CONDITIONS AND TIMING OF EMPLACEMENT OF THE MARCY ANORTHOSITE, NE ADIRONDACK HIGHLANDS, INFERRED FROM ANATECTITES IN ITS AUREOLE: PART 2, U-PB GEOCHRONOLOGY

Attempts to date the granulite grade Marcy metanorthosite have led to equivocal results due to the difficulty of interpreting the growth histories of a complex zircon population. The main obstacle has been in distinguishing between metamorphic zircon, magmatic zircon and inherited zircon components. Analyses of milligram-sized fractions of zircon have at best placed loose constraints on emplacement age. Single-grain analyses, once considered a holy grail of zircon analytical procedure, have produced an apparent continuum of ages ranging from 970 to 1170 Ma. Internal images of anorthosite zircons using cathodoluminescence (CL) and back-scattered electrons (BSE) indicate multiple growth but are often diffuse in appearance and difficult to interpret. By contrast, zircons from anatectic gneiss in the aureole of the Marcy massif display intricate and well-defined internal growth zones in BSE. Since anatexis in surrounding metagabbro, charnockite and mangerite gneiss (New Russia complex) can be shown to be associated with emplacment of the anorthosite and to be at least in part post-tectonic, the youngest magmatic zircons in the gneiss should date anatexis during the thermal peak and by implication give the approximate intrusive age of the Marcy anorthosite and a minimum age for penetrative deformation in the anatectites. From three main crystal morphologies (brown prismatic with oscillatory zoning, clear embayed prisms with oscillatory to patchy zoning, and platy to equant with no zoning), microprobe analyses indicated 4 stages of chemically distinct zircon growth clearly recognized in BSE images. Images served as a map for single- and partial-grain high age-resolution TIMS U-Pb analysis. Three discrete thermal events that correlate directly with zircon growth stages are recognized: An event prior to 1140 Ma interpreted as an inherited component; an event at 1112 ± 2 Ma interpreted as charnockite emplacement age; and an event at 1040 ± 1 Ma interpreted as the age of anatexis and, by proxy, massif emplacement age. An age of 1003 ± 4 Ma was determined for equant, unzoned zircons interpreted to represent retrograde metamorphic zircon growth. All other zircon ages from this sample are demonstrably mixed ages resulting from zircons that display multiple growth stages.