Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer
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 t...
| Published in: | Precambrian Research |
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| Main Authors: | , , |
| Format: | Article in Journal/Newspaper |
| Language: | unknown |
| Published: |
Elsevier
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| Subjects: | |
| Online Access: | http://hdl.handle.net/1885/89358 https://doi.org/10.1016/S0301-9268(01)00166-8 https://openresearch-repository.anu.edu.au/bitstream/1885/89358/5/MigratedxPub19353_RSD_2001.pdf.jpg https://openresearch-repository.anu.edu.au/bitstream/1885/89358/7/01_Rutland_Age_of_deformation_episodes_in_2001.pdf.jpg |
| Summary: | 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 (D1), prior to the deposition of the Skellefte stratigraphic sequence during an extensional tectonic episode. A younger episode of deformation, D2, 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 D2 shear zones allows the dating of D2 at ≈ 1860 Ma. Other planar granitoid bodies, which cut the D1 structures, confirm the age of this event. In the context of earlier published data, we conclude that the correlation of D2 in the Robertsfors Group with the main folding in the Skellefte Group is valid. Another granitoid body, folded by D2 but emplaced after D1, 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 D1. However, we infer, using other evidence, that D1 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 ... |
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