obtained in the Pare and Usambara Mountains, a similar age difference as observed with the
monazite.
6. Discussion
6
.
1
. Discussion of pre6ious geochronological results
The geochronologic results from this study can be combined with published mineral ages and
P – T estimates to derive quantitative P – T – t paths for the different granulite segments of the
Mozambique Belt.
Previously published
geochronological data for the granulites of eastern Tanzania are summarised in Table 3. They are
recalculated using modern decay constants when necessary and re-interpreted using the closure
temperatures summarised in Table 1. All ages are combined to discuss and compare the cooling
histories of the different Pan-African granulite complexes in Tanzania.
Zircon ages are available from four of the granulite complexes Coolen et al., 1982; Maboko
et al., 1985; Muhongo and Lenoir, 1994. These ages span a period of 70 Ma between 645 Ma and
715 Ma and were originally interpreted as the time of high grade metamorphism. Upper discor-
dia intercepts at around 700 Ma Maboko et al. 1985, Fig. 9a are re-interpreted as intrusion
ages. However, the U – Pb results on the large magnetic and size fractions yield mostly short
discordias intersecting concordia at a low angle Maboko
et al.,
1985 and
are, therefore,
imprecise. Studies which allow direct comparison of Rb –
Sr with K – Ar data from the same terrane An- driessen et al., 1985; Priem et al., 1979 or direct
comparison of K – Ar and Ar – Ar data Maboko et al., 1989 reveal that many of the K – Ar and
some Ar – Ar ages of biotite and hornblende are too old and may be influenced by excess Ar see
Fig. 9b and c. Biotite Rb – Sr data and muscovite K – Ar and Ar – Ar data, however, are consistent
with other geochronological results.
For the Wami River granulites correlation of the re-interpreted results yields a slow integrated
Fig. 9. Cooling paths reconstructed for Pan-African granulite complexes in the Mozambique Belt of Tanzania with pub-
lished thermochronological data. a: 1, results of U-Pb on zircon and K – Ar on biotite for the Wami River granulite
complex Maboko et al., 1985. b: 2, Intrusion age of the anorthosite re-interpreted from U – Pb on zircon of Muhongo
and Lenoir 1994; 3, U – Pb on zircon and monazite this study; 4, Ar – Ar and K – Ar on hornblende, biotite, muscovite
and K-feldspar for the NW-Uluguru Mountains and sur- rounding migmatite gneisses Maboko et al., 1989; 5, Sm – Nd
gamet-whole rock isochrons Maboko and Nakamura, 1995. Tentative cooling path is indicated by dashed line. c: 6,
U – Pb on zircon Coolen et al., 1982; 7, K – Ar on hornblende Andriessen et al., 1985; 8, K – Ar on biotite and muscovite
and Rb – Sr on biotite and muscovite for the Furua granulite complex and surrounding migmatite gneisses Priem et al.,
1979. Error bars are given for the assumed uncertainties in the closure temperatures of the minerals 9 25 – 9 30°C.
Width of the symbols corresponds to approximately 5 Ma which is larger than the analytical error for most of the U – Pb
data. Light areas indicate the maximum range of the inte- grated cooling paths a and c or an unlikely fast alternative
cooling trajectory b.
A .
