U-Pb ages of plutonic and volcanic rocks in the Svecofennian Bothnian Basin, central Sweden, and their implications for the Palaeoproterozoic evolution of the Basin
T. Lundqvist et al., U-Pb ages of plutonic and volcanic rocks in the Svecofennian Bothnian Basin, central Sweden, and their implications for the Palaeoproterozoic evolution of the Basin, GFF, 120, 1998, pp. 357-363
New U-Pb age data on two early orogenic granodiorites (ca. 1930 Ma) and one
felsic metavolcanite (1870 Ma) from the Svecofennian Bothnian Basin area o
f central Sweden are presented. Combined with literature data they suggest
an extended, at least ca. 80 m.y. long, evolution of this part of the Bothn
ian Basin. Sedimentation started before 1950 Ma, and continued up to at lea
st 1870 Ma. Early-orogenic plutonic activity started at ca. 1950 Ma, perhap
s even earlier, and continued up to ca. 1850-1840 Ma. Detrital zircons foun
d in low-grade greywackes of the region range in age from ca. 1880 to 2020
Ma. According to the tentative model suggested here, these zircons may eman
ate from granitoids intruded into the greywacke sequence of the Bothnian Ba
sin. They were subsequently exposed to erosion and included in upper parts
of the sedimentary column. These upper parts could (statistically) be of a
lower metamorphic grade than older greywackes, which are generally migmatiz
ed.