MORE ABOUT THE MORETOWN TERRANE IN THE BRISTOL DOME, WESTERN CONNECTICUT

To test last year’s discovery of 550 Ma zircons in the Bristol dome, we have analyzed the other three members of the Taine Mountain Fm (TMF). We now have data form one sample each (in ascending order) of the Wildcat Mountain (Otw), Scranton Mt (Ots), and Whigville (Otwv) members, and one from the Bristol Gneiss. We analyzed 60 grains and 79 spots from the inner Otw unit. The largest (50%) of the grains (150 – 200 µm long and subhedral) have a weighted mean age of ~540 Ma. Lesser populations are ~582, ~630, 1200 to 1500, and 1900 to 2300 Ma. The dominant population at ~540 Ma confirms the presence of Macdonald’s et al. (2014) “Moretown terrane” in the provenance of these metasedimentary rocks. Paleoproterozoic cores in two grains with 540 Ma rims show that the Moretown terrane was derived from older crust. E(Hf)_t values from -2 to-4 are consistent with this, but other grains with values of +1 to +2 suggest more than one magmatic source. Eight grains have ages of ~620 Ma and these have E(Hf)_t values of +9 to -9, showing that sediments of the Otw were also derived from multiple peri-Gondwanan sources. Six 540 Ma grains have partial metamorphic rims with ages of ~505 to ~485 Ma. These abraded (detrital) rims suggest that the Moretown terrane experienced a metamorphic event in the late Cambrian prior to erosion and deposition in the TMF. The mantling Ots member is similar with 36 of 46 grains having ages of ~550 Ma, and 6 grains of ~635 Ma. Zircon yield of the upper Otwv member was poor, but ten of 22 grains have ages between 590 and 670 Ma, with no grains at ~540 Ma. Two grains with ages of ~640 Ma have E(Hf)_t values of -8, suggesting an evolved peri-Gondwanan provenance. The few and small grains from the outer Bristol gneiss were metamorphic, with ages of ~380 Ma. Only a few grains had small, inherited cores; attempts to date these cores failed. Taken together these results show that the TMF was deposited after the oldest detrital (abraded) metamorphic rim (~485) and before the oldest in situ metamorphic overgrowth (~430 Ma) or broadly in the Ordovician, with sediments derived from the metamorphosed Moretown terrane (arc?) and from an evolved peri-Gondwanan (Gander?) terrane. One may wonder if the complete lack of Ordovician detrital grains reflects either an early Ordovician depositional age or a forearc setting of the Moretown arc, isolated from eastern Ordovician arcs.