IAPETUS SHRUGGED: THE EARLY RIFTING RECORD OF LAURENTIA IN THE NEW ENGLAND AND QUÉBEC APPALACHIANS AND THE GENERATION OF PERI-LAURENTIAN MICROCONTINENTS

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. The Argentine Precordillera (Astini et al., 1995) is a far-travelled Laurentian-derived microcontinent. Waldron and van Staal (2001) suggested that the Dashwoods block in Newfoundland is a peri-Laurentian fragment that was reaccreted during the Taconic orogeny.

Dated rift volcanic rocks in the New England and Québec Appalachians include a 571 ± 5 Ma felsite from the Pinney Hollow Fm in Vermont (Walsh and Aleinikoff, 1999) and a 554 +4/-2 Ma felsite from the Tibbit Hill Fm in Québec (Kumparapeli et al., 1989). We used CA-IDTIMS to date 7 zircon grains from the Pinney Hollow Formation. Our revised date is 562.37 +/- 0.14 Ma. We also used LA-ICPMS to date detrital zircon from undated basal rift clastic rocks and discovered that the detritus was 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 Pinney Hollow Fm 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 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.