Inhibited eclogitization and consequences for geophysical rock properties and delamination models: Constraints from cratonic lower crustal xenoliths

Abstract

Studies on lower crustal and mantle xenoliths as well as geophysical data provide important information on the cratonic lithosphere. While geothermobarometric calculations of a majority of mantle xenoliths are in agreement with the typically low surface heat flow values of a craton (~40mW/m2), P–T estimates for lower crustal xenoliths deviate significantly from the cratonic geotherms. Independent from the individual cratonic history, the temperatures are ~200–300°C higher than what is expected at the base of the lower crust (~500–600°C at ~1.3–1.6GPa). Possible explanations may be a lack of equilibration to the cratonic geotherm or a relatively recent localized heat input. The presence of granulitic rocks under eclogite-facies conditions which are expected to prevail in the lower cratonic crust has consequences for the interpretation of geophysical rock properties. A mafic granulite which has been preserved under eclogite-facies conditions has densities and P-wave velocities similar to a felsic composition equilibrated to eclogite-facies conditions. Furthermore, phase diagrams calculated from xenolith bulk compositions demonstrate that eclogitization at relatively high temperatures as required for delamination of continental crust can only be triggered at significantly higher pressures than lithostatic at the base of the lower crust. As long as P–T conditions and the rock composition entail the assemblage to be granulitic, the addition of fluid at temperatures above 800°C will not result in eclogitization, but rather in melt generation. This can also lead to an increase in density of up to 3%, however, this is strongly dependent on the amount of water saturation.