Anorogenic early permian dykes in the Western Mongolian altai - petrography, geochemistry and K-Ar geochronology

Abstract

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 alkaline- and 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.