REE GEOCHEMISTRY IN METAMORPHIC AMPHIBOLES

Abstract

The study of the geochemistry of rare-earth and trace elements in calcic amphibole from the Nyurundukan mafic complex, northwestern Baikal region, showed that their abundance and distribution depend on the grade of metamorphism. The total REE abundance and La/Yb ratio in amphiboles tend to decrease with the decreasing temperature of metamorphism. The constant partition coefficients of trace and rare-earth elements between coexisting amphiboles and garnets provide evidence of equilibrium and are attained best in the granulite-facies rocks. Such a regular REE distribution is disturbed in the amphibolite-facies rocks by metasomatic processes. The chondrite-normalized REE patterns of the metasomatic amphiboles differ from those of metamorphic-peak amphiboles, often being inherited from the earlier mineral. This makes it possible to determine the sequence of mineral formation in the metasomatically altered rocks. The comparison of REE patterns in amphiboles from the Nyurundukan Complex and the high-pressure Lapland granulite complex at the Baltic Shield suggests that the REE abundances and pattern are independent of pressure, being mainly defined by the temperature of metamorphism.