Geological significance of U-Pb and Rb-Sr ages for two pre- tectonic granites from the central metasedimentary belt, Ontario.

The geological history of two pre-tectonic granitic rocks from the CMB was investigated by comparing U/Pb zircon and sphene and Rb/Sr whole-rock ages determined using samples collected from the same site. The emplacement ages for the Tallan Lake granodiorite (1254 m.y.) and Methuen quartz monzonite (1242 m.y.) are recorded by the U/Pb zircon method, whereas the corresponding Rb/Sr whole-rock ages are too young and appear to have been disturbed. The Rb/Sr whole-rock age of 1129 m.y. for the Methuen body is between the age of emplacement (1242 m.y.) and the age of regional metamorphism (1085 m.y.), defined by a U/Pb sphene age from the same outcrop, and is inferred to be geologically meaningless. In the case of the Tallan Lake granodiorite, two geologically meaningful Rb/Sr whole-rock ages are obtained when the samples are divided into fracture-bearing and fracture-free groups. The isotopic ages determined indicate that at least three discrete geological events occurred; the generation and emplacement of granitic magma at approx 1250 m.y., regional deformation and metamorphism approx 1085 m.y. and a late-stage fracturing event and related fluid migration at 1023 m.y. An important conclusion from the Rb-Sr data is that the majority of units have initial Sr ratios within the range 0.703-0.704, indicating that most of this granitic magma was derived in the crust or mantle with little or no involvement of relatively old upper crustal material.-J.M.H.