Claude Hubert

Accretion of Archean oceanic plateau fragments in the Abitibi, greenstone belt, Canada

The arc-arc collision model is often applied to the evolution of Archean greenstone belts. This model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic-tholeiitic affinities toward calc-alkalic affinities. New geological data from the Malartic composite block of the Abitibi greenstone belt indicate that this volcanic pile, rather than representing a conformable sequence, is composed of four distinct mafic lithotectonic domains that are overlain by a calc-alkalic sequence. The mafic domains are thought to be examples of accreted Archean oceanic plateau material that were deformed during their collage state. The younger calc-alkalic sequence represents extension-related volcanism that erupted through the ac- creted and deformed plateau material, possibly as a consequence of ridge subduction in a regime of oblique convergence. The relations described here may represent a common feature of Archean terranes.

The tectonic evolution of the Abitibi greenstone belt of Canada

Based on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics. The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma. The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province☆

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone. Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico. Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.

Geologic evolution of the Late Archean Abitibi greenstone belt of Canada

The southern volcanic zone (SVZ) of the Late Archean Abitibi belt of the Superior province of Canada is dominated by komatiitic to tholeiitic volcanic plateaus and large, bimodal, mafic-felsic volcanic centers. These volcanic rocks were erupted between about 2717 and 2700 Ma in a series of rift basins that formed as a result of wrench-fault tectonics. They overlie and juxtapose a volcano-plutonic assemblage characterized in the northern Abitibi belt. The age of the assemblage is about 2720 Ma or older, and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics, cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ± 2 Ma. A tectonic model is proposed in which the SVZ formed in a series of rift basins that dissected an earlier formed volcanic arc. Comparisons are made with rift environments that have been postulated for Phanerozoic areas such as the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand, and the Sumatra and Nicaragua arcs.