Palaeoproterozoic mafic intrusions along the Avesta-Osthammar belt, east-central Sweden:mineralogy, geochemistry, and magmatic evolution

摘要

An E-W-trending belt of mafic intrusions, called the Avesta-Osthammar belt in this article, extends across east-central Sweden for approximately 130 km, surrounded by 1.90-1.86 thousand million year old early orogenic Svecofennian granitoids. In terms of chemical composition, these intrusions range from gabbroic to dioritic, with 44—60 wt.% SiO_2. Their Mg# mostly range from 77 to 49, with a few scattering all the way down to 13. These rocks are variably deformed and metamorphosed, and could be termed as amphibolites from a mineralogical perspective, being dominated by amphibole and plagioclase. Although remains of pyroxene are found in a few more primitive samples, more evolved rocks contain relatively abundant quartz and biotite, as well as some ilmenite or magnetite. Garnet has been recorded in one sample. In terms of isotopic composition, they are variably enriched in radiogenic Sr, with initial (87)~Sr/~(86)Sr ratios of 0.7021-0.7034 (εSr —2 to +16), and mildly depleted when it comes to radiogenic Nd, with initial (εNd )~of —0.6 to +2.2. Similar to many other mafic intrusive rocks in Sweden, their magma thus seems to be derived from a depleted mantle source enriched by fluids or melts from subducting oceanic crust. They most likely originated in a subduction-related volcanic arc in close relation to the surrounding granitoids. Although they are chemically more evolved than the ultrabasic to basic cumulate gabbros in the nearby southern Roslagen area, they appear to be derived from a slightly more primitive high-magnesia basalt magma. A model is presented where these gabbros and diorites are formed by differentiation through fractional crystallization of plagioclase, olivine, and clinopyroxene from such a parental magma.