Tectonic evolution of the Late Archean Pontiac Subprovince, Superior Province, Canada: Structural, metamorphic, and geochronological studies.

Abstract

The Pontiac Subprovince is a Late Archean (ca. 2.7 Ga) metasedimentary-metavolanic-granitoid-gneiss terrane situated along the southeastern margin of the Superior Province in Quebec. Detailed structural study of the northwestern part of this amphibolite facies metasedimentary belt has revealed a protracted history of deformation in Late Archean and Early Proterozoic time. An early contractional event (D$\sb1$) resulted in development of steep foliations now preserved as folded or straight relics within microlithons of S$\sb2$ foliation. During this stage, the Cadillac-Larder Lake fault zone acted as a major fault zone along which greenstones for the Abitibi Subprovince were thrust over the Pontiac Subprovince. Regional metamorphism resulted from both thickening of the crust, and intrusion of voluminous I-type graintes. D$\sb2$ structures record peak metamorphic high-temperature deformation of the crustal rocks during which large-scale D$\sb2$ nappes moved towards the south-southeast. The basal thrust faults of these nappes are preserved as high-strain shear zones within the study area. Second order east-trending recumbent F$\sb2$ folds, a penetrative S$\sb2$ crenulation foliation, and a north-northwest-trending L$\sb2$ elongation lineation are other important structures of this deformational event. A later D$\sb3$ contractional event superimposed east-trending upright folds on all earlier structures. Extensional D$\sb4$ structures are associated with reactivation of the Cadillac-Larder Lake fault zone as a normal fault zone, and are only recorded in or close to normal faults within this fault zone. Finally, brittle D$\sb5$ thrust faults and kink structures are superimposed on all older structures in the northwestern Pontiac Subprovince. Gneisses of the Lac Opasatica area record pre-D$\sb1$ penetrative structures that are not present within the other rock types of the study area. Increasing metamorphic grade from biotite zone to sillimanite zone is evident from north (the Cadillac-Larder Lake fault zone) to south in the study area, close to outcrops of the S-type granites. Thermobarometry of samples from the study area indicates that regional metamorphism of the rocks in the northwestern Pontiac Subprovince occurred at about 590$\sp\circ$C and 6.2 kbar. Investigation of the metamorphic and structural history of these Late Archean rocks suggests a clockwise PT path similar to that of Phanerozoic collisional belts. $\sp{40}$Ar/$\sp{39}$Ar age constraints from this study combined with other geochronologic indicate a slow cooling rate of about 2$\sp\circ$ to 6$\sp\circ$C/Ma, and reveal that temperatures as high as 350$\sp\circ$C and 280$\sp\circ$C were persistent in crustal rocks of the area until ca. 150 Ma after attainment of peak metamorphic conditions. (Abstract shortened by UMI.)