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

Abstract

The fluids associated with eclogite-facies metabasites and metapelites of the Usagaran belt in Tanzania, which are relies of a former oceanic crust subducted 2 Ga ago (Moller 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 degreesC. 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-N-2-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 N-2 in the eclogite-facies fluid. The presence of some water in the original fluid is deduced from relic H2O-NaCI+/-Ca/MgCl2 inclusions associated with primary CO2 inclusions in the metapelite, suggesting the coexistence of immiscible aqueous and carbonic fluids. In metabasite, relic H2O-N-2+/-NaCl fluid inclusions occur side by side with the dominant CO2+/-N-2 inclusions. Ca/Mg-carbonate daughter minerals, enclosed or attached to primary CO2, CO2-N-2 and H2O-N-2 fluid inclusions, point to the presence of dissolved Ca and Mg components in the original H2O-CO2+/-N-2 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.