Origin of Madagascar

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Mozambique belt with crust domains and ages
Mozambique Belt Structural and metamorphic facies. Madagascar with the presumed fringes of the Mozambique and Malagasy oceans

The emergence of Madagascar is characterized by an accumulation of different geological structures from different geographical areas of origin and development phases over time. It includes paleoarchaic and paleoproterozoic kraton fragments descended from other cratons, as well as neoproterozoic juveniles, i. H. newly formed island arch components from various ocean basins.

During the Pan-African Orogeny , these land masses united when the East African Orogen formed until about 550 million years ago (millions of years in the following text abbreviated as mya ).

India with Madagascar broke away from Africa at around 160 mya . At around 90 mya, Madagascar was separated from India. Since then, Madagascar has been separated from Africa by the Indian Ocean .

Geological structures, tectonic shifts

Cratons

The cratons of Central Madagascar are of particular importance with regard to the formation of Madagascar. They include the Antogil block and the Kraton Antananarivo with the Tsaratanana plate and the Itremo plate.

Both Malagasy cratons were separated by ocean basins, which after their closure around 719 to 530 mya created the Betsimisaraka suture zone. This contact zone consists of highly metamorphic rocks that have been transformed by temperature and / or pressure with mineral components that are typical of ophiolites and thus characterize a collision zone between eastern and western Gondwana.

It is believed that the Antogil Block, and thus Madagascar, broke away from the Indian Dharwar craton by 96 to 84 mya after India, including Madagascar, broke away from East Africa by 160 to 158 mya.

Antogil block

The Antogil block consists of two Kraton fragments, the Antogil Kraton and the Masora Kraton. The Antogil Kraton is in the northeast, while the Masora Kraton is in the central east, both on the fringes of Madagascar.

According to the so-called Out-of-Africa-Hypothesis or the Azania-Terrane-Model, the Antogil block was originally part of the Indian Dharwar kraton.

During the Pan-African Orogeny, the Antogil block united with Azania and the east African coast in the area of ​​the then still connected Congo-São Francisco (Congo-SF) craton and the Tanzania craton and the Bangweulu block.

This is derived from the metamorphic and recrystallizing events between 792 and 553 mya. They are the only ones that occurred on the Antogil Block during this period.

Antananarivo Kraton

The Antananarivo craton seems to have formed around 2500 mya and was originally part of the East African coast with the cratons Congo-SF and Tanzania and the Bangweulu block.

The separation of this kraton is related to the formation of Azania. Azania, an old name for areas of the East African coast, presumably extended from what is today Madagascar, Somalia, Ethiopia to the Afif terran on the Arab-Nubian shield . Azania detached itself from the eastern flanks of the cratons of the Congo-SF, Tanzania and the Bangweulu Bloc during the submergence of the oceanic plate of the Mozambique Ocean under the African east coast with the formation of crustal clods (microcontinent, terrane and similar blocks) of archaic and palaeoproterozoic ages . The period of this separation is not yet clear.

It is believed that the subduction of the Mozambique Ocean caused the formation of oceanic island arcs with backarc and forearc basins , which stretched the African continental crust, which eventually led to the detachment of these parts of the crust (slab roll-back).

The widespread metasedimentary sediments of the Itremo Plate, transformed by pressure and / or temperature and deposited on the craton from 1855 mya, are very similar to the rocks of the Tanzania craton and the Bangweulu block. From this it can be concluded that these formations were connected at that time.

Another indication of this thesis is the deformation (deformation) of the Itremo slab before gabbros and granites intruded (invaded) into it. The period is still very vague between approx. 1700 to 800 mya. It is presumably comparable to the deformation of the Irumide Belt between the Tanzania / Bangweulu Kraton and the Kalahari Kraton and occurs before the drift of Azania from the east coast of Africa, probably 1050 to 850 mya.

The Tsaratanana plate, which contains rocks that are more than 200 my older than those of the Antananarivo craton, show contacts with Africa in the form of metamorphic granulite - rock properties (facies) as they also occur on the northern part of the Tanzania craton .

Sedimentary rock units

The further development of Madagascar is documented in neoproterozoic, juvenile crustal areas made of sedimentary rocks (deposit rocks), such as the Bemarivo belt in the extreme north, the Vohibory domaine in the extreme south, the Androyen unit in the south, the molo area in the southwestern center as well the Betsimisaraka suture zone in the eastern areas.

These juvenile crust areas are related to the formation of the Mozambique belt . Geophysical patterns, structural properties and geochronology suggest that the Mozambique Belt continues to Madagascar.

