Tesfaye Kebede - Academia.edu (original) (raw)
Papers by Tesfaye Kebede
Journal of African Earth Sciences, 2017
Despite being the longest river and the fourth in drainage area, Nile River has the lowest discha... more Despite being the longest river and the fourth in drainage area, Nile River has the lowest discharge per unit areas among the top ten rivers of the world. Understanding the hydrologic significance of the regional litho-stratigraphy and structures help to better understand the hydrodynamics. This work is aimed at characterizing the Baro-Akobo-Sobbat sub-basin of Nile and determine trans-basin flows. Integrated method is used to characterize the basin and determine the Baro-Akobo-Sobbat sub-basin's relationship with African Mesozoic Rifts. Oil and water well drilling logs; aeromagnetic, gravity and vertical electrical sounding data; and various study reports are used to establish regional lithostratigraphic correlations and determine trans-regional hydrogeological connectivity. A total of 633 samples collected from wells, springs, rivers, lakes, swamps and rain water are analysed for their chemical, stable isotopes, tritium and radon properties. The Baro-Akobo river basin is commonly presumed to have good groundwater potential, particularly in its lowland plain. However, it has poor exploitable groundwater potential and recharge rate due to the extensive clay cover, limited retention capacity and the loss of the bulk of the groundwaters through regional geological structures to the deep seated continental sediments; presumably reaching the hydraulically connected African Mesozoic Rifts; mainly Melut and Muglad. The deep underground northward flows, along Nile River is, presumably, retarded by Central African Shear Zone in the Sudan.
Mineralogy and Petrology, 2001
Precambrian Research, 2005
The Wadi Mubarak belt in Egypt strikes west-east (and even northeast-southwest) and crosscuts the... more The Wadi Mubarak belt in Egypt strikes west-east (and even northeast-southwest) and crosscuts the principal northwest-southeast trend of the Najd Fault System in the Central Eastern Desert of Egypt. The belt therefore appears to be a structural feature that formed postdate to the Najd Fault System. In contrast, it is shown here that the deformation in the Wadi Mubarak belt can be correlated with the accepted scheme of deformation events in the Eastern Desert of Egypt and that its geometry and apparently cross-cutting orientation is controlled by a large granite complex that intruded prior to the structural evolution. Structural correlation is facilitated by a series of intrusions that intrude the Wadi Mubarak belt and resemble other intrusions in the Eastern Desert. These intrusions include: (1) an older gabbro generation, (2) an older granite, (3) a younger gabbro and (4) a younger granite. The structural evolution is interpreted to be characterized by early northwest directed transport that formed several major thrusts in the belt. This event is correlated with the main deformation event in the Eastern Desert, elsewhere known as D2. During this event the regional fabric of the Wadi Mubarak belt was wrapped around the El Umra granite complex in a west-east orientation. The Wadi Mubarak belt was subsequently affected during D3 by west-east and northwest-southeast trending sinistral conjugate strike-slip shear zones. This event is related to the formation of the Najd Fault System. Detailed resolution of superimposed shear sense indicators suggest that D3 consisted of an older and a younger phase that reflect the change of transpression direction from east-southeast-west-northwest to eastnortheast-westouthwest. The El Umra granite complex is dated here with single zircon ages to consist of intrusion pulses at 654 and 690 my. These ages conform with the interpretation that it intruded prior to D2 and that the structural pattern of the Wadi Mubarak belt was initiated early during D2.
Variscan and pre-Variscan basement units of the Tauern Window (TW) were strongly over-printed by ... more Variscan and pre-Variscan basement units of the Tauern Window (TW) were strongly over-printed by Alpine orogeny; and, hence, delimiting the basement assemblages into their respective components remains to be challenging. Although considerable knowledge is accumulated from previous geological and geochronological investigations on the TW basement units; distinction between the pre-Variscan and Variscan components of the basement rocks is far from comple-tion and, therefore, necessitates systematic approaches. Here we present new single-grain and within-grain zircon U-Pb ages (±2s) of selected lithotectonic units (Basi-samphibolit, Biotitporphyroblastenschiefer and equivalent lithologies, Zwolferzug, Habach Phyllite, and Zentralgneis) of the central TW basement sequence. The new and published previous age data are combined to place constrains on outstanding geologic problems such as the relationship of the Basisamphibo-lit to the Zwolferzug garnet amphibolite, the strati-graphic posit...
