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The Phanerozoic thermo-tectonic evolution of northern Mozambique constrained by _1hn4_1hn0Ar-_1hn3_1hn9Ar, fission track and (U-Th)-He analyses [Elektronische Ressource] / vorgelegt von Matthias Ch. Daßinnies

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THE PHANEROZOIC THERMO-TECTONIC
EVOLUTION OF NORTHERN MOZAMBIQUE
40 39
CONSTRAINED BY AR/ AR, FISSION TRACK
AND (U-TH)/HE ANALYSES

















Dissertation zur Erlangung des Doktorgrades der
Naturwissenschaften am Fachbereich Geowissenschaften
der Universität Bremen


Vorgelegt von
Matthias Ch. Daßinnies
Bremen, 2006





















Tag des Kolloquiums: 22.12.2006
Gutachter: Prof. Dr. J. Jacobs
Prof. Dr. W. Bach
Prüfer: Prof. Dr. T. Mörz
Prof. A. Kopf
Contents
CONTENTS
ACKNOWLEDGEMENTS ...................................................................................................... v
SUMMARY........................................................................................................................ vii
ZUSAMMENFASSUNG ......................................................................................................... x
CHAPTER 1
INTRODUCTION..................................................................................................................1
1.1 Scope of thesis.................................................................................................. 1
1.2 Research objectives and methods..................................................................... 3
1.3 Outline of thesis ............................................................................................... 4
CHAPTER 2
THERMOCHRONOLOGICAL METHODS AND ANALYTICS ...................................................... 6
40 39 2.1 Ar/ Ar dating method................................................................................... 6
2.1.1 Argon isotope measurements and age calculation.............................. 7
2.1.2 Theoretical considerations and definitions......................................... 9
2.1.3 Analytical procedures.......................................................................14
2.2 Fission track dating method ........................................................................... 17
2.2.1 Fission track age and error calculations ........................................... 18
2.2.2 External detector method.................................................................. 22
2.2.3 Fission track length and track annealing .......................................... 24
2.2.4 Modelling of t-T paths from apatite fission track data ..................... 31 2.2.5 Analytical procedures33
2.2.6 Fission track data presentation ......................................................... 38
2.3 (U-Th)/He dating of apatite............................................................................ 40
2.3.1 Helium isotope measurements and age calculation.......................... 41
2.3.2 α-emission balance and helium age correction................................. 44
2.3.3 Age error estimation ......................................................................... 47
2.3.4 Helium diffusion in apatite............................................................... 48
2.3.5 Analytical procedures.......................................................................51
4 2.3.6 Forward modelling of He ages........................................................ 54
4 2.3.7 Remarks on the sample pre-screening and on excess He................ 56
2.4 Estimates on denudation................................................................................. 57

