DETRITAL ZIRCON GEOCHRONOLOGY OF THE FREDERICTON TROUGH, NEW
BRUNSWICK: EVIDENCE FOR SILURIAN CLOSURE OF AN IAPETUS REMNANT
R.J. Dokken, J.W.F. Waldron, S.A. DuFrane
The Fredericton Trough cover succession, in the Appalachians of southwestern New Brunswick, consists of the Silurian Kingsclear Group, and is situated between the Ganderian St. Croix and Miramichi terranes, which represent crust rifted from Amazonia on the margin of the Iapetus Ocean. South of the northeast trending Fredericton Fault, which bisects the trough, the Kingsclear Group unconformably overlies the Kendall Mountain Formation of the St. Croix terrane, interpreted to have been deposited on the Ganderian passive margin of the Tetagouche-Exploits backarc basin. To the north of the Fredericton
Fault, the Kingsclear Group is in faulted contact with rocks of the Miramichi terrane, including the
Brunswick subduction complex. The Fredericton Trough has thus been interpreted to represent a marine foredeep formed during Salinic orogenesis, filling as the Tetagouche-Exploits seaway closed to bring together northern and southern components of Ganderia. Detrital zircon samples were collected from known fossil localities, where possible, in four formations of the Kingsclear Group for U-Pb dating by
LA-MC-ICP-MS. South of the Fredericton Fault, detrital zircon populations in the early Llandovery
Digdeguash Formation include a prominent statistical peak in the late Neoproterozoic (ca. 620 Ma) – typical of peri-Gondwanan detritus – with a minor peak in the Cambrian (ca. 500 Ma), and a small proportion of zircons in the 1.0 to 1.9 Ga range. The overlying late Llandovery to Wenlock Flume Ridge
Formation shows a significant Ordovician population (ca. 465 Ma), and an asymmetric peak at 1.0 Ga with a tail of older Proterozoic zircons, characteristic of Laurentian input. North of the Fredericton fault, the early Llandovery Hayes Brook Formation shows a dominant peak in the Ordovician (ca. 465 Ma), and a peak at 1.0 Ga with a range of Proterozoic zircons (ca. 1.0 to 1.8 Ga), similarly consistent with
Laurentian provenance. The Burtts Corner Formation, of Wenlock to Ludlow age, has a mixed signature including peaks in the Paleozoic (ca. 480 Ma), Neoproterozoic (ca. 660 Ma), and in the Proterozoic (ca.
1.0 to 2.2 Ga), suggesting that later input from an uplifted Miramichi terrane added material to the
Laurentian sources. Contrasting detrital zircon signatures between approximately synchronous formations suggest a significant role for the Fredericton Fault. To the south of the fault, where the Kingsclear Group sits unconformably upon rocks of the Ganderian St. Croix terrane, Laurentian detritus is not observed until at least the late Llandovery, indicating that a remnant of the Iapetus Ocean separated Laurentia from southern components of Ganderia until this time.