The Bemarivo Belt in northern Madagascar consists of two terrans that were created from island arch components in different ocean areas. The amalgamation of the two crustal blocks took place between 563 and 532 mya and both with the northern Madagascar craton area around 540 to 530 mya.

The Vohibory-Domaine in southern Madagascar contains metasediments (sediments transformed by pressure and / or temperature) that were deposited after 800 mya. They show island arch components from intra-oceanic rift fractures , which also caused Azania to be separated from the African cratons Congo-SF, Tanzania and Bangweulu.

Based on the sedimentary units of the Bemarivo Belt, the Vohibory Domaine and the Betsimisaraka Suturzone, it can be concluded that Azania was surrounded by the Mozambique Ocean in the west and the Malagasy Ocean in the east. The fringe of the Mozambique Ocean can be traced from the Vohibory Domain to the Arab-Nubian Shield, while that of the Malagasy Ocean stretched from the Betsimisaraka Suture across the Bemarivo Belt to the Seychelles.

Rocks, metamorphoses, facies

Paleoarchaic cratons

Antogil block

The Antogil block (Antogil / Masora-Kraton) is the oldest crustal block and consists of archaic tonalitic ortho- and paragneiss, 3320 to 3154 mya, into which several mafic dykes (fissure-shaped igneous rock formations) penetrate combined with a sedimentation sequence after a deep deformation and migmatic transformation of the rocks. The age of the metasediment deposition is 3178 mya, while the migmatization is 2597 my old. In a subsequent phase, voluminous, little stratified granites were created. These contain components from both older and younger crust.

From 2570 to 2513 mya, granodiorite - and later monzogranite-dominated volcanic rocks - penetrated the rock. This was followed by the formation of dyke swarms from metamorphosed gabbro around 2147 mya.

Metamorphic and recrystallizing events are detected between 792 and 763 and 553 mya. They are the only ones in the Pan-African orogeny period and do not show any high-grade thermal / tectonic processes. The rocks range from green slate to lower amphibolite facies.

Psammitic metasediment layers are discordant (angular or irregular) on the northern and western crystalline bedrock. In the northern area these sediments merge into the Bemarivo belt. To the west, the wide Betsimisaraka contact zone (suture) separates the Antogil block from the Antananarivo craton.

This highly deformed suture consists of graphitic pelite with outcrops of Harzburgite , Chromite and emerald-greenish deposits. It is defined as the edge zone of the Malagasy Ocean between the cratons Antogil and Antananarivo.

Paleoproterozian cratons

Antananarivo Kraton

The Antananarivo craton occupies the largest and central area of ​​Madagascar. It consists of granitoids, 2550 to 2500 mya. Between 824 and 719 mya, voluminous volcanic rocks made of granite, syenite and gabbro penetrated . The chemism of these volcanic rocks corresponds to magmas that arise in areas of subduction zones.

The entire craton was thermally and tectonically affected between 700 and 532 mya. This gave the original rocks a granulite facies with the development of a gneiss-like arrangement.

Granitoid magmatism occurred between 630 and 561 mya, the lava flows of which formed plates of very different thickness, from 100 meters to several kilometers thick rock layers. This magmatism occurred during severe deformations of the craton, which are manifested in the Betsileo shear zone to the southwest of the craton.

The eastern craton area was deformed between 630 and 530 mya when the cratons Antananarivo and Antogil collided. In particular, an east-west contraction occurred between 615 and 530 mya.

The rocks have a granulite facies to gneiss facies.

Tsaratanana plate

The rocks of the Tsaratanana plate lie on the Antananarivo craton. The plate is formed by the three belts of similar rock texture , geochronology and structure Maevatanana, Andriamena and Beforona. The belts are separated from the Antananarivo craton below by mylonite zones.

The Tsaratanana plate contains mafic gneiss, chromite- containing ultramafic rocks, tonalites and meta- pelites , which were formed between 2750 and 2490 mya and were deformed by 2500 mya and metamorphically overprinted under ultra-hot temperatures.

Gabbros invaded the rock between 800 and 770 mya. Granitoids rose by 637 mya, each associated with further deformations and metamorphoses.

Itremo plate

The Itremo plate joins the Antananarivo Kraton in the SW. It consists of dolomitic structures, quartzites , pelites and reshaped silt rocks , which are unevenly supported by amphibolites and gneisses, which in turn are comparable to the orthogneiss of the Antananarivo craton.

These sedimentary rocks, which may have originated from East African sources, were deposited between 1700 and 804 mya and deformed in lying, horizontal folds. Between 804 and 779 mya, various gabbros and syenites penetrated this area , which are chemically related to supra-subduction zones ( subduction zones with ophiolites in the forearc area ). These rocks formed predominantly in areas of colliding ocean plates.

