GEOCHRONOLOGY AND GEOCHEMISTRY OF THE ANNIDALE GROUP, SOUTHERN NEW BRUNSWICK, CANADA: CAMBRO-ORDOVICIAN ARC-RELATED MAGMATISM ALONG THE MARGIN OF GANDERIA

Late Cambrian - Early Ordovician rocks of the Annidale Group are preserved within a northeast-trending, fold-thrust belt along the northwestern margin of the peri-Gondwanan New River terrane in southern New Brunswick. The group comprises a complex assemblage of marine mafic-intermediate volcanic and sedimentary rocks, rhyolite flows and domes, gabbro and plagiogranite. New U/Pb data indicates that tectonic interleaving of the c. 497 – 489 Ma Annidale Group and its juxtaposition with New River basement occurred prior to c. 476 Ma, the age of the Stewarton gabbro that stitches the faulted contact. Younger (c.478-472 Ma) felsic volcanic rocks and associated rhyolite domes are interpreted to lie unconformably on the Annidale Group.

Geochemical signatures of the Annidale Group are indicative of supra-subduction zone magmatism. Mafic-intermediate volcanic rocks are subalkaline, mostly tholeiitic to transitional basaltic andesites that exhibit both arc and non-arc geochemical signatures. Most samples show weak to strong negative Nb and Ta and positive Th anomalies on extended rare earth plots and fall clearly within volcanic-arc fields on trace-element discrimination diagrams. One exception is a group of extreme LREE-depleted basalts associated with plagiogranite intrusions, which display concave-upwards REE patterns typical of N-MORB affinity, but with uncharacteristically low TiO₂ (0.48-0.54%) more typical of arc tholeiites. These basalts are also magnesian-rich (13.7 – 17.2% MgO) and strongly enriched in Cr (1180-4910 ppm) and Ni (390-880ppm).

These new data bearing on the age and tectonic setting of the Annidale Group are in accord with recent models for the tectonic development of the Exploits Subzone in Newfoundland and confirm that Latest Cambrian – Early Ordovician magmatism in the Annidale Group can be related to the development of the Penobscot arc along the Gander margin. Latest Arenig to Middle Ordovician (post-Penobscottian) magmatism, possibly related to the initiation of the Victoria arc, affected both the Annidale Group and adjacent Neoproterozoic – Early Cambrian New River terrane. A recent ⁴⁰Ar/³⁹Ar age of 444 +/- 5 Ma for metamorphic muscovite in schistose felsic tuff of the Annidale Group indicates that unroofing of the arc volcanics was coincident with the final closure of Iapetus.