Lp. Bedard et Jn. Ludden, ND-ISOTOPE EVOLUTION OF ARCHEAN PLUTONIC ROCKS IN SOUTHEASTERN SUPERIOR PROVINCE, Canadian journal of earth sciences, 34(3), 1997, pp. 286-298
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.