Geochemistry and volcanic setting of the Ordovician Forbordfjell and Jonsvatn greenstones,Trondheim Region,central Norwegian Caledonides |
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| Authors: | Tor Grenne David Roberts |
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| Institution: | (1) Geologisk Institutt, Universitetet i Trondheim, N-7034 Trondheim-N.T.H., Norway;(2) Norges geologiske undersøkelse, P.O. Box 3006, N-7001 Trondheim, Norway |
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| Abstract: | Geochemical patterns in two, correlative, Ordovician, basaltic greenstone formations from the central Norwegian Caledonides corroborate field indications of a tripartite stratigraphic subdivision of the units. Lower and upper members show similar chemical traits and are transitional to LREE-enriched, high-magnesia to low-magnesia basalts. The thicker, more massive, middle member carries the geochemical signature of LREE-depleted,  normal mid-oceanic ridge tholeiites. Stratigraphic-upward petrochemical variation discerned within the individual members points to processes of either closed or open-system low-P fractional crystallization. The geochemical trends can be ascribed to a mechanism of spreading accompanied by lithospheric thinning in a marginal ocean basin situation, followed by renewed crustal thickening. Geochemical and lithovolcanic polarity indicators denote that the basin was located to the east of an Arenig-Llanvirn magmatic arc sited within the Caledonian Iapetus Ocean. |
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