Geochemistry of the dolerite dykes in Sodermanland, eastern central Sweden
Title:
Geochemistry of the dolerite dykes in Sodermanland, eastern central Sweden
Author:
Risku-Norja, Helmi
Appeared in:
GFF
Paging:
Volume 114 (1992) nr. 1 pages 67-91
Year:
1992-03-10
Contents:
The numerous middle to late Proterozoic mafic, doleritic dykes occurring in the province of Sodermanland, east central Sweden, have been studied geochemically and in the field. The chemically defined dyke groups do not necessarily coincide with the groups defined on the basis of dominant dyke trends, although this is usually the case. Thus, Group I dolerites are tholeiitic to mildly alkali-enriched and trend E-W to ESE-WNW, defining the traditional Breven-Hallefors swarm. Group II is alkali-basaltic representing the N-S trending Blekinge Dalarna Dolerite dykes (BDD) and Group III is truly tholeiitic being a swarm of SE-NW to N-S trending dykes concentrated in the eastern and southern parts of the studied area (the Sodertalje dykes and some dykes previously rendered to the Breven-Hallefors swarm). Group IV is also truly tholeiitic comprising rare ESE-WNW to SSE-NNW trending dykes coinciding directionally with swarms/groups I and III and not very much different from Group I dykes in the field. All these swarms were emplaced after the cratonization of the crust following the Svecofennian orogeny 1.9-1.8 Ga ago, and they are, thus, distinct from the intraorogenic 1850 Ma old, Svecofennian dykes also found in the same area. Group I has Rb/Sr ages of about 1530 Ma and Group II about 1000 Ma. No age data are available on Groups III and IV, which makes particularily the definition of Group IV hypothetical at present. Although the contents of the trace elements within each group vary, the ratios of the incompatible trace elements are specific for each swarm suggesting that the distinctive characteristics of the swarms have been inherited from mantle sources. A crustal overprint was received during the residence of the magmas in crustal reservoirs or during their ascent to the upper crust. Group-specificity is also demonstrated by trace element spidergrams and differentiation trends. There are differences also in the mode of occurrence in the field; the dyke widths, trends and textures. Although mineralogical alteration may be extensive, the chemical effects of alteration are of minor importance and are expressed as irregular variations in the contents of the mobile minor and trace elements and, rarely, also of Ti, Nb and Zr. These effects are confined to the most narrow dykes, and within the clustered Group III dykes, presumably to the later intruded dykes. The total evidence strongly suggests separate sources for Group I, II, III and IV magmas. The compositions of the Group IV dolerites overlap to some extent those of Groups I and III, but the trace-element spidergrams are quite distinctive. Comparison of the characteristics of the presently analyzed dykes with those of the large, composite Hallefors dyke, suggests a genetic link between Group I and the Hallefors dyke and, by implication, also between Group I and the Breven dolerite.
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