TACONIC VERSUS MIDDLE PROTEROZOIC ANATEXIS AND DEFORMATION IN THE BERKSHIRE MASSIF, WESTERN NEW ENGLAND

Middle Proterozoic rocks of the Laurentian margin were thrust westward over Cambrian and Ordovician continental shelf deposits during the Taconic orogeny in the Berkshire massif. Dating this deformation would constrain the time of collision between Laurentia and the Ordovician Taconic arc and help discriminate between competing models for the orogeny. Discontinuous tabular bodies of alaskite were mapped by Ratcliffe (1984a,b; 1985) within the Berkshire massif and interpreted by him as syntectonic anatectic melts that intruded Taconic thrusts. The alaskite sills are most commonly found in the granitic Middle Proterozoic Tyringham Gneiss and many of the mapped Taconic thrusts within the massif closely follow the distribution of the alaskite bodies. We collected one sample of the Tyringham Gneiss and fourteen samples of alaskite, ranging in composition from granite to trondhjemite, for SHRIMP analysis to date the age of thrusting in the Berkshire massif. Zircons from the Tyringham gneiss contain cores with oscillatory zoning and thin homogeneous rims. The weighted average of eight ²⁰⁶Pb/²³⁸U analyses from the cores is 1179 ± 9 Ma, while nine ²⁰⁶Pb/²³⁸U spot analyses from the rims yield an age of 1004 ± 9 Ma; we interpret these to represent the crystallization age of the Tyringham Gneiss protolith and subsequent high grade metamorphism, respectively. Zircons from two samples of alaskite commonly contain xenocrystic cores that yield a wide range of ages from approximately 1050 to 1200 Ma surrounded by broad rims which commonly display oscillatory zoning. Many grains also show oscillatory zoning with no cores. The weighted average of sixteen ²⁰⁶Pb/²³⁸U analyses from grains without cores and rims of grains with cores in one alaskite sample is 997 ± 5 Ma, and the weighted average of eight analyses from the second sample is 1004 ± 19 Ma. We suggest that the alaskite bodies formed during the Ottawan phase of the Grenville orogeny and that they have no connection to the Taconic orogeny. The basal contact between Middle Proterozoic rocks of the Berkshire massif and underlying Early Paleozoic rocks is clearly a thrust, but many contacts within the Berkshire massif mapped as Taconic thrusts must either be Middle Proterozoic faults or, more likely, intrusive contacts between older basement gneisses and younger anatectic melts.