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

• Main Authors: Markl, Gregor, Piazolo, Sandra
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1998
• Subjects: Antarc*; Antarctica; Dronning Maud Land; East Antarctica
• Online Access: https://researchers.mq.edu.au/en/publications/17cc82dc-bd34-4c79-976c-8c2bd7fc8895; http://www.scopus.com/inward/record.url?scp=0001513478&partnerID=8YFLogxK

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 X NaCl 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(f H2O /f HF ) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f H2O /f HF ) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f H2O /f HCl ) 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 H 2 O, 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(f H2O /f HF )] derived from the late-magmatic stages of the syenites with both ...