The Proterozoic geological history of the Irumide belt, Zambia

Abstract

The Irumide belt is an elongate crustal province characterised by Mesoproterozoic tectonism and magmatism that stretches over a distance of approximately 900 kilometers from central Zambia to the Zambia-Tanzania border and northern Malawi. It is bounded to the northwest by largely undeformed Palaeoproterozoic basement lithologies of the Bangweulu block and is truncated to the northeast by Mesoproterozoic and Neoproterozoic transcurrent shear zones within reactivated parts of the Palaeoproterozoic Ubendian belt. To the southeast and south, Irumide lithologies were reworked within the Neoproterozoic Lufilian and Zambezi belts, and to the east by the East African Orogen. Lithologies in the Irumide belt comprise a Palaeo- to Mesoproterozoic complex of gneisses and granitoids and a supracrustal succession of quartzites and pelites. A three-fold subdivision was accepted prior to this study; (1) Palaeoproterozoic granites and gneisses forming the Irumide basement, (2) a supracrustal succession of quartzites and metapelites called the Muva Supergroup, (3) various deformed and undeformed granitoids intruding both the Palaeoproterozoic basement and Muva Supergroup and considered to be pre-Irumide (1.4 Ga) and syn-, late- to post-Irumide (1.1-0.95 Ga). The age of Irumide tectonism itself was poorly constrained between 1.4 and 1.0 Ga. The basement units comprise the Mkushi Gneiss in the southwest and the Luwalizi Granite, Mwambwa River and Mulungwizi Gneisses in the northeast. These units have been correlated with the Palaeoproterozoic Bangweulu block and Ubendian belt in the past. These basement units are structurally and in places unconformably overlain by a metasedimentary succession of quartzites and metapelites, which in the southwest has been called the Kanona Group, and in the northeast the Manshya River Group.Both sequences have been correlated with similar quartzite-pelite successions on the Bangweulu block, termed the Mporokoso Group, and, together with a second cycle reworked unit on the Bangweulu block called the Kasama Formation, were collectively grouped into the Muva Supergroup. Both basement and supracrustals have been deformed, metamorphosed and intruded by a host of granitoids which, based on structural fabrics, were subdivided into pre-, syn-, late- and post-tectonic suites with respect to Irumide tectonism. Due to the lack of reliable geochronological constraints, this subdivision had remained untested until now. All units in the Irumide belt have been strongly affected by compressional tectonics, resulting in northwest-directed thrusting onto the Bangweulu block basement and extensive crustal shortening. Minor southeast-verging structures form part of locally developed backthrusts within an overall northwest-vergent tectonic regime. At least parts of the Irumide basement were affected by Irumide tectonism, but large-scale thrusting was mainly accommodated along a basal decollement at the basement-cover interface. Extensive shortening is exemplified by tight- to isoclinal folding within the supracrustal sequence, ranging from upright to recumbent. Thrusts developed where shortening could not be accommodated by tight folding, which produced tectonic duplication within the metasedimentary pile, making formation-to-formation correlations across the belt tenuous at best. Irumide tectonism has been reported to affect the base of the Mporokoso Group on the Bangweulu block, where folding along the Luongo shear zone occurred contemporaneously with thermal resetting of biotite dated at ~1.0 Ga (K-Ar dates). Metamorphic parageneses record low- to medium-pressure/medium- to high-temperature conditions.Metamorphic grades range from greenschist facies in the northwestern foreland, to upper amphibolite facies in the southeast, with local granulites. Peak Irumide metamorphism, recorded in metamorphic zircon rim overgrowths, has been dated in this study at 1.02 Ga. Metamorphism to the southeast, across the younger Karoo grabens, had previously been constrained at 1.05 Ga, indicating an across strike diachronous development of metamorphism for the Irumide belt. The lithological units identified and dated as part of this study in the Irumide belt include: (1) limited Neoarchaean rocks emplaced at 2.73 Ga and representing the oldest rocks in the Bangweulu block; (2) ca. 2.05-1.85 Ga volcano-plutonic complexes and gneisses representing the most important components in the Bangweulu block; (3) an extensive quartzite-metapelite succession with minor carbonate forming the Mporokoso, Kanona and Manshya River groups, and deposited at ca. 1.8 Ga; (4) granitoids emplaced between 1.65-1.55 Ga; (5) deposition of the Kasama Formation between 1.43 and 1.05 Ga (second-cycle reworking of the Mporokoso Group); (6) voluminous syn- to post-kinematic Irumide granitoids emplaced between 1.05-0.95 Ga. In addition, a minor suite of 1.36-1.33 Ga anorogenic plutons (nepheline syenite and biotite granite) have been identified in the far northeastern Irumide belt, but were not included in this study. Whole-rock geochemical data for magmatic rocks in northern Zambia, predominantly from within the Irumide belt, indicate uniform crust-dominated patterns. Overall high REE contents and trace element characteristics indicate the significant participation of older crust in the generation of all magmatic suites.The data are insufficient to conclusively demonstrate that this crustal melting was associated with either intra-plate, volcanic arc or post-collisional/extensional collapse. A limited number of Sm/Nd isotopic data for the entire range of magmatic suites corroborate the highly reworked nature of parent magmas, with all samples characterised by strongly negative åNd(T) values and TDM model ages between 2.2 and 3.2 Ga. The geochronological data presented in this thesis show that the Irumide belt includes a Palaeoproterozoic basement complex comprising units as old as 2.73 Ga, but mostly made up of granitic gneisses ranging in age between 2.05 and 1.93 Ga, while granitic and volcanic units of the Bangweulu block to the northwest were dated at 1.87-1.86 Ga. Detrital zircon age data from quartzites and zircon crystallisation ages of interlayered tuffs within the Muva Supergroup indicate a depositional age of between 1.88 and 1.85 Ga, with local derivation from locally recognised basement units, although similarly aged rocks of the Tanzania craton to the northeast are also a possible source. The detrital record of the Muva Supergroup shows that the various components of the Bangweulu block, including 2.73, 2.05-1.93 and 1.87-1.86 Ga units, were assembled by the time of deposition of the Muva Supergroup at around 1.8 Ga. Both the basement units and the Muva Supergroup were intruded by a previously unknown magmatic suite of biotite granites between 1.65-1.55 Ga, the first record of such a magmatic event in central Africa. The new data presented in this thesis allow a critical assessment of previously proposed regional correlations between Mesoproterozoic teranes in central and southern Africa.Significant temporal differences between the Irumide belt and the Kibaran belt, Choma-Kalomo block and Namaqua-Natal belts had previously not been detected due to the poor quality, low resolution or limited size of isotopic data sets. The new data set produced in this study indicates a distinct and separate tectono-magmatic history for each of these terranes, therefore precluding previously suggested correlations. In particular, the presumed southeastward continuation of the Irumide belt across the Neoproterozoic Zambezi belt into the Choma-Kalomo block is precluded by the data presented in this thesis. This new geochronological framework allows for significant spatial separation of the Kalahari and Congo cratons prior to the Neoproterozoic closure of the Damara-Lufilian-Zambezi ocean, and is therefore in support of palaeogeographic models of Rodinia which either place the Congo and Kalahari cratons as distinct and separate fragments within the supercontinent, or show one or both of the two cratons not to form part of it. Currently, available data are not able to determine the tectonic setting or the palaeogeographic location of the Irumide belt, and as a result it is unclear whether it developed within Rodinia as a collisional orogen, at its margin as an accretionary orogen, or was not associated with Rodinia at all.