

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Date deposited: 28 Jan 2014 14:33
Last modified: 18 Jul 2017 02:59

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Author: J.F. Wilson
Author: R.W. Nesbitt
Author: C.M. Fanning

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