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
For a valid interpretation of mineral ages, infor- mation on the closure temperature T
c
for parent daughter
systems is
essential. The
closure temperature is defined as the temperature of the
system at the time given by its apparent ages Dodson, 1979. This T
c
depends on grain size and shape as well as on geologic parameters like cool-
ing rate and also on crystallographic parameters.
Table 1 Summary of approximate closure temperatures
a
Mineral Grain size mm
T
c
°C References
U–Pb system 100–300
Schenk 1980, 1990, Bingen and van Breemen 1998, Parrish and 800 peak
Monazite metamorphism
Whitehouse 1999, this study 200–30 000
Titanite Mezger et al. 1991, Gromet 1991, Cherniak 1993, Scott and St-Onge
630–730 1995, Zhang and Scha¨rer 1996
200–500 650
This study 130–430
Mezger et al. 1989 380–420
Rutile K–Ar and Ar–Ar system
160 450–500
e.g. Harrison 1981 Hornblende
– Muscovite
e.g. Hanson and Gast 1967 350–400
– 300
e.g. Harrison et al. 1985 Biotite
Microcline 125–250
150–200 Harrison and McDougall 1982
Rb–Sr system Muscovite
450–500 –
e.g. Harrison and McDougall 1982 –
350 e.g. Harrison and McDougall 1982
Biotite
a
T
c
values are chosen for selected minerals at different grain sizes for slow cooling rates of 1–10°CMa.
For some parentdaughter systems in some minerals experimental data is available e.g. U –
Pb in titanite, Cherniak 1993; K – Ar in horn- blende, Harrison 1981 U – Pb in monazite,
Smith and Giletti 1997. For other systems only empirical values are available and many of them
may need further refinement. A correlation of experimental results with well controlled natural
geologic settings is still wanting for many miner- als. Table 1 summarises T
c
for minerals relevant to this study and the choice preferred by the
authors, which is pivotal for the interpretation of the geochronological data and the cooling his-
tory.
4
.
1
. Monazite It is generally accepted that the T
c
of monazite is at least 700°C for slowly cooled rocks. How-
ever, there is ample evidence that the T
c
may be significantly higher as indicated by field data
from the Hercynian crustal section of Calabria, Italy, Schenk, 1980, 1990 or the Valhalla com-
plex in British Columbia Spear and Parrish, 1996. A single grain U – Pb study by Bingen and
van Breemen 1998 in amphibolite to granulite facies rocks shows that monazite growth ages
can be preserved through 850°C metamorphism under dry conditions. A study by Parrish and
Whitehouse 1999 also suggests higher T
c
. Re- cent experiments on Pb diffusion rates in monaz-
ite by Smith and Giletti 1997 suggest that circular or elongate monazite grains of 100 mm
radius should have closure temperatures of only 630 – 720°C in regions which cool at rates be-
tween 1 and 10°CMa. The authors caution that uncertainties in their closure temperature calcula-
tions may be as high as 140°C. Comparison with the examples from geochronological field studies
in granulites see above leads us to conclude that the calculations of Smith and Giletti 1997
underestimate the closure temperature of monaz- ite and are not accurate enough for application.
We conclude that monazite ages from this study may be interpreted as growth ages and
thus date the peak of the granulite facies meta- morphic event in eastern Tanzania which reached
temperatures of 810 9 40°C Appel et al., 1998 in most areas. Estimates of a lower T
c
may then be due either to growth of new monazite at tem-
peratures below its T
c
or, alternatively, to recrys- tallisation or Pb-loss induced by deformation
andor fluids rather than by diffusion.
4
.
2
. Titanite Experimental as well as empirical estimates for
the T
c
of the U – Pb system in titanite are avail- able. Mezger et al. 1991 estimated a closure
temperature of 630°C for titanite crystals of 1 cm diameter at a cooling rate of 2°CMa from field
studies in the Adirondack Mountains. In other parts of the Grenville Orogen, titanite preserved
their
U – Pb ages
although the
surrounding gneisses were later migmatised, which indicates
that T
c
may be at least as high as 650°C Mezger et al., 1992 for larger grains. Experimental stud-
ies of Cherniak 1993 yield a closure temperature of approximately 630°C for a diffusion radius of
500 mm at 2°CMa cooling rate. Cherniak 1993 thus concluded that effective diffusion radius may
be smaller than grain size.
