Eclogites in the Sclm: Are Any Subducted?

W.L. Griffin, Suzanne Y. O'Reilly and N.J. Pearson GEMOC, Dept of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia [email protected] It has become conventional wisdom that xenoliths of eclogite and garnet pyroxenite derived from cratonic subcontinental lithospheric mantle (SCLM) represent fragments of subducted ocean floor, implying that the SCLM has grown by a lithosphere-stacking mechanism involving repeated shallow subduction beneath cratons. However, the inferred behaviour of these ancient “slabs” is markedly different from what we observe in the modern Earth; seismic-tomography images clearly show slabs descending steeply to at least 660 km depth, rather than layering at shallow depths beneath the continents. Xenolith suites in basalts from young terrains (Tectons: eg E. China, E. Australia, western USA, Hawaii) commonly contain garnet pyroxenites that display exsolution microstructures clearly reflecting their origin as high-T cumulates or crystallised melts. Similar microstructures, though less common, also occur in cratonic eclogites. The compositional field of Tecton garnet pyroxenites can be expressed by mixing of high-T, high-Al cpx ± opx ± gnt. This compositional field is coincident with that of cratonic eclogites; both rock types are distinct in composition from the clearly crustal eclogites found in HP/UHP metamorphic belts.