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New field, petrographic, geochemical and geochronological data from the Togtokhinshil Complex (western margin of the Lake Zone, Central Asian Orogenic Belt) reveal the presence of two separate Early Palaeozoic magmatic pulses: Mid-Ordovician and Late Devonian. The Mid-Ordovician (c. 460 Ma) magmatism produced various types of mafic plutonic rocks (gabbro-diorite suite) emplaced into the volcanosedimentary sequence of the Baatar Fm. Gabbros to gabbrodiorites have metaluminous and mostly low to normal-K calc-alkaline or possibly tholeiitic character. The majorand trace-element as well as Nd isotopic signatures suggest its magmatic-arc geotectonic setting and imply the derivation either exclusively from the mantle or in part from the juvenile mafic crust. The Late Devonian (c. 376 Ma) granite suite intruded the rocks of both the gabbro-diorite suite and the Baatar Fm. Granites are subaluminous and have a high-K calc-alkaline to shoshonitic character. Their magmatic arc-like geochemical signature is interpreted as inherited from the recycled arc-related crustal source. The granite suite seems a product of extensive mantle heat-induced crustal anatexis. The Depleted Mantle Nd model ages (c. 0.75 Ga) indicate derivation of the magmatic rocks of the both suites from relatively juvenile, and geochemically immature, sources. These results provide a clear evidence of the Mid-Ordovician arc-related magmatism in the western part of the Lake Zone. They thus prolong the assumed period of magmatic-arc activity and constrain onset of accretionary processes in this part of the Central Asian Orogenic Belt. The gabbro-diorite suite represents either part of a long-lived magmatic-arc or, more likely, a member of multiple island-arc system along the western margin of the Lake Zone. Newly described Late Devonian magmatism in the Togtokhinshil Complex provides further evidence of tectono-thermal event that was widespread in the Central Asian Orogenic Belt. The geodynamic cause for this event remains uncertain, but effects of lithospheric thinning and/or asthenospheric mantle upwelling are considered the most likely.

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