THE NATURE OF THE LOWER CONTINENTAL-CRUST OF EUROPE - PETROLOGICAL AND GEOCHEMICAL EVIDENCE FROM XENOLITHS

Studies of lower crustal xenoliths in alkaline volcanics of Europe have revealed that a wide range of rock types exists in the lower crust. These range from eclogites, mafic garnet-bearing and garnet-free granulites, through felsic granulites to high-grade metasediments. Xenolith suites, and by inference the lower crust, are very variable in chemistry and mineralogy. Estimates of lower crustal composition vary according to the proportions of different rock types included in a xenolith suite, from basic (< 50 wt.% SiO2) to intermediate (almost-equal-to 55 wt.% SiO2). Ages within the lower crust are very poorly constrained, but zircons from crustal xenoliths are often younger than the oldest rocks exposed at the surface. This suggests that much of the mafic lower crust has been added to the base of the pre-existing crust by processes of intrusion and underplating. Such additions to the crust have probably occurred during periods of subduction, rifting and orogenic collapse. Evidence from radiogenic and stable isotopes suggests that extensive contamination of mafic magmas by the pre-existing crust has occurred. Seismic velocity constraints indicate that in some parts of Europe, e.g. Germany, mafic granulites are confined to the base of the crust, but in other regions (northwest Scotland, Hungary) they may occupy much of the lower crust. In the northern part of the Baltic shield (Kola) the thick crust contains variably retrogressed and metasomatized eclogites at depths between 30 and 48 km.