Tonian and Permian magmatic episodes in the Zavkhan Terrane (Western Mongolia): evidence from the Khasagt Mountains

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We provide the first account of whole-rock geochemistry and U-Pb SHRIMP geochronology applied to the igneous rocks exposed in the Khasagt Mountains at the margin of the Zavkhan Terrane in Western Mongolia (Central Asian Orogenic Belt). The results show that the area experienced magmatic episodes in the Tonian Period of the Neoproterozoic: at 815–800 Ma recorded by the emplacement of granite, Q-monzonite and alkali gabbro, followed by a subsequent pulse of latibasalt lavas and subvolcanic dykes at 787–769 Ma. Subsequently, in the northern part of the Khasagt region, zircons from alkaline granites and volcanic rocks revealed an early Permian igneous episode at 276–268 Ma. Both late Tonian and early Permian events have been recorded in several terranes of the Mongolian Collage, facilitating more detailed correlation. Our study of the Khasagt Mountains highlights the significance of using multiple geochemical discriminators. These intrusive suites provide evidence of bimodal magmatism, with an alkaline signature (i.e. Nb/Yb >4), in both late Tonian and early Permian time, and also indicate other geochemical features such as high Th/U ratios in zircons, typical of an extension regime. These new investigations of the Khasagt igneous rocks have showed links to giant rift systems and global processes. The Tonian episode was related to an early stage of Rodinia rifting, and the Permian episode to the Central Asian intra-continental rift system.