BIOTITE AND APATITE CHEMISTRY IN METAPELITES FROM THE TACONIC BARROVIAN ZONAL SEQUENCE IN DUTCHESS COUNTY, NEW YORK

Biotite and apatite occur in metapelites from greenschist to granulite facies, and serve as sources and/or sinks for halogens and water during metamorphism. These phases may be used to infer intensive/extensive variables, including composition of fluid attending metamorphism, and to document ingress of fluid. We present Bt and Ap chemistry from metapelites from a Barrovian metamorphic field gradient (Bt–Grt–St–Ky–Sil–Sil+Kfs). Detailed studies of the metamorphism are available and are not duplicated in this study, in which our objective is to evaluate Ap and Bt chemistry as a function of metamorphic grade in rocks of similar composition.

Bt is present from the Bt to Sil+Kfs zones. Mg/(Mg+Fe) in Bt is variable (from ~0.33 in Grt to ~0.75 in Sil zone). Bt inclusions in Grt and St have Mg/(Mg+Fe) indistinguishable from Bt in the matrix. Experimental studies show F in Bt increases with metamorphic grade at the expense of OH. In this study, F/OH (molar) in Bt is variable, ranging between 0.11 and 0.17 from Bt through Sil zones, except in the lower St zone, where higher values of F/OH of 0.26 occur. Bt from the highest grade rocks has the lowest F/OH (0.074-0.096), which may suggest retrograde interaction with externally-derived fluid. Ti in Bt from Gr and Ilm- or Rt-bearing metapelites may be used to estimate equilibration temperature (Henry & Guidotti 2005). Ti in Bt from the Grt zone yields T of 525^oC and from Sill zone yields T of 660^oC.

F and OH concentrations in Ap are similar irrespective of grade (FAp ~0.80 mole fraction; HAp ~0.19), except for Ap in the Ky zone, where FAp = 0.69 and HAp = 0.30. Ky zone rocks record anomalously low P-T conditions, inferred to be due in part to infiltration of fluid (Whitney et al., 1996), which is reflected in the high HAp mole fraction. Cl is high and variable in protolith Ap (CAp ~0.018), and decreases with increasing grade.

Cl-rich Ap in the protolith may indicate growth in Cl-rich apatite-saturated seawater, or alteration of Ap in the immediate post-depositional environment. Our results show that Cl in Ap decreases rapidly with increasing grade in metapelites, and Ap and Bt have the potential to yield information about post-peak fluid infiltration. Post-peak fluid infiltration in higher grade rocks is consistent with W-vergent Taconic thrusting of higher over lower grade rocks and dehydration as the source.