Teklewold Ayalew - Academia.edu (original) (raw)

Papers by Teklewold Ayalew

Constraints on Pan African metamorphism of the western Ethiopian Shield

Abstracts with Programs - Geological Society of America, May 1, 2001

Office of Environmental Health Hazard Assessment

This report has been reviewed by the staff of the Air Resources Board and approved for publicatio... more This report has been reviewed by the staff of the Air Resources Board and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Air Resources Board. Mention of trade names or commercial products does not constitute endorsement or recommendation for their use. ii Acknowledgements

Composition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara, Lake Tana Province

Mineralogy and Petrology, 2007

Characterisation of the Lithosphere Beneath the North-Western Ethiopian Plateau: Spinel Lherzolites from Injibara (Lake Tana Province)

Ofioliti, Jan 7, 2005

... Simona Ferrando, Maria Luce Frezzotti, Else-Ragnhild Neumann, Angelo Peccerillo, Ayalew Derej... more ... Simona Ferrando, Maria Luce Frezzotti, Else-Ragnhild Neumann, Angelo Peccerillo, Ayalew Dereje, Yirgu Gezahegn, Ayalew Teklewold. Download RIS. ... Previous studies of mantle rocks from the Main Rift (eg Mega, South Ethiopia; Bedini et al., 1997), have indicated that ...

Metamorphic rocks are igneous, sedimentary, or other metamorphic rocks whose texture and composit... more Metamorphic rocks are igneous, sedimentary, or other metamorphic rocks whose texture and composition has been changed by metamorphism. Metamorphism occurs as a response to changes in the physical or chemical environment of any pre-existing rock, such as variations in pressure or temperature, strain, or the infiltration of fluids. It involves recrystallization of existing minerals into new grains and/or the appearance of new mineral phases and breakdown of others. Metamorphic processes take place

Final report of the Gore-Gambela Geotraverse, Western Ethiopia

Includes map in back pocketThe table of contents for this item can be shared with the requester. ... more Includes map in back pocketThe table of contents for this item can be shared with the requester. The requester may then choose one chapter, up to 10% of the item, as per the Fair Dealing provision of the Canadian Copyright Ac

Relation Ship Sbetween Geology, Mor Phology and Soil Associations of Bichena Area ( S. E. GOJAM )

SINET: Ethiopian Journal of Science, 2002

The western Ethiopian Shield comprises three lithotectonic units. The Birbir domain, an assemblag... more The western Ethiopian Shield comprises three lithotectonic units. The Birbir domain, an assemblage of mafic to felsic intrusive and extrusive rocks and mainly volcanogenic sedimentary rocks, is enclosed between the dominantly orthogneissic Baro and Geba domains. The earliest recorded deformation event (D1) resulted in the formation of a subhorizontal gneissosity within the gneissic terranes which was synchronous with an early upper amphibolite-facies metamorphic peak (M1) at 800-770 Ma which locally caused partial melting. All terrains were subsequently deformed in the D2 event which was the result of severe E-W crustal shortening. An anticlockwise P-T-t path is implied. Subsequent D3 deformation was concentrated within mylonitised domain boundaries which record major transcurrent movement. These structures were reactivated and suffered fluid incursion resulting in isotopic reequilibration at 635-580 Ma. A second metamorphic event, M2, related to crustal thickening and consequent granite genesis, occurred after the cessation of D3 shearing. Bulk chemical analyses show that the metamorphosed plutonic and volcanic rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern magmatic arcs. They belong in part to the low-K series, suggesting an oceanic environment. The evolution of the region can be explained in terms of the melting of a subducting slab, intrusion, metamorphism and the formation of an oceanic island arc complex. Continued plate convergence caused severe east-west shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx, a second thermal event and accretion of microcontinents.

SINET: Ethiopian Journal of Science, 1997

Precambrian Research, 2004

The Western Ethiopian Shield comprises three lithotectonic units: the Birbir domain, an assemblag... more The Western Ethiopian Shield comprises three lithotectonic units: the Birbir domain, an assemblage of deformed and metamorphosed volcano-sedimentary rocks and associated ultramafic to felsic magmatic rocks, is bounded by the dominantly orthogneissic Baro and Geba domains. Geochemical considerations suggest the igneous rocks of the Birbir domain are predominantly calc-alkaline and similar to those generated by subduction in modern oceanic island-arcs, broadly consistent with Sm-Nd data that indicate an early crust-forming event at ca. 0.95-1.1 Ga. This complex was accreted during the Neoproterozoic East African Orogeny and formed the core and source area for subsequent plutonism and metamorphism. The earliest recorded deformation resulted in a sub-horizontal gneissosity within the gneissic terrains, synchronous with an early (ca. 770-800 Ma) metamorphic event (M 1), which records amphibolite facies conditions (600-800 • C and ca. 6-8 kbar) in the gneissic terrains and greenschist to amphibolite facies conditions in the Birbir domain. The igneous protoliths to the orthogneisses (i.e., pre-dating M 1) are dated at 830-785 Ma. All terrains were subsequently deformed in the D 2 event, the result of oblique westerly directed transpression, with consequent folding and thrusting concentrated at domain boundaries. Subsequent D 3 deformation was focussed within mylonitized zones that likely represent reactivated D 2 thrusts and record both dextral (early) and sinistral (late) transcurrent movement. A second protracted metamorphic event, M 2 , at around 650-550 Ma, resulted in rehydration of M 1 parageneses and was fluid-driven. M 2 metamorphism was synchronous with D 3 shearing. Fluid incursion was concentrated within sheared domain boundaries and caused isotopic re-equilibration. The WES records a long-lived but episodic tectonothermal event that can be explained in terms of arc-continent collision. Melting of a subducting slab and associated intrusion caused metamorphism and the formation of an oceanic island-arc complex. Continued plate convergence caused severe E-W shortening and basin closure. Further attenuation gave rise to transcurrent shearing, fluid influx and a second thermal event during accretion and orogenic closure.

