Bulletin of the Geological Society of Finland (Dec 2009)
Mantle transect of the Karelian Craton from margin to core based on P-T data from garnet and clinopyroxene xenocrysts in kimberlites
Abstract
Peridotitic garnet and clinopyroxene xenocrysts from the 1.2 Ga Lentiira-Kuhmo and the 760 Ma Kuusamo kimberlite fields, emplaced within the Central Karelian Craton, have been studied using major and trace element geochemistry to obtain information on the stratigraphy, compositional variability, and evolutionary history of the underlying lithospheric mantle. An earlier study on the 600 Ma Kaavi-Kuopio kimberlite field showed that near the SW margin of the craton the 230-km-thick subcontinental lithospheric mantle (SCLM) exhibits a well-developed 3-layer structure indicative of episodic construction. New Ni in garnet thermometry on samples farther into the craton, from Kuhmo, Lentiira, and southern and northern Kuusamo, gives temperature ranges of 800–1400, 700–1450, 700–1450 and 600–1300 °C, respectively, and when extrapolated to the geotherm determined for the Kaavi-Kuopio area, indicates sampling intervals of c. 100–240, 80–250, 80–250 and 65–210 km, respectively. The results demonstrate that the SCLM of the Karelian Craton reaches its greatest thickness in the Lentiira-Kuhmo and southern Kuusamo area and thins towards the North and South. The mantle stratigraphy of the craton core, compared to the craton edge, shows less compositional variation, with only two distinguishable horizons corresponding generally to the two deepest layers at Kaavi-Kuopio. Additionally, based on the Mg# of pyropes, the level of depletion of peridotites comprising the mantle lithosphere in the central craton is significantly higher than at the craton edge represented by Kaavi-Kuopio. Coupled with the rarity of mantle-derived chrome diopside, the implication is that this portion of the mantle either underwent high levels of partial melting to produce very refractory residua or experienced less melt-modification than average SCLM subsequent to formation. The craton core provides a wide diamond window up to 110 km thick compared to c. 40 km at the craton edge.
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