Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land, Antarctica: Monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Gregor Markl*, Sandra Piazolo

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    60 Citations (Scopus)

    Abstract

    Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20°C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with XNaCl in the range 0.15-0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650°C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800°C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650°C) assemblages. Biotite in all but two syenite samples crystallized at log(fH2O/fHF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(fH2O/fHF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(fH2O/fHCl) derived from biotite covers the range 0.5-2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510-2790 bars for H2O, 1.3-5.3 bars for HF and 7-600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(fH2O/fHF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(fH2O/fHF) increases significantly. Chlorine preferably partitions into the fluid and hence log(fH2O/fHCl) does not change markedly during fluid-rock interaction.

    Original languageEnglish
    Pages (from-to)246-268
    Number of pages23
    JournalContributions to Mineralogy and Petrology
    Volume132
    Issue number3
    Publication statusPublished - 1998

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