USING APATITE AS A MONITOR OF FLUID EVOLUTION AND INFLUX DURING METAMORPHISM: AN EXAMPLE FROM THE BARROVIAN ZONAL SEQUENCE IN DUTCHESS COUNTY, NEW YORK

Apatite is a ubiquitous accessory phase used to infer intensive/extensive variables, including composition of the fluid attending metamorphism as a function of distance from an intrusion in contact environments and documenting ingress of fluids in regional environments. Here we present data from Dutchess County, NY, where rocks of similar lithology were sampled along a metamorphic field gradient that preserves a Barrovian zonal sequence (Chl – Bt – Grt – St – Ky – Sil – Sil+Kfs). Detailed studies of the metamorphic evolution are available and do not need to be duplicated for this study, in which our objective is to evaluate Ap chemistry as a function of metamorphic grade in rocks of similar bulk composition.

The protolith comprises shales (Austin Glen Member, Normanskill Formation), and the metamorphism is Taconic (c. 445 Ma; Hames et al., 1991), although rocks E of the Hudson River may have been affected by an Acadian overprint. Higher grade zones record Ts of 500-700^oC at 5-6 kbar (Whitney et al.,1996). Ap in lower-grade rocks (protolith to St zone) is commonly zoned, with mottled cores (low in F, Fe and Mn) imaged using backscattered electrons and cathodoluminesence and irregular boundaries, whereas Ap in higher-grade rocks (St to Sil+Kfs zones) appears unzoned with subhedral boundaries.

Overall, F and OH concentrations in Ap are similar irrespective of metamorphic 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 peak P-T conditions, inferred to be due in part to infiltration of fluid associated with modification of Grt compositions (Whitney et al., 1996). This infiltration is reflected in high HAp mole fractions in Ky zone Ap. Cl is high and variable in Ap from the protolith (CAp ~0.018), and decreases, albeit not systematically, with increasing metamorphic grade. The Cl-rich nature of Ap in the protolith may be indicative of growth in a Cl-rich apatite-saturated seawater environment, or of alteration of Ap in the immediate post depositional environment. Our results demonstrate: 1) Cl in Ap decreases with increasing metamorphic grade in pelites; 2) Ap in granulite facies rocks is rich in F, irrespective of bulk composition (our work on the mafic Barro-Alto UHT metamorphic complex, Brazil; Piccoli et al., 2003); 3) Ap has the potential to yield information about post-peak fluid infiltration.