Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study |
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Authors: | Hielke A Jelsma Michael L Vinyu Jan R Wijbrans E A T Verdurmen P J Valbracht G R Davies P J Valbracht |
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Institution: | (1) Faculty of Earth Sciences, Free University, de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands, NL;(2) Department of Geology, University of Zimbabwe, P.O. Box MP167, Harare, Zimbabwe, ZW;(3) Institut de Physique du Globe, Laboratoire de Geochimie et Cosmochimie, Université Paris VII, 2 Place Jussieu, F-75252, Paris Cedex 05, France, FR |
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Abstract: | The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and
Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain
and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships
and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic,
continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of
the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related
to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction
with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone
sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon
ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination
of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence
corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and
supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic
gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone
sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter
intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic
evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns
in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant
influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon
ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma
and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees
well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide
event and heralding cratonization. The similarity between U-Pb zircon ages and TDM model ages (2.65–2.62 Ga) and the positive ɛNdT values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism
was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and
minor contamination with older crust. The rather high model μ1 values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from
a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older
TDM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived
by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction
between depleted mantle-derived melts and older crustal material.
Received: 15 August 1995 / Accepted: 12 January 1996 |
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