Disturbance of Pb, Sr, and Nd isotope systems related to medium and low grade metamorphism illustrated with data from the polymetamorphic Early Proterozoic (Rombak) - Sjangeli basement window of the Caledonides of Northern Sweden
Sammanfattning: The Sjangeli supracrustal belt forms the easternmost belt of a series approximately N-S striking, steeply dipping Early Proterozoic belts of the Rombak-Sjangeli basement window, which formed in an island arc setting. The volcanic units of the Sjangeli area consist of metatuffs, which contain abundant Cu-mineralizations, metalava flows and pillowed metalavas and are intercalated with banded carbonatesilicate rocks and micaschists, which contain various amounts of volcanic material. The volcanic rocks are basic to ultrabasic and consist predominantly of high-Mg tholeiitic basalts which vary to komatiitic basalts, and minor serpentinites and high-Fe tholeiitic basalts, which probably originated from extensive melting of a depleted mantle source. This is indicated by high contents of MgO Ni, and Cr, the flat REE pattern, and TiO2/P2O5 , Ti/Zr, and platinum group element ratios, and positive ?Nd values. Rb-Sr whole-rock isotope data from the metalava flows and from a basic metadike yield ages of 2324 ± 15 My and 2252 ± 71 My, respectively. Mixing plots indicate, however, that these data could be too high by 100 to 150 My. The supracrustal rocks became intruded by syenites and granites at c. 1.7 - 1.8 AE, and were subjected to an amphibolite facies metamorphism which, in the basic rocks, resulted in a mineral assemblage consisting of hornblende + andesine + ilmenite + (biotite) ± quartz ± calcite ± epidote + magnetite. This assemblage indicates metamorphic conditions of c. 6 kbar and c. 560°C. Mylonitic zones, which predominantly occur at unit contacts, show retrogradation to a greenschist facies mineral paragenesis consisting of actinolite + albite + quartz + biatite + epidote + chlorite + calcite + titanite which probably formed at c. 4 kbar and c. 480°C. The mylonites represent Proterozoic structures and their retrogradation occurred during the Svecokarelian metamorphism at c. 1.6 - 1.8 AE. The Caledonian overthrust, which followed a phase of deep erosion and discordant sedimentation of Cambrian epiclastic sediments, reached greenschist facies conditions. Rb-Sr whole-rock isotope data from the metatuffs define a mixing line with an apparent age of 488 ± 64 My. Mixing diagrams indicate c. 400 My as time of mixing. The resulting greenschist facies mineral assemblages therefore had formed during two completely different metamorphisms. Both greenschist facies metamorphic events were associated with disturbances of the isotopic systems, which resulted from infiltration of U, Th, and Pb at c. 430 My and at c. 1800 My, and the mobilization of Sr. The isotopic remobilization of the greenschist facies metamorphism resulted in a partial homogenization, causing mineralwhole-rock systems to be in isotopic disequilibrium. The mixing of the Sr whole-rock system and the addition of U, Th, and radiogenic Pb are strongest in the metatuffs which are strongly mylonitized. In the little or non-schistose metalava flows the addition of radiogenic lead is minor and the Rb-Sr system is apparently not disturbed by the metamorphic events. The amphibolites, which are the least deformed unit with the highest grade mineral assemblage, show the least radiogenic lead composition.
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