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

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.

Titaniferous Magnetite Deposits Associated with Archean Greenstone Belt in the East Indian Sheild

“…Another occurrence of Archean titaniferous magnetite deposit hosted by gabbro-anorthositic suite, synchronous with Archean greenstone belt is reported from Nuggihalli Schist Belt (> 3.1 Ga) of Karnataka in South India (Subba Rao [4]). Such associations of Archean mafic intrusions with Archean greenstone belts have also been reported from elsewhere like Barberton greenstone belt (3.5 Ga), Kaapval Craton, South Africa (Shackleton [8]); Late Archean greenstone belt of Zimbabwe (the upper Bulawayan Supergroup, 2.85-2.66 Ga) (Wilson, Nesbitt and Fanning [9]); Greenstone belt of Pilbara Craton, Australia (3.5 to 2.9 Ga) (Trendall [10]); Hebei Province, China (Choukroune, Bouhallie and Arndt [11]); Yilgarn Craton, Western Australia (Myers [12]); Abitibi belt, Canada (Windley [13]). In this paper, the authors tried to establish the orthomagmatic magnetite mineralization associated with gabbro-anorthositic intrusives and chronostratigraphically correlating with the IOG.…”

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

“…The Shurugwi Greenstone Belt belongs to the Bulawayan Supergroup, a well-preserved greenstone belt sequence in the Zimbabwe craton. Isotope age data of this sequence range from 2.88 to 2.65 Ga (Moorbath et al, 1987;Wilson et al, 1995). The evolution and the tectonic setting of the Bulawayan Greenstone Belt Sequence are still subjects to controversy.…”

Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8–3.1 Ga detrital zircons

“…Wilson et al, 1995) and the Rhodesdale granitoid-gneiss terrain east of the Midlands greenstone belt (Robertson, 1976) (Fig.1). The old gneiss terrains include banded, migmatitic varieties and homogeneous, foliated varieties of tonalitic to granitic composition.…”

“…These intrusions were regarded as post-kinematic with respect to the formation of the regional north-oriented fabric and intrusive into Sebakwian Group greenstones, thus providing a minimum age for the Sebakwian Group (Wilson, 1968 1). These rocks have been assigned to the Lower Greenstones (Wilson, 1979), and include rocks of the 2.90-2.88 Ga Belingwean Group and rocks of the 2.83-2.79 Ga Lower Bulawayan Group (Horstwood et al, 1999;Wilson et al, 1995;Prendergast and Wingate, 2013). TTG-type granitoids of the Chingezi Suite (Wilson et al, 1995) are associated with the Lower Greenstones, and occur as intrusions within the Tokwe Segment to the west.…”

“…(iii) to undertake a search for the most ancient zircons preserved to better delineate the extent of the oldest parts of the craton ("Tokwe segment": Wilson et al, 1995;Horstwood et al, 1999);…”