Abstract:

Dawsonite cement is abundant in pyroclastic rocks in Tongbomiao Formation in Tanan sag of Tamtsag basin in Mongolia. The types, diagenetic sequence of cements and authigenic minerals of dawsonite-bearing pyroclastic rocks are investigated by palarizering microscope, SEM and X-ray diffraction. Their diagenetic sequence is clay coating cementation-calcite-early microcrystalline quartz cementation-early kaolinite cementation-dawsonite cementation, later microcrystalline quartz cementation, later kaolinite-ankerite cementation. The stable isotope compositions of dawsonite and ankerite are investigated by MAT253 stable isotope mass spectrometer. Dawsonite δ¹³C values range from -8.15‰ to -2.81‰, and the calculated values of CO₂ gas in isotopic equilibrium with dawsonite range from -12.91‰ to -7.55‰, with analogy of Hailaer basin and BGS basins, which show that CO₂ for forming dawsonite almost from mantle-magma, and andesite debris, crystal feldspar and detrital feldspar, vitric debris and volcanic ash supplying Na⁺ and Al³⁺.Ankerite δ¹³C values range from -6.76‰ to -2.65‰,which are similar with dawsonite, with related geological and geochemical simulation examples, which note that CO₂ for forming ankerite also from mantle-magma. Andesite debris, vitric debris, volcanic ash and montmorillonite which contain ferrum supply metal ions for the formation of ankerite.

Key words: dawsonite, ankerite, generation, pyroclastic rock, Tamtsag basin, mineralogy