Geochronology of gneisses adjacent to the Mylonite Zone in southwestern Sweden: evidence of a tectonic window?
(2020) In Dissertation in geology GEOR02 20172Department of Geology
- Abstract
- The relationship between the Eastern Segment and Idefjorden terrane prior to the Sveconorwegian orogeny has long been debated. Evidence of a common history is minimal and localised; at the same time, the notion that the Sveconorwegian orogeny formed as a result of a collisional event is also disputed. In this thesis, I analyse a region of the Idefjorden terrane, colloquially known as ‘B-Bulan’, close to the boundary with the Eastern Segment, which shows evidence of Eastern Segment-age (1.81-1.65 Ga) signatures and possibly Hallandian-age (1.47-1.38 Ga) signatures previously unseen in the Idefjorden terrane. Ages were determined using U-Pb spot dating of zircon using Laser Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS). The... (More)
- The relationship between the Eastern Segment and Idefjorden terrane prior to the Sveconorwegian orogeny has long been debated. Evidence of a common history is minimal and localised; at the same time, the notion that the Sveconorwegian orogeny formed as a result of a collisional event is also disputed. In this thesis, I analyse a region of the Idefjorden terrane, colloquially known as ‘B-Bulan’, close to the boundary with the Eastern Segment, which shows evidence of Eastern Segment-age (1.81-1.65 Ga) signatures and possibly Hallandian-age (1.47-1.38 Ga) signatures previously unseen in the Idefjorden terrane. Ages were determined using U-Pb spot dating of zircon using Laser Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS). The 207Pb/206Pb ages of samples analysed shows the presence of Idefjorden and Eastern Segment signatures, as well as a potential Hallandian signature in a small number of samples. Idefjorden signatures are recorded in three samples at three locations (4f-i, 4g-i and 3m-ii). They show weighted mean 207Pb/206Pb ages of 1625 ± 5 Ma (4f-i), 1603 ± 4 Ma (4gi) and 1641 ± 11 Ma (3m-ii). Eastern Segment protolith ages are recorded in five samples at two locations (2.5j and 3a). At location 3a, samples show weighted mean 207Pb/206Pb ages of 1693 ± 12 Ma (3a-v), 1692 ± 3 Ma (3a-iii) and 1705 ± 3 Ma (3a-i). At location 2.5j, zircon populations of Eastern Segment age are observed between 1850 and 1650 Ma (2.5j-i) and around 1685 Ma (2.5j-iv). Both samples show multiple zircon populations (including Hallandian signatures) and complex field relationships which require closer examination. This study shows the presence of Eastern Segment-aged units and Hallandian signatures within B-Bulan, further west than previously acknowledged. (Less)
- Popular Abstract
- The origin of a mountain-building event known as the Sveconorwegian orogeny, which took place in southwestern Sweden between around 1050 and 970 million years ago, is hotly debated. The region is defined by two rock suites, the Eastern Segment in the east, and the Idefjorden terrane in the west, which are separated by a zone of highly deformed rocks, the Mylonite Zone.
Rocks are like diaries, preserving a record of the major events that happened over the course of their lifetimes, and if you were to compare the diary entries of the rocks in Borås with those in Gothenburg, you would find minimal evidence of a common history. For example, the Hallandian orogeny, which occurred around 400 million years before the Sveconorwegian orogeny, is... (More) - The origin of a mountain-building event known as the Sveconorwegian orogeny, which took place in southwestern Sweden between around 1050 and 970 million years ago, is hotly debated. The region is defined by two rock suites, the Eastern Segment in the east, and the Idefjorden terrane in the west, which are separated by a zone of highly deformed rocks, the Mylonite Zone.
Rocks are like diaries, preserving a record of the major events that happened over the course of their lifetimes, and if you were to compare the diary entries of the rocks in Borås with those in Gothenburg, you would find minimal evidence of a common history. For example, the Hallandian orogeny, which occurred around 400 million years before the Sveconorwegian orogeny, is present in the diaries of Eastern Segment rocks, but is completely absent from those of the Idefjorden terrane. If these two suites shared a common history prior to the Sveonorwegian orogeny, you would expect such a significant event to be preserved in the Idefjorden terrane, wouldn’t you?
But to complicate matters, geologists at the Swedish Geological Survey recorded evidence of localised Hallandian-age diary entries to the west of the Mylonite Zone in an isolated area between Alingsås and Skepplanda known colloquially as ‘B-Bulan’. This finding aroused great interest, but without the surrounding context the result was open to interpretation. Could it be that the Hallandian mountain-building event of around 1450 to 1380 million years ago was recorded in the diary entries of rocks in the Eastern Segment and the Idefjorden terrane? If so, the obvious conclusion would be that they shared a common history, that they were lying next to each other almost 400 million years before the Sveconorwegian orogeny. Alternatively, could this area in fact be part of the Eastern Segment? In which case, our previous ideas about the location of the Mylonite Zone need to be modified.
