Densities of metapelitic rocks at high to ultrahigh pressure conditions: What are the geodynamic consequences?

Current geodynamic models of continental collision involving (ultra)high pressure complexes imply that even deeply subducted continental crust is significantly lighter than the ultrabasic upper mantle. To test this implication, we have investigated density changes of major components of continental crust, in particular metagreywacke and metapelite, as a function of pressure and temperature using a Gibbs free energy minimization approach. Pseudosections were calculated for fixed chemical compositions and the P–T range of 10–40 kbar, 600–1000 °C. Selected compositions were those of natural psammopelitic rocks, average crustal components, various theoretical mixtures of quartz, plagioclase, illite, chlorite and Fe,Ti-oxides, and finally mid-ocean ridge basalt and lherzolite for comparison. Calculated densities were presented as density maps (isochors in P–T diagrams).In general, observed densities of psammopelitic rocks increase with rising pressure due to the formation of advancing amounts of garnet, Na-pyroxene, and kyanite. A common assemblage, for instance, at 25 kbar/800 °C consists of phengite, quartz, jadeite, garnet, kyanite, magnetite, and rutile. After overstepping the quartz–coesite transition the density of a mean psammopelitic rock (3.35 g/cm3) is almost as high as that of garnet lherzolite. Calculations with other pelitic compositions demonstrate that the resulting densities (up to 3.5 g/cm3) can even exceed that of a garnet lherzolite due to high contents of garnet.Our calculations suggest that (i) even non-basic crustal material can sink into the Earth's mantle to fertilize it and (ii) the proportion of low-density granitic rocks in deeply subducted continental crust must be relatively high to claim buoyancy forces for a return of this crust to the surface.