Rotated lineations in a metamorphic and geochronologic transition zone, central Massachusetts: Implications for Late Pennsylvanian transport

New geochronologic data combined with structural and petrologic studies have radically changed concepts of the tectonic framework in the Bronson Hill anticlinorium, central Massachusetts. The authors focus on a striking coincidence of structural, metamorphic and geochronologic transitions on the east limb of the Warwick dome and Kempfield anticline. The zone was first identified by Robinson as a lineation swirl'', across which the dominant N-S orogen-parallel lineation makes a nearly 180[degree] rotation in map view. From west to east, lineations curve continuously from gently N-plunging to E-plunging, down-dip, to gently S-plunging. Eastward from the swirl'', metamorphic grade increases from the sill-sta-mus zone to the gar-crd-sill-ksp zone in south-central Massachusetts. All U-Pb data indicates this metamorphism is late Acadian, about 360 Ma. Along the granulite-facies margin, late Acadian, syn- to post-peak metamorphic dextral motion in the Conant Brook shear zone produced N-S lineations, evidence that N-S lineations east of the swirl'' are Acadian. Rocks west of the swirl'' near the Pelham dome are in the kya-sta-mus zone. These minerals grew during pervasive shearing that produced a strong N-S lineation with top-side-south kinematics. This translates to dextral motion on the east flank of the Pelham dome and overturned west flank of the Warwickmore » dome. Isolated tectonic slices of cover enclosed in gneissic basement in the Pelham dome area preserve sill-ksp-kya schists with an Acadian metamorphic age (367 Ma), E-W lineation, and top-side-E kinematics. N-S lineations observed across central Massachusetts appear to be a composite of Acadian and Late Pennsylvanian deformations with the latter dominant in the Pelham dome and western part of the Warwick dome. Two different models for development of the swirl'' are proposed.« less