Evidence for a higher closure temperature of titanite in slowly cooled rocks was presented by
Scott and St-Onge 1995. Their combination of thermobarometry and U – Pb dating suggests that
the T
c
of 100 mm – 1 mm diameter titanite lies in the range 660 – 700°C, higher than all previous
estimates. This conclusion is now supported by other studies e.g. Corfu 1996, Verts et al.
1996 and by discordance patterns observed in rocks which experience brief thermal events,
where discordant titanite data can be interpreted with the episodic Pb-loss model e.g. Tucker et al.
1986, Haggart et al. 1992. Similar evidence was presented by Gromet 1991, where titanite
grains of 500 to 2000 mm diameter showed strong discordance to an upper intercept and even titan-
ite grains of 250 mm diameter showed some inher- itance although this rock experienced only about
650°C during a metamorphic event, possibly re- lated to the brevity of the overprint. From inher-
ited magmatic titanite in a syenite intrusion, Zhang and Scha¨rer 1996 deduced a closure tem-
perature for volume diffusion of Pb in excess of 710°C. They suggest that titanite is always closed
to Pb at its crystallisation temperature and that the closure temperature concept may be mislead-
ing for metamorphic titanite, a contention not supported by the data of this study. Important
factors in all these studies are the time of titanite growth relative to the onset of cooling and the
duration of the metamorphic event in case the titanite had formed previously. Both may limit the
ability to determine a closure temperature from these mineral ages for slowly cooled terranes.
Most titanite fractions analysed in this study consist of whole grains with a diameter of 200 –
500 mm. Based on the studies of Cherniak 1993, Gromet 1991, and Scott and St-Onge 1995 T
c
of 650°C for titanite from granulites-facies rocks with slow cooling rates has been used in this study
as a conservative estimate.
4
.
3
. Rutile An estimate for the closure temperature for Pb
in rutile was given by Mezger et al. 1989, based on comparison with K – Ar and
40
Ar
39
Ar ages of hornblende and biotite. It was suggested that the
closure temperature is ca. 430 9 30°C for slow cooling rates of 2 – 10°CMa. This value is consis-
tent with U – Pb ages on rutile obtained by Scha¨rer et al. 1986. Most published U – Pb ages
of rutile, are concordant or only slightly discor- dant. Inheritance of older age information is only
likely when a metamorphic overprint does not exceed greenschist-facies conditions Mo¨ller et al.,
1995.
4
.
4
. Mica and amphibole For the K – Ar and Ar – Ar method, the state of
recrystallisation of the minerals Dallmeyer et al., 1990, and their chemical composition Lee, 1993
appear to be critical in the control of Ar release. Excess Ar is a particular problem for biotite and
hornblende. In general this excess Ar can be taken in from the growth environment or it can be
inherited from an older event. However, it is also possible that minerals lose K, but not Ar, during
low temperature alteration and this also results in an apparently old age. Since biotite, but not mus-
covite, is generally much more prone to yield old Ar – Ar ages, this may indicate that chloritisation
is the cause for the apparent excess ages. There- fore, only perfectly fresh biotite and hornblende
can be used for high precision K – Ar and Ar – Ar geochronology in metamorphic rocks.
Table 1 shows that Ar – Ar ages from horn- blende are expected to be close to, but younger
than U – Pb titanite ages from the same area and older than U – Pb ages of rutile. Similarly, Ar – Ar
ages of biotite are expected to be close to, but younger than, U – Pb ages obtained on rutile. This
is important in the discussion of inherited or excess Ar, which appears to be very common in
biotite and hornblende of granulites from the Mozambique Belt data of Priem et al. 1979 and
Maboko et al. 1989, discussed below.
5. Results
