FELSIC MINERAL INCLUSIONS IN ZIRCON FROM THE PORT LEYDEN NELSONITE, WESTERN ADIRONDACK HIGHLANDS, NEW YORK: A PRODUCT OF MAGMA MIXING?

The Port Leyden nelsonite is small magnetite-apatite-ilmenite ore body occurring in Mesoproterozoic metapelitic gneiss on the western margin of the Adirondack Highlands. It is unusual in that no compositionally adequate parent magma (e.g. jotunite or oxide-apatite gabbronorite) has been identified in the area (Darling and Florence, 1995).

The nelsonite typically contains elevated levels of Zr (1400 to 2500 ppm) largely present in abundant modal zircon. The Zr abundances are considerably higher than normal levels of Zr solubility in non-peralkaline melts and suggests that some of the zircon modal fraction is inherited (Hanchar and Harrison, 2003).

The zircon grains display both euhedral, oscillatory zoned cores (interpreted as igneous) and anhedral, irregular, compositionally homogeneous rims (interpreted as metamorphic or igneous). The oscillatory zoned cores contain small (2-10 micrometer), solid inclusions that have energy-dispersive X-ray spectra (EDS) consistent with quartz, K-feldspar, plagioclase, biotite, and apatite. Remarkably, no low-silica mafic mineral inclusions (e.g. orthopyroxene, clinopyroxene, olivine) were observed in zircon.

Felsic mineral inclusions in zircon from an igneous rock that has mafic magma affinities provides further evidence that the included cores of zircons in the Port Leyden nelsonite are inherited. This unusual occurrence may be possible considering that the mafic igneous rocks described above are part of the bimodal anorthosite-mangerite-charnockite-granite (AMCG) magmatic complex in the Adirondacks (McLelland et al, 1988). It is conceivable that during magma mixing, zircon from granite or charnockite may have become incorporated into coeval jotunite or oxide-apatite gabbronorite. Subsequently, the latter magma experienced either unmixing (Philpotts, 1967) or crystal settling (Dymek and Owens, 2001) to produce the Port Leyden nelsonite.