The granulite to eclogite transition beneath the eastern margin of the Australian craton

Xenolith suites in kimberlitic and basaltic dykes on the eastern margin of the Australian craton (EMAC) are dominated by mafic rock types, with subordinate spinel and garnet lherzolite and felsic granulite. The mafic rocks include garnet websterites, mafic granulites (+ or -kyanite) and eclogites. A range of microstructures demonstrates the transformation of primary igneous assemblages to granulites and eclogites, through exsolution, corona growth, breakdown of plagioclase and deformation. Eclogites and granulites contain similar compositional ranges of garnet and clinopyroxene ; P-T estimates indicate that these two rock types coexisted over the depth range from 35 to 75 km. The P-T array defined by the EM AC xenoliths extends from ca. 10 kbar/800 degrees C to 22 kbar/1020 degrees C, and shows the strong curvature typical of advective heat transport. The mafic rocks show a wide range in composition, and demonstrate the control of bulk composition on the stability of eclogite vs. granulite mineral assemblages. Ne-normative rocks with high normative olivine + pyroxene are mainly eclogites ; hy-normative rocks with high normative feldspar are mainly granulites. The microstructural and mineralogical changes are interpreted to reflect cooling of igneous intrusions in the lower crust and uppermost mantle. As cooling proceeded towards the steady-state conductive geotherms, an increasing proportion of mafic compositions entered the eclogite stability field. The conversion of granulite to eclogite was also influenced by kinetic factors, as shown by the arrested reactions preserved in many samples. The compositional diversity shown by the EMAC mafic xenoliths suggests that the igneous protoliths are the products of several episodes of magmatic underplating and tectonic activity, but have equilibrated toward a common geotherm, sampled at the time of entrainment.