Late-stage orogenic loading revealed by contact metamorphism in the northern Appalachians, New York.

Pelitic schists from contact aureoles surrounding mafic-ultramafic plutons in Westchester County, NY record a high- P (~0.8 GPa) high- T (~790 °C) contact overprint on a Taconic regional metamorphic assemblage (~0.5 GPa). The contact metamorphic assemblage of a pelitic sample in the innermost aureole of the Croton Falls pluton, a small (<10 km2) gabbroic body, consists of quartz-plagioclase-biotite-garnet-sillimanite-ilmenite-graphite-Zn-rich Al-spinel. Both K-feldspar and muscovite are absent, and abundant biotite, plagioclase, sillimanite, quartz and ilmenite inclusions are found within subhedral garnet crystals. Unusually low bulk-rock Na and K contents imply depletion of alkalic components and silica through anatexis and melt extraction during contact heating relative to typical metapelites outside the aureole. Thermobarometry on nearby samples lacking a contact overprint yields 620-640 °C and 0.5-0.6 GPa. In the aureole sample, WDS X-ray chemical maps show distinct Ca-enriched rims on both garnet and matrix plagioclase. Furthermore, biotite inclusions within garnet have significantly higher Mg concentration than matrix biotite. Thermobarometry using GASP and garnet-biotite Mg-Fe exchange equilibria on inclusions and adjacent garnet host interior to the high-Ca rim zone yield ~0.5 ± 0.1 GPa and ~620 ± 50 °C. Pairs in the modified garnet rim zone yield ~0.9 ± 0.1 GPa and ~790 ± 50 °C. Thermocalc average P-T calculations yield similar results for core (~0.5 ± ~0.1 GPa, ~640 ± ~80 °C) and rim (~0.9 ± ~0.1 GPa, ~800 ± ~90 °C) equilibria. The core assemblages are interpreted to record the P-T conditions of peak metamorphism during the Taconic regional event whereas the rim compositions and matrix assemblages are interpreted to record the P-T conditions during the contact event. The high pressures deduced for this later event are interpreted to reflect loading due to the emplacement of Taconic allochthons in the northern Appalachians during the waning stages of regional metamorphism (after c. 465 Ma) and before contact metamorphism ( c. 435 Ma). In the absence of contact metamorphism-induced recrystallization, it is likely that this regional-scale loading would remain cryptic or unrecorded. [ABSTRACT FROM AUTHOR]