The Carboniferous ophiolite in the middle of the Qiangtang terrane, Northern Tibet: SHRIMP U-Pb dating, geochemical and Sr-Nd-Hf isotopic characteristics

摘要

Ophiolite plays a key role in identifying paleo-ocean and paleo-plate and rebuilding the evolutionary history of ancient orogea Mafic-ultramafic roclcs are distributed in a broadly E-W direction in the middle of the Qiangtang terrane, northern Tibetan plateau. However, interpretation of these rocks as ophiolite and subsequent tectonic implications have been much disputed, and they were regarded by some to have an origin in a continental rift setting. A detailed zircon dating and geochemical and Sr-Nd-Hf isotopic study of this suite of rocks has been undertaken in order to clarify this important issue. The mafic-ultramafic suite was collected from the Gangma Co and Guoganjianian localities and is composed of cumulate and isotropic gabbro, basalt, actinolitite and plagiogranite. All basaltic rocks are tholeiitic and have low rare earth element (REE) abundances with variable REE patterns and slight negative Nb and Ti anomalies. These features are comparable with those of normal mid-ocean ridge basalts (N-MORB) and/or enriched mid-ocean ridge basalts (E-MORB). The positive whole-rock ε_(Nd)(t) and zircon ε_(Hf)(t) values indicate that these rocks were derived from a long-term depleted mantle source. Zircon U-Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on two cumulate gabbros and two plagiogranites samples yielded Carboniferous ages of 357 ±2.5 Ma, 356.1 ±3.0 Ma,354.7±4.7 Ma and 345.4 ±4.6 Ma. The ophiolite marks a Paleo-Tethys Ocean basin in middle of the Qiangtang terrane, and it is interpreted as the western extension of the Changning-Menglian Paleo-Tethys ophiolite in the eastern margin of the Tibetan plateau.