The composition of the lower crust and the nature of the continental Moho - xenolith evidence

Lower crustal xenoliths provide information on the nature and depth of the continental Moho, the composition of the lower crust, and its physical properties. Minimum criteria for recognition of lower crustal xenoliths are that similar rock types do not occur in nearby outcrops and that mineral assemblages and P-T estimates are consistent with inferred lower-crust conditions. A worldwide survey of xenolith suites indicates that the lower continental crust beneath rift zones and orogenic belts (ancient or young) consists predominantly of mafic granulites, with subordinate amounts of felsic meta-igneous rocks and rare metasediments. In these regions the Moho appears to be a 5-20 km transition zone, in which mafic rocks are interlayered with other lithologies. The proportion of felsic rocks (spinel lherzolite) becomes greater at depth. The 'seismic Moho' lies near the bottom of this zone, well below the 'petrological Moho'. Seismic data, a few xenolith suites, and some exposed deep crustal sections, suggest that the cratonic lower crust may be less than mafic, and the Moho more clearly defined.-J.M.H.