A mantle or mafic crustal source for Proterozoic anorthosites?

Calculations of fractional crystallization (FC) and assimilation fractional crystallization (AFC) at 11 kb for a variety of primitive magmatic compositions and a mafic assimilant demonstrate that none of them has a bulk composition suitable to be parental to massif anorthosites. Mafic compositions thought to be parental to massif anorthosites have Mg′ values of 0.6 to 0.4 and form coherent arrays with moderately steep slopes on plots of TiO₂, K₂O, and P₂O₅ versus Mg′. The calculated liquid lines of descent (LLD) of basaltic magmas undergoing FC or AFC processes pass through the arrays of anorthosite parent magma compositions with much shallower slopes than the natural arrays, which indicates that the arrays of natural parental magmas were produced by a process other than FC/AFC. Also, by the time most crystallizing basaltic magmas with or without assimilation reach plagioclase saturation, their residual liquids have Mg′ values that are too low to be parental to anorthosites. MORB-like olivine tholeiites and high-aluminum olivine tholeiites (HAOT) from convergent plate margins do reach plagioclase saturation while sufficiently magnesian, but their Wo (Wollastonite) contents are too high such that they reach plagioclase saturation coexisting only with augite and do not reach orthopyroxene saturation (if at all) until Mg′ is too low. Calculations show it is not possible to produce a high-Al melt from typical mantle peridotites that has sufficient TiO₂ to make andesine-type anorthosite.

Calculation of partial melting for an average mafic crustal composition at 11 kbar provides a much closer match to the array of natural parental compositions in terms of minor element concentrations and proportions of mineral components. However, accounting for the entire array requires a more magnesian source composition. Such compositions exist in several crustal xenolith localities. Similar results were obtained using the bulk composition of the Stillwater Complex, which is used as a model mafic source (here the premise is that overdense crustal intrusions might sink back into the mantle). As with the terrain composition, this particular layered intrusion composition is not sufficiently magnesian, however, the fit improves when mixtures of early and late stage portions of the complex (i.e., the denser portions) were run as potential source regions.     

Ray path interpretation of the crustal structure beneath Saudi Arabia

Interpretation of a long-range seismic refraction line in Saudi Arabia has shown that beneath the Arabian Shield velocity generally increases with depth, from about 6 km s⁻¹ at the surface to about 7 km s⁻¹ at the top of the crust-mantle transition zone. The base of this transition zone (Moho) occurs at 37–44 km in depth. Intracrustal discontinuities can also be recognized, the most important being in the 10–20 km-depth range and separating the upper from the lower crust. Laterally, the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions. Beneath the Red Sea shelf and coastal plain the crust, including 4 km of sediments, is only 15–17.5 km thick. With the aid of both seismic and gravity data an abrupt, steeply dipping transition from the crust of the Red Sea shelf and coastal plain to that of the Arabian Shield has been derived. With a jump of more than 20 km in Moho depth, this appears to be the major discontinuity between the Red Sea depression and the Arabian continental shield.     

Geochemistry and fore-arc evolution of upper mantle peridotites in the Cryogenian Bir Umq ophiolite,Arabian Shield,Saudi Arabia

ABSTRACT

The Bir Umq ophiolite is one of the most important ophiolitic successions in the Arabian Shield, and represents an excellent case for the study of the tectonomagmatic evolution of the earliest Precambrian events in the juvenile part of the Arabian-Nubian Shield (ANS). It is a dismembered ophiolite, which includes a serpentinized peridotite with small amounts of gabbro and mélange, and is overlain by the Sumayir formation. The mantle section of the Bir Umq ophiolite has been pervasively sheared and folded during its emplacement and is extensively serpentinized, carbonated and silicified, resulting in the common development of magnesite and listwaenite along the shear zones. Listwaenite occurs in the form of upstanding ridges due to its resistance to erosion. Antigorite is the main serpentine mineral, which, however, has low amounts of lizardite and chrysotile, indicating that the present serpentinites formed by prograde metamorphism. The ophiolitic rocks of Bir Umq have undergone regional metamorphism up to the greenschist to amphibolite facies. The presence of mesh and bastite textures indicates harzburgite and dunite protoliths. The serpentinized peridotite preserves rare relicts of primary minerals such as olivine, pyroxene and Cr-spinel. The serpentinized ultramafics of Bir Umq have high Mg# [molar Mg/(Mg+Fe²⁺); 0.90–0.93), low CaO, and Al₂O₃ contents similar to that of the environment of the suprasubduction zone. Additionally, they are characterized by the depletion of some compatible trace elements (e.g., Nb, Sr, Ta, Zr, Hf and REE), but show a wide variation in the Rb and Ba. Moreover, they are enriched in some elements that have affinities for Mg-rich minerals such as Ni, Cr, V, and Co. Fresh relics of olivine have high Fo (av. 0.91) and NiO (av. 0.42) contents, similar to those in the mantle olivine. The fresh Cr-spinel has high Cr# (0.68) and low TiO₂ content (av. 0.11), similar to those in modern fore-arc peridotites. The composition of both orth- and clinopyroxenes confirms the fore-arc affinity of the studied ultramafics. The present study indicates that the protoliths of the serpentinized ultramafics of Bir Umq have high partial melt degrees, which is consistent with the characteristics of ultramafic rocks formed in a subarc environment (fore-arc) within a suprasubduction zone system.     

