NEW ZIRCON AND MONAZITE U-TH-PB SIMS AGES FOR METAPLUTONIC AND METAPELITIC GNEISSES IN THE PALMER ZONE OF TRANSPRESSION, CENTRAL MA AND CT: CONSTRAINTS ON TIMING OF DEFORMATION, HIGH GRADE METAMORPHISM AND DELAMINATION(?)

Oblique convergence between Avalon-Meguma and Laurentia in the mid-late Paleozoic resulted in crustal shortening of at least 5:1 between Maine and southern New England. The Palmer Zone of Transpression (PZoT), developed within the Bronson Hill (BHZ) and western Central Maine zones (CMZ), illustrates the structural style that formed in response to oblique convergence. Orogen-normal flattening/reverse displacement and orogen-parallel elongation/dextral displacement were each strongly partitioned within the PZoT into zones dominated by pure shear (western and eastern Monson orthogneiss) and simple shear (metastratified cover). All plutons within the PZoT and adjoining areas are deformed (flattened, boudinaged) by the regional transpression system. All metapelites contain foliations/lineations that are synkinematic with high-grade Sil-Kfs-Grt-Bt+Crd assemblages or post-kinematic with retrograde Grt+Kfs => Sil-Bt assemblages. Both observations indicate shortening continued after crustal thermal conditions peaked. Zircon U-Pb (Zrc) and monazite Th-Pb (Mnz) SIMS ages for variably deformed plutons and Rangeley Fm. paragneisses constrain timing of transpressional deformation. The majority of leucopegmatites in paragneiss, which formed in response to high-grade metamorphism, range from 365-352 Ma. The dominant magmatic event is the Hardwick-Walker Mtn. suite at 360-350 Ma. Metamorphic Zrc in paragneiss yields 367 +/- 12 and 358 +/- 7 Ma (all 2σ). Monazite in 3 samples of paragneiss yield weighted mean ages of 327 (4), 339 (4), and 340 (6) Ma, with one sample preserving an older age mode at 384 (8) Ma. The Zrc ages from plutons constrain the onset of transpression to be younger than ca. 350 Ma. Peak metamorphism preceded this onset, as indicated by leucopegmatite crystallization and metamorphic Zrc ages. Mnz ages indicate transpression continued through at least ~330 Ma. Monazite U-Th-Pb microprobe chemical ages linked to transpressional microstructures indicate deformation continued to ca. 300 Ma. The extreme crustal shortening of the BHZ-CMZ from Maine to southern New England, pre-transpressional magmatism, and high grade metamorphism appear to have occurred in the absence of subduction. Lithospheric delamination at ca. 365 Ma is a tenable model for explaining these contemporaneous phenomena.