New progress in the study of the Devon-Carboniferous Nanduan Formation in the eastern Changning-Menglian Belt and its implications for the evolution of the Tethys Ocean

The tectonic transformation from the Proto-to the Paleo-Tethys Ocean has always been a topic of heated debates amongst both domestic and international geological communities. The Nanduan Formation constitutes an important part of the sedimentary strata on the eastern side of the Changning-Menglian Belt. Its depositional age and environments as well as its relation to the evolution of the Tethys Ocean remain controversial, which inhibits the reconstruction the evolution of the Changning-Menglian Ocean and by extension of the East Tethys Ocean. In this contribution, petrological and petrographic characterizations, major and trace element geochemical analyses, and U-Pb dating of detrital zircons were carried out on the metamorphic sandstone samples of the Nanduan Formation in attempts to put the above-mentioned controversy to rest and to provide constraints on the evolution of the Tethys Ocean. The results show that the Nanduan Formation was deposited in a neritic setting on a passive continental margin during the Devonian-Carboniferous, and originated from a mature continental source. Three distinct age peaks, of 560 Ma, 950 Ma and 1130 Ma respectively, have been identified in the detrital zircon age spectrum of the Nanduan Formation, which can be correlated with the upper Paleozoic detrital zircon age spectrum of the Lhasa terrane from the northern margin of Australia. This in turn indicates an affinity of the Nanduan Formation to the Gondwanan continents. However, the paleo-geographical location of the terrane during the Devonian-Carboniferous transition remains to be constrained. The two age peaks of 440 Ma and 950 Ma identified from the Devonian Manxin Formation on the eastern margin of the Baoshan block differ significantly from those in the Nanduan Formation. The newly-obtained Devonian detrital zircon age spectrums on both sides of the Changning-Menglian Ocean in this study, therefore provide strong sedimentological evidences for the continuity of the Proto-and the Paleo-Tethys Oceans.