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University of Kiel The Faculties Faculty of Mathematics and
Natural Sciences Organizations Institute of Geosciences Metamorphic and Magmatic Petrology
Scientific Profile People Laboratories Projects Teaching Field Trips Publications
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Formation and evolution of the continents
The continental crust is a product of differentiation of the Earth's mantle. During orogenic reworking, the crust itself is involved in differentiation processes, which are responsible for, e.g., its layered structure. By knowing the structure of the crust, it is possible to conclude on its genesis. Because we do not have direct access to the lower continental crust, we have to study crustal cross-sections, which crop out at the Earth's surface. Examples for such crustal profiles are:
- Calabria (southern Italy): Varistic crust, with 7 km thick lower crust
- Sri-Lanka: Pan-African lower crust, from 14 to 35 km depth
It is observed that the granulite-facies lower crust is dominated by basic rocks. The middle crust consists mainly of granitoids, which are overlain by low-grade metamorphic gneisses and schists of the upper crust. Our examinations are aimed to reconstruct the interaction between metamorphism and differentiation of the crust during orogenic reworking and to unravel its timing using geochronological methods.
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Folded gneisses of the continental crust
(Sandriver, South Africa).
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Pressure-temperature-time paths of metamorphism and orogenesis
Geodynamic processes during orogenesis cause changes in pressure and temperature conditions (P-T conditions), which affect the rocks of an orogen. The opposite way around, reconstructing the P-T evolution of a metamorphic rock allows to determine the geodynamic causes of metamorphism (e.g., subduction, tectonic thickening, extension during orogenic collapse, erosion, magmatic underplating).
The chemical composition of minerals depends on pressure and temperature. Therefore, a chemical zonation in a mineral records a change in pressure and temperature during the formation of this mineral. In addition, P-T changes cause reactions between minerals, which are identified by characteristic reaction textures. Combining the observations of mineral chemistry and reaction textures, it is possible to reconstruct the P-T changes during the formation of the rock.
Pressure-temperature evolution of an eclogite
from a precambrian subduction zone.
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Refolded folding with gamekeeper in the "Lower Zambezi National Park" (Zambia).
This reaction texture in a precambrian eclogite shows subduction and uplift from great depth (ca. 70 km; width of view is 3.5 mm).
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Geochronology of metamorphic processes
The geodynamic interpretation of pressure-temperature paths of metamorphic rocks is often difficult. To get a more complete picture, we need to know the age and the duration of the metamorphic event.
Generally, metamorphic minerals are examined with isotopic-geochronological methods. Adequate minerals for different isotopic systems are
- monazite, zircon, titanite, rutile (U-Pb)
- garnet (Sm-Nd)
- muscovite, biotite (Rb-Sr)
During temperature changes, these minerals show different behaviour with respect to recrystallisation or isotopic loss due to diffusion. Therefore, combination of different isotopic systems can give the ages of different metamorphic stages.
In addition, the amount of uranium, thorium and radiogenic lead in monazite can be used to date this mineral ("CHIME"). This method is an excellent choice to identify polyorogenic overprinting of precambrian crust.
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This monazite (backscattered electron image) shows an original magmatic growth pattern and metamorphic overprinting at the rim.
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Subduction zone metamorphism
By examining rocks from palaeo-subduction zones, tectonic and thermal processes during subduction can be identified. We also examine changes in partitioning of elements and fluids (fluid-rock interactions).
Our main focus is:
| Reconstruction of the P-T evolution of the subduction zone: |
We examine rocks from the Dora Maira Massiv (Western Alps). Here, continental crust has been buried to mantle depth and suffered ultrahigh pressure (UHP) metamorphism. |
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| Geodynamics in the Earth's history: |
With increasing age of the Earth, the frequency of subduction zone rocks decreases. Up to 1995 the eclogites of Glenelg (Scotland) were thought to be the oldest subduction-related rocks (1 Ga). In Tanzania we found 2 Ga old eclogites in an old orogen. They show that even in the early Proterozoic geodynamic processes were similar to the recent conditions. The rarity of old eclogites seems to be a problem of conservation. |
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| Palaeo-geographic reconstructions: |
Occurrences of subduction zone rocks define the sites of former suture zones. E.g., we found MORB-type eclogites in Zambia, which define a suture between the Archaean Kalahari and Tanzania Craton. |
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Coarse-grained eclogite from the italian Alps.
Schematic cross-section through a subduction zone.
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Fluids and fluid-rock interactions
- Fluids in subduction zones: Devolatilisation during prograde burial of crustal rocks is an important process, influencing many element cycles in the Earth. Released fluids cause melting within the mantle wedge and the associated volcanism. The study of palaeo-subduction zones is important to get information about mechanisms of fluid transport in the subducted lithosphere and about their effects on chemistry and mineralogy of the rocks. The results are important for an understanding of recent subduction zones.
- Fluids in the lower continental crust: The composition of fluid phases in rocks has an effect on the crustal rheology and melting processes. The internal differentiation of the crust (formation and ascension of granites) is largely influenced by fluids. For a better understanding of such processes, we determine the composition of the fluid inclusions using microthermometrical methods (H2O, CO2, N2, CH4) and we analyse the fluid contents in channels of the mineral cordierite. Thus we get information about presence and composition of fluids in the lower crust.
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Fluid inclusion in quartz (width of view 82 µm).
CO2-rich cordierite (blue), which has lost
CO2 along a crack and at the rims (yellow).
(width of view 3.5 mm).
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