Mo ¨ller
et al
. Precambrian
Research
104 2000
123 –
146
Table 3 Summary of published geochronological data for Pan-African granulites in E-Tanzania
a
Area Reference
Rock type Method
Mineral Age Ma
Remarks, re-interpretation Muhongo and Lenoir
u.i., metamorphic 645 9 10
Zrn S-Pare Mountains
U–Pb Enderbite
1994 503 9 20
b
Cahen and Snelling Pegmatite
K–Ar Bt
1966, p.27 590 9 25
b
Excess Ar K–Ar
Granulitic gneiss Bt
585 9 20
b
Granulitic gneiss Excess Ar
Cahen and Snelling K–Ar
Hbl 20 km W of N-Pare
1966, p.27 Mountains
Cahen and Snelling Hbl–Scp-Bt gneiss
20 km N of Usambara K–Ar
Bt 488 9 20
Mountains 1966, p.27
u.i., intrusions Maboko et al. 1985
882–1600, 701–715 Zrn
U–Pb Ortho-granulites
Wami River
Ortho-granulites u.i., Pb-loss, metamorphic
Maboko et al. 1985 U-Pb
Zrn 476–538, 620 – 642
Ortho-granulites Five results
Maboko et al. 1985 Rb–Sr
Bt 458–485
Maboko et al. 1985 71 9 8
Ap Fission track
Ortho-granulites 2566 9 9, 47 9 9
l.i., l.i. Maboko et al. 1985
Para-gneiss
20 km N of Wami River
U–Pb Zrn
Muhongo and Lenoir 695 9 4
u.i., intrusion Uluguru Mountains
Zrn Anorthosite
U–Pb 1994
633 9 7, 618 9 16 Anorthosite,
Two results Maboko and Nakamura
Sm–Nd Grt-wr
1995 Ortho-granulite
536 9 3, 550 9 10, Three results
Muhongo 1990 Rb–Sr
Bt Ortho-granulites
577 9 30 K–Ar 702 9 14
Maboko et al. 1989 Ar–Ar
Hbl 628 9 3
Excess Ar Ar–Ar Maboko et al. 1989
K–Ar Bt
978 9 20, 1537 9 33 560 9 11, 529 9 10
–, –,580, –, no plateau 487 9 2, 495 9 2
K–Ar 488 9 10, 500 9 10 Maboko et al. 1989
Ar–Ar Ms
Maboko et al. 1989 K–Ar 434 9 9, 452 9 9
Ar–Ar 422 9 2, 450 9 2
Kfs Approx. ages at 115, 80 and
Ortho-granulite Fission track
Noble et al. 1997 Ap
300, 80, 30 20°C
Coolen et al. 1982 Furua complex
Grt-2Px granulite U–Pb
Zrn 652 9 10
l.i., metamorphic Priem et al. 1979
Bt Fourteen Results
Av. 463 388–511 Rb–Sr
Furua complex Ortho-granulites
Two results Priem et al. 1979
Rb–Sr Ms
524 9 10, 531 9 12 Surrounds
414–560 Fourteen results
Priem et al. 1979 K–Ar
Bt Priem et al. 1979
K–Ar Ms
482 9 14, 483 9 14, Three results
487 9 15 Andriessen et al. 1985
K–Ar Hbl
614–665 Six results
a
Note: biotite and hornblende K–Ar data of Andriessen et al. 1985 and Priem et al. 1979, and biotite Rb–Sr Maboko et al., 1985 are too numerous to list individually and only the age span of all analyses is given, refer to the original papers for details; u.i. is the upper intercept of U–Pb discordia, l.i. is the lower intercept.
b
Recalculated using the values recommended by Steiger and Ja¨ger 1977 for t2 of K.
cooling rate between 2 and 4.5°CMa Fig. 9a for granulites which have experienced similar P – T
conditions to the Pare, Usambara and Uluguru Mountains Appel et al., 1998. Maboko et al.
1989 used K – Ar as well as the
40
Ar –
39
Ar step- heating technique on hornblende, biotite, muscov-
ite and K-feldspar from samples collected at the NW edge of the Uluguru Mountains, close to the
northern edge of the anorthosite complex and to location T46 of this study Fig. 2. Interpretation
of the data of Maboko et al. 1989 is complicated by the fact that only one of the samples in their
study yielding the hornblende analysis and one of the biotite results is from the granulite complex
itself, whereas the other three samples biotite, muscovite and K-feldspar were collected from
felsic gneisses and migmatites surrounding the granulite complex. Growth of muscovite in these
rocks may be retrograde related to rehydration reactions in rocks which re-equilibrated to lower
T than the granulite complex itself. Therefore, the muscovite ages may have to be regarded as mini-
mum ages. Despite their higher closure tempera- ture, K – Ar muscovite ages are without exception
significantly younger than K – Ar biotite ages Maboko et al., 1989. High K – Ar and Ar – Ar
ages on hornblende and biotite from the granulite sample and biotite K – Ar and Ar – Ar results of
migmatite samples are interpreted to reflect excess Ar or were already interpreted in this way by
Maboko et al. 1989. The hornblende Ar – Ar age is identical to the zircon and monazite ages T46,
this study as well as to two garnet Sm – Nd isochrons ages for granulites from the area
Maboko and Nakamura, 1995. Because instan- taneous cooling by about 350°C down to 450°C
shaded area in Fig. 9b is considered very un- likely for these deep crustal ca. 10 kb granulites
which show no petrological evidence for decom- pression, this result is also interpreted to reflect
excess Ar. A tentative cooling path of about 3°CMa is indicated by a dashed line Fig. 9b.
