The Palaeoproterozoic Malmback Formation in S Sweden: age,composition and tectonic setting

摘要

The Palaeoproterozoic Malmback Formation in southern Sweden comprises mafic to felsic volcanic rocks and syn-eruptive volcaniclastic and volcanogenic sedimentary deposits. Field observations indicate terrestrial to fluvial deposits – predominantly with massive to highly vesicular lavas, but also with ignimbrites, ashes, redeposited volcanic rocks and conglomerates. Three groups of volcanic rocks can be distinguished geochemically: (1) primitive basalts to basaltic andesites, (2) dacitesandesites and (3) rhyolites. The groups constitute a calc-alkaline to shoshonitic series. Large ion lithophile elements and light rare earth elements enrichment, combined with depletion in heavy rare earth elements and high field strength elements, particularly in Nb (and Ta), are characteristic of typical arc rocks. Discriminant diagrams suggest emplacement along a one-time active continental margin, which is now marked by the Transscandinavian Igneous Belt (TIB): an array of ～1.85–1.65 Ga granitic to gabbroic plutonic rocks and associated volcanic rocks more than 1000 km long. In the TIB, the Malmback Formation forms mega-xenoliths and other inclusions. U–Pb secondary ion mass spectrometry dating of zircon from a rhyolite of the Malmback Formation yielded a concordia age of 1796±7 Ma, which is interpreted as the crystallisation age of the rhyolite and the age of deposition of the Malmback Formation, during a relatively early part of the formation period of the TIB. Altogether, the setting, composition and age of the Malmback Formation and the TIB confirm the existence of an Andean-type active continental margin in the present southern part of the TIB at c.1.8 Ga.