36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Distinguishing Grenville Basement from Neoproterozoic to Cambrian Cover Rocks in the Western New England Appalachians

Paper number: 36-5
Program: Technical Sessions
Time: 9:25 AM – 9:45 AM

Authors

Abstract

Neoproterozoic rifting of Rodinia created the Laurentian margin of eastern North America. Neoproterozoic to Cambrian rift to drift sediments deposited on the newly created margin were locally sourced and are compositionally similar to the underlying Mesoproterozoic Grenville basement metasediments in the Green Mountain massif in Vermont. The cover sedimentary rocks were deformed and metamorphosed during Paleozoic orogenies while the basement rocks were overprinted and locally retrograded making distinctions between them difficult in areas where cover rocks experienced high-grade Paleozoic metamorphism, such as the Chester dome, and in high-strain fault zones, where fluid infiltration and retrograde metamorphism altered basement rocks. Clearly, a first-order problem in interpreting the structural and tectonic history of this complex region depends on correctly assigning rocks to the basement or cover sequence. Detrital zircon separated from quartz-rich rocks provide a valuable test for stratigraphic assignments. Because rift volcanic rocks are uncommon in Vermont, ca. 550 Ma detrital zircons are relatively rare in the cover metasediments. Fortunately, there are abundant 950 Ma detrital zircon grains derived the Stamford Granite and the Bull Hill Gneiss, which intruded after all phases of the Grenville orogeny ended. Grains of this age are commonly found in the cover rocks but should be absent in the older basement metasediments.

We dated detrital zircon from the Cavendish, Wilcox, and Saltash Formations, which Doll et al. assigned to the cover sequence, but Ratcliffe et al. (2011) interpreted as part of the Grenville basement. We also dated samples of the Dalton Formation from Bald Mountain in Mendon, VT, which Ratcliffe et al. (2011) reassigned to Grenville basement. Finally, we analyzed samples of Okemo Quartzite, part of the Grenville basement, for comparison. Detrital zircon grains from two samples of the Okemo Quartzite form a single peak centered at 1.32 Ga. In contrast, other samples contain numerous grains approximately 950 Ma, indicating that they are part of the Neoproterozoic to Cambrian cover sequence. In particular, the Cavendish Formation, which Ratcliffe et al. (2011) assigned to one of the oldest sequences in the Grenville basement, older than 1.37 Ga, must be part of the cover sequence, as correctly interpreted by Doll et al. (1961).

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411416

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