Retrograde fluid infiltration in the high-grade Modum Complex South Norway: evidence for age,source and REE mobility |
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| Authors: | Ingrid Anne Munz David Wayne Håkon Austrheim |
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| Institution: | (1) Mineralogisk-Geologisk Museum, Sarsgt. 1, N-0562 Oslo, Norway;(2) Department of Earth Sciences, University of Leeds, LS2 9JT Leeds, UK;(3) Present address: Institutt for energiteknikk, P.O. Box 40, N-2007 Kjeller, Norway;(4) Present address: Hydrogeology Program, MS 168, MacKay School of Mines, University of Nevada-Reno, 89557-0138 Reno, NV, USA |
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| Abstract: | The high-grade metamorphic basement of the Modum Complex, South Norway,exhibits retrogradation and alteration due to late stage fluid infiltration.Extensive alteration zones of albite-and calcite-rich veining occur especiallywithin and around numerous metagabbros. The gabbros, intruded at 1224±15Ma, are now partly altered to amphibolites due to the subsequent high-grade metamorphism.Two generations of albite-rich rocks have been recognized: (1) a fine-grained, foliated type;(2) a coarse-grained, crosscutting type. Both types show a typical greenschist facies mineralassemblage; albite ± actinolite ± chlorite ± talc. The calcite veins/dykesrepresent a younger generation of veins than both albite-rich types. U–Pb data for spheneof type (1) yielded an age of 1080±3 Ma, determining a point on the retrogradeP-T-t path of the Modum Complex. Increasing albitisation of themetagabbros leads to a decrease in(143ND/144Nd)oand an increase in (87Sr/86)o.Albite- and calcite-rich samples show negative  Nd and positive Sr, suggesting that fluids which interacted with the metagabbros originated from a crustal reservoir. The Nd and Sr isotopic data show disequilibrium at the microscale as well as at the macroscale. Negative Sm–Nd model ages of the albite-rich rocks demonstrate that rare-earth elements (REEs) were mobile and fractionated during albitisation. |
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