Granulite-facies Xenoliths in Rift Basalts of Northern Tanzania: age, Composition and Origin of Archean Lower Crust

Abstract

Granulite xenoliths entrained within Quaternary rift basalts in northern Tanzania document the composition, equilibration conditions, age and petrogenesis of the present-day lower crust beneath the eastern margin of the Tanzanian Craton (Labait) and the adjacent Mozambique Belt (Lashaine and Naibor Soito). Mafic to intermediate Archean lithologies dominate throughout the suites (∼2·66 Ga based on U–Pb zircon ages), demonstrating that deep-seated Archean lithosphere extends far to the east of the margin of the Tanzanian Craton within the Mozambique Belt. There is no evidence for significant additions to the crust via magmatic underplating since that time. Many of the lower crustal xenoliths share compositional similarities with lavas from the greenstone belts of the Tanzanian Craton, suggesting that they crystallized from similar magmas. Extreme depletions of highly incompatible elements (e.g. Cs, Rb, Th and U) in the granulites, relative to the lavas, coupled with unradiogenic 143Nd/144Nd (0·5114–0·5122) and 87Sr/86Sr (0·7040–0·7051), suggest that these depletions occurred coincident with or shortly after the rocks crystallized, possibly through partial melting associated with metamorphism. These samples may thus represent high-grade lower crustal complements to the greenstone belt lavas. Compared with the craton-margin samples, granulite xenoliths from within the Mozambique Belt record very high peak equilibration pressures at moderate temperatures (>1·2 to >1·7 GPa, 750–960°C, based on pseudosections), documenting their equilibration deep within thickened continental crust during the East African Orogeny, c. 560 Myr ago. These samples therefore offer an unusual window into the deepest reaches of the crust in a continental orogen. Despite the fact that the Mozambique Belt experienced significant crustal thickening followed by post-orogenic collapse, there is no evidence for loss of the deep lithosphere associated with these processes.