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

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 varied evolution. The Unegt Uul Crystalline Complex is a tectonic mélange 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 temperatures of c. 650 °C and pressures of 6-7.5 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 and 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).

Geochemical and structural constraints on the magmatic history of the Chandman Massif of the eastern Mongolian Altay Range, SW Mongolia

“…17), the area represents a sliver of older material bounded to the south by younger Devonian granites, metavolcanics and metasedimentary sequences, and to the north (across the Bogd fault zone) by Permian volcanics and granites. These interpretations, however, are inconsistent with geochronological findings and the geological character of the Chandman Khayrkhan Crystalline Complex, as described below, in Hrdličková et al (2008) and in geological maps (Hanžl and Aichler 2007).…”

“…Magmatic rocks of the Chandman Massif are further characterized into metaluminous and peraluminous units based on field evidence, geochemistry, and petrography. The timing and conditions of intrusion are constrained by previous geochronology (Hrdličková et al 2008) and new geothermobarometry. These data are combined to create a model of the metamorphic and intrusive history of the area.…”

Burd Gol Granite Massif as a product of the Late Cambrian post-orogenic magmatism in the SE part of the Lake Zone, Gobi Altay, SW Mongolia

“…It could be well correlated with leucogranites in tectonic mélange of the Unegt Uul Mt. (further east) which yielded zircons 518 ± 5 Ma old (Hrdličková et al 2008).…”

“…The Gobi Altay Zone is interpreted to having originated in a backarc/forearc basin (Badarch et al 2002). Lower Paleozoic metamorphic complexes intruded by Variscan granite plutons are documented in this part of the Gobi Altay Zone (Economos et al 2008;Hrdličková et al 2008).…”

Evolution of the Early Permian volcanic-plutonic complex in the western part of the Permian Gobi-Altay Rift (Khar Argalant Mts., SW Mongolia)

“…) and high-grade metamorphic rocks of the Tsakhir Uul Crystalline Complex (Hanžl and Aichler eds. 2007;Hrdličková et al 2008;Kröner et al 2010;Lehmann et al 2010;Štípská et al 2010). The volcanic and sedimentary rocks in the Khar Argalant Mts.…”