关键词: 医巫闾山, 变质核杂岩, 石英EBSD组构, 锆石U-Pb年代学, 韧性剪切带, 伸展变形

Abstract: The Yiwulüshan area in western Liaoning constitutes a key Late Mesozoic deformational tectonic unit in the northeastern segment of the North China craton (NCC). Its multi-stage deformation events preserve critical information regarding the transition of the regional tectonic regime. Through systematic analysis of the macro- and microstructural characteristics of strongly deformed rocks in Yiwulüshan metamorphic core complex, combined with quartz EBSD fabric analysis, this study identifies two distinct phases of extensional ductile shearing events: i.e., Late Jurassic high-temperature (550-650 ℃) ductile shearing (under low amphibolite facies conditions) and Early Cretaceous low-temperature (400-500 ℃) ductile shearing (under greenschist facies conditions). The early-stage deformation is characterized by LS-type tectonites dominated by dextral shearing, exhibiting prominent simple shear properties. In contrast, the late-stage deformation forms L=S type tectonites with sinistral shearing behavior, where the contributions of simple shear and pure shear are comparable. Zircon U-Pb geochronological constraints indicate that these two phases of events occurred during 156.9-146.7 Ma (Late Jurassic) and 132.6-126.8 Ma (Early Cretaceous), respectively. Analysis of the recrystallized grain size of quartz reveals that the high-temperature shear zone was associated with lower differential stress (20.53-22.16 MPa) and higher strain rate (8.41×10-13-5.56×10-11 s-1), whereas the low-temperature shear zone showed the relative slower strain rate (differential stress: 20.62-24.07 MPa; strain rate: 1.34×10-15-9.69×10-13 s-1). The early high-temperature extensional deformation is linked to the transition of the tectonic regime in the northeastern NCC—from the north-southward closure of the Mongol-Okhotsk Ocean to the subduction of the Paleo-Pacific plate. The late low-temperature extensional deformation is related to the roll-back of the subducting Paleo-Pacific plate.

Key words: Yiwulüshan, metamorphic core complex, quartz EBSD fabric, Zircon U-Pb geochronology, ductile shear zone, extensional deformation