Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature
granulites of Rogaland (SW Norway)
Andreas Möller
Patrick J. O’Brien
Allen Kennedy
Alfred Kröner
10.6084/m9.figshare.3454187.v1
https://geolsoc.figshare.com/articles/dataset/Linking_growth_episodes_of_zircon_and_metamorphic_textures_to_zircon_chemistry_an_example_from_the_ultrahigh-temperature___granulites_of_Rogaland_SW_Norway_/3454187
<p><em>In-situ</em> U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence
imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel
the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland
anorthosite-norite intrusive complex; and (b) to constrain the age of M<sub>2</sub> ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M<sub>3</sub> event. All samples yield magmatic inherited zircon of <em>c.</em> 1035 Ma, some an additional group at <em>c.</em> 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme
crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing)
migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results
for the M<sub>1</sub> metamorphic event at <em>c.</em> 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence
for zircon growth during the M<sub>2</sub> metamorphic episode.
</p> <p>Zircon intergrown with or included within M<sub>2</sub> metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2<em>σ, n</em> = 20). The youngest observed results are found in zircon outside M<sub>2</sub> minerals, some overgrown by M<sub>3</sub> mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger
pooled age of 908 ± 9 Ma (2<em>σ, n</em> = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the
UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last
magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the
regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element
behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and
between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use
of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth
or recrystallization during metamorphism.
</p>
2016-06-21 11:57:34
zircon chemistry
growth episodes
M 2
trace element analyses
M 2 minerals
UHT
M 3 mineral assemblages
trace element behaviour
SW
zircon growth
M 3 event
1000 Ma BP
Geology