Norwegian Garnet Websterites: Analogues for Mantle Metasomatism?

In the southern Norwegian Caledonides, garnet websterite forms small bodies enclosed in gneiss, or within layered complexes also comprising peridotite, clinopyroxenite and eclogite, probably originating as crustal, mafic-ultramafic
intrusions. Websterites are variably enriched in phlogopite and carbonate and have graphite-bearing fluid inclusions, and are frequently very coarse grained. Calculated pressures of 4-5GPa [1] are consistent with the presence of micro-diamond [3]. Bulk geochemistry suggests an olivine-rich protolith, but
subsequent enrichment in Si, C, K, P, other LIL and HFSE. Sr isotopic data for cpx [2] show unsupported, high radiogenic Sr, suggesting an influx of fluids from the enclosing gneisses. Interlayered eclogite is cut by veins of zircon- and apatite-rich glimmerite and coesite-garnetite with depletion haloes of
clinopyroxenite. Coarse magnesite is patchily distributed within the websterite. Near Molde [3] microdiamond-bearing garnetite, glimmerite and websterite form veins cutting peridotite, demonstrating fracture-metasomatism of an olivinerich
protolith. The Norwegian external garnet websterites therefore provide rare evidence for metasomatism of ultramafic rocks by siliceous, C-O-H-rich fluids/melts generated in the host gneisses under UHP conditions.
Characteristics of the websterites are similar to those of xenoliths attributed to metasomatism of mantle peridotite. Thus, importantly, they provide field-scale examples of metasomatic features not available in mantle xenoliths, and
could open new perspectives on mantle metasomatism, diamond genesis and crust-mantle interactions.