The garnet Sm – Nd isochron age of 618 9 16 Ma on texturally late, undeformed garnet coronas in
the anorthosite Maboko and Nakamura, 1995 is important evidence in support of a single gran-
ulite facies event, because it indicates that garnet growth during retrograde isobaric cooling oc-
curred just after or contemporaneously with zir- con and monazite growth in meta-qtz-diorite T46.
In the Furua complex, the combination of a lower U – Pb intercept of zircon at 652 9 10 Ma
Archaean upper intercept; Coolen et al. 1982 and Rb – Sr biotite and muscovite ages Priem et
al., 1979 yields a prolonged history of slow inte- grated cooling at a rate of about 2.5°CMa for the
granulite-facies
rocks, and
the surrounding
migmatitic gneisses Fig. 9c. The results of K – Ar on hornblende and biotite Andriessen et al.,
1985; Priem et al., 1979 are again interpreted as unreliable possibly because of the presence of
excess Ar. The reconstruction of the cooling his- tory is thus based on Rb – Sr biotite and muscov-
ite ages and K – Ar results on muscovite.
The geochronological data available prior to this study and their interpretation did not provide
a conclusive picture of the precise age of Pan- African metamorphism in Tanzania, its spatial
distribution and the post-metamorphic cooling history. One of the problems appears to be the
ubiquitous presence of excess Ar in hornblende and biotite.
6
.
2
. Cooling histories of the granulites Interpretation of the published U – Pb data on
zircon for the Wami River complex, the Furua complex and data from the Pare Mountains
Muhongo and Lenoir, 1994, in combination with the new mineral data presented here, sug-
gests that the granulite-facies event reached its peak between 620 Ma and about 640 – 650 Ma.
The range of ages from published U – Pb studies on zircon is consistent with the results of this
study. The data allow the tentative reconstruction of cooling histories for the granulites of the Wami
River complex and the Furua Complex and possi- bly the NW Uluguru Mountains Fig. 9, indicat-
ing similar prolonged slow cooling with integrated cooling rates of 2 – 5°CMa over time intervals of
140 – 240 Ma.
Different age domains can be distinguished within the granulites of eastern Tanzania. The
monazite age of 640 Ma in the Pare Mountains is interpreted as reflecting the time of peak meta-
morphic conditions at about 800°C Appel et al.,
1998, consistent with a U – Pb zircon upper inter- cept age of 645 9 10 Ma on a granulite facies
gneiss from the Pare Mountains Muhongo and Lenoir, 1994. Monazite from the Usambara
Mountains yields ages of 625 – 630 Ma which are about 15 Ma younger than the time of peak
metamoiphism in the Pare Mountains. The mon- azite and titanite ages of 617 – 620 Ma from the
Umba Steppe semipelites and marbles are inte- grated to date the peak of metamorphism, esti-
mated at 750 – 800°C by Mo¨ller 1995 and significantly younger than the time of metamor-
phism determined for the Pare and the Usambara Mountains.
Within the northern and eastern Uluguru Mountains monazite ages span an age range of at
least 11 m.y., possibly 23 m.y. It is unclear whether the different ages reflect true differences
in the time of peak metamorphism or whether they can, in part, also be attributed to inheritance
or preservation of prograde growth, the latter being the preferred interpretation. The results
may be seen as the age envelope in which to place granulite facies metamorphism in this part of the
Uluguru Mountains. More obvious diachronism of Pan-African metamorphism is indicated by the
20 m.y. younger monazite and zircon ages on meta-qtz-diorite sample T46 in the NW Uluguru
Mountains than of monazite in the northern and eastern Uluguru Mountains Fig. 10b. The mini-
mum age difference is 20 m.y. Congruence of the monazite and zircon age of T46 where the zircon
fraction is interpreted as the product of crystalli- sation of partial melt during the earliest stages of
cooling from peak of metamorphism is taken as strong evidence for peak metamorphism at this
time and a high T
c
of monazite. The upper zircon
intercept of
695 9 4 Ma
from the
anorthosite Muhongo and Lenoir, 1994 may be interpreted as the age of crystallisation of the
anorthosite body Fig. 9b and Fig. 10b. It can be concluded that there appear to be differences in
the timing of high grade metamorphism between different granulite complexes in the Pan-African
Mozambique Belt of Tanzania on a relatively small scale of less than 100 km.