Saudi Arabian seismic-refraction profile: A traveltime interpretation of crustal and upper mantle structure

The crustal and upper mantle compressional-wave velocity structure across the southwestern Arabian Shield has been investigated by a 1000-km-long seismic refraction profile. The profile begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, trends southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan, and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, and six shot points were used, including one in the Red Sea.Two-dimensional ray-tracing techniques, used to analyze amplitude-normalized record sections indicate that the Arabian Shield is composed, to first order, of two layers, each about 20 km thick, with average velocities of about 6.3 km/s and 7.0 km/s, respectively. West of the Shield-Red Sea margin, the crust thins to a total thickness of less than 20 km, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust.A major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition. Elsewhere along the profile, several high-velocity anomalies in the upper crust correlate with mapped gneiss domes, the most prominent of which is the Khamis Mushayt gneiss. Based on their velocities, these domes may constitute areas where lower crustal rocks have been raised some 20 km. Two intracrustal reflectors in the center of the Shield at 13 km depth probably represent the tops of mafic intrusives.The Mohorovičić discontinuity beneath the Shield varies from a depth of 43 km and mantle velocity of 8.2 km/s in the northeast to a depth of 38 km and mantle velocity of 8.0 km/s depth in the southwest near the Shield-Red Sea transition. Two velocity discontinuities occur in the upper mantle, at 59 and 70 km depth.The crustal and upper mantle velocity structure of the Arabian Shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the Precarnbrian. The Shield is currently flanked by the active spreading boundary in the Red Sea.     

Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt,southern India: implications for Late Proterozoic crustal evolution in East Gondwana

Two stages of granitic magmatism occurred during the Pan-African evolution of the Kerala Khondalite Belt (KKB) in southern India. Granitic gneisses were derived from porphyritic granites, which intruded prior to the main stage of deformation and peak-metamorphism. Subsequently, leucogranites and leucotonalites formed during fluid-absent melting and intruded the gneiss sequences. Monazites from granitic gneisses, leucogranites and a leucotonalite were investigated by conventional U-Pb and electron microprobe dating in order to distinguish the different stages of magma emplacement. U-Pb monazite dating yielded a wide range of ages between 590–520 Ma which are interpreted to date high-grade metamorphism rather than magma emplacement. The results of this study indicate that the KKB experienced protracted heating (>50 Ma) at temperatures above 750–800 °C during the Pan-African orogeny. The tectonometamorphic evolution of the study area is comparable to southern Madagascar which underwent a similar sequence of events earlier than the KKB. The results of this study further substantiate previous assertions that the timing of high-grade metamorphism in East Gondwana shifted from west to east during the Late Proterozoic.     

Recent trends of temperature and precipitation proxies in Saudi Arabia: implications for climate change

The objective of the present study was to reconstruct a short-term (12–14 years) trend of surface temperature and precipitation patterns using their surrogates as provided by satellite images for selected locations along the Red Sea mountains in Saudi Arabia. Time series land surface temperature (LST) and normalized difference vegetation index (NDVI) data acquired from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite were temporally plotted to delineate the trend and the decadal rates of change of both parameters. Results showed that real climate change is reported in the study area during the study period. There is a net increasing in the surface temperatures by 0.45 to 1.2 °C/decade and a net decrease in annual rainfall between 2001 and 2014. Findings of the present study show that the region is under a warming of the climate and a declining of wetness, which coincide with the air temperature and rainfall trends obtained from meteorological stations.     

Structure and composition of mantle peridotites at the boundary with crustal complexes of ophiolites in the Syumkeu massif,Polar Urals

Intense viscous-ductile deformations with multiorder flow folds and thin banding have been established in lherzolite and harzburgite of the Syumkeu massif 1.0–1.5 km below the boundary with crustal complexes. Intense shear deformation of mantle restites is traced along the entire boundary zone. The mineral composition of lherzolite and harzburgite in this zone occupies a transitional position between peridotite restites and olivine websterite from the lower part of the banded dunite-wehrlite-pyroxenite-gabbro complex. This implies that the mantle rocks from the crust-mantle transition zone were substantially transformed under transpressional intense shear stress settings along with a high-temperature ductile flow of mantle restites interacting with the supplied melt at a depth of more than 10 km. This type of transition zones differs from those known elsewhere in the Urals and supplements our knowledge on modes of mantle restite juxtaposition with crustal plutonic rocks.     

