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Madagascar Madagascar is the world's fourth largest island, covering 388,740 square miles (627,000 km2) in the western Indian Ocean off the coast of southeast Africa. Madagascar consists of a highland plateau fringed by a lowland coastal strip on the east, with a very steep escarpment dropping thousands of feet from the plateau to the coast over a distance of only 50-100 miles (80.5-161 km). Madagascar was separated from Africa by continental drift, and so is geologically part of the African mainland. The highest point in Madagascar is Mount Maromokotro in the north, which rises to 9,450 feet (2,882 m); the Ankaratra Mountains in the center of the plateau rise to 8,670 feet (2,645 m). The plateau dips gently to the western coast of the island toward the Mozambique Channel, where wide beaches are located. Several islands surround the main island, including Isle St. Marie in the northeast and Nosy-Be in the north. Most of the high plateau of Madagascar was once heavily forested, but intense logging over the last century has left most of the plateau a barren, rapidly eroding soil and bedrock-covered terrain. Red soil eroded from the plateau has filled many of the river estuaries along the coast.
Precambrian rocks underlie the eastern two-thirds of Madagascar, and the western third of the island is underlain by sedimentary and minor volcanic rocks that preserve a near complete record of sedimentation from the Devonian to Recent. The Ranotsara fault zone divides the Precambrian bedrock of Madagascar into two geologically different parts. The northern part is underlain by Middle and Late Archean orthogneisses, variably reworked in the Early and Late Neoproterozoic, whereas the southern part, known as the Bekily Block, consists domi-nantly of graphite-bearing paragneisses, bounded by north-south trending shear zones that separate belts with prominent fold-interference patterns. All rocks south of the Ranotsara fault zone have been strongly reworked and metamorphosed to granulite conditions in the latest Neoproterozoic. Because the Ranotsara and other sinistral fault zones in Madagascar are subvertical, their intersections with Madagascar's continental margin provide ideal piercing points to match with neighboring continents in the East African oro-gen. Thus, the Ranotsara fault zone is considered an extension of the Surma fault zone or the Ashwa fault zone in East Africa, or the Achankovil or Palghat-Cauvery fault zones in India. The Palghat-Cauvery fault zone changes strike (orientation relative to geographic north) to a north-south direction near the pre-breakup position of the India-Madagascar border and continues across northern Madagascar. The Precambrian rocks of northern Madagascar can be divided into three north-south trending tectonic belts defined, in part, by the regional metamorphic grade. These belts include the Bemarivo Block, the Antongil Block, and the Antananarivo Block.
The Bemarivo Block of northernmost Madagascar is underlain by calc-alkaline intrusive igneous rocks (Andriba Group) with geochemical compositions suggestive of rapid derivation from depleted mantle sources. These rocks are strikingly similar in age, chemistry, and isotopic characteristics to the granitoids of the Seychelles and Rajasthan (India). The Andriba granitoids are overlain by the Dara-ina-Milanoa Group (~750-714 million years old) in the north, and juxtaposed against the Sambirano Group in the south. A probable collision zone separates the Sambirano Group from the Andriba Group. The Daraina-Milanoa Group consists of two parts: a lower, largely clastic metasedimentary sequence and an upper volcanic sequence dominated by andesite with lesser basalt and rhyolite. Like the Andriba Group, volcanic rocks of the Daraina-Milanoa Group are calc-alkaline in chemistry and have geochemical signatures indicating that these rocks were derived directly from melting in the mantle. Copper (Cu) and
| | Paleozoic and younger rocks Bemarivo Block
□ Daraina-Milanoa Group (Neoproterozoic calc-alkaline volcanics and sedimentary rocks)
□ Andriba Group (Neoproterozoic calc-alkaline intrusives)
■ Sambirano Group (Mesoproterozoic high-grade clastic rocks)
□ Late Archean biot'rte granite and granodiorite, orthogneiss, and migmatite
■ Middle Archean paragneiss and migmatite gneiss
Itremo-Amboropotsy Groups, quartzite, schist, carbonate, mafic gneiss
□ Late Archean greenstone belts, biot'rte gneiss, amphibolite, gabbro
□ Angavo-Nondiana belt high-grade late Archean and Neoproterozoic rocks
□ Early Neoproterozoic (790-640 Ma) arc-related granitoids intruding older gneiss and migmatite
I Late Archean gneiss and migmatite
I Late Neoproterozoic granitoids Bekily Block
I Vohibory, Graphite, and Androyan Sequences, ^^^ granulrte, charnock'rte, marble, and undifferentiated granulrte and amphibolite fades rocks n
Neoproterozoic Shear Zones
^ Ampanihy 0 Vorokafortra A Ranotsara
Cape St. Marie
Cape St. Marie
Compiled from Besarie (1964); Windley, et al. (1994); Kroner, et al. (2000); and Tucker (1999). O Infobase Publishing
Geological map of Madagascar showing the distribution of Precambrian rocks and shear zones in the east and the Paleozoic basins in the west gold (Au) mineralization occurs throughout the belt. The Sambirano Group consists of pelitic schist, and lesser quartzite and marble, which are variably metamorphosed to greenschist grade (in the northeast) and amphibolite grade (in the southwest). In its central part, the Sambirano Group is invaded by major massifs of migmatite gneiss and charnockite. The depositional age and provenance of the Sambirano Group is unknown.
