T249 Getting to the Root of It—Metamorphism, Tectonics, and Crustal Evolution (Posters)

Wednesday, 7 November 2012
9:00 AM - 6:00 PM
Hall B

THE SUBCONTINENTAL LITHOSPHERIC MANTLE OF THE NE SIBERIAN CRATON: PERIDOTITES FROM OBNAZHENNAYA

Paper number: T249-7
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Kimberlites are “windows” into the mantle of the Earth, some 75-80% by volume. As such, the Siberian Platform offers a view into the subcontinental lithospheric mantle (SCLM) of the Siberian Craton. By obtaining geochemical information on mantle xenoliths from this region, the relationship between the SCLM and the underlying mantle can be better understood. The Siberian Craton was amalgamated during the mid-Proterozoic and is divided into provinces based upon when the crustal pieces were accreted onto the craton. Three of these provinces contain kimberlite intrusions, trending N-NE, from oldest to youngest across the craton. The Obnazhennaya kimberlite, the youngest of the kimberlite pipes, is located in the Olenek province at the most northeastern region of the platform. It was emplaced (~140 Ma) after the eruption of the Siberian Flood Basalts (~250 Ma). Obnazhennaya is a relatively “shallow” kimberlite and is barren of diamonds. Because of this, in congruence with its late Jurassic age, mantle xenoliths from the Obnazhennaya kimberlite can offer a view into a different portion of the SCLM. These xenoliths are mainly comprised of coarse-grained peridotites, eclogites, and websterites formed at temperatures of ±900 °C and pressures of ~37 kbar.

This study is focussed on the mineralogical and chemical compositions of the peridotites, with an emphasis on garnet chemical zonation, as a reflection of a ‘basaltic-type’ of metasomatism. These xenoliths have a mineral assemblage of Gt + Ol + Cpx + Opx, with a secondary assemblage of Chl + Serp + Phlog. Chemical variations in these rocks are discerned through changes in mineralogy – e.g., as the Ol content decreases, the content of Px + Gt increase. The compositions of the primary mineral assemblage are used to constrain pressure and temperature conditions at time of formation of the peridotites whereas secondary features, such as exsolution and alteration, provide evidence for the interaction between fluids and the original rock during metasomatic events. This in-depth study of the primary and secondary features of these mantle xenoliths provides for an understanding of the processes involved in the formation and evolution of the SCLM of the NE Siberian Craton.