GEOCHRONOLOGY OF SKARNS AND ANORTHOSITE IN THE ADIRONDACK HIGH PEAKS, AND IMPLICATIONS FOR AGE AND EMPLACEMENT DEPTH OF THE MASSIF

The Marcy anorthosite massif dominates the High Peaks region of the Mesoproterozoic Adirondack Highlands. Generally thought to be emplaced ca. 1155 Ma and metamorphosed during the 1090–1020 Ma Ottawan phase of the Grenvillian Orogeny, some workers pose the possibility of both 1160–1150 Ma and 1040–1020 Ma phases in the batholith. Low oxygen isotope ratios in metasomatic rocks of the Willsboro–Lewis wollastonite skarn belt suggest that large volumes of meteoric water circulated through anorthosite’s northeastern border zone during emplacement. Deformed rocks from this belt have been shown to have Grenvillian ages using a variety of methods, but the age of skarn formation (and meteoric water infiltration) has not been directly constrained. To address this, we separated zircon from a variety of Lewis skarn lithologies to date by LA-ICPMS. Zircon from three pyroxene skarns with igneous protoliths generally have 1050–1010 Ma metamorphic ages with rare 1170–1130 Ma cores. These are similar to metamorphic and igneous ages reported in Adirondack anorthosite by Peck et al. (Am Min 2018). 1160–1140 Ma cores are also seen in zircon from a cross-cutting pyroxene skarn (dike?), and in the matrix of an undeformed garnetite, in addition to metamorphic ages that range from 1050 Ma to ca. 980 Ma. Interestingly, zircon from undeformed coarse wollastonite and a second garnetite have young metamorphic ages, averaging 1000 and 970 Ma respectively. The structural and thermal manifestation of the 1000–900 Ma Rigolet phase of the Grenvillian Orogeny within the Adirondacks is poorly characterized, but these young metamorphic dates and others in and around the Marcy massif suggest that Rigolet deformation and/or fluid flow may be locally important here.

We also analyzed zircon from three samples of deformed, low-δ¹⁸O anorthosite at the southern boundary of the anorthosite. These samples have similar age distributions to anorthosite elsewhere, having 1180–1130 Ma cores (interpreted as igneous protolith ages) and 1100–950 Ma rims (metamorphic ages). Taken together, ages of low-δ¹⁸O anorthosite and skarn are consistent with anorthosite emplacement at ca. 1155 Ma followed by metamorphism during the Ottawan and Rigolet phases of the Grenvillian orogeny.