Anatexis of Metapelites in the Adirondacks: The Importance of Shawinigan Orogenesis

Studies of timing of anatexis in Adirondack metapelites by SHRIMP U-Pb dating of zircons have produced data demonstrating the importance of ca. 1200-1160 Ma Shawinigan orogenesis. Initial studies of metapelites from the Adirondack Lowlands and southwestern Highlands found that zircon populations included detrital cores, yielding maximum ages of ca. 1340 Ma and interpreted as detrital grains derived from early arc rocks similar to the Dysart- Mt. Holly suite, as well as sector-zoned (“fir-tree”) overgrowths and single grains that yielded ages of ca. 1180 -1155 Ma. No zircons were found to record ca. 1050 Ottawan ages, despite the fact that titanite ages in the Highlands are commonly ca. 1050-1030 Ma and monazite grains also record growth during Ottawan events. These results are interpreted as indicating that the metapelites accumulated in restricted basins that received detritus only from an early arc similar to the Dysart- Mt. Holly suite, and that zircon growth was restricted to high-grade metamorphism during Shawinigan orogenesis and subsequent ca. 1155 Ma AMCG igneous activity. Although rocks in the southwestern Highlands experienced high-grade Ottawan metamorphism, they became so dehydrated during Shawinigan events that new zircon growth and anatexis did not occur.

A subsequent study showed that metapelites in the eastern Adirondacks had a similar provenance (ca 1340 Ma detrital grains) and display overgrowths and single grains with ca. 1170 Ma Shawinigan ages. However, these metapelites also display ca. 1050 Ma Ottawan grains and overgrowths suggesting that Ottawan anatexis was relatively local and related to availability of fluids moving through major fault systems.

These results indicate that, although the Grenville orogenic cycle culminated in the ca. 1050 Ma Himalayan-style collisional Ottawan orogeny and its attendant high-grade metamorphism and magmatism, earlier events related to ca.1210-1160 Ma Shawinigan orogenesis were profound, widespread, and important in the tectonic history of the Grenville Province.