East African Orogeny Research Articles

Geochronology of metamorphism, deformation and fluid circulation: A comparison between Rb-Sr and Ar-Ar phyllosilicate and U-Pb apatite systematics in the Karagwe-Ankole Belt (Central Africa)

This work presents an integrated geochronology study combining step-heating Ar-Ar and in-situ Rb-Sr phyllosilicate geochronology with EPMA elemental maps and U-Pb apatite geochronology in order to determine the timing and duration of regional deformation and fluid circulation processes in the Karagwe-Ankole Belt in Central Africa. The Kibuye-Gitarama-Gatumba study area (West Rwanda) forms part of the Western Domain of this Mesoproterozoic belt, where the metamorphic and deformation history, geothermometric evolution and mineralization processes (both genesis and geochronology) have been well-documented in previous studies.Our geochronological results and interpretations show the importance of thermal diffusion throughout the elevated temperature regime present in the Neoproterozoic. During the Ediacaran, metamorphism and fluid alteration in the Gatumba area was associated with deformation, as independently demonstrated by U-Pb apatite and Ar-Ar phyllosilicate geochronology. From the end of the Ediacaran until the Late Cambrian, saline fluid activity is recorded and fluid-induced (re)crystallization processes dominate the Gatumba biotite isotope record. The direct comparison between and Ar-Ar geochronology suggests a parentless 40Ar-component is preserved during this fluid-induced (re)crystallization due to its saline, (earth) alkali-rich composition. Additionally, this multi-method approach allows us to place the history of the Western Domain of the Karagwe-Ankole belt in relation to that of the surrounding areas, i.e. the Eastern Domain and the Congo and Tanzania Cratons. Deformation and epidote-amphibolite facies metamorphism (at ~590–570 Ma), and fluid fluxes (at ~590–495 Ma) occurred in the Kibuye-Gitarama-Gatumba area after the West Gondwana Orogeny but from the onset of the East African Orogeny onwards till after the culmination of the Gondwana amalgamation, demonstrating the progressively eastward-moving orogenic front in this supercontinent assembly.

Tectonic and sedimentary history of the neoproterozoic metavolcanic–volcaniclastic rocks of the El-Dabbah Group, Central Eastern Desert, Egypt

The El-Dabbah Group is a >7500 m-thick sequence of metavolcanic rocks, volcaniclastics, and banded iron-formation located within the Nubian Shield, Central Eastern Desert, Egypt. The sequence preserves lower greenschist-facies island arc rocks. We used detailed mapping to reconstruct the stratigraphic and tectonic history of the El-Dabbah Group and the surrounding sedimentary basins. The El-Dabbah Group was identified as deposition at central rift zone of an oceanic island arc and was overlies by the subaerial sedimentary rocks (the Atshan Formation and the Hammamat Group). Four faulting were distinguished within the El-Dabbah area: (1) top-to-the-S low-angle thrusting; (2) NNE side down normal faulting; (3) sinistral displacement on north–south faults; and (4) WNE trend N-side-down normal faulting with the Hammamat Group. We obtained the crystallization ages of a granite (638.1 ± 2.9 Ma) and a quartz porphyry (659.6 ± 3.0 Ma). The detrital zircon age of the Hammamat Group show peaks at 650, 680, and 790 Ma. These evidences suggest, three sedimentation-deformation stages were identified in this area. Stage 1 involved sedimentation of island-arc-derived volcaniclastic rocks at 770-700 Ma and to thrust related accretion at 700–680 Ma. Stage 2 involved collisional and transtensional deformation related to formation of the subaerial basin that contains the Atshan Formation at 670-640 Ma, the normal fault, the strike-slip fault and granite intrusion during 660–620 Ma. Stage 3 involved orogenic collapse, regional strike-slip-related sedimentation and deformation of the Hammamat Group at 610–570 Ma. We show that the El-Dabbah Group records a history of oceanic island arc accretion, collision, and collapse within the northern East African Orogeny during 770–570 Ma in Neoproterozoic.

Metamorphic and metasomatic evolution of the Western Domain of the Karagwe-Ankole Belt (Central Africa)

The tectonometamorphic evolution of the Western Domain of the Karagwe-Ankole Belt, containing widespread granite-related W–Nb–Ta–Sn mineralization in pegmatites and hydrothermal quartz veins of Early Neoproterozoic age, is largely unknown. This study aims to characterize the Meso- and Neoproterozoic metamorphism and metasomatism in the Karagwe-Ankole Belt, to reconstruct the temperature evolution and to investigate its temporal relation to deformation and the widespread granite magmatism and mineralization. A quantitative geothermometric study was conducted on metasiltstones and amphibolites, and applies thin section petrography, garnet–biotite and chlorite geothermometry on samples collected in the representative Kibuye-Gitarama-Gatumba area in West Rwanda. The presence of garnet, staurolite and kyanite in metasiltstones, and hornblende and andesine-labradorite feldspar in amphibolites indicates prograde Barrovian metamorphism up to syn-to post-deformational (D1 or D2) lower amphibolite facies (up to 630 °C). This peak metamorphism was followed by post-D2 greenschist facies metamorphism (c. 525 °C–440 °C; garnet, biotite, chlorite, muscovite in metasiltstone), probably related to the East African Orogeny as part of the Gondwana assembly. A geothermometric evolution with high temperature conditions (>500 °C) at least from the flare-up of Early Neoproterozoic tin granites and their metasomatic haloes onwards for most of the Neoproterozoic is proposed, in close correspondence with the geodynamic evolution of the neighboring terranes. Additionally, this high temperature regime is an important factor to be taken into account when interpreting thermal diffusion-sensitive geochronological data.