After 789 mya, the Itremo plate was newly deformed into upright, open folds, counter-rotating or strike-slip faults ( blade displacements ), which was closed by granitoid intrusions between 570 and 539.

The metamorphic grade of the rocks increases from the east at the Betsileo shear zone to the Antananarivo craton with lower green schist facies to the west.

The Ranotsara shear zone separates the Itremo plate and thus the northern and central area of ​​Madagascar from the south-west adjoining Androyen region, which consists of various sedimentary rocks and was metamorphically transformed between 645 and 530 mya and penetrated by molten rock.

Neoproterozoic juvenile crustal domains

Betsimisaraka suture zone

The broad Betsimisaraka suture zone separates the cratons Antananarivo and Antogil. It consists of metasediments and contains numerous ultramafic and mafic rocks, which are considered to be the remains of an oceanic lithosphere (upper geological layers of the earth). From this it is concluded that this suture was the subduction edge zone of the Malagasy Ocean between the two cratons.

The sediments come from the Indian Dharwar craton and were deposited between 800 and 550 mya.

Bemarivo belt

The Bemarivo Belt contains two different juvenile terranos that developed as island arch components in different ocean areas. In the southern, older terran, highly metamorphosed paragneiss developed from metasedimentary, i.e. that is, transformed sedimentary rocks formed Paleoproterozoic age. Extensive metamorphic magmas from original island arc components penetrated this paragneiss around 750 mya. The rocks of the southern terran have an upper amphibolite to granulite facies.

The northern, younger terran originated in the eastern area of ​​Azania and contains mostly metamorphic supracrustal rocks from igneous and igneous-sedimentary sequences, 750 to 740 and 720 mya. These, too, were penetrated, 718 and 705 mya, by volcanic rocks consisting of migmatites and orthogneiss, which developed from metamorphically shaped island arch components. The facies of the northern block are marked as amphibolite to green schist. These can be associated with the Seychelles and Northwest India .

The two crustal blocks merged with each other between 563 and 532 mya and both with the northern Madagascar craton area around 540 to 520 mya, each connected with final penetrations of granotoids and metamorphic overprints. They are separated by a shear zone .

Vohibory domain

The Vohibory-Domaine consists of juvenile crust with mafic granulites, amphibolites and sedimentary rocks, which were formed from island arch components under intra-oceanic conditions between 910 and 760 mya. Between 630 and 600 mya they were subject to tectonic and thermal changes when the crustal components collided.

The Vohibory-Domaine is similar to the Eastern Granulite Belt in Tanzania with granulite facies, 650 to 610 mya, so that the juvenile crust in the Mozambique Belt extends from southern Kenya through Tanzania to Madagascar.

Androyene unit

The Androyen Unit forms an area south of the Rantosara Shear Zone of highly transformed metasediments. The conversion took place at ultra-high temperatures. An archaic basement under the sediment cover has not yet been confirmed. Possibly the sediments of the Androyen unit resemble those of the Molo area and the provinces of the Itremo plates.

The sediments were deposited between 620 and 560 mya.

Molo area

The Molo area metasediments were deposited in a triangular zone between the Itremo Plate and the Ranotsara Shear Zone.

The parent rocks with an age of 620 to 560 mya come from a basin that separated central Madagascar from East Africa.

literature

  • BF Windley and others: Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal. Springer Link Geological Review. October 1994, Volume 83, Edition 3, pp. 642-659 doi: 10.1007 / BF00194168 .
  • Alan S. Collins and Brian F. Windley: The Tectonic Evolution of Central and Northern Madagascar and Its Place in the Final Assembly of Gondwana. The Journal of Geology, 2002, Volume 110, pp. 325-339 doi: 10.1086 / 339535 .
  • De Waele and others: U-Pb detrital zircon geochronological provenance patterns of supracrustal successions in central and northern Madagascar. bdewaele.be PDF (English)
  • Peter Luger and others: Comparison of the Jurassic and Cretaceous sedimentary cycles of Somalia and Madagascar: implications for the Gondwana breakup. Geologische Rundschau, December 1994, Volume 83, Issue 4, pp. 711–727 doi: 10.1007 / BF00251070 .