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits.Semi-consolidated new crust was affected by late Svecokarelian deformation (D_n) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the F_n folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the D_n deformation.

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Zusammenfassung Die kontinentale Kruste des zentralen Baltischen Schildes entwickelte sich durch nach Westen gerichtetes Anwachsen während der Svecokarelischen Orogenese vor 1700 bis 2200 Ma. Die folgenden Erscheinungsformen lassen sich mit einem plattentektonischen Mechanismus in Einklang bringen, der Subduktion von ozeanischer Kruste unter einen Archaischen Kraton im Osten einschließt: Flysch-Sedimente mit Serpentinit-Massen und Kissenlaven, lineare hochmetamorphe Zonen, vulkanische Gürtel vom Inselbogen-Typ und spättektonische Batholithe mit porphyrischen Cu-Mo-Lagerstätten.Die halbkonsolidierte neue Kruste wurde durch späte Svecokarelische Deformation (D_n) nach 1850 Ma erfaßt; NNE-orientierte Falten mit Krenulationsschieferung wurden älteren Strukturen aufgeprägt in Verbindung mit NW-streichenden, plastischen Transcurrent-Scherzonen, die die F_n-Falten in sanfte S- und Z-Formen verbiegen. Die spättektonischen Batholithe intrudierten teils während, teils nach der D_n-Deformation.

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Résumé La croûte continentale du Boucher baltique central a évolué par voie d'accrétion vers l'ouest durant l'orogénie svécocarélienne 1700–2200 Ma. Les événements suivants sont en accord avec un mécanisme de tectonique de plaques impliquant la subduction d'une croûte océanique sous un craton archéen à l'est: sédiments flyschoïdes avec masses de serpentinite et de laves en coussins, zones linéaires à haut degré de métamorphisme, ceintures volcaniques du type guirlande d'îles et batholithes tectoniques tardifs avec gisements porphyriques de type Cu-Mo.La nouvelle croûte à semi-consolidée fut affectée par une déformation svécocarélienne tardive (D_n) postérieure à 1850 Ma. Des plis de direction NNE avec clivage de crénulation ont été superposés sur des structures plus anciennes, associés à des zones de cisaillement transcurrentes de direction NW qui ont incurvé les plis F_n suivant des formes en S et Z. Le batholithe tectonique tardif s'est mis en place en partie au même moment que, et en partie après, la déformation D_n.

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, , 1500–2200 . , , : ; ; - . , 1850 ; NNE- , , NW , , F_n S Z. , .

     

The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt,and the implications for late Archaean crustal evolution

Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated ${}^{238}\text{U}{}^{204}\text{Pb}$ values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements.     

Aeromagnetic data over Harrat Lunayyir and surrounding areas,western Saudi Arabia

Harrat Lunayyir is one of the smaller lava fields in western Saudi Arabia that is of current interest due to a dike intrusion episode in 2009, an ongoing swarm of earthquakes and the possible hazard that pose. In addition to seismology, other geophysical data have been used to study the structure of the area, and the available aeromagnetic information is shown and discussed here. The reduced to the pole (RTP) magnetic grid and its enhancements have been used to define some of the main anomalies, and these have been correlated with the known geology, including the Red Sea coastal dike system. There are numerous linear features that are possibly related to the dikes or faulting, but within the area of the harrat, details due to the underlying structure are largely obscured by the magnetic surface lavas. Northeasterly trends in the magnetic data may indicate old zones of weakness that intersect one of the main coastal dikes at the location of the recent seismic activity and surface fissure, suggesting that this is a point of weakness resulting in the volcanism and seismicity that also appears to be largely limited to two of the NE trends. The association of the recent seismicity with a known geological and aeromagnetic feature is important in determining the seismic hazard for the region, especially if the location of future activity can be used to reduce overall uncertainty in the analysis by identifying potential fault sources. Here, the seismicity appears to lie on one of the NNW-trending coastal dikes that have been reactivated recently along a section between two NE-trending older faults.     

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the _Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative _Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative _Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host.The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle — both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart.     

