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Sveconorwegian influence on the ca. 1.36 Ga old Tjärnesjö granite, and associated pyroxene bearing quartz-monzonites in southwestern Sweden

Andersson, Jenny (1996) In Dissertations in Geology at Lund University
Department of Geology
Abstract
The granite massif in the Tjärnesjö area, southwestern Sweden, was deformed and metamorphosed under granulite facies conditions in late Sveconorwegian time. The intrusive is dominated by a coarse-grained, megacrystic orthoclase granite. Pb-Pb evaporation datings on single zircons from the granite yield an age of ca. 1.36 Ga, interpreted as the intrusion age. A similar age was obtained on zircons from quartz-monzonitic pyroxene bearing parts within the granite, here referred to as charnockitoids. Correlations with the ca. 1.38 Ga Torpa granite and its charnockitic associations in the Varberg region further to the west are supported by the intrusion-age of the Tjärnesjö granitoids, as well as the textural and chemical character of the two... (More)
The granite massif in the Tjärnesjö area, southwestern Sweden, was deformed and metamorphosed under granulite facies conditions in late Sveconorwegian time. The intrusive is dominated by a coarse-grained, megacrystic orthoclase granite. Pb-Pb evaporation datings on single zircons from the granite yield an age of ca. 1.36 Ga, interpreted as the intrusion age. A similar age was obtained on zircons from quartz-monzonitic pyroxene bearing parts within the granite, here referred to as charnockitoids. Correlations with the ca. 1.38 Ga Torpa granite and its charnockitic associations in the Varberg region further to the west are supported by the intrusion-age of the Tjärnesjö granitoids, as well as the textural and chemical character of the two intrusives.
Rocks within the Tjärnesjö massif vary from almost massive, to intensely deformed, sometimes even mylonitic rocks. Within large parts of the intrusive, the orthoclase megacrysts are recrystallized and transformed into mixed crystalline aggregates with components of intermediate microcline, where the proportion of microcline component is increased by the degree of deformation. However, in other deformed parts of the intrusive, the orthoclase is stable and the K-feldspar megacrysts may appear as fractured, surrounded by fine grained recrystallized domains of quartz, plagioclase and myrmekite. The different appearance of the K-feldspar megacrysts associated with deformation suggests an early event of deformation where orthoclase was stable, at high T-conditions, followed by a later deformational phase in which orthoclase was transformed into a mixed crystalline state with components of intermediate microcline. Furthermore, rocks with stable orthoclase appear homogeneously foliated whereas rocks that display a distinct structural change within the K-feldspar megaerysts mainly occur as discrete deformation zones.
Zircons from an undeformed pegmatitic dyke, intersecting the gneissosity in a strongly deformed part of the granite, yield an age of ca. 955 Ma by Pb-Pb evaporation. This age is the minimum age of the regional deformation that affected the Tjärnesjö massif. The maximum age is set by the ca. 1.36 Ga intrusion age of the granite massif.
The granulite facies mineral assemblages present in the charnockitoids define a high grade foliation at deformed sites. This high-grade event is interpreted to be late Sveconorwegian. (Less)
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author
Andersson, Jenny
supervisor
organization
year
type
H1 - Master's Degree (One Year)
subject
keywords
Sveconorwegian influence, ca 1.36 Ga, geology, geography, Tjärnesjö granite, pyroxene, quartz-monzonites, southwestern Sweden
publication/series
Dissertations in Geology at Lund University
report number
67
language
English
id
2369918
date added to LUP
2012-10-23 10:38:50
date last changed
2012-10-23 12:00:14
@misc{2369918,
  abstract     = {The granite massif in the Tjärnesjö area, southwestern Sweden, was deformed and metamorphosed under granulite facies conditions in late Sveconorwegian time. The intrusive is dominated by a coarse-grained, megacrystic orthoclase granite. Pb-Pb evaporation datings on single zircons from the granite yield an age of ca. 1.36 Ga, interpreted as the intrusion age. A similar age was obtained on zircons from quartz-monzonitic pyroxene bearing parts within the granite, here referred to as charnockitoids. Correlations with the ca. 1.38 Ga Torpa granite and its charnockitic associations in the Varberg region further to the west are supported by the intrusion-age of the Tjärnesjö granitoids, as well as the textural and chemical character of the two intrusives.
Rocks within the Tjärnesjö massif vary from almost massive, to intensely deformed, sometimes even mylonitic rocks. Within large parts of the intrusive, the orthoclase megacrysts are recrystallized and transformed into mixed crystalline aggregates with components of intermediate microcline, where the proportion of microcline component is increased by the degree of deformation. However, in other deformed parts of the intrusive, the orthoclase is stable and the K-feldspar megacrysts may appear as fractured, surrounded by fine grained recrystallized domains of quartz, plagioclase and myrmekite. The different appearance of the K-feldspar megacrysts associated with deformation suggests an early event of deformation where orthoclase was stable, at high T-conditions, followed by a later deformational phase in which orthoclase was transformed into a mixed crystalline state with components of intermediate microcline. Furthermore, rocks with stable orthoclase appear homogeneously foliated whereas rocks that display a distinct structural change within the K-feldspar megaerysts mainly occur as discrete deformation zones.
Zircons from an undeformed pegmatitic dyke, intersecting the gneissosity in a strongly deformed part of the granite, yield an age of ca. 955 Ma by Pb-Pb evaporation. This age is the minimum age of the regional deformation that affected the Tjärnesjö massif. The maximum age is set by the ca. 1.36 Ga intrusion age of the granite massif.
The granulite facies mineral assemblages present in the charnockitoids define a high grade foliation at deformed sites. This high-grade event is interpreted to be late Sveconorwegian.},
  author       = {Andersson, Jenny},
  keyword      = {Sveconorwegian influence,ca 1.36 Ga,geology,geography,Tjärnesjö granite,pyroxene,quartz-monzonites,southwestern Sweden},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Sveconorwegian influence on the ca. 1.36 Ga old Tjärnesjö granite, and associated pyroxene bearing quartz-monzonites in southwestern Sweden},
  year         = {1996},
}