Abstract

Zircon U–Pb dating on rhyolites from three different localities in the upper Aillik group, a volcano-sedimentary sequence of the Makkovik Orogen in Labrador, indicates two distinct pulses of volcanic activity at about 1860 and 1807 Ma. The individual rhyolite ages of graphic, 1856 ± 2, and 1807 ± 3 Ma are 40–150 Ma older than Rb–Sr ages determined previously on the same rocks. A slight scatter of zircon data observed for one of the rhyolites and the presence of old zircon cores suggest that the felsic magmas were derived in part from older continental material. Analyses of monazite and titanite from two anatectic rocks, a migmatite and leucogranite from a gneiss terrain adjacent to the supracrustals, document a period of high-grade metamorphism in the interval 1794 ± 2 to 1761 ± 2 Ma. This period is considered the terminal phase of the Makkovikian Orogeny. Zircons in the migmatite yield minimum ages ranging from about 1800 to 2340 Ma, indicating a derivation from latest Archean to early Proterozoic basement rocks.Zircon dating on a rhyolite from the Bruce River Group within the ~1650 Ma old Trans-Labrador batholith yields an age of 1649 ± 1 Ma. This age suggests that Bruce River acid volcanism was related to Trans-Labrador batholith emplacement.

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