PATTERNS OF TRACE-ELEMENT DISTRIBUTION IN CALCIC AMPHIBOLES AS A FUNCTION OF METAMORPHIC GRADE

Abstract

The distribution of the rare earths and other trace elements in calcic amphiboles from the Njurundukan mafic complex, northwestern Baikal region, in Russia, depends on metamorphic grade. Both total REE and La/Yb in amphiboles decrease with decreasing temperature of metamorphism. Consistent partition-coefficients for trace elements in amphibole and coexisting garnet. suggestive of equilibrium, are best developed in granulite-facies rocks. Such a consistent distribution of the REE is disturbed in amphibolite-facies rocks owing to incipient metasomatism. Amphiboles from metasomatic rocks show REE distributions that differ from those in amphiboles belonging to the metamorphic culmination, and commonly are inherited from precursor minerals. This property leads to the establishment of the sequence of mineral formation in metasomatically altered rocks. Comparison with amphiboles from the high-pressure Lapland granulite complex, Baltic Shield, suggests that pressure does not affect REE abundance and distribution in amphiboles.