40Ar/39Ar Age Constraints on Two NNE-Trending Ductile Shear Zones, Yanshan Intraplate Orogen, North China Craton

Abstract

Localized regions of large-magnitude extensional strain typified by metamorphic core complexes are prominent geologic and physiographic features of the Yanshan intraplate orogen. Compared to typical metamorphic core complexes defined in the Basin and Range Province of the western United States, an exceptional structural feature is the coexistence of seemingly incompatible earlier strike-slip ductile shear zones and later normal faults in metamorphic core complexes within the orogen. Two such important extensional structures are the Louzidian ductile shear zone and the related Louzidian normal fault in the Kalaqin metamorphic core complex, Inner Mongolia, and the Waziyu ductile shear zone and the related marginal normal fault of the Fuxin Basin in the Yiwulüshan metamorphic core complex, western Liaoning Province. Field observations and structural analyses reveal that these two NNE-trending ductile shear zones were genetically related to sinistral strike-slip and extensional faulting, and formed under greenschist-facies conditions. Two samples from the Louzidian ductile shear zone yield a 40Ar/39Ar biotite plateau age of 133 Ma and a 40Ar/39Ar K-feldspar plateau age of 126 Ma, whereas three synkinematic biotites from the Waziyu ductile shear zone give a 40Ar/39Ar plateau age span of 130-116 Ma. These plateau ages record the time of ductile deformation and mylonitization with sinistral strike-slip and extensional movement along the Louzidian and Waziyu ductile shear zones, and thus establish an Early Cretaceous age for such structures. The similarity of these ages to the time when regional strike-slip occurred on the Tan-Lu fault, large-scale gold metallogenesis took place in Eastern China, and the catastrophic mass mortality event resulted in exceptional preservation of the famous Jehol biota imply that these ages not only provide important timing constraints on the first-phase unroofing of the metamorphic core complexes within the Yanshan orogen, but also suggest strong genetic links between these structural phenomena and important metallogenic and paleoenvironmental events.