Neoproterozoic Crust Formation and Terrane Suturing in the East African Orogen: Geological, Geochemical, and Geochronological Investigation in northern Madagascar

Sambirano river, northern Madagascar

The time interval by the Late Precambrian and its transition into the Cambrian is one of the most significant in Earth’s history. Major tectonic, geochemical, and biologic events occurred on a global scale, including dramatic fluctuation in sea level and climate, evolution and diversification of soft bodied and shelly metazoans, and the reorganisation and growth of the Gondwanan supercontinent. The formation of supercontinents has been a topic of wide interest over many years. Reconstructing the assembly of Gondwanaland is, however, hampered by a lack of reliable data on the tectono-thermal evolution of the adjoining Pan-African belts, thus hindering the correlation of geologic events among these belts.

In its predrift position near the dawn of the Cambrian, Madagascar was situated in the interior of Gondwana near the eastern edge of the East African Orogen (EAO). This orogen marks the join between East and West Gondwana, and it is generally believed to be site of the former Mozambique ocean. Madagascar is thus crucially positioned to reveal three important features of the EAO essentially for understanding the formation of Gondwana: (1) the nature of the terranes in the central part of the accretionary orogen and their possible extensions in Somalia, Ethiopia, and Yemen; (2) the structure of the estern part of the EAO in the key transitional area from dominantly terrane accreated geology (in the north) to continental collision and crustal overthickening I (in the south), and (3) the chronology of terrane suturing and continental collision events in the critical join between East and West Gondwanaland.

The Precambrian rocks of northern and central Madagascar may be divided into three tectonic elements: (1) a northernmost, largely juvenile Neoproterozoic (~750-725 Ma) volcanic/plutonic arc terrane, perhaps once contiguous with the Seychelles; (2) a southernmost continental shield consisting of middle-to-late Archean granitoid and mafic gneisses and lesser paragneiss; and an intervening collision zone that changes character from west to east. In the east, the zone consists principally of pelitic shist, gneiss, and migmatite intruded by rare plutons of charnockite, and in the west, it consists of migmatized and deformed calc-alkaline gneissesCordirite gneiss, Lokoho river, northern Madagascar invaded by leucogranites. The Neoproterozoic igneous rocks of North Madagascar are strikingly similar in age, chemistry, and isotopic characteristics to the igneous rocks of the Seychelles and Rajasthan (India), and hence an Andean-type arc origin has been proposed for all of them. The Sambirano Group (3) consists of metaconglomerate, graywacke, and pelite, as well as lesser orthoquartzite and calc-silicate gneiss, variably metamorphosed to greenschist grade (in the northeast) and amphibolite grade (in the southwest) constrained by garnet-cordierite thermobarometry to peak conditions of ~850 °C and ~7 kbar. Similar temperatures and pressures were obtained from stromatic pelitic migmatite using GASP barometry. Based on U-Pb geochronology of monazite in crd-gar-kspar-sill gneiss (~520 Ma), zircon in charnockite (~521 Ma), and recrystallized sphene (~511 Ma) in terrane 1, peak metamorphism occurred in ~Middle Cambrian time. Thus the Sambirano Group and related charnockites, are interpreted to be part of a larger Cambrian-age orogenic belt bounded on the south by the Tsarantana thrust zone that separates a juvenile volcanic/plutonic arc terrane (North Madagascar) from an Archean craton (Central Madagascar) of unknown Rodinian affinity (East or West Gondwanan, or neither).
Additionally rocks from the Sambirano and Mahavavy River, which crosscut the northern Malagasy suture zone have been analysed geochronologically and geochemically. Three different magmatic rock types are exposed along the traverse: highly foliated migmatitic gneisses, which are intermingled and mixed with restitic amphibolites, and lastly late crosscutting pink granitic intrusions. Preliminary U-Pb geochronological results for single zircons from the gneisses indicate that the dominant zircon population crystallized within the amphibolite at around 580 Ma. In addition the migmatitic gneisses show a different zircon population, which yield ages of ~880 Ma. Furthermore zircons from the crosscutting granite imply an emplacement age of ~560 Ma.

New geochemical data indicate that the rocks along the Sambirano River is depleted in HFS elements (Nb, Zr, Ti etc.) and exhibits low HFS/LIL element ratios (Rb/Zr, etc.) this may be attributed to subduction zone enrichment (LIL elements) of the source of the observed rock types.

The magmatic rock suite along the Sambirano River record a complex polyphase orogenic history and can be interpreted either as rocks produced during an arc-continent collision or as reworked rocks, which had been produced during and earlier Neoproterozoic arc-continent collision. They also form part of a distinct set of temporal, spatial and compositional characteristics for the Malagasy shield that provide important constraints on possible Neoproterozoic reconstructions. Currently the relationship of this terrane towards other areas within the EAO is examined and other section might shed light into the positioning of this important part of central Gondwanaland.

Tucker, R.D., Kusky, T.M., Buchwaldt, R., and Handke, M.J.: Neoproterozoic nappes and superimposed folding of the Itremo Group, west-central Madagascar.- Precambrian Research, in press (2002).

Buchwaldt R., Tucker, R. D., Dymek, R. F.: Geothermobarometry and U-Pb Geochronology of metapelitic granulites and pelitic migmatites from the Lokoho region, Northern Madagascar.- American Mineralogist, 88, 1751-1764 (December 2003).

Buchwaldt, R Tucker, R. D., Dymek, R. F. (2002): Petrogenetic implication of three contrasting terranes in northern Madagascar.- Geological Society of America, 2002 annual meeting, Abstracts with Programs - Geological Society of America, 34, no. 6, 272.

Tucker, R. D.; Kusky, T. M.; Buchwaldt, R. (2001): Neoproterozoic nappes and superimposed folding of the Itremo Group, west-central Madagascar.- Geological Society of America, 2001 annual meeting, Abstracts with Programs - Geological Society of America, 33, no. 6, 448.

Buchwaldt, R. ; Tucker, R. D. (2001): P-T-time constraints on the metamorphic rocks of North Madagascar and their relevance on the assembly of Gondwanaland.- Geological Society of America, 2001 annual meeting, Abstracts with Programs - Geological Society of America, 33, no. 6, 436.