i
Contents
CHAPTER 3
POST PAN-AFRICAN THERMO-TECTONIC EVOLUTION OF THE NORTH MOZAMBICAN
BASEMENT AND ITS IMPLICATION FOR THE GONDWANA RIFTING: INFERENCES FROM
40 39AR/ AR HORNBLENDE, BIOTITE AND TITANITE FISSION TRACK DATING........................59
3.1 Introduction.....................................................................................................60
3.2 Geological setting and previous geochronology.............................................62
3.2.1 Axial Granulite Complex (Unango Group) ......................................62
3.2.2 The eastern domain and the Lurio Belt foreland...............................64
3.2.3 Late Palaeozoic to Early Mesozoic intracontinental rift basin .........66
3.2.4 Marginal rift basins ...........................................................................68
3.3 Analytical procedures.....................................................................................68
40 39 3.3.1 Ar/ Ar analysis..............................................................................68
3.3.2 Titanite fission track analysis............................................................70
3.4 Results ..........................................................................................................71
40 39 3.4.1 Ar/ Ar hornblende analysis...........................................................71
40 393.4.2 Ar/ Ar biotite analysis...................................................................72
3.4.3 78
3.5 Interpretation...................................................................................................83
40 39 3.5.1 Ar/ Ar hornblende data .................................................................83
40 393.5.2 Ar/ Ar biotite data.........................................................................84
40 39 3.5.3 Cooling rates for Ar/ Ar hornblende and biotite results ...............85
3.5.4 Titanite fission track data..................................................................86
3.6 Discussion.......................................................................................................89
3.6.1 Proterozoic to Early Palaeozoic cooling in the south western Axial
Granulite Complex............................................................................89
3.6.2 Early Palaeozoic cooling in the southern basement..........................89
3.6.3 Late Palaeozoic cooling and denudation history...............................92
3.7 Conclusions...................................................................................................100
CHAPTER 4
CENTRAL EASTERN AFRICA DURING GONDWANAS RIFT, BREAK-UP AND DRIFT EVOLUTION
SINCE THE MESOZOIC .....................................................................................................111
CHAPTER 5
PREVIOUS APATITE FISSION TRACK STUDIES IN CENTRAL EAST AFRICA AND EAST
ANTARCTICA..................................................................................................................121
ii
Contents
CHAPTER 6
APATITE FISSION TRACK ANALYSIS – RESULTS AND INTERPRETATION ........................... 125
6.1 Axial Granulite Complex.....................................................................130
6.1.1 Results............................................................................................130 6.1.2 Interpretation..................................................................................
6.2 Mount Tumbine137
6.2.1 Results6.2.2 Interpretation137
6.3 Southern basement.......................................................................................142
6.3.1 Results............................................................................................6.3.2 Interpretation142
6.3.3 Remarks on the D values of the southern basement samples...... 155 par
CHAPTER 7
APATITE (U-TH)/HE ANALYSIS – RESULTS AND INTERPRETATION ................................. 157
4 7.1 Apparently reliable He ages (I)................................................................... 158
4 7.2 Ambiguous He ages (II).............................................................................. 162
4 7.3 Rejected He ages (III) ................................................................................. 162
7.4 Comments on apatite (U-Th)/He dating....................................................... 165
7.5 Results of forward t-T path modelling of (U-Th)He ages............................ 165
7.5.1 Slow and protracted cooling path models....................................... 166
7.5.2 Cooling path models involving reheating events ........................... 168
CHAPTER 8
DISCUSSION – MESOZOIC-CENOZOIC COOLING AND DENUDATION HISTORY
OF THE NORTH MOZAMBICAN BASEMENT ...................................................................... 173
8.1 Axial Granulite Complex ............................................................................. 173
8.2 Mount Tumbine............................................................................................180
8.3 Southern basement.......................................................................................182
8.3.1 The eastern margin.........................................................................
8.3.2 The central southern basement and the Lurio Belt......................... 189
8.4 Denudation estimates for the Permo-Jurassic period ................................... 194
8.5 Inferences on the Mesozoic Gondwana break-up ........................................ 195
CHAPTER 9
THE NORTH MOZAMBICAN – SOUTH CENTRAL MALAGASY RELATIONS: INFERENCES FROM
FISSION TRACK ANALYSES.............................................................................................. 197
CHAPTER 10
CONCLUSION ................................................................................................................. 207
iii
Contents
BIBLIOGRAPHY...............................................................................................................213
APPENDIX A
TITANITE FISSION TRACK – RADIAL PLOTS ..........................................................................I
APPENDIX B
APATITE FISSION TRACK – DATA PLOTS...........................................................................III
B.1 Western Axial Granulite Complex .................................................................III
B.2 Mount Tumbine..............................................................................................IV
B.3 Southern Basement – C group ........................................................................VI
B.4 ent – M ....................................................................XIX
APPENDIX C
BASH SCRIPT – GMT 4.0 ............................................................................................ XXIII


iv ACKNOWLEDGEMENTS
This PhD study was funded by the University of Bremen (FNK) research grant ZF
05/101/01.
Special thanks go to my doctoral advisor Prof. Dr. J. Jacobs for the opportunity to carry
out this research, for his supervision, interest and constant support throughout.
Furthermore, I would like to thank Prof. Dr. W. Bach for taking on to be the second
expert.
I am obliged to Prof. Dr. M. Olesch for providing working facilities as well as access to
the fission track laboratory at University of Bremen. Dr. G. Grantham is especially
acknowledged for providing sample material for thermochronological analyses. I am
indebted to NGU and Dr. B. Thomas for the possibility to join the mapping project and
using their logistic setup to accomplish field work in northern Mozambique. Prof. A.
Gleadow and Dr. B. Kohn are thanked for their invitation to spend a research period in
the (U-Th)/He and fission track laboratories at Melbourne University. I wish to thank
40 39Dr. J-A. Wartho for fruitful e-mail discussions on Ar/ Ar dating.
The technical and administrative support of V. Kolb, B. Schröder, P. Witte and the
enthusiastic hints on sample preparation of Mr. Schulz are highly appreciated.
Many thanks go to my current and former colleges Dr. B. Emmel, PD Dr. F. Lisker, Dr.
M. Geiger, Dr. E. Guasti, Dr. M. Lorencak, Dr. U. Weber, Dr. B. Ventura, M. Heldt and
R.Tjalingii for helpful comments, suggestions and pleasant times at the Universities of
Bremen and Melbourne.
Above all, I wish to express my thanks to my wife, my parents, my brothers and my
friends, whose steady encouraging support gave motivation and balance to me.




