Rb-Sr and Sm-Nd relationship in dyke and sill intrusions in the Oslo Rift and related areas.

Abstract

Eighteen dykes and three sills from the Oslo Rift and related areas in SW Sweden have been investigated by the Rb-Sr and Sm-Nd methods. Rb-Sr isochron ages in the range 287 to 248 Ma were obtained for twelve of the dykes. All dyke ages fall within the time-span previously established for the Oslo Rift magmatism: 305-240 Ma.An early group of tholeiitic dolerites and microsyenitic dykes fall in the period 297-285 Ma. The intrusion pattern of these dykes suggests strike-slip movements during an early phase of rift evolution. Rhomb-porphyry (RP) dykes are generally later (280-270 Ma) than the main period of plateau volcanism (295-275 Ma), and are contemporaneous with the emplacement of monzonitic (larvikite) plutons. The age 249 +- 3 Ma) of a per-alkaline microgranite dyke from the Nittedal caldera is compatible with a derivation of the dyke from late differentiates of the alkali-syenite dyke from Scania, Sweden, yielded an age (261 +- 6 Ma) considerably younger than the dolerites (-300 Ma), supporting other evidence of a prolonged period of intrusive activity in this area. The Mjøndalen-Etnedal RP dyke shows evidence of at least three intrusion pulses along its strike, of magmas with different Sr isotopic signature. The Nd and Sr initial ratios of the dykes fall within the field covered by other Oslo Rift igneous rocks, indicating that the mixing model involving two mantle sources and a crustal component is also applicable to the dyke rocks. Two dykes show evidence for in-situ interaction between magma and a selective radiogeniC87Sr contaminant, most likely crustal fluids.