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

摘要: Pelitic schists from contact aureoles surrounding mafic-ultramafic plutons in Westchester County, NY record a high-P (~0.8 GPa) high-T (~790 °C) overprint on Taconic regional metamorphic assemblage (~0.5 GPa). The of pelitic sample the innermost aureole Croton Falls pluton, small (<10 km 2 ) gabbroic body, consists quartz-plagioclase- biotite-garnet-sillimanite-ilmenite-graphite-Zn-rich Al-spinel. Both K-feldspar and muscovite are absent, abundant biotite, plagioclase, sillimanite, quartz ilmenite inclusions found within subhedral garnet crystals. Unusually low bulk-rock Na K contents imply depletion alkalic components silica through anatexis melt extraction during heating relative to typical metapelites outside aureole. Thermobarometry nearby samples lacking yields 620-640 °C 0.5-0.6 GPa. In sample, WDS X-ray chemical maps show distinct Ca-enriched rims both matrix plagioclase. Furthermore, biotite have significantly higher Mg concentration than biotite. using GASP garnet-biotite Mg-Fe exchange equilibria adjacent host interior high- Ca rim zone yield ~0.5 � 0.1 GPa ~620 50 °C. Pairs modified ~0.9 ~790 THERMOCALC average P-T calculations similar results for core ~0.1 GPa, ~640 ~80 (~0.9 ~800 ~90 equilibria. assemblages interpreted conditions peak metamorphism Ta- conic event whereas compositions event. high pressures deduced this later inter- preted reflect loading due emplacement allochthons northern Appalachians waning stages (after c. 465 Ma) before metamor- phism (c. 435 Ma). absence metamorphism-induced recrystallization, it is likely that regional-scale would remain cryptic or unrecorded.