Geochemistry, provenance, and tectonic significance of sedimentary rocks of the Middle to Late Devonian McArras Brook and Visean Martin Road formations, Merigomish Subbasin, northern Antigonish Highlands, Nova Scotia

摘要

The Merigomish Subbasin occurs along the southern flank of the composite Late Devonian-Permian Maritimes Basin of Atlantic Canada. The subbasin consists predominantly of Visean to Westphalian clastic rocks that overlie Middle to Late Devonian basalt and continental clastic rocks of the McArras Brook Formation. The McArras Brook Formation is unconformably overlain by a 120 meter thick Late Visean clastic sequence of the Martin Road Formation, which represents the oldest rocks deposited in the Merigomish Subbasin. Geochemical and isotopic attributes of representative samples of the clastic rocks in the McArras Brook and Martin Road formations were investigated in order to constrain their respective provenance and to investigate the regional tectonic significance of the angular unconformity that separates them. The clastic rocks selected for analysis have clast sizes ranging from 1 to 4 mm in diameter. The geochemistry and isotopic signatures of the clastic rocks in these formations arc so similar that they arc interpreted to have been derived from the same source region. Both formations are characterized by a limited range in SiO_2 (65-78 wt.), wider ranges in other major elements such as Mg0 and Fe_2O_3, relatively constant Ti/Nb, moderate LREE enrichment (La/Sm_n = 3.6-1.1), relatively flat HREE patterns (Tb/Yb_n = 1.3-1.4) strongly negative epsilon_(Nd) values (-6.5 to -7.7, calculated for a 320 Ma depositional age), and high T_(DM) values (ranging from ca. 1.5 to 1.6 Ga). Taken together, the geochemical and isotopic signature of the McArras Brook and Martin Road formations suggest that they were predominantly derived from local Silurian siliciclastic rocks of the Arisaig Group in the Antigonish Highlands. The isotopic data also suggest an important contribution from the Meguma terrane to the south. The data are compatible with local sources, and the geochemical similarity between the McArras Brook and Martin Road formations suggests that the angular unconformity that separates them is of local, rather than regional, significance.