Carbonic fluid inclusions in South Indian granulites: evidence for entrapment during charnockite formation |
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| Authors: | M Santosh D H Jackson N B W Harris D P Mattey |
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| Institution: | (1) Department of Earth Sciences, The Open University, Walton Hall, MK7 6AA Milton Keynes, UK;(2) Department of Geology, Royal Holloway and Bedford New College, Egham Hill, TW20 0EX Egham, Surrey, UK;(3) Present address: Centre for Earth Science Studies, Akkulam, Post Box No. 7250, 695 031 Trivandrum, India |
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| Abstract: | Field evidence and fluid inclusion studies on South Indian incipient charnockites suggest that charnockite formation occurred
during a decompressional brittle regime following the ‘peak’ of metamorphism and regional deformation. The most abundant type
of inclusions in quartz and garnet grains in these charnockites contain high-density carbonic fluids, although lower-density
fluids occur in younger arrays of inclusions. Discrete fluid inclusion generations optically are observed to decrepitate over
well-defined temperature intervals, and quantitative measurements of CO2 abundance released from these inclusions by stepped thermal decrepitation show up to a four-fold increase (by volume) in
the incipient charnockites relative to the adjacent gneisses from which they are derived. Studies based on optical thermometry,
visual decrepitation and stepped-heating inclusion release together indicate that entrapment of carbonic fluids coincided
with charnockite formation. We confirm that an influx of carbon dioxide-rich fluids is associated with the amphibolite-granulite
transition, as recorded by the incipient charnockites, the remnants of which are commonly preserved as the earliest generation
of high-density fluid inclusions. |
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