Paper No. 342-5
Presentation Time: 2:30 PM
DATING DEFORMATION IN THE PALMER ZONE OF TRANSPRESSION, CENTRAL MASSACHUSETTS: TEMPORAL CONSTRAINTS ON MODELS FOR PROGRESSIVE DEFORMATION IN THE MIDDLE CRUST
The Palmer Zone of Transpression (PZoT) in the Bronson Hill zone of central Massachusetts is an expression of late Paleozoic oblique convergence. The steeply W-dipping, N-S trending PZoT is defined by bounding high strain zones (Mt. Dumplin on west, Cental Maine and Conant Brook on east) of opposing shear sense enclosing the Monson orthogneiss. Research was designed to establish the timing of deformation to test the hypothesis that strain in transpressional systems occurs contemporaneously across the zone. An understanding of the timing of deformation in this zone could elucidate the mechanisms that formed the zone and contribute to a greater overall understanding of fabric evolution in transpressional systems. U-Pb SIMS and SHRIMP ages of zircon in deformed pegmatite in Monson orthogneiss and Paxton and Rangeley paragneisses are 369 to 364 Ma. Slightly younger ages of 359, 358, 357, and 356 Ma in Petersham diorite, Hardwick tonalite, West Warren diorite, and Walker Mtn. tonalitic orthogneiss, respectively, date emplacement of plutons that exhibit various states of strain. Plutons and pegmatites that contain all fabrics associated with progressive transpression – lineations ranging from subhorizontal (initial) to steeply-plunging parallel to dip (final) – mark the maximum age of deformation. These ages are 356, 354, and 353 Ma in the Walker Mtn. orthogneiss, Fitzwilliam granite, and a Walker Mtn. pegmatite, respectively, and indicate transpression in the PZoT initiated after ~355 Ma. Monazite from Rangeley metapelites selected in the context of petrofabrics and syn-deformational mineral assemblages yield U-Th-Pb EPMA chemical ages that indicate dextral transpression occurred continuously for ~15-20 Ma; dates range from 349 to 335 Ma. A synthesis of these ages shows that tonalitic to dioritic intrusion initiated ~10 Ma before the start of mid-crustal monazite growth at 350 Ma. Broadly speaking, monazite ages tend to decrease from the outer portions of the PZoT (East Brookfield quad) to the center (Conant Brook shear zone and Greenwich syncline), perhaps indicating that deformation migrated across the PZoT. Further analysis of monazite in additional rocks of the Greenwich syncline and west-bounding Mt. Dumplin high strain zone will constrain timing of deformation across the entire PZoT.