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

摘要

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 (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 ℃. 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. CO_(2-) and CO_2-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 CO_2 and N_2 in the eclogite-facies fluid. The presence of some water in the original fluid is deduced from relic H_2O-NaCl±Ca/MgCl_2 inclusions associated with primary CO_2 inclusions in the metapelite, suggesting the coexistence of immiscible aqueous and carbonic fluids. In metabasite, relic H_2O-N_2±NaCl fluid inclusions occur side by side with the dominant CO_2±N_2 inclusions. Ca/Mg-carbonate daughter minerals, enclosed or attached to primary CO_2, CO_2-N_2 and H_2O-N_2 fluid inclusions, point to the presence of dissolved Ca and Mg components in the original H_2O-CO_2±N_2 fluid. The primary carbonic fluid inclusions, which re-equilibrated under graulite-facies conditions, and secondary fluid inclusions of similar density and composition trapped at this stage, are preserved, because latter cooling and decompression followed a near-isochoric P-T path.