Roland Gorbatschev

The Precambrian development of southern Sweden

Abstract The Precambrian of southern Sweden represents a gradual accretion of the Baltic Shield. The Svecofennian region in the east was stabilized by about 1750 Ma. In the west, an orogenic event between ca. 1600 and 1700 Ma added new continental crust to the shield, possibly also reworking a marginal segment of the Svecofennian. A continuous belt of granites and porphyries marks the boundary of the south-western orogen with the stable Svecofennian region. The subsequent development of southern Sweden was essentially ensialic, periods of granite intrusion and metamorphism interrupting anorogenic developments. In a final culmination of activity around 900–1100 Ma, southern Sweden played the role of a crustal segment marginal to the Grenvillian orogen.

The RbSr age of the Hästefjorden granite and its bearing on the Precambrian evolution of southwestern Sweden

Abstract On the basis of a seven-point isochron, the RbSr whole-rock age of the Hastefjorden granite in southwestern Sweden is 1240 ± 30 Ma using a decay constant of 1.39·10 −11 yr −1 . The initial 87 Sr/ 86 Sr ratio is 0.712. This dating sets a lower limit for the beginning of the Dalslandian (Dal sequence) sedimentation and supports suggestions of a chronological diversity of the so-called Gothian Amal—Kroppefjall granite complex in western Sweden. It also invalidates correlation of the Amal—Kroppefjall complex as a whole with the Smaland plutonics of southeastern Sweden. This proves that rocks previously considered as being formed during the “Gothian Cycle” are very different in age.

Fundamental subdivisions of Precambrian granitoids in the Åmål mega-unit and the evolution of the south-western Baltic Shield, Sweden

Abstract The “Amal-Kroppefjall granites” have hitherto been regarded as belonging to the “Gothian orogeny” of southern Scandinavia and have been considered equivalent to the Smaland-Varmland granites of south-eastern Sweden. In the Amal-Kroppefjall region there actually occur several groups of granitoid intrusives of widely different ages and lithotectonic characteristics. The good state of preservation of the rocks in this region is due to its synclinorial subsidence during and after a major event of regional metamorphism and migmatization affecting much of south-western Scandinavia. It is concluded that the concept of a single coherent development of the “Amal-Kroppefjall granites” and the previously suggested correlations and definitions of the “Gothian” in southern Scandinavia are untenable.

Rb-Sr age of granitoid gneisses in the “Pregothian” area of south-western Sweden

Abstract Grey granodioritic to tonalitic gneisses in the migmatite terrain of south-western Sweden, sampled along a traverse between Vara and Vanersborg, yield a six-point isochron Rb-Sr age of 1735 ± 85 Ma at an initial 87Sr/86Sr ratio of 0.7027 ± 0.0008. This large-sample whole-rock age fails to support the concept of an early, Presvecokarelian development of the migmatites in this part of the “Pregothian”. It suggests a major geological event of age and, in comparison, only subordinate influence in this area of later events (900 Ma, 1250 Ma, etc.), which do not reset the isotope system on a regional scale. The low initial Sr-isotope ratio argues against a long prehistory of upper crustal evolution of the dated rocks.

The Central Scandinavian Dolerite Group in Jämtland, central Sweden

Abstract Inz north central Sweden, the ca. 1250-Ma old Central Scandinavian Dolerite Group mainly comprises large flat-lying sheets essentially of alkali-basaltic olivine dolerite. The content of incompatible elements is somewhat lower than is average in alkali basalts. Geochemically, the dolerites are rather homogeneous and follow a liquid-descent line which is modified somewhat by the cumulation of early olivine. Mineralogical control of crystallization is particularly obvious in the Ni/Cr and K/Rb distributions. The contents and proportions of minor chemical constituents are discussed in relation to systems proposed for tectonic classification. The intrusion of the Central Scandinavian Dolerite Group was an intra-plate event coinciding in time with the early stages of the Sveconorwegian orogeny in western Scandinavia.

