Tourmaline-rich pseudomorphs in sillimanite zone metapelites: Demarcation of an infiltration front

Textural features combined with mineral chemistry preserved in metamorphic rocks provide insights into metamorphic reaction mechanisms as well as open vs. closed system processes. Prograde tourmaline-rich muscovite pseudomorp hs after staurolite develop in sillimanite zone merapelites adjacent to per aluminous granitoid intrusives in NW Maine. Tourmalines occur in discrete d omains restricted to central regions of muscovite-rich, quartz-poor pseudom orphs with biotite-rich margins. These tourmaline grains are relatively lar ge (less than or equal to 1.0 mm), lack detrital cores and exhibit only min or compositional zoning, in sharp contrast to matrix tourmaline from other samples. These features suggest fluid-infiltration as the causative mechani sm for the formation of these tourmaline-rich mica pseudomorphs after staur olite. Irreversible thermodynamic models of local reactions and material transport in combination with mineral chemistry allow evaluation of reaction mechani sms that produced these pseudomorphs. Thermodynamic models in the NKCMTFASH OB system mimic the observed textural features if a three-stage process is used. Stage 1: Staurolite replacement is initiated by infiltration of an aq ueous phase that adds K+Na+H2O to the rock with the concomitant removal of Al+Fe. Because the system is initially undersaturated with respect to tourm aline, a pseudomorph containing muscovite with minor biotite develops at th e expense of staurolite. Stage 2: With continued infiltration, concentratio n of B increases, tourmaline saturation is exceeded, tourmaline nucleates a nd grows. Local material transport constraints mandate that tourmaline prec ipitation be spatially restricted to regions of staurolite dissolution. Con sequently, tourmaline forms in clusters at sites containing the last vestig es of staurolite in the pseudomorph core, also evidenced by staurolite incl usions within several tourmaline grains. Resultant domains of staurolite re placement during this stage contain about equal amounts of muscovite and to urmaline. Typical staurolite poikiloblast pseudomorphing reactions require silica transport, matrix quartz dissolves from the surrounding host resulti ng in a local enrichment of biotite and plagioclase at the pseudomorph marg in. Stage 3: Small amounts of sillimanite nucleate and grow throughout the rock. Late-stage aqueous fluids from the adjacent monzonitic intrusive are likely to be the primary B source. Theoretical, textural, and compositional modeling combined with observation al data indicate that boron must have been derived externally from the rock , that the modal amount of tourmaline is very sensitive to the B content of the fluid, that tourmaline is stable throughout the sillimanite zone depen ding on other cation activities and pH of the fluid, and that these pseudom orphs provide insight into B contents of metamorphic fluids and the timing of the B influx. The outer geographic extent of the tourmaline-bearing pseu domorphs marks the boundary of a reactive geochemical front, and thus defin es an advective iso,orad. Interpretation of subtle textural features preser ved in the rock in conjunction with irreversible textural modeling provides a powerful tool with which to understand the chemical evolution of metamor phic rocks and the fluids involved.