Late Archaean Kenogamissi complex, Abitibi Subprovince, Ontario, Canada: doming, folding and deformation-assisted melt remobilisation during syntectonic batholith emplacement

Abstract

The Kenogamissi complex represents a large exposure of folded Late Archaean crystalline crust exposed within the Abitibi Subprovince, Ontario, Canada. It is composed of an heterogeneous amphibolite-grade orthogneiss unit, and several generations of batholiths and plutons of tonalite, granodiorite and granite composition. Together, the various units represent granitic magmatism during the period from 2740 Ma to 2660 Ma. Structural mapping and petrographic studies were focused on the orthogneiss unit (2723 Ma), on the newly defined Roblin tonalitegranodiorite batholith (ca. 2713 Ma) and on the highly strained metavolcanic rocks within the deformation aureole that surrounds the Kenogamissi complex. Structural analysis indicates that the Kenogamissi complex was emplaced into the greenstones as a dome that caused severe flattening and recumbent F2 refolding of earlier F1 folds in the deformation aureole. Doming is interpreted to be caused by the emplacement and inflation of tonalite-granodiorite batholiths, such as the Roblin Batholith, into the actively folding Swayze greenstone belt. Continued regional folding resulted in F3 refolding of F1 and F2 in the deformation aureole. Continued regional folding also deformed and folded the Kenogamissi complex and resulted in further uplift and emplacement of the complex into the greenstone belt. The early-formed magmatic foliation and compositional layering in the Roblin Batholith were folded by F3 while the batholith was still a crystal mush, and an F3 axial-surface magmatic foliation was locally formed. Folding of the partially molten Roblin Batholith also resulted in the remobilisation of fractionated liquids into shear zones which formed on the limbs of the F3 magmatic folds. Similar structures are present in the orthogneiss unit and are interpreted to represent remobilisation of melts which intruded the orthogneiss at the time of emplacement of the Roblin Batholith. The formation of the dykes on sheared fold limbs may be attributed to increased dilatancy during localised shearing of the crystal mush. Deformation-assisted remobilisation and extraction of fractionated liquids, and the possible transport of the fractionated liquids to higher levels in the crystallising Roblin Batholith, may have played a role in its magmatic differentiation.