Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania

The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzan ia was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and la ter the rocks suffered retrogression during isobaric cooling (IBC) at the a mphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670-610 degrees C and pressures of 6.5-6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite p orphyroblasts at the expense of sillimanite in graphitic paragneisses as we ll as by garnet zoning in the same rock. Further retrogression was due to h ydration along shear zones and faults. This led to the alteration of forste rite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X-H2O = 0.5 in calcsilicate rocks. These findings, combined with the textural and thermobarometric data from t he Uluguru Mountains and the Furua area, suggest that the IBC P-T path was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Bel t rocks in Tanzania. This was most probably caused by addition of igneous r ocks (underplating) at the initial stage of the formation of the granulite- facies rocks in the belt; followed by their residence and cooling in the mi ddle and upper crustal levels. The slight decompression during this stage c ould be explained by either extensional or transtensional deformation. Fina lly, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed. (C) 1999 Elsevier Science Limited. All rights reserved.