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Petrology, volcanology, and diamonds of archean calc-alkaline lamprophyres, Wawa, Ontario, Canada Lefebvre, Nathalie Suzanne

Abstract

Atypical diamondiferous polymict volcaniclastic breccias and lamprophyre dikes have been recently discovered within the Wawa subprovince of the Superior craton. These rocks comprise part of a subduction-related volcanic sequence of the Michipicoten greenstone belt. Dated at 2.67 Ga, they are the oldest known primary diamondiferous rocks. Detailed mapping of 9 trenches and 7 outcrops showed that the rocks comprise both dikes and thick continuous units of polymict volcaniclastic breccia. Both are mafic, metamorphosed to greenschist facies and deformed, with little primary magmatic texture preserved. Magmatic predecessors, determined from detailed mineralogical and petrographic observations, are calcalkaline lamprophyres. The only preserved magmatic phenocryst is coarse, oscillatory-zoned edenitic and pargasitic amphibole. The parent magmas may have contained phenocrysts of clinopyroxene and phlogopite, and are similar in bulk composition to that of Abitibi lamprophyres and other Archean calc-alkaline lamprophyres. The breccias are interpreted to be volcaniclastic debris flow deposits based on the stratigraphy, wide range in fragment lithologies (which include pyroclastic material), poor sorting, and paucity of sedimentary structures. The explosive eruption style of the Wawa calcalkaline lamprophyric magma is, however, atypical for lamprophyres. The calc-alkaline lamprophyric magma formed in the mantle at depths of ~150 km in a convergent tectonic regime, thus sampling the cool diamondiferous subducting slab. The magma was emplaced very rapidly ensuring preservation of diamonds during ascent. The Wawa metavolcanic rocks represent the first confirmed occurrence of diamonds in calc-alkaline lamprophyric rocks. The Wawa metavolcanic rocks host a diamond suite dominated by microdiamonds. Eighty diamonds less than 1 mm in one dimension were studied by a variety of mineralogical methods. Morphological studies show that the majority of the diamonds are colourless, weakly resorbed, octahedral single crystals and aggregates. A variety of diamond colours, as well as cubic and cubo-octaheral single crystals and aggregates were also observed. Infrared spectroscopy determined that the diamonds have nitrogen contents ranging from 0 to 740 ppm and two modes of nitrogen aggregation at 0 -30% B and 60-95% B-centers. These states of nitrogen aggregation suggest high (1050 - 1300°C) temperatures of residence in the mantle. The diamonds exhibit a wide range of mineralogical characteristics and therefore must have formed under varying physical and chemical conditions. The morphology and nitrogen characteristics of the Wawa diamonds are comparable to those of xenocrystal cratonic diamonds and are unlike those of orogenic diamonds formed in subduction zones. It is enigmatic that the Wawa diamonds were emplaced into a subducted-related setting but show characteristics typical of xenocrystal cratonic diamonds.

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