Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

David Buriánek, Vojtěch Janoušek, Pavel Hanžl, Yingde Jiang, Karel Schulmann, Ondrej Lexa, Battushig Altanbaatar

Petrogenesis of the Late Carboniferous Sagsai Pluton in the SE Mongolian Altai

Journal of Geosciences, volume 61 (2016), issue 1, 67 - 92

DOI: http://doi.org/10.3190/jgeosci.207

Sagsai Pluton intruded the low-grade metavolcanosedimentary complex of the Cambrian Tugrug Fm. in the Sagsai river valley SW of Tsogt soum, Gobi-Altai aimag (SE Mongolian Altai). This oval body (~9 × 12 km) is composed mainly of amphibole monzogabbros to quartz diorites, biotite to amphibole-biotite syenites to quartz monzonites. It is intruded by anatectic two-mica granites along the W rim. The abundant mafic microgranular enclaves enclosed by syenites to quartz monzonites are interpreted as an evidence of vigorous magma mingling and mixing. The newly obtained LA ICP-MS zircon ages for two samples of quartz monzonites gave emplacement ages of 307 ± 1 Ma and 309 ± 2 Ma (2σ), respectively. They have ε^t_Hf values varying broadly from +2.4 to +11.6 ruling out a participation of an old, mature crust. One gabbro body gave zircon U-Pb age of 322 ± 2 Ma with positive ε^t_Hf values of +7.4 to +11.5. The latter suggest an exclusively juvenile, depleted mantle-derived magma source. According to calculated P-T conditions, we propose three main stages in the Sagsai Pluton evolution. The gabbro started to grow at >1000 °C and 0.8-0.9 GPa. The ensuing magma mixing with crustal melts and final crystallization of gabbro occurred at similar P-T conditions (740-830 °C and 0.5-0.6 GPa) as partial melting of migmatite xenoliths in the Sagsai Pluton. The P-T conditions of the Sagsai Pluton final emplacement calculated from quartz syenite (c. 600 to 670 °C and 0.4-0.5 GPa) are consistent with the mineral assemblage in the contact aureole (cordierite-biotite and biotite-chlorite schists). The magmatic evolution of Sagsai Pluton confirms a model of deep lithosphere thermal anomaly associated with heterogeneous melting of crust in a post-subduction extensional regime, perhaps shortly after the slab break-off.