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 2.1 40Ar/39Ar 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 procedures....................................................................... 33 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 2.3.6 Forward modelling of 4He ages........................................................ 54 2.3.7 Remarks on the sample pre-screening and on excess 4He................ 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 39 AR/ 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 3.3.1 40Ar/39Ar analysis..............................................................................68 3.3.2 Titanite fission track analysis............................................................70 3.4 Results ..........................................................................................................71 3.4.1 40Ar/39Ar hornblende analysis...........................................................71 3.4.2 40Ar/39Ar biotite analysis...................................................................72 3.4.3 Titanite fission track analysis............................................................78 3.5 Interpretation...................................................................................................83 3.5.1 40Ar/39Ar hornblende data .................................................................83 3.5.2 40Ar/39Ar biotite data.........................................................................84 3.5.3 Cooling rates for 40Ar/39Ar 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.................................................................................. 130 6.2 Mount Tumbine............................................................................................ 137 6.2.1 Results............................................................................................ 137 6.2.2 Interpretation.................................................................................. 137 6.3 Southern basement ....................................................................................... 142 6.3.1 Results............................................................................................ 142 6.3.2 Interpretation.................................................................................. 142 6.3.3 Remarks on the Dpar values of the southern basement samples...... 155 CHAPTER 7 APATITE (U-TH)/HE ANALYSIS – RESULTS AND INTERPRETATION ................................. 157 7.1 Apparently reliable 4He ages (I)................................................................... 158 7.2 Ambiguous 4He ages (II).............................................................................. 162 7.3 Rejected 4He 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 ......................................................................... 182 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 ................................................................................................................