Uranium and boron distributions related to metamorphic microstructure-evidence for metamorphic fluid activity

Abstract

The distribution of uranium and boron in polymetamorphosed, granulite facies schists and gneisses has been studied using particle track methods. The concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into: (1) the behaviour of U and B in metamorphism, (2) the activity of a fluid phase in the metamorphic processes and (3) the nature of chemical processes during schistosity development. A low concentration of primary U occurs in micro-inclusions of apatite and zircon (many are metamict) in the granulite facies M1 assemblage. This assemblage which lacks B, except for zoned sillimanite, has undergone a localized retrograde metamorphism RM1 characterised by hydrous alteration products containing abundant U and B. The RM1 metamorphism is attributed to fluids generated during granulite facies dehydration reactions. A schistosity S2 defined by M2 fibrolite aggregates overprints the M1 events. It is associated with (1) intragranular U concentrated in M2 apatite and titanium bearing minerals and (2) abundant intergranular U within the fibrolite aggregates. High B contents also occur with the fibrolite. The S2 schistosity appears to develop in a metamorphic environment containing a fluid enriched in U and B.