Geochemistry and tectonic setting of Tonian igneous and sedimentary rocks in the southern Cobequid Highlands, Nova Scotia, Canada: Implications for early history of Avalonia

The Cobequid Highlands of mainland Nova Scotia are part of Avalonia in the northern Appalachian orogen. U-Pb (zircon) dating has shown that, in addition to the Ediacaran igneous and sedimentary rocks that characterize Avalonia elsewhere, the Cobequid Highlands contain igneous and sedimentary rocks of Tonian age. In the Mount Ephraim block, volcanic rocks of the Dalhousie Mountain Formation, quartz diorite of the Six Mile Brook pluton, and dioritic to granitic rocks of the Mount Ephraim Plutonic Suite have yielded ages of ca. 760–730 Ma. All are calc-alkalic and formed in a continental margin subduction zone. In the Bass River block, interlayered sedimentary and volcanic rocks of the Gamble Brook (GBF) and Folly River (FRF) formations are also Tonian based on a maximum depositional age of ca. 780 Ma from a lithic tuff sample. The FRF is dominated by autobrecciated pillow basalt and aquagene tuff, related gabbro dykes and sills, chemically precipitated iron formation, and minor lithic tuff and clastic sedimentary rocks. The GBF is composed mainly of arkosic wacke interlayered with siltstone, mudstone, and minor calcareous rocks, together with mafic volcanic rocks and sills/dykes like those in the FRF. Although petrographically similar, mafic rocks in both formations display three types of chemical characteristics: N-MORB, E-MORB/continental tholeiite, and tholeiitic transitional to alkalic ocean-island basalt. Based on petrological characteristics and ages, the FRF formed in an extensional environment, possibly a back-arc basin associated with the magmatic arc represented by volcanic and plutonic rocks in the Mount Ephraim block, or an older arc.