GEOCHRONOLOGICAL CONSTRAINTS ON THE EMPLACEMENT OF BASEMENT UPLIFTS IN THE NORTHERN APPALACHIANS

Middle Proterozoic basement of the Berkshire massif in MA and CT was thrust westward over early Paleozoic shelf rocks of the Laurentian margin. Although, it is widely accepted that displacement on the frontal thrust occurred during the Ordovician Taconic orogeny, evidence for the age of thrusting is limited. We used zircon SHRIMP methods to date five felsic sills previously interpreted as syn-tectonic intrusives into Taconic thrusts. Two of these samples are from the eastern margin of the massif, and give Silurian igneous crystallization ages of 432 +/- 3 Ma (n=11) and 434 +/- 5 Ma (n=8). Xenocrystic zircon cores (950 to 1250 Ma) indicate contamination by Grenville basement. Igneous zircon rims from three felsic sills located in the interior and western margin of the massif yield SHRIMP ages of 997 +/- 10 Ma (n=16), 1004 +/- 19 Ma (n=8), and 1003 +/- 8 Ma (n=7). Cores from these zircons range from 1050 to 1230 Ma. Thus, none of the sills formed during the Ordovician Taconic orogeny.

The well-exposed frontal thrust zone on the western margin of the massif contains spectacular mylonites with steep strain gradients over tens to hundreds of meters. Electron microprobe dating of monazite from fault-zone samples does not support a Taconic age for thrusting. Monazite from a highly deformed quartzite exposed in the hanging-wall of the Dry Hill fault gives an age of 392 +/- 14 Ma (1s.e., n=16). Quartz-rich schist from a fault zone at Umpachene Falls contains monazite grains with multiple age populations that peak at 530, 435, 380, and 290 Ma. Monazite grains in a graphitic schist from Benton Hill 200 m below a major thrust give a weighted average age of 436 +/- 8 Ma (n=21). Two localities from Yale Farm near the western frontal thrust of the Berkshire massif give weighted average monazite ages of 401 +/- 9 Ma (n=26) and 400 +/- 10 (n=40).

The preliminary monazite ages from frontal thrust zone suggest that motion on faults along the western margin of the Berkshire massif occurred during the Acadian orogeny. We found no clear evidence for internal thrusting within the massif; it apparently behaved as a rigid block during uplift. Silurian sills near the eastern margin of the massif are distal arc-related rocks that appear to have intruded normal faults along the western margin of an incipient back-arc basin that evolved into the Connecticut Valley trough.