Stratigraphy, geochemistry and zircon geochronology of the midlands greenstone belt, Zimbabwe
Stratigraphy, geochemistry and zircon geochronology of the midlands greenstone belt, Zimbabwe
Fieldwork, petrography, major, trace and rare-earth element geochemistry, Sm-Nd isotope geochemistry and single-grain zircon geochronology have been conducted on rocks from the Midlands Greenstone Belt, Zimbabwe and other regionally important rocks from around the craton.
A 2880± 8Ma volcanic sequence has been identified by single-grain zircon geochronology within what had formally been considered the 2.7Ga Upper Bulawayan stratigraphy of the Midlands Greenstone Belt. The Mafic Formation is therefore redefined as Lower Belingwean in age in accordance with the stratigraphic nomenclature of Wilson et al. (1995). The Maliyami and Felsic Formations are dated at 2739 ± 33Ma by Sm-Nd isochron determination, agreeing with the 2701 ± 6Ma zircon age of Wilson et al. (1995). The Mafic Formation is recognised as comprising bimodal volcanism with siliciclastic and chemical sedimentation deposited on an ancient granitoid basement. The Maliyami and Felsic Formations exhibit basalt-andesite-rhyodacite mingled volcanism reflecting increased volcanism, with decreased siliciclastic and chemical sedimentation, and an oolitic limestone indicating an increase in the oxygen content of the environment. The environment of deposition is therefore considered to reflect a shallow, continental marginal sea or basin.
Petrographic and geochemical data suggest that the volcanic lithologies can be divided into two distinct suites: basalt and andesite-rhyodacite. The basalts have fractionated clinopyroxene and plagioclase whilst the andesite-rhyodacite suite reflect ilmenite, amphibole and plagioclase fractionation. Continental crustal contamination is present in some samples but the variation of εNd(t) seen in most samples reflects a heterogeneous mantle source. The 2.9Ga and 2.7Ga basalts formed by melting of a 3.2-3.4Ga depleted sub-continental lithospheric mantle component, whilst the 2.7ga andesite-rhyodacite suite reflects melting of an underlying 2.9Ga basaltic underplate emplaced during this earlier event. As such, a model of formation for the Midlands Greenstone Belt is envisioned through magmagenesis resulting from impingement of a thermal anomaly beneath the Zimbabwe Craton at 2.9Ga and 2.7Ga.
University of Southampton
Horstwood, Matthew Simon Anthony
2330787d-4335-49b9-be02-b7855e36e2bb
1998
Horstwood, Matthew Simon Anthony
2330787d-4335-49b9-be02-b7855e36e2bb
Nesbitt, Robert
fa2d2f0f-c147-49ad-9dc0-ad98aaef48d4
Horstwood, Matthew Simon Anthony
(1998)
Stratigraphy, geochemistry and zircon geochronology of the midlands greenstone belt, Zimbabwe.
University of Southampton, Doctoral Thesis, 235pp.
Record type:
Thesis
(Doctoral)
Abstract
Fieldwork, petrography, major, trace and rare-earth element geochemistry, Sm-Nd isotope geochemistry and single-grain zircon geochronology have been conducted on rocks from the Midlands Greenstone Belt, Zimbabwe and other regionally important rocks from around the craton.
A 2880± 8Ma volcanic sequence has been identified by single-grain zircon geochronology within what had formally been considered the 2.7Ga Upper Bulawayan stratigraphy of the Midlands Greenstone Belt. The Mafic Formation is therefore redefined as Lower Belingwean in age in accordance with the stratigraphic nomenclature of Wilson et al. (1995). The Maliyami and Felsic Formations are dated at 2739 ± 33Ma by Sm-Nd isochron determination, agreeing with the 2701 ± 6Ma zircon age of Wilson et al. (1995). The Mafic Formation is recognised as comprising bimodal volcanism with siliciclastic and chemical sedimentation deposited on an ancient granitoid basement. The Maliyami and Felsic Formations exhibit basalt-andesite-rhyodacite mingled volcanism reflecting increased volcanism, with decreased siliciclastic and chemical sedimentation, and an oolitic limestone indicating an increase in the oxygen content of the environment. The environment of deposition is therefore considered to reflect a shallow, continental marginal sea or basin.
Petrographic and geochemical data suggest that the volcanic lithologies can be divided into two distinct suites: basalt and andesite-rhyodacite. The basalts have fractionated clinopyroxene and plagioclase whilst the andesite-rhyodacite suite reflect ilmenite, amphibole and plagioclase fractionation. Continental crustal contamination is present in some samples but the variation of εNd(t) seen in most samples reflects a heterogeneous mantle source. The 2.9Ga and 2.7Ga basalts formed by melting of a 3.2-3.4Ga depleted sub-continental lithospheric mantle component, whilst the 2.7ga andesite-rhyodacite suite reflects melting of an underlying 2.9Ga basaltic underplate emplaced during this earlier event. As such, a model of formation for the Midlands Greenstone Belt is envisioned through magmagenesis resulting from impingement of a thermal anomaly beneath the Zimbabwe Craton at 2.9Ga and 2.7Ga.
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Horstwood 1998 Thesis
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Published date: 1998
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Local EPrints ID: 463331
URI: http://eprints.soton.ac.uk/id/eprint/463331
PURE UUID: 7138c1a0-fbca-49a4-b499-ad13b503b164
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Date deposited: 04 Jul 2022 20:50
Last modified: 16 Mar 2024 19:03
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Author:
Matthew Simon Anthony Horstwood
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