Peridotites from Attawapiskat, Canada: Mesoproterozoic Reworking of Palaeoarchaean Lithospheric Mantle beneath the Northern Superior Superterrane

Abstract

The Jurassic Attawapiskat kimberlites allow the study of the association of a world-class primary diamond deposit (Victor Mine) with a post-Archaean rift system, the Midcontinent Rift, which affected the southern Superior Craton at ∼1·1 Ga. Peridotite xenoliths and xenocrysts from the Attawapiskat kimberlites have been analysed to understand the processes of craton formation and modification in the Superior lithospheric mantle. Chondrite-normalized platinum group element (PGEN) signatures in olivine are complex and highly variable, but correlate with Os isotopic compositions. The existence of a depleted mantle reservoir beneath the Attawapiskat area since the Palaeoarchaean is indicated by ∼3·6 Ga TRD ages preserved in peridotitic olivine. An Mg# up to 93·6 in these olivines requires that protolith formation involved high degrees of partial melting, leading to harzburgitic and dunitic residues. Cr-rich garnets with positive slopes in depleted chondrite-normalized heavy rare earth elements (HREEN) are consistent with fractional polybaric melt extraction continuing from the garnet into the spinel stability field. TRD ages of ∼2·7 Ga in olivines with residual PGEN patterns probably reflect residual PGE alloy or refractory PGE sulphide inclusions and indicate that additional melting occurred in the mantle at the time of subduction–accretion, with hydrous melts infiltrating the overlying mantle wedge leading to iridium-group PGE (I-PGE) alloy formation. Metasomatic melts related to the Midcontinent Rift (1·1 Ga) interacted with variably depleted peridotite, leading to platinum-group PGE (P-PGE) enrichment and Mesoproterozoic TRD ages. Older depleted domains are, however, preserved (e.g. sinusoidal REEN patterns in lherzolitic garnet). After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere via cooling, and diamonds sampled by post-rift kimberlites, such as Victor (∼180 Ma), must have formed after the Midcontinent Rift. These diamonds are likely to be of mixed parageneses: high-pressure compositions indicative of diamond stability are observed in both lherzolitic and high-Mg eclogitic to pyroxenitic garnets at Victor.