Petrology, tectonic setting, and Ar-40/Ar-39 (hornblende) dating of the Late Ordovician-Early Silurian Belie Cote Road orthogneiss, western Cape Breton Highlands, Nova Scotia

The Belie Cote Road orthogneiss is a major component of the western Cape Br eton Highlands, where it forms a belt approximately 60 km in length. Previo us U-Pb dating has shown that the granodioritic to tonalitic protolith of t he gneiss crystallized at 442 +/- 3 Ma, providing a minimum age for the met avolcanic and metasedimentary units of the Aspy terrane intruded by the ort hogneiss. The gneissic fabric in the orthogneiss is mainly conformable with the regional fabric, and generally trends north-south, except in the south ern part of the unit where it is oriented east-west. Typical orthogneiss co ntains quartz, plagioclase, and biotite, with variable amounts of K-feldspa r and muscovite and rarely epidote or garnet, A tonalitic variant contains amphibole. The orthogneiss is peraluminous, with A/CNK values of 1 to 1.2. Petrochemical characteristics are consistent with syntectonic emplacement i n the roots of a volcanic are built on continental crust. Ar-40/Ar-39 datin g was done on hornblende from three samples of orthogneiss and seven sample s of amphibolite from xenoliths in the orthogneiss and an adjacent amphibol ite unit. Eight of these samples yielded cooling ages ranging between 384 a nd 370 Ma. Two younger ages (ca. 363 and 353 Ma) may reflect localized effe cts of younger plutonism and/or shearing. The Ar-40/Ar-39 data combined wit h previous U-Pb data from titanite indicate that the orthogneiss and associ ated units experienced rapid cooling from ca. 600 to 400 degrees C between ca. 386 Ma and 370 Ma, perhaps related to uplift associated with ongoing te rrane amalgamation in Cape Breton Island.