Magmatic versus metamorphic origin of olivine-plagioclase coronas |
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| Authors: | Geert-Jan L de Haas Timo G Nijland Peter J Valbracht Cees Maijer Rob Verschure Tom Andersen |
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| Institution: | 1.Laboratorium for isotopgeologi, Mineralogisk-Geologisk Museum, Sars'gate 1, 0562 Oslo, Norway,;2.Rooseveltlaan 964, 3526 BP Utrecht, The Netherlands,;3.Institute of Earth Sciences, Free University, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands,;4.Department of Geochemistry, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands, |
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| Abstract: | An SEM, REE, and mineral Sm-Nd isotope study on olivine-plagioclase coronas in the Vestre Dale gabbro, Norway, was carried out in order to solve the temporal relationships within these coronas. It is demonstrated that in contrast to common models, corona formation is a multistage, late-magmatic process, starting with the formation of orthopyroxene by (partial) dissolution of olivine, followed by the nucleation and rapid outward growth of orthopyroxene+spinel symplectites at its outer margin, and concluded by replacement of this precursory outer shell by calcic amphibole. Isotopic equilibrium between inner shell orthopyroxene and the magmatic assemblage, and the REE contents of orthopyroxene and outer shell amphibole also provide strong arguments for a magmatic origin, with the local availability of fractionated magma acting as a limiting factor. The absence of replacement of intercumulus clinopyroxene by amphibole also favours a late stage magmatic rather than a metamorphic origin for corona amphibole. |
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