The Antongil Block, surrounding the Bay of Antongil and Isle St. Marie, consists of late Archean biotite granite and granodiorite, migmatite, and tonalitic and amphibolitic gneiss, bounded on the west by a belt of Middle Archean metasedimentary gneiss and migmatite. These tonalitic gneisses of this region are the oldest rocks known on the island of Madagascar, dated using radiometric methods to be 3.2 billion years old. The older gneisses and migmatites are intruded by circa 2.5 billion-year-old epidote-bearing granite and granodiorite. Late Archean gneisses and migmatites near the coast in the Ambositra area may be equivalent to those near the bay of Antongil, although geochronological studies are sparse and have not yet identified middle Archean rocks in this area. Rocks of the Antongil Block have greenschist to lower-amphibolite meta-morphic assemblages, in contrast to gneisses in the Antananarivo Block which tend to be metamorphosed to granulite facies. This suggests that the Antongil Block may have escaped high-grade Neo-proterozoic events that affected most of the rest of the island. Gneisses in this block are broadly similar in age and lithology to the peninsular gneisses of southern India. High-grade psammites of the Sam-birano Group unconformably overlie the northern part of the Antongil Block, and become increasingly deformed toward the north in the Tsarantana thrust zone, a Neoproterozoic or Cambrian collision zone between the Bemarivo Block and central Madagascar. The western margin of the Antongil Block is demarcated by a 30-mile (50-km) wide belt of pelitic metasediments with tectonic blocks of gabbro, harz-burgite, and chromitites, with nickel and emerald deposits. This belt, named the Betsimisiraka suture, may mark the location of the closure of a strand of the Mozambique Ocean that separated the Antongil Block (and southern India?) from the Antananarivo Block within the Gondwanan supercontinent.
The Antananarivo Block is the largest Precam-brian unit in Madagascar, consisting mainly of 2.552.49 billion-year-old granitoid gneisses, migmatites, and schist intruded by 1,000-640 million-year-old calc-alkaline granites, gabbro, and syenite. Rocks of the Antananarivo Block were strongly reworked by high-grade Neoproterozoic tectonism between 750 and 500 million years ago, and metamorphosed to granulite facies. Large, sheet-like granitoids of the stratoid series intruded the region, perhaps during a phase of extensional tectonism. Rocks of the Antananarivo Block were thrust to the east on the Bet-simisiraka suture over the Antongil Block between 630-515 million years ago, then intruded by post-collisional granites (such as the 537-527 million-year-old Carion granite, and the Filarivo and Tomy granites) between 570-520 million years ago.
The Séries Quartzo-Schisto-Calcaire or QSC (also known as the Itremo Group) consists of a thick sequence of Mesoproterozoic stratified rocks comprising, from presumed bottom to top, quartzite, pelite, and marble. Although strongly deformed in latest Neoproterozoic time (~570-540 million years ago), the QSC is presumed to rest unconformably on the Archean gneisses of central Madagascar because both the QSC and its basement are intruded by Early Neoproterozoic (~800 million-year-old) granitoids and no intervening period of tectonism is recognized. The minimum depositional age of the QSC is ~800 million years ago, and its maximum age, of ~1,850 million years ago, is defined by U-Pb detrital zircon geochronology. The QSC has been variably metamorphosed (~570-540 million years ago; green-schist grade in the east; amphibolite grade in the west) and repeatedly folded and faulted, but original sedimentary structures and facing-directions are well preserved. Quartzite displays features indicative of shallow subaqueous deposition, such as flat lamination, wave ripples, current ripple cross lamination, and dune cross-bedding, and carbonate rocks have preserved domal and pseudo-columnar stromatolites. To the west of the Itremo Group, rocks of the Amboropotsy and Malakialana Groups have been metamorphosed to higher grade, but include pelites, carbonates, and gabbro that may be deeper water equivalents of the Itremo Group. A few areas of gabbro/amphibolite-facies pillow lava/marble may represent strongly metamorphosed and dismembered ophiolite complexes.
