A multielement geochronologic study of the Great Dyke, Zimbabwe: significance of the robust and reset ages

doi:10.1016/S0012-821X(98)00228-3

Earth and Planetary Science Letters

Volume 164, Issues 1–2, 15 December 1998, Pages 353-369

https://doi.org/10.1016/S0012-821X(98)00228-3 Get rights and content

Abstract

New Sm–Nd, U–Pb, and Pb–Pb age determinations indicate that the Great Dyke of Zimbabwe, an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide, is over 100 Ma older than previously believed based on Rb–Sr ages. The intrusion was emplaced as a series of subchambers with similar stratigraphy, comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite, the top sections of which include Pt-enriched sulfide zones, and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite. The Sm–Nd method has yielded a combined mineral/whole-rock isochron of 2586±16 Ma and ε_Nd(t) of +1.1 for samples from the Darwendale, Sebakwe, and Wedza Subchambers as well as the satellite East Dyke. This isochron age is in excellent agreement with the U–Pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the Selukwe Subchamber with an error-weighted mean at 2587±8 Ma. Two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant, and although not well constrained, suggest Pb loss from the zircons at ca. 830 Ma. This may be related to the onset of the widespread and diachronous Pan-African tectonothermal event in southern Africa. Whole–rock samples and clinopyroxene and plagioclase separates from a Darwendale Subchamber drill core yielded a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb isochron age of 2596±14 Ma, which is in agreement with the Sm–Nd isochron and the rutile U–Pb crystallization age. This new age information shows that emplacement of the Great Dyke and its satellite dikes closely followed the amalgamation of the Kaapvaal and Zimbabwe Cratons, and was contemporaneous with emplacement of the youngest of the trondhjemite–tonalite–granodiorite granitoid suite in the Zimbabwe Craton. Assuming that amalgamation of the Kaapvaal and Zimbabwe Cratons was largely by NNW-directed convergence, it follows that the source of the Great Dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes. Isochrons of ²⁰⁶Pb/²⁰⁴Pb vs. ²³⁸U/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb vs. ²³⁵U/²⁰⁴Pb yield ages with large errors, but well constrained initial Pb ratios (²⁰⁶Pb/²⁰⁴Pb = 14.15±0.30 and ²⁰⁷Pb/²⁰⁴Pb = 15.04±0.06). Assuming a two-stage model for common lead evolution, this result yields a μ value of 9.5. Along with the calculated initial Sr and Nd isotopic compositions, these data are consistent with derivation of the Great Dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction. While a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the Great Dyke, the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source.

Introduction

The Great Dyke is a major intrusion of mafic and ultramafic rocks that cuts across the dominantly Archean rocks of the Zimbabwe Craton and is regarded as a major magmatic event that marks the Archean–Proterozoic boundary in that province [1]. Based on Nd model ages, the Archean craton of Zimbabwe appears to have undergone two episodes of crustal generation at 3.5 and 2.9 Ga [2] by the development of dominantly tonalite–trondhjemite intrusions subsequently metamorphosed to orthogneisses. These granitoid domains were the basement to the major greenstone belt sequences and comprise up to 60% of preserved Archean crust in Zimbabwe [3]. The craton is bounded by the Zambezi metamorphic belt in the north, the Mozambique belt to the east and the Limpopo belt to the south. Proterozoic and Phanerozoic basin deposits cover the Archean rocks in the northern part of the craton (Fig. 1).

The earliest widespread formation of greenstones in Zimbabwe, together with associated granitoid suites, was the Lower Greenstone sequences of the Belingwean Group emplaced at 2.9–2.8 Ga. These comprised mafic volcanic sequences in the lower parts passing upwards into a dominantly bimodal succession with alternating mafic and felsic volcanic rocks. The upper part of the Belingwean Group is a komatiite and mafic rock volcanic sequence that also incorporates sedimentary rocks [4]. An unconformity separates the Upper Belingwean from the base of the Upper Greenstone succession. The latter is called the Lower Bulawayan and consists of intermediate to felsic volcanic rocks and volcaniclastic sediments yielding single zircon U–Pb ages of 2.83–2.79 Ga [4]. Where not in contact with Belingwean rocks it is intruded by granitoids. The extensive Upper Bulawayan Group includes clastic sediments, argillites and limestones together with basalts and komatiitic basalts. The Upper Bulawayan Group has yielded Rb–Sr whole rock ages of 2.66±7 to 2.48±14 Ga [5][6]. Single zircon U–Pb dating [4] shows the Upper Bulawayan to have been emplaced at 2.70–2.64 Ga. The locally occurring Shamvaian Group comprises sedimentary associations intercalated with felsic volcanic rocks and is the youngest of the greenstone successions but single zircon U–Pb determinations yielded an array for which the concordia upper intercept is 2.66±17 Ga [4]. Intrusive into the Lower Greenstone successions are a number of granitoid bodies, some of which may be correlated with felsic volcanism [4].