Emplacement and thermal effect of post-collisional Chewo Pluton (Arabian-Nubian Shield); implication for late East-African Orogeny

The Chewo pluton built by pyroxene-amphibole to amphibole-biotite monzodiorite and quartz monzonite is a typical post-collisional intrusive body emplaced into a low-grade Neoproterozoic Tambien Group belonging to the Tokar-Barka Terrane in the southern Arabian–Nubian Shield. The pluton shows a high-K calc-alkaline and metaluminous composition with significant enrichment in both LREE and LILE due to hybridization and magma mixing between crustal and mantle-derived melts. Estimated P-T conditions of quartz monzonite magma solidification at T: 703 ± 23 °C and P: 0.32 ± 0.08 GPa and thermal overprint in the pluton aureole at T: ̴200–755 °C and P: 0.28 ± 0.06 GPa indicate that the Chewo pluton intruded at a depth of between ca. 10–13 km. The Chewo Pluton was emplaced diapirically, which was driven by a local extension in the hinge of the large-scale asymmetric syncline during the last increments of regional deformation – orogen-perpendicular WNW(NW)-ESE(SE) compression. It indicates that the regional WNW(NW)-ESE(SE) shortening is the main geodynamic event of the East African Orogen in the upper-crustal Tokar-Barka Terrane resulting in the assembly of the eastern and western Gondwana continents. In concordance, the zircon U/Pb age of the Chewo pluton (618.1 ± 1.5 Ma) provides the upper limit for the regional deformation.

Nickel-Copper Sulfide Mineralization in the Ntaka Hill Ultramafic Complex, Nachingwea Region, Tanzania

Abstract The Ntaka Hill Ni-Cu deposit in Tanzania lies within the Mozambique belt, a complex, long-lived Neoproterozoic suture zone that formed during the amalgamation of Gondwana. The deposit is hosted within the moderately deformed Ntaka Ultramafic Complex, which was emplaced at ~660 Ma, around 20 m.y. before the East African orogeny. Due to strain partitioning into amphibolites and metapyroxenites at the margins of the sills, magmatic cumulate textures are very well preserved. The Ntaka Hill mineralization is predominantly disseminated sulfides within orthopyroxenites and harzburgites. Sulfides show typical magmatic mineralogy and occur as isolated blebs and interstitial networks as well as in soft-walled segregation veins that form localized shoot-like high-grade zones. Deformation had relatively little effect on the deposition of sulfides other than minor localization of strain within some preexisting sulfide-dominant veins. Grade shells define elongate tubular channels that coincide broadly with the more magnesian cumulate rocks. Base and precious metal tenors are widely variable, ranging from 1 to 17% Ni and 0.5 to 10% Cu with no particular correlation with host-rock type, although the tenor range in the more magnesian cumulates is more restricted, with the low-tenor component (<3% Ni, <1% Cu) missing. Olivine compositions vary widely, with forsterite contents ranging from Fo84 to Fo89 with up to 0.85% NiO. These unusually high Ni contents are attributed to equilibration with high-Ni-tenor sulfides percolating through the intercumulus pore space. Complex zoning patterns in the Cr content of orthopyroxenes record fluctuating crystallization conditions attributed to wall-rock assimilation. The intrusion emplacement setting is interpreted to be derived from a flow of xenolith-, crystal-, and sulfideladen magma through channelized sills where much of the sulfide was incompletely equilibrated with the host magma, resulting in highly variable R-factors. Despite the widespread presence of graphite in the marginal pyroxenites, olivine-sulfide Fe/Ni partitioning records typical redox conditions that are slightly more oxidized than quartz-fayalite-magnetite (QFM)—further evidence of a lack of extensive large-scale equilibrium within the intrusive complex. The Ntaka Ultramafic Complex preserves an arrested stage of the amalgamation of small sulfide-bearing sill-sediment complex intrusions into larger convecting magma chambers, forming one end of a process continuum including Kevitsa (Finland) as an intermediate stage and Mirabela (Brazil) as the layered intrusion end member.

