Evidence for non-lithostatic pressure in subducted continental crust

Continental crust buried during collisional orogeny typically records pressures of 3 GPa or lower; however, pressures much higher than this are recorded locally, which would suggest burial to mantle depths. Deep continental subduction is not observed in active orogens and should be hindered by the positive buoyancy of sialic crust relative to the mantle. Non-lithostatic pressure caused by mechanical contrasts between rock types provides an alternative explanation for extreme pressures recorded in buried continental crust; however, its occurrence and significance in natural systems is debated. Mechanical pressure heterogeneities were proposed specifically to explain extreme pressures of c. 5.5 GPa obtained in enstatite eclogite veins in the archetypal subducted continental terrane, the Western Gneiss Complex (WGC) in Norway. In this study, we use Lu-Hf garnet geochronology to test when, and thus, in what part of the burial cycle of the WGC the enstatite eclogite assemblages actually equilibrated. The results show that equilibration occurred at c. 393 Ma, which is much later than the typical ages obtained from 'normal' eclogites in the WGC and represents a time when the terrane was already at crustal depths (< 2.5 GPa). Finite element modeling of mechanical pressure distribution demonstrates that late extreme pressure excursions are feasible for the given rock system and could explain the seemingly spurious conditions recorded in these unusual rocks. The recognition of non-lithostatic ultrahigh-pressure in deeply buried continental crust allows crucial simplification of models for continental subduction and validates the importance of rock thermo-mechanics in interpreting observations from collision zones.