Microstructural features of ultrahigh-pressure garnet-peridotite from the Donghai district in the Sulu terrane: New evidence of rapid exhumation.

The microstructural characteristics of garnets, olivines and diopsides in ultrahigh-pressure garnet-peridotite, sampled from the borehole of Chinese Continental Scientific Drilling (CCSD), have been studied by transmission electron microscopy (TEM). Free dislocations with high density, dislocation walls and sub grains have been observed in some very local areas of garnet, olivine and diopside. Dislocation density of garnet is about 1.34 x 10(9)/cm(2), and the dominant slip system of it is 1/2 < 111 > {110}. A partial melting belt, consisting of melt and magnetite grains, has been observed in a garnet crystal containing small dislocation loops. Dislocation density of olivine is about 2.64 X 10(8)/cm(2). Ilmentite and magnetite exsolutions are present in olivine. Polygonal areas with small offset of orientation between each other are found in diopside. This indicates that, after the garnet-peridotite entered the subduction channel, strong plastic deformation took place in the garnet-peridotite during the ultrahigh-pressure metamorphic peak stage and the first rapid exhumation stage; this explains the abundance of free dislocations, dislocation walls and sub grains in garnet, olivine and diopside. Then, partial melting and static state recrystallization occurred in these minerals, so the dislocations in the most part areas of garnet, olivine and diopside vanished. The partial melting belt existing in garnet indicates that the subsequent exhumation process must had a very high uplift rate and a very high cooling rate.