PETROSTRUCTURAL ANISOTROPY OF AN ARCHEAN GNEISS DOME: EXAMPLE OF THE PUKASKWA BATHOLITH, SUPERIOR PROVINCE, CANADA

Large volumes of Archean shields consist in granitic gneiss domes. The internal fabric, petrostructural and seismic anisotropy of these large masses of crustal rocks are mostly unknown. Yet, they could significantly contribute to total shear wave splitting. Here we present the results of a pilot study conducted on the Pukaskwa batholith to evaluate the degree of internal organization of this body. We use the anisotropy of magnetic susceptibility (AMS) method to quantify the strength and orientation of mineral fabric.

The Pukaskwa batholith is a remarkable example of an Archean terrain. The Pukaskwa batholith is a heterogeneous assemblage of tonalo-trondjhemo-granodiorite (TTG) gneisses. Since the densities of the plutonic rocks are typically less in the Pukaskwa batholith than in the Hemlo greestone belt, some authors have proposed a diapiric rise. However, others argued that TTG gneisses adjacent to greenstone belts were deformed by strike-slip tectonics. AMS has proved useful in other studies using plutonic rocks in order to evaluate the mode of emplacement.

The goal of this study is to determine the internal fabrics of the Pukaskwa rocks. This data could also help with exploitation of economic deposits hosted in the Hemlo greenstone belt.

Preliminary results from the north-south transect of the Oiseau Bay section (20 km, 20 stations, 520 specimens) suggest that the Pukaskwa batholith has magnetic lineations of a broad northern trend with moderate 50° dips and foliations striking WNW - ESE (112°) with moderate 50° dip angles. The magnetic susceptibility ranges widely from 35 to 8543 x 10^-6 [SI] (average ~ 927 x 10^-6 [SI]). The degree of magnetic anisotropy (P_j = 1.184 on average) is high and consistent with a dominant magnetostatic/ferromagnetic origin. Thermomagnetic experiments have confirmed the ferromagnetic component is low ulvöspinel titanomagnetite with a Curie temperature of 551°C.

The new AMS data, analyzed in a geospatial reference framework, permits a discussion of different kinematic models and clarifies the respective roles played by either vertical or horizontal tectonics in the Hemlo – Pukaskwa system.