scholarly journals Isotopic evidence for a lithospheric origin of alkaline rocks and carbonatites: an example from southern Africa

2016 ◽  
Vol 53 (11) ◽  
pp. 1216-1226 ◽  
Author(s):  
Lewis D. Ashwal ◽  
Madelein Patzelt ◽  
Mark D. Schmitz ◽  
Kevin Burke
Keyword(s):  
South Africa ◽  
Southern Africa ◽  
Alkaline Rock ◽  
Time Span ◽  
Mantle Lithosphere ◽  
Alkaline Rocks ◽  
Isotopic Compositions ◽  
Magmatic Rocks ◽  
Mantle Sources ◽  
Source Component

Alkaline rocks and carbonatites, including nepheline syenites, are well established as mantle-derived magmatic products, but the nature and location of their mantle sources is debated. Some workers have used isotopic compositions to infer mixed mantle plume type sources such as EM1, HIMU, and FOZO, implying derivation from the subcontinental asthenosphere. Other models favour an entirely lithospheric source, whereby the magmas, originally formed during intracontinental rifting, became deformed and subducted into the mantle lithosphere during later continental collisions, and constituted part of a source component for later rift-related alkaline and carbonatite magmatism. We tested this model using Sr, Nd, and Hf isotopic compositions of deformed and undeformed nepheline syenites and carbonatites from three occurrences in southern Africa, representing emplacement over a ∼1 Ga time span. These include Bull’s Run, South Africa (1134 Ma); Tambani, Malawi (726 Ma); and the Chilwa Alkaline Province, Malawi (130 Ma). Mixing modelling indicates that the isotopic compositions of the early Cretaceous Chilwa samples can be accounted for if their source consisted of a blend of ∼99% depleted subcontinental mantle lithosphere and ∼0.5%–1% of a subducted component similar to the Neoproterozoic Bull’s Run nepheline syenites. We do not consider the Bull’s Run material specifically as the component involved in the Chilwa source, but our model illustrates an example of how recycled, older, alkaline magmatic rocks can contribute to the mantle sources of younger alkaline rock and carbonatite magmatism. This model accounts for the observation of recurrent alkaline rock and carbonatite magmatism over hundreds of millions of years in spatially restricted areas like southern Africa. Carbonatite and related alkaline magmatic rocks, therefore, need not owe their origin to deep, sublithospheric melting processes.

scholarly journals Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins

2020 ◽  
Vol 47 (3) ◽  
pp. 119-142
Author(s):  
Roger H. Mitchell
Keyword(s):  
Partial Melting ◽  
Lithospheric Mantle ◽  
Tectonic Setting ◽  
Alkaline Rock ◽  
Mantle Metasomatism ◽  
Geochemical Evolution ◽  
Alkaline Rocks ◽  
Isotopic Compositions ◽  
Economic Significance ◽  
Rock Types

Lamproite is a rare ultrapotassic alkaline rock of petrological importance as it is considered to be derived from metasomatized lithospheric mantle, and of economic significance, being the host of major diamond deposits. A review of the nomenclature of lamproite results in the recommendation that members of the lamproite petrological clan be named using mineralogical-genetic classifications to distinguish them from other genetically unrelated potassic alkaline rocks, kimberlite, and diverse lamprophyres. The names “Group 2 kimberlite” and “orangeite” must be abandoned as these rock types are varieties of bona fide lamproite restricted to the Kaapvaal Craton. Lamproites exhibit extreme diversity in their mineralogy which ranges from olivine phlogopite lamproite, through phlogopite leucite lamproite and potassic titanian richterite-diopside lamproite, to leucite sanidine lamproite. Diamondiferous olivine lamproites are hybrid rocks extensively contaminated by mantle-derived xenocrystic olivine. Currently, lamproites are divided into cratonic (e.g. Leucite Hills, USA; Baifen, China) and orogenic (Mediterranean) varieties (e.g. Murcia-Almeria, Spain; Afyon, Turkey; Xungba, Tibet). Each cratonic and orogenic lamproite province differs significantly in tectonic setting and Sr–Nd–Pb–Hf isotopic compositions. Isotopic compositions indicate derivation from enriched mantle sources, having long-term low Sm/Nd and high Rb/Sr ratios, relative to bulk earth and depleted asthenospheric mantle. All lamproites are considered, on the basis of their geochemistry, to be derived from ancient mineralogically complex K–Ti–Ba–REE-rich veins, or metasomes, in the lithospheric mantle with, or without, subsequent contributions from recent asthenospheric or subducted components at the time of genesis. Lamproite primary magmas are considered to be relatively silica-rich (~50–60 wt.% SiO2), MgO-poor (3–12 wt.%), and ultrapotassic (~8–12 wt.% K2O) as exemplified by hyalo-phlogopite lamproites from the Leucite Hills (Wyoming) or Smoky Butte (Montana). Brief descriptions are given of the most important phreatomagmatic diamondiferous lamproite vents. The tectonic processes which lead to partial melting of metasomes, and/or initiation of magmatism, are described for examples of cratonic and orogenic lamproites. As each lamproite province differs with respect to its mineralogy, geochemical evolution, and tectonic setting there is no simple or common petrogenetic model for their genesis. Each province must be considered as the unique expression of the times and vagaries of ancient mantle metasomatism, coupled with diverse and complex partial melting processes, together with mixing of younger asthenospheric and lithospheric material, and, in the case of many orogenic lamproites, with Paleogene to Recent subducted material.

