摘要:

Seismic refraction studies have been carried out along several profiles through the Caledonian front in Jämtland in order to investigate the configuration of the Precambrian crystalline basement beneath the Caledonian sedimentary cover. The seismic sections were calculated according to the time-delay method and by assuming a linear increase of velocity with depth. Recordings from each of the 3 shotpoints along the profiles allowed the construction of continuous profiles. A method was applied whereby the basement velocity and the velocity gradients were varied until the best fit with respect to the calculated depths from different shotpoints was obtained. The seismic models are compared with drill-hole data, interpretation of aeromagnetic data and surface geology. Depths calculated from the seismic data compare well with the information from the few deep drillholes that reach down to the autochthonous basement. The magnetic and seismic models concur well along most of the profiles but differ greatly locally, thus demonstrating the importance of the application of various geophysical techniques, particularly when investigating complex stratigraphies, as in the Caledonides. Both the seismic and the magnetic models, suggest the existence of an undulating basement surface in the Caledonian front that coincides with some of the major folds in the sedimentary cover. East of the Offerdal synform the basement surface has previously been interpreted to dip gently (about 2°) westwards. The new seismic and magnetic data suggest a more irregular basement configuration than has previously been demonstrated.

ISSN:0016-786X    

DOI:10.1080/11035898409454597

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The Boda Pb-Zn deposit is located in the eastern part of the Siljan impact structure in central Sweden. It consists of calcite, baritc, sphalerite, galena, pyrite and marcasite, mainly as fracture and breccia fillings along a thrust zone in Ordovician limestones. Boda, and the smaller but similar Sollerön deposit, probably formed in close connection with the tectonic disturbance following the Devonian meteorite impact considered to have created the Siljan Ring structure. A geochemical profile through the ore-hosting limestone at Boda shows that wall rock alteration is not very marked. Lead isotope data point to the Proterozoic basement as the direct or indirect source of the ore lead, and to a large contribution of redeposited Proterozoic lead in the Ordovician limestone, presumably contained in the clastic silicate fraction. The extreme radiogenicity of the Boda ore lead may be explained by selective leaching of the more radiogenic lead fraction from the source rocks, rather than leaching of average rock lead. Relatively heavy sulfur isotope values in the galena may point to a sulfate source in the Paleozoic sediments for the sulfur. The schalenblende texture of the sphalerite ore suggests repeated pulses of rapid precipitation from supersaturated ore solutions.

ISSN:0016-786X    

DOI:10.1080/11035898409454598

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0016-786X    

DOI:10.1080/11035898409454599

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Sulphur isotope ratios in a pyritic sulphide mineralization associated with Svecokarelidic (2.2–1.8 Ga old) spilitic rocks at Naatikkalampi in Kuusamo, NE Finland, have been determined. The results of 18 determinations show a relatively narrow range of variation from +2.8% to +3.4% relative to the Canon Diablo standard. The mean value is 3.1%. The origin of the sulphur in the sulphide phase is discussed. It is concluded that the sulphur isotope ratios of the mineralization are consistent with an igneous hydrothermal model suggested for the quartz-sulphide veins by field evidence. As there is not enough knowledge concerning the sulphur isotope ratios of Precambrian seawater, a sedimentary or a bacteriogenic origin of sulphur can, however, not be excluded on the basis of the results.

ISSN:0016-786X    

DOI:10.1080/11035898409454600

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0016-786X    

DOI:10.1080/11035898409454601

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Galena in drill core samples from sandstone-lead deposits in the Dorotea district in the Caledonian Front was analysed for its lead isotope composition. Six samples were taken from Bellvikserg and three from the minor deposit of Ormsjö. The composition of all samples was highly radiogenic with206Pb/204Pb-ratios between 22 and 23, and207Pb/204Pb-ratios in the range 16.0 to 16.2. Plotted on a207Pb/204Pb versus206Pb/204Pb diagram, the Dorotea lead follows a linear trend together with lead from Laisvall and Vassbo, suggesting their radiogenic component to be derived from source rocks of Svecokarelian age.

ISSN:0016-786X    

DOI:10.1080/11035898409454602

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0016-786X    

DOI:10.1080/11035898409454603

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Granite, gabbro and anorthosite together constitute the Nordingrå massif. These igneous rocks have intruded a sequence of Svecokarelian metasediments and postdate the regional metamorphism and folding. Available field observations demonstrate that the granite intrudes the gabbro-anorthosite complex. Zircons separated from the granite and the anorthosite have an age of 1578 ± 19 Ma, which is considered to indicate the time of magmatic crystallization. The Rb-Sr whole rock age 1552 ± 18 Ma of the gabbro and anorthosite is slightly lower than the zircon age, whereas the Rb-Sr reference age of the granite, 1416 Ma, is considerably lower than the age of the zircons. Hypothetical models of genesis and history are discussed to explain this discordancy. It is suggested that post-crystallization hydrothermal fluids, circulating in the bedrock during the uplift and cooling of the orogenic belt and/or during subsequent epeirogenic movements, transported Rb and Sr isotopes in the whole rock system. This led to an apparent low Rb-Sr age and a high initial87Sr/86ratio. The conclusions are supported by δ18O analyses, which indicate that the granite has been depleted in18O, and by microscopical observation of mineral alterations. K-Ar ages of separated minerals from granite and gabbro may also have been influenced by these alterations. A thermal imprint on the bedrock at the intrusion of the Jotnian dolerite sheets may also have contributed to the observed considerable spread of the K-Ar ages from 1600 to 1100 Ma.

ISSN:0016-786X    

DOI:10.1080/11035898409454604

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0016-786X    

DOI:10.1080/11035898409454605

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Room-temperature optical absorption spectra, microprobe analyses and X-ray cell parameters of a manganese-bearing chlorite from Långban, Sweden are presented. The optical absorption spectra are characterized by four broad and intense absorption bands at 23300, 21900, 19700 and 11300 cm−1, which are assigned to Mn3+spin-allowed d-d transitions. The structure of the crystal field spectra of Mn3+in chlorite and the obtained crystal field splitting parameter, 10Dq = 16000 cm−1, indicate that Mn3+is located at a six-coordinated site, where the mean Mn3+-ligand bond distance is ∼ 0.200 nm and the site symmetry is lower than C2v. The most likely location of Mn3+in the chlorite structure is the small M(4)-site of the brucite-type layer.

ISSN:0016-786X    

DOI:10.1080/11035898409454606

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor