Project title: Mesoproterozoic anorogenic granitoid
magmatism in active shear zones
Major supervisor: Svetlana Bogdanova
The Mesoproterozoic period between 1.6 and 1.4
Ga is commonly thought to be one of crustal stabilization in the almost entire
North Atlantic realm. However, this time was also marked by convergent plate
tectonics and accretion of new portions of juvenile crust at 1.6 to 1.4 Ga
(e.g. Gower and Åhäll, 1998; Rivers, 1998). The palaeomagnetic data of Elming
and others (1993) even indicate that was some shift of position of Fennoscandia
and the whole East European Craton relative to Laurentia and Greenland. Thus
the crust both in the south of the North American Craton and in the west of the
East European Craton was strongly disturbed tectonically and experienced
intense magmatism.
Numerous anorogenic-type intrusions,
including some of the rapakivi-anorthosite type, penetrated the crust between
ca. 1.6 and 1.4 Ga in the Scania and Blekinge, on Bornholm Island and to the
east of the Baltic Sea in NE Poland, Lithuania and Belarus. EW- trending zones of shearing appear to
have controlled the siting of these anorogenic intrusions. This maybe applied
both to the large Riga rapakivi anorthosite pluton of 1.58 Ga age and 1.50 Ga
intrusions in Poland and Lithuania (Sundblad et al., 1994; Claesson et al.,
1995). The zones of shearing and anorogenic magmatism in the Peri-Baltic
Belarus region are associated not only with local reheating of the crust but
also with amphibolite-facies metamorphism in the country rocks and the
intrusions themselves. This is reflected by 40Ar/39Ar
amphibole ages of 1.55 to 1.4 Ga (Bogdanova, Page et al., 1996 and in press).
Seismic data suggest that the 1.65 to 1.5 Ga old igneous rocks may have
underplated the West Lithuanian Granulite Domain, forming a low-velocity layer
at depths of 12 to 16 km (EUROBRIDGE
Seismic Group, 1997).
Major EW-trending shear zones complicate the
crustal structure of the Palaeoproterozoic orogen also in southeastern Sweden.
The best known of these zones is the Blekinge Shear Zone (BSZ) that is stitched
by ca 1.5-1.4 Ga Karlshamn-group granite intrusions. These granites somewhat
resemble rapakivi-type rocks and show magmatic flow fabrics. The rock units on
Bornholm Island are generally similar to those in Blekinge and Scania (cf.
Jøgart, 1993). Previous structural observations may suggest that the main
deformation and folding in SW Blekinge and on Bornholm occurred during
movements along the EW shear zones.
The main problem to be solved by this project
is to establish structural and temporal relationships between BSZ and the
Mesoproterozoic granitic intrusions. Two alternatives may be expected:
1)
the
shear zone was active BEFORE the granites intruded and the magma just used
zones of weakness in the crust; in this case no particular structural link
between the zone and the intrusions can be found, except elongated form
of the batholiths. The age of metamorphic minerals in country rocks is older
than emplacement/crystallization of the intrusion.
2)
the
shearing was active DURING the emplacement and strongly uniformed structural
pattern within and outside the intrusion. Ages of the shearing and emplacement
are the same.
Key
localities and objectives
Österlen, S Sweden
Granites with glomeroporphyritic which texture intruded gneisses of the unknown origin and age in the area around Stenshuvud were described as granite porphyries by Törnebohm and Hennig (1904). However, later these rocks were classified as volcanites or subvolcanites (SGU, map sheet, Ba 40). There is another group of gneissic, medium-grained granites in the area also remain of unknown age and origin.
Geochemical-petrological and structural investigations of these granites and their country rocks are needed for understanding geological history of the region and for correlation of these granites with those in adjacent areas like Blekinge and western Scania.
Between 1.5 and 1.4 Ga numerous granitic bodies
intruded into the Palaeoproterozoic crust of the region. Two large batholiths,
Karlshamn and Eringsboda, represent a significant part of this magmatism. These
granites are coarse grained, porphyric with often rapakivi and antirapakivi
textures and usually show magmatic flow fabrics. Multiple magma emplacements
are suggested by SGU.
Some preliminary structural studies (Bogdanova and Cecys, 1999) have shown that emplacement of these batholiths are related to the Blekinge Shear Zone (BSZ) which is one of the prominent EW-trending shear zones known around the southern Baltic Sea.
Structural,
petrological and geochemical investigations are required to establish temporal
and spatial relationships between this zone and the granitic bodies. The study should
include revealing of the internal structure of the batholiths (anisotropy of
magnetic susceptibility (AMS) and field structural observations) as well as
defining different magma pulses (geochemistry). Timing of the BSZ activity and
different magma pulses forming the batholiths can show whether the emplacements
occurred in tectonic setting of active shear zone.
Åhäll, K.-I. and Gower, C.F., 1997. The Gothian
and Labradorian orogens: variations in accretionary tectonism along a late
Paleoproterozoic Laurentia-Baltica margin. GFF, 119: 181-191.
Bogdanova, S. and Cecys, A., 1999. Mesoproterozoic
active shear zones controlling anorogenic magmatism in the western part of the
East European Craton, EUG10, Strasbourg, pp. 688.
