Origin and composition of the Hällefors composite dyke, central Sweden

Sammanfattning: The Hällefors dyke is a large, wedge-shaped high-level intrusion that trends E-W and has a subvertical dip, the overall dimensions being about 42 km in length and 1 km in width. It was emplaced ca 1550 Ma ago in gneisses of the Svecokarelian (1950-1750 Ma) orogenic belt in south central Sweden. The dyke is composite and consists of: (1) a slightly differentiated marginal border group of mainly olivine dolerites with affinity to continental high-Al tholei- ites, and (2) a Fe-Ti(-P) - rich differentiated and crudely layered central series of porphyritic, largely cumulitic rocks: ferrodolerite, apatite-rich ferrodiorite and microferrodiorite. Compositionally, the last, most evolved unit approaches continental tholeiitic andesites. The marginal border group and the central series are separated by a zone of coarse-grained dolerites. Minor rock types associated with the dyke include: patches and inclusions of intermediate and, to a smaller extent, felsic rocks (quartz syenite porphyry), patches of monzodolerite, internal veins of felsic rocks (mainly of quartz- albite composition), and small granophyre dykes in the outer contact zone of the dyke.The marginal border group and the central series appear to be related to a common parental magma through fractional crystallization, at an early stage of evolution, of mainly olivine and plagioclase. A possible parent is a mantle- -derived high-Al basaltic magma. The rocks exhibit an overall potassic character, a feature most likely due to selective assimilation of crustal material during the ascent of the magma. Therefore, a model of assimilation-fractional crystallization is suggested for the evolution of the Hällefors magma.Olivine-clinopyroxene geothermometry and the nickel contents in olivine suggest reasonable magmatic temperatures (940-1025°C). Fe-Ti oxide thermometry yields a wide temperature range (955-385°C), considered to partly reflect the high-temperature oxidation of the Fe-Ti oxides and olivine during cooling from magmatic temperatures, and partly low-grade metamorphism or alteration. During these processes, the oxygen fugacity was relatively low (-logfC>2= 12-30), close to that of the FMO buffer.The origin of the minor rock types is unsettled except for granophyre; this last rock probably formed by remelting of the country-rock gneiss. The patches and inclusions of intermediate and partly felsic compositions may represent either fused xenoliths of gneiss or late-stage sodic residual differentiates from the crystallization of dolerite and ferrodiorite. The former alternative is favoured, although it does not explain the origin of quartz syenite porphyry or the distribution of some trace elements (Zr and Y) in these rocks. The felsic rocks of veins may represent felsic melts produced during the assimilation of gneiss xenoliths and the interaction of the basaltic magma with the country-rock gneiss; however, differentiation by fractional crystallization or by liquid immiscibility cannot be ruled out. The monzodolerite probably originated by differentiation of coarse-grained dolerite, strongly modified locally by contamination with crustal material.Features such as rapid cooling, chilling, the occurrence of gneiss xenoliths and local autobrecciation (in the microferrodiorite) suggest emplacement at shallow depths. Some non-dnational relations show that stoping has probably played a role at an early stage in the emplacement of the dyke.The intrusion of the Hällefors dyke and other E-W dolerites in south central Sweden may be the result of aborted rifting ca 1550 Ma ago. The E-W dyke swarm may be an aulacogene branch of a triple junction which was active during the late Precambrian in this part of Sweden.