Geochronological characterization of a transition zone between the Mozambique Belt and Unango-Marrupa Complex in SE Tanzania

We present geochronological and petrological data of orthogneisses, granitoids, and supracrustal rocks from SE Tanzania to characterize the transition zone between the Mesoproterozoic Unango-Marrupa Complex and the Neoproterozoic Mozambique Belt.Geochronological data (U-Pb zircon; LA-ICP-MS) indicate that the emplacement of orthogneisses (protolith) and granitoids in SE Tanzania occurred in Stenian, Cryogenian, and Ediacaran Periods. The magmatic emplacement ages are determined between 1056 ± 19 Ma and 554 ± 3.9 Ma. No older ages that that were determined.The Stenian orthogneisses (biotite granitic gneiss dates at 1056 ± 19 Ma and the associated migmatites, charnockites, amphibolite) represents Mesoproterozoic magmatic inliers in the Neoproterozoic rocks and the extension of the Unango-Marrupa Complex in SE Tanzania.Supracrustal rocks (marble and calc-silicate, garnet-biotite (hornblende) gneisses, quartzo-feldspathic gneisses, graphite schists, and quartzites) are interlayered with granitoids dated between 790 ± 8.7 Ma and 614 ± 8.1 Ma. The supracrustal rocks represent the upper part of the Mozambique Belt (Eastern Granulites/Cabo-Delgado nappes), and are possibly relics of continental shelf sedimentary deposits of the Mozambique Ocean.Upon assessing metamorphic ages (U-Pb zircon) in SE Tanzania we determined the Ediacaran metamorphic event dated between 606 ± 6.6 and 560 ± 37 Ma, which reworked the Stenian – Cryogenian crusts. The U-Pb zircon metamorphic ages are supported by a precise U-Th-Pb monazite metamorphic age at 589 ± 5 Ma obtained from a biotite garnet gneiss rock sample (amphibolite facies). The Ediacaran metamorphism in SE Tanzania was followed by the emplacement of biotite alkali granite at 554 ± 3.9 Ma, which probably marks crustal relaxation and extension after metamorphism as the result of collision or accretion.Magmatic rocks of the Mozambique Belt in SE Tanzania were emplaced during the East African Orogeny. Metamorphic reworking occurred during the final amalgamation of Gondwana between 600 and 500 Ma (Kuunga Orogenic Cycle) in line with tectono-magmatic history of the neighbouring Unango-Marrupa Complex, the Ubendian Belt, and the Irumide-Southern Irumide Belt.

A relic of the Mozambique Ocean in south-east Tanzania

The Neoproterozoic geology of East Africa records the events leading up to the formation of Gondwana and subsequently Pangea, and hence understanding the geodynamic history of this area offers a unique insight into supercontinent development. Eighteen new U-Pb zircon and two U-Pb monazite ages were collected in conjunction with 110 whole-rock lithogeochemistry analyses from a region of south-east Tanzania we designate the ‘Ntaka Terrane’. These new data suggest this area of the East African Orogen constitutes an exotic nappe terrane that was part of the Mozambique Ocean in the Neoproterozoic. The lithological sequence in the region consists of a 770–720 Ma ‘basement’ of intermediate-felsic paragneiss possibly derived from a volcano-sedimentary sequence and recording arc-like geochemical signatures. These are intruded by 700–650 Ma amphibolites (±garnet) with mafic-intermediate magmatic protoliths that also have arc-like geochemistry. This sequence is then intruded by a set of ca. 670–650 Ma ultramafic, dominantly pyroxenitic sills collectively termed the Ntaka Ultramafic Complex, which have a much more mantle-like, non-arc origin, possibly formed as the island-arc intersected a mantle plume ‘hot spot’. The mafic-intermediate intrusives occur synchronous with, and outlast, the high-Mg basalt magmatism, suggesting that this latter event was transient in the context of the overall tectonic regime. The final magmatic event in the terrane is the intrusion of 640–620 Ma syn-orogenic granites that are coeval with 640–600 Ma upper amphibolite-granulite facies metamorphism. The broader geodynamic setting consisted of the Congo-Tanzania-Bangweulu Block of east Africa to the west and the Azania Block to the east of the Neomozambique Ocean, both of which contain Archean-Paleoproterozoic detritus. Based on the island arc-like geochemical characteristics of the 700–650 Ma mafic intrusive rocks and the lack of detrital zircons >1000 Ma, we suggest that the Ntaka Terrane represents a Neoproterozoic intra-oceanic island arc which was accreted to either East Africa or Azania shortly before full continental collision (the East African Orogeny) at ca. 640–620 Ma. Data from the Ntaka Terrane correlate with the Vohibory Terrane (Madagascar), Eastern Granulites (northern Tanzania) and Cabo Delgado Nappe Complex (Mozambique), suggesting these areas represent a set of broadly correlative oceanic terranes.

