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| Titel |
Mineral replacement and element mobility during scapolitisation and albitisation of metagabbro from the Bamble sector, SE-Norway |
| VerfasserIn |
A. K. Engvik, A. Korneliussen, P. Ihlen, H. Austrheim |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065087
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| Zusammenfassung |
| Widespread metasomatism affected the 100 km long and 25 km wide Proterozoic
Bamble and Modum-Kongsberg Sectors, South Norway, resulting in the chemical and
mineralogical transformation of wide segments of continental crust. Scapolitisation was
associated with veining, and was followed by albitisation, transforming metagabbros
pervasively over large areas. The transformation of gabbro to scapolite metagabbro is
observed as a fluid front replacing the primary magmatic mineral assemblage in three
stages: During an incipient amphibolitisation stage the primary mafic minerals
were replaced by anthophyllite or hastingsite, followed by pargasitic and edenitic
Ca-amphibole. Magnetite was dissolved, while rutile formed by the breakdown
of ilmenite. Plagioclase was replaced by Cl-rich scapolite (Me19-42) reflecting
Cl-saturation while K- and Mg-saturation produced phlogopite, enstatite, sapphirine
and rare corundum. The high modal contents of chlorapatite and tourmaline in the
scapolite-metagabbro imply infiltration of B and P. The albitites consist dominantly
of albite (Ab95-98) with varying, generally small, amounts of chlorite, calcite,
rutile, epidote and pumpellyite. Constant volume isocon plots were constructed
for sample sets of increasingly altered samples crossing fluid fronts. The results
imply that metagabbro undergoing scapolitisation is strongly depleted in Fe2O3,
progressively depleted in Al2O3 and undergo an increase in MgO and Na2O and a
smaller increase in SiO2. Albititsation shows the same extensive depletion in Fe2O3,
in addition to depletion in TiO2, MgO and CaO, while Na2O, Al2O3 and SiO2
increase.
The above described mineral replacement reactions and chemical evolution document
element mobilization. Fluids played an active role in these reactions, forming H2O-, CO2-
and Cl-bearing phases at the expense of the primary volatile-free minerals. To summarize,
scapolitisation caused infiltration of K, Mg, Na, B and P, and albitisation infiltration of Na.
Characteristically, the metasomatism shows a strong depletion in Fe which led to higher Mg#
of the Fe-Mg-bearing phases. We regard the metasomatism as an important mineral- and
rock-forming process, and speculate about its role in the formation of the Fe-ores and
Mg-Al-rich lithologies such as orthoamphibole-cordierite schist and sillimanite-nodular
gneisses in the Bamble and Modum-Kongsberg Sectors. |
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