v


vi
Summary

SUMMARY
40 39Results of thermochronometric investigations comprising Ar/ Ar hornblende and
biotite, titanite and apatite fission track (FT) and apatite (U-Th)/He dating on 102
basement rock samples from northern Mozambique record a cooling and denudation
history since Early Palaeozoic times. In the north Mozambican sector, these results
place new temporal and geometric constraints on the initial rift and subsequent drift
configuration during the Gondwana supercontinent dispersal as well as on the post
break-up evolution of the sheared margin of central East Africa. Furthermore, they
highlight the influence exerted by ductile basement structures of Pan-African age on the
loci of tectonically active zone and associated denudation since the Late Palaeozoic.
40 39Ar/ Ar hornblende and biotite ages range from c. 542 Ma to 456 Ma and c.
448 Ma to 428 Ma, respectively. They record the cooling from the latest Pan-African
metamorphic imprint, presumably related to the formation of the Namama Thrust Belt
at c. 550-500 Ma, at slow rates of about 11°-7°C/Ma from 525°C to 305°C in Early to
Late Ordovician times. Locally, the thermal influences of syn- to post-tectonic
granitoid / pegmatite emplacements at about 500-450 Ma delayed cooling. Widespread
basement cooling to < 350°C occurred in Late Ordovician to Early Silurian times.
The titanite fission track ages fall into two age groups of c. 378 Ma to 327 Ma and
c. 284 Ma to 219 Ma. Very slow cooling since the Late Ordovician/Early Silurian at
rates of < 1°C/Ma to below 275 ± 25°C in the Late Devonian/Early Carboniferous is
deduced from the older titanite FT ages. It is related to decreasing denudation in
response to the establishment of pre-Karoo peneplains within central Gondwana. The
younger titanite FT ages record the cooling of a denuding and approximately E-W
trending uplifted rift flank whose formation marks the onset of rifting and incipient
Gondwana disintegration in the Early to Late Permian. Associated crustal extension
proceeded obliquely to a NW-SE tensional stress field by brittle reactivation of easterly
trending ductile basement fabrics and presumably, linked to the Zambezi Rift system.
Titanite FT results indicate ≤ 9-12 km of crust removal since the Permo-Carboniferous.
vii
Summary
Apatite FT ages vary between c. 169 Ma and 61 Ma whereby ages of ≥ 100 Ma record
complex, and in part multistage cooling and denudation histories that are generally
restricted to zones of crustal weakness along the Mozambique Belt yielding northerly
trending ductile fabrics; along the western Axial Granulite Complex and along the
present eastern continental margin. Apatite FT data imply that denudation was related to
brittle reactivation of ductile basement fabrics by rifting, transtension and/or
transpression along southern Tanganyika-Rukwa-Malawi System and along the Davie
Fracture Zone at the eastern margin. Modelled time-temperature paths indicate two
periods of more rapid cooling (c. 5°-3°C/Ma) to below c. 110°C in the Early to Late
Jurassic and Early to Late Cretaceous. The Early/Middle Jurassic rifting, break-up and
subsequent seafloor spreading within the Somali and Mozambique basins triggered the
Jurassic periods, which were accompanied by the deposition of up to 2.5 km of volcanic
rocks and associated reheating along the eastern margin at about 180-160 Ma. Far field
stresses linked to global plate reorganizations due to the opening of the Atlantic and
Indian oceans are accounted for the Cretaceous cooling periods. The apatite FT results
indicate < 6 km and < 7 km of maximum denudation in the Axial Granulite Complex
and along the eastern margin since the Jurassic, respectively. Apatite FT ages ≤ 100 Ma
from the central part of basement record a uniform cooling and denudation pattern.
Modelled time-temperature paths indicate a more rapid cooling step (c. 5°-3°C/Ma) to
below c. 110°C in Early to Late Cretaceous times. It is associated with the erosional
compensation of a potential local base level gradient between the central southern
basement and its bounding N-S trending zones of contemporaneous crustal extension
and exhumation to the west and east. These apatite FT results indicate < 4 km of
denudation since the Cretaceous. In Palaeogene times, a basement reheating to c. 60°C,
presumably due to heat advection by fluids is inferred from time-temperature models of
samples located in zones of crustal weakness and is synchronous with the initiation of
the East African Rift System at c. 30 Ma.
Apatite (U-Th)/He ages span from 150 Ma to 40 Ma. Results of forward modelled
time-temperature paths indicate a widespread slow and protracted cooling associated
with persistent slow denudation from c. 80°C in the Late Cretaceous to subaerial
conditions (c. 40°C) in Palaeogene/Neogene times. Along the eastern margin and the
viii