Abstract
Combustion metamorphic (CM) rocks (clinker and paralava) occur in abundance in the eastern and southern margins of the Goose Lake in Western Transbaikalia and form five fields. The sections we studied in natural outcrops exposed in numerous gullies and in quarries comprise the full range of CM varieties from low-grade to fused paralavas and clinkers. The tridymite-plagioclase-cordierite and tridymite-cordierite paralava and clinker have medium to high K/Ca ratios (∼2.5–4.5 wt.%) with K restricted to K-rich (∼4–6 wt.% K2O) high-silica glass, making the bulk samples suitable for 40Ar/39Ar dating. Regional-scale combustion metamorphic events were triggered by reactivation of faults in the Goose Lake Basin causing repeated erosion of gently dipping coal-bearing sediments that exposed coal beds to oxidation resulting in their spontaneous ignition. Geological evidence indicates that the earliest natural coal fire and formation of CM rocks occurred at the end of the Early Cretaceous. Geological and preliminary geochronological data indicate that large-scale coal fires occurred in the Early Pleistocene (no later than 1.8 ± 0.4 Ma ago) and in Late Pleistocene (0.02 ± 0.01 Ma and 0.03 ± 0.03 Ma).
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