Evolvement And Geological Significance Of Yiwulüshan Metamorphic Core Complex, Western Liaoning Province, China

A series of Mesozoic metamorphic core complex (MCC) are located in andaround North China Craton. The Yiwulüshan MCC is located at northeast margin ofNorth China Craton, and neighboring with east segment of Xing’an-Mongolianorogenic belt and Songliao Basin. This paper makes sure the characteristics ofgeometry and deformation about the Yiwulüshan MCC, based on analysis ofmacrostructure, microstructure and quartz Lattice Preferred Orientation (LPO). TheYiwulüshan MCC is comprised of metamorphic core, middle ductile rheological bedand supra-detachment basin, which are segregated by ductile detachment shear zoneand master detachment fault, respectively. The MCC was formed by three stages ofdeformation, the early stage was extensional shearing top-to-the SW at amphibolitefacies in middle crust, the middle stage was weakly E-W directional compression, andthe late stage was extensional shearing top-to-the NWW at greenschist facies in uppercrust. Due to the SHRIMP U-Pb dating of the zircon in syn-tectonic granite and40Ar/39Ar dating of the muscovite in tectonites, we suggest that the three stages ofdeformation were occurred at Middle-Late Jurassic, from late Late Jurassic to earlyEarly Cretaceous, and late Early Cretaceous.Tectonic-magmatic model of the MCC is founded via combining the analysis offinite strain and kinematics vorticity with the occurrence and deformation of thesyn-extension granite and wall rock. The approximately level extension top-to-the SWat Middle-Late Jurassic induced the decompression melting of middle-lower crust.Extensional shearing led to the finite strain of the ductile shear zone formed at thisstage behaves as the domination of monaxial elongate strain. Subsequently, theperpendicular pressure from magma to wall rock caused the increase of plane strain.Finally the strain types of the ductile shear zone shows elongate strain near planestrain in logarithmic Flinn diagram. And the monaxial elongate and pressure ofmagma induced that the ductile shear zone is pure dominated general shearing. Compression between the two stages of extension was weak and without magmatism,so that the strain type of the ductile shear zone was influenced unconspicuous. Thelate Early Cretaceous magmatism in the MCC was weaker than Middle-Late Jurassic.The horizontal extension was occurred at the beginning then it transferred to verticaluplift and formed the MCC. The deformation process during late Early Cretaceous ledto the ductile shear zone formed at this period exhibit plane strain with equally simpleand pure shearing.In all the pole Mole circle diagrams, we have ξ1> ξ2, indicating that thinnedshear zones formed in extensional setting during Middle-Late Jurassic and late EarlyCretaceous. From the thickness, occurrence and thinning rate of the ductile shearzones, the reduced crustal thickness due to ductile shearing was estimated to beapproximately3.72km. Based on structural analysis, contact relationships, andgeochronological data, we propose that intense extensional detachment contributed tothe stratigraphic gap along the ductile detachment shear zone at the contact betweenProterozoic metasedimentary rocks and the Archean basement. After correlating thestratigraphy of the Yiwulüshan area with regions adjacent to it, we conclude that a1468—1690m-thick section of Proterozoic is missing along the ductile detachmentshear zone. We estimate that the crustal thickness in the Yiwulüshan region has beenreduced by more than5.41km during two stages of extensional detachment.The two stages of extension in Yiwulüshan area are both accompanied by high-Kcalc-alkaline volcanic rocks and bimodal igneous rocks. Contrasting between thespatial and temporal variations of Mesozoic volcanic rocks and characteristics ofmagmatism in research area, combined with the comparability between thetransformation of the kinematic senses of Yiwulüshan MCC and the drifting directionsof plates in western Pacific, indicates that the Mesozoic extensions in Yiwulüshanarea was controlled by subductions of Paleo-Pacific plate, and the subduction atMiddle-Late Jurassic was weak. Furthermore, influence from the subductions of thePaleo-Pacific plate beneath the Eurasian continent behaved magmatism of mantle andlower crust in eastern China during Early-Middle Jurassic, and the extensional deformations in crustal scale which were begun to occurred at Middle-Late Jurassicand extensively developed at Early Cretaceous.