Blackwell Publishing Journal Backfiles 1879-2005
Weathering and transportation studies of the chemical composition of sediments have determined how surface fractionation processes modify the elemental signature due to provenance and tectonic setting of siliciclastic rocks. Although the bulk of the exposed upper continental crust comprises granitoids, metamorphic rocks from the intermediate to lower crust may be, in some geological contexts, the provenance of siliciclastic sediments. A preferential enrichment of the LREE relative to the HREE is observed in weathered, garnet-rich, kinzigitic paragneisses from the Calabrian Arc, 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. However, a significant part of HREE, released during garnet alteration, is trapped by secondary minerals in the clay-sized fraction of the soil, in a manner similar to Pb2+ and Cs+, cations of some concern in environmental geochemistry. In the weathered material monazite is also important in controlling the Eu-anomaly, the negative size of which increases with increasing Th addition. The Eu-anomaly in the clay-sized fraction of the soil is very similar to that of the fresh rock, suggesting that the Eu/Eu* index in pelitic sediments deriving from the intermediate to lower crust may be regarded as a reliable indicator of parental affinity. Other provenance indicators include La/Th, which share the same mineralogical control; indicators of contrasting mafic and felsic provenance, e.g. Sc/Th, should be used with care.
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