SYNACCRETIONARY SEDIMENTARY AND VOLCANIC ROCKS IN THE ORDOVICIAN TETAGOUCHE BACKARC BASIN, NEW BRUNSWICK, CANADA: EVIDENCE FOR A TRANSITION FROM FOREDEEP TO FOREARC BASIN SEDIMENTATION

摘要

Development of an Upper Ordovician to lower Silurian (Llandovery) accretionary wedge (Brunswick subduction complex) along the composite Laurentian margin accompanied subduction of the Tetagouche backarc basin and coincided with synaccretionary sedimentation in the Bathurst and Fournier supergroups in northern New Brunswick. These dominantly turbiditic synaccretionary units conformably to unconformably overlie Upper Ordovician pelagic shale and chert in the upper stratigraphic levels of several of the nappes that compose the imbricate thrust stack of the subduction complex. Local occurrences of tectonic melange at the contacts between the turbidites and their pelagic substrate are consistent with deposition of at least some of the former during thrust-related deformation in the accretionary wedge. Detrital zircon data indicate maximum depositional ages ranging from 454 +/- 3 to 459 +/- 8 Ma, coeval with initiation of subduction of the Tetagouche backarc basin. In the Elmtree inlier, arc tholeiitic basalt disconformably or unconformably overlies Darriwilian MORB-type pillowed flows that constitute backarc oceanic crust of the Tetagouche basin. Sedimentary rocks immediately below and intercalated with the arc tholeiites contain detrital zircons ranging from 444 +/- 6 Ma to 455 +/- 10 Ma, suggesting that the tholeiites are related to backarc subduction.