PALEOZOIC EVOLUTION OF THE NORTHEASTERN TIBETAN PLATEAU: A NEW MODEL INVOLVING THE NORTH QAIDAM OPHIOLITE

The Qilian and Qaidam terranes of the northeastern Tibetan Plateau consist of four basic tectonic units from NE to SW: the North Qilian Ordovician Franciscan-type ophiolite, the Central Qilian Precambrian passive continental margin sequence, the South Qilian Early Paleozoic metasedimentary sequence, and the North Qaidam Early Paleozoic ultrahigh-pressure metamorphic belt. The North Qilian ophiolite is thought to have been the suture zone between the Qilian/Qaidam terranes and the Sino-Korean and Tarim cratons during the Late Silurian–Early Devonian collision.

Unfortunately, Paleozoic history of the area has been extensively masked by Mesozoic and Cenozoic deformation, making it difficult to determine the true evolution of the northeastern Tibetan Plateau. A series of tectonic models have been proposed for the formation of this area, including: i) the collision of multiple island arcs and micro-continents; ii) an evolving mélange complex enlarged by tectonic accretion and strike-slip faulting, and iii) a single N or S facing subduction zone.

Here we propose a different tectonic model for the formation of the northeastern Tibetan Plateau based on new data regarding an additional oceanic suture zone (the North Qaidam ophiolite) exposed along the southern edge of the Qaidam terrane. The North Qaidam ophiolite consists of serpentinized peridotite, a thick meta-gabbro sequence with plagiogranite lenses, meta-basalts with apparent sheeted dike geometries, and a thin, discontinuous sequence of vesicular basalt interlayered with chert and marble; it is associated with a thick metasedimentary unit interpreted as flysch. The ophiolite is exposed in the hanging wall of the Luliang Shan detachment zone and has been complexly deformed and interfolded with the North Qaidam UHP metamorphic belt across the detachment. The rocks have been metamorphosed to epidote-amphibolite facies. Zircons from two plagiogranite lenses reveal the Cambrian age of the ophiolite and record geochemical signatures indicative of ocean crustal origins. Major and trace element geochemistry of the meta-gabbros and meta-basalts reveal LREE-enrichment, depletion of HFSE’s, low wt % TiO2, and enrichment in slab-derived trace elements, suggesting formation in a supra-subduction zone setting.