The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent,with a Neoproterozoic metamorphic overprint |
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| Institution: | 1. Institute of Petrology and Structural Geology, Charles University, Albertov 6, 14328 Prague, Czech Republic;2. Department of Materials Science and Physics, University of Salzburg, Hellbrunnerstraße 34, A-5020 Salzburg, Austria;3. Centre of Isotopic Research, VSEGEI, 74 Sredny prospect, 199106 St. Petersburg, Russia;4. Department of Geological Sciences, Masaryk University, Kotlá?ská 267/2, Brno, Czech Republic;5. Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava, Slovak Republic;1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. State Key Laboratory of Oil and Gas Geology and Exploitation, School of Resource and Environment, Southwest Petroleum University, Chengdu 610500, China;1. Department of Earth & Environmental Sciences, 227 Hutchison Hall, University of Rochester, Rochester, NY 14627, USA;2. INSA Honorary Scientist, 103, Sector 7, Panchkula, Haryana 134109, India;3. Department of Geology, Banaras Hindu University, Varanasi, UP 221005, India;4. Department of Geology, Panjab University, Sector-14, Chandigarh-160014, India;1. Department of Applied Geology, Curtin University, Perth WA 6845, Australia;2. School of Geosciences, King''s College, University of Aberdeen, AB24 3UE United Kingdom;3. Department of Earth Sciences, The University of Adelaide SA 5005, Australia;4. School of Earth Science & Resources, Chinese University of Geosciences, Beijing, China |
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| Abstract: | We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons. |
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