Heavy and light marker minerals around the East African Rift System (EARS) with special reference to the evolution of Zanzibar Island, Tanzania
Graphical abstract
Introduction
The East African Rift System (EARS) forms the most prominent continental rift system of the African continent starting off far north in the Dead Sea Transform Fold (DSTF). It is marked by the Jordan Graben, extends into the Gulf of Aqaba and the Red Sea while bordering the African against the Arabian Plates, before it crosses into what is called geographically the African Continent (Sharland et al., 2001; Smit et al., 2008; Dill et al., 2010). Here, the several thousand kilometers long EARS forms two branches which towards the SSE merge again into what is called the Mozambique Channel (Chorowicz, 2005; Calais et al., 2006; Macheyeki et al., 2008; Delvaux and Barth, 2010). The geodynamic structure is said to be related to a mantle plume beneath eastern Africa (Sepulchre et al., 2006) and laterally bound by Archean basement domes such as the Tanzanian Craton while following its Proterozoic mobile belts at its margin (Calais et al., 2006; Albaric et al., 2009). The latter comes next to the Isle of Zanzibar (Fig. 1). Numerous studies have been centered around this suture zone so that only some brief overviews have been given above. It is in stark contrast to the Isle of Zanzibar where only a few scientific papers about the geology occur and the focus is placed more frequently on the geo-ecology of the mangrove swamps along the western coast while geology is only a side-effect of them (Arthurton et al., 1999; Camoin et al., 2004; Nyandwi and Kangwe, 2006; Mwandya et al., 2010; Punwong et al., 2013; Woodroffe et al., 2015; Zavala-Garay et al., 2015) (Fig. 2). The Isle of Zanzibar forms part of the continental shelf. It is made up of Pleistocene reefal limestones covered, in places, by alluvial sediments (Camoin et al., 2004). One crucial point is the island's temporary connection with the mainland which is not only the result of a glacial sea-level lowstand but also related to tectonic processes. The current study addresses an issue hitherto not dealt with during research into the island's geology, the mineralogy of Quaternary sediments and their age. It is not treated as a stand-alone topic restricted to the Isle of Zanzibar but compared with similar mineral associations encountered along the EARS in mainland Tanzania, and its surrounding geodynamic units. Particular attention is paid to marker mineral associations which are used as a tool for different issues, e.g., from provenance to weathering intensity and in view of that may also contribute to the geodynamic evolution of this island in East Africa during the most recent geological history.
Section snippets
Methodology
Samples have been taken on the Isle of Zanzibar, in and around the Ngorongoro region, on various basement and magmatic rocks adjacent to the EARS in the Tanzania Craton and neighboring countries mainly during the senior author's studies in E Africa. The sedimentary composition of the samples was determined using a stereomicroscope and subsequently investigated by means of the petrographic microscope for their mineral assemblage. Routine optical mineralogical examination was supplemented by XRD
Geological setting
The regional geology of mainland Tanzania is dominated by the large crystalline basement block of the Archean Tanzania Craton which is surrounded by Proterozoic mobile belts (Fritz et al., 2013) (Fig. 1c and d). From the Late Carboniferous through the Early Jurassic, Karroo sedimentation characterized by terrigenous sediments of alluvial, fluvial, deltaic and lacustrine type mark the break apart of Pangea (Wopfner, 2002; Macgregor, 2017; Tuck-Martin et al., 2018). Rifting continued into the
Geological and chronological results
The samples were taken from the NE coastal zone of Zanzibar terminated towards the Indian Ocean by the most seaward part of the foreshore and landward by the most elevated coastal terrace, reflecting an ancient beach or shore zone (Fig. 2c) (Bird, 2008). It is a catena from unconsolidated calcareous bioclastic coastal sands near the sea to (semi) consolidated bioclastic limestones which on the raised shore zone are blanketed with red soils (Terra rossa/regolith). The youngest part proximal to
Mineralogical results
The mineral associations are composed of heavy (abbreviated to HM) and light minerals (LM) separated from each other by the specific gravity level of 2.9 g/cm3 (Table 1). The third group of minerals called phyllosilicates sensu lato or clay minerals (CM) are treated as separate entity because of their transitional character between LM and HM. There are clay minerals which by definition are HM, such as Fe-enriched biotite (annite), whereas others are true LM,e.g., muscovite (Table 1).
Age and origin of the impure calcareous rocks
The youngest unconsolidated bioclastic rocks aged 2119 to 1858 yrs cal BP are dominated by aragonite, high Mn–Mg calcite and little low Mn–Mg calcite. Aragonite and high-Mg calcite are known from different modern strongly evaporated saline waters and coastal marine environments where their production rates are very high (Quinn and Saller, 2004; Lerman and Clauer, 2007; Rey et al., 2016). The aragonite - Mn–Mg calcite association is typical of modern marine calcareous rocks. In a normal marine
Conclusions
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HM assemblages in context with LM can be used as marker minerals for geodynamics and supergene alteration processes. The difference between the mineral assemblage along the east coast of Zanzibar and the mineral assemblage of the mainland fed by the large fluvial drainage systems points to the presence of a local magmatic HM source.
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The varied metamorphic and magmatic lithology of the basement and the rift-related magmatic rocks created in around the EARS an ideal platform to use this technique
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Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The senior author expresses his gratitude to N. Dill and D. Navarro-Navas for field assistance and also to K. Ufer for his support during XRD analysis. We are grateful for the comments to the first version of our paper made by Andrew Morton and for the editorial handling of it by Damien Delvaux, Editor-in-Chief of the Journal of African Earth Sciences. We extend our gratitude also to the Journal Manager, Praveen Johnson and his production team who took great efforts to finish this publication
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