Establishment of the North Himalayan double gneiss domes: evidence from field identification of the Cuonadong dome, south Tibet

The Cuonadong dome, which is firstly reported in this study, is exposed as an east-southern important part of the North Himalayan gneiss domes (NHGD), . The Cuonadong dome is located in the southern part of the Zhaxikang ore concentration area, which is divided into three tectono-lithostratigraphic units by two circle faults around the dome, which is geometrically from upper to lower (or from outer to inner) in order of cap rocks, decollement system and core, with the outer fault being upper detachment fault and the inner fault being lower detachment fault. The Cuonadong dome is a magmatic orthogneiss and leucogranite mantled by orthogneiss and metasedimentary rocks, which in turn are overlain by Jurassic metasedimentary and sedimentary rocks. The grades of metamorphism and structural deformation increase towards the core, which is consistent with the low to high degree metamorphic schist, tourmaline granitic biotite gneiss, garnet mica gneiss, and mylonitic quartz mica gneiss of the Ridang Formation. Their typical metamorphic minerals include garnet, staurolite, sillimanite and kyanite. The Cuonadong dome has preserved evidence for four major deformational events:southward thrust, early approximately N-S extensional deformation, main approximately E-W extensional deformation, and late collapse structural deformation around the core of the Cuonadong dome, which are consistent with four groups of lineation, respectively. The form of the Cuonadong dome resulted from the main E-W extension which resulted from eastward flow of middle or lower crust from beneath Tibet accommodated by northward oblique underthrusting of Indian crust beneath Tibet. The establishment of the Cuonadong dome has enhanced and enriched the significance of the E-W extension of the NHGD, which is further divided into two structural dome zones according to the different extensional directions:approximately NS extensional North Himalayan gneiss domes (NS-NHGD) and E-W extensional North Himalayan gneiss domes (EW-NHGD).