Résumé
The North Tanzanian rift is the southern part of the east branch of the East African Rift. It represents early stage rifting and is divided into 2 different volcanic and seismic activities: (1) in the north part, the Natron basin with shallow seismicity and intense volcanism (2-0.75 Ma) and (2) in the south part, the Manyara basin with deep crustal earthquakes and sparse volcanism (0.4 and 0.9 Ma). In this study, we used geochemical signature of magmas and deep fluids that percolate into the lithosphere beneath Manyara basin, to define the composition of magmas and fluids at depth beneath the south part of the North Tanzanian rift. The Manyara basin has distinct volcanic activities with mafic melilitite (Labait), Mg-nephelinites (Kwaraha) and evolved Mg-poor nephelinites (Hanang). Melilitites and Mg-nephelinites are olivine-rich primary magmas recording high-pressure crystallization environment at 4 GPa and 1 GPa, respectively and anhydrous conditions (0.1 and 0.4 wt% H (sub 2) O in primary melt). Geochemical modelling suggests that primary magmas result from a low degree of partial melting (< or =1%) of a peridotitic source with garnet and phlogopite. At crustal conditions, melilitite and Mg-nephelinite magmas evolved to Mg-poor nephelinite by fractional crystallization. The crystallization of cpx in Mg-poor nephelinite occurred at low pressure (340-640 MPa and 1075 degrees C) from silicate melt with low water content (<0.35 wt% H (sub 2) O), whereas melt inclusions entrapped in nepheline crystal indicate that interstitial CO (sub 2) -rich and H (sub 2) O-poor phonolitic melt with 6 wt% CO (sub 2) was present at 700-1000 MPa. At the early stage of rifting, volcanism in Manyara basin erupted CO (sub 2) -rich and H (sub 2) O-poor mafic magmas from at least 120 km below the rift escarpment, whereas few magmas evolved during ascent at mantle and crustal conditions. Manyara volcanism has similarities with the North Tanzania rift-axis (including Lengai) with a deep garnet-phlogopite-bearing source, CO (sub 2) -rich magmas (silicate lavas and carbonatite) but differs by deeper crystallisation environment, more oxidized conditions and very low water content in magmas. The small amount of volcanic rocks erupted in Manyara basin may indicate that trapping of CO (sub 2) -rich magmas/fluids at depth may have occurred and are potential trigger of deep crustal earthquakes.