DETRITAL ZIRCON INVESTIGATION INTO THE LATE PROTEROZOIC TO EARLY PALEOZOIC TECTONIC HISTORY OF THE QING SHAN REGION OF THE NAN SHAN, NORTHEAST TIBET
Bektas, Serdar 1982-
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The northeastern Tibetan Plateau consists of the Qilian Shan, Nan Shan and Qiadam terranes. This region has been extensively studied due to its complicated tectonic history, and has yielded different tectonic models for the Late Proterozoic to Early Paleozoic. These models include the Archipelago model, the North-facing magmatic arc model, and more complicated models such as the Multiple-accretionary and the Back-arc extension models. These models propose that the region was formed by various oceanic basin subductions and collisions between the different terranes. In contrast, the Broad Convergent model proposes that the terrane assemblages which grew progressively southwestward by addition of accretionary complexes and magmatic arcs. However, there is no concensus about subduction polarities, and tectonic formation of region. Analysis of detrital zircon from these terranes provides a potential opportunity to differentiate between some of these models. In order to address the tectonic evolution of the southern Nan Shan, I conducted U-Pb detrital zircon analyses on five samples of Proterozoic to Early Paleozoic lithologies in the Qing Shan mountains at the southern margin of the Nan Shan-Qilian Shan region. One sample from a Proterozoic unit yields a strong Neoproterozoic age population (~800 Ma) and might represent the regional basement or local igneous sources that resulted from rifting of continental crust. This sample reveals that the basement beneath Qing Shan is as old as late Proterozoic. This result is consistent with the North-facing Magmatic Arc model, the Archipelago, and the Multiple-accretionary model, since the models suggest that the basement age of the Nan Shan region consists of late Proterozoic units. Zircon grains from the Proterozoic sample may have been derived from magmatism related to the breakup of Rodinia due to their mafic origin. All other samples yielded Early Paleozoic (Ordovician to Silurian) maximum depositional ages, and broadly similar age distributions. The Early Paleozoic detrital zircon ages are interpreted to be derived from arc magmatism, as most of the grains yield an igneous U/Th signature. These ages might be related to the southward subduction of the North Qilian Ocean beneath the Qilian Shan, which is consistent with the North-facing Magmatic Arc model, or northward subduction of an ocean basin between the Qaidam and the Nan Shan terranes during the Late Cambrian to Early Silurian, which is consistent with the Archipelago and Multiple-accretionary models.