Precambrian Research 104 2000 123 – 146
U – Pb dating of metamorphic minerals: Pan-African metamorphism and prolonged slow cooling of high pressure
granulites in Tanzania, East Africa
Andreas Mo¨ller
a,b,
, Klaus Mezger
b,1
, Volker Schenk
a
a
Mineralogisch-Petrographisches Institut, Uni6ersita¨t Kiel,
24098
Kiel, Germany
b
Max-Planck-Institut fu¨r Chemie, Postfach
3060
,
55020
Mainz, Germany Received 1 November 1999; accepted 4 May 2000
Abstract
U – Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamor- phosed during a Pan-African tectonothermal episode. These mineral ages range from 610 to 655 Ma and indicate that
peak metamorphic conditions were diachronous in the different granulite domains. U – Pb titanite and rutile ages define integrated cooling rates of 2 – 5°CMa for all investigated granulite areas, and suggest a common process for
the post-metamorphic histories of the different granulite areas. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening
during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East- and West-Gondwana as the cause of
granulite facies metamorphism. Palaeomagnetic data have shown that this collision did not occur until 550 Ma, when the Pan-African granulites in Tanzania had already cooled below 500°C. The high pressure granulites of eastern
Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Mozambique Belt; Pan-African orogeny; U – Pb geochronology; Monazite; Titanite; Rutile www.elsevier.comlocateprecamres
1. Introduction
Metamorphic pressure – temperature – time P – T – t paths provide essential constraints for any
models that relate metamorphism to tectonic pro- cesses. The different tectonic settings can be in-
dicative of the plate-tectonic scenario that led to metamorphism and the formation of an orogenic
belt. In order to unravel the evolution of a com-
Corresponding author. Present address: Institut fu¨r Geo- wissenschaften, Johannes Gutenberg Universita¨t Mainz, Post-
fach 3980, D-55099 Mainz, Germany. Tel.: +
49-6131- 3925584; fax: + 49-6131-3924769.
E-mail address
:
amoellermail.uni-mainz.de A. Mo¨ller.
1
Present address: Institut fu¨r Mineralogie, Universita¨t Mu¨n- ster, Corrensstr. 24, 48149 Mu¨nster, Germany.
0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 8 6 - 3
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
