Eric Welin

The depositional evolution of the Svecofennian supracrustal sequence in Finland and Sweden

Abstract On the basis of six new zircon ages from metavolcanic rocks, together with other isotopic, geophysical and geological data, the chronological and tectono-stratigraphic evolution of the early Proterozoic Svecofennian supracrustal rocks in Finland and Sweden are discussed. These data suggest that a NW-SE displacement from the Bothnian Bay to Lake Ladoga and a sinking of the southwestern block was initiated 1950–2000 Ma ago. Accumulation of pelitic sediments and greywackes on the subsiding Archaean basement continued until about 1860 Ma In south-central and northern Sweden, shallow submarine and subaerial volcanics were extruded 1880–1890 Ma ago. The thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860–1890 Ma ago.

Tabulation of recalculated radiometric ages published 1960–1979 for rocks and minerals in Sweden

Abstract Radiometric ages published 1960–1979 for Swedish rocks and minerals have been tabulated and, when appropriate, recalculated with the new recommended decay constants (R. H. Steiper & E. Jager, 1977: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362).

Age measurements on radioactive minerals from Sweden

Abstract Age measurements with the U‐Pb method have been carried out on Swedish pegmatite minerals. As a result two periods of pegmatite formation have been established. The younger pegmatites are situated in south‐western Sweden and have an age of 930 m. y. The older group consists of late Svecofennian pegmatites with an age of 1810 m. y. in central and northern Sweden. Some indications that pegmatites occur in Sundsvall‐Ostersund area with an age of 1380 m. y. have also been encountered but need further verification. The U‐Pb ages are compared with older K‐Ar determinations.

Further Age Measurements on Radioactive Minerals from Sweden

Abstract Age determinations utilizing the U/Pb method have been carried out on radioactive minerals in the Precambrian bedrock of Sweden. As a result, minerals from northern Sweden have been found to have an age of 1775–1845 m.y. but a more precise result could not be obtained. The pegmatites in the Ostersund—Sundsvall area and the pegmatites and aplites in southeastern Sweden are 1380 and 1745 m.y. old respectively. A pegmatite associated with the Karlshamn granite and a granite on Romeleasen each have ages of 1455 m.y. Some previously determined K/Ar ages are discussed and it is concluded that the uplift of the gneiss block in southwestern Sweden occurred during or after a period of igneous activity in western Sweden 935–1000 m.y. ago. The age data also indicate a contemporaneity between the deposition of the Dal formation in western Sweden and the Jotnian sandstones, the former being of marine lithology, the latter representing a terrestrial facies.

Rb/Sr Whole Rock Age Data on Some Swedish Precambrian Rocks

Abstract Rb/Sr whole rock age dating of sub-Jotnian volcanics in the Los region give an age of 1685 m.y., and of red Vaxjo-granitc in the Stralsnas area an age of 1740 m.y. The results are in good agreement with U/Pb age data and disregard the existence of a Gothian cycle.

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.

RbSr isotope systematics at amphibolite facies conditions, Uppsala Region, Eastern Sweden

Abstract On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for 87Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic 87Sr and on the fixation of 87Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.

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 radiometric age of a Proterozoic hyperite diabase in Värmland, western Sweden

Abstract Across Varmland County there stretches in N-S direction a broad belt of black diabases known as hyperites. To the east it borders on a diffuse tectonic zone separating the gneiss complex of southwestern Sweden from the Svecokarelian orogenic belt. To the west, the diabases are partly involved in a late, middle Proterozoic thrust zone, the Mylonite Zone. The hyperite diabases appear as variably boudinated sills and dikes which generally follow the northwest strike of the older gneissic rocks. A pegmatoid variety from the interior of a large dike was collected for radiometric dating. Separated zircons plot on a regression line with an upper intercept with the Concordia at 1550 Ma, which is interpreted to correspond to the crystallization of the hyperite diabase. The lower intercept is at 880 Ma, which coincides with a Rb-Sr total rock-mineral isochron age of 975 Ma within the limits of error. It is concluded that a metamorphic event at 900–1000 Ma has caused Sr isotopic homogenization within the an...

K-Ar ages of Jotnian dolerites in Västernorrland County, Central Sweden

Abstract Olivine dolerites ascribed to the Jotnian (or post-Jotnian) have been dated by the K-Ar method. Model ages of minerals and whole rocks show a spread from 875 to 1230 m.y. An isochron calculation of the age gives a result of 1245 m.y. This is regarded as the time of retention of argon, and most probably also closely corresponds to the time of crystallization of the dolerites.