DETRITAL ZIRCON GEOCHRONOLOGY IN GANDERIAN AND AVALONIAN TERRANES OF CAPE BRETON ISLAND AND SOUTHERN NEW BRUNSWICK, CANADA: CHALLENGES AND OPPORTUNITIES

Abstract

The definition of Ganderian and Avalonian terranes in the northern Appalachian orogen is based on a long history of inquiry on multiple fronts. Differences in magmatic and metamorphic events and in isotopic characteristics have been used most commonly to separate terranes from each other, and to subdivide them internally. However, detrital zircon geochronology has made increasingly important contributions in recent years and has provided information regarding source areas for Ganderian and Avalonian terranes during their pre-Appalachian evolution. This work is hampered by difficulties in interpreting the provenance of Meso- to Neoproterozoic zircon grains typically interpreted as derived from the West African and/or Amazonian cratons. For example, 1.4 – 1.0 Ga zircon grains can be derived from a number of long-lived magmatic systems in arcs and continental settings associated with the Grenville orogen. The well-documented preservation bias for Grenville-aged zircon grains makes this problem even more difficult to address. In syn- and post-collisional strata all along the Appalachian orogen, distinctions among detrital zircon populations become even more problematic because of the potential Laurentian contribution, which mirrors many of the Meso- to Neoproterozoic source areas in the Amazonian and West African cratons. These younger strata also commonly contain large populations of magmatic zircon grains derived from arc and other magmatic activity during the Ordovician through Devonian accretionary and collisional phases of the Appalachian orogen, making it even more difficult to see the Precambrian contributions and to interpret source areas from small populations of grains. On-going work in Precambrian rocks of the Ganderian Bras d’Or and Brookville terranes in Nova Scotia and New Brunswick has shown that these terranes preserve rocks from a former passive margin of Gondwana, arc rocks formed in Pan-African subduction zones, and rocks formed during the initial stages of rifting of Ganderia from Gondwana. The dominant contributions of zircon grains are from the Amazonian craton, with signatures distinct from those typical of Avalonian rocks of the same age range. In post-collisional strata from the same terranes, consistent distinctions are much more difficult to observe.