Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer

Abstract

It is proposed in a companion paper that the volcanic rocks of the Skellefte Group in northern Sweden were deposited on a pre-1.9 Ga basement, rather than formed as a juvenile volcanic arc and subsequently accreted to the continent. It is suggested that the proposed basement, the Robertsfors Group to the south of Skellefteå, was already deformed during an early episode of deformation (D 1), prior to the deposition of the Skellefte stratigraphic sequence during an extensional tectonic episode. A younger episode of deformation, D 2, then affected both areas. This hypothesis has important implications for the evolution of the Svecofennian Province. To test it, by constraining the ages of the principal deformation episodes, we have undertaken both isotope dilution and ion microprobe studies of zircon and monazite from three localities in the Robertsfors Group. A phase of migmatisation directly related to D 2 shear zones allows the dating of D 2 at ≈1860 Ma. Other planar granitoid bodies, which cut the D 1 structures, confirm the age of this event. In the context of earlier published data, we conclude that the correlation of D 2 in the Robertsfors Group with the main folding in the Skellefte Group is valid. Another granitoid body, folded by D 2 but emplaced after D 1, contains a suite of zircons of apparent magmatic aspect, but with a spread of ages from 1870 to 1900 Ma. Because of the high MSWD, the suite was divided into two groups with ages of ≈1896 and 1874 Ma. Recognising the possibilities of inheritance and of updating by younger events, we have preferred to interpret the younger age as a minimum age for magmatic intrusion. This is consistent with the basement hypothesis but is our only definite constraint on the age of D 1. However, we infer, using other evidence, that D 1 is probably older than 1900 Ma. Xenocrysts, interpreted as detrital zircons derived from the associated metasediments, give ages between 2716 and 1941±20 Ma. The latter age provides a maximum age of deposition for part of the Robertsfors Group and a maximum age for D 1. Other sequences, to the south and west, considered broadly equivalent to the Robertsfors Group, are intruded by granitoids dated between ≈1920 and 1960 Ma, providing local minimum ages of deposition for the sequences involved. Regional correlation with respect to age and deformation style can be made with the pre-1.9 Ga Kalevian rocks to the NE, deposited on Archaean basement on the SW margin of the Karelian Province. We suggest that pre-1.9 Ga complexes, equivalent to the Robertsfors Group, are widespread in the Svecofennian Province, and were deposited in a marginal basin which may have been initiated as early as 2.2 Ga ago. This hypothesis of an extensive basement crustal layer evidently requires that a substantial volume of crustal growth occurred prior to 1.9 Ga, in contrast to previous hypotheses. Erosion of the pre-1.9 Ga basin rocks after D 1 is also the likely source of the 2.1–1.9 Ga detrital zircons in younger sediments associated with the overlying post-1.9 Ga volcanics. The source of these detrital zircons has previously been problematical.