Anorogenic Early Permian dykes in the western Mongolian Altai - petrography, geochemistry and K-Ar geochronology

A variety of felsic and mafic dykes grouped into swarms intruded the Lower Palaeozoic volcano-sedimentary sequences (flysch) and Late Devonian to Early Carboniferous plutonic rocks in the Hovd and Altai zones of the Central Asian Orogenic Belt (CAOB), western Mongolian Altai. The dykes reach a thickness of 0.5–20 m, length of approximately 50–2,500 m and strike mostly SW–NE or E–W. The felsic rocks chemically correspond to high-K calc-alkaline to alkaline rhyolites. Compositional trends of mafic rocks pass from alkalineand calc-alkaline basalts to trachyandesite. The bimodal nature of the association and the transitional calc-alkaline to alkaline character of the dykes indicate magma production through partial melting of the mantle and continental crust in an intra-plate (rift) geodynamic setting. The new conventional whole-rock K–Ar dating of mafic and felsic dykes yielded ages ranging from 300 ± 9 to 281 ± 9 Ma (1σ). This indicates anorogenic volcanic activity associated with Late Carboniferous to Early Permian extension coeval with magmatism in the Gobi–Altai Rift and in the adjacent parts of the Chinese Altai. The calculated crystallization pressures of 1–2 kbar and 0.3–0.4 kbar for felsic and mafic rocks, respectively, indicate emplacement at shallow levels.