SHRIMPII GEOCHRONOLOGY OF THE ADIRONDACK AMCG SUITE, III: INTERPRETATION OF METAMORPHIC EFFECTS ON ZIRCON AGES

SHRIMP-II dating of zircons from Adirondack anorthosites and associated ferrogabbros, coronitic metagabbros, and charnockites reveals complex data consistent with a primary age of ca. 1150 Ma for anorthosites and metagabbros and 1170-1150 Ma for charnockites. However, many spot analyses, including those from zircon domains preserving igneous features, yield arrays of analytical points between 1150 and ca. 1000 Ma. Whereas some discrete grains that yielded ages of 1040-1000 Ma have morphology and textures suggesting metamorphic zircon growth, and some obvious young overgrowths can be identified, most zircons yielding ages less than 1150 Ma have internal features, morphology, and Th/U ratios (between 1.0 and about 0.3) suggesting primary igneous origin. The most likely mechanism to account for these specific young U-Pb ages is pervasive Pb-loss within domains of primary zircons rather than new zircon growth. Because the SHRIMP analyses are mostly concordant and the analytical points form arrays between ca. 1150 and 1000 Ma, it is likely that Pb-loss was attendant upon a late stage, or stages, of the Ottawan orogeny at ca. 1040-1000 Ma. Experimental studies have shown that solid-state diffusional Pb-loss is ineffective; thus fluids were presumably associated with these events. These relationships are consistent with the younger ages for Adirondack AMCG suites obtained by earlier conventional multigrain and even single-grain zircon U-Pb analyses. This behavior in zircons from anorthosites and metagabbros is in strong contrast with zircons recovered from anatectic melt bodies formed in Adirondack metapelites. These zircons show discrete core and overgrowth morphologies. The largest cores range in age from ca.1220 Ma to about 1140 Ma, but there are striking high-U, ca. 1020-1040 Ma Ottawan overgrowths that display distinctive growth zonations. These relationships in zircons from metapelite anatectites are consistent with new zircon growth in a reactive rock matrix rich in water and other fluids, whereas instances of pervasive Pb-loss, with little or no new zircon growth, appear to be related to Ottawan effects in primary igneous rocks.