Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Belleterre-Angliers Greenstone Belt in the Pontiac Subprovince is the youngest and southernmost of the greenstone belts in the Archaean Superior Province of the Canadian shield. The greenstone belt has been disrupted into three fragments: from north to south the Baby, Belleterre and Lac des Bois groups, respectively. The BeUeterre-Angliers Greenstone Belt has sheared contacts with younger Pontiac metasediments and older tonalites. The Pontiac metasediments dip beneath the Baby Group and are tectonically interlayered with the tonalites. Thus the Belleterre-Angliers belt is interpreted to be a thrust sheet (6 km thick in the north, thinner in the south) above a mid-crustal duplex consisting of metasedimentary and tonalite imbricates. The thrust sheet was transported from the north, but age and geochemical constraints rule out derivation from the Abitibi Subprovince. It probably roots in a band of ultramafic rocks that mark a major south-vergent thrust in the northern Pontiac Subprovince. After emplacement the Belleterre-Angliers sheet and the adjacent Pontiac metasediments were deformed by a system of linked, extensional shears and transfer faults that border dome or antiformal areas in a style similar to Phanerozoic metamorphic core complexes. The orientation of shear zones and foliations indicates that the regional far-field stresses were compressional both before and after extensional shearing. Since the system of extensional shears is intruded by a suite of monzodiorite, granodiorite, syenodiorite and syenite plutons (with high Mg number) it is concluded that crustal inflation caused by the injection (intraplating) of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting.