PEGMATITES FROM THE ADIRONDACK MOUNTAINS, NY: SYSTEMATIC, MINERALOGY AND GEOCHRONOLOGY

The pegmatites of New York, mostly of NYF-type, occur in marble in the Adirondack Lowlands and in the metasedimentary sequences south and east of the Marcy anorthosite (1155 Ma) in the Highlands. Their mineral composition is simple or complex and most of the southern Adirondack pegmatites display compositional zoning. Post-emplacement metamorphic features are dominant for pegmatites in the northwestern Adirondacks (Lowlands), but become less intense in those in the southern and eastern Adirondacks.

In order to constrain the timing of their emplacement and igneous affiliation, U-Pb zircon ages were determined by LA-MC-ICP-MS. The zircon crystals selected under the binocular were large (up to 2 cm) or small (2-5 mm) size; their U content varied from as much as 4000 ppm, in large, dark, partially metamict grains to 20 ppm in the small, clear crystals. Around 20-40 individual analyzes with a 30 micron beam were performed on a single large grain or on a grain population from the same hand specimen.

The feldspar-type pegmatites from the Adirondack Lowlands yielded ages of 1212±17 Ma for one of the Rossie pegmatite bodies with an inherited component of ca. 1312 Ma. The McLear pegmatite (diopside, danburite, fluoro-edenite, fluorian tremolite, uraninite) yielded an age of 1217±17 Ma. These data provide a minimum age for the supracrustal sequence and expand the timing and type of early Shawinigan magmatism in the Adirondack Lowlands.

The REE - pegmatites from the Highlands yielded the following ages: Scott’s Farm (K-feldspar, titanite, diopside, fluorite) 1063±9 Ma, Roe Spar Bed 1042±21 Ma, Crown Point 1024±1.4 Ma, Sugar Hill 1051±17 Ma, and Barton Hill mine 1028+20/-48 Ma. They are the result of the Ottawan igneous events and were emplaced probably during/after the A-type Lyon Mountain granite (1070-1050 Ma) intrusion. The age of the beryl-type Batchellerville pegmatite in southern Adirondacks is 1122±87 Ma. The small Hull’s Falls pegmatite in eastern Highlands yielded the oldest age of 1180+15/-7 Ma and the Mayfield pegmatite yielded the youngest age at 986±53 Ma. The small post-skarn pegmatite at the Lewis Quarry in eastern Adirondacks yielded 995±17 Ma.

Chemical ages on monazite from some of the pegmatite bodies will be performed to compare/confirm the U-Pb zircon ages.