Metasomatic evolution of amphibolites during cooling and uplift - The origin and significance of zonation in amphiboles and other hydrous minerals

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Sّrensen, Bjّrn Eske
Holding Date 08 October 2008

This study addresses the metasomatic alteration of ortho-amphibolites, in the Froland area in the Bamble sector, South Norway. High salinity brines with near constant salinities of c. 30 wt% solvents, infiltrated and metasomatised the amphibolites throughout cooling and uplift. Potassic alteration and biotite formation comprise the earliest deducible alteration event and is seen as overgrowth on amphiboles. Alteration begins by formation of ferrotschermakite/ferropargasite rims on the amphibole cores. Rims gradually becomes richer in Mg and depleted in Fe, K, Na, Al and Cl and, finally, terminates with Cl poor actinolite (XMg c. 0.9). Simultaneously biotite experiences metasomatic alteration that is strongly correlated with the coexisting calcic amphibole.
The compositional changes reflect the complex interaction between brine fluids and hydrous minerals during cooling and uplift. Accordingly, the brine fluids fully control the composition of Fe-Mg silicates by metasomatosis. Therefore the models arguing that the Cl content of biotite is a function of the original Fe/Mg ratio of the mineral, fluid composition and PT-conditions (Munoz and Swenson 1981) can not explain the Froland amphiboles and biotites because silicate and whole rock chemistries are changed by interaction with the fluid. The composition of the halogen bearing Fe-Mg silicates is partially controlled by the aqueous fluid composition along with P and T. The compositional zoning reflects interaction with a fluid having constant halogen contents during gradually changing PT-conditions.
In intensively altered areas the original amphibolite mineralogy is entirely replaced. Here we observe the large-scale metasomatic processes in a small scale version, for example following this pattern toward the centre of a vein: Amphibolite with small amount of biotite (host rock), Zone1: biotite-plagioclase zone, Zone2: amphibole-plagioclase, Zone3: Plagioclase + calcite + apatite zone, Zone3a: pyrrhotite + ilmenite (rutile +titanite) ± amphibole + calcite+ chalcopyrite + apatite. This small scale observation reflects how ore forming elements, such as Cu, Mo, Au and Fe, are mobilised from the one part to an other part of the Bamble shear zone complex at a larger scale (see also Cameron, et al. 1993).
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