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Petrografisk studie av pseudotachylitisk breccia från nedslagskratern Siljan

Bäckehag, Kristoffer LU (2025) In Examensarbeten i geologi vid Lunds universitet GEOL02 20251
Department of Geology
Abstract (Swedish)
När en kosmisk kollision mellan jorden och en annan himlakropp sker, då bildas det nya bergarter. En av dessa bergarter är den omdebatterade pseudotachylitiska breccian (PTB). Bergarten PTB består av en snabbt avkyld smälta, med inneslutna rundade fragment från omgivande litologi. PTB har kristalliserats i djupa sprickor bildade vid nedslag, de varierar i storlek från mikrometer till hundratals meter i mäktighet. Nedslagskratern Siljan, som är i fokus för denna studie är en peak-ring crater och har en relativt stor förekomst av PTB vid den inre kraterplatån. I denna studie har PTB i 11 stycken tunnslip från borrkärnan Hättberg undersökts med polarisationsmikroskop, SEM och EBSD. Debatten rörande PTB är hur och när den har bildats och... (More)
När en kosmisk kollision mellan jorden och en annan himlakropp sker, då bildas det nya bergarter. En av dessa bergarter är den omdebatterade pseudotachylitiska breccian (PTB). Bergarten PTB består av en snabbt avkyld smälta, med inneslutna rundade fragment från omgivande litologi. PTB har kristalliserats i djupa sprickor bildade vid nedslag, de varierar i storlek från mikrometer till hundratals meter i mäktighet. Nedslagskratern Siljan, som är i fokus för denna studie är en peak-ring crater och har en relativt stor förekomst av PTB vid den inre kraterplatån. I denna studie har PTB i 11 stycken tunnslip från borrkärnan Hättberg undersökts med polarisationsmikroskop, SEM och EBSD. Debatten rörande PTB är hur och när den har bildats och kristalliserats, skedde det vid nedslagsögonblicket (A-typ) eller senare när kratern kollapsade (B-typ). Denna studie försöker klassificera PTB som en pseudotachylit typ- A. Bevis för detta är den första dokumenterade och analyserade förekomsten av granulär zirkon, innesluten i nedslagskratern Siljans PTB. Men ytterligare geologiska observationer indikerar att PTB kan vara pseudotachylit typ- B och bildats när kratern kollapsade. Även strukturgeologiska observationer av s-foliation och shear-sense indicators, visar att PTB har deponerats med en kombination av injektion och skjuvning. Förekomsten av granulär zirkon skulle kunna ge en möjlighet till en ännu säkrare datering av nedslagskratern Siljan med U-Pb datering. Den granulära zirkonen och klassifikationen av PTB kan få djupgående inverkan på fortsatta studier av nedslagskratern Siljan. (Less)
Abstract
When a cosmic collision occurs between earth and another celestial body, new rock types are formed. One of these rock types is the controversial pseudotachylitic breccia PTB. The PTB consists of a rapidly cooled melt, with enveloped rounded fragments from the surrounding lithologi. The PTB has crystallized in deep cracks formed during impact, varying in size from micrometers to hundreds of meters in thickness. The impact crater Siljan, which is the focus of this study, is a peak-ring crater and has a relatively large occurrence of PTB at the inner crater plateau. In this study, PTB in 11 thin sections from the Hättberg drill core have been examined with polarization microscopy, SEM and EBSD. The debate regarding PTB is how and when it was... (More)
When a cosmic collision occurs between earth and another celestial body, new rock types are formed. One of these rock types is the controversial pseudotachylitic breccia PTB. The PTB consists of a rapidly cooled melt, with enveloped rounded fragments from the surrounding lithologi. The PTB has crystallized in deep cracks formed during impact, varying in size from micrometers to hundreds of meters in thickness. The impact crater Siljan, which is the focus of this study, is a peak-ring crater and has a relatively large occurrence of PTB at the inner crater plateau. In this study, PTB in 11 thin sections from the Hättberg drill core have been examined with polarization microscopy, SEM and EBSD. The debate regarding PTB is how and when it was formed and crystallized, was it at the moment of impact (A-Type) or later when the crater collapsed (B-type). This study attempts to classify PTB as a pseudotachylite type- A. Evidence for this is the first documented and analyzed occurrence of granular zircon, contained in PTB from the impact crater Siljan. However, several geological observations indicate that PTB may be pseudotachylite type-B and formed during the crater collapse. Structural geological observations of s-foliation and shear-sense indicators also show that PTB was deposited by a combination of injection and shear. The occurrence of granular zircon could provide an opportunity for an even more reliable dating of the impact crater Siljan, with the U-Pb dating. The granular zircon and the classification of PTB may have a deep impact on further studies of the impact crater Siljan. (Less)
Please use this url to cite or link to this publication:
author
Bäckehag, Kristoffer LU
supervisor
organization
course
GEOL02 20251
year
type
M2 - Bachelor Degree
subject
keywords
Siljan, nedslagskrater, PTB, pseudotachylit, granulär zirkon.
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
706
language
Swedish
id
9200176
date added to LUP
2025-06-16 13:10:54
date last changed
2025-06-16 13:10:54
@misc{9200176,
  abstract     = {{When a cosmic collision occurs between earth and another celestial body, new rock types are formed. One of these rock types is the controversial pseudotachylitic breccia PTB. The PTB consists of a rapidly cooled melt, with enveloped rounded fragments from the surrounding lithologi. The PTB has crystallized in deep cracks formed during impact, varying in size from micrometers to hundreds of meters in thickness. The impact crater Siljan, which is the focus of this study, is a peak-ring crater and has a relatively large occurrence of PTB at the inner crater plateau. In this study, PTB in 11 thin sections from the Hättberg drill core have been examined with polarization microscopy, SEM and EBSD. The debate regarding PTB is how and when it was formed and crystallized, was it at the moment of impact (A-Type) or later when the crater collapsed (B-type). This study attempts to classify PTB as a pseudotachylite type- A. Evidence for this is the first documented and analyzed occurrence of granular zircon, contained in PTB from the impact crater Siljan. However, several geological observations indicate that PTB may be pseudotachylite type-B and formed during the crater collapse. Structural geological observations of s-foliation and shear-sense indicators also show that PTB was deposited by a combination of injection and shear. The occurrence of granular zircon could provide an opportunity for an even more reliable dating of the impact crater Siljan, with the U-Pb dating. The granular zircon and the classification of PTB may have a deep impact on further studies of the impact crater Siljan.}},
  author       = {{Bäckehag, Kristoffer}},
  language     = {{swe}},
  note         = {{Student Paper}},
  series       = {{Examensarbeten i geologi vid Lunds universitet}},
  title        = {{Petrografisk studie av pseudotachylitisk breccia från nedslagskratern Siljan}},
  year         = {{2025}},
}