Deformation structure and exhumation process of the Laojunshan gneiss dome in southeastern Yunnan of China

Middle-lower crust and mantle rocks are generally widely exposed in metamorphic core complex or gneiss dome,which is an ideal place to study the exhumation process related to regional extension and rheology.The Laojunshan metamorphic complex in southeastern Yunnan is located in a special tectonic position surrounded by the Cathaysia,Yangtze and Indochina blocks.It is composed of different metamorphic-deformation rocks and granitic intrusions.There also are many economic deposits(e.g.,tin and tungsten)that are spatially and genetically associated with the formation and exhumation of the Laojunshan gneiss dome.Based on detailed analysis of macro-and microscopic structure,stress field distribution and deformation condition,the tectonic units of the Laojunshan metamorphic complex show obvious characteristics of doming,as well as of typical structural units of metamorphic core complex.It has strongly deformed metamorphic gneiss core(footwall),detachment fault system and sedimentary cover(hanging wall)with lightly metamorphism and deformation.The footwall of gneiss dome presents a strongly ductile deformation domain,accompanied by different ages of granitic intrusions.The distribution of developed foliation and lineation within granitic gneisses are arc-shaped and radial,respectively,with a nearly N-S trending from the footwall to the hanging wall.Mylonitization of deformed rocks gradually weakens and transits to orthogneiss as it moves away from the detachment fault toward the footwall.The low angle detachment fault between the footwall and the hanging wall shows an arc-like shape feature.Mylonite fabrics are preserved in the deformed rocks of the detachment fault,which are mainly composed of chloritized schist,fault breccia,cataclasite and fault gouge.A large number of normal faults are developed in detachment faults and hanging wall,and their stress fields radiate in an arc around the footwall.Zircon U-Pb ages of amphibolite and granitic gneiss from the footwall range from 445 to 420 Ma,indicating the timing of Caledonian magmatic emplacement and the main formation period of the Laojunshan gneiss dome.U-Pb ages of the zircon metamorphic rims are 241-230 Ma,representing the timing of high temperature metamorphism and shortened deformation of the Indosinian collision.In this period,the Laojunshan gneiss dome experienced the tectonic compression in association with high temperature metamorphism-deformation,which was superimposed by detachment and extensional exhumation in association with intense hydrothermal interaction and mineralization in the late stage.

Three-dimensional electrical resistivity structure beneath the Cuonadong dome in the Northern Himalayas revealed by magnetotelluric data and its implication

The North Himalayan gneiss domes(NHGD),as one of the extensional structures widely distributed across the southern Tibetan Plateau,are an important window for studying post-collisional diastrophism and magmation as well as polymetallic mineralization.However,the deep mechanism for the formation of NHGD remains controversial.The magnetotelluric(MT)method was adopted to study the deep structure of the Cuonadong dome in the Northern Himalayas.The characteristics of the dome were explored by using the MT sounding curves and phase tensors.Three-dimensional(3D)MT inversion was performed to determine the electrical resistivity structure beneath the Cuonadong dome.The preferred 3D electrical resistivity model shows that an obvious low-resistivity anomaly develops beneath the Cuonadong dome which is overlaid by a high-resistivity body and surrounded by an apparent subcircular zone of low-resistivity anomalies.The integrated conductivity(longitudinal conductance)from depths of 1-20 km indicates that the average longitudinal conductance at the core of the Cuonadong dome is about 10,000 S.The high-conductivity anomaly at the core is found to be analogous to that of lava,mainly resulting from the crustal partial melting,and the estimated melt content is 11.0-17.3%.The high conductance surrounding the dome reaches 20,000 S on average,which is mainly attributed to saline fluids.MT results in this study support that the Cuonadong dome experienced magmatic diapirism.Taken together with previous geological and geochemical studies,we suggest that under the east-west(E-W)extensional tectonic setting in southern Tibet,deep crustal partial melting constantly accumulated beneath the dome,and therefore the magmatic diapirism resulted in the formation of the Cuonadong dome.In addition,the MT results also indicate that the development of the Cuonadong dome provides abundant mineralizing fluids and the space for migration of metallogenic fluids for(rare-metal)polymetallic mineralization.