pervasively, indicating a later Svecokarelian evolution in that area, i.e. that the granitoids are located in domains with separate orogenic evolutions. © 2001 Elsevier Science B.V. All rights reserved.
Keywords
:
Palaeoproterozic; Shear zone; Svecofennium; Transpression; U – Pb geochronology
1. Introduction
1
.
1
. Background and aim A spatial connection between granitic intru-
sions and large-scale deformation zones is com- mon and well-established, while evidence for
coeval intrusive and tectonic activities is a more recent finding Hutton, 1988; D’Lemos et al.,
1992; Saint Blanquat et al., 1998; Brown and Solar, 1998a,b. The study of such conditions has
led to refined models concerning e.g. diagnostic internal structures with respect to tectonic envi-
ronment and feed-back relations between melt transfer and shear zone activity e.g. Brown and
Solar, 1998a,b.
In the central part of the Fennoscandian shield, the relationship between shear zones and granitic
intrusions is fundamental for the understanding of the Palaeoproterozoic crustal evolution, granitic
intrusions of various ages and origins are frequent Stephens et al., 1994, 1997; Lundqvist, 1995 and
the number of identified shear zones is increasing e.g. Ehlers et al., 1993; Ka¨rki et al., 1993;
Bergman and Sjo¨stro¨m, 1994; Stephens et al., 1994, 1997; Wijbrans et al., 1995; Beunk et al.,
1996.
So far, the few published ages of tectonic fabric in ductile shear zones, as well as indirect evidence,
show that shearing overlapped the periods of granitic plutonism temporally Ho¨gdahl and
Sjo¨stro¨m, 1999; Ho¨gdahl et al., 1995, 1996; Korja and Heikkinen, 1995; Sjo¨stro¨m and Bergman,
1995;
Stephens and
Wahlgren, 1995,
1996; Wahlgren and Stephens, 1996; Lindroos et al.,
1996. This general condition is valid both for intrusions derived from the deeper levels of the
crust as well as S-type granites derived from shal- lower crustal levels. However, in the Svecofennian
domain, the relationship between specific intru- sives and shear zones is not well understood,
which
emphasises the
need for
combined geochronological and structural investigations.
Our study focuses on such a relationship by defining the magmatic age of a granitic rock as
well as the age of a ductile shear zone within the intrusion. Microstructures are studied to find out
the physical conditions during deformation, and the role of minerals in the deformational fabric,
which are suitable for U – Pb analyses, e.g. titanite and zircon. Combined with structural analysis in
the field, the microstructures are used to sort out the kinematic conditions along the shear zone and
along one of the margins of the granitoid. Ages and characteristic features of adjacent shear zones
within the granitoid are presented briefly and the local conditions are integrated into the regional
picture.
Three main factors outline regional implica- tions of this work, 1 The area represents the
junction of the two largest shear zones in central Sweden, one of which may continue to southern
Finland Sjo¨stro¨m et al., 2000. 2 The investi- gated intrusion has previously been referred to as
a part of the Revsund granitoids Ho¨gbom, 1894; Lundega˚rdh et al., 1984. These granitoids make
up a major Palaeoproterozoic intrusive suite of north central Sweden and compared with their
areal extent, existing age determinations are scarce. Our results have consequences for the
definition of a Revsund granitoid in one of the type areas. 3 The results also indicate that per-
vasive Svecokarelian deformation in the area may have been earlier than generally assumed for the
Palaeoproterozoic evolution of north central Sweden.
1
.
2
. The Re6sund granitoids
:
regional occurrence and local character
Large massifs of Revsund granitoids Ho¨gbom, 1894 occur in the Palaeoproterozoic paragneisses
of the Bothnian Basin in north central Sweden Fig. 1. The granitoids, dated previously at 1.80 –
1.77 Ga Patchett et al., 1987; Skio¨ld, 1988; Claes-
son and Lundqvist, 1995; Delin, 1996; Delin and Aaro, 2000, are interpreted to have a deep crustal
origin Claesson and Lundqvist, 1995 with input from the mantle Gorbatschev et al., 1997. Their
emplacement is assumed to have occurred after Svecokarelian peak metamorphism and deforma-
tion and they have, therefore, been classified as post-orogenic Gaa´l and Gorbatschev, 1987, or
post-kinematic. It has been suggested that the Revsund suite belongs to the Transscandinavian
Igneous Belt TIB Gorbatschev and Bogdanova, 1993 extending from southern Sweden to northern
Norway, partly below the Scandinavian Cale- donides Fig. 1.
The Revsund granitoids in Ja¨mtland county constitute the southernmost component of this
intrusive suite. Based on differences in major ele- ment composition, the granitoids in the two areas
first described by Ho¨gbom 1894, were treated subsequently as separate massifs Gorbatschev et
al., 1997, a northern Fja¨llsjo¨ and a southern massif Fig. 2. Both intrude older Svecofennian
granitoids and metasupracrustal rocks. The southern of these massifs is divided into an
eastern and a western part Gorbatschev et al., 1997 by a boundary more or less coinciding with
the extension of the Hassela shear zone HSZ, while the western margin is affected by the
Storsjo¨n – Edsbyn
deformation zone
SEDZ, Bergman and Sjo¨stro¨m, 1994 Fig. 2. Along these
zones, the granitoid has been affected by ductile deformation that has been overprinted partly by
brittle structures.
The composition of the Revsund granitoids ranges regionally from granite sensu stricto to
granodiorite with a metaluminous affinity Gor- batschev, 1990; Claesson and Lundqvist, 1995. In
the northern Fja¨llsjo¨ massif, there are also varieties with quartz-monzodioritic composition Persson,
1978. The colour varies successively from pale grey to red and a typical feature is a coarse porphyritic
texture cf. Fig. 4a. In contrast to other areas, the Revsund granitoids in the southern
massif
of Ja¨mtland
are associated
with
Fig. 1. Simplified geological map of the Fennoscandian shield including the ductile shear zones in central Sweden. Thick, broken line shows the inferred continuation of the HSZ towards the shear zone system in southern Finland modified after Gaa´l and
Gorbatschev, 1987; Bergman and Sjo¨stro¨m, 1994; Stephens et al., 1994; Sjo¨stro¨m et al., 2000.
Fig. 2. The Revsund granitoids in county Ja¨mtland consist of the northern Fja¨llsjo¨ Massif and the Southern Massif, which is divided into a western investigated here and an eastern part. This boundary approximately coincides with the HSZ and the SEDZ outlines
the western margin of the massif. These zones envelope the 1.85 – 1.84 Ga Ljusdal batholith. Both zones display bulk dextral kinematics horizontal component. In the western part of the Southern Revsund Massif the SEDZ and the HSZ converge and form
a ca. 50-km wide pattern of anastomosing shear zones modified after Bergman and Sjo¨stro¨m, 1994; Lundqvist, 1995.
minor pegmatites and fine-grained uranium- and thorium-rich granitic dykes Ho¨gdahl et al., 1998.
2. Regional deformation zones: the Storsjo¨n – Edsbyn deformation zone and the
