Abstract
This PhD-thesis investigates the geological formation environment of c. 3200-3000 million-year-old volcanic rocks from SW Greenland, using whole-rock geochemical data in combination with U-Pb, Sm-Nd and Lu-Hf isotope data.
The following three supracrustal areas were studied: (1) The Tartoq Group comprises a rock assemblage, which is interpreted as a section thought arc-related oceanic crust. A new age of 3189 ± 65 Ma is presented from a Lu-Hf errorchron and therefore this mafic sequence may represent one of the oldest ophiolites on Earth (2) The Ikkattup Nunaa supracrustal association displays geochemical mixing trends between juvenile felsic and mafic end-members. This suggests that partial melting and magma mixing by mafic underplating and slab melt metasomatism of the mantle source region were important in forming rocks of andesitic composition, which is also observed in some modern island arcs. (3) The Nunatak 1390 area contains ultramafic rocks with geochemical characteristics similar to high-Ti komatiites, but they show geochemical signs of a juvenile slab melt contribution. Their high MgO contents may thus reflect flux-melting of a refractory mantle source, as is the case for boninites, rather than high degrees of partial melting at elevated temperatures, as commonly proposed for such rocks.
In conclusion, the main finding of this PhD-thesis is that all of the three studied areas have geochemical- and field-characteristics, which can be related to a subduction zone environment. This confirms previous research from SW Greenland and suggests that modern-style tectonic processes have been in operation for at least 3200 million years.
The following three supracrustal areas were studied: (1) The Tartoq Group comprises a rock assemblage, which is interpreted as a section thought arc-related oceanic crust. A new age of 3189 ± 65 Ma is presented from a Lu-Hf errorchron and therefore this mafic sequence may represent one of the oldest ophiolites on Earth (2) The Ikkattup Nunaa supracrustal association displays geochemical mixing trends between juvenile felsic and mafic end-members. This suggests that partial melting and magma mixing by mafic underplating and slab melt metasomatism of the mantle source region were important in forming rocks of andesitic composition, which is also observed in some modern island arcs. (3) The Nunatak 1390 area contains ultramafic rocks with geochemical characteristics similar to high-Ti komatiites, but they show geochemical signs of a juvenile slab melt contribution. Their high MgO contents may thus reflect flux-melting of a refractory mantle source, as is the case for boninites, rather than high degrees of partial melting at elevated temperatures, as commonly proposed for such rocks.
In conclusion, the main finding of this PhD-thesis is that all of the three studied areas have geochemical- and field-characteristics, which can be related to a subduction zone environment. This confirms previous research from SW Greenland and suggests that modern-style tectonic processes have been in operation for at least 3200 million years.
| Originalsprog | Engelsk |
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| Forlag | Department of Geography and Geology, University of Copenhagen |
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| Antal sider | 240 |
| Status | Udgivet - 2012 |