DEFINITION OF The Northwest Lowlands are located in the northwest portionof the Adirondack Mountains. On the basis of lithologies,the Lowlands are closely related to the Frontenacterrane of the Canadian metasedimentary belt and arethought to be connected via the Frontenac Arch. The NorthwestLowlands are smaller in area, have lower topographicrelief than the Highlands, and are dominated by metasedimentaryrocks interlayered with leucocratic gneisses. Bothlithologies are metamorphosed to upper amphibolite grade.The metasedimentary rocks are mostly marbles but also containunits of quartzites and mica schists, suggesting a platformsedimentary sequence provenance. The protoliths of theleucocratic gneisses are controversial. Some geologists considerthe leucocratic gneisses to be basal rhyolitic and daciticash-flow tuff deposits that have been metamorphosed, basedon geochemical signatures and the absence of xenoliths in theformations. However, others question this interpretation andsuggest that the leucocratic bodies are intrusive in nature,based on crosscutting field evidence and geothermometry.The geothermometry on the leucocratic gneiss yields a temperatureof 1,436°F–1,490°F (780°C–810°C). This is ananomalously high metamorphic temperature compared withother rocks in the region, suggesting that they may be igneouscrystallization temperatures.
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