The relationship between 1.88 Ga old magmatism and the Baltic-Bothnian shear zone in northern Sweden

• New geochemical data and U-Pb ages for Proterozoic mafic and felsic dykes and their granitic host rocks in the Bläsberget Massif, proximal to the Baltic-Bothnian shear zone, indicate that these rocks are part of a terrane distinct from Svecofennian terranes in Central and Southern Sweden. The dykes comprise a NNE-trending swarm which can be traced over a strike length of about 20 km. Most of the mafic dykes are internally deformed and have systematic internal structures showing a sinistral horizontal component and an eastern-side-up sense of movement. This sinistral shearing is also observed within the nearby Baltic-Bothnian shear zone.The mafic dykes have a uniformly basaltic composition. High content of elements such as Ti and Zr illustrate the typical ‘within-plate’ character. The felsic dykes are chemically homogeneous and have a SiO2 content of about 77 wt%. High content of Y, Yb and Nb confirms the ‘within-plate’ character of the dyke swarm.Zircons extracted from one of the felsic dykes indicate a U-Pb age of c. 1.88 Ga. Approximately the same age has been obtained from the host rock granite and from a nearby occurrence of a similar late-orogenic granite. Since the dykes cross-cut most deformation and migmatite structures and most likely represent the waning stages of orogenic deformation in the area, it is evident that the timing of the orogenic events in the area is different and older than the Svecofennian development in central and southern Sweden.A genetic relationship is suggested between the magmatism and the development of the Baltic-Bothnian shear zone.© The Geological Society 1996

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

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Applied Geology

Tillämpad geologi

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