END(T) variation in an Early Proterozoic actinolite skarn, Sweden, reflecting f elsic volcanite-seawater interaction

Abstract

Two neodymium isotopic analyses of least-altered metavolcanites from the Hjulsjö area and six analyses of albitized and actinolitized metavolcanites from Ställbergstorp farm south of Hjulsjö show variations in εNd(T) values that point to two-stage mixing involving seawater- and rock-dominated fluid-rock interaction. In a first stage felsic volcanites are albitized and leached in LREE. Some of these extremely LREE leached felsic volcanites show very negative εNd(T) values. Thus, leaching occurred by a fluid with a long crustal history, probably modified seawater containing Nd derived from an Archaean erosional area. In a second stage redeposition of LREE previously leached from the volcanic pile accompanies the formation of actinolite aggregates in the leached metavolcanite and results in an increase of εNd(T).