Metasomatic alteration of a felsic metavolcanite to an actinolite skarn near Stallbergstorp: evidence for high LREE mobility

Abstract

Felsic metavolcanites of the Lower Leptite Group of the 1.9-1.8 Ga Svecokarelian Bergslagen Supracrustal Series, Sweden, are locally pervasively altered into actinolite-magnetite skarns, showing euhedral magnetite blasts in an amphibole matrix. The alteration comprises two stages: after complete albitization the felsic metavolcanites are characterized by extremely low K and high Na contents and LREE depleted patterns relative to least altered metavolcanites. Incipient replacement of albite by amphibole leads to enrichment in MgO, CaO and FeO*, whereas the light rare earth element (LREE) abundances remain unchanged. Subsequent massive actinolite growth in the matrix and in veins is characterized by a strong enrichment in MgO, CaO, FeO* and LREE.