U-PB AGE, TRACE ELEMENT AND HF ISOTOPIC COMPOSITIONS OF ZIRCON FROM SILURIAN AND EARLY DEVONIAN GRANITE, GABBRO, AND PYROXENITE IN(?) THE BLAIR RIVER INLIER, NORTHERN CAPE BRETON ISLAND, NOVA SCOTIA, CANADA

Relationships between Silurian igneous rocks and Proterozoic metamorphic rocks In the Blair River Inlier (BRI) of northern Cape Breton Island, Nova Scotia, have been brought into focus by modern microanalytical methods. Granite and diorite in the southernmost BRI have been known to be Silurian since the 1990s and metamorphosed gabbro and anorthosite yielded only Mesoproterozoic U-Pb zircon ages with a strong Silurian metamorphic overprint. More recently, small bodies of unmetamorphosed anorthosite, gabbro, and pyroxenite have all yielded Silurian or early Devonian U-Pb zircon ages, calling into question the Mesoproterozoic age interpretation of the large massif-type anorthosite and the broader significance of the BRI in the northern Appalachian orogen.

We present laser-ablation U-Pb ages along with trace element and isotopic compositions of zircon that expand the compositional range of mid-Paleozoic igneous rocks in the southernmost BRI and help to distinguish between these rocks and metamorphosed Mesoproterozoic to early Neoproterozoic rocks of similar composition. Two granite samples yield U-Pb zircon ages of 432.4 ± 4.5 Ma and 413 ± 4 Ma with epsilon Hf of -1 to +3. Gabbro and pyroxenite samples yield U-Pb zircon ages of 413 ± 8 Ma and 418 ± 2 Ma, respectively. Gabbro zircon has epsilon Hf in the range +3 to +9 and pyroxenite has epsilon Hf of -10 to -13. None of these rocks contain abundant Proterozoic zircon xenocrysts. Mesoproterozoic and early Neoproterozoic rocks of the BRI have complicated U-Pb systematics due to Pb-loss and possibly multiple overgrowth events; however, none of the previously dated Proterozoic units contain Silurian or Devonian magmatic zircon.

The mid-Paleozoic plutonic rocks are located in a structurally complex zone in the southern BRI where the bounding Wilkie Brook and Red River faults converge, and outcrop is limited. It is possible that a Silurian to early Devonian gabbroic complex intruded, metamorphosed, and locally melted part of the southernmost BRI. It is also possible that the southwestern boundary needs to be modified, and an unmapped fault exists between the unmetamorphosed igneous rocks and the metamorphosed Proterozoic units of the BRI. Resolution awaits a new mapping campaign in the southern BRI.