Experimental study of phase relationships in metasediments at high pressure and high temperature. Application to subducted continental crust
Experiments have been carried out on a metagreywacke and a petapelite in a large P-T range in order to study phase relationships of such rocks in the subducted continental crust and to determine the variations of mineral compositions. In the case of metagreywacke, the mineralogical assemblage phe + cpx + grt + qtz/coes is stable above 2.3 GPa.Contrary to basic rocks, at T<900°C, a hydrous mineral -phe-remains stable at UHP up to 8.0 GPa. The presence of a hydrous mineral at peak metamorphic conditions and during the exhumation has profound consequences on both partial melting and retrogression. The influx of HéO-rich fluid is not essential for retrogression and partial melting of subducted-exhumed acid rocks. The survey of the main ultra-hight pressure metaporphic regions confirms the role of partial melting during exhumation in most cases.