Imaging the Late Triassic lithospheric architecture of the Yidun Terrane, eastern Tibetan Plateau: Observations and interpretations

2021 ◽  
Author(s):  
Qiong-Yao Zhan ◽  
Di-Cheng Zhu ◽  
Qing Wang ◽  
Peter A. Cawood ◽  
Jin-Cheng Xie ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Oceanic Lithosphere ◽  
Late Triassic ◽  
Eastern Tibetan Plateau ◽  
Magmatic Differentiation ◽  
Isotopic Compositions ◽  
Magmatic Rocks ◽  
Mantle Sources ◽  
Magma Sources ◽  
Nonlinear Trends

The present-day lithospheric architecture of modern and ancient orogens can be imaged by geophysical techniques. For ancient orogens, unravelling their architecture at the time of formation is hindered by later tectono-magmatic events. In this paper, we use spatial variations in radiogenic isotopic compositions of Late Triassic magmatism from the Yidun Terrane, eastern Tibetan Plateau, to establish its lithospheric architecture during the Triassic. Comprehensive geochemical and isotopic data of Late Triassic magmatic rocks from four transects across the Yidun Terrane document eastward enrichment in whole-rock Nd, Sr, and zircon Hf isotopic compositions. Mafic and felsic rocks of major plutons show coherent and nonlinear trends in the Zr and P2O5 systematics and have limited variation of isotopic compositions. This indicates that Late Triassic magmatic differentiation was dominated by fractionation of mantle-derived mafic magmas. The spatial isotopic trends result from changing mantle sources, including variable contributions of isotopically depleted asthenospheric mantle and isotopically enriched subcontinental lithospheric mantle (SCLM) to magma sources. The spatial variation of mantle sources suggests a westward thinning of the SCLM during the Triassic. We propose that this architecture is most likely associated with eastward subduction of oceanic lithosphere of the Jinshajiang Ocean located at the west of the Yidun Terrane, immediately prior to the Late Triassic magmatism.

Region of Birth and Child Mortality among Black Migrants to South Africa: Is there a foreign-born advantage?

2016 ◽  
Vol 13 (3) ◽  
pp. 359-376 ◽  
Author(s):  
Tiffany L Green ◽  
Amos C Peters
Keyword(s):  
South Africa ◽  
South African ◽  
Child Mortality ◽  
Southern Africa ◽  
Strong Relationship ◽  
Developed Countries ◽  
Black Immigrants ◽  
Foreign Born ◽  
Region Of Origin ◽  
Child Deaths

Much of the existing evidence for the healthy immigrant advantage comes from developed countries. We investigate whether an immigrant health advantage exists in South Africa, an important emerging economy.  Using the 2001 South African Census, this study examines differences in child mortality between native-born South African and immigrant blacks.  We find that accounting for region of origin is critical: immigrants from southern Africa are more likely to experience higher lifetime child mortality compared to the native-born population.  Further, immigrants from outside of southern Africa are less likely than both groups to experience child deaths.  Finally, in contrast to patterns observed in developed countries, we detect a strong relationship between schooling and child mortality among black immigrants.

