Experimental evidence of decompression melting during exhumation of subducted continental crust

Experiments have been carried out on a metagreywacke at 800, 850 and 900 degrees C, in the pressure range 0.5-5 GPa to locate the solidus and the eclogite/amphibolite facies transition in felsic rocks, identify the nature of the reactions responsible for major mineralogical changes, and determine the proportions of phases as a function of pressure. The mineral assemblage phengite + clinopyroxene + garnet + quartz/coesite is stable above 2.3 GPa while biotite + plagioclase + garnet + quartz is stable below 2 GPa. The model reaction for the eclogite/amphibolite facies transition in metagreywackes is: Phe + Cpx + Qtz = Bt + Pl + Grt + melt with melt on the low pressure-high temperature side of the reaction. The modal proportion and calcium content of garnet change with pressure. Both decrease from 5 to 2.5 GPa, then increase at the eclogite/amphibolite facies transition, and finally decrease with decreasing pressure below 2.3 GPa. The grossular content in garnet is thus a potential marker of the eclogite/amphibolite facies transition during retrogression. The modal proportion of melt progressively increases with decreasing pressure from 5 to 2.5 GPa, then shows a sudden and marked increase between 2.5 and 2.3 GPa, and finally decreases between 2.3 and 1 GPa. Thus, a melting pulse occurs at the eclogite/amphibolite facies transition during decompression of subducted continental crust. A survey of the main UHP metamorphic regions and the P-T paths followed during their geotectonic history indicates that partial melting may have played a role during their exhumation. A striking feature of retrogressed UHP felsic rocks is that garnet rims are commonly enriched in grossular. Our experiments explain this observation and demonstrate that a grossular-rich growth zone in garnet is not necessarily indicative of highest pressures reached during metamorphism but may correspond to a decompression stage.