Evolution of metamorphic volatiles during exhumation of microdiamond-bearing granulites in the Western Gneiss Region, Norway |
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| Authors: | Rune B Larsen Elizabeth A Eide Ernst A J Burke |
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| Institution: | (1) Geological Survey of Norway, P.O.B. 3006, N-7002 Trondheim, Norway; E-mail: rune.larsen@ngu.no, NO;(2) Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, NL-1081 HV Amsterdam, The Netherlands, NL |
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| Abstract: | Fluid inclusions in garnet, kyanite and quartz from microdiamond-bearing granulites in the Western Gneiss Region, Norway,
document a conspicuous fluid evolution as the rocks were exhumed following Caledonian high- and ultrahigh-pressure (HP–UHP) metamorphism. The most important of the various fluid mixtures and daughter minerals in these rocks are: (N2 + CO2 + magnesian calcite), (N2 + CO2 + CH4 + graphite + magnesian calcite), (N2 + CH4), (N2 + CH4 + H2O), (CO2) and (H2O + NaCl + CaCl2 + nahcolite). Rutile also occurs in the N2 + CO2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions. Volatiles composed of N2 + CO2 + magnesian calcite characterise the ambient metamorphic environment between HP–UHP (peak) and early retrograde metamorphism. During progressive decompression, the mole fraction of N2 increased in the fluid mixtures; as amphibolite-facies conditions were reached, CH4 and later, H2O, appeared in the fluids, concomitant with the disappearance of CO2 and magnesian calcite. Graphite is ubiquitous in the host lithologies and fluid inclusions. Thermodynamic modelling of the
metamorphic volatiles in a graphite-buffered C-O-H system demonstrates that the observed metamorphic volatile evolution was
attainable only if the f
O2 increased from c. −3.5 (±0.3) to −0.8 (±0.3) log units relative to the FMQ oxygen buffer. External introduction of oxidising
aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the f
O2. The oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental
sediments previously subducted during an extended period of continental collision in conjunction with the Caledonian orogeny.
Received: 15 December 1997 / Accepted: 25 May 1998 |
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