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Doctoral thesis
English

Geological evolution of the Bodsjö area, Central-Sweden

ContributorsGinet, Claude B.
DirectorsJaffe, Felice
Number of pages242
Imprimatur date1980-06-17
Abstract

The area studied is situated in the Bodsjö region, 80 km south of Ostersund, central Sweden, and is part of the Svecokarelian belt of Proterozoic age.

The geology of the area (approximately 250 square kilometres) which were mapped at a scale of 1/50,000 reveals the following main rock groupes:

Meta-sediments, which include meta-arkoses, meta-quartzites, meta-pelites, meta-sub-pelites wich subordinate amounts of tuffaceous rocks, represent a shallow water sedimentation at the transition between a marine and terrestrial environment. The stratigraphic relationship between the various meta-sedimentary units as well as the original sedimentary features are completely obliterated by high grade metamorphism, migmatization and polyphased deformations. A model for the evolution of the sedimentary sequence, based on lithology, is presented and probably stratigraphic correlations with the surrounding areas are proposed.

Basic and acid metavolcanics are represented by amphibolitic and quartzo-feldspathic rocks, respectively. Their chemistry suggests that they originated in an island arc environment, basic metavolcanics being possibly an early tholeiitic series and acid metavolcanics a subsequent calc-alkaline series.

Early orogenic intrusives are composed of gabbros, granodiorites, and granites. Within this group deformed granites (augen gneiss) are the most abundant.

Late-kinematic granites form small intrusions throughout the whole area. These granites have an eutectic normative composition and may be anatectic.

The Revsund granite is represented by a typical coarse-grained porphyritic type in the northern part of the area and by numerous small intrusions and dykes of varied composition throughout the central part.

Granitic intrusions of the Bodsjö area are classified as I- or S-types. These rocks are discussed in the context of Wilson's (1980) classification of Swedish granites.

Two distinct types of dolerite are the youngest intrusive bodies of the area. The Turinge (2-pyroxenes) dolerite, forming a flat lying sheeted intrusion elongated along the main structural trend of the area, and the Jotnian (1-pyroxene) dolerite mainly represented by scattered dykes and sills.

Various contact phenomena near the Turinge dolerite are observed, producing a variety of remobilized unfoliated granites particularly within the early granites.

The type and degree of metamorphism was studied in detail within the metasedimentary rocks by petrographic and microprobe methods. It was found that the main mineralogical assemblages are :

-Cordierite-biotite-andalusite-sillimanite

-Cordierite-garnet-sillimanite-biotite

-Sillimanite-biotite

-Andalusite-biotite

Microprobe date show that the petrographic observations and calculated Fe-Mg repartition (KD value) between cordierite and biotite are closely correlated.

The sequence of prograde reactions and textural relationships observed reflects the progressive decrease in pH20 (dehydration) and the relative increase in oxygen fugacity.

The highest possible P-T conditions are deduced from the composition of cordierite at the equilibrium of the reaction :

Cordierite+K-feldspar+H20 <-> sillimanite+biotite.

The cordierite composition, within the Bodsjö area, varies between Fe 35 and Fe 40. (...)

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Citation (ISO format)
GINET, Claude B. Geological evolution of the Bodsjö area, Central-Sweden. 1980. doi: 10.13097/archive-ouverte/unige:174350
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