Elemental mobility during the weathering of exposed lower crust: the kinzigitic paragneisses from the Serre, Calabria, southern Italy

Weathering and transportation studies of the chemical composition of sedime nts have determined how surface fractionation processes modify the elementa l signature due to provenance and tectonic: setting of siliciclastic rocks. Although the bulk of the exposed upper continental crust comprises granito ids, metamorphic rocks from the intermediate to lower crust may be, in some geological contexts, the provenance of siliciclastic sediments. a preferen tial enrichment of the LREE relative to the HREE is observed in weathered, garnet-rich, kinzigitic paragneisses from the Calabrian Are, southern Italy . This fractionation is due mostly to the mineralogical control exerted by monazite, which is concentrated in the silt-size fraction of the soil. Howe ver, a significant part of HREE, released during garnet alteration. is trap ped by secondary minerals in the clay-sized fraction of the soil, in a mann er similar to Pb2+ and Cs+, cations of some concern in environmental geoche mistry. In the weathered material monazite is also important in controlling the Eu-anomaly, the negative size of which increases with increasing Th ad dition. The Eu-anomaly in the clay-sized fraction of the soil is very simil ar to that of the fresh rock, suggesting that the Eu/Eu* index in pelitic s ediments deriving from the intermediate to lower crust may be regarded as a reliable indicator of parental affinity. Other provenance indicators inclu de La/Th, which share the same mineralogical control; indicators of contras ting mafic and felsic provenance, e.g. Sc/Th, should be used with care.