Petrographic and geochemical evidence for major and trace element metasomatism in recrystallized f elsic metatuffites from the Persberg area, Bergslagen, Central Sweden

Abstract

Felsic rocks with randomly orientated biotite crystals up to 1.5 cm long, occurring NE of Persberg, Sweden, are strongly recrystallized felsic metatuffites. There is a gradual transition from the surrounding weakly recrystallized to the strongly recrystallized metatuffites. Recrystallization resulted from hydrothermal alteration of the metavolcanic rocks, which also caused the development of mafic aggregates, mainly consisting of biotite and/or cordierite. Considerable mobility of major, trace and rare earth elements accompanied the hydrothermal alteration: the strongly recrystallized metatuffites are depleted in K, Rb, Ba and Sr, and enriched in Na, Fe, Mg, Zn, Zr, REE, Hf and W. Compared to their host rocks the mafic aggregates are depleted in Na, Si, Y, W, Th and REE, and enriched in K, Fe, Mg, Sc, Cr, Co, Ni, Ti, Zn, Rb, Sr, Ba and Cs. Mobility of Zr is illustrated by the morphological characteristics of the zircons present in the strongly recrystallized metatuffites and mafic aggregates. Many of the larger grains show a conspicuous zoning and it is argued that unusual clusters of small zircon grains, often occurring on the boundaries of quartz and albite grains, are probably non-magmatic. The hydrothermal alteration is thought to be driven by the ascending nearby Horsjö granite and enhanced by the Hyttsjö gabbro-tonalite-granite which intruded shortly after the Horssjö granite and probably before the collapse of the hydrothermal system.