ND-ISOTOPE EVOLUTION OF ARCHEAN PLUTONIC ROCKS IN SOUTHEASTERN SUPERIOR PROVINCE

The Opatica plutonic belt, Abitibi greenstone belt, and Pontiac Subpro vince represent a major proportion of the southeastern Superior Provin ce, which was formed and accreted rapidly between approximately 2.9 an d 2.8 Ga. Plutons in these belts are grouped into four types: (i) tron dhjemite-tonalite-granodiorite (TTG) suite (2.82-2.69 Ga), (ii) monzod iorite (MZD) suite (2.697-2.659 Ga), (iii) late alkaline granitoid (AL I() suite (2.68-2.67 Ga), and (iv) anatectic granite and monzonite (AN A) suite (2.69-2.64 Ga). The four suites are represented in all belts and show similar petrography and geochemistry. In terms of Nd-isotope composition, the TTG, MZD, and ALK suites are typical of destructive p late margin magmatism and have +1.4 < initial epsilon(Nd) < +3.7, valu es which are very similar to that of the Abitibi mantle (epsilon(Nd) 2.5). The lower values for the ANA suites (epsilon(Nd) +0.1 to +2.4) r esult from recycling of crustal components. In the Opatica belt the AN A granitoids fall on the Nd-isotope evolution curve defined by the Opa tica TTG plutons, and are thus considered to be melt products of this suite. However, Abitibi and Pontiac ANA suites show a larger range of epsilon(Nd) from +0.1 to +2.4, compared with +1.0 to +1.3 for the Opat ica, suggesting more heterogeneous crustal source rock. Recent geologi cal mapping and geophysical studies associated with the Lithoprobe pro ject have suggested that the Opatica belt represents a plutonic belt a gainst which the Abitibi was accreted by subduction-related collision and that the Pontiac Subprovince is dominated by imbricated metasedime nts related to the final stages of collision in the Abitibi region. Th e Nd-isotope data provide support for these arguments. Early plutonic suites are mantle derived and related to arc-accretion processes. As t he collision process progresses, a more evolved isotopic component is introduced, possibly in relation to sediment subduction into the mantl e. Anatexis of the crust in the central Opatica belt and the core of t he Pontiac Subprovince resulted in the formation of granites with a cr ustal signature for Nd isotopes.