Journal of African Earth Sciences, 2001
Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a hig... more Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a high-grade migmatitic terrane. It is composed of feldspars + quartz + muscovite + biotite ± garnet + zircon ± allanite ± apatite + Fe-Ti oxides + Fe sulphide. Textural studies and microprobe analyses revealed two generations of almandine-spessartine-rich magmatic garnet. The first is euhedral, fine-grained (300-350 pm), commonly occurs as inclusions in plagioclase and alkali feldspars, and exhibits chemical zoning with almandine-rich cores and spessartine-rich rims. In contrast, the second variety is medium-to coarse-grained (1-7 mm) and shows reverse zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary muscovites were discriminated based on the concentrations of Ti, Fe, Mn and Na. Biotite is characterised by a higher alumina saturation index than biotites of other granitoids in the area, suggesting considerable alumina concentration in the source magma. Garnet-biotite thermometry and phengite barometry were used to estimate the P-T conditions of crystallisation for the Suqii-Wagga two-mica granite pluton at-7 kbar and-670°C. Mineral paragenesis, the composition of aluminous minerals and the P-T conditions of crystallisation indicate that samples containing fine-grained garnet crystallised earlier than those containing medium-to coarse-grained garnet. Field and petrographic investigations, mineral chemistry, and whole rock major and trace element studies suggest that the Suqii-Wagga two-mica granite has the characteristics of anatectic granite. Highly variable normative Ab/Or ratios suggested melting under varying a,2 o conditions and/or source characteristics. The relatively high Rb/Sr, Rb/Ba and low CaO/Na20 (< 0.3) ratios indicate the derivation of the granitic magma from a plagioclase-poor politic source. Moreover, pronounced negative Eu anomalies and large ion lithophile element modelling suggested crystal fractionation involving plagioclase. The presence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal material to the magmatic evolution of the area.
Environmental Earth Sciences, 2017
Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviati... more Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviation from signatures in other Ethiopian river basins. In this study, hydrogeochemical and isotope methods were employed to determine regional and local hydrogeology and characteristics of the basin. Optical, thermal and radar remote sensing products were used to update geological and structural maps of the basin and determine sampling points using the judgment sampling method. A total of 363 samples from wells, springs, rivers, lakes, swamps and rain were collected for this study, and an additional 270 water quality data sets were added from previous studies. These data were analyzed for their hydrogeochemical characteristics and isotope signatures. Analysis of the oxygen, deuterium and tritium isotopes shows the groundwater of the basin is modern water. Among all basins in Ethiopia, the Baro-Akobo Basin shows the highest enrichment. This indicates the proximity of the rainfall sources, which presumably are the Sudd and other wetlands in South Sudan. The hydrochemical properties of the waters show evapotranspiration is the dominant hydrologic process in the basin and explains the large amount of water that is lost in the lowland plain. Analysis of radon-222 shows no significant groundwater flux over the wetlands, which are part of Machar Marshes. This shows evaporation to be dominant hydrologic process in this zone. Results from all analyses help explain the limited holding capacity of the aquifers in the recharge zone and their vulnerability to anthropogenic impacts and climate variability. There is a trend of decreasing surface flow and rainfall and increasing water soil erosion.
International Journal of Earth Sciences, 2005
New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting ou... more New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwo¨lferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwo¨lferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwo¨lfer-zug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.
Mineralogical Magazine, 1998
Journal of African Earth Sciences, 1999
Crystalline rocks from the western Ethiopian Precambrian terrain comprise two major rock groups: ... more Crystalline rocks from the western Ethiopian Precambrian terrain comprise two major rock groups: (1) the often migmatised eastern and western high-grade gneisses; and (2) the central low-grade metavolcanosedimentary rocks. Granitoid bodies of different ages and compositions intrude these rocks. Field observations, petrographic investigations, and geochemical features support a two-fold classification of the granitoid rocks. The volcanic arc granitoids (VAG) are
Chemical Geology, 2007
A zircon 'microvein' composed of several hundred crystals occurs in peralkaline granitic gneiss o... more A zircon 'microvein' composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians. U-Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, and Mn) abundances of the 'microvein' and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the 'microvein' zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively, reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr. The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that darkgrey BSE domains were formed by dissolution-reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals. The bright and dark-grey BSE domains in Type-I zircon yield U-Pb ages of 779 ± 69 Ma and 780 ± 35 Ma, and similar domains in Type-II zircon dated at 778 ± 49 Ma and 780 ± 31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical, within analytical uncertainties, to the 776 ± 12 Ma zircon U-Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming 'microvein' and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatichydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of orthomagmatic or hydrothermal fluids containing fluorides as a major constituent, which expelled a considerable amount of trace elements, namely, Hf, U, Th, Y, and the REEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼0.5 to 1.0). Furthermore, the ca. 780-776 Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.
Precambrian Research, 2003
A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplac... more A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low-and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600-630 • C and 5-6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr/ 86 Sr of 0.70281 and positive ε Nd(625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age (T DM) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded ε Nd(625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ∼ = 1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic.
Precambrian Research, 2003
A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplac... more A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low-and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600-630 • C and 5-6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr/ 86 Sr of 0.70281 and positive ε Nd(625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age (T DM) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded ε Nd(625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ∼ = 1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic.
