Petrology of a Late Archaean, Highly Potassic, Sanukitoid Pluton from the Baltic Shield: Insights into Late Archaean Mantle Metasomatism

The late Archaean Panozero pluton in Central Karelia (Baltic Shield) is a multi-phase high-Mg, high-K intrusion with sanukitoid affinities, emplaced at 2·74 Ga. The magmatic history of the intrusion may be subdivided into three cycles and includes monzonitic and lamprophyric magmas. Compositional variations are most extreme in the monzonite series and these are interpreted as the result of fractional crystallization. Estimates of the composition of the parental magmas to the monzonites and lamprophyres show that they are enriched in light rare earth elements, Sr, Ba, Cr, Ni and P but have low contents of high field strength elements. Radiogenic isotope data indicate a low U/Pb, high Th/U, high Rb/Sr, low Sm/Nd source. The magmatic rocks of the Panozero intrusion are also enriched in H2O and CO2; carbon isotope data are consistent with mantle values, indicating a fluid-enriched mantle source. The similarity in trace element character of all the Panozero parental magmas indicates that all the magmas were derived from a similar mantle source. The pattern of trace element enrichment is consistent with a mantle source enriched by fluids released from a subducting slab. Nd-isotope data suggest that this enrichment took place at c. 2·8 Ga, during the main episode of greenstone belt and tonalite–trondhjemite–granodiorite formation in Central Karelia. Sixty million years later, at 2·74 Ga, the subcontinental mantle melted to form the Panozero magmas. Experimental studies suggest that the monzonitic magmas originated by the melting of pargasite–phlogopite lherzolite in the subcontinental mantle lithosphere at 1–1·5 GPa. The precise cause of the melting event at 2·74 Ga is not known, although a model involving upwelling of asthenospheric mantle following slab break-off is consistent with the geochemical evidence for the enrichment of the Karelian subcontinental mantle lithosphere by subduction fluids.