Age and Origin of the Tsavorite and Tanzanite Mineralizing Fluids In the Neoproterozoic Mozambique Metamorphic Belt

Abstract The genetic model previously proposed for tsavorite- (and tanzanite-) bearing mineralization hosted in the Neoproterozoic Metamorphic Mozambique Belt (stretching from Kenya through Tanzania to Madagascar) is refined on the basis of new Sm-Nd age determinations and detailed Sr-O-S isotope and fluid-inclusion studies. The deposits are hosted within meta-sedimentary series composed of quartzites, graphitic gneisses, calc-silicate rocks intercalated with meta-evaporites, and marbles. Tsavorite occurs either in nodules (also called “boudins”) oriented parallel to the metamorphic foliation in all of the deposits in the metamorphic belt or in quartz veins and lenses located at the hinges of anticlinal folds (Lelatema fold belt and Ruangwa deposits, Tanzania). Gem tanzanite occurs in pockets and lenses in the Lelatema fold belt of northern Tanzania. The Sm-Nd isotopic data for tsavorites and tanzanites hosted in quartz veins and lenses from Merelani demonstrate that they formed at 600 Ma, during the retrograde metamorphic episode associated with the East African Orogeny. The tsavorites hosted in nodules do not provide reliable ages: their sedimentary protoliths had heterogeneous compositions and their Sm-Nd system was not completely rehomogenized, even at the local scale, by the fluid-absent metamorphic recrystallization. The initial 87 Sr/ 86 Sr isotopic ratios of calcite from marble and tanzanites from Merelani fit with the strontium isotopic composition of Neoproterozoic marine carbonates. Seawater sediment deposition in the Mozambique Ocean took place around 720 Ma. The quartz-zoisite O-isotopic thermometer indicates a temperature of formation for zoisite between 385 and 448 °C. The sulfur isotopic composition of pyrite (between –7.8 and –1.3‰ V-CDT) associated with tsavorite in the Lelatema fold belt deposits suggests the contribution of reduced marine sulfate. The sulfur in pyrite in the marbles was likely derived from bacterial sulfate reduction which produced H 2 S. Fluid inclusion data from tsavorite and tanzanite samples from the Merelani mine indicate the presence of a dominant H 2 S-S 8 ±(CH 4 )±(N 2 )±(H 2 O)-bearing fluid. In the deposits in Kenya and Madagascar, the replacement of sulfate by tsavorite in the nodules and the boron isotopic composition of tourmaline associated with tsavorite are strong arguments in favor of the participation of evaporites in garnet formation.

Tectonic environments of sapphire and ruby revealed by a global oxygen isotope compilation

ABSTRACTMany sapphire and ruby occurrences are spatially linked with orogenic belts such as the Pan-African Orogen, the Himalayas, and regions of active or former subduction along the western margin of the Pacific Ocean. These gemstones have oxygen isotope compositions (δ18O) that span >45‰, reflecting the wide range of environments and conditions involved in corundum (Al2O3) formation. We compiled a global data base of sapphire and ruby δ18O, from which the following major groups of gemstones emerge: a dominant population of sapphires with δ18O centred around 5.5‰ (the mantle value) that is spatially related to regions of former subduction; a lesser population of sapphires and rubies with slightly higher δ18O that are associated with skarn and pegmatite; rubies with relatively low δ18O of 0‰–7‰ that occur in hydrothermally altered ultramafic metamorphic rocks in collision zones; and rubies with high δ18O of 14‰–25‰ that are found, almost exclusively, in Himalayan marble. The spatial distribution of the δ18O groups relative to plate boundaries provides insight into the two major periods of continental collision involved in sapphire and ruby formation: the Ediacaran collision of East and West Gondwana (the East African Orogeny) and the Cenozoic collision of India and Asia.

The Kenya rift revisited: insights into lithospheric strength through data-driven 3-D gravity and thermal modelling

Abstract. We present three-dimensional (3-D) models that describe the present-day thermal and rheological state of the lithosphere of the greater Kenya rift region aiming at a better understanding of the rift evolution, with a particular focus on plume–lithosphere interactions. The key methodology applied is the 3-D integration of diverse geological and geophysical observations using gravity modelling. Accordingly, the resulting lithospheric-scale 3-D density model is consistent with (i) reviewed descriptions of lithological variations in the sedimentary and volcanic cover, (ii) known trends in crust and mantle seismic velocities as revealed by seismic and seismological data and (iii) the observed gravity field. This data-based model is the first to image a 3-D density configuration of the crystalline crust for the entire region of Kenya and northern Tanzania. An upper and a basal crustal layer are differentiated, each composed of several domains of different average densities. We interpret these domains to trace back to the Precambrian terrane amalgamation associated with the East African Orogeny and to magmatic processes during Mesozoic and Cenozoic rifting phases. In combination with seismic velocities, the densities of these crustal domains indicate compositional differences. The derived lithological trends have been used to parameterise steady-state thermal and rheological models. These models indicate that crustal and mantle temperatures decrease from the Kenya rift in the west to eastern Kenya, while the integrated strength of the lithosphere increases. Thereby, the detailed strength configuration appears strongly controlled by the complex inherited crustal structure, which may have been decisive for the onset, localisation and propagation of rifting.