Several large greenstone belts crop out in the northern part of the Antananarivo Block. These include the Maevatana, Andriamena, and Befor-ana-Alaotra greenstone belts, collectively called the Tsarantana sheet. Rocks in these belts include metamorphosed gabbro, mafic gneiss, tonalites, norite, and chromitites, along with pelites and minor magnetite-iron formation. Some early intrusions in these belts have been dated by Robert Tucker of Washington university in St. Louis to be between 2.75 and 2.49 billion years old, with some 3.26 billion-year-old zircon xenocrysts and Middle Archean neodym-ium (Nd) isotopic signatures. The chemistry, age, and nature of chromite mineralization all suggest an arc setting for the mafic rocks of the Tsarantana sheet, which is in thrust contact with underlying gneisses of the Antananarivo Block. The thrust zone is not yet well documented, but limited studies indicate east-directed thrusting. Gabbro intrusions that are 800-770 million years old cut early fabrics, but are deformed into east-vergent asymmetric folds cut by east-directed thrust faults.
The effects of Neoproterozoic orogenic processes are widespread throughout the Antananarivo Block. Archean gneisses and Mesoproterozoic stratified rocks are interpreted as the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from about 1,000 to ~720 million years ago, which were emplaced prior to the onset of regional metamorphism and deformation. Continental collision related to Gondwana's amalgamation began after ~720 million years ago and before ~570 million years ago and continued throughout the Neoprotero-zoic with thermal effects that lasted until about 520 million years ago. The oldest structures produced during this collision are kilometer-scale fold-and-thrust nappes with east or southeast-directed vergence (present-day direction). They resulted in the inversion and repetition of Archean and Proterozoic rocks throughout the region. During this early phase of convergence, warm rocks were thrust over cool rocks, thereby producing the present distribution of regional metamorphic isograds. The vergence of the nappes and the distribution of metamorphic rocks are consistent with their formation within a zone of west- or northwest-dipping continental convergence (present-day direction). Later upright folding of the nappes (and related folds and thrusts) produced kilometer-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with east-west horizontal shortening (present-day direction) within a sinistral transpressive regime. This final phase of deformation may be related to motion along the Ranotsara and related shear zones of south Madagascar and to the initial phases of lower crustal exhumation and extensional tectonics within greater Gondwana.
South Madagascar, known as the Bekily Block, consists of upper amphibolite and higher-grade paragneiss bounded by north-south-striking shear zones that separate belts with prominent fold interference patterns. Archean rocks south of the Ranot-sara shear zone have not been positively identified but certain orthogneisses have Archean ages (~2.9 billion years) that may represent continental basement to the paragneisses of the region. All rocks south of the Ranotsara shear zone have been strongly reworked and metamorphosed in the latest Neoproterozoic. The finite strain pattern of refolded folds results from the superimposition of at least two late Neoproterozoic deformation events characterized by early sub-horizontal foliations and a later network of kilometer-scale vertical shear zones bounding intensely folded domains. These latest upright shears are clearly related to late Neoproterozoic horizontal shortening in a transpressive regime under granulite facies conditions.
The western third of Madagascar is covered by Upper Carboniferous (300 million years old) to mid-Jurassic (180 million years old) basinal deposits that are equivalents to the Karoo and other Gondwanan sequences of Africa and India. From south to north, these include the Morondova, Majunga (or Maha-janga), and Diego (or Ambilobe) basins. Each has a similar three-fold stratigraphic division including the Sakoa, Sakamena, and Isalo Groups consisting of mainly sandstones, limestones, and basalts, overlain by unconsolidated sands in the south and along the western coast. These basins formed during rifting of Madagascar from Africa, and have conjugate margins along the east coast of southern and central Africa. The base of the Morondova basin, the oldest of the three, has spectacular glacial deposits including diamictites, tillites, and glacial outwash gravels. These are overlain by coals and arkoses, along with plant fossil (Glossopteris) rich mudstones thought to represent meandering stream deposits. Marine limestones cap the sakoa Group. Fossiliferous deltaic and lake deposits of the Sakamena group prograde (from the east) over the Sakoa Group. The uppermost Isalo Group is 0.6-3.7 miles (1-6 km) thick, consisting of large-scale cross-bedded sandstones, overlain by red beds and fluvial deposits reflecting arid conditions. Mid-Jurassic limestones (Ankara and Kelifely Formations) mark a change to subaqueous conditions throughout the region.
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