Reverend Doctor Sox Leleki and His Role in the Black Methodist Consultation

2019 ◽  
Vol 6 (2) ◽  
Author(s):  
Sonwabile Mancotywa
Keyword(s):  
South Africa ◽  
Southern Africa ◽  
Political Influence ◽  
Individual Member ◽  
Methodist Church ◽  
Role Players

The Methodist Church of Southern Africa (MCSA) is one of the mainline Christian denominations with a very good history. However, it was not immune from the larger political influence of South Africa that was polarised by apartheid. This article is intended to look at the formation known as the Black Methodist Consultation (BMC), which at that time had an individual member who played an important role in its development and activities. Sox Leleki was one of the key role players of this movement inside the Methodist Church

Ores Sources, Smelters and Archaeometallurgy: Exploring Iron Age Metal Production in the Southern Waterberg, South Africa

2013 ◽  
Vol 11 (2) ◽  
pp. 243-267 ◽  
Author(s):  
Foreman Bandama ◽  
Shadreck Chirikure ◽  
Simon Hall
Keyword(s):  
South Africa ◽  
Iron Age ◽  
Southern Africa ◽  
Mineral Resources ◽  
Limpopo Province ◽  
Metal Working ◽  
Cultural Interaction ◽  
Metal Production ◽  
European Colonization ◽  
Sem Analyses

The Southern Waterberg in Limpopo Province is archaeologically rich, especially when it comes to evidence of pre-colonial mining and metal working. Geologically, the area hosts important mineral resources such as copper, tin and iron which were smelted by agriculturalists in the precolonial period. In this region however, tin seems to be the major attraction given that Rooiberg is still the only source of cassiterite in southern Africa to have provided evidence of mining before European colonization. This paper reports the results of archaeological and archaeometallurgical work which was carried out in order to reconstruct the technology of metalworking as well as the cultural interaction in the study area and beyond. The ceramic evidence shows that from the Eiland Phase (1000–1300 AD) onwards there was cross borrowing of characteristic decorative traits amongst extant groups that later on culminated in the creation of a new ceramic group known as Rooiberg. In terms of mining and metal working, XRF and SEM analyses, when coupled with optical microscopy, indicate the use of indigenous bloomery techniques that are widespread in pre-colonial southern Africa. Tin and bronze production was also represented and their production remains also pin down this metallurgy to particular sites and excludes the possibility of importing of finished tin and bronze objects into this area.

Participatory catchment management — an opportunity for Southern Africa

1995 ◽  
Vol 32 (5-6) ◽  
pp. 145-151
Author(s):  
D. B. Versfeld
Keyword(s):  
South Africa ◽  
Land Management ◽  
Southern Africa ◽  
Participatory Rural Appraisal ◽  
Catchment Management ◽  
Management Process ◽  
Participatory Methods ◽  
Top Down ◽  
Kwazulu Natal ◽  
Rural Appraisal

South Africa has hundreds of thousands of hectares of heavily populated and badly degraded landscapes. Past attempts at land management have been either through avoidance or the top-down imposition of “betterment” schemes. Participatory methods offer a new opportunity for communities living within these catchments to share their knowledge and to become involved in planning and implementing the management process. This paper discusses the use of Participatory Rural Appraisal (PRA) in a catchment rehabilitation programme in rural KwaZulu/Natal, the lessons learnt and the prospects for wider application.

Carnivore Size and Quaternary Climatic Change in Southern Africa

1986 ◽  
Vol 26 (1) ◽  
pp. 153-170 ◽  
Author(s):  
Richard G. Klein
Keyword(s):  
South Africa ◽  
Southern Africa ◽  
Late Quaternary ◽  
Geochemical Data ◽  
Mathematical Relationship ◽  
Carnivore Species ◽  
Quantitative Estimates ◽  
The Relationship ◽  
Problematic Aspect ◽  
Cool Conditions

The relationship between carnassial length and latitude south is analyzed for 17 African carnivore species to determine if individuals tend to be larger in cooler climates, as predicted by Bergmann's Rule. With modern data in support, middle and late Quaternary temperatures might then be inferred from mean carnassial length in fossil samples, such as those from Equus Cave, Elandsfontein, Sea Harvest. Duinefontein, and Swartklip in the Cape Province of South Africa. One problematic aspect of the study is the use of carnassial length and latitude as necessary but imperfect substitutes for body size and temperature, respectively. For some species, another difficulty is the relatively small number of available modern specimens, combined with their uneven latitudinal spread. Still, in 14 of the species, carnassial length does tend to increase with latitude south, while mean carnassial length in the same species tends to be greater in those fossil samples which accumulated under relatively cool conditions, as inferred from sedimentologic, palynological, or geochemical data. Given larger modern samples from a wide variety of latitudes, refinement of the mathematical relationship between carnassial length and latitude in various species may even permit quantitative estimates of past temperatures in southern Africa.

