Source Rocks and Provenances of the Ladoga GroupSiliciclastic Metasediments (Svecofennian Foldbelt, Baltic Shield):Results of Geochemical and Sm— Nd Isotopic Study

摘要

Siliciclastic metasediments of the Ladoga Group that is the Kalevian stratotype in Karelia correlative with the Kalevian siliciclastic succession in Finland are studied in terms of geochemistry and Sm–Nd iso-topic systematics. The results obtained show that rocks in the Ladoga Group lower part are enriched, as compared to rocks of the upper part, in Ti0_2, Fe_20_3, MgO, Cr, Co, Ni, and Sc, being comparatively depleted in Al_20_3 and Th that is a result of compositional changes in provenances. The Sm- Nd isotopic data evidence that siliciclastic sediments of the Ladoga Group have accumulated during the erosion of rocks, which originated at the time of the Archean and Early Proterozoic crust-forming processes. Siliciclastic material with the Archean and Early Proterozoic T_Nd(DM) values, which are characteristic of metasediments in the group lower part, was derived respectively from granite gneisses of the Archean basement in the Karelian megablock of the Baltic Shield and from volcanic rocks of the Sortavala Group. Volcanic rocks of island-arc terranes of the Svecofen-nian foldbelt represented main source of siliciclastic material that accumulated in upper part of the succession.