Mid-crustal Metasomatic Reaction Veins ina Spinel Peridotite

In Central Dronning Maud Land, East Antarctica, rare metre-sizedlenses of spinel peridotite are enclosed in high-grade metamorphicrocks. The rocks experienced a medium-P granulite-facies metamorphismat 575 Ma and a low-P amphibolite-facies overprint at 530 Ma.The latter is probably related to extensive granitoid magmatismbetween 530 and 500 Ma, which produced large volumes (abouthalf of the outcrops today) of granitic to syenitic rocks aswell as abundant K-feldspar–quartz pegmatites. One ofthe spinel peridotite lenses in the Schirmacher Oasis of CentralDronning Maud Land is crosscut by several small (up to 10 cmwide) veins with a characteristic zoned sequence of mineralassemblages, which was formed by reaction of a hydrous, SiO₂-saturatedfluid or pegmatitic melt with the peridotite. The zoned sequenceconsists of the following mineral assemblages (from the centreof 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 investigatedwith respect to its conditions of formation, modal mineralogy,mineral chemistry, fluid inclusions, and oxygen and hydrogenisotope compositions of selected minerals. Based on the stabilityof cummingtonite and on equilibrium calculations in the MgO–SiO₂–H₂Osystem and on quartz–biotite oxygen isotope thermometry,the reaction vein formed at 650°C, which is in accord withtypical pegmatite crystallization temperatures. The pegmatiteof zone 0 is interpreted to have formed in an open fissure whereas,on textural grounds, zone 3 replaces former peridotite. On thebasis of mass balance constraints, the boundary between zones1 and 2 is interpreted to approximately represent the formerboundary between peridotite and the open fissure before reaction.Oxygen isotope systematics show that the infiltrating fluidhad an isotopic composition of 9–10 SMOW. All mineralsof the reaction vein with the exception of the inherited spineland olivine in the adjacent peridotite are in equilibrium withsuch a fluid. Spinel in the peridotite is depleted in ¹⁸O comparedwith coexisting olivine, which suggests isotopic disequilibrium.Spinel in zone 3 has a distinctly different isotopic compositioncompared with that in the peridotite, apparently approachingbut not reaching equilibrium. The combination of mineral chemistryand mass balance constraints of the modal mineralogy constrainsthe volume change during metasomatism and the direction of elementaldiffusion. It is indicated that Mg, Cr and Ni always diffusedtowards the vein, whereas Si, Al, K, Na, H₂O and possibly Fediffused into the peridotite. KEY WORDS: peridotite; metasomatism; pegmatite; diffusion; reaction