MINERALOGIC REACTION ZONES AT A CALC-SILICATE METAPELITE INTERFACE - AN EXAMPLE OF TRACE-ELEMENT MOBILITY IN A METAMORPHIC ENVIRONMENT

Metasomatic interaction on a cm scale between calc-silicate pods and t he enclosing sillimanite + biotite + tourmaline gneiss at Partridge Br east Lake, northern Manitoba, Canada, led to the development of an inn er (by calc-silicate rock), hornblende-rich reaction zone and an outer , biotite-rich zone. The boundary between the reaction zones is interp reted as the original calc-silicate/metapelite interface. Compared wit h its metapelitic protolith, the biotite zone shows a two- to twenty-f old depletion in the concentrations of incompatible trace elements (no tably the light rare earths, U, Th, Nb, Ta, Zr and Hf). In contrast, t he relative concentrations of trace elements remained nearly constant during the mineralogical transformation of the calc-silicate rock to t he hornblende zone. The depletion of trace elements in the biotite zon e is attributed to the dissolution of accessory phases (e.g. monazite) . Although stable at the metamorphic conditions (approximately 600-650 -degrees-C at approximately 4.5 kbar) prevalent during metasomatism, M g-rich tourmaline is absent in the biotite zone, suggesting that eithe r the pH or composition (e.g. the (Al + Si)/(Ca + Mg + Fe) ratio) of t he aqueous fluid phase was inappropriate for the preservation of this mineral.