Refined geological history of the polyphase plutonometamorphic complex in the Thala Hills area (Enderby Land, East Antarctica) from zircon SHRIMP dating and implications for Neoproterozoic amalgamation of Gondwanaland

Abstract The Thala Hills area occupies a key position in Gondwanaland reconstructions near the India–Sri Lanka–Antarctica junction. We present U–Pb zircon isotopic age determinations from SHRIMP II obtained on four granite gneiss samples. Three high-temperature tectonomagmatic episodes may be distinguished in the study area at c. 980–970, c. 780–720 and c. 545–530 Ma. The c. 980–970 Ma event corresponds to the Rayner Structural Episode that affected East Antarctica, including the Sør Rondane Mountains to the west and Kemp Land to the east. The c. 780–720 Ma episode included two events at approximately 780 Ma (high-grade anatexis) and 720 Ma (syntectonic granitoid emplacement), and was roughly coeval with tectonomagmatic events in Dronning Maud Land of East Antarctica, as well as in other Gondwanaland regions, such as Madagascar, Sri Lanka and eastern Africa. The c. 780–720 Ma episode may be correlated with the East African Orogeny. These correlations argue for a similar geological evolution and conjugate position for Dronning Maud Land and Enderby Land despite a postulated Cambrian Lützow–Holm Bay suture separating them. The Cambrian ( c. 545–530 Ma) episode was manifested by high-grade anatexis that confirms previously identified late Neoproterozoic–Cambrian tectonothermal activities in the study area. Supplementary material: Trace element compositions of zircons and whole-rock chemical compositions obtained by previous studies are available at https://doi.org/10.6084/m9.figshare.c.3738332

Geology, Geochemistry and Geochronology of the Kenticha Rare Metal Granite Pegmatite, Adola Belt, Southern Ethiopia: A Review

This work reviews the geology, geochemistry and geochronology and discusses the spatial and temporal relationship of the granite pegmatite and the rare metal mineralization of the Kenticha granite pegmatite, southern Ethiopia using published and unpublished works to give a comprehensive understanding about the formation of the mineral deposit. The Kenticha rare metal pegmatite belt comprises several groups of pegmatites which show a high magmatic fractionation, regional and compositional zoning, mineralogical assemblage, and secondary alterations. The internal zonation shows high degree of evolution from the border to the core zone during crystallization and solidification of the leucogranitic to pegmatitic melt. Tantalum mineralization at Kenticha includes zoned tantalite-(Mn) and columbite-(Mn), as well as microlite, pyrochlore, uranmicrolite, and rare tapiolite, ixiolite/wodginite and Ta-bearing rutile. The tectonic setting of the Kenticha granite pegmatite in the Within Plate Granite (WPG) to syn-Collisional Granite (syn-COLG) granite and probably sourced from extreme fractionation of syn-to late tectonic granites or anatexis process of the metasedimentary rocks in the area. The emplacement of the Kenticha pegmatite was at ca. 530 Ma and temporally related to the post-collisional phase of granitic magmatism at 570 - 520 Ma, after the last tectonic stage of east African orogeny during the late stage of Gondwana assembly.

Metamorphic modifications of the Muremera mafic–ultramafic intrusions, eastern Burundi, and their effect on chromite compositions

The Muremera mafic–ultramafic intrusions were emplaced into metasedimentary rocks of the Karagwe–Ankole Belt in eastern Burundi, as part of the Mesoproterozoic Kibaran tectonomagmatic event. Igneous minerals of the Muremera intrusions have been partly altered to hydrous and carbonated metamorphic assemblages, although in most cases, the original igneous textures are well-preserved. Rounded, subhedral cumulus olivine has been partially and pseudomorphically replaced by lizardite–magnetite mesh-rim and lizardite–brucite mesh-centre assemblages, while anhedral interstitial plagioclase has been replaced by chlorite–tremolite. A later and localized event results in prograde alteration to antigorite–magnetite–chlorite–talc–carbonate and talc–carbonate–chlorite assemblages. The rocks are inferred to have undergone at least three separate metamorphic/alteration events resulting in: AS1 – an early alteration assemblage (mesh-rim lizardite–magnetite) characterized by very low fluid/rock ratios and widespread distribution; AS2 – a later, widespread low-temperature retrogressive (mesh-centre lizardite–brucite) assemblage associated with abundant close-spaced parallel veins; AS3 – later, prograde (antigorite–magnetite) and AT4 (talc–chlorite–carbonate) assemblages associated with more localized shearing and higher fluid/rock ratios. The AS1 assemblage most likely represents deuteric alteration that occurred soon after intrusion and cooling. The AS2 assemblage may relate to a continuation of this cooling, or may be correlated with the regional upright D2 folding event, while the AS3 and AT4 alteration assemblages are most likely correlated with the N–S oriented D3 faulting episode linked to the distal East African Orogeny. Euhedral to subhedral chromite grains are essentially unaltered where enclosed in primary unaltered olivine, pyroxene or plagioclase, as well as in AS1 lizardite–magnetite and AS2 lizardite–brucite altered olivine or pyroxene. In samples which show alteration to AS3 antigorite–magnetite and AT4 talc–carbonate–chlorite assemblages, the chromite grains are zoned to Mg–Al-poor ferritchromite rims. The width and the composition of these alteration rims are related to the degree of alteration of the silicate assemblages. However, it is concluded that chromite found in rocks that have avoided late/localized AS3–AT4 alteration preserve their magmatic to late-magmatic chemical and textural characteristics, and can therefore be used in regional geological studies and exploration.

