Origin of layered continental mantle (Karelian craton, Finland):: Geochemical and Re-Os isotope constraints

Studies of mantle xenolith and xenocryst studies have indicated that the subcontinental lithospheric mantle (SCLM) at the Karelian Craton margin (Fennoscandian Shield) is stratified into at least three distinct layers cited A, B, and C. The origin and age of this layering has, however, remained unconstrained. In order to address this question, we have determined Re-Os isotope composition and a comprehensive set of major and trace elements, from xenoliths representing all these three layers. These are the first Re-Os data from the SCLM of the vast East European Craton. Xenoliths derived from the middle lever B (at similar to 110-180 km depth), which is the main source of harzburgitic garnets and peridotitic diamonds in these kimberlites, are characterised by unradiogenic Os isotopic composition. Os-187/Os-188 shows a good correlation with indices of partial melting implying an age of similar to 3.3. Ga for melt extraction. This age corresponds with the oldest formation ages of the overlying crust, suggesting that layer B represents the unmodified SCLM stabilised during the Paleoarchean. Underlying layer C (at 180-250 km depths) is the main source of Ti-rich pyropes of megacrystic composition but is lacking harzburgitic pyropes. The osmium isotopic composition of layer C xenoliths is more radiogenic compared to layer B, yielding only Proterozoic T-RD ages. Layer C is interpreted to represent a melt metasomatised equivalent to layer B. This metasomatism most likely occurred at ca. 2.0 Ga when the present craton margin formed following continental break-up. Shallow later A (at similar to 60-110 km depth) has knife-sharp lower contact against layer B indicative of shear zone and episodic construction of SCLM. Layer A peridotites have ultradepleted arc mantle-type compositions, and have been metasomatised by radiogenic Os-187/Os-188, presumably from slab-derived fluids. Since layer A is absent in the core of the craton, its origin can be related to Proterozoic processes at the craton margin. We interpret it to represent the lithosphere of a Proterozoic arc complex (subduction wedge mantle) that became underthtusted beneath the craton margin crust during continental collision similar to 1.9 Ga ago. (C) 2006 Elsevier B.V. All rights reserved.