PETROGENESIS OF THE WILLSBORO-LEWIS WOLLASTONITE SKARNS, NORTHEAST ADIRONDACK MTS, NEW YORK

Detailed studies of stable isotope, trace element, and major element chemistry have been made of the Willsboro-Lewis skarn belt, N.E. Adirondack Mts, NY. Low d ¹⁸O (-2 to 2‰, VSMOW) wollastonite skarn (WS)(Wo+Gt+Cpx) and associated garnet skarn (GS) and garnet pyroxene skarn (GPS) developed adjacent to massif anorthosite, and were metamorphosed to granulite facies during the Ottawan phase of the Grenville orogeny.

Garnet from WS ranges from X_Adr=0.1 to 0.9. High X_Adr is correlated to lower d ¹⁸O. WS garnet is in isotopic equilibrium with wollastonite. Results indicate equilibrium Grs-Adr fractionation (~0.8‰) in agreement with previous studies. Additionally, Ti (1-0 wt% TiO₂) and Zr (1200-50ppm) content decreases with increasing Fe in garnet (8-30 wt% Fe₂O₃). Garnet from GS occurs in a restricted compositional range (X_Adr=0.17 to 0.30). Euhedral, 1-3 cm diameter, oscillatory zoned garnet with correlated sub-mm to mm-scale zonation of X_Adr and d ¹⁸O occur in the GS. Higher X_Adr is correlated with higher d ¹⁸O, as well as greater Ti and Zr content. Concentric garnet zones preserve d ¹⁸O differences from changing fluid composition during growth of skarn garnet. Zones did not equilibrate during granulite facies metamorphism. REE patterns from garnet distinguish between the GS and WS. However, distinct REE pattern differences are not seen between different zones of zoned garnet. Garnet from WS has a concave up REE pattern with a maximum at Pr, and large distinct +Eu anomalies. Garnet from GS display HREE enrichment and small -Eu anomalies.

The Willsboro-Lewis skarns formed by mixing of metasomatic fluids from meteroic, magmatic, and contact metamorphic sources. Low d ¹⁸O indicates all skarns formed by massive infiltration of heated meteoric water. Zoned garnet from GS formed by variable mixing of fluids of meteoric and magmatic origin. High X_Adr and d ¹⁸O zones represent greater influx of magmatic and metamorphic fluids. Correlation of lower d ¹⁸O with greater X_Adr of garnet from WS suggests complex fluid flow different than that forming GS. The occurrence of low d ¹⁸O minerals, zoned garnet, sharp isotopic gradients, and their close spatial proximity to anorthosite, support the shallow intrusion of anorthosite followed by a fluid absent, granulite facies metamorphism.