Signature of Cenozoic orogenic movements in combustion metamorphic rocks: mineralogy and geochronology (example of the Salair-Kuznetsk Basin transition)

Cenozoic combustion metamorphic (CM) complexes produced by fossil natural coal fires are widespread at range-basin junctions worldwide. Large-scale fires accompany the initial orogenic phases as fresh coal-bearing strata become drawn into the aeration zone as a result of crustal deformation. In combustion metamorphism, the protolith melts to different degrees either into ferrous basic paralava or in glassy clinker. The melt rocks have a phase composition favorable for 40Ar/39Ar dating of ignition coeval with the onset of each episode in Late Cenozoic orogenic events. We suggest an algorithm providing correct 40Ar/39Ar age determination of CM rocks followed by well-grounded geological interpretation and test the new approach on melt rocks from the Kuznetsk Coal Basin.Paralava samples were dated by 40Ar/39Ar incremental heating and the isotope ratios were corrected for Ca-, Cl-, and K-derived Ar isotopic interferences. The interpretation of age-spectrum results was checked against internal and external criteria. The former were plateau and isochrone ages and the latter included the so-called “couple criterion” and conventional relative ages inferred from geological and stratigraphic evidence. As a result, we distinguished two groups of dates for combustion metamorphic events bracketed between 1.2±0.4 and 0.2±0.3 Ma. The older ages represent rocks in the western edge of the Prokopievsk-Kiselevsk block of the Salair zone and the younger dates correspond to those in its eastern edge. The reported dates record the time when the fault boundaries of the blocks were rejuvenated during recent activity and the block accreted to the Salair orogenic area as a submontane step. The suggested approach to the choice of objects, classification of rocks, and interpretation of 40Ar/39Ar spectra is universal and can be practiced in any area of combustion metamorphism.