THE COBEQUID HIGHLANDS OF NORTHERN MAINLAND NOVA SCOTIA, CANADA: EVIDENCE FOR TONIAN AND EDIACARAN MAGMATIC ARCS IN AVALONIA

Relations among Precambrian metamorphic and igneous rocks in the Cobequid Highlands of Nova Scotia are obscured by Silurian and Devonian sedimentary rocks, Carboniferous bimodal volcanic rocks, and abundant intrusions of Carboniferous gabbro and granite. In addition, the rocks are cut by numerous faults that make correlations difficult. Despite these obstacles, a combination of mapping, petrological studies, and U-Pb zircon dating mainly by laser ablation-inductively couple plasma-mass spectrometry is demonstrating the critical importance of the area in understanding the origin and early history of Avalonia. New combined with previously published ages demonstrate that the 612-600 Ma continental margin magmatic arc gabbroic to granitic plutons of the Bass River block intruded the mainly metavolcanic Folly River and mainly metasedimentary Gamble Brook formations. Based on petrological characteristics, both formations are interpreted to have been formed in a Tonian back-arc basin associated with a 770-725 Ma continental margin magmatic arc represented by mafic to felsic volcanic rocks of the Dalhousie Mountain Formation and related Mount Ephraim Plutonic Suite. The latter suite intruded the metasedimentary Mount Thom Formation, which has a likely maximum depositional age of ca. 800 Ma. Detrital zircon spectra from metasedimentary formations in the Mount Thom, Gamble Brook, and Folly River formations are dominated by Meso- and Paleoproterozoic zircon with peaks at 1200, 1500, and 1980 Ma, and all three units are interpreted to be of similar Tonian age (800-750 Ma). The Folly River Formation consists mainly of autobrecciated pillow basalt and aquagene tuff, related gabbro dykes and sills, and abundant ironstone. It includes lithic tuff with intermediate to felsic clasts that yielded only two large zircon populations at ca. 780 Ma and ca. 890 Ma, direct evidence for the oldest arc magmatism yet reported in Avalonia. In contrast, the Jeffers block in the northern Cobequid Highlands contains mainly mafic to felsic tuffaceous rocks with ages of ca. 630-600 Ma and plutons of similar Ediacaran ages. However, arc-related plutonic rocks of Tonian age also occur in the Jeffers block. Although their extent is not yet well defined, their presence supports a link among the Bass River, Mount Ephraim, and Jeffers blocks.