PATTERNS OF PALEOZOIC METAMORPHISMS IN THE AVALON TERRANE OF SOUTHEASTERN NEW ENGLAND: EVIDENCE FOR SYN-AND POST-METAMORPHIC FAULTING FROM 40AR/39AR MINERAL AGES AND THERMOBAROMETRY

⁴⁰Ar/³⁹Ar cooling ages of amphibole, white mica, biotite, and K-feldspar together with existing thermobarometry are used to characterize the metamorphic field gradient in the Avalon terrane of SE New England. The dated samples are from two east-west traverses across the Avalon and Nashoba terranes in Mass. and other sites in R.I. that were analyzed to differentiate between Acadian and Alleghanian orogenic events. Amphibole from a weakly metamorphosed gabbro near the town of Ashland yielded a cooling age of 588 Ma, consistent with its Neoproterozoic crystallization age. Two samples of amphibole from the Lexington Mafic Complex reproduce similar results with maximum cooling ages of ~430 Ma, consistent with a U-Pb zircon crystallization age of ~440 Ma. A single diorite sample produced a disturbed amphibole age spectrum with a maximum cooling age of 377 Ma. This age is interpreted to reflect resetting by contact metamorphism by a nearby 378 Ma (U-Pb zircon age) granite intrusion.

Approaching the terrane-bounding Bloody Bluff fault (BBF) from the E. in Mass., cooling ages become younger with increasing strength of overprinting gneissosity. Cooling ages of amphibole and biotite fall to 268-250 Ma as the BBF is approached. These and existing thermochronologic data define NW-trending cooling age isopleths that carry the Alleghanian index mineral isograds in the Narragansett basin NW to the BBF without any clear direct evidence for Acadian metamorphism. Avalon thermochronologic data contrast strongly with our new and existing Devonian-Carboniferous amphibole cooling ages from adjacent high-grade rocks of the Nashoba terrane. The abrupt truncation of Alleghanian isopleths in the Avalon terrane by the BBF and the Nashoba terrane indicates km-scale normal motion in post-Alleghanian (Mesozoic) time along this fault.