U-Pb zircon ages for Hyde School Gneiss indicate that it crystallized around 1285 to 1230 Ma. U-Pb zircon ages for lithologically similar Lyon Mountain Gneiss indicate crystallization around 1080 to 1050 Ma. Despite similar appearances, the two gneisses are of a different origin. [Information is from section by J. McLelland and others, within chapter 5.]
Source: GNU records (USGS DDS-6; Reston GNULEX).
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