- Email: [email protected]
Tectono-Thermal History of Eastern Eritrea: The Eastern Margin of West Gondwana
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tectonic contact along the eastern margin of the Itremo Group is related to orogenic collapse, as proposed by Collins et al. (2000), this event must be younger than 500 Ma. The Itremo Region provides evidence that final assembly of Gondwana in central Madagascar continued into late Cambrian-Ordovician times.
References Andriamampihantona, M.J. (1992) Contribution a l’etude du complexe alcalin d’ambatofinandrahana et de ses min6ralisations A lanthanides (Region centrale de Madagascar). Thitse de Doctorat, UniversitC Joseph Fourier, Grenoble I. Collins, A.S., Razakamanana, T. and Windley, B.F. (2000) Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African orogen. Geol. Mag., v. 137, pp. 39-51. Cox, R., Armstrong, R. and Ashwal, L. (1998) Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for preGondwana palaeogeography. J. Geol. SOC.London, v. 155, pp. 1009-1024.
Fernandez, A., Huber, S. and Schreurs, G. (2000) Evidence for late Cambrian-Ordovician final assembly of Gondwana in central Madagascar. In: Geol. SOC.Amer. Annual Meeting, Reno, Nevada. Handke, M., Ncker, R. and Ashwal, L. (1999) Neoproterozoic continental arc magmatism in west-central Madagascar. Geology v. 27, pp. 351-354. Huber, S. (2000) Geologie und Geochronologie der Itremo Sedimente im Gebiet des Mont Ibity (Zentral Madagaskar). M.Sc. thesis, University of Bern, Switzerland, 97p. Moine, B. (1974) Charactkres de sedimentation et de metamorphisme de series precambriennes epizonales a catazonales du centre de Madagascar. Sci. de la Terre Memoire, v. 31, 293p. Stern, R.J. (1994) Arc assembly and continental collision in the Neoproterozoic East African orogen: implications for the consolidation of Gondwanaland. Ann. Rev. Earth Planet. Sci., V. 22, pp. 319-351. Windley, B.F., Razafiniparany, A., Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal. Geol. Rundsch., v. 83, pp. 642-659.
Gondwana Research, V 4, No. 2, pp. 168-169. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Tectono-Thermal History of Eastern Eritrea: The Eastern Margin of West Gondwana Woldai Ghebreab Department of Earth Sciences, University of Asmara, P.O. Box 1220, Asmara, Eritrea, E-mail: [email protected]
Neoproterozoic rocks of the East African Orogen (EAO) in eastern Eritrea comprise two lithotectonic domains, Ghedem and Bizen. The Ghedem domain consists of highgrade metamorphic rocks with low angle tectonic grains dominant. The Bizen domain consists of low-grade metamorphic rocks with steep tectonic grain dominant, though sub-horizontal collapse structures are also common. These structures include flat-lying folds and lowangle detachments with top-to-west/northwest senses of shear. A major westward dipping low-angle discontinuity separates the two domains with the Bizen domain in the hanging wall. Along parts of this discontinuity pyrite, chalcopyrite (with or without malachite stains),
ferruginization, silicification and chloritization, invite future detailed investigation towards mineral exploration. Three Pan-African D,., phases of deformation are recognized. D, deformation is characterized by a steep penetrative foliation S, which is axial planar to upright F, folds. F, folds were distorted by low-angle shear zones and F, recumbent folds during D,. D, resulted in steep N-S/NNE-SSW strike-slip shear zones and a steep S, spaced cleavage axial planar to open upright NNW trending F, folds. Garnet rim-matrix and inclusion geothermobarometry indicate that rocks of the Ghedem domain experienced syn-metamorphic deformation and were exhumed from
169
as deep as 50 km. Exhumation was accomplished by erosion and gravity-driven orogenic collapse and isostatic root recovery. Geothermobarometric studies further indicate that metamorphism peaked during early D, when P-T conditions were at 12 kbar and 650°C. The peak metamorphism was dated at 593f 5 Ma (U-Pb monazite
ages) and is attributed to terminal convergence between East and West Gondwana to form the EAO. 40Ar/39Arages of hornblende and muscovite concentrates indicate that the rocks underwent cooling from 576 Ma to - 561 Ma at a rate of 10-15"C/Ma on the way to exhumation during late D, deformation. Exhumation rate was at 2 - 3 mm/a.
