EMPLACEMENT AND METAMORPHISM OF THE MARCY ANORTHOSITE: NEW CONSTRAINTS FROM GEOCHRONOLOGY AND OXYGEN ISOTOPES

The 3000 sq km Marcy anorthosite dominates the Adirondack Highlands (Grenville Province, NY). The massif preserves igneous textures with metamorphic coronas and is deformed near its margins. Historically, the relationship between anorthosite emplacement and metamorphism has been controversial, with many workers concluding that anorthosite emplacement coincided with metamorphism. Valley and O’Neil (Nature, 1982) proposed that metamorphism of the anorthosite (~30 km depth) could not be the same event that formed wollastonite skarns, which have low δ¹⁸O and formed in the presence of meteoric water during shallow emplacement (<10 km). Subsequent geochronology supports the polymetamorphic model, but the timing and depth of metamorphism and magmatism in the Adirondacks have been debated for >35 years. This study presents new data that constrain the timing of metamorphic mineral growth in the anorthosite and shallow intrusion of the anorthosite.

Four anorthosite samples from the southern massif (all deformed and two coronitic) were selected for geochronology. In these samples zircon formed during garnet growth from breakdown of igneous hemoilmenite and plagioclase. Metamorphic zircon with for the most part dark and/or patchy CL was analyzed in situ in thin sections by laser ICPMS, and have ages in the range 1050-1035 Ma. One sample contains a few large euhedral grains with concentric CL zoning and 1160-1130 Ma ages that we interpret as igneous. These data are consistent with ca. 1155 Ma magmatism and later granulite facies metamorphism at ca. 8 kbar during the 1090-1020 Ma Ottawan orogeny.

Anorthosite with low δ¹⁸O at the southern margin of the Marcy massif was interpreted by Morrison and Valley (CMP 1988) as supporting shallow intrusion. New laser fluorination analyses confirm δ¹⁸O(Pl) as low as 1.2‰ SMOW in the contact zone, which is in equilibrium with metamorphic garnet. The average fractionation for these samples (ΔPl-Gt=1.65±0.70‰, n=5) is the same as for samples with igneous δ¹⁸O (ΔPl-Gt=1.46±0.71‰, n=18), which corresponds with cooling temperatures after the peak of metamorphism. This points to the low δ¹⁸O signature pre-dating Ottawan metamorphism, and thus hydrothermal alteration by surface fluids being an earlier event, most likely caused by emplacement of the massif at 1155 Ma.