METAMORPHIC HISTORY OF THE PRECAMBRIAN HONEY BROOK UPLAND AND ASSOCIATED PALEOZOIC COVER SEQUENCE, SOUTHEASTERN PENNSYLVANIA PIEDMONT: REVIEW AND REFINEMENT

The Honey Brook Upland is one of several Precambrian crustal blocks present in the SE Pennsylvania Piedmont. This study focuses on retrieval of recorded variations in temperature and ambient fluid composition in these rocks, and temporal constraint of these variations, through study of major-mineral reaction textures and phosphate accessory-phase paragenesis. Previous metamorphic models for the Honey Brook Upland identify: i) Anorthosite intrusion and coeval (?) volcanism. ii) Metamorphism of the plutonic-volcanic complex to upper-amphibolite-/lower-granulite-facies conditions during the Grenville Orogeny. iii) Cooling of the Honey Brook Upland below biotite Ar-Ar closure temperature by ~850 Ma. iv) Late Proterozoic thermal disturbance as indicated by a biotite Rb-Sr thermal closure age of ~570 Ma. v) Temporally-unconstrained greenschist-facies retrogression (Honey Brook Upland) and metamorphism of its quartzite-carbonate cover sequence during the Paleozoic. This study adds the following refinements, many of which correlate with earlier observations: i) Re-integrated host and exsolved pyroxene in granulite-facies meta-plutonic rock (opdalite) yields intrusive temperatures of 915-945°C. ii) Modal Cl-apatite and three-stage Cum:Qtz+Bt+Cc:Bt+(K, Cl)-Hbl coronas developed on igneous Opx indicate the presence of H2O-CO2-KCl fluids during granulite-facies metamorphism at an upper temperature limit of ~720-730°C. iii) Compositionally-delimited domains within monazite in amphibolite-facies felsic gneisses yield growth ages of 1009±4 Ma (wtd. avg. ± 2 s.e.), 965±6 Ma, and 876±10 Ma; maximum monazite-xenotime temperatures for the first domain range between 620-700°C, and the low-actinide, high Th/U ratios (200-250) of monazite rims are indicative of growth during low-temperature hydrothermal alteration at ~880 Ma. iv) Secondary monazite in altered coronas in meta-opdalite is dated at 586±88 Ma, and texturally-associated, oriented monazite+xenotime needles in altered Cl-apatite yield equilibration temperatures of 325-425°C, correlating with the Late Proterozoic thermal disturbance mentioned above. v) Coexisting monazite and xenotime in Paleozoic Chickies Quartzite records Middle Devonian (378±28 Ma) and Lower Permian (272±44 Ma) growth at temperatures of 260-320°C.