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

摘要

There are two 300-500-km long belts of Palaeoproterozoic layered intrusions: the Northern (Kola) Beltudand the Southern (Fenno-Karelian) Belt in the Baltic (Fennoscandian) Shield. New U-Pb (TIMS) ages andudradiogenic isotopic (Nd-Sr-He) data have been determined for mafic-ultramafic Cu-Ni-Ti-Cr and PGEbearingudlayered intrusions of the Kola Belt. U-Pb ages on zircon and baddeleyite for gabbronorite andudanorthosite from the Fedorovo-Pansky, Monchepluton and Main Ridge (Monchetundra and Chunatundra),udMt. Generalskaya intrusions and gabbronorite and dykes from the Imandra lopolith of the Kola Belt define audtime interval of more than 130 million years, from ca. 2.52 Ga to 2.39 Ga. At least four intrusive phases haveudbeen distinguished: three PGE-bearing, and one barren. This spread of ages is wider than that for intrusionsudof the Fenno-Karelian Belt which clusters at 2.44 Ga. Nd isotopic values for the Northern Belt range from -ud1.1 to -2.4, implying an enriched mantle “EM-1 type” reservoir for these layered intrusions. Initial Srudisotopic data for the rocks of the intrusions are radiogenic relative to bulk mantle, with ISr values from 0.703udto 0.704. Geochemical data and 4He /3He isotopic ratios of the minerals reflect a significant contributionudfrom a mantle source rather than the influence of crustal processes during emplacement. The geological andudgeochronological data indicate that in the eastern part of the Baltic Shield, mafic – ultramafic intrusiveudmagmatism was active over a protracted period and was related to plume magmatism associated withudcontinental breakup that also involved the Superior and Wyoming provinces.