CARBONIFEROUS OPHIOLITE IN THE MIDDLE OF THE QIANGTANG BLOCK, NORTHERN TIBET: GEOCHEMISTRY, SHRIMP U-PB DATING AND TECTONIC IMPLICATIONS

The Qiangtang Block is a major tectonic unit of the Tibetan Plateau. Geological studies of the Qiangtang area have been severely limited by the poor accessibility due to its high elevation (>5000 m) and rudimentary road network. E-W trending mafic and ultramafic rocks occur widely in the middle part of the Qiangtang Block. They were interpreted by some workers as an ophiolite sequence, but such interpretation has not been universally accepted. Due to the importance of the Qiangtang Block in the reconstruction of the tectonic evolution of the Tibet Plateau, we undertook a detailed zircon geochronology and geochemical-isotopic study of these rocks. The new data will be used to discuss the tectonic implications and arbitrate the controversy of the ophiolite interpretation.

The Carboniferous ophiolitic mélange mainly occurs in the Gangma Co and Guoganjianian areas of the central Qiangtang Block. The mélange is composed of serpentite, harzburgite, pyroxenite, cumulate gabbro, basalt, hornblendite and plagiogranite. All the rock types have been metamorphosed to the greenschist facies. SHRIMP U-Pb dating on zircons yielded weighted mean ages of 345 ± 5 Ma and 357 ± 3 Ma for cumulate gabbros, and 356 ± 3 Ma and 354 ± 3 Ma for plagiogranites. Geochemical data indicate that these rocks have typical N-MORB characteristics, with LREE-depleted or flat REE patterns. Whole-rock isotopic analyses gave positive e_Nd(T) values (+1 to +8.3), and zircon e_Hf(T) values range from +4.8 to +13.4 for cumulate gabbros, and +4.2 to +11.5 for plagiogranites. The data allow us to conform that this suite of rocks represents a Carboniferous ophiolitic mélange.

The occurrence of the ophiolitic mélange suites suggests that a Carboniferous Ocean basin (Paleo-Tethys) existed in middle of the Qiangtang Block. The ocean was closed in the Middle to Late Triassic as inferred from the metamorphic ages of eclogite and blueschist that occur nearby. We hypothesize that the ophiolitic mélange in the middle of the Qiangtang block represents the western extension of the Sanjiang Paleo-Tethys ophiolite. A tectonic framework of the Qiangtang block and its implications will be presented.