THE MOUNT EPHRAIM BLOCK, COBEQUID HIGHLANDS, NOVA SCOTIA, CANADA: A SIGNIFICANT CA. 750-735 MA MAGMATIC ARC EVENT IN AVALONIA

The fault-bounded Mount Ephraim block forms the eastern part of the Avalonian Cobequid Highlands of northern mainland Nova Scotia. Mapping, petrological work, and new U-Pb zircon dating have shown that the block includes ca. 800 Ma quartzofeldspathic, semi-pelitic, and pelitic gneiss and schist of the Mount Thom Formation, ca. 750 Ma volcanic rocks of the Dalhousie Mountain Formation, 780(?) to 730 Ma gabbroic/dioritic to granitic plutons of the Mount Ephraim suite, and ca. 740-730 Ma dioritic rocks of the Six Mile Brook pluton. The plutonic rocks are magnesian and calc-alkalic and formed in a continental margin subduction zone. Eight samples from the Mount Ephraim suite have epsilon Nd ranging from -1.7 (monzogranite) to +1.7 (gabbro). These older units are intruded by the ca. 635 Ma calc-alkalic arc-related granodioritic Gunshot Brook Pluton (epsilon Nd 0.5 and 0.6). An elongate body of within-plate syenite to alkali-feldspar granite and gabbro intruded the Mount Thom Formation and Mount Ephraim suite along their southern margin yielded zircon ages of 482–480 Ma indicating a link to similar alkalic rocks in the Antigonish Highlands by the Ordovician.

The Mount Ephraim block contrasts with other components of the Cobequid Highlands in the Bass River and Jeffers blocks. The Bass River block along the southernmost margin of the highlands consists of mafic metavolcanic rocks interbedded with quartzite, metawacke, and minor marble and ironstone of the Folly River and Gamble Brook formations, with maximum depositional ages of ca. 1 Ga. These units are intruded by abundant ca. 622 and ca. 610 Ma calc-alkalic arc-related plutonic rocks. The Rockland Brook mylonite zone separates the Bass River block from ca. 625 to ca. 590 Ma volcanic rocks and co-magmatic calc-alkaline granitoid rocks of the Jeffers block, considered typical of Avalonia. However, a U-Pb zircon age of ca. 745 Ma from rhyolite porphyry in the Jeffers block may indicate a link between the Jeffers and Mount Ephraim blocks. Although similar in age to the Mount Ephraim rocks, the Burin Group in the Newfoundland consists of pillowed basalt and other mainly mafic rocks interpreted to have formed in an ensimatic arc. The magmatic events recorded in the Mount Ephraim block provide potential sources for Cryogenian and Tonian inherited/detrital zircon elsewhere in Avalonia.