TIMING OF ANORTHOSITE EMPLACEMENT, SKARN FORMATION, AND METEORIC WATER INFILTRATION IN THE WILLSBORO-LEWIS WOLLASTONITE DISTRICT (ADIRONDACK HIGHLANDS, NEW YORK)

The Willsboro-Lewis wollastonite district occurs along the margin of the 1.15 Ga Marcy anorthosite massif in the Adirondack Highlands (New York), and records evidence for formation in the anorthosite’s low-pressure metamorphic contact aureole. Wollastonite – garnet – pyroxene gneisses in the ca. 25 km long, 1.5 km thick skarn belt are intercalated with massive garnetite and pyroxene ± garnet skarns, all of which have low oxygen isotope ratios indicating circulation of heated meteoric water and relatively shallow depths above the brittle-ductile transition during the late part of the Shawinigan orogeny. Anorthosite, skarns, and country rocks were all variably deformed and recrystallized at depths of 25-30 km during the 1.09–1.02 Ga Ottawan phase, and locally altered during the 1.01–0.98 Ga Rigolet phase, of the Grenvillian orogeny. This study examined rare zircon in skarn rocks to constrain the timing of surface-derived meteoric water infiltration. Zircon yield a spectrum of ages and oxygen isotope ratios reflecting the polymetamorphic history of these rocks. Most samples are dominated by metamorphic zircon having Ottawan or Rigolet ages and are in oxygen isotopic equilibrium with their host rocks. Relict 1150–1140 Ma ages are preserved in some zircon cores, which are taken as the ages of igneous protoliths. Some of these 1150–1140 Ma cores preserve low oxygen isotope ratios; a record of interaction with meteoric water. Ages seen in the Willsboro-Lewis skarns reproduce the span of igneous, disturbed, and metamorphic ages in Adirondack anorthosite, and point to contemporaneous anorthosite emplacement, meteoric water circulation, and skarn formation at ca. 1150 Ma. This result is consistent with shallow emplacement of the Marcy anorthosite massif during crustal thinning related to the collapse of the 1.19–1.14 Ga Shawinigan orogeny, and later Ottawan granulite facies overprinting.