Title: Halogen-rich scapolite and biotite; implications for metamorphic fluid-rock interaction
Abstract: Abundant Cl-rich scapolite and biotite indicate high Cl activities during greenschistthrough 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 ofscapolite 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 l0 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 ofCl-rich phases decreases abruptly with the appearance ofzoisite-bearing assemblages in the metasedimentary rocks, suggesting infiltration of low-Cl HrO at higher grades. Scapolite compositions range from 32 to 62 equivalent anorthite (EqAn) and 0.7 6 to 0.06 Xo. There is a general increase in EqAn and decrease in X., with increasing metamorphic grade. However, local variations in scapolite composition suggest hat metamorphic fluid compositions were heterogeneous and internally bufered. Biotites coexisting with scapolite contain up to 0.51 wto/o Cl (biotite grade) and 2.58 wto/o F (kyanite + sillimanite grade). Biotite compositions indicate significant differences inlogffir/fr.:) values of fluids in equilibrium with low-grade, biotiteand carbonate-rich granofels. Differences in the inferred metamorphic fluid compositions are observed, in some cases, down t o a s c a l e o f < l c m . 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, suggests that metamorphic fluid flow was highly channelized and aqueous fluid-rock interaction was limited. High and variable Cl activities were not smoothed out by aqueous fluid-rock interaction through two regional metamorphic events.
Publication Year: 1989
Publication Date: 1989-08-01
Language: en
Type: article
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Cited By Count: 102
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