RARE EARTH ELEMENT COMPOSITION OF APATITE, ZIRCON AND MONAZITE FROM, AND U-PB ZIRCON AGE OF, THE PORT LEYDEN NELSONITE, WESTERN ADIRONDACK HIGHLANDS, NY

LA-ICP-MS analysis of rare earth elements (REE) in apatite, zircon, and monazite from the Port Leyden nelsonite, western Adirondack Highlands, were obtained at Rensselaer Polytechnic Institute and the University of Arizona.

Apatite, comprising 30 to 45 vol.% of the nelsonite, contains 1.1 to 2.1 wt.% total rare earth element oxides. Chondrite normalized plots of REE abundances show: 1) LREE enrichment in apatite and monazite, 2) a similar (in magnitude) negative Eu anomaly for monazite, apatite, and zircon, 3) HREE enrichment in zircon, and 4) a positive Ce anomaly in zircon. CL imaging of zircon reveals a population of homogenous grains and grains showing both oscillatory zoned cores and homogeneous rims. No systematic difference was recognized in the REE patterns of both zircon types. Furthermore, the REE patterns are typical for igneous zircon (Hoskin and Schaltegger 2003). The negative Eu anomalies of all three phases suggest significant plagioclase crystallization in the parent magma. The Th/U ratio of all zircon ranges from 0.07 to 0.90, with an average of 0.37, and is suggestive, but not definitive, of an igneous origin (Rubatto, 2017). Although the Port Leyden nelsonite contains small amounts of metamorphic garnet, the HREE enrichment in zircon indicates no zircon grew in the presence of garnet.

The concordant age on oscillatory zoned cores (n = 14) is 1145.5 ± 4.2 Ma, whereas the concordant age on homogeneous grains and rims (n = 16) is 1036.8 ± 3.8 Ma. Due to the presence of quartz and feldspar mineral inclusions (Darling, 2016), the oscillatory zoned cores are interpreted as xenocrysts. The homogeneous grains and rims are interpreted to have formed during igneous crystallization of the Port Leyden nelsonite. The 1036.8 ± 3.8 Ma age of igneous crystallization is similar to an obtained zircon age (1032.1 ± 3.8 Ma) on a mapped, stratiform jotunite / anorthosite body, lacking xenocrysts, occurring along strike of the regional foliation, 25 km to the northeast, near Crooked Creek (Whitney et al. 2002). This strongly suggests that regionally extensive, folded jotunite / anorthosite layers mapped in the western and central Adirondacks are a likely parental magma source for the Port Leyden nelsonite.