AN UNUSUAL BUT POTENTIAL POWERFUL MICROSTRUCTURE FOR LITHOSPHERE EVOLUTION

Fossil UHP continental subduction zones, now exposed at the surface of the earth, consist dominantly of subducted continental crust with subordinate mantle fragments (subduction zone- and / or relict peridotites (BRUECKNER & MEDARIS, 2000)) incorporated within them. In such terranes characteristic mineral indicators for UHPM conditions (CHOPIN, 2003), such as coesite, micro-diamond, ellenbergite or super-silicic garnet microstructures can be combined with more classical petrological techniques to make estimates about maximum exhumation depths of these fossil continental subduction zones. In principle the characteristic UHP mineral indicators / classical petrological techniques should be applied to all rock types: i.e. the subducted continental crust AND the intercalated mantle fragments. If this is not done the two rock types may originate from different depth levels putting severe constraints on possible exhumation mechanism. In this respect the super-silicic (or majoritic) garnet microstructure forms an unusual, but potential powerful, UHP mineral indicator. Sofar it has been recognized in mantle xenoliths, brought to the surface by kimberlite-related magmas, and in orogenic peridotites associated with UHP subduction zones formed by continental collision. In the latter case the super-silici