Contrasting P‐T‐t paths: Thermochronologic evidence for a Late Paleozoic final assembly of the Avalon Composite Terrane in the New England Appalachians

Strongly contrasting pressure‐temperature‐time paths for the Avalon composite terrane and the structurally overlying Putnam‐Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U‐Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam‐Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One‐dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high‐grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high‐grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post‐Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.