FIELD RELATIONSHIPS, PETROLOGY, AND PETROGENESIS OF NEOPROTEROZOIC PLUTONS IN THE SOUTHERN COBEQUID HIGHLANDS, NOVA SCOTIA, CANADA

The Cobequid Highlands in northern mainland Nova Scotia form an enigmatic part of Avalonia in the northern Appalachian orogen. They include Neoproterozoic arc-related volcanic and plutonic rocks, but apparently do not contain Cambrian cover sequences. Historically the Cobequids have been divided into two parts separated by the Rockland Brook Fault, the Jeffers block in the north and the Bass River block in the south, which is the focus of this study. Four Late Neoproterozoic plutonic units are identified in the southern Cobequid Highlands: Gunshot Brook, McCallum Settlement, Debert River, and Frog Lake. With the exception of Frog Lake, these plutons range in composition from diorite to tonalite, granodiorite, granite and alkali-feldspar granite. Previously reported U-Pb (zircon) data include a poorly constrained age of 575±5 Ma from granite in the McCallum Settlement pluton, ages of 609±4 Ma and 612±4 Ma from granodiorite samples from the Debert River pluton, and 605±5 Ma for granite from the Gunshot Brook pluton. Farther west, four strongly mylonitic granitoid samples yielded U-Pb ages ranging from 580±5 to 605±5 Ma and are interpreted to be related to the Debert River and McCallum Settlement plutons. In the westernmost part of the block, mylonitic granodiorite in Economy River yielded a previously published U-Pb (zircon) age of about 734 Ma. In contrast to the other plutons, Frog Lake consists of heterogeneous, variably mylonitic bodies of diorite (and locally tonalite) scattered throughout the southwestern part of the highlands. A ⁴⁰Ar/³⁹Ar age of 622±3 Ma was reported previously from hornblende in one of these diorite bodies. In spite of their apparent range in ages, new and previously published geochemical data from all of these plutons suggest that the tonalitic to granitic units represent a co-genetic suite of calc-alkaline granitoid rocks formed in a subduction zone setting. However, at least some dioritic samples from the Frog Lake bodies have high Ti, V, and FeO^t/MgO ratio, and display tholeiitic trends, indicating that they may not be genetically linked to the calc-alkaline suite. Preliminary results indicate that the Late Neoproterozoic plutons of the southern Cobequid Highlands are similar to Neoproterozoic plutonic and volcanic units in the Jeffers block to the north.