Mid-crustal metasomatic reaction veins in a spinel peridotite


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Mid-crustal metasomatic reaction veins in a spinel peridotite
Journal of Petrology
Markl G., Abart R., Vennemann T.W., Sommer H.
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In Central Dronning Maud Land, East Antarctica, rare metre-sized lenses
of spinel peridotite are enclosed in high-grade metamorphic rocks. The
rocks experienced a medium-P granulite-facies metamorphism at similar
to575 Ma and a low-P amphibolite-facies overprint at similar to530 Ma.
The latter is probably related to extensive granitoid magmatism between
530 and 500 Ma, which produced large volumes (about half of the outcrops
today) of granitic to syenitic rocks as well as abundant
K-feldspar-quartz pegmatites. One of the spinel peridotite lenses in the
Schirmacher Oasis of Central Dronning Maud Land is crosscut by several
small (up to 10 cm wide) veins with a characteristic zoned sequence of
mineral assemblages, which was formed by reaction of a hydrous,
SiO2-saturated fluid or pegmatitic melt with the peridotite. The zoned
sequence consists of the following mineral assemblages (from the centre
of the vein towards the outer margin): zone 0, plagioclase + quartz;
zone 1, green biotite intergrown with zircon + clinoamphibole; zone 2,
cummingtonite + dark brown biotite intergrown with rutile +
clinoamphibole; zone 3, cummingtonite + light brown biotite + spinel;
zone 4, olivine + orthopyroxene + spinel +/- clinopyroxene (unaltered
peridotite). This sequence was investigated with respect to its
conditions of formation, modal mineralogy, mineral chemistry, fluid
inclusions, and oxygen and hydrogen isotope compositions of selected
minerals. Based on the stability of cummingtonite and on equilibrium
calculations in the MgO-SiO2-H2O system and on quartz-biotite oxygen
isotope thermometry, the reaction vein formed at similar to650degreesC,
which is in accord with typical pegmatite crystallization temperatures.
The pegmatite of zone 0 is interpreted to have formed in an open fissure
whereas, on textural grounds, zone 3 replaces former peridotite. On the
basis of mass balance constraints, the boundary between zones 1 and 2 is
interpreted to approximately represent the former boundary between
peridotite and the open fissure before reaction. Oxygen isotope
systematics show that the infiltrating fluid had an isotopic composition
of 9-10parts per thousand SMOW. All minerals of the reaction vein with
the exception of the inherited spinel and olivine in the adjacent
peridotite are in equilibrium with such a fluid. Spinel in the
peridotite is depleted in O-18 compared with coexisting olivine, which
suggests isotopic disequilibrium. Spinel in zone 3 has a distinctly
different isotopic composition compared with that in the peridotite,
apparently approaching but not reaching equilibrium. The combination of
mineral chemistry and mass balance constraints of the modal mineralogy
constrains the volume change during metasomatism and the direction of
elemental diffusion. It is indicated that Mg, Cr and Ni always diffused
towards the vein, whereas Si, Al, K, Na, H2O and possibly Fe diffused
into the peridotite.
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Create date
29/09/2012 17:23
Last modification date
20/08/2019 14:34
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