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Abstract 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 mineralogical and isotopic evidence for formation in the anorthosite’s low-pressure metamorphic contact aureole. Wollastonite–garnet–pyroxene gneisses in the ~25-km-long, 1.5-km-thick skarn belt are mined for wollastonite and 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 their formation. Anorthosite, skarns, and country rocks were all variably deformed and recrystallized at depths of 25 to 30 km during the 1.09- to 1.02-Ga Ottawan phase, and locally altered during the 1.01- to 0.98-Ga Rigolet phase, of the Grenvillian orogeny. This study examined rare zircon in low-δ18O skarn rocks to constrain the timing of surface-derived meteoric water infiltration. Zircon was dated, and trace elements were measured by laser-ablation ICPMS, and oxygen isotopes were measured by ion microprobe, yielding 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 207Pb/206Pb ages and are in high-temperature oxygen isotopic equilibrium with host wollastonite, garnet and/or pyroxene. Several samples contain igneous zircon with disturbed U–Pb isotope systematics, reflecting some combination of new zircon growth and recrystallization during subsequent metamorphism. Relict 1150–1140 Ma ages are preserved in some zircon cores, which are taken as the ages of igneous zircon incorporated during skarn formation or from protoliths. Some of these 1150 to 1140 Ma cores preserve the low-δ18O 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- to 1.14-Ga Shawinigan orogeny, and that granulite facies overprinting was a later tectonic event. 

Massif-type anorthosites, enormous and enigmatic plagioclase-rich cumulate intrusions emplaced into Earth’s crust, formed in large numbers only between 1 and 2 billion years ago. Conflicting hypotheses for massif-type anorthosite formation, including melting of upwelling mantle, lower crustal melting, and arc magmatism above subduction zones, have stymied consensus on what parental magmas crystallized the anorthosites and why the rocks are temporally restricted. Using B, O, Nd, and Sr isotope analyses, bulk chemistry, and petrogenetic modeling, we demonstrate that the magmas parental to the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen, require large input from mafic melts derived from slab-top altered oceanic crust. The anorthosites also record B isotopic signatures corresponding to other slab lithologies such as subducted abyssal serpentinite. We propose that anorthosite massifs formed underneath convergent continental margins wherein a subducted or subducting slab melted extensively and link massif-type anorthosite formation to Earth’s thermal and tectonic evolution.