-
Gondwana Research, V 4, No. 2, p p . 169-1 70. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Characteristic Features of the Late Precambrian Felsic Magmatism in Western Anatolia: Implications for the Pan-African Evolution in NW Perigondwana S. Gursul and M.C. Gonciioglu* General Directorate of Mineral Research and Exploration, Ankara, Turkey Middle East Technical University, Ankara, Turkey
It is prevalently accepted that the Tauride-Anatolide terrane is the NW continuation of the Arabian microcontinent and part of Perigondwana. The Pre-Cambrian basement of the Tauride-Anatolide terrane is mainly characterized by high-grade metamorphic complexes (e.g. Bitlis Massif). However, limited outcrop$ of slightly metamorphic volcanic assemblages and sedimentary/ volcano-sedimentary successions have also been reported from different parts of the terrane (Kozlu and Goncuoglu, 1996). To analyze the Pre-Cambrian events, the Sandikli Basement Complex, described in this study, is the most critical one of these units. The Sandikli Basement Complex is located to the west of Sandikli town in western central Anatolia. It is composed of the low-grade metamorphic Guvercinoluk Formation that is intruded by the Kestelqayi Porphyroid Suite and unconformably overlain by anchi-metamorphic Cambrian units. The GiivercinolukFormation consists of an alternation of dark-coloured meta-siltstones and meta-sandstones with rare black lydite and cherty meta-dolomite lenses. The middle part is represented by meta-turbidites and
olistostromal conglomerates. The uppermost part of the sequence is made up of dark coloured beccoidal limestones, cherty and laminated limestones interlayered with sandstones. The unit is intruded by quartz-porphyry dykes. The meta-clastic rocks are well-foliated and display the metamorphic paragenesis: quartz+albite+ muscovite +calcite indicating low-grade metamorphism. The thickness of the unit reaches up to 800 meters and lateral as well as vertical facies changes are commonly observed. No fossils have been yet determined from this member. The Kestelqayi Porphyroid Suite is characterized by a deformed and highly sheared dome-shaped rhyoliticdacitic body with a granitic core. Granitic dykes intrude the volcanic carapace and the meta-sediments of the Guvercinoluk Formation. A narrow contact metamorphic zone with hydothermal mineralization is locally preserved. Both the granitoids and the rhyolites-dacites are typically mylonitic rocks with relict igneous textures. Within the rhyolites, corroded quartz and microdperthitic feldspar phorphyroclasts are embedded in a mylonitic matrix with elongated fine-grained quartz and white mica. Preliminary geochemical data indicate that the granitoids are
tectonic contact along the eastern margin of the Itremo Group is related to orogenic collapse, as proposed by Collins et al. (2000), this event must be younger than 500 Ma. The Itremo Region provides evidence that final assembly of Gondwana in central Madagascar continued into late Cambrian-Ordovician times.
References Andriamampihantona, M.J. (1992) Contribution a l’etude du complexe alcalin d’ambatofinandrahana et de ses min6ralisations A lanthanides (Region centrale de Madagascar). Thitse de Doctorat, UniversitC Joseph Fourier, Grenoble I. Collins, A.S., Razakamanana, T. and Windley, B.F. (2000) Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African orogen. Geol. Mag., v. 137, pp. 39-51. Cox, R., Armstrong, R. and Ashwal, L. (1998) Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for preGondwana palaeogeography. J. Geol. SOC.London, v. 155, pp. 1009-1024.
Fernandez, A., Huber, S. and Schreurs, G. (2000) Evidence for late Cambrian-Ordovician final assembly of Gondwana in central Madagascar. In: Geol. SOC.Amer. Annual Meeting, Reno, Nevada. Handke, M., Ncker, R. and Ashwal, L. (1999) Neoproterozoic continental arc magmatism in west-central Madagascar. Geology v. 27, pp. 351-354. Huber, S. (2000) Geologie und Geochronologie der Itremo Sedimente im Gebiet des Mont Ibity (Zentral Madagaskar). M.Sc. thesis, University of Bern, Switzerland, 97p. Moine, B. (1974) Charactkres de sedimentation et de metamorphisme de series precambriennes epizonales a catazonales du centre de Madagascar. Sci. de la Terre Memoire, v. 31, 293p. Stern, R.J. (1994) Arc assembly and continental collision in the Neoproterozoic East African orogen: implications for the consolidation of Gondwanaland. Ann. Rev. Earth Planet. Sci., V. 22, pp. 319-351. Windley, B.F., Razafiniparany, A., Razakamanana, T. and Ackermand, D. (1994) Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal. Geol. Rundsch., v. 83, pp. 642-659.