Using a T
c
of 650°C for titanite results in an initial cooling rate of about 5°CMa for the Pare
and the Uluguru Mountains. Diachronism in the thermal history within the granulites of north-
eastern Tanzania is supported by rutile and some biotite cooling ages which show the same pattern
of regional age distribution as the monazite in many areas Fig. 2 and Fig. 10. Rutile ages are
about 100 Ma younger than monazite ages, indi- cating an integrated cooling rate of about 4°C
Ma. This supports the interpretation that the age difference observed in the monazite results has a
geological cause associated with Pan-African metamorphism Fig. 10a and the subsequent un-
roofing history. The Umba Steppe and the Usam- bara Mountains underwent a cooling history
Fig. 10. Cooling paths reconstructed for the different granulite complexes studied in the Pan-African part of the Tanzanian
Mozambique Belt. a Pare and Usambara Mountains and Umba Steppe: 1, U – Pb age on zircon from Muhongo and
Lenoir 1994; 2, K – Ar ages of biotite from Cahen and Snelling 1966. b Uluguru Mountains; 3, Rb – Sr ages on
biotite from Muhongo 1990; 4, age of anorthosite re-inter- preted from U – Pb on zircon, Muhongo and Lenoir, 1994; 5,
Sm – Nd garnet-whole rock isochrons on anorthosite and or- thogneiss Maboko and Nakamura, 1995. Dashed path for
NW-Uluguru from Fig. 9.
Muhongo and Lenoir, 1994 and thus provide a reliable estimate for the time of the Pan-African
metamorphism in the Mozambique Belt of Tanza- nia. The observed cooling paths are consistent
with the anti-clockwise isobaric cooling ACW- IBC P – T path deduced from petrological obser-
vations and thermobarometry Appel et al., 1998.
The cooling histories of the granulite terranes, are parallel but offset Fig. 11. supporting the
notion that there are real age differences between mountain ranges Pare Mountains and Usambara
Mountains and Umba Steppe and within a single mountain range Uluguru Mountains. These dif-
ferences may be taken as evidence that these age domains are separated by important but hidden
faults, or may be explained by variations in the location of an external heat source. The progres-
sive slowing of cooling rates is consistent with the hypothesis that a granulite-facies event is at least
in part driven by an external heat source, e.g. the intrusion of magmas into the lower crust Anovitz
and Chase, 1990; Oxburgh, 1990 and slow uplift rates.
7
.
2
. Tectonic scenario for granulite metamorphism in eastern Tanzania
Previous geochronological results for the Tan- zanian granulites were interpreted to indicate fast
uplift following tectonic crustal thickening after plate-collision during the formation of Gondwana
e.g. Maboko et al., 1985, 1989; Muhongo and Lenoir, 1994; Maboko and Nakamura, 1995. A
collision process would cause rapid decompres- sion after or during the thermal peak of metamor-
phism, leaving behind decompression textures in the rock record. Such rapid decompression is
invariably associated with fast cooling rates Eng- land and Thompson, 1984; Bohlen, 1991 of \
20°CMa as shown by data from the Alps and Himalayas e.g. von Blanckenburg et al., 1989
and contrast with the slow integrated cooling rates in the Tanzanian granulites. It is concluded
that a continent collision scenario is not compat- ible with mineral texture Coolen, 1980; Mo¨ller,
1995; Appel et al., 1998, fluid inclusion Herms and Schenk, 1998 and geochronological evidence
this study.
Fig. 11. Compilation of the integrated cooling paths of gran- ulite complexes from the Pan-African Belt of NE Tanzania:
Pare and Usambara Mountains and Umba Steppe dark paths and different parts of the Uluguru Mountains light
paths. The range of cooling paths in the studied granulite areas is indicated by the light shaded area. Paths constructed
from previous geochronological data for the Wami River 1: Maboko et al., 1985 and Furua complex granulites 2: Coolen
et al., 1982; Andriessen et al., 1985; Priem et al., 1979 shown for comparison black paths fall on the same swath of cooling
paths.
similar but time-parallel to that of the Pare Mountains, with initial cooling rates of about
5°CMa.
7. Conclusions
