Combustion metamorphic(pyrometamorphic)complexes produced by prehistoric natural coal fires are widespread inCentral Asia,namely at the interfaces between mountain systems and the flanking sedimentary basins.Large-sca...Combustion metamorphic(pyrometamorphic)complexes produced by prehistoric natural coal fires are widespread inCentral Asia,namely at the interfaces between mountain systems and the flanking sedimentary basins.Large-scale and prolonged firesaccompanied the initial orogenic stages as unweathered coal-bearing formations became exposed into the aeration zone.Pyrometamorphic rocks are comparable to sanidinite facies rocks in formation conditions and in alteration of sedimentary material but,unlike these,their protolith underwent different melting degrees to produce either ferrous basic paralavas or glazed clinkers.The phasecomposition of the newly-formed melted rocks are favorable for^(40)Ar/^(39)Ar dating of combustion metamorphic events which are coeval tothe onset of the main stage of recent orogenic events.We suggest a new algorithm providing correct ^(40)Ar/^(39)Ar dating ofpyrometamorphic rocks followed by well-grounded geological interpretation.We studied pyrometamorphic rocks in the western Salairzone of the Kuznetsk coal basin where combustion metamorphism under temperatures above 1000℃acted upon large volumes of coal-bearing sediments.Samples of paralavas were dated by the step heating ^(40)Ar/^(39)Ar method checked against internal(plateau andisochrone ages)and external('criterions of couple')mineralogical criterions,and against preliminary dating from geological andstratigraphic evidence.As a result,we distinguished two groups of dates for combustion metamorphic events.The first one(1.2±0.4Ma)is drawn towards the west boundary of Prokopyevsk-Kiselevsk block of Salair zone,while the second one(0.2±0.3Ma)isconfined to its east boundary.The former ages represent rocks in the western edge of the Prokopievsk-Kiselevsk block of the Salair zoneand the latter ages correspond to those in its eastern edge.The dates record the time when the fault boundaries of the blocks wererejuvenated during recent activity and the block accreted to the Salair orogenic area as a piedmont step.These are the first absolute agesobtained for the onset of uplift of the northern edge of the Ahai-Sayan area,the key event of its neotectonic history.The suggestedapproach to the choice of objects,classification of rocks,and interpretation of^(40)Ar/^(39)Ar data is universal and can be practiced in anyarea subjected to combustion metamorphism.展开更多
基金The study was supported by grant SS-4922. 2006. 5 from the president of the Russian Federation for leading science schools ("Siberian metamorphic school")grants 05-05-65036 from the Russian Foundation for Basic Researchit was carried out as part of Integration Project 105 of the Siberian Branch of the Russian Academy of Sciences.
文摘Combustion metamorphic(pyrometamorphic)complexes produced by prehistoric natural coal fires are widespread inCentral Asia,namely at the interfaces between mountain systems and the flanking sedimentary basins.Large-scale and prolonged firesaccompanied the initial orogenic stages as unweathered coal-bearing formations became exposed into the aeration zone.Pyrometamorphic rocks are comparable to sanidinite facies rocks in formation conditions and in alteration of sedimentary material but,unlike these,their protolith underwent different melting degrees to produce either ferrous basic paralavas or glazed clinkers.The phasecomposition of the newly-formed melted rocks are favorable for^(40)Ar/^(39)Ar dating of combustion metamorphic events which are coeval tothe onset of the main stage of recent orogenic events.We suggest a new algorithm providing correct ^(40)Ar/^(39)Ar dating ofpyrometamorphic rocks followed by well-grounded geological interpretation.We studied pyrometamorphic rocks in the western Salairzone of the Kuznetsk coal basin where combustion metamorphism under temperatures above 1000℃acted upon large volumes of coal-bearing sediments.Samples of paralavas were dated by the step heating ^(40)Ar/^(39)Ar method checked against internal(plateau andisochrone ages)and external('criterions of couple')mineralogical criterions,and against preliminary dating from geological andstratigraphic evidence.As a result,we distinguished two groups of dates for combustion metamorphic events.The first one(1.2±0.4Ma)is drawn towards the west boundary of Prokopyevsk-Kiselevsk block of Salair zone,while the second one(0.2±0.3Ma)isconfined to its east boundary.The former ages represent rocks in the western edge of the Prokopievsk-Kiselevsk block of the Salair zoneand the latter ages correspond to those in its eastern edge.The dates record the time when the fault boundaries of the blocks wererejuvenated during recent activity and the block accreted to the Salair orogenic area as a piedmont step.These are the first absolute agesobtained for the onset of uplift of the northern edge of the Ahai-Sayan area,the key event of its neotectonic history.The suggestedapproach to the choice of objects,classification of rocks,and interpretation of^(40)Ar/^(39)Ar data is universal and can be practiced in anyarea subjected to combustion metamorphism.
