Contact Relationship between the high-pressure and ultrahigh-pressure metamorphic belts in the southern Sulu high-pressure-ultrahigh-pressure metamorphic terrane and its geological significance

Abstract：The determination of the contact relationship between the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belts at the southern margin of the Sulu HP-UHP metamorphic terrane has great significance for establishing the tectonic framework of the area. The basal conglomeratic bed of the Jingping Group in the HP metamorphic belt at the southern margin of the Sulu HP-UHP metamorphic terrane unconformably overlies the Qushan granite-gneiss in the southern part of the UHP metamorphic belt. The gravels in the conglomeratic bed underwent late-stage plastic deformation but their geological characteristics still display the unique properties of sedimentary gravels in the lower part of the strata. In addition, isotope chronological study has revealed that the protoliths of the Qushan granite-gneiss and Jinping Group metamorphic rocks formed at 859 Ma and 814 Ma respectively, indicating that the contact between the Qushan granite-gneiss and Jinping Group metamorphic rocks is unconformable. They together underwent HP-UHP metamorphism-deformation in the process of the late-stage tectonic movement. During their exhumation the Jingping Group metamorphic rocks were subducted toward the WNW, forming a ductile shear zone on the unconformity.