Abstract
The Svecofennian orogeny in southern Finland has traditionally been divided into two broadly defined compressional stages, the 1.89–1.87 Ga “synorogenic” stage and the ca 1.84–1.81 Ga “lateorogenic” stage. The term “intraorogenic” is used to describe a less studied stage that occurred between these two, with some overlap with both.
Mafic and intermediate intrusions collectively named as the Kaiplot gabbros are situated on a number of islands and islets in Nagu (Nauvo) in the southwestern archipelago of Finland. The outcrops occur as dykes as well as plutonic bodies and have been emplaced in at least two separate pulses. Net-veining and other structures suggesting incomplete mixing between mafic and felsic magmas are found. The main plutonic body is a hornblende gabbronorite. U-Pb dating (TIMS, zircon) gives an age of 1865 ± 2 Ma for it.
Geochemically, the most primitive Kaiplot gabbros are tholeiitic and show affinity to back-arc basin basalts, indicating generation in an extensional tectonic environment. Their parental magmas appear to have originated from relatively high-degree partial melting of a shallow spinel-bearing and slightly subduction-modified depleted mantle. During transport and emplacement, both differentiation and assimilation of crustal material have taken place. The Kaiplot gabbros endorse an extensional tectonic episode of the Svecofennian orogeny at around 1865 Ma, possibly as a result of tectonic switching propagating southwestwards.
Mafic and intermediate intrusions collectively named as the Kaiplot gabbros are situated on a number of islands and islets in Nagu (Nauvo) in the southwestern archipelago of Finland. The outcrops occur as dykes as well as plutonic bodies and have been emplaced in at least two separate pulses. Net-veining and other structures suggesting incomplete mixing between mafic and felsic magmas are found. The main plutonic body is a hornblende gabbronorite. U-Pb dating (TIMS, zircon) gives an age of 1865 ± 2 Ma for it.
Geochemically, the most primitive Kaiplot gabbros are tholeiitic and show affinity to back-arc basin basalts, indicating generation in an extensional tectonic environment. Their parental magmas appear to have originated from relatively high-degree partial melting of a shallow spinel-bearing and slightly subduction-modified depleted mantle. During transport and emplacement, both differentiation and assimilation of crustal material have taken place. The Kaiplot gabbros endorse an extensional tectonic episode of the Svecofennian orogeny at around 1865 Ma, possibly as a result of tectonic switching propagating southwestwards.
| Original language | English |
|---|---|
| Pages (from-to) | 5-30 |
| Number of pages | 25 |
| Journal | Bulletin of the Geological Society of Finland |
| Volume | 97 |
| Issue number | 1 |
| DOIs | |
| Publication status | Published - 19 Jun 2025 |
| MoE publication type | A1 Journal article-refereed |
Funding
The first author thanks Professor Emeritus Carl Ehlers for introducing her to the subject and Tom Stålfors for valuable field work company. Laboratory engineer Sören Fröjdö assisted with the SEM analysis and data handling. The thin sections were prepared at the Geohouse in Turku by laboratory technician Arto Peltola. The U-Pb-dating was performed by Irmeli Mänttäri at GTK, Espoo. The authors are grateful for the critical and constructive comments by the reviewers J. Pownall and R. Lahtinen and for clear instructions from the editors J. Kohonen and A. Luttinen. This project was financially supported by the Åbo Akademi University Graduate School.
Keywords
- mafic magmatism
- Svecofennian orogeny
- tholeiitic
- geochemistry
- zircon U-Pb age
- Nagu
- SW Finland
- Nauvo
- extensional tectonics
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