Claesson, S., Sundblad, K., Ryka, W. and
Motuza, G., 1995. The Mazury complex - An extension of the Transscandinavian
Igneous Belt (TIB) into the East European Platform?, Terra nova, EUG 8 abstracts, pp. 107.
Elming, S.-Å. et al., 1993. The drift of the
Fennoscandian and Ukrainian shields during the Precambrian: a palaeomagnetic
analysis. Tectonophysics, 223: 117-198.
Jögart, T., 1993. The basement geology of
Bornholm. Exkursionsfufrer
auf Rugen und Bornhom: 120-149.
Rivers, T., 1997. Lithotectonic elements of the
Grenville Province: review and tectonic implications. Precambrian Research, 86:
117-154.
Sundblad, K., Mansfeld, J., Motuza, G., Ahl, M.
and Claesson, S., 1994. Geology, geochemistry and age of a Cu-Mo-bearing
granite at Kabeliai, Southern Lithuania. Mineralogy and Petrology, 50: 43-57.
Törnebohm, A. E. & Hennig, A., 1904: Beskrifning
till blad 1 & 2 omfattande de topografiska kartbladen Landskrona, Lund,
Krostianstad, Malmö, Ystad, Simrishamn. Sveriges Geologiska Undersökning.
Subproject 1:
The Stenshuvud
and Tåghusa granites: new representatives of the Mesoproterozoic granitic
magmatism in southern Sweden.
Aims: Which period of
the crust formation in the Österlen area do the granites represent and to which
tectonic setting do they belong? What is the origin of the granites and how
their glomeroporphyritic texture was formed?
Time plan: fieldwork
was carried out during period 1997-1999, all analyses are done and manuscript
will be completed until the end of 2000.
Laboratories: Some of chemical analyses were done at Luleå
and some at ACME lab in Canada. Age determinations were carried out in the
Museum of Natural History, Stockholm (NRM) and the Vernadsky inst. of
Geochemistry and Analytical Chemistry, Moskow, Russia.
Economy: Apart from S.
Bogdanova’s research grants (NFR), early stages of study were supported by the
Swedish Institute.
Co-workers: Svetlana Bogdanova, Elena Bibikova (Moscow) and
Karl-Axel Kornfält (SGU).
Subproject 2:
The NORDSIM study of 1.5-1.4 AMCG rocks in
southern Sweden.
Aims: In southern Sweden
many 1.5-1.4 Ga granitic intrusions show inhomogeneous zircon grains indicating
multiple events of the rock formation. Because conventional methods have
yielded discordant ages NORDSIM investigation could reveal exact timing of
crystallization of granites along with overprinting and inherited events.
Time plan: fieldwork
will be completed in 2000-2001 and NORDSIM studies will be carried out during
2001.
Laboratories: NORDSIM facility in NRM, Stockholm
Economy: S. Bogdanova’s research grants (NFR).
Co-workers: Svetlana
Bogdanova, Elena Bibikova (Moscow) and Åke Johansson (NRM).
Subproject 3:
The internal structure of the Karlshamn granite intrusion, SE Sweden.
Aims: To reveal the
internal structure (different magma phases, magma flow pattern) of the Karlshamn granite
intrusion using AMS, structural field observations and geochemistry; to
establish relationships with the country rocks and BSZ using isotopic datings
of magma pulses, enclaves/xenoliths and country rocks, to create 3D
gravity-magnetic model, and finally, to model geochemical and
structural-tectonic evolution of the intrusion.
Time plan: fieldwork
will be completed in 2000, analytical part and manuscript in first half of
2001.
Laboratories: Geochemical analyses will be done in Liege
and ACME lab, geophysical modeling in Geological Survey of Lithuania (GSL), AMS
measurements in Lund and isotopic work will be done in NRM, Stockholm.
Economy: S. Bogdanova’s research grants (NFR).
Co-workers: G. Bylund,
L. Korabliova (GSL), J.-C. Duchesne (Liege), S. Claesson (NRM) and Åke
Johansson (NRM).
Subproject 4:
The internal structure of the Eringsboda granite intrusion, SE Sweden.
This subproject is analogous to that Nr. 3 and has the same tasks to
study another large Eringsboda intrusion related to BSZ. The fieldwork and
analytics will be done in 2001, and manuscript should be ready in late
2001-early 2002.
Subproject 5:
Granite emplacement in active shear zones: a case from SE Sweden.
Aims: First, to study BSZ
structurally employing AMS and field observations. Then, from available own
data collected during subprojects above as well as from data in the literature,
to establish relationships between the Mesoproterozoic granitic intrusions in
SE Sweden and EW-trending shear zones (BSZ): timing, structural links etc.
Time plan: fieldwork
(BSZ) will be completed in 2001, analytical part and manuscript in 2002.
Laboratories: AMS measurements will be done in Lund and
additional, if required, geochemical analyses in Liege and ACME lab.
Economy: S. Bogdanova’s research grants (NFR).
Co-workers: S.
Bogdanova, G. Bylund, L. Page and S. Claesson (NRM).