Implication of isotope data on the metamorphism of the basic volcanites from the Sjangeli Window, northern Sweden

Abstract

The Lower Proterozoic supracrustal rocks of the Sjangeli area form an approximately N-S striking belt. The supracrustal belt consists of three metamorphosed volcanic and volcanosedimentary units, which are intercalated with metasedimentary units, and has been intruded by granites and syenites 1780 \u00B1 85 My ago. The basic volcanic rocks in the central section of the Sjangeli supracrustal belt have a Middle Proterozoic amphibolite facies mineral assemblage (c. 6 kbar and c. 560 \u00B0C), while the retrogressive basic volcanic rocks at the borders of the supracrustal belt (tuffs, lava flows, and pillow lavas) have Caledonian greenschist facies mineral assemblages (c. 4 kbar and c. 480 \u00B0C). Rb-Sr whole-rock isotope data from the metamorphosed lava flows and from a basic dike yield ages of 2324 \u00B1 15 My and 2252 \u00B1 71 My, respectively. Mixing plots indicate, however, that these data could be too high by 100 to 150 My. Rb-Sr whole-rock isotope data from the tuffs define a mixing line with an appearent age 488 \u00B1 64 My. Mixing diagrams indicate c. 400 My as time of mixing. Lead isotope data from the lava flows and the basic dike give mixing time of 425-500 My. These mixing times are interpreted to be related with the Caledonian metamorphism. Sm-Nd data from the amphibolites indicate a Middle Proterozoic age of the amphibolite facies metamorphism, which probably was related to the intrusion of the granites and syenites in the Sjangeli area. A Middel Proterozoic model age (1820 \u00B1 20 My) is also obtained for the Pb-Pb data from the metalva flows.