scholarly journals Circulation pattern controls of wet days and dry days in Free State, South Africa

2021 ◽  
Author(s):  
Chibuike Chiedozie Ibebuchi
Keyword(s):  
South Africa ◽  
Indian Ocean ◽  
Atmospheric Circulation ◽  
Southern Africa ◽  
Austral Summer ◽  
Free State ◽  
Austral Spring ◽  
Study Region ◽  
Southwest Indian Ocean ◽  
South Indian

AbstractAtmospheric circulation is a vital process in the transport of heat, moisture, and pollutants around the globe. The variability of rainfall depends to some extent on the atmospheric circulation. This paper investigates synoptic situations in southern Africa that can be associated with wet days and dry days in Free State, South Africa, in addition to the underlying dynamics. Principal component analysis was applied to the T-mode matrix (variable is time series and observation is grid points at which the field was observed) of daily mean sea level pressure field from 1979 to 2018 in classifying the circulation patterns in southern Africa. 18 circulation types (CTs) were classified in the study region. From the linkage of the CTs to the observed rainfall data, from 11 stations in Free State, it was found that dominant austral winter and late austral autumn CTs have a higher probability of being associated with dry days in Free State. Dominant austral summer and late austral spring CTs were found to have a higher probability of being associated with wet days in Free State. Cyclonic/anti-cyclonic activity over the southwest Indian Ocean, explained to a good extent, the inter-seasonal variability of rainfall in Free State. The synoptic state associated with a stronger anti-cyclonic circulation at the western branch of the South Indian Ocean high-pressure, during austral summer, leading to enhanced low-level moisture transport by southeast winds was found to have the highest probability of being associated with above-average rainfall in most regions in Free State. On the other hand, the synoptic state associated with enhanced transport of cold dry air, by the extratropical westerlies, was found to have the highest probability of being associated with (winter) dryness in Free State.

Le terrane de Slide Mountain (Cordillères canadiennes) : une lithosphère océanique marquée par des points chauds

2003 ◽  
Vol 40 (6) ◽  
pp. 833-852 ◽  
Author(s):  
M Tardy ◽  
H Lapierre ◽  
D Bosch ◽  
A Cadoux ◽  
A Narros ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Oceanic Island ◽  
Light Rare Earth ◽  
Isotopic Compositions ◽  
Incompatible Elements ◽  
Depleted Mantle ◽  
Mantle Sources ◽  
British Colombia ◽  
Ultramafic Cumulates ◽  
Back Arc

The Slide Mountain Terrane consists of Devonian to Permian siliceous and detrital sediments in which are interbedded basalts and dolerites. Locally, ultramafic cumulates intrude these sediments. The Slide Mountain Terrane is considered to represent a back-arc basin related to the Quesnellia Paleozoic arc-terrane. However, the Slide Mountain mafic volcanic rocks exposed in central British Colombia do not exhibit features of back-arc basin basalts (BABB) but those of mid-oceanic ridge (MORB) and oceanic island (OIB) basalts. The N-MORB-type volcanic rocks are characterized by light rare-earth element (LREE)-depleted patterns, La/Nb ratios ranging between 1 and 2. Moreover, their Nd and Pb isotopic compositions suggest that they derived from a depleted mantle source. The within-plate basalts differ from those of MORB affinity by LREE-enriched patterns; higher TiO2, Nb, Ta, and Th abundances; lower εNd values; and correlatively higher isotopic Pb ratios. The Nd and Pb isotopic compositions of the ultramafic cumulates are similar to those of MORB-type volcanic rocks. The correlations between εNd and incompatible elements suggest that part of the Slide Mountain volcanic rocks derive from the mixing of two mantle sources: a depleted N-MORB type and an enriched OIB type. This indicates that some volcanic rocks of the Slide Mountain basin likely developed from a ridge-centered or near-ridge hotspot. The activity of this hotspot is probably related to the worldwide important mantle plume activity that occurred at the end of Permian times, notably in Siberia.

Sign in / Sign up

Export Citation Format

Share Document