Halogen-rich scapolite and biotite; implications for metamorphic fluid-rock interaction

Abundant Cl-rich scapolite and biotite indicate high Cl activities during greenschist-through amphibolite-facies regional metamorphism of metasedimentary rocks northwest of the Idaho batholith. Biotite-zone granofels contain up to 40 modal percent Cl-rich scapolite. The strict stratigraphic control on the occurrence of scapolite and the fine-scale interlayering of Cl-rich and Cl-poor metasedimentary rocks suggest that scapolite was formed during regional metamorphism by reaction of plagioclase with calcite and halite. Mass-balance constraints on the formation of scapolite show that, prior to metamorphism, sedimentary units containing 10 modal percent halite along restricted sedimentary horizons, 0.5 to 20 cm thick, would have been sufficient to form the most scapolite-rich layers. The abundance of Cl-rich phasesd ecreasesa bruptly with the appearanceo fzoisite-bearing assemblagesin the metasedimentaryr ocks, suggestingin filtration of low-Cl H₂O at higher grades. Scapolite compositions range from 32 to 62 equivalent anorthite (EqAn) and 0.7 6 to 0.06 X_Cl. There is a general increase in EqAn and decrease in X_Cl with increasing metamorphic grade. However, local variations in scapolitec omposition suggesth at metamorphic fluid compositions were heterogeneousa nd internally bufered. Biotites coexisting with scapolite contain up to 0.51 wt% Cl (biotite grade) and 2.58 wt% F (kyanite + sillimanite grade). Biotite compositions indicate significant differences in log (f_HF/f_HCl) values of fluids in equilibrium with low-grade, biotite- and carbonate-rich granofels. Differences in the inferred metamorphic fluid compositions are observed, in some cases, down to a scale of < 1 cm.

Cl is strongly partitioned into an aqueous fluid relative to solid phases and is removed by metamorphic devolatilization and infiltration by dilute fluids. Therefore, the occurrence of bufered activity gradients in Cl, as indicated by scapolite and biotite compositions, suggeststh at metamorphic fluid flow was highly channelizeda nd aqueousf luid-rock interaction was limited. High and variable Cl activities were not smoothed out by aqueous fluid-rock interaction through two regional metamorphic events.