A NEW MODEL FOR PALEOZOIC TECTONICS IN SOUTHWESTERN NEW ENGLAND: CONSTRAINTS ON THERMAL HISTORY FROM MONAZITE U-TH-PB GEOCHRONOLOGY AND AR THERMOCHRONOLOGY

Tectonic evolution of SW New England (W CT and MA, SE NY) includes: 1) Neo-Proterozoic to L-M Ordovician sedimentation on Laurentian passive margin; 2) L to M Ordovician collision of one or more “Taconic” arcs by 450-445 Ma; 3) Intrusion of separate felsic and mafic to intermediate suites into the terrane at 440-450 Ma; 4) Development of S-D depositional basins (CT and MA); 5) Acadian deformation and high-P regional metamorphism (up to sil grade) at ca. 390 Ma; 6) Intrusion of granitoids at 360 Ma; and 7) Rapid exhumation, particularly from 390 – 375 Ma. New age constraints from microprobe monazite dating of nearly 40 samples from CT, MA and NY requires a substantial modification of the current model, including potential redefinition of the fundamental meaning of Cameron’s Line (CL). 15 samples of S-D Straits Schist east of CL yielded a dominant thermal-event age of 388 Ma (15 samples, with a 1s of the mean ages=4). Two other age populations in these samples are 422 Ma (10 samples, with 1s of 5) and 359 Ma (10 samples, with 1s of 6). 8 samples from Pre-S units east of CL show the same ages (388 Ma – 5 samples; 421 Ma – 5 samples; 359 Ma – 2 samples). Only 3 pre-S samples east of CL show any evidence for Taconic monazite growth, and all are in structural culminations that may be windows to a lower structural level. The 422 Ma age does not fit the conventional model and most closely matches pre-400 Ma ages in rocks from central MA, east of the Bronson Hill arc. Ordovician granitoids and mafic-intermediate igneous rocks (“Brookfield Diorite” on the CT state map) are all moderately to intensely mylonitic, are restricted to the E side of CL and resemble similar rocks in the Bronson Hill arc in both age and petrology. Samples from Taconic allochthons west of CL in CT and NY show no evidence of an Acadian (ca. 390) age in monazites, but do show a robust age population at 450 Ma (consistent with the Taconic age of Hames et al., 1991) and also evidence of an older (ca. 490-500 Ma) thermal event. Thus chronologic and structural evidence suggest greater complexity than previously thought: in light of the new age data, CL may be the surface expression of a shallowly dipping post-Taconic detachment surface bounding a package of far-traveled rocks that originated on, or east of, the current location of the Bronson Hill arc and was emplaced onto the Taconic forearc sometime after 420 Ma but before 390 Ma.