Abstract

New U-Pb ages from zircon, titanite, or monazite separates from 12 units in the Aspy and Bras d'Or terranes, Nova Scotia, range from late Precambrian to Devonian and generally support the interpretation that the 2 terranes had differing histories until juxtaposition in the Devonian. Five plutons of dioritic, tonalitic, and granodioritic compositions in the northern Bras d'Or terrane yielded zircon ages between 575 (maximum) and 556 ± 4 m.y. Younger titanite ages of 548 ± 2 and 521 ± 2 m.y. from two of these plutons may indicate younger metamorphism. Previously undocumented Early Ordovician plutonism (493 ± 2 Ma, zircon) and metamorphism (496 ± 5 Ma, titanite) also occurred in the Bras d'Or terrane.

In the Aspy terrane, a zircon age of 433 +7/-4 m.y. was obtained for a rhyolite unit in the upper part of a widespread metavolcanic-metasedimentary sequence, which together with higher-grade, probably correlative units, is characteristic of the terrane. The age is interpreted to indicate that most metamorphic rocks in the terrane are Ordovician to Early Silurian in age, in contrast to Precambrian ages in the Bras d'Or terrane. Devonian plutonism was widespread in the Aspy terrane, whereas Devonian plutonism in the Bras d'Or terrane appears restricted to the vicinity of the boundary with the Aspy terrane. Zircon, titanite, or monazite ages from 4 plutonic and orthogneissic units in the Aspy terrane and in the Bras d'Or terrane adjacent to the terrane boundary range from 403 ± 3 to 375 +5/-4 m.y.

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