THE DYNAMICS OF ARCHEAN LITHOSPHERE IN NORTHERN TANZANIA

Archean crust in the Mozambique belt in northern Tanzania has been deformed during two Proterozoic orogenies and is currently rifting. Since Archean cratons are stabilized by deep lithospheric keels, the massive re-working seen in the Mozambique belt suggests that thick, stable lithosphere either never developed or was removed. The objective of our work is to understand why the Mozambique belt is more easily reworked than cratonic lithosphere, such as the Tanzanian Craton immediately to the west.

Both outcrops of granulite-facies rocks and xenoliths entrained in rift volcanics are being studied in order to determine the composition and thermal evolution of the lower crust of the Mozambique belt. Petrography and mineral chemistry are used to constrain the P-T histories of the samples. Lower crustal xenoliths from Lashaine yield preliminary P-T estimates between 860 and 985 °C and 1.0 and 1.6 GPa. These conditions suggest a very high geotherm and hence the absence of a lithospheric root, but may not accurately reflect peak conditions due to disequilibrium.

Whole-rock Nd and in situ zircon Hf isotopic compositions will be used to evaluate the flux of mantle-derived melt into the lower crust during orogeny and rifting, which will, in turn, reflect the presence or absence of cratonic lithosphere. U-Pb geochronology performed on a variety of U-bearing accessory phases (zircon, monazite, titanite, apatite, and rutile) with different closure temperatures will be used to infer the thermal history of the lower crust of the Mozambique belt. Phases with low closure temperatures should record the onset of a cool geotherm associated with the presence of a deep lithospheric root, if one formed. The thermal histories of the samples may also be used to infer the sources of heat during orogeny and the effects of rifting on the lower crust.