Zircon geochronology and Hf isotope compositions of the granitic gneiss from Cuonadong in South Tibet and its insights for the evolution of the Proto-Tethys

The Cuonadong dome is a newly discovered gneiss dome in the Tethys-Himalaya area of southern Tibet. Early Paleozoic augen gneiss is developed in the core of the dome. Based on field investigation, the authors conducted LA-(MC)-ICP-MS U-Pb dating and Lu-Hf isotopic analysis for two samples from the granitic gneiss. Core-mantle-rim texture is well developed in the zircons from the gneiss in CL images:the core is the inherited zircon with erosion embayed texture, the mantle is the igneous zircon with oscillatory zone, and the rim is the black zircon with re-melting metamorphic genesis. The weighted mean ²⁰⁶Pb/²³⁸U age of igneous zircon varies in the range of (500.6±2.6) Ma-(501.1±2.5) Ma, which represents the Early Paleozoic magmatic crystallized age, whereas the Cenozoic re-melting age of margin metamorphic zircon is (37.7±0.5) Ma, which represents the onset of the southern-Tibet detachment. The ε_Hf(t) values and two-stage model ages (T_DM2) of mantle Paleozoic igneous zircons range from -2.1 to +5.3 (averagely +2.2) and from 1.1 to 1.6 Ga (averagely 1.3 Ga), respectively, indicating that the source was derived from the partial melting of the High Himalaya Paleoproterozoic strata. Considering the regional Early Paleozoic magmatism and Cenozoic metamorphic event, the authors hold that the Cuonadong granitic gneiss was formed in the orogeny triggered by the Early Paleozoic Proto-Tethyan Oceanic subduction beneath the Gondwana continent, and recorded the Cenozoic post-collisional metamorphic and anatexis events.