Geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the Gardar Province, South Greenland: implications for crystallisation processes in anorthositic magmas |
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| Authors: | Ralf Halama Tod Waight Gregor Markl |
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| Institution: | Institut für Geowissenschaften, Eberhard-Karls-Universit?t Tübingen, Wilhelmstr. 56, 72074 Tübingen, Germany,
Danish Lithosphere Centre, ?ster Voldgade 10 L, 1350 Copenhagen, Denmark,
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| Abstract: | Chemical and Sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the Gardar Province can be used to elucidate magma-chamber and emplacement processes. The megacrysts occur either as single crystals or assembled as anorthosite xenoliths. The size of the megacrysts varies from <1 cm to 1 m. They consist of a large core with variable zonation (An58-39) and a relatively small (<600 µm), normally zoned rim (An62-27). The contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol% An. This gap is interpreted as having formed during dyke emplacement due to a sudden pressure release. Some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy-oscillatory zoning which has not been reported elsewhere to our knowledge. The oscillatory zoning has wavelengths of up to 2,500 µm and a maximum amplitude of 7 mol% An. It is interpreted as reflecting movements of the crystals in the magma reservoir. The Sr isotopic composition of one crystal shows a radiogenic inner core ((87Sr/86Sr)i=0.7044) and a less radiogenic outer core ((87Sr/86Sr)i=0.7039-0.7036). The lack of a significant change between outer core and rim ((87Sr/86Sr)i=0.7037) is consistent with formation of the more An-rich rim due to pressure release. Variations in the core may be related to movements of the crystal and/or magma mixing. A trace-element profile across a megacryst shows a small increase in Sr and small decreases in Ba and La contents of the recalculated melt composition across the core-rim boundary, whereas P, Ce, Nd and Eu remain constant. Melt compositional changes upon emplacement are therefore considered to be of minor importance. Constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace-element budgets. |
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