The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U–Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil–Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo–Mesoarchean (3.32–3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55–2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the “Betsimisaraka suture” is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro–Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560–520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo–Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.
Sutures between continental plates/blocks provide key evidence for the existence of former oceans and for collisional tectonics throughout the geological record. The correct identification and delineation of these sutures is of critical importance for regional tectonic models and, as these suture zones themselves host economically important lithologies (ultramafic pods) and are important conduits for the mobilisation of precious metals, such identification can be of tremendous economic value. Unfortunately, the correct identification of suture zones in the Precambrian is hampered by various factors. The majority of sutures recorded in the literature are located in the upper crust of orogens, where the sutures have a greenschist grade of metamorphism and therefore have an identifiable anatomy that can be directly related to present-day suture zones. Older orogens have, however, invariably been highly eroded to expose predominant granulite and high amphibolite tonalite-trondhjemitegranodiorite (TTG) gneisses, as in Madagascar, and lack modern equivalents for comparison. This, together with the poor preservation of diagnostic rock assemblages (e.g. eclogite, whiteschist) adds to the difficulty in identifying ancient sutures. In current tectonic models, Madagascar comprises two Archaean toPalaeoproterozoic crustal provinces, the Anatananarivo and Antongil blocks, separated by a purported north-south Betsimisaraka suture. An east-west suture separates these older units from a younger arc terrane, the Bemarivo Block, to the north. Due to deep weathering profiles and a lack of accessible critical sections, there is very little tangible evidence for the presence of these sutures in the field. Moreover, the age data reported in the literature show a desperate shortage of data for central and northern Madagascar. Recent zircon U-Pb SHRIMP dating has confirmed that the Antongil Block comprises igneous components of 3.2 and 2.5 Ga, while in the Antananarivo Block only magmatic units of around 2.5 Ga occur. In the Antananarivo Block these Archaean units have been intruded by younger plutons at ~800 and ~550 Ma. Both the Antananarivo and Antongil blocks were strongly affected by a thermal/metamorphic event at around 550 Ma. Ages on granitoids and volcanic units in the Bemarivo Block to the north indicate a plutono-volcanic event at ~720 Ma, with no record of inheritance. The data so far thus shows some similarities between the Antongil and Antananarivo Block, which both record profuse crustal growth at 2.5 Ga. The lack of recorded 3.18 Ga dates to the west of the possible suture zone may merely reflect a scarcity of data on the Antananarivo Block, upon which further geochronological work is ongoing. The Bemarivo Block has been confirmed as a ~720 Ma arc terrane, in which a lack of Archaean inheritance suggests it to be exotic to the Antananarivo-Antongil blocks until its final docking at ~550 Ma. Work is in progress to characterise the detrital age populations of supracrustal units of central and northern Madagascar, in an effort to fingerprint the different terranes and thus reveal the cryptic sutures between the Antananarivo, Antongil and Bemarivo blocks.
Our recent geological survey of the basement of central and northern Madagascar allowed us to reevaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO).Five crustal domains are recognised, characterised by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts.Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex).The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks.The older rocks were intruded by plutonic suites at about 1000 Ma, and at 820 to 760 Ma, 630 to 595 Ma and 560 to 520 Ma.The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: 1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures; 2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins.Subsequent mid-Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism.The global 'Terminal Pan-African' event (from 560 to 490 Ma) finally stitched together the mid-Neoproterozoic cratons to form Gondwana.
Abstract Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa–Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820–760 Ma, 630–595 Ma and 560–520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid-Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global ‘Terminal Pan-African’ event (560–490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana.
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