Detrital zircon geochronology and provenance of the Neoproterozoic Hammamat Group (Igla Basin), Egypt and the Thalbah Group, NW Saudi Arabia: Implications for regional collision tectonics

Detrital zircon U–Pb SIMS dating is used to evaluate the provenance of two correlative basins in the Arabian-Nubian Shield (ANS). The Wadi Igla Formation in the Central Eastern Desert (CED) of Egypt and the Thalbah Group in the Midyan Terrane (MT) of NW Saudi Arabia are considered to be post-amalgamation terrestrial basins, developed during closure of the Mozambique Ocean and amalgamation of the ANS in Cryogenian-early Ediacaran time. The analytical results indicate that the upper-part of the Wadi Igla Formation has a maximum depositional age of 628±6Ma, contains 98% Neoproterozoic zircon with ages between 815 and 628Ma, and has two distinct peaks at 690Ma and 652Ma. A rhyolite clast from the upper-part of the Wadi Igla Formation gives a U–Pb age of 700±6Ma. This age significantly pre-dates Dokhan volcanism, indicating that the dominant rhyolitic clasts within the Wadi Igla Formation are not from the Dokhan Volcanics, as previously believed. Analytical results from the Thalbah Group suggest multiphase basin formation and development. The lower part of the Thalbah Group is intruded by monzogranites of the Liban complex, has a minimum depositional age of 635±5Ma, resembling that of the Wadi Igla Formation. Its middle part has a maximum age of 612±7Ma and is comprised of 90% Neoproterozoic zircon with ages ranging from 820 to 612Ma. The upper part of the Thalbah Group has a maximum age of 596±10Ma and contains a wider range of Neoproterozoic detritus with ages between 985 and 596Ma. The basement of the Thalbah Group is represented by metasediments and metavolcanics of the Zaam Group. The sample collected from the uppermost part of the Zaam Group (Um Ashsh Formation) contains zircon of mostly Cryogenian age (ca. 812–697Ma) and has a maximum age of 700±4Ma, suggesting that the Zaam Group might be correlative with the subduction-related metavolcanic and metasedimentary rocks that are overlain unconformably by the Wadi Igla Formation in the CED.The Wadi Igla basin and the lower and middle parts of the Thalbah basin have similar provenance, record a Cryogenian-early Ediacaran age, and represent syn-subduction (rather than post-amalgamation) basins. The upper part of the Thalbah Group, in contrast, has a distinct provenance representing an Ediacaran syn-collisional basin. The narrow age range of the Wadi Igla Formation and the lower and middle parts of the Thalbah Group indicates a restricted source from the CED and MT island arc basement, whereas the wide age range for the upper part of the Thalbah Group indicates a contribution from other parts of the ANS. The sediment sources and the age patterns of detrital zircons change abruptly at ca. 596Ma. This may coincide with the onset of collision of the CED and MT basements with the older Hijaz-Gebeit terrane (850–680Ma) to the south along the Yanbu-Onib-Sol Hamed-Gerf-Allaqi-Heiani (YOSHGAH) suture in the ANS during the East African Orogeny.

Water content of the Tanzanian lithosphere from magnetotelluric data: Implications for cratonic growth and stability

Hydrogen strongly influences mantle rheology and is therefore an important factor in the growth and stability of cratons. Hydrogen also strongly affects electrical conductivity so it is possible to infer the hydrogen content of the lithospheric mantle in-situ and test models of craton formation using magnetotelluric data. Tanzania is an ideal natural laboratory to test hypotheses on lithospheric hydrogen content since it contains regions with very different tectonic regimes including the stable Tanzanian Craton and the East Africa Rift that is reworking lithosphere previously deformed in the East African Orogeny. Additionally, the lithosphere is well sampled by voluminous xenoliths that constrain lithospheric composition and the geotherm, which also affect electrical conductivity. Hydrogen contents were calculated for two locations in Tanzania: the first in the stable central Tanzanian Craton and the second on the eastern margin of the craton where incipient rifting is occurring. The central Tanzanian Craton was found to have a high lithospheric mantle water content of ∼10−2 wt% which is comparable to that of the oceanic asthenosphere and is hard to reconcile with the long-term survival of the craton. It is possible that the water was introduced into the lithosphere recently by kimberlite volcanism or that, if the lithosphere has had a high water content throughout its history, the central craton has been shielded from deformation by weaker orogens that surround it. The eastern margin of the craton has a water content of 10−3 to 10−4 wt% throughout much of the lithospheric mantle that decreases to 10−4 to 10−5 wt% at the base of the lithosphere and at depths corresponding to the uppermost plume head. Xenolith data show evidence for partial melting of the plume head and the base of the lithosphere in this dehydrated region. The partial melting and dehydration of a plume head beneath a craton is a present-day observation of the processes that may have formed cratonic roots.

