DETRITAL ZIRCONS: INTERPRETING AGES THAT ARE MISSING

We have dated detrital zircons from several quartzites in eastern CT both to constrain the ages of deposition and gain insight into the likely provenance areas, specifically Laurentia or peri-Gondwanan. Clough Quartzite in the Bronson Hill terrane in Vernon, CT contains zircons with ages suggesting derivation from the underlying Bronson Hill arcs, as well as both peri-Gondwana and Laurentian sources. An unnamed quartzite from the Bronson Hill terrane in Essex, CT contains zircons with mostly middle Ordovician ages, suggesting local derivation from the Bronson Hill arc. These contrast with ages of detrital zircon from two unnamed quartzites correlated with the Plainfield Formation by Rodgers (1985) and associated with the 600–650 Ma peri-Gondwanan rocks of the Lyme dome and the Avalonian Selden Neck block. These unnamed quartzites and local paragneiss, form thin lenses surrounded by long, continuous sheets of tonalitic and granitic orthogneisses interpreted by some (e.g. Rodgers, 1985) to be volcanic rocks as are present in the Boston platform. However, zircons from these quartzites are exclusively older than about 925 Ma, and contain no zircons with the late Neoproterozoic age of the surrounding rocks. An explanation for the lack of zircon ages between 600–650 Ma is that the surrounding orthogneisses are younger and intruded into the quartzites and metasedimentary sequence. Similarly, the very narrow range of zircon ages (615–600 Ma) from the enclosing orthogneisses is inconsistent with detrital mixing typical of paragneisses but characteristic of intrusive magmas. An explanation of the map pattern of lenses of quartzite in long, continuous orthogneiss sheets is high ductile strain associated with the thrust-wedging of the Avalonian block between Gander cover and magmatic rocks along the Honey Hill and Hunts Brook faults. In short, the lack of late Neoproterozoic zircons in peri-Gondwanan quartzites was the first clue to discriminating between volcanogenic sedimentary rocks and strongly attenuated plutonic rocks to explain igneous sheets of rocks with igneous compositions tens of Km long.