Get access

Petrogenesis and geotectonic setting of early Svecofennian arc cumulates in the Roslagen area, east-central Sweden


Å. Johansson, Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden. E-mail:


Several intrusions of ultrabasic to basic composition occur in the Roslagen area of east-central Sweden in close spatial and temporal association with the surrounding 1.90–1.87 Ga old early orogenic Svecofennian granitoids. An imprecise Sm-Nd WR errorchron yields an age of 1895 ± 71 Ma. In spite of the penetrative deformation in the granitoids, the basic–ultrabasic rocks mostly appear undeformed and largely preserve magmatic textures with plagioclase, olivine (in some rock types), orthopyroxene and clinopyroxene, and amphibole as major constituents. The plagioclase is typically very anorthitic (ca. An90).

The Roslagen intrusions range in composition from primitive to evolved (Mg# 80 to 49) but contain only 40–50 wt% SiO2. Many samples are highly elevated in Al2O3 (up to 30 wt%), CaO (up to 16 wt%) and Sr (up to 800 ppm), with strongly positive Eu and Sr anomalies, in line with being plagioclase cumulates. Although masked by cumulus effects, the relative trace element contents indicate a volcanic arc signature. The initial Nd isotope composition is homogeneously ‘mildly depleted’, with εNd of +0.3 to +1.1, and the initial Sr isotope composition ‘mildly enriched’, with εSr of +8 to +15.

Non-cumulus rocks with small Eu and Sr anomalies can be used to deduce the composition of the parental magma. This LILE- and LREE-enriched and HFSE-depleted high-alumina basalt magma, with Mg# of ca. 50–60 and Ca# of ca. 80, most likely formed by partial melting of mantle material, enriched by fluids in a subduction environment, at 1.9 Ga. The cumulate rocks apparently crystallized from a somewhat more evolved water-rich magma with Mg# of ca. 40. Crystallization was followed by the development of late-magmatic to post-magmatic coronas between olivine and plagioclase in the presence of H2O-rich fluids. The subduction-related setting would make these intrusions Palaeoproterozoic counterparts of Alaskan-type ultramafic intrusions, but they differ from those in being plagioclase enriched, possibly reflecting different levels of exposure. Copyright © 2012 John Wiley & Sons, Ltd.