AGE, ORIGIN, AND TECTONIC SIGNIFICANCE OF MESOPROTEROZOIC AND SILURIAN FELSIC SILLS IN THE BERKSHIRE MASSIF, MASSACHUSETTS

Discontinuous sills of felsic gneiss in the interior and western margin of the Berkshire massif and granite sills on the eastern margin of the massif were correlated by Ratcliffe (1984a, 1984b, 1985) and Ratcliffe and Hatch (1979), and interpreted by them as syntectonic anatectic melts that intruded Taconic thrusts. We dated three felsic gneiss sills and two granite sills in an attempt to constrain the age of Taconic thrusting, but discovered that the sills are not coeval. Rather, they were intruded during two widely separated episodes, one during the Mesoproterozoic at approximately 1000 Ma and the other during the Silurian at approximately 430 to 435 Ma. The 1000 Ma sills of felsic gneiss in the interior of the massif are located in Mesoproterozoic units and many of the mapped Taconic thrusts within the massif closely follow the distribution of this unit, here informally called the felsic gneiss of Harmon Brook. These 1000 Ma sills formed during the Ottawan or Rigolet orogeny and they have no connection to the Taconic orogeny. The 430 to 435 Ma granite sills along the eastern margin of the massif, informally called the granite of Becket Quarry, are found in both Mesoproterozoic basement and the Neoproterozoic Hoosac Formation. The sills are too young to have intruded during the Ordovician Taconic orogeny, but they may have formed during later faulting near the contact between Mesoproterozoic basement and Neoproterozoic cover rocks. The Tyringham Gneiss is one of the most common Mesoproterozoic units in the Berkshire massif. Zircons from the Tyringham Gneiss contain cores with oscillatory zoning and thin unzoned rims. The weighted average of eight 206 Pb/ 238 U analyses from the cores is 1179 /- 9 Ma, whereas nine spot analyses from the rims yield an age of 1004 /- 9 Ma. We interpret these two ages to represent the crystallization of the Tyringham Gneiss protolith and a subsequent high grade metamorphism, coeval with the intrusion of the felsic gneiss of Harmon Brook. The western contact between Mesoproterozoic rocks of the Berkshire massif and underlying Early Paleozoic rocks is clearly a thrust, but there is no independent evidence that movement occurred during the Taconic orogeny; displacement may also have occurred during the Silurian Salinic or the Devonian Acadian orogeny. Many contacts mapped as Taconic thrusts within the Berkshire massif follow the distribution of the 1000 Ma felsic gneiss of Harmon Brook. The age of the sills is clearly incompatible with this interpretation, and evidence for faulting along these mapped thrusts is lacking. Instead of being deformed into an imbricate stack, the massif behaved as a rigid block during Paleozoic uplift. Finally, the age of granite sills along the eastern margin of the massif does not constrain the basement-cover contact to be a Taconic thrust, as previously interpreted. The contact may be a Silurian fault, possibly related to extension and the opening of the Connecticut Valley trough as a back-arc basin. According to this model, the magma for the granite sills was generated above a west-dipping subduction zone under the Laurentian margin, which developed after the Taconic orogeny.