Granulite facies lower crustal xenoliths from the Eifel,West Germany: petrological and geochemical aspects

Petrographic, petrological and geochemical data for 16 mafic meta-igneous, granulite facies lower crustal xenoliths from the East Eifel were collected in order to develop a model for the lower crustal history for this region. The xenoliths consist of plagioclase±amphibole±clinopyroxene±garnet±orthopyroxene±scapolite + opaque minerals±apatite±rutile±zircon. Garnet has reacted to a variable extent with plagioclase and clinopyroxene to form a corona of plagioclase_II+ amphibole + orthopyroxene_II. Pyroxenes and plagioclases show complex zoning patterns with regard to Al and Ca which can be interpreted in terms of P, T history. Decreasing temperature and pressure conditions are recorded by decreasing Al in clinopyroxene rims coexisting with increasing anorthite contents in plagioclase rims and the breakdown of garnet. In addition, a young heating event that affected the granulites to different degrees is inferred from the complementary Ca-zoning patterns in clino- and orthopyroxenes. Rare earth element (REE) patterns of whole rocks together with the trends displayed and fractionated liquids. REE analyses of the mineral separates display equilibrium partitioning patterns for amphibole and clinopyroxene, although isotopic data show that amphibole contains externally-derived Sr and Nd components not recognized in other minerals. At least a 4-stage history for the granulites is recorded: (1) intrusion and crystal fractionation of basaltic magmas in the lower crust, probably accompanied by crustal assimilation, (2) granulite facies metamorphism, (3) a decrease in temperature and pressure, and (4) a later heating event. The complicated thermal history is reflected in Sm–Nd mineral isochron ages which range from about 170 Ma down to about 100 Ma and cannot be assigned to distinct geological events. These ages correlate with inferred temperatures; the low ages are measured for xenoliths with the highest temperatures. In some cases the young heating event is likely to be responsible for partial resetting of the mineral isochrons.