Konstantinos Papapavlou : Titanite as strain chronometer (09-02-2017)

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Konstantinos Papapavlou (U. Portsmouth)

Titanite as strain chronometer: A perspective from the largest impact craters on the planet

Jeudi 9 février 2017 à 13h30/ Thursday, February 9, 2017, 1:30pm

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

Résumé / Abstract:

Titanite (CaTiSiO5) is a uranium-bearing accessory mineral, abundant in a variety of crustal lithologies. This mineral has been used extensively, using U-Pb isotope systematics, as chronometer of magmatic, metamorphic, and metasomatic events. The potential of titanite as deformation chronometer remains poorly understood. In this presentation, I will show how, combining a variety of microanalytical techniques, this mineral can be used as recorder and chronometer of impact-induced and tectonic deformation in the two largest and oldest impact craters of the planet: the 2.02 Ga Vredefort (South Africa) and the 1.85 Ga Sudbury (Canada) structures. The Sudbury impact structure has been deformed by a network of epidote-amphibolite to greenschist facies ductile shear zones. Plastically deformed titanite grains with fluid-mediated textural features within these shear zones record three age populations of ca. 1.75 Ga, 1.65 Ga, and 1.45 Ga that are interpreted as reworking episodes during three distinct orogenic pulses. The textural population of 1.75 Ga titanite grains hosts as inclusions shock metamorphosed titanite grains that show identical crystallographic relationships with shocked titanite grains from the Vredefort structure. These grains are interpreted as the first microstructurally-justified terrestrial examples of shock metamorphism in titanite. The shocked titanite grains from the Sudbury structure exhibit total lead loss and yield a 207Pb/206Pb age of 1851 ± 12 Ma, indistinguishable from the impact age (i.e. 1850 ± 2 Ma). Those from Vredefort show partial lead loss and yield a 207Pb/206Pb age of 2620±24 Ma. The differential response to impact age resetting is attributed to variations in the development of syn-impact diffusion pathways and post-shock thermal annealing processes. Overall, the microstructural geochronology of titanite is suggested to be a new tool to unravel the tectonic and impact bombardment history of planetary lithospheres.