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Petrology and age of metamorphosed rock in tectonic slices inside the Palaeozoic sediments of the eastern Mongolian Altay, SW Mongolia

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Author(s): Hrdlickova K | Bolormaa K | Burianek D | Hanzl P | Gerdes A | Janousek V

Journal: Journal of Geosciences
ISSN 1802-6222

Volume: 53;
Issue: 2;
Start page: 139;
Date: 2008;
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Keywords: Variscan metamorphism | P-T calculations | ICP-MS | U-Pb zircon dating | Mongolian Altay | Gobi Altay Terrane

ABSTRACT
Three crystalline complexes were newly described in the area of the junction between the Mongolian Altay and Gobi Altay in the surroundings of Chandman Sum, SW Mongolia. Khan Khayrkhan, Chandman Khayrkhan and Unegt Uul crystalline complexes were distinguished on the basis of their geological position, distinct petrography, metamorphic style and contrasting geochronological data. These units are situated along the northern margin of the Gobi-Altay Terrane and at first sight seem to have a very simple and uniform history. However, detailed studies reveal a more complex and different evolution. The Unegt Uul Crystalline Complex is a tectonic melange of leucogranites, amphibolites and mica schists exposed between the branch of the Bogd fault in the north and Lower Palaeozoic sediments in the south. Metamorphic rocks suffered a prograde metamorphic event reaching peak temperatures of c. 800 ± 70 °C and pressures of 9.6 ± 1 kbar estimated from amphibolites. This Complex represents the oldest of the three units; the age of leucogranite formation corresponds to Cambrian (518 ± 5 Ma). Chandman Khayrkhan Crystalline Complex is also restricted by Bogd fault from the north but southern boundary is limited by an intrusive contact with the Chandman Massif. It is composed of orthogneisses and migmatites with amphibolite and calc-silicate lenses. The Complex was affected by a HT, likely periplutonic, metamorphic event with subsequent retrogression. While the rocks of the Chandman Khayrkhan Crystalline Complex remain undated, the age of the granitic rocks in the Chandman Massif itself are Early Carboniferous (345 ± 2 Ma). The Khan Khayrkhan Crystalline Complex has tectonic contacts with the surrounding Palaeozoic volcanosedimentary units. It is built by orthogneisses, amphibolites, paragneisses and mica schists. Metamorphic style corresponds to a prograde event with peak at c. 670 °C and 7-10 kbar. The crystallization age for the granitic protolith to the orthogneisses was the latest Devonian (363 ± 3 Ma).

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