Journal of African Earth Sciences, 2017
Despite being the longest river and the fourth in drainage area, Nile River has the lowest discha... more Despite being the longest river and the fourth in drainage area, Nile River has the lowest discharge per unit areas among the top ten rivers of the world. Understanding the hydrologic significance of the regional litho-stratigraphy and structures help to better understand the hydrodynamics. This work is aimed at characterizing the Baro-Akobo-Sobbat sub-basin of Nile and determine trans-basin flows. Integrated method is used to characterize the basin and determine the Baro-Akobo-Sobbat sub-basin's relationship with African Mesozoic Rifts. Oil and water well drilling logs; aeromagnetic, gravity and vertical electrical sounding data; and various study reports are used to establish regional lithostratigraphic correlations and determine trans-regional hydrogeological connectivity. A total of 633 samples collected from wells, springs, rivers, lakes, swamps and rain water are analysed for their chemical, stable isotopes, tritium and radon properties. The Baro-Akobo river basin is commonly presumed to have good groundwater potential, particularly in its lowland plain. However, it has poor exploitable groundwater potential and recharge rate due to the extensive clay cover, limited retention capacity and the loss of the bulk of the groundwaters through regional geological structures to the deep seated continental sediments; presumably reaching the hydraulically connected African Mesozoic Rifts; mainly Melut and Muglad. The deep underground northward flows, along Nile River is, presumably, retarded by Central African Shear Zone in the Sudan.
Mineralogy and Petrology, 2001
Precambrian Research, 2005
The Wadi Mubarak belt in Egypt strikes west-east (and even northeast-southwest) and crosscuts the... more The Wadi Mubarak belt in Egypt strikes west-east (and even northeast-southwest) and crosscuts the principal northwest-southeast trend of the Najd Fault System in the Central Eastern Desert of Egypt. The belt therefore appears to be a structural feature that formed postdate to the Najd Fault System. In contrast, it is shown here that the deformation in the Wadi Mubarak belt can be correlated with the accepted scheme of deformation events in the Eastern Desert of Egypt and that its geometry and apparently cross-cutting orientation is controlled by a large granite complex that intruded prior to the structural evolution. Structural correlation is facilitated by a series of intrusions that intrude the Wadi Mubarak belt and resemble other intrusions in the Eastern Desert. These intrusions include: (1) an older gabbro generation, (2) an older granite, (3) a younger gabbro and (4) a younger granite. The structural evolution is interpreted to be characterized by early northwest directed transport that formed several major thrusts in the belt. This event is correlated with the main deformation event in the Eastern Desert, elsewhere known as D2. During this event the regional fabric of the Wadi Mubarak belt was wrapped around the El Umra granite complex in a west-east orientation. The Wadi Mubarak belt was subsequently affected during D3 by west-east and northwest-southeast trending sinistral conjugate strike-slip shear zones. This event is related to the formation of the Najd Fault System. Detailed resolution of superimposed shear sense indicators suggest that D3 consisted of an older and a younger phase that reflect the change of transpression direction from east-southeast-west-northwest to eastnortheast-westouthwest. The El Umra granite complex is dated here with single zircon ages to consist of intrusion pulses at 654 and 690 my. These ages conform with the interpretation that it intruded prior to D2 and that the structural pattern of the Wadi Mubarak belt was initiated early during D2.
Variscan and pre-Variscan basement units of the Tauern Window (TW) were strongly over-printed by ... more Variscan and pre-Variscan basement units of the Tauern Window (TW) were strongly over-printed by Alpine orogeny; and, hence, delimiting the basement assemblages into their respective components remains to be challenging. Although considerable knowledge is accumulated from previous geological and geochronological investigations on the TW basement units; distinction between the pre-Variscan and Variscan components of the basement rocks is far from comple-tion and, therefore, necessitates systematic approaches. Here we present new single-grain and within-grain zircon U-Pb ages (±2s) of selected lithotectonic units (Basi-samphibolit, Biotitporphyroblastenschiefer and equivalent lithologies, Zwolferzug, Habach Phyllite, and Zentralgneis) of the central TW basement sequence. The new and published previous age data are combined to place constrains on outstanding geologic problems such as the relationship of the Basisamphibo-lit to the Zwolferzug garnet amphibolite, the strati-graphic posit...
