A thermomechanical model of diapriric flow during UHP metamorphism and its applications in using P-T-t data to infer upper-mantle rheology and thermal structures

Some of the UHP terranes in the world occurred within the coeval magmatic arcs during oceanic subduction. This observation requires upward transport of the subducted continental materials from a depth of > 70-100 km across the upper mantle wedge to the upper crust. Upward transport of the UHP rocks may have been accomplished by rising diapirs launched from the oceanic subducting slabs, which carried a large amount of continental materials mixed with fragments of oceanic crust and upper mantle. To test this hypothesis, we developed a quantitative thermomechanical model that integrates existing knowledge on thermal structures of modern oceanic subduction zones with the mechanics of diapir transport. Using this model, we are able to track P-T and T-t paths of individual diapirs and compare them with the observed P-T and T-t paths from the North Qaidam metamorphic terrane in northern Tibet. The main physical insight gained from our modeling is that the large variation of the observed P-T paths from North Qaidam can be explained by a combination of temporal and spatial variation of thermal structure and mechanical strength of the upper mantle wedge above the early Paleozoic Qilian subduction slab. Hotter P-T trajectories can be explained by a high initial temperature (~800°C) of a diapir that traveled across a relatively strong mantle wedge (i.e., olivine activation energy E = 350 kJ/mol for dry olivine), while cooler P-T paths may be explained by a diapir with a low initial temperature (~700°C) that traveled through a weaker mantle wedge, with its strength at least two orders of magnitude lower than that of dry olivine. The latter condition could have been achieved by hydraulic weakening of olivine aggregates in the mantle wedge by fluid percolation through the whole mantle wedge.