Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield : metallogenic, petrological and geodynamic implications

There are two 300-500-km long belts of Palaeoproterozoic layered intrusions: the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt in the Baltic (Fennoscandian) Shield. New U-Pb (TIMS) ages and radiogenic isotopic (Nd-Sr-He) data have been determined for mafic-ultramafic Cu-Ni-Ti-Cr and PGEbearing layered intrusions of the Kola Belt. U-Pb ages on zircon and baddeleyite for gabbronorite and anorthosite from the Fedorovo-Pansky, Monchepluton and Main Ridge (Monchetundra and Chunatundra), Mt. Generalskaya intrusions and gabbronorite and dykes from the Imandra lopolith of the Kola Belt define a time interval of more than 130 million years, from ca. 2.52 Ga to 2.39 Ga. At least four intrusive phases have been distinguished: three PGE-bearing, and one barren. This spread of ages is wider than that for intrusions of the Fenno-Karelian Belt which clusters at 2.44 Ga. Nd isotopic values for the Northern Belt range from - 1.1 to -2.4, implying an enriched mantle “EM-1 type” reservoir for these layered intrusions. Initial Sr isotopic data for the rocks of the intrusions are radiogenic relative to bulk mantle, with ISr values from 0.703 to 0.704. Geochemical data and 4He /3He isotopic ratios of the minerals reflect a significant contribution from a mantle source rather than the influence of crustal processes during emplacement. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic – ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.