Le Geotop - Dr. Karen V. Smit : Formation and evolution... (31-03-2017)

Dr. Karen V. Smit : Formation and evolution... (31-03-2017)

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Dr. Karen V. Smit (Gemological Institute of America)

Formation and evolution of the Palaeoarchaean western Superior lithosphere: evidence from peridotite and diamond ages

Vendredi 31 mars 2017à 12h30 / Friday, March 31, 2017, 12:30pm

Local PK-7605, 201 ave. Président-Kennedy, UQAM

Résumé / Abstract:

Peridotitic and eclogitic xenoliths as well as diamond samples from the Attawapiskat area in the western Superior craton provide an opportunity to understand the processes of craton formation and ongoing Archaean and Proterozoic modification in the Superior lithospheric mantle. Existence of a Palaeoarchaean depleted mantle reservoir beneath the Attawapiskat area is indicated by 3.6-3.5 Rhenium-depletion (TRD) ages in peridotitic olivine. This Paleaoarchaean mantle likely represents the depleted reservoir for Palaeoarchaean to Hadean crustal extraction required in the northern Superior superterrane, and formed the proto-Superior core for later accretion of terranes from the south. Low-Mg eclogites have shallow origins as plagioclase-bearing protoliths likely in former oceanic crust, emplaced into the SCLM by subduction, demonstrating an important role for 2.7 Ga subduction processes in Superior craton modification and stabilisation.

Influx of Proterozoic Midcontinent Rift-related magmas is seen in P-PGE-enrichment and Mesoproterozoic TRD ages in peridotite. Since older depleted Palaeoarchaean domains were preserved, any Rift-related effects on the lithosphere are due to small-degree melts rather than widespread melt overprinting. The thermal impact of the Proterozoic Midcontinent Rift is seen in an elevated geotherm and narrow "diamond window" in 1.1 Ga kimberlites, and resulted in destruction of any Archaean diamonds in the lithosphere. After the thermal impact of the rift subsided, diamond-stable conditions were extended to shallower depths in the lithosphere. Diamonds that formed after the Midcontinent Rift, as sampled by post-rift kimberlites at Attawapiskat, form the basis for diamond mining at the Victor deposit and demonstrate that traditional undisturbed Archaean terranes should not be the only preferred setting for diamond exploration programmes.