plex orogenic belt such as the Mozambique Belt MB of East Africa, the direct coupling of
geochronologic data with petrologic information Appel et al., 1998 and crustal residence ages
Mo¨ller et al., 1998 is of paramount importance. In this study, both the prograde and retrograde
thermal histories are reconstructed using U – Pb ages obtained on metamorphic minerals with dif-
ferent closure temperatures including monazite, titanite and rutile. Because most of the minerals
sampled in this study were extracted from gran- ulite facies metasediments they can be considered
to be most likely of metamorphic origin. This study also compares published U – Pb zircon ages
and K – Ar, Ar – Ar, Rb – Sr on hornblende, biotite and muscovite data from different granulite ter-
ranes in Tanzania for their consistency with new U – Pb ages. The scarcity of age data for the
Pan-African orogen of East Africa has led some authors to use ages determined on different gran-
ulite complexes for an integrated interpretation of the whole orogenic belt Maboko et al., 1985,
1989; Muhongo and Lenoir, 1994. However, it can be shown that it is important to know the age
of metamorphism for each area separately for P – T – t path construction, because rock units jux-
taposed today may have been at different crustal levels and experienced different P – T histories, but
the same tectonic and metamorphic processes. Samples from 17 locations metapelitic gneisses,
orthogneisses, marbles and calcsilicates within the MB were chosen to cover the different parts
of the respective granulite complexes in eastern Tanzania.
2. Geologic setting: granulite complexes in the Pan-African Belt of Tanzania
One of the most influential contributions that shaped the understanding of the African Precam-
brian geology was the definition of the Mozam- bique Belt by Holmes 1951. He recognised the
discontinuity of geological structural trends be- tween the Tanzania craton and its eastern hinter-
land and showed that these areas had to be younger than the craton. Subsequently Shackleton
1967 proposed that the MB has a complex his- tory and suggested that the belt is composed of
Archaean basement and several younger metasedi- mentary sequences. The MB then served as one of
the classical examples for rejuvenation i.e. no new crustal material added during orogenic cycle
of Archaean and Early Proterozoic basement Watson, 1976. However, it was also proposed
that the MB is a product of late Precambrian plate collision following ocean closure Burke et
al., 1977 McWilliams, 1981.
Stern 1994 proposed the term ‘East African orogen’ for the areas covered by the older terms
‘Arabian – Nubian shield’
and ‘Mozambique
Belt’’, because it is appropriate to view the whole area as the product of one Neoproterozoic
Fig. 1. Simplified geological map of eastern Tanzania, modified from Coolen 1980. Important granulite domains in
the Mozainbique Belt are indicated by shaded areas. Newly recognised granulite occurrences in the Mozambique Belt after
Appel et al. 1998. The western limit of Pan-African meta- morphic influence on the Proterozoic Usagaran Belt is indi-
cated by a dashed line after Gabert and Wendt 1974 and Priem et al. 1979.
Wilson-cycle see inset of Fig. 1. The Arabian – Nubian shield contains large tracts of Pan-African
juvenile crust and abundant ophiolites and is in- terpreted by Stern 1994 as a collage of accreted
terranes. In contrast, the MB with its high-grade gneisses resembles the deeply eroded root of an
orogen formed by a single collision event between East- and West-Gondwana. The MB experienced
further uplift during Phanerozoic rifting, some of it associated with the development of the East
African Rift. This interpretation supports the model of Hoffman 1991, i.e. the MB was formed
by fan-like closure of a previously existing Mozambique ocean, with the hinge of the fan
somewhere in South Africa. Since this fan never fully closed, crustal shortening was most intense
in the southern part of the belt. Stern 1994 argues further that the exposure of granulites at
the surface in Kenya and Tanzania is evidence that crustal thickness of the orogen was greatest
and collision most intense in these areas, because today the granulites are found within the crust of
normal thickness of approximately 35km e.g. KRISP Working Party, 1995.
Within Tanzania,
geochronological results
show that the Mozambique Belt of Holmes 1951 has to be subdivided into a Pan-African late
Proterozoic domain to the east and an Usagaran = Ubendian, Early Proterozoic domain to the
west Fig. 1. A tentative subdivision in southern Tanzania was based on progressively older Rb – Sr
biotite ages towards the west Wendt et al., 1972; Priem et al., 1979 interpreted as the result of the
decreasing Pan-African thermal overprint on the Early Proterozoic rocks Gabert and Wendt,
1974. U – Pb dating of metamorphic monazite and titanite from eclogite-facies rocks places the
main metamorphic event in the Usagaran domain at 2000 Ma Mo¨ller et al., 1995. Appel et al.
1998 suggest that distinctive decompression tex- tures in the Usagaran Belt and cooling textures in
the Pan-African granulites can be used to distin- guish the two belts.
To distinguish the two metamorphic events we endorse the use of the terms ‘Pan-African Belt of
East Africa’ or the ‘East-African Orogen’ pro- posed by Stern 1994 for the Pan-African gran-
ulite facies gneisses of eastern Tanzania and use the
name ‘Usagaran
Belt’ or
‘Ubendian – Usagaran Belt’ for the region where the main
metamorphic event occurred at about 2 Ga Fig. 1. The term Pan-African is used in this study for
the time span from about 650 to 550 Ma, relevant to and encompassing metamorphic events in the
circum-Indic region related to the formation of Gondwana.
The Pan-African Belt in Tanzania consists of Archaean to Proterozoic rocks e.g. Mo¨ller et al.,
1998 metamorphosed under granulite facies con- ditions e.g. Bagnall, 1963; Sampson and Wright,
1964; Coolen, 1980; Appel et al., 1998 during the Pan-African orogeny e.g. Coolen et al., 1982;
Maboko et al., 1985, this study. Some of the granulite complexes Fig. 1 apparently form fault
bounded mountain ranges, interpreted as tectonic klippen e.g. Shackleton, 1986, namely the Pare
and Usambara Mountains Bagnall, 1963; Bagnall et al., 1963 and the Uluguru Mountains Samp-
son and Wright, 1964.
Previous petrologic and geochronological stud- ies have been carried out mainly on the Furua
complex Coolen, 1980; Coolen et al., 1982, the Wami River complex Maboko et al., 1985, and
the Uluguru
Mountains Muhongo,
1990; Maboko et al., 1989. The granulite complexes
within the Mozambique Belt exhibit striking simi- larities in lithology, structure and grade of meta-
morphism Coolen, 1980; Appel et al., 1998. Petrologic studies reveal similar peak metamor-
phic conditions of 810 9 40°C and 9.5 to 11 kbar and a similar P – T path for an extensive area
within the Pan-African Belt including the Pare, Usambara and Uluguru Mountains granulite
complexes and some adjacent lowland areas Ap- pel et al., 1998.
3. Analytical methods