A Rb-Sr geochronological study of the older granitoids in the Åmål tectonic mega-unit, south-western Sweden

Abstract Rocks of the tonalitic—granodioritic provisional Amal I plutonio complex in the vicinity of Uddevalla, western Sweden, were subjected to a Rb-Sr geochronological study. The area was selected on account of the comparatively weak local manifestations of plutonic activity and metamorphism referrable to the younger, provisional, Amal II group and later events. The sampled rocks nevertheless show a partial opening of the regional Rb-Sr isotopic system and consequently yield a poor-fit seven-point age reference line of 1690 Ma, the initial 87Sr/86Sr ratio being 0.7011 ± 0.0027. Feldspar-biotite whole-rock isochrons obtained from a study of two of the dated samples suggest a disturbance of the Rb-Sr isotope system at 975 ± 60 Ma. This age fits the known Rb-Sr and K-Ar ages of the late Sveconorwegian (Dalslandian) activity in the south-western Baltic shield.

The Rb—Sr age of the Ursand granite on the boundary between the Åmål and “Pregothian” mega-units of southwestern Sweden

Abstract The Rb-Sr age of the Ursand granite massif is 1250 ± 50 m.y. Within the limits of experimental error, this is identical with the Rb—Sr age of the Hastefjorden massif, situated close to the central axis of the Amal mega-unit in south-western Sweden. The age determination confirms that granites of the Hastefjorden group transgress the limits between the Amal and “Pregothian” mega-units. It also suggests that this event of granite formation has regional significance in parts of the south-western Baltic Shield.

Rb—Sr dating of rocks in the Värmland granite group in Sweden

Abstract Thirteen samples of Varmland granites from south-eastern Varmland, Sweden, have been subjected to a Rb—Sr dating study. Most of the studied samples subdivide into two groups defining two separate isochrons of closely similar age but different initial 87Sr/86Sr ratios. Five samples from a southern sub-area of Filipstad granite around Filipstad and Karlskoga have yielded a Rb-Sr age of 1700 ± 37 Ma at an initial 87Sr/86Sr ratio of 0.7068 ± 0.0021. Five samples from the northern sub-area of Hagfors—Eksharad—Uvana, together with two samples from the vicinity of Kristinehamn, form another isochron of 1689 ± 30 Ma with an initial Sr-isotopic ratio of 0.7021 ± 0.0006. One sample could not be assigned to either isochron and is intermediate between them. The differentiation of Varmland-group granites into separate complexes and types is also brought out by field data and geochemical characteristics.

Rb-Sr isotopic relations of a tonalitic intrusion on Tjörn Island, south-western Sweden

Abstract Eight samples from the Ronnang tonalite–granodiorite intrusion on Tjorn Island, south-western Sweden, were investigated in a Rb-Sr geochronological study. The whole-rock Rb-Sr system is disturbed and the eight samples describe a 1615-Ma age-reference line, which does not qualify as an isochron. However, an isochron age of 1710 ± 115 Ma (λ = 1.39 × 10−11 a −1) at a low, initial, 87Sr/86Sr ratio is obtained when only four of the samples poorest in Rb are employed. This age is interpreted as dating the intrusion. The event which disturbed the whole-rock Rb-Sr system was identified from combined mineral and whole-rock data. An isochron of 1010 ±105 Ma and a reference line of 1015 Ma were obtained. This age coincides with the age of Sveconorwegian intrusions and metamorphism. A compilation of Rb-Sr data from southwestern Sweden identifies a zone of late Sveconorwegian regeneration of the whole-rock isotopic systems.

Metamorphism in extensive nappe terrains: A study of the Central Scandinavian Caledonides

Abstract Metamorphism in the nappe terrains of the Central Scandinavian Caledonides is characterized by inverted gradients and other indications of disequilibrium caused by the superposition of nappes of various metamorphic grade and the translation of the nappe pile onto the margin of the Baltic Shield. In the lower nappes there is a gradual westward increase of grade, but some inverted gradients persist. In the upper allochthon, the variation of metamorphism is essentially vertical. It is possible to identify and evaluate parageneses formed prior to, during, and after nappe emplacement. Their interpretation in terms of separate Caledonian or Precambrian events is often ambiguous. Early Caledonian metamorphism apparently represented a range of P-T-conditions. The build-up and translation of the nappe pile was characterized by transient high pressure conditions. A metamorphic pulse, manifest particularly in the west, succeeded nappe emplacement onto the Baltic Shield. Strain-induced metamorphism in the vi...