Diapirism and sagduction as a mechanism for deposition and burial of 'Timiskaming-type' sedimentary sequences, Superior Province: Evidence from detrital zircon geochronology and implications for the Borden Lake conglomerate in the exposed middle to lower crust in the Kapuskasing uplift

摘要

Neoarchean "Timiskaming-type" sedimentary sequences are the youngest supracrustal rocks in many Archean greenstone belts. Detrital zircon ages from such a sequence in the northwestern Superior Province match the known ages of volcanism and plutonism in the surrounding area. They show a marked change in zircon provenance from the lower to upper part of the sequence, in a clear pattern that can be best explained by a model involving basin formation by diapirism and sagduction. In this model, detritus were derived from down-stratigraphy erosion above rising granitoid domes and deposited in a sinking synclinal keel between them. These new data thus support the presence of vertical tectonism in the Superior Province at a time in which abundant evidence for horizontal tectonism has been documented. Diapirism and sagduction provides a mechanism to bring supracrustal rocks deep into the crust below the synclinal keel, without involving crustal thickening and potentially during crustal extension. This can readily explain the observation in the Kapuskasing uplift that the Borden Lake conglomerate, a Timiskaming-type sedimentary rock, was buried up to 30 km deep in the crust and experienced upper amphibolite to granulite-facies metamorphism very shortly after its deposition.