Geochemistry of mafic dikes in the Adirondack mountains: implications for late Proterozoic continental rifting

Mafic dikes of late Proterozoic age which cut Grenvillian crust in the northeastern Adirondack Mountains are mostly mildly alkaline basalts except for a few tholeiitic examples. All dikes are high in Ti, P, K, Zr, Y, and LREE, and plot in within-plate fields on tectonic discriminant diagrams. The dikes are similar in composition to Hudson Highland dikes in southern New York and New Jersey and to the Bakersville dike swarm in the southern Appalachians. They differ from the Grenville dike swarm in Ontario and Quebec in being alkaline and having higher Ti and P contents. Mesozoic alkaline dikes in the same geographic area as the Proterozoic ones are strongly enriched in Ba, K, Rb and LREE, and approach lamprophyre in composition. The Proterozoic dikes have low La/Nb and La/Ta ratios, suggesting that subduction-modified mantle lithosphere was not substantially involved in their genesis. This contrasts with certain Mesozoic tholeiitic dikes, associated with the opening of the Atlantic, which show sharp negative Nb or Ta anomalies relative to La indicating they were derived from subduction-modified lithospheric mantle. The trace element chemistry suggests that the source for the Proterozoic dikes was trace element-enriched asthenosphere (OIB-like source), as postulated for certain basalts erupted in the East African Rift system, and in parts of the Basin and Range Province of the southwestern United States of America. Finally, the Proterozoic dikes are chemically similar to rift volcanics from the western Vermont Appalachians, and thus they are thought to represent magmatism associated with extension of the Grenvillian crust prior to opening of the Iapetus ocean.     

The extent of desertification on Saudi Arabia

Desertification is the process that turns productive deserts into non-productive deserts as a result of poor land-management. Desertification reduces the ability of land to support life, affecting wild species, domestic animals, agricultural crops and humans. The reduction in plant cover that accompanies desertification leads to accelerated soil erosion by wind and water. South Africa is losing approximately 300–400 million tons of topsoil every year. As vegetation cover and soil layer are reduced, rain fall impact and run-off increases. This paper discusses the extent of desertification, its potential threat to sustained irrigated agriculture and possible measures adopted to control ongoing desertification processes to minimize the loss of agricultural productivity in an arid country such as the Kingdom of Saudi Arabia.     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.     

Asymmetry in oceanic crustal structure of the South China Sea basin and its implications on mantle geodynamics

ABSTRACT

We investigated the oceanic crustal structure and lithospheric dynamics of the South China Sea (SCS) basin through a comprehensive analysis of residual gravity anomaly and bathymetry combined with seismic constraints and interpretation from geodynamic modelling. We first calculated the residual mantle Bouguer anomaly (RMBA) of the oceanic crustal regions of the SCS by removing from free-air gravity anomaly the predicted gravitational attractions of water-sediment, sediment-crust, and crust-mantle interfaces, as well as the effects of lithospheric plate cooling, using the latest crustal age constraints including IODP Expedition 349 and recent deep-tow magnetic surveys. We then calculated models of the gravity-derived crustal thickness and calibrated them using the available seismic refraction profiles of the SCS. The gravity-derived crustal thickness models correlate positively with seismically determined crustal thickness values. Our analysis revealed that the isochron-averaged RMBA are consistently more negative over the northern flank of the SCS basin than the southern conjugate for magnetic anomaly chrons C8n (~25.18 Ma) to C5Dn (~17.38 Ma), implying warmer mantle and/or thicker crust over much of the northern flank. Computational geodynamic modelling yielded the following interpretations: (1) Models of asymmetric and variable spreading rates based on the relatively high-resolution deep-tow magnetic analysis would predict alternating thicker and thinner crust at the northern flank than the southern conjugate, which is inconsistent with the observed systematically thicker crust on the northern flank. (2) Models of episodic southward ridge jumps could reproduce the observed N-S asymmetry, but only for crustal age of 23.6–20 Ma. (3) Southward migration of the SCS ridge axis would predict slightly thinner crust at the northern flank, which is inconsistent with the observations. (4) Models of higher mantle temperatures of up to 25–50°C or >2% less depleted mantle sources on the northern flank could produce large enough anomalies to explain the observed N-S asymmetries.     

New geochronological data on volcanic rocks from northeast Sudan and their implication for crustal evolution

The basement volcano-sedimentary rocks of northeast Sudan form part of the Nubian Shield of northeast Africa. Volcanic rocks from the Kadawēb area yield Rb—Sr wholerock isochron ages of 718 and 722 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7027 and 0.7029. In the Homogar area, 150 km to the south, volcanic rocks yield a Rb—Sr whole-rock isochron age of 671 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7034. Although all of these lavas have been altered by a low-grade greenschist facies event, isotopic and geochemical evidence indicates limited open system behaviour. Thus these dates most probably represent extrusive ages indicating two episodes of volcanic activity during the evolution of the Nubian Shield. These results place some important constraints on the nature of crustal evolution in northeast Africa.