Archean calc-alkaline lamprophyres of Wawa, Ontario, Canada: Unconventional diamondiferous volcaniclastic rocks

Unusual diamondiferous rocks are found in the Wawa subprovince of the Southern Superior Craton. They are dated at 2.67-2.7 Ga and comprise part of a calc-alkaline volcanic sequence of the Michipicoten Greenstone Belt. Detailed mapping of an ∼40 km2 area showed that the rocks are metamorphosed polymict volcaniclastic breccia (PVB) and lamprophyre. The breccia occurs as thick, 60-110 m conformable beds traceable in intermittent outcrops along strike for more than 4 km, whereas younger lamprophyre occurs as 0.5-3 m dykes. Magmatic predecessors for the metavolcanic rocks were determined on the basis of detailed mineralogical and petrographic observations, and are found to be calc-alkaline lamprophyres. The only preserved magmatic phenocryst phase is coarse, oscillatory-zoned amphibole of edenitic and pargasitic compositions. The parent magmas are similar in bulk composition to that of Abitibi lamprophyres and other Archean calc-alkaline lamprophyres, and may have thus also contained phenocrystal clinopyroxene and phlogopite. The Wawa lamprophyric magmas formed contemporaneously with felsic to mafic volcanic rocks and late orogenic intrusives of cycle 3 of the Michipicoten Greenstone Belt. They were emplaced episodically in local extensional areas, in an active Archean subduction zone. The breccia formed as a volcaniclastic deposit and contains fragments of pyroclastic lapilli and juvenile material. Stratigraphy, a wide range in clast lithologies, poor sorting and paucity of sedimentary structures suggests the breccia formed in a debris flow. The Wawa diamondiferous rocks may be ancient equivalents of modern lamprophyric cinder cones and demonstrably associated epiclastic deposits.