Abstract:

Situated in an archetypal continental collision zone the Tibetan Plateau developed world-class porphyry Cu and carbonate REE metallogenetic belts, yet there is no scientific consensus about the mechanism of continental collision control of Cu-REE mineralization in the region. The outstanding issues include the exact trigger mechanism for melting of the thickened lithosphere, the relationship between the lithospheric framework and Cu-REE metallogenesis, and the source of Cu, REE and volatiles and their depositional processes. To better understand the deep process and lithospheric architectural control of Cu-REE metallogenesis, this paper summarizes the lithospheric architecture of the Gangdese forward and oblique collision zones had been imaged and analyzed using joint inversion of surface wave and satellite gravity data, combined with magnetotelluric (MT) array and geochemical data. During the India-Asia collision in the Cenozoic the subducting Indian lithosphere experienced significant tearing, allowing asthenospheric upwelling to rework the overlying Asian lithosphere and cause melting. The resulting ultrapotassic melt ascended and accumulated at the bottom of the crust, providing high heat flow and volatiles for the melting of the juvenile lower crust. In the hydrous melt, amphibole fractionation led to melt oxidation, promoting Cu recycling and enrichment. The above results revealed three key factors controlling the formation of collisional porphyry deposits: moderate-angle subduction, slab tearing, and reactivation of the sulfide-enriched juvenile lower crust. At the margin of the Yangtze craton, Sanjiang oblique collision zone, vertical upwelling and horizontal flow of asthenosphere, driven by subduction of the Indian plate or mantle convection, caused thermal erosion and partial melting of the cratonic continental lithosphere. The lithosphere beneath the craton margin was REE enriched due to prior metasomatism by REE/CO₂-rich fluid from recycled oceanic sediment, and enriched REEs were carried by ascending carbonate melt along the lithosphere discontinuity (e.g., strike-slip fault, rift) to form large scale carbonate REE deposits. Without prior lithospheric metasomatism, carbonatic, ultrapotassic and mafic melts produced from the melting of the cratonic lithosphere had limited potential to form carbonate REE deposits.

Key words: continental plate subduction, continental collision, slab tearing, Hf isotopic mapping, juvenile crust