DOES THE PALEOZOIC TECTONIC HISTORY OF QIANGTANG RECORD THE FORMATION AND BREAK-UP OF A DEVONIAN SUPERCONTINENT?

Detailed geological studies within the Qiangtang terrane of north-central Tibet provide new insights about its pre-Cenozoic tectonic evolution. In the Gangma Co area (34°N, 84°15'E), a Late Triassic - Early Jurassic metamorphic core complex juxtaposes melange in the footwall against Upper Carboniferous passive margin strata in its hanging wall. Hanging-wall strata include glaciomarine deposits that link Qiangtang to Gondwana during this time and are intruded by mafic sills. The footwall melange includes a sliver of amphibolite-facies (700 ± 50°C, 7 ± 2 kbar) gneiss that is inferred to represent Qiangtang crystalline basement. Twelve zircons from the gneiss yield U-Pb ion-microprobe ages that range from 419 to 556 Ma. Amphiboles from the same gneiss yield ⁴⁰Ar/³⁹Ar ages between 355 and 380 Ma. The older zircon ages are similar to those from gneisses within the Lhasa terrane and the Lesser and Greater Himalaya, and imply that Qiangtang was contiguous with these terranes along the northern margin of Gondwana during earliest Paleozoic time. The amphibole and younger zircon ages are consistent with high-grade metamorphism prior to Carboniferous time. The tectonic evolution of the Kunlun terrane is similarly characterized by middle Paleozoic orogenesis followed by deposition of upper Paleozoic passive margin strata. We propose a Devonian continental collision between the Qiangtang and Kunlun terranes. Deposition of Carboniferous - Permian passive margin strata within both the Kunlun and Qiangtang terranes resulted from subsequent opening of the Paleo-Tethyan Ocean between them. Late Permian - Early Jurassic closure of this ocean resulted in a second accretion of the Qiangtang terrane to the southern Eurasian margin. If a similar two-stage collision history is envisioned for the North and South China blocks, this hypothesis implies that the triangular geometry of the Songpan-Ganzi oceanic basin may be attributed to break-up of a Devonian supercontinent (Kunlun + North China + South China + Qiangtang + Lhasa + Gondwana), as opposed to being the sole consequence of collision between three irregularly-shaped blocks.