PB AND O ISOTOPIC CONSTRAINTS ON THE SOURCE OF GRANITIC-ROCKS FROM CAPE-BRETON ISLAND, NOVA-SCOTIA, CANADA

Pb isotopic compositions of leached feldspars from twenty-three pluton s in-Cape Breton Island can be divided into two groups: anorthosite, s yenite, and granite in the Blair River Complex, which have the least r adiogenic compositions on the Island, and granitic rocks from terranes (Aspy, Bras d'Or, and Mira) to the south. Pb isotopic data for the Bl air River Complex ((206)pb/(204)pb = 17.399-18.107; Pb-207/Pb-204 = 15 .505-15.560; Pb-208/Pb-204 = 36.689-37.733) are consistent with an old source region ultimately derived from the mantle and contaminated py sialic crust. Oxygen isotopic compositions of syenite in the Blair Riv er Complex (delta(18)O = +8.0 to +8.5 permil) are slightly higher than anorthosite (+7.0 to +8.3 permil); a Silurian granite in the Blair Ri ver Complex has delta(18)O = +7.5 permil. Cambrian to Devonian plutons in the Aspy, Bras d'Or, and Mira terranes are more radiogenic (Pb-206 /Pb-204 = 18.192-18.981; Pb-207/Pb-204 = 15.574-15.712; Pb-208/ Pb-204 =37.815-38.936) than the Blair River Complex and were generated from s ource regions having a predominant crustal Pb signature (high CL). The delta(18)O values of granites and granodiorites in the Aspy terrane ( +7.5 to +9.2 permil; avg +8.6 permil) and Bras d'Or (+3.7 to +11.3 per mil; avg +9.4 permil) are also consistent with involvement of sialic c rust. Many Late Proterozoic granites from the Mira terrane have anomal ously low delta(18)O values (+0.2 to +5.9 permil), perhaps produced fr om protoliths that had undergone hydrothermal alteration prior to melt ing. Paleozoic granitic rocks from the Aspy, Bras d'Or, and Mira terra nes cannot be uniquely distinguished on the basis of their Pb and O is otopic compositions. The granitic rocks could have been generated duri ng terrane amalgamation from combinations of unradiogenic (Grenville-l ike) and more radiogenic (Avalon-like) sources.