Basement Influence on the Supracrustal and Plutonic Evolution and Deformation of an Archean Greenstone Belt

Abstract

The 10–14 km thick Archean volcano-sedimentary sequence in the Chibougamau region is composed of two mafic to felsic cycles, and of two thick, predominantly epiclastic, sedimentary sequences. The supracrustal suite is isoclinally folded along east-west axes, and punctuated by syn-volcanic, syn-kinematic and post-kinematic plutons. Tonalitic basement occurs along the northern margin, in a central horst, and as large rafts in later batholiths. Events affecting the supracrustal rocks are recorded in the tectono-magnetic history of the basement gneiss superimposed on the events related to basement formation.

The first volcanic cycle consists of a mafic lava plain with felsic volcanic centers rising above sea level. Stabilization by the intrusion of a sub-volcanic batholith above the basement led to the erosion of much of the volcanic edifice, which terminated activity in the southern portion of the belt. The second volcanic cycle built on the first cycle in the north and terminated with the intrusion of a subvolcanic differentiated complex and batholith, uplifting the volcanic edifice and shedding elastics into marginal basins.

Major east-west folding was coupled with the injection of elongate monzodiorite and tonalité batholiths along the basement supracrustal boundaries. The deformation pattern is strongly affected by the presence of basement and pre-kinematic plutons.

A tentative model favors a continent margin with periodic rifting controlling the development of the supracrustal sequence. The basement fracture pattern established at the outset has influenced every stage in the deposition, intrusion, and deformation of the belt.