Lithospheric mantle evolution monitored by overlapping large igneous provinces: case study in southern Africa

Most of the studies on the large igneous provinces (LIPs) focus on Phanerozoic times, and in particular, thoserelated to the disruption of Pangea (e.g. CAMP, Karoo, Parana–Etendeka) while Precambrian LIPs (e.g.Ventersdorpf, Fortescue) remain less studied. Although the investigation of Precambrian LIPs is difficultbecause they are relatively poorly preserved, assessment of their geochemical characteristics in parallel withyounger overlapping LIP is fundamental for monitoring the evolution of the mantle composition through time.Recent 40Ar/39Ar dating of the Okavango giant dyke swarm (and related sills) in southern Africa showed that~90% of the dykes were emplaced at 179±1Ma and belong to the Karoo large igneous province whereas ~10% ofdykes yielded Proterozoic ages (~1–1.1 Ga). Here,weprovide newmajor, trace and rare earth elements analysesof the low-Ti Proterozoic Okavango dyke swarm (PODS) that suggest, combined with age data, a cognate originwith the 1.1 Ga Umkondo large igneous province (UIP), southern Africa.The geochemical characteristics of the PODS and UIP basalts are comparable to those of overlapping low-TiKaroo basalts, and suggest that both LIPs were derived from similar enriched mantle sources. A mantle plumeorigin for these LIPs is not easily reconciled with the geochemical dataset and the coincidence of twocompositionally similar mantle plumes acting 900 Myr apart is unlikely. Instead,we propose that the Umkondoand Karoo large igneous provinces monitored the slight evolution of a shallow enriched lithospheric mantlefrom Proterozoic to Jurassic

Dyke swarms and associated lava formations in the northern Lebombo monocline, Karoo Large Igneous Province, South Africa

As one of the early classical examples of plume-generated Large Igneous Provinces (LIPs), the Karoo is characterized by a bulls-eye distribution of High-Ti basalts, picrites, and earliest nephelinites onto a conspicuous triple rift junction (i.e., Okavango dyke swarm and the two Mwenezi and Lebombo monoclines), surrounded by Low-Ti basalts. It is noted that this regional distribution between high-and low-Ti basalts within the Jurassic Karoo LIP differs from that of the Permian Emeishan LIP; thereby undermining the use of this particular feature as evidence for mantle plume involvement

Evaluation of cardiovascular risk- lowering health benefits accruing from laboratory-based, community-based and exercise-referral exercise programmes

_____________________________________________________________ This article is brought to you by Swansea University. Any person downloading material is agreeing to abide by the terms of the repository licence. Authors are personally responsible for adhering to publisher restrictions or conditions. When uploading content they are required to comply with their publisher agreement and the SHERPA RoMEO database to judge whether or not it is copyright safe to add this version of the paper to this repository. http://www.swansea.ac.uk/iss/researchsupport/cronfa-support/ Evaluation of cardiovascular risklowering health benefits accruing from laboratory-based, community-based and exercise-referral exercise programme

40Ar/39Ar ages of the sill complex of the Karoo large igneous province: implications for the Pliensbachian-Toarcian climate change.

Reliable geochronological results gathered so far (n = 76) have considerably constrained the timing of the emplacement of the Karoo large igneous province (LIP). Yet strikingly missing from this dating effortis the huge southern sill complex cropping out in the >0.6 x 10(6) km2 Main Karoo sedimentary basin. We present 16 new 40Ar/39Ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a N-S trend in the eastern part of the basin. The results show a large range of plateau and miniplateau ages (176.2 +- 1.3 to 183.8 +- 2.4 Ma), with most dates suggesting a -3 Ma (181-184 Ma) duration for the main sill events. The available age database allows correlation of the Karoo LIP emplacement with the Pliensbachian-Toarcian second-order biotic extinction, the global warming, and the Toarcian anoxic event (provided that adequate calibration between the 40K and 238U decay constant ismade). The mass extinction and the isotopic excursions recorded at the base of the Toarcian appear to be synchronous with both the increase of magma emission of the Karoo LIP and the emplacement of the sills.The CO2 and SO2 derived from both volcanic emissions as well as carbon-rich sedimentary layers intrudedby sills might be the main culprits of the Pliensbachian-Toarcian climate perturbations. We propose that the relatively low eruption rate of the Karoo LIP is one of the main reasons explaining why its impact on thebiosphere is relatively low contrary to, e.g., the CAMP (Triassic-Jurassic) and Siberia (Permo-Triassic) LIPs

Tectonic implications from the geochemistry of Mfongosi Group metasediments, Natal Metamorphic Province, South Africa

The formation of the Mesoproterozoic Natal Metamorphic Province occurred during the closing stages of the assembly of the supercontinent Rodinia (∼1150 to ∼950 Ma), forming part of the ∼1190 to ∼950 Ma Grenvillian orogeny. This was preceded by the accretion, at 1210 Ma, of Mesoproterozoic island arcs along the southern margin of the Archaean Kaapvaal Craton. The Mfongosi Group represents an enigmatic sequence of rocks found between the Kaapvaal Craton and the partially ophiolitic Natal Nappe Zone to the north of these accreted island arcs. Mfongosi metagreywackes adjacent to the Kaapvaal Craton have ocean island arc major element geochemical signatures. These contrast with metagreywackes approximately two kilometres further south, near the contact zone between the Natal Thrust Front and Natal Nappe Zone, which have active continental margin major element geochemical signatures. A third type of metasediment is represented in both areas by geochemically distinct low-Ca+Na, high-K meta-arkoses to meta-lithic arkoses, which were formed by relatively minor sedimentation from a passive continental margin. The Mfongosi Group is a fore-arc complex, incorporating elements of a fore- arc basin and an accretionary prism deposited in a trench, which filled during final oblique collision between the Kaapvaal Craton and an oceanic island arc to the south. The inclusion of an active continental margin signature in the metasediments is enigmatic, although the geochemistry of the metagreywackes suggests that the margin of the Grunehogna Province is a probable source. Subsequent closure of this basin resulted in inversion and accretion of the metasedimentary sequence onto the southern margin of the Kaapvaal Craton. [ABSTRACT FROM AUTHOR

The Zambezi Channel: a new perspective on submarine channel evolution at low latitudes

The submarine Zambezi Channel is the deep, stable, north-south orientated, lower portion of a channel system draining the continental slope of central Mozambique; transporting material southwards into the Mozambique Channel and Basin, southwest Indian Ocean. Using recently collected Multi Beam Echo Sounder and PARASOUND data we discuss the geomorphology of the Zambezi Channel. This system is enigmatic in that the main channel is stable, with low sinuosity despite being at a low latitude where rivers seasonally deliver fine grained sediment. A further enigma is that system does not now continue upslope to the Zambezi River, the largest river in southern Africa. Instead this river flows into the northern Mozambique basin to the south-west of the small channels. The Zambezi Channel is compared to small-scale physical models in an attempt to better understand the geomorphology of the channel. The geomorphological features of the main channel show a quite remarkable resemblance to an analogue model produced within a purely erosive environment. To explain these enigmas, it is proposed that geomorphology of the main Zambezi Channel was produced by periodic, high-volume pulses of flood water, and associated sediment, from the Zambezi River, the second largest river in Africa. These events are considered to be due to minor tectonic movements along the Chobe Fault in the Kalahari that permitted the draining of several palaeo-lake systems between the Early Pleistocene through to the early Mid-Pleistocene. Such repetitive draining of palaeo-lakes would have produced flooding comparable to glacial dam bursts. Such events would deliver significantly more sediment laden flood water to the region than “normal” flow conditions. We hypothesise that these significant flood events have influenced the geomorphology of the Zambezi River to the extent that it is no longer comparable to other low-latitude systems, and exhibits characteristics akin to high-latitude systems with highly variable sediment input

Zambezi continental margin: compartmentalized sediment transfer routes to the abyssal Mozambique Channel

Sediment delivery to the abyssal regions of the oceans is an integral process in the source to sink cycle of material derived from adjacent continents and islands. The Zambezi River, the largest in southern Africa, delivers vast amounts of material to the inner continental shelf of central Mozambique. The aim of this contribution is to better constrain sediment transport pathways to the abyssal plains using the latest, regional, high-resolution multibeam bathymetry data available, taking into account the effects of bottom water circulation, antecedent basin morphology and sea-level change. Results show that sediment transport and delivery to the abyssal plains is partitioned into three distinct domains; southern, central and northern. Sediment partitioning is primarily controlled by changes in continental shelf and shelf-break morphology under the influence of a clockwise rotating shelf circulation system. However, changes in sealevel have an overarching control on sediment delivery to particular domains. During highstand conditions, such as today, limited sediment delivery to the submarine Zambezi Valley and Channel is proposed, with increased sediment delivery to the deepwater basin being envisaged during regression and lowstand conditions. However, there is a pronounced along-strike variation in sediment transport during the sea-level cycle due to changes in the width, depth and orientation of the shelf. This combination of features outlines a sequence stratigraphic concept not generally considered in the strike-aligned shelf-slope-abyssal continuum

Palaeoceanographic changes identified using seismic and radiogenic isotope data from the Mozambique Ridge, SW Indian Ocean

Seismic reflection data from the southern Mozambique Ridge, SW Indian Ocean, show indications for a modification in the oceanic circulation system. In the absence of an age-depth model based on a drill site we used information gathered from the study of radiogenic isotopes of ferromanganese nodules and crusts. Those were recovered via dredges from the Mozambique Ridge to gain knowledge on the origin of the modifications of the oceanic circulation system, which is documented in the distinct change in reflection characteristics. This is found to have occurred at 9 Ma