Lithogeochemical and Sm-Nd and U-Pb isotope data from the Silurian-Lower Devonian Arisaig Group clastic rocks,Avalon terrane,Nova Scotia:A record of terrane accretion in the Appalachian-Caledonide orogen

摘要

The Silurian-Lower Devonian Arisaig Group,in the Avalon terrane of Nova Scotia,consists of a thick (approx1900 m) sequence of unmetamorphosed fossiliferous siliciclastic strata deposited during terrane accretion-ary events in the Appalachian-Caledonide orogen.Geochemistry and Sm-Nd and U-Pb (detrital zircon) isotope data of Arisaig Group rocks contrast with the underlying Avalonian rocks,indicating that they were not derived from Avalonian basement.All sedimentary rocks are characterized by strongly negative epsilon_(Nd(t)) values (from -4.8 to -9.3,t=430 Ma) and T_(DM) ages older than 1.5 Ga;the overall trend from the base to the top of the group is toward increasingly negative epsilon_(Nd) values.The fact that some Silurian sedimentary rocks contain detrital zircons of similar age suggests that basin formation was broadly coeval with active volcanism in the orogen.These samples also contain abundant Neoproterozoic-Early Cambrian zircons (ca.620-520 Ma) and lesser abundances at ca.1200-900 and 2200-1500 Ma.Archean zircons are very minor.The sample of Lower Devonian strata contains Late Silurian and Early Ordovician zircons and,in comparison to the Silurian samples,less abundant Cambrian (ca.520-510 Ma) and Neoproterozoic (610-550 Ma;834 Ma) zircons and subordinate Mesoproterozoic (1000-1200 Ma),Mesoproterozoic (1400-1600 Ma),and Paleoproterozoic (2000-2100 Ma) zircons.There are no Archean zircons.A comparison between the U-Pb detrital-zircon data and the age of tectonothermal events in potential source areas,together with regional geologic data,suggests that Silurian strata of the Arisaig Group were primarily derived from Baltica,but that there was increasing input from Laurentia by the time of deposition of the Lower Devonian strata of the group.The Arisaig Group is interpreted to have been deposited adjacent to the trailing edge of Avalonia during Appalachian accretionary events,and the geochemical and isotopic characteristics of its strata clearly record the signatures of these regional tectonic events.