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
Heavy minerals were separated using routine procedures which involved steel jaw-crusher and
steel roller-mill, Wilfley table, Frantz magnetic separator and heavy liquids. Mineral fractions
were then hand-picked under a binocular micro- scope to avoid inclusions and obtain grain or
fragment fractions of similar size, shape and
colour. Titanite was cleaned in pure alcohol in an ultrasonic bath for about 15 min, washed in warm
distilled 3 N HCl for about 10 min to remove surface contamination, and twice rinsed in dis-
tilled water. Monazite was washed in warm dis- tilled water only prior to dissolution. Rutile was
washed in warm 0.5 N HF for about half an hour, zircon in hot 6 N HCL for about 15 min.
Uranium and Pb concentrations were deter- mined by isotope dissolution with a
233
U
205
Pb mixed spike, added before dissolution to allow
optimum homogenisation with the sample. Ele- ment concentrations in weighed mineral fractions
are known to about 0.2, calculated from analy- tical errors alone. All zircon-, monazite- and ru-
tile-fractions were digested in 3 ml Savillex
®
screw-top beakers in a Krogh-style or Parr
®
Teflon
®
bomb within a screw top steel container at 210°C. Monazite dissolved in 0.5 ml 7 N HNO
3
and 0.5 ml 6.2 N HCl after 1 – 3 days. Rutile dissolved within a few days in a mixture of 0.5 ml
concentrated HF and five drops of 7 N HNO
3
. Titanite fractions were digested overnight in the
oven in a mixture of 0.5 ml concentrated HF and ten drops of 7 N HNO
3
after boiling for 12 – 24 h on the hot-plate. The zircon fraction dissolved in
concentrated HF and ten drops of 7 N HNO
3
in the oven within 10 days. Dissolution was checked
optically for each sample, under a microscope where necessary.
Uranium and Pb were separated with ion-ex- change Teflon
®
columns filled with about 0.5 ml of DOWEX AG 1X8
®
anion exchange resin e.g. Krogh, 1973; Tilton, 1973. Pb chemistry for mon-
azite, rutile, titanite, and feldspar employed the HBr – HCl method, whereas Pb from zircon was
separated with HCl. Uranium was separated with the HCl – HNO
3
method. Five total procedural blanks were determined between 44 and 123 pg
with an average of 80 pg. The Pb-isotope ratios measured for the blank were:
206
Pb
204
Pb: 18.53;
207
Pb
204
Pb: 15.69;
208
Pb
204
Pb: 35.90. Isotope ratios were measured on a Finnigan
MAT 261 mass-spectrometer in multi-collector static mode on Faraday cups, using single Re
filaments. A secondary electron multiplier SEM was used for measuring
204
Pb when high ratios made it necessary, and for some U analyses in
dynamic mode. Pb was loaded with H
3
PO
4
and silica-gel Cameron et al., 1969. The measured Pb
isotopic ratios were corrected for fractionation with a mass discrimination factor of 0. 1amu,
based on 23 analyses of 50 ng of equal atom SRM-982, measured during this study in compari-
son with the values recommended by Todt et al. 1996. Reproducibility of the
207
Pb
206
Pb ratio of the SRM-982 standard average: 0.466512 was
0.033. Within-run reproducibility was much higher, with an average of 0.0021 at 2s confi-
dence level. The measurements of
206
Pb
204
Pb ra- tios with
204
Pb on the SEM were corrected with a factor of 1.0038, determined from five measure-
ments of SRM-982. Most U was measured as oxide after loading with H
3
PO
4
and silica-gel. Based on repeated analyses of 100 ng SRM-U500
standard, a mass fractionation correction factor of 0.01amu was applied to samples measured in
static mode and a correction factor of 0.3amu to SEM dynamic measurements. Reproducibility
for the
235
U
238
U ratio of the standard static mode was 0.29, with an average within-run
reproducibility of better than 0.04. For some samples, U was loaded with graphite
dispersed in a wateralcohol-solution and mea- sured as U
+
at temperatures between 1650 and 1740°C. Reproducibility estimated from seven
U500 standards loaded with graphite was 0.28 for Faraday cup in static mode. Fractionation was
corrected with a factor of 0. 1amu. Mass frac- tionation was strongly time-dependent with these
graphite loaded samples and care was taken to heat up all samples in the same manner and avoid
acquisition times longer than approximately five blocks of 20 measurements each.
4. Closure temperature estimates
