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Fluids in a 2 Ga old subduction zone — deduced from eclogite-facies rocks of the Usagaran belt, Tanzania

Institut für Geowissenschaften, Universität Kiel, D-24098 Kiel, Germany

e-mail: ph{at}min.uni-kiel.de

The fluids associated with eclogite-facies metabasites and metapelites of the Usagaran belt in Tanzania, which are relics of a former oceanic crust subducted 2 Ga ago (Möller et al., 1995), were investigated using fluid inclusions. Geothermobarometry on both metabasites and metapelites indicates an eclogite-facies metamorphism at pressures of at least 15 kbar and temperatures of 750–800 °C. Near-isothermal uplift brought the rocks to lower crustal levels (7–10 kbar) before cooling and further uplift. Correlation of the P-V-T-X data for texturally primary fluid inclusions with the petrologically deduced P-T path indicates a re-equilibration of the fluid inclusions during isothermal decompression. CO₂- and CO₂-N₂-rich fluid inclusions dominate in quartz as well as in garnet, kyanite and pyroxene, and their occurrence in armoured quartz enclosed in garnet and kyanite indicates the presence of CO₂ and N₂ in the eclogite-facies fluid. The presence of some water in the original fluid is deduced from relic H₂O-NaCl±Ca/MgCl₂ inclusions associated with primary CO₂ inclusions in the metapelite, suggesting the coexistence of immiscible aqueous and carbonic fluids. In metabasite, relic H₂O-N₂±NaCl fluid inclusions occur side by side with the dominant CO₂±N₂ inclusions. Ca/Mg-carbonate daughter minerals, enclosed or attached to primary CO₂, CO₂-N₂ and H₂O-N₂ fluid inclusions, point to the presence of dissolved Ca and Mg components in the original H₂O-CO₂±N₂ fluid. The primary carbonic fluid inclusions, which re-equilibrated under granulite-facies conditions, and secondary fluid inclusions of similar density and composition trapped at this stage, are preserved, because later cooling and decompression followed a near-isochoric P-T path.

Key-words: fluid inclusions, 2 Ga old eclogites, metapelites, eclogite-facies fluid, Tanzania.

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