The younger granites post-date the Upper Greenstones and the Shamvaian Group and consist of two groups [7]: (1) the earlier tonalite–granodiorite Sesombi suite was dated at approximately 2.7 Ga [5][8] and more recently has been dated at 2673±5 Ma [9]; (2) the later group comprises tabular monzogranites of large areal extent and generally similar composition. This craton-wide multiphase granitoid Chilimanzi suite marks the last major pre-Great Dyke granitic event [4]. The post-tectonic Murahwa granite (north), also associated with the Chilimanzi granitoid suite, has been dated at 2601±14 [10] utilizing U–Pb, which agrees well with the Rb–Sr ages of 2574±14 Ma [11] and 2583±52 Ma [12] for the southern area of this large body near Masvingo. The compositionally and tectonically equivalent Glendale tonalite in the north has been dated using U–Pb at 2618±6 Ma. These post-tectonic granitoids are characterized by relatively high Sr initial ratios (0.7040±10 to 0.706±14) and were generated through remobilization of basement granitoid [10]. The Great Dyke is the first magmatic event in the stabilized Zimbabwe Craton, and therefore, new age data on this major intrusion have important implications for the stabilization of the crust as well as the intrusion's temporal relation to the prevailing tectonism and extensive late granitoid magmatism. Hence the main purpose of this study is to determine the time of crystallization for the Great Dyke using the Sm–Nd and U–Pb methods and to compare the results to our new and previously published Rb–Sr ages.

Section snippets

General geology of the Great Dyke

The Great Dyke (Fig. 2), aligned approximately NNE, is 550 km in length and 3 to 11 km wide [13]. Parallel to the intrusion and almost over the same length are a number of gabbro or quartz gabbro satellite dikes. The most prominent of these are the Umvimeela and East Dykes situated on the west and east sides of the Great Dyke, respectively, and between 3 and 25 km distance from it. These satellite dikes are closely associated with a major fracture pattern postulated to be the result of the

Previous isotopic studies on the Great Dyke

All previous age determinations of the Great Dyke were carried out using Rb–Sr (all Rb–Sr ages discussed here were recalculated for an ⁸⁷Rb–decay constant of 1.42×10⁻¹¹ yr⁻¹). The first isotopic investigation of the Great Dyke was by Faure et al. [24] which suggested an equivalent age to the Bushveld Complex of 1950±150 Ma obtained by Nicolaysen et al. [25]. Allsopp [26] on the basis of Rb–Sr and Ar–Ar age determination on two biotites set a lower limit of 2476±30 Ma for the layered sequence.

Analytical procedures

Samples were chosen as those having the least alteration. Rocks of the Great Dyke show strong alteration in general and care must be exercised in their selection. In particular the gabbroic rocks show some alteration of the feldspars even in least altered samples. Pyroxenite may show pervasive and intense alteration even in drill core from depths up to 400 m. This is generally the result of the high magnesian nature of the rocks which renders them susceptible to alteration and also the

Rb–Sr

Rb–Sr data for whole–rock samples of orthopyroxenite, gabbronorite and websterite and mineral separates of clinopyroxene and plagioclase from the GDX17 drill core of the Darwendale Subchamber (Table 1) cannot be fitted to a straight line if it is assumed that all scatter is due to analytical error. If some of the scatter is attributed to normally distributed errors in the initial ⁸⁷Sr/⁸⁶Sr, then an errorchron of 2467±85 Ma and an initial Sr ratio of 0.7026±4 are produced (Fig. 4). Although

Significance of the Rb–Sr ages

There is fairly good agreement between the Rb–Sr isochron of 2467±85 Ma presented here (Table 1 and Fig. 4) and decay constant-adjusted isochrons of 2455±16 Ma and 2477±90 Ma by Hamilton [29] and Davies et al. [27], respectively. Our isochron and the one by Davies et al. [27] show similar scatter and imprecision compared to the isochron produced by Hamilton [29]. However, these ages are at least 100 million years younger than ages obtained by the Sm–Nd isochron and rutile U–Pb methods. Sm–Nd

Conclusions

Our new Sm–Nd, U–Pb, and Pb–Pb age determinations indicate that the Great Dyke of Zimbabwe and its satellite dikes are over 100 Myr older than previously believed based on Rb–Sr ages. The Rb–Sr method records a resetting probably related to high-temperature hydrothermal processes.

The Sm–Nd method has yielded a combined mineral/whole–rock isochron of 2586±16 Ma and ε_Nd(t) of +1.1 for samples from the Darwendale, Sebakwe, and Wedza Subchambers as well as the satellite East Dyke. This isochron age

Supplementary data

Acknowledgements

We are indebted to Charlie Murahwi of Anglo American Corporation, Zimbabwe, for facilitating our field work and the sampling of drill cores, and Sandy Zeff for the analytical work performed at the University of Michigan. We also acknowledge the constructive reviews provided by K. Condie and S.A. Morse. This work was supported by a grant from the Office of the Vice Provost for Academic and Multicultural Affairs at the University of Michigan. [CL]

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A multielement geochronologic study of the Great Dyke, Zimbabwe: significance of the robust and reset ages

Abstract

Introduction

Section snippets

General geology of the Great Dyke

Previous isotopic studies on the Great Dyke

Analytical procedures

Rb–Sr

Significance of the Rb–Sr ages

Conclusions

Supplementary data

Acknowledgements

Precambrian Res.

Precambrian Res.

Geochim. Cosmochim. Acta

Chem. Geol.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

J. Afr. Earth Sci.

Precambrian Res.

The generation of continental crust: An integrated study of crust-forming processes in the Archean of Zimbabwe

J. Petrol.

Age relationships between greenstone belts and `granites' in the Rhodesian Archean craton

Earth Planet. Sci. Lett.

On the age of Rhodesian greenstones

Contrib. Mineral. Petrol.

The granite–greenstone terrains of the Rhodesian Archaean Craton

Nature

Constraints on Archean crustal evolution of the Zimbabwe craton: A U–Pb zircon, Sm–Nd and Pb–Pb whole–rock isotope study

Contrib. Mineral. Petrol.

The differentiation and structure of the Great Dyke of Southern Rhodesia

Trans. Geol. Soc. S. Afr.