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Timestamp: 2019-04-20 07:06:47+00:00

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Central Madagascar is presently receiving much attention by geologists interested in unravelling the complex plate interactions responsible for the closure of the Mozambique Ocean and the amalgamation of Gondwana. There are a number of good reasons for this interest. First and foremost is that Madagascar preserves an exciting array of meta-sedimentary and meta-igneous rocks that formed in a number of different tectonic environments and, as Tucker et al. (1999b) showed, have a long history of intrusion dating back to the Archaean. Secondly, both zircon geochronology (Kroner et al., 2000; Kroner et al., 1999a; Tucker et al., 1999b) and Rb/Sr isotopes (Vachette and Hottin, 1971) show that the east of the island was not affected by Neoproterozoic thermal metamorphism. In other words, the thermal front of the East African Orogen passes through central Madagascar.
Up until recently, very little work on the tectonic evolution of central and north Madagascar had been done (Windley et al., 1994 being a notable exception). In contrast, much work over the last decade has been done on the U-Pb geochronology of the area (Cox et al., 1998; Guerrot et al., 1993; Handke et al., 1997; Ito et al., 1997; Kroner et al., 1999a; Kroner et al., 1999b; Pacquette and Nkdklec, 1998; Tucker et al., 1999a; Tucker et al., 1999b). This combined work identified four major periods of zircon growth in central and north Madagascar 1) pre-2.6 Ga; 2) 2.6-2.4 Ga; 3) 830-715 Ma; and 4) 630-500 Ma. However, very little information was published as to how the complex metamorphism and multiphase deformation prevalent in the island relates to these ages. The work of the group I have been involved with at the Universities of Leicester, U.K., Mainz, Germany, Toliara, Madagascar and the Tectonics Special Research Centre, Perth, Australia, aims to redress this balance by identifying the main tectonic units of Madagascar and examining the structural and time relationships between them.
As the first stage of this work, we have identified five tectonic units on the basis of depositional, intrusion and metamorphic history and are separated from each other by major tectonic boundaries (Collins et al., 2000a; Collins et al., 2000b; Collins et al., 2000c; Collins et al., 2000d; Kroner et al., 2000). These units are 1) the Antongil Block; 2) the Antananarivo Block; 3) the Tsaratanana Thrust Sheet; 4) The Itremo Sheet; and 5) the Bemarivo Orogenic Belt (Fig. 1).
The Antongil block consists of a granitic and gneiss core, semi-encircled by a series of metasediments. It is characterized by the presence of lower temperature metamorphic assemblages (greenschist-lower amphibolite facies) than those found within rocks from the centre of the island (Hottin, 1976). The crystalline core of the Antongil block consists of ortho- and paragneiss dating back to 3127 Ma intruded by -2550 Ma granite bodies. Rb/Sr (Vachette and Hottin, 1971) and U/Pb zircon (Collins et al., 2000e; Tucker et al., 1999b) are not reset, demonstrating that the Antongil block was not affected by the Proterozoic tectono-thermal events so characteristic of the rest of Madagascar. A series of psammitic metasediments unconformably (Hottin, 1976) overlies the crystalline core to the north and west of the outcrop. In the north, these sediments pass up into the overthrust Bemarivo orogenic belt. To the west, these sediments pass up into a highly deformed belt of graphitic pelites in association with podiform harzburgites, chromitites, and emerald deposits that separate the Antongil block from the structurally overlying Antananarivo block. This boundary zone is interpreted as a strand of the Mozambique Ocean suture.
Fig. 1. The five tectonic units of central and northern Madagascar. BS = approximate extent of the Betsimisaraka suture. BSZ = Betsileo shear zone. RS = Ranotsara shear zone. Masoala = Masoala Peninsula. Antongil = Bay of Antongil. A = Antananarivo. ToC = Tropic of Capricorn.
gneiss fabrics. Contractional deformation, related to the thrusting of the Antananarivo block over the Antongil block, is concentrated in the east. A later phase of granitoid magmatism occurred between 630 and 561 'Ma and produced the stratoid granites so characteristic of this region. These late granites were intruded coeval with extensional deformation (NCdClec et al., 1995) associated with the crustal-scale extensional Betsileo shear zone. Deformation is sealed by the 530 Ma Carion granite (Kroner et al., 2000).
The Tsaratanana thrust sheet is composed of basic gneiss, tonalites, podiform chromite-bearing ultrabasic rocks and pelites that have been metamorphosed to ultra- high temperatures (Nicollet, 1990). Early intrusions have been dated as between 2.75 and 2.49 Ga with zircon xenocrysts passing back to 3.26 Ga (Collins et al., 2000c; Tucker et al., 1999b). 800-770 Ma gabbros cut the already deformed older rocks (Guerrot et al., 1993), however, they are themselves deformed into asymmetric folds and are cut by east-directed thrusts. This deformation occurred after a -630 Ma phase of granitoid intrusion. A mylonite zone that locally preserves evidence for top-to-the-east thrusting everywhere underlies the Tsaratanana thrust sheet. It was tectonically emplaced prior to the 780 Ma magmatism as these intrusions are found throughout central Madagascar.
(the Betsileo shear zone, Collins et al., 2000d). The Itremo sheet itself has not been deformed during extensional deformation and appears to have passively slid on the Betsileo shear zone.
The Bemarivo orogenic belt is a discrete tectonic region in the north of Madagascar. At map-scale this tectonic unit crosscuts the Antananarivo block, the Antongil block and the Betsimisaraka suture (Fig. 1). The Bemarivo orogenic belt comprises two discrete regions. A southern region dominated by upper amphibolite- and granulite- grade metasedimentary gneiss and a northern region characterised by granitic dome-like massifs that intrude through migmatites and orthogneiss (Jourde et al., 1974). Three major meta-volcanosedimentary regions also occur in this northern region: the Daraina, Milanoa and Betsiaka series (together forming the Daraina Group). The Dariana Gp. formed at -715 Ma (Tucker et al., 1999a) was later deformed into upright isoclinal folds. The southern region was deformed by top-to-the-south thrusting coeval with metamorphism of granulite-grade temperatures. Tucker et al. (1999a) dated this metamorphic event by U-Pb in monazite and sphene as between 510-520 Ma. These ages are the youngest for metamorphic minerals so far discovered on Madagascar indicating that the Bemarivo orogenic belt was thrust over the already amalgamated collage of central Madagascar in Cambrian times. The tectonic unit experienced a phase of extensional collapse after this thrusting.
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Arrested charnockite formation has been reported in several earlier studies from southern India and Sri Lanka (e.g., Srikantappa et al., 1985; Hansen et al., 1987; Santosh et al., 1990), and has been central to the theme of fluid-controlled granulite formation in the East Gondwana crustal fragments (Yoshida and Santosh, 1994). The development of orthopyroxene-bearing anhydrous assemblages within upper amphibolite facies gneisses on a mesoscopic scale are taken to indicate the role of C0,-rich fluids in dehydration and granulite formation (Santosh, 1991).
(Fig. l), Pyroxene granulites and calc-silicates occur subordinately. The leptynites and khondalites carry disseminations of graphite. Needles of sillimanite occur along foliation planes in the khondalites. In many localities, garnet in the gneisses and metapelites occur as porphyroblasts, often aligned or stretched parallel to the foliation of the rock. At Kunnathukal, feldspar porphyroblasts define sharp augen structure. These features are being reported here for the first time, and might indicate the existence of a shear zone at the southern margin of the KJB, dividing the TGB and NGB, similar to the Achankovil Shear Zone which marks the northern boundary of the KKB.

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