Fluid-rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes

Taetz, Stephan, John, Timm, Bröcker, Michael, and Spandler, Carl (2016) Fluid-rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes. Contributions to Mineralogy and Petrology, 171 (90). pp. 1-27.

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Abstract

A complex high-pressure/low-temperature vein system that cross-cuts eclogitic host rocks of the Pouébo Eclogite Melange (northern New Caledonia) records the prograde blueschist-to-eclogite transition and associated formation of garnet–quartz–phengite veins. Geothermobarometry (Grt–Cpx–Ph, Zr-in-rutile) and pseudosection calculations indicate peak metamorphic conditions of ca. 540 °C and 1.9–2.2 GPa. Petrological and geochemical observations as well as pseudosection modelling suggest that the main vein network is formed by dehydration processes that collected internally derived fluids related to the breakdown of hydrous phases (amphibole, chlorite, epidote) during prograde metamorphism. The lower solid volume of the newly formed phases and the associated increase in pore fluid pressure lead to the formation of veins that allowed for accumulation and channelized evacuation of these fluids. Such veins do not show metasomatic alteration selvages because the fluid–rock system had been in chemical equilibrium. A second vein type (transport veins) records the superimposed influx of external fluids with slightly different composition that most likely are related to similar dehydration reactions in other parts of the subducting slab. Due to the source-rock-imposed compositional differences, these fluids are not in equilibrium with the infiltrated rock volume and induce the formation of distinct metasomatic selvages by dissolution–precipitation processes. Mass-balance calculations show that Ca, Na and Li are added to the selvage by the external fluid. LILE and to a lesser extend also HREE are mobilized and removed from the selvage. The LREE are predominantly buffered by newly formed minerals (e.g. epidote). Petrological evidence implies that the studied vein system formed while the sample was still part of a coherent subducting slab. Rb–Sr geochronology indicates that this occurred at 38.2 ± 0.3 Ma. This age is ca. 6 myr younger than the hitherto presumed peak metamorphic age, suggesting a more complex and protracted metamorphic history for the PEM.

Item ID: 47127
Item Type: Article (Research - C1)
ISSN: 0010-7999
Keywords: eclogite, slab dehydration, fluid-rock interaction, metasomatism, Rb-Sr geochronology, mass-balance calculations, New Caledonia
Additional Information:

The online version of this article (doi:10.1007/s00410-016-1295-z) contains supplementary material, which is available to authorized users.

Funders: Deutsche Forschungsgemeinschaft (DFG)
Projects and Grants: DFG grant JO 349/5-1
Date Deposited: 04 Jan 2017 07:55
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
04 EARTH SCIENCES > 0402 Geochemistry > 040202 Inorganic Geochemistry @ 50%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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