Mineralogic Reaction Zones at a Calc-Silicate/Metapelite Interface: An Example of Trace Element Mobility in a Metamorphic Environment

J. V. Owen, J. Dostal and B. N. Church
Department of Geology, Saint Mary's University, Halifax, N.S., Canada B3H 3C3
Department of Energy, Mines and Petroleum Resources, Victoria, B.C., Canada V8V 1X4

Abstract: Metasomatic interaction on a cm scale between calc-silicate pods and the enclosing sillimanite+ biotite + tourmaline gneiss at Partridge Breast Lake, northern Manitoba, Canada, led to the development of an inner (by calc-silicate rock), hornblende-rich reaction zone and an outer, biotite-rich zone. The boundary between the reaction zones is interpreted as the original calc-silicate/metapelite interface. Compared with its metapelitic protolith, the biotite zone shows a two- to twenty-fold depletion in the concentrations of incompatible trace elements (notably the light rare earths, U, Th, Nb, Ta, Zr and Hf). In contrast, the relative concentrations of trace elements remained nearly constant during the mineralogical transformation of the calc-silicate rock to the hornblende zone. The depletion of trace elements in the biotite zone is attributed to the dissolution of accessory phases (e.g. monazite). Although stable at the metamorphic conditions (∼600–650°C at ∼ 4.5 kbar) prevalent during metasomatism, Mg-rich tourmaline is absent in the biotite zone, suggesting that either the pH or composition (e.g. the (Al + Si)/(Ca + Mg + Fe) ratio) of the aqueous fluid phase was inappropriate for the preservation of this mineral.

Keywords: metasomatism • reaction zone • calc-silicate • metapelite • trace element • Manitoba

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 205-214; DOI: 10.1180/minmag.1994.058.391.03
© 1994, The Mineralogical Society
Mineralogical Society (www.minersoc.org)