Further support for a Cretaceous age for the feathered-dinosaur beds of Liaoning,China:New 40 Ar÷ 39 Ar dating of the Yixian and Tuchengzi Formations

Chinese Science Bulletin (Impact Factor: 1.37). 01/2002; 47(2):136-139. DOI: 10.1360/02tb9031

ABSTRACT We report new 40Ar÷39Ar dating results obtained from total fusion and incremental-heating analyses of sanidine and biotite from three tuffs found
interbedded within the fossil-bearing deposits of Liaoning, northeast China. The first is a new sample of the Bed 6 Sihetun
tuff from the Yixian Formation, previously dated by our team as middle Early Cretaceous, and recently considered by Lo et
al., partially reset due to metamorphism from a nearby basaltic sill. The second is the Yixian Bed 9 tuff from Hengdaozi considered
by Lo et al. to be unaffected by metamorphism and whose age, based on total fusion 40Ar÷39Ar dating of biotite, argues for a Jurassic age for the Yixian Formation. The third tuff is a previously undated tuff from
the upper part of the underlying Tuchengzi Formation. Single crystal total fusion 40Ar÷39Ar analyses of the Sihetun sanidine showed homogeneous radiogenic Ar, Ca÷K ratios, excellent reproducibility and gave a mean
age of 125.0±0.18 (1SD)±0.04 (SE) Ma. Single sanidine crystal total fusion 40Ar÷39Ar analyses of the Hengdaozi tuff gave a mean age of 125.0±0.19 (1SD)±0.04 (SE) Ma, which is indistinguishable from the Sihetun
tuff. The Tuchengzi Formation tuff gave a mean age of 139.4±0.19 (1SD)±0.05 (SE) Ma. Detailed laser incremental-heating analyses
of biotite from Sihetun, Hengdaozi, and Tuchengzi tuffs show disturbed Ar release patterns and evidence of trapped argon components.
We conclude from these analyses that the total fusion dates on biotite by Lo et al. are erroneously old and isotopic dating
of both biotite and sanidine from tuffs of the Yixian Formation point to a middle Early Cretaceous age. The upper part of
the Tuchengzi Formation can be referred to the Early Cretaceous.

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