IAPETUS SHRUGGED: THE EARLY RIFTING RECORD OF LAURENTIA IN THE NEW ENGLAND AND QUÉBEC APPALACHIANS

The Ediacaran breakup of Rodinia created the Laurentian margin and set the stage for the Paleozoic accretion of terranes during the Appalachian orogenic cycle. Cawood et al. (2001) proposed two pulses of rifting in the northern Appalachians at ca 570 and 540 Ma and suggested that the first separated Laurentia from Gondwanan cratons and that the second created Laurentian microcontinents. Waldron and van Staal (2001) suggested that the Dashwoods block in Newfoundland is a peri-Laurentian fragment that was reaccreted during the Taconic orogeny.

Rift volcanic rocks in the New England and Québec Appalachians include a 571 ± 5 Ma felsite from the Pinney Hollow Fm (PHF) in Vermont (Walsh and Aleinikoff, 1999) and a 554 +4/-2 Ma felsite from the Tibbit Hill Fm (THF) in Québec (Kumparapeli et al., 1989). We used CA-IDTIMS to date 7 zircon grains from the PHF- 562.37 ± .14 Ma, 6 grains from one THF sample- 553.70 ± .06 Ma, and 4 grains from another THF sample- 552.24 ± .07 Ma. We also used LA-ICPMS to date detrital zircon from undated basal rift clastic rocks that were locally derived from subjacent basement rocks in isolated rift graben in the Hoosac Range in northwestern MA and in the Chester dome in southeastern VT. In contrast, the Rowe Schist in MA, correlative with the PHF in VT, contains detrital zircon derived from many different sources in Laurentia, suggesting deposition by long-shore currents on an ocean-facing continental margin.

Our data suggest that deposition in deep water on a (hyper-?) extended continental margin was under way by 562 Ma and that the rifted blocks were never far removed from the Laurentian margin. Relict oceanic crust and obducted mantle occur outboard of the Pinney Hollow/Rowe belt along the contact with the Gondwanan-derived Moretown Fm. Evidence for reaccreted peri-Laurentian crust can be reinterpreted as Ordovician deposits in intra-arc or syn-collisional basins.