Journal of African Earth Sciences, 2001
Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a hig... more Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a high-grade migmatitic terrane. It is composed of feldspars + quartz + muscovite + biotite ± garnet + zircon ± allanite ± apatite + Fe-Ti oxides + Fe sulphide. Textural studies and microprobe analyses revealed two generations of almandine-spessartine-rich magmatic garnet. The first is euhedral, fine-grained (300-350 pm), commonly occurs as inclusions in plagioclase and alkali feldspars, and exhibits chemical zoning with almandine-rich cores and spessartine-rich rims. In contrast, the second variety is medium-to coarse-grained (1-7 mm) and shows reverse zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary muscovites were discriminated based on the concentrations of Ti, Fe, Mn and Na. Biotite is characterised by a higher alumina saturation index than biotites of other granitoids in the area, suggesting considerable alumina concentration in the source magma. Garnet-biotite thermometry and phengite barometry were used to estimate the P-T conditions of crystallisation for the Suqii-Wagga two-mica granite pluton at-7 kbar and-670°C. Mineral paragenesis, the composition of aluminous minerals and the P-T conditions of crystallisation indicate that samples containing fine-grained garnet crystallised earlier than those containing medium-to coarse-grained garnet. Field and petrographic investigations, mineral chemistry, and whole rock major and trace element studies suggest that the Suqii-Wagga two-mica granite has the characteristics of anatectic granite. Highly variable normative Ab/Or ratios suggested melting under varying a,2 o conditions and/or source characteristics. The relatively high Rb/Sr, Rb/Ba and low CaO/Na20 (< 0.3) ratios indicate the derivation of the granitic magma from a plagioclase-poor politic source. Moreover, pronounced negative Eu anomalies and large ion lithophile element modelling suggested crystal fractionation involving plagioclase. The presence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal material to the magmatic evolution of the area.
Environmental Earth Sciences, 2017
Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviati... more Hydrogeochemical and isotopic signatures of the waters of the Baro-Akobo River Basin show deviation from signatures in other Ethiopian river basins. In this study, hydrogeochemical and isotope methods were employed to determine regional and local hydrogeology and characteristics of the basin. Optical, thermal and radar remote sensing products were used to update geological and structural maps of the basin and determine sampling points using the judgment sampling method. A total of 363 samples from wells, springs, rivers, lakes, swamps and rain were collected for this study, and an additional 270 water quality data sets were added from previous studies. These data were analyzed for their hydrogeochemical characteristics and isotope signatures. Analysis of the oxygen, deuterium and tritium isotopes shows the groundwater of the basin is modern water. Among all basins in Ethiopia, the Baro-Akobo Basin shows the highest enrichment. This indicates the proximity of the rainfall sources, which presumably are the Sudd and other wetlands in South Sudan. The hydrochemical properties of the waters show evapotranspiration is the dominant hydrologic process in the basin and explains the large amount of water that is lost in the lowland plain. Analysis of radon-222 shows no significant groundwater flux over the wetlands, which are part of Machar Marshes. This shows evaporation to be dominant hydrologic process in this zone. Results from all analyses help explain the limited holding capacity of the aquifers in the recharge zone and their vulnerability to anthropogenic impacts and climate variability. There is a trend of decreasing surface flow and rainfall and increasing water soil erosion.
International Journal of Earth Sciences, 2005
New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting ou... more New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwo¨lferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwo¨lferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwo¨lfer-zug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.
Mineralogical Magazine, 1998
Journal of African Earth Sciences, 1999
Crystalline rocks from the western Ethiopian Precambrian terrain comprise two major rock groups: ... more Crystalline rocks from the western Ethiopian Precambrian terrain comprise two major rock groups: (1) the often migmatised eastern and western high-grade gneisses; and (2) the central low-grade metavolcanosedimentary rocks. Granitoid bodies of different ages and compositions intrude these rocks. Field observations, petrographic investigations, and geochemical features support a two-fold classification of the granitoid rocks. The volcanic arc granitoids (VAG) are
Chemical Geology, 2007
A zircon 'microvein' composed of several hundred crystals occurs in peralkaline granitic gneiss o... more A zircon 'microvein' composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians. U-Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, and Mn) abundances of the 'microvein' and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the 'microvein' zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively, reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr. The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that darkgrey BSE domains were formed by dissolution-reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals. The bright and dark-grey BSE domains in Type-I zircon yield U-Pb ages of 779 ± 69 Ma and 780 ± 35 Ma, and similar domains in Type-II zircon dated at 778 ± 49 Ma and 780 ± 31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical, within analytical uncertainties, to the 776 ± 12 Ma zircon U-Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming 'microvein' and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatichydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of orthomagmatic or hydrothermal fluids containing fluorides as a major constituent, which expelled a considerable amount of trace elements, namely, Hf, U, Th, Y, and the REEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼0.5 to 1.0). Furthermore, the ca. 780-776 Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.
Precambrian Research, 2003
A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplac... more A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low-and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600-630 • C and 5-6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr/ 86 Sr of 0.70281 and positive ε Nd(625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age (T DM) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded ε Nd(625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ∼ = 1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic.
Precambrian Research, 2003
A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplac... more A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low-and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600-630 • C and 5-6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr/ 86 Sr of 0.70281 and positive ε Nd(625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age (T DM) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded ε Nd(625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ∼ = 1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic.