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Geological Society, London, Special Publications; 1987; v. 33; p. 95-107;
DOI: 10.1144/GSL.SP.1987.033.01.08
© 1987 Geological Society of London

Early Proterozoic Volcanic Suites of the Baltic Shield

The volcano-tectonic setting and mineralization of the early Proterozoic Kemiö-Orijärvi-Lohja belt, SW Finland

H. Colley

Department of Geology and Physical Sciences, Oxford Polytechnic, Headington, Oxford, OX3 0BP, UK

L. Westra

Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

The Kemiö-Orijäirvi-Lohja region of SW Finland, which consists of metabasic rocks, metagreywackes, metavolcanic rocks of felsic composition (leptites) and syngenetic gabbro-tonalite bodies, is a westerly striking portion of the early Proterozoic (1900 Ma) Svecofennian Belt. This belt, which extends westward into Central Sweden is flanked by large post-kinematic granites. The grade of metamorphism decreases westward from amphibolite-granulite transitional conditions around Orijärvi to greenschist conditions in Central Sweden.

The volcanic sequence begins with submarine basic-intermediate lavas followed by turbiditic metasediments and ends with the deposition of felsic pyroclastic rocks. Sheets of gabbro related to larger synkinematic intrusive bodies occur within the felsic rocks. Disseminated Cu-Fe sulphides occur in the lower basic rocks and massive Cu-Pb-Zn ore bodies, of probable volcanogenic origin, are found in the upper felsic rocks. Mg-alteration and the formation of cordierite-anthophyllite rock characterizes the latter ore deposits.

The geochemistry of the metavolcanic rocks indicates a transition between alkaline and subalkaline compositions, and the geochemistry of intrusive bodies suggests they are comagmatic with the volcanic rocks they intrude. In more strongly deformed areas mobility of Fe, Si and K is indicated. The origin of some high Fe basic lavas is unclear. After consideration of the geology and geochemistry of the Svecofennian Belt it is suggested that it may represent an ancient analogue of present day New Zealand where ‘back-arc’ rifting is occurring in both oceanic areas and mature arc crust.





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L. Rajavuori and L. M. Kriegsman
Fluorine in orthoamphibole dominated Zn-Cu-Pb deposits: examples from Finland and Australia
Geological Society, London, Special Publications, 2002; 204: 337 - 353.
[Abstract] [PDF]