Garnet amphibolites from the Ganzi-Litang fault zone, eastern Tibetan Plateau: mineralogy, geochemistry, and implications for evolution of the eastern Palaeo-Tethys Realm

摘要

The Ganzi-Litang fault zone, an outstanding tectonic element in the eastern Tibetan Plateau has been intensively debated as an in-situ suture zone marking relict of a subducted Palaeo-Tethyan oceanic crust or a failed intracontinental rift. This paper reports the garnet amphibolites discovered along the Ganzi-Litang fault zone, eastern Tibetan Plateau. These garnet amphibolites are characterized by the garnet-hornblende-rutile-sphene-plagioclase-quartz assemblage. Conventional geothermobarometry figures out the metamorphic temperature and pressure conditions at 582-626 degrees C and 1.61-1.82 GPa, respectively. Geochemical analysis (no Nb-Ta deletions and left-inclined to flat patterns of rare-earth elements) indicates that the garnet amphibolites could represent metamorphic product of the mid-ocean-ridge (MORB)-type mafic rocks that were contaminated by a mantle plume. The protolith of the garnet amphibolites was dated at 236 Ma using in-situ U-Pb zircon method, and the retrograde metamorphism was dated at 218 Ma using in-situ U-Pb sphene method. A comprehensive analysis combined with the development of the Palaeo-Tethys Ocean and the Yidun arc through geologic time indicates a Triassic to Early Jurassic age (236-195 Ma) for the metamorphism of the garnet amphibolites. The low geothermal gradient of 9.8 degrees C/km and the N-MORB nature of the garnet amphibolites suggest a subduction-zone environment for the high-pressure metamorphism. Therefore, the Ganzi-Litang fault zone is a Palaeo-Tethyan suture separating the Yidun arc and the Songpan-Ganzi terrane, representing the relics of a branch of the Palaeo-Tethys Ocean that was contaminated by a mantle plume.