From genesis to juxtaposition: the evolution of the Ivisârtoq greenstone belt, southwest Greenland

Mader, Marianne M. (2005) From genesis to juxtaposition: the evolution of the Ivisârtoq greenstone belt, southwest Greenland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Ivisârtoq greenstone belt is situated ~40 km south of the Isua greenstone belt within the Godthåbsfjord region of southwest Greenland. This region is part of the Archean gneiss complex of Greenland, a major component of the 3800-2500 Ma North Atlantic Craton. Compared to the Isua greenstone belt, which has been the focus of intense study because it contains some of the oldest known (~ 3.8-3.7 Ga) rocks on Earth, little work has been carried out on the Ivisârtoq greenstone belt. The Ivisârtoq greenstone belt is exceptionally well-exposed in three dimensions with relief of 1200m, and primary features are better preserved here than in any other Archean greenstone belt in Greenland. Consequently, this belt provides an exceptionally good opportunity to characterise early to middle Archean magmatism, deformational processes, and tectonic environments. -- The Ivisârtoq greenstone belt was multiply deformed and metamorphosed at amphibolite facies. The belt forms an upright southwest-closing, V-shaped synform. This study concentrated on the 3 km thick southern limb of the synform, where the most complete section of the supracrustal rocks is preserved, and where there are pillow lava structures with way-up indicators. The southern limb was found to comprise a tectonostratigraphy made up of two main components, termed the northern and southern parts. These composite rock units differ in composition and intensity of deformation and are separated by a major high strain zone. -- The northern part of the belt is generally less deformed than the southern part and is characterized by heterogeneously deformed amphibolite, derived from variolitic and homogeneous basaltic pillow lavas, interlayered with boudins of olivine-bearing ultramafic rocks and quartz-feldspar-mica gneiss and schist. The southern part consists predominantly of strongly deformed rocks including banded amphibolite, homogeneous granitoid, gabbroic rocks, and a thick layer of predominantly quartz-feldspar-biotite- schist. -- New major and trace element geochemical data, LAM ICP-MS U-Pb zircon geochronology, and field mapping have shed new light on the origin and evolution of the Ivisârtoq greenstone belt. This new work indicates that the northern and southern parts of the belt represent discrete packages of rocks that formed in different environments and were juxtaposed after -2895 Ma, after the main volcanism associated with each part had ceased. Each package of rocks suffered separate deformational events before juxtaposition: two discrete deformational phases were recognized in the less deformed northern part, whereas the southern part was much more intensely deformed and individual events could not be differentiated. -- Crustal contamination signatures of the amphibolites in the northern part of the belt suggest formation in a back-arc basin environment, in which the basaltic magma ascended through thin, patchy or immature crust. A U-Pb zircon age of an intrusive tonalite sheet demonstrates that the northern basaltic pillow lavas were deposited before -3165 Ma. -- In contrast, the southern part of the belt appears to have formed on a thick continental crust. This interpretation is based on several lines of evidence: crustal contamination signatures of the banded amphibolite, the presence of a thick sequence of quartzofeldspathic metasedimentary rocks, an A-type granitoid, and a horizon of metamorphosed meymechite, which is indicative of deep melting of subcontinental mantle. A maximum age of-2895 Ma was determined for the deposition of the southern metasedimentary rocks. -- The interpreted juxtaposition of the northern and southern packages of the Ivisârtoq belt is consistent with lateral collisions of terranes perhaps by plate tectonic processes.

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