Individual evidence

  1. ^ Alan S. Collins: Madagascar and the amalgamation of Central Gondwana. In: Continental Evolution Research Group, Geology and Geophysics. Received August 4, 2005; accepted 25 October 2005, Available online 10 January 2006. adelaide.edu: PDF (English)
  2. ^ H. Fritz and others: Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution. In: Journal of African Earth Sciences, Volume 86, October 2013, Pages 65–106, doi: 10.1016 / j.jafrearsci.2013.06.004
  3. ^ John C. Briggs: The biogeographic and tectonic history of India. In: Journal of Biogeography. 30, 2003, p. 381, doi : 10.1046 / j.1365-2699.2003.00809.x .
  4. ↑ GEOLOGY OF DHARWAR CRATON. In: Shodhganga @INFLIBNET Center shodhganga.inflibnet.ac: PDF (English)
  5. ↑ Fernandez-Alonso and others: THE PROTEROZOIC HISTORY OF THE PROTO-CONGO CRATON OF CENTRAL AFRICA. In: Department of Earth Sciences, Royal Museum for Central Africa, B-3080 Tervuren, Belgium africamuseum.be: PDF (English)
  6. ↑ GEOLOGICAL FRAMEWORK AND REGIONAL METALLOGENY OF TANZANIA. In: kilimanjarominingcompany.com: PDF with 79 pages ( memento of the original from April 26, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English)  @1@ 2Template: Webachiv / IABot / www.kilimanjarominingcompany.com
  7. ↑ LS Andersen, R. Unrug: Geodynamic evolution of the Bangweulu Block, northern Zambia. In: Precambrian Research, Volume 25, Issues 1-3, August 1984, Pages 187-212 doi: 10.1016 / 0301-9268 (84) 90032-9 .
  8. ^ Lecture Regional Geology of the Earth, Neoproterozoic 4, (Arabia, Egypt) WS 2006/07. In: erdwissenschaften.uni-graz.at: PDF
  9. ↑ Tomoeki Nakakuki, Erika Mura: Dynamics of slab rollback and induced back-arc basin formation. In: ResearchGate, Article in EARTH AND PLANETARY SCIENCE LETTERS 361: 287–297 JANUARY 2013 doi: 10.1016 / j.epsl.2012.10.031
  10. ^ B. De Waele and others: High-temperature, low-pressure tectono-thermal evolution of the Irumide Belt, central, Southern Africa: Lithosphere delamination during arc-accretion. In: Frontier Research on Earth Evolution Report 2002–2004, V2. 9p. bdewaele.be: PDF (English)
  11. ↑ Armin Zeh and others: Archean Accretion and Crustal Evolution of the Kalahari Craton — the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton / Swaziland to the Francistown Arc. In: Journal of Petrology. 50, 2009, p. 933, doi : 10.1093 / petrology / egp027 .
  12. ↑ Robert J. Stern and others: Formation of juvenile continental crust in the Arabian-Nubian shield: evidence from granitic rocks of the Nakasib suture, NE Sudan. In: Geologische Rundschau. 87, 1998, p. 150, doi : 10.1007 / s005310050196 .
  13. ↑ DI Schofield and others: Geological evolution of the Antongil Craton, NE Madagascar. In: Precambrian Research Volume 182, Issue 3, 1 October 2010, Pages 187–203, doi: 10.1016 / j.precamres.2010.07.006
  14. ↑ Guido Schreurs and others: The role of the Ranotsara Zone in southern Madagascar for Gondwana correlations. In: Institute of Geological Sciences, TSK 11 Göttingen 2006. wwwuser.gwdg.de PDF (English)
  15. ↑ Tsilavo Raharimahefa, Timothy M. Kusky: Structural and remote sensing analysis of the Betsimisaraka Suture in northeastern Madagascar. In: Gondwana Research Volume 15, Issue 1, February 2009, Pages 14–27, doi: 10.1016 / j.gr.2008.07.004
  16. ↑ RJ Thomas and others: Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar. In: Precambrian Research Volume 172, Issues 3–4, August 2009, Pages 279–300 doi: 10.1016 / j.precamres.2009.04.008
  17. ↑ LD Ashwal and others, Petrogenesis of Neoproterozoic Granitoids and Related Rocks from the Seychelles: the Case for an Andean-type Arc Origin. In: In: Journal of Petrology. 43, 2002, pp. 45-83, doi : 10.1093 / petrology / 43.1.45 .
  18. ^ Alan S. and others: Depositional age, provenance and metamorphic age of metasedimentary rocks from southern Madagascar. In: Gondwana Research, Volume 21, Issues 2–3, March 2012, Pages 353–361, Special Issue: Western Gondwana doi: 10.1016 / j.gr.2010.12.006
  19. ↑ V. Tenczet and other: Anorthosites in the Eastern Granulites of Tanzania-New SIMS zircon U-Pb age data, petrography and geochemistry. In: Precambrian Research, Volume 148, Issues 1–2, 20 July 2006, Pages 85–114 doi: 10.1016 / j.precamres.2006.03.004