In this thesis, I aimed to determine which of these two possibilities was more likely. I started by analysing the same rock type that had produced the Hallandian-age diary entry, on the eastern boundary of the B-Bulan. I traced this Brobacka-type rock into the B-Bulan to see how far it extended, and to find the rocks that lie above and below it. I was trying to determine whether this Brobacka-type was associated with rocks from the Eastern Segment or the Idefjorden terrane. I collected samples from different areas within the B-Bulan and chose a selection to analyse using uranium-lead isotope dating, a method commonly used in geochronology. If I could determine the age of my samples, I could better understand the relationship between the Brobacka-type rocks and the surrounding rocks.
The results, unfortunately, were less conclusive than expected, largely because of calibration errors in the measuring equipment. Nevertheless, my thesis shows the presence of Hallandian-age rocks further west than previously recorded, which suggests that the rocks in the B-Bulan region share a common history with those rocks in the Eastern Segment. This is an important finding, but it was expected. I also found Eastern Segment rocks within the B-Bulan – rocks that had not been recorded west of the Mylonite Zone before – which may suggest that the B-Bulan is a part of the Eastern Segment, and perhaps even that the location of the Mylonite Zone should be reconsidered to account for this. Finally, the relationship between Hallandian-age signatures and the Idefjorden terrane – the bombshell that I was hoping for – proved inconclusive. At one location, I found potential evidence of Hallandian signatures along with Idefjorden terrane and Eastern Segment signatures, but I was unable to determine whether the results were representative, or whether they might have represented a mixing of different ages. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9025299
- author
- Davies, James LU
- supervisor
- organization
- course
- GEOR02 20172
- year
- 2020
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- Geochronology, Hallandian orogeny, Mylonite Zone, Sveconorwegian orogeny, U-Pb, Idefjorden terrane, Eastern Segment, B-Bulan, Brobacka
- publication/series
- Dissertation in geology
- report number
- 584
- funder
- Lunds Geologiska Fältklubb
- language
- English
- id
- 9025299
- date added to LUP
- 2020-07-22 20:35:34
- date last changed
- 2020-07-22 20:35:34
@misc{9025299,
abstract = {{The relationship between the Eastern Segment and Idefjorden terrane prior to the Sveconorwegian orogeny has long been debated. Evidence of a common history is minimal and localised; at the same time, the notion that the Sveconorwegian orogeny formed as a result of a collisional event is also disputed. In this thesis, I analyse a region of the Idefjorden terrane, colloquially known as ‘B-Bulan’, close to the boundary with the Eastern Segment, which shows evidence of Eastern Segment-age (1.81-1.65 Ga) signatures and possibly Hallandian-age (1.47-1.38 Ga) signatures previously unseen in the Idefjorden terrane. Ages were determined using U-Pb spot dating of zircon using Laser Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS). The 207Pb/206Pb ages of samples analysed shows the presence of Idefjorden and Eastern Segment signatures, as well as a potential Hallandian signature in a small number of samples. Idefjorden signatures are recorded in three samples at three locations (4f-i, 4g-i and 3m-ii). They show weighted mean 207Pb/206Pb ages of 1625 ± 5 Ma (4f-i), 1603 ± 4 Ma (4gi) and 1641 ± 11 Ma (3m-ii). Eastern Segment protolith ages are recorded in five samples at two locations (2.5j and 3a). At location 3a, samples show weighted mean 207Pb/206Pb ages of 1693 ± 12 Ma (3a-v), 1692 ± 3 Ma (3a-iii) and 1705 ± 3 Ma (3a-i). At location 2.5j, zircon populations of Eastern Segment age are observed between 1850 and 1650 Ma (2.5j-i) and around 1685 Ma (2.5j-iv). Both samples show multiple zircon populations (including Hallandian signatures) and complex field relationships which require closer examination. This study shows the presence of Eastern Segment-aged units and Hallandian signatures within B-Bulan, further west than previously acknowledged.}},
author = {{Davies, James}},
language = {{eng}},
note = {{Student Paper}},
series = {{Dissertation in geology}},
title = {{Geochronology of gneisses adjacent to the Mylonite Zone in southwestern Sweden: evidence of a tectonic window?}},
year = {{2020}},
}