Geospatial technology and structural analysis for geological mapping and tectonic evolution of Feiran–Solaf metamorphic complex, South Sinai, Egypt

The Sinai Peninsula constitutes an important district of the Egyptian lands where it forms a triangular portion in northeastern Egypt. The southern Sinai metamorphic complexes are the northern uppermost part of the Arabian–Nubian Shield revealing the upper and middle crust from the East African Orogeny, in which they tectonically evolved. The Feiran–Solaf metamorphic complex (FSMC) of Sinai, Egypt is one of the highest grades metamorphic complexes of a series of basement domes that trends NW and crops out throughout the Arabian–Nubian Shield. The main aim of the present study is to apply the geospatial technology and to represent the capability of the geospatial technology to estimate the combined influence of lithology and structure studies, and to construct the lithological and structural maps of FSMC. Furthermore, detailed structural analysis is carried out to reveal the different ductile and brittle deformational events and proposed the tectonic evolutionary model for the study area. Mainly geospatial technology and structural analysis software have been used to go well with the aim of the present study. Developing specific image processing of satellite images and structural analysis were succeeded to discriminate the various lithological rock units, and the geological structural features of the FSMC, using geographic information system tools to construct the different thematic maps, were extracted. The present detailed investigations of the enhanced satellite images, structural analysis, and field verification reveal that the FSMC reached its present tectonic setting through more than four deformational phases concluding that the Pan-African Najd Fault System continued in Sinai and was reactivated during Red Sea tectonics as indicated by the dextral shear zone (Rihba) bordering the northern side of the FSMC.

Comparison of the metamorphic history of the Monapo Complex, northern Mozambique and Balchenfjella and Austhameren areas, Sør Rondane, Antarctica: Implications for the Kuunga Orogeny and the amalgamation of N and S. Gondwana

Reconstructions of Gondwana place Dronning Maud Land, Antarctica (DML) adjacent to N. Mozambique prior to fragmentation. The Monapo Complex outlier klippen overlying the Nampula Terrane in N. Mozambique has been correlated with rocks in eastern and central DML. Metamorphic assemblages and P–T conditions from the two areas are compared as well the timing of metamorphism. Granulite grade assemblages preserved in the Monapo Complex suggest P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of isothermal decompression, isobaric cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where near isobaric cooling and hydration at between ∼4–7kb and 550–700°C is recognized. Granulite grade assemblages from Balchenfjella in eastern Sør Rondane, DML suggest initial P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of decompression, cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where retrogression at between ∼6–7kb and 600–700°C is recognized.SHRIMP zircon data from six samples from eastern Sør Rondane show episodic zircon growth over a period from ∼630Ma to ∼530Ma with five metamorphic pulses being defined. Undeformed granitic intrusions with ages of ∼550Ma intrude Sør Rondane. SHRIMP zircon data from samples from alkaline intrusions and mafic granulites in the Monapo Complex yield crystallization and metamorphic ages of ∼635Ma and 570-590Ma respectively. The complex is intruded by granitic veins with ages of ∼550Ma.The similarities in metamorphic mineral assemblages, P–T estimates and zircon geochronology data indicate a common geological history between Sør Rondane and the Monapo Complex of northern Mozambique. This common history is interpreted to result from both areas being part of a mega-nappe structure which was emplaced in a transpressional setting involving collision between N. Gondwana (comprising rocks of the East African Orogeny) and south Gondwana (comprising the Nampula Terrane of northern Mozambique and the Maud Province of western Dronning Maud Land) along the Damara-Zambesi-Lurio-CDML collision boundary.Other erosional remnants of this nappe include the Mugeba Complex in northern Mozambique, the Kataragama klippe in Sri Lanka, the Urungwe klippe in Zimbabwe, the Masosa Suite and Mavhuradohna Complex in western Mozambique and NE Zimbabwe.

Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania

A study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern part of the craton comprises typical Neoarchaean grey granodioritic orthogneisses dated with the LA-ICP-MS method by U–Pb zircon at ca. 2.7Ga. A gradual eastward increase in strain in these rocks culminates in a 1–2km wide, locally imbricated, north–south-trending, ductile thrust/shear zone with an oblique top-to-the-NW sense of movement. East of the craton-edge shear zone, high-grade supracrustal rocks are termed the “Mpwapwa Group” in view of uncertain regional correlations. There is an apparent lithological zonation of the Mpwapwa Group parallel to the craton margin shear zone. In the west, the group consists of typical “shelf facies” metasedimentary rocks (marbles, calc-silicates, quartzites, etc.). U–Pb dating of detrital zircons from two quartzites reveal only Archaean detritus, constraining their maximum depositional age to <ca. 2.6Ga. The shelf rocks pass eastwards into semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, which may have volcanic protoliths and are intruded by multiple mafic sills. Dating of a felsic gneiss showed it to probably be a Neoarchaean rock which underwent Palaeoproterozoic metamorphism during the Usagaran event at ca. 1930Ma. This date likely represents initial juxtaposition of the (Archaean) Mpwapwa Group against the Tanzania Craton. Consequently the Group are not considered to be stratigraphic correlates of the Usagaran Konse Group or Isimani Suite seen further south, but rather form part of the Neoarchaean crust of the “Western Granulite” terrane of the East African Orogen. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1873±31Ma. Zircons in the granite have metamorphic rims dated between 550 and 650Ma that grew during the East African orogenic event.Distinctive para- and orthogneisses, including charnockite dated at 2707±21Ma, outcrop in the southern part of the area. A sliver of southern tonalitic orthogneiss, entrained in the craton margin shear zone, is an igneous rock of probable early Palaeoproterozoic or Archaean age (>ca. 2300Ma) with a strong metamorphic overprint at ca. 1960Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990Ma, again believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks. The role of the Neoproterozoic East African orogeny in the evolution of the craton margin is unclear, but the geometry of the shear zone, the presence of Neoproterozoic zircon rims in the Palaeoproterozoic granite and published studies from nearby, all suggest that the latest movements on the shear zone may be Neoproterozoic in age and that the structure may represent the local western front of the East African Orogeny.

Crustal age domains and metamorphic reworking of the deep crust in Northern-Central Tanzania: a U/Pb zircon and monazite age study

Geochronological data are presented from Northern Tanzania, where deep-crustal terranes of different age are exposed. Stacking of these terranes was diachronous with one peak around 640 Ma, defined as East African Orogeny, and final consolidation at 550–580 Ma, that is defined as Kuunga Orogeny. This later event is predominant in the Western Granulite Belt of northern Tanzania and related to thrusting onto the Tanzanian Craton. The Tanzania Craton itself experienced a polycyclic history; age domains around 2.64 Ga prevail in the studied samples. There is no evidence of the Paleoproterozoic Usagaran Belt in northern Tanzania. Here the gneisses contain relicts of reworked Archean basement and are therefore considered part of the Western Granulites. Inliers of the Western Granulites are also found in the cores of marble antiforms that are part of the upper, sedimentary sequence of the Eastern Granulites. Those inliers formed during the Kuungan orogenic phase when the Eastern Granulites have taken their final position and were folded together with the Western Granulites.

Infracambrian superimposed tectonics in the Late Proterozoic units of Mount Ablah area, southern Asir Terrane, Arabian Shield, Saudi Arabia

Mount Ablah is a mining prospect, hosted by a dioritic igneous body that is bound to the east by greenschist grade metamorphic rocks and to the west by Ablah group volcano–sedimentary rocks. Rock units of Mount Ablah area were remapped through field investigations, petrological studies, and analysis of enhanced TM Landsat data. Ablah group rocks were divided into lower tuffaceous and upper epiclastic units. The epiclastic unit was divided into three subunits. During remapping, a sliver of serpentinite was discovered, which occurs between the dioritic igneous body and Ablah group rocks. The greenschist grade metamorphic rocks were intruded by Late Proterozoic quartz diorite prior to deposition of Ablah group rocks. The epiclastic unit is an infracambrian molassic unit that filled a graben, known as the Ablah graben. The Mount Ablah area was intruded by post-tectonic granitic rocks and affected by two superimposed F1-F2 folding events, associated with thrust and dextral faults, respectively. The first folding event involved N–S folding and thrusting. Simultaneously, stress partitioning at E–W accommodations zones produced E–W minor folds (F2) and associated E–W dextral faults. The F1–F2 folding events are contemporaneous with the Pan African deformation event, also known as the East African Orogeny (EAO). The EAO is infracambrian in age and culminated in development of the Najd sinistral fault system. The E–W dextral faults were probably reactivated during Cenozoic Red Sea rifting. The Ablah graben's infracambrian sedimentary rocks, such as siltstone, sandstone, and limestone that are mainly bound within the Ablah graben were not deformed prior to F1–F2 folding. Thus, the upper epiclastic unit of the Ablah group rocks is rheologically different from the surrounding greenschist rocks, responded to the late E–W compression in a more ductile manner than the surrounding greenschist rocks. Therefore, the Ablah graben was inverted, refolded, and crosscut by E–W dextral faults during the infracambrian EAO event, prior to development of Najd sinistral fault system, which are evident in Asir Terrane and crosscut Ablah graben.