Gondwana Research, V 4, No. 2, pp. 168-169. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Tectono-Thermal History of Eastern Eritrea: The Eastern Margin of West Gondwana Woldai Ghebreab Department of Earth Sciences, University of Asmara, P.O. Box 1220, Asmara, Eritrea, E-mail: [email protected]
Neoproterozoic rocks of the East African Orogen (EAO) in eastern Eritrea comprise two lithotectonic domains, Ghedem and Bizen. The Ghedem domain consists of highgrade metamorphic rocks with low angle tectonic grains dominant. The Bizen domain consists of low-grade metamorphic rocks with steep tectonic grain dominant, though sub-horizontal collapse structures are also common. These structures include flat-lying folds and lowangle detachments with top-to-west/northwest senses of shear. A major westward dipping low-angle discontinuity separates the two domains with the Bizen domain in the hanging wall. Along parts of this discontinuity pyrite, chalcopyrite (with or without malachite stains),
ferruginization, silicification and chloritization, invite future detailed investigation towards mineral exploration. Three Pan-African D,., phases of deformation are recognized. D, deformation is characterized by a steep penetrative foliation S, which is axial planar to upright F, folds. F, folds were distorted by low-angle shear zones and F, recumbent folds during D,. D, resulted in steep N-S/NNE-SSW strike-slip shear zones and a steep S, spaced cleavage axial planar to open upright NNW trending F, folds. Garnet rim-matrix and inclusion geothermobarometry indicate that rocks of the Ghedem domain experienced syn-metamorphic deformation and were exhumed from
169
as deep as 50 km. Exhumation was accomplished by erosion and gravity-driven orogenic collapse and isostatic root recovery. Geothermobarometric studies further indicate that metamorphism peaked during early D, when P-T conditions were at 12 kbar and 650°C. The peak metamorphism was dated at 593f 5 Ma (U-Pb monazite
ages) and is attributed to terminal convergence between East and West Gondwana to form the EAO. 40Ar/39Arages of hornblende and muscovite concentrates indicate that the rocks underwent cooling from 576 Ma to - 561 Ma at a rate of 10-15"C/Ma on the way to exhumation during late D, deformation. Exhumation rate was at 2 - 3 mm/a.
-
Gondwana Research, V 4, No. 2, p p . 169-1 70. 02001 International Association for Gondwana Research, Japan. ISSN: 1342-937X
GR
Gondwana Research
Characteristic Features of the Late Precambrian Felsic Magmatism in Western Anatolia: Implications for the Pan-African Evolution in NW Perigondwana S. Gursul and M.C. Gonciioglu* General Directorate of Mineral Research and Exploration, Ankara, Turkey Middle East Technical University, Ankara, Turkey
It is prevalently accepted that the Tauride-Anatolide terrane is the NW continuation of the Arabian microcontinent and part of Perigondwana. The Pre-Cambrian basement of the Tauride-Anatolide terrane is mainly characterized by high-grade metamorphic complexes (e.g. Bitlis Massif). However, limited outcrop$ of slightly metamorphic volcanic assemblages and sedimentary/ volcano-sedimentary successions have also been reported from different parts of the terrane (Kozlu and Goncuoglu, 1996). To analyze the Pre-Cambrian events, the Sandikli Basement Complex, described in this study, is the most critical one of these units. The Sandikli Basement Complex is located to the west of Sandikli town in western central Anatolia. It is composed of the low-grade metamorphic Guvercinoluk Formation that is intruded by the Kestelqayi Porphyroid Suite and unconformably overlain by anchi-metamorphic Cambrian units. The GiivercinolukFormation consists of an alternation of dark-coloured meta-siltstones and meta-sandstones with rare black lydite and cherty meta-dolomite lenses. The middle part is represented by meta-turbidites and
olistostromal conglomerates. The uppermost part of the sequence is made up of dark coloured beccoidal limestones, cherty and laminated limestones interlayered with sandstones. The unit is intruded by quartz-porphyry dykes. The meta-clastic rocks are well-foliated and display the metamorphic paragenesis: quartz+albite+ muscovite +calcite indicating low-grade metamorphism. The thickness of the unit reaches up to 800 meters and lateral as well as vertical facies changes are commonly observed. No fossils have been yet determined from this member. The Kestelqayi Porphyroid Suite is characterized by a deformed and highly sheared dome-shaped rhyoliticdacitic body with a granitic core. Granitic dykes intrude the volcanic carapace and the meta-sediments of the Guvercinoluk Formation. A narrow contact metamorphic zone with hydothermal mineralization is locally preserved. Both the granitoids and the rhyolites-dacites are typically mylonitic rocks with relict igneous textures. Within the rhyolites, corroded quartz and microdperthitic feldspar phorphyroclasts are embedded in a mylonitic matrix with elongated fine-grained quartz and white mica. Preliminary geochemical data indicate that the granitoids are