Journal of African Earth Sciences, 1998

Major and trace element data for granitoid rocks from the Gore-Gambella Precambrian terranes of w... more Major and trace element data for granitoid rocks from the Gore-Gambella Precambrian terranes of western Ethiopia have been used to infer their petrogenetic history and the tectonic environment in which the magmas were emplaced. The plutons are found in the lithotectonic domains of Birbir, Baro and Geba, and were emplaced during the interval between 830 and 540 Ma. Geochemical data show that the Neoproterozoic, pre-to synkinematic metaplutonic rocks of the Birbir domain are predominantly talc-alkaline with minor low K trondhjemitic granodiorites and tonalites. Some syn-kinematic granitoids, occurring as sheets or lenticular bodies within high-grade metamorphic rocks of the Baro domain, are peraluminous and show a wide range of incompatible trace elements such as Y and Zr. The post-kinematic plutons are enriched in K and incompatible elements relative to older intrusives, suggesting a shoshonitic petrochemical affinity. Petrological and geochemical data show that the bulk of the pre-to syn-kinematic rocks represent mantle-derived talc-alkaline and arc-tholeiitic magmas. Major and trace element variations suggest a multistage, probably polybaric evolution for these intrusions. The synkinematic granitoid lenses of the Baro domain have petrological and geochemical characteristics that suggest a genesis by crustal melting. The late-to post-kinematic plutons are probably related to the melting of talc-alkaline continental crust, followed by fractional crystallisation or AFC. Overall, the plutonic activity reflects the geotectonic evolution of the region through time. The pre-to syn-kinematic rocks from the Birbir domain are compositionally akin to those generated by subduction in modern magmatic arcs; they belong in part to a low K series and are interpreted as emplaced in an intraoceanic island-arc environment. The occurrence of greywacke turbidites and rare carbonates of submarine deposition supports this hypothesis. The late-to post-kinematic intrusions possess geochemical signatures that imply a role for both subduction-related and intraplate components. o 7998 Elsevier Science Limited. All rights reserved. Resume-Les elements majeurs et traces avec des granitoides provenant des terrains precambriens de Gore-Gambella en Ethiopie occidentrale on Bte utilises afin d'inferer leurs histoires petrogenetiques et I'environnement tectonique dans lequel les magmas se sont mis en place. Les plutons se situent dans les domains lithotectoniques de Birbir, de Baro et de Geba, et se sont mis en place dans I'intervalle 830 a 540 Ma. Les donnees geochimiques indiquent que les roches metaplutoniques pre-a syn-cinematiques du domaine Birbir sont principalement calco-alcalines avec des quantites mineures de granodiorites trondhjemitiques et de tonalites. Quelques granitoides syn-cinematiques, en feuillets ou en lentilles au sein des roches tres metamorphiques du domaine de Baro, sont hyperalumineux et presentent une gamme dtendue de teneurs en elements traces incompatibles tels que le endue de teneurs en elements traces incompatibles tels que le Y et le Zr. Les plutons post-cinematiques sont enrichis en K et en elements incompatibles par rapport aux intrusions anciennes, suggerant une affinite shoshonitique.

Timing of East African Rift development in southern Ethiopia: Implication for mantle plume activity and evolution of topography

Geology, 2008

... 86K): [in this window] [in a new window], Figure 1. Topography of East Africa showing the ...... more ... 86K): [in this window] [in a new window], Figure 1. Topography of East Africa showing the ... In such a model, the old He age of the southern sample (4.8 °N) corresponds to ... Because of associatederosion during shoulder formation, the real uplift of the topography was more limited ...

U-Pb and Rb-Sr geochronology of the Western Ethiopian Shield

Geological Society of America Bulletin, 1990

Page 1. U-Pb and Rb-Sr geochronology of the Western Ethiopian Shield TEKLEWOLD AYALEW* \ KEITH BE... more Page 1. U-Pb and Rb-Sr geochronology of the Western Ethiopian Shield TEKLEWOLD AYALEW* \ KEITH BELL &amp;amp;gt; Ottawa-Carleton Geoscience Centre and Department of Earth Sciences, Carleton University, Ottawa, Canada K1S 5B6 JOHN M. MOORE J RANDALL R. PARRISH ...