EVIDENCE FOR THE METAMORPHIC ORIGIN OF 1.05-1.0 GA ZIRCON RIMS FROM THE LYON MOUNTAIN GRANITE GNEISS, ADIRONDACK HIGHLANDS, NEW YORK

The Lyon Mountain Granite Gneiss (LMG) is a weakly foliated magnetite-bearing granitic rock that occurs throughout the Adirondack Highlands. Zircon from the LMG exhibits oscillatory-zoned cores (~1.15 Ga), and multiple layers of faintly zoned to unzoned rims (~1.05-1.0 Ga). Traditionally, the cores were thought to be inherited because LMG intruded Hawkeye Granite Gneiss (HGG), previously dated by multi-grain TIMS at ~1.10 Ga; this led to the conclusion that the rims record the time of igneous crystallization of LMG. However, because new SHRIMP dating shows that HGG is 1.16 Ga, LMG igneous cores reflect either the time of igneous crystallization or inheritance. Accurate interpretation of LMG zircon requires high-resolution imagery combined with high spatial resolution SHRIMP geochronology and geochemistry of cores and rims to determine their igneous or metamorphic origins.

New evidence for the metamorphic origin of LMG zircon rims includes: (1) unzoned or broad zoning in CL vs. fine concentric oscillatory zoning in igneous cores, (2) Th/U mostly <0.2, whereas most igneous cores have Th/U>0.2, (3) REE patterns usually display large spread in MREE, whereas REE in cores is quite consistent, (4) small or absent Eu/Eu*, whereas igneous cores have large negative Eu anomalies, and (5) occasional depletion of HREE, suggesting co-precipitation with garnet. Interpretation of a metamorphic origin for LMG zircon rims implies that the igneous cores formed during late Shawinigan emplacement of the LMG; most cores are not xenocrystic. This conclusion has widespread implications for petrologic and structural histories, tectonic models, and paradigms of Fe ore formation.

Six samples of typical LMG yield zircon core ages of ~1.16-1.15 Ga, and two samples of fayalite granite are ~1.14 Ga. Our large SHRIMP U-Pb data set of rim ages from LMG and HGG zircon (n=168) ranges from ~1100-850 Ma. Deconvolution using the Unmix algorithm of Isoplot 3 yields periods of metamorphic growth at 1082 ± 5, 1046 ± 3, and 994 ± 6 Ma. These data support previous interpretations that the Ottawan orogeny was a long-lived event, punctuated by several pulses over a period of about 100 m.y. Ages of regional garnet (~1.02-1.01 Ga), monazite (1.02 Ga), and titanite (~1.0-0.93 Ga) suggest that the Highlands remained hot throughout the Ottawan and Rigolet orogenies.