TECTONIC EROSION, DIAPIRIC FLOW, AND EMPLACEMENT OF UHP ROCKS DURING OCEANIC SUBDUCTION: ORIGIN OF THE NORTH QAIDAM UHP GNEISSES

The origin of UHP gneisses in North Qaidam of western China has been attributed to early Paleozoic continental collision. This interpretation is inconsistent with the fact that they occurred as smaller outcrops (<20x20 km2) separated 70-150 km from one another. The early Paleozoic collision model is also inconsistent with the observation that arc magmatism of 510-380 Ma spatially overlaps the UHP belt and is temporally coeval with UHP metamorphism (495-443 Ma). In addition, the gneiss complex hosting UHP rocks lies unconformably below Sinian-early Ordovician shallow marine sediments and yields 950-850 U-Pb zircon ages, suggesting that the early Paleozoic UHP rocks were tectonically embedded within the Proterozoic basement. We suggest the following tectonic process for the formation of the North Qaidam UHP rocks. (1) 510-500 Ma: southward subduction of the Qilian ocean initiated an arc on the Qaidam continental terrane. (2) 500-440 Ma: accretionary materials and pieces of Qaidam continental crust were tectonically eroded by the subducting oceanic slab and were carried down to depths of 75-90 km. (3) Subducted continental materials together with fragments of mantle and oceanic crustal materials rose to middle/lower crustal levels within the arc as large diapirs (10-20 km in diameter) and ascended rapidly (5-10 cm/yr?) from the top of the subducting slab, allowing nearly isothermal decompression (600-700°C) of the UHP rocks from >2.8 GPa to 0.5-1.0 GPa within probably 1-2 m.y. (4) 440-400 Ma: Intra-arc thrusting brought UHP rocks from the middle/lower crust to upper-crustal levels. (5) 40-0 Ma: Southward thrusting during the Indo-Asian collision finally exhumed the UHP rocks to the surface. Our model implies that emplacement of UHP rocks during oceanic subduction may be a common process. However, UHP rocks are rarely observed in an arc setting because they are volumetrically small and have to be exhumed to the surface by large-magnitude thrusting or detachment faulting.