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Journal of Petrology | Volume 44 | Number 6 | Pages 1097-1120 | 2003
© Oxford University Press 2003
Mid-crustal Metasomatic Reaction Veins ina Spinel Peridotite
1 INSTITUT FÜR MINERALOGIE, PETROLOGIE UND GEOCHEMIE, EBERHARD-KARLS-UNIVERSITÄt, WILHELMSTRASSE 56, D-72074 TÜBINGEN, GERMANY
2 MINERALOGISCH–PETROGRAPHISCHES INSTITUT, BERNOULLISTRASSE 30, CH-4056 BASEL, SWITZERLAND
E-mail: markl{at}uni-tuebingen.de
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 
575 Ma and a low-P amphibolite-facies overprint at 530 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 650°C, 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 9–10
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 18O 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.
KEY WORDS: peridotite; metasomatism; pegmatite; diffusion; reaction