The Precambrian mobile belt of southern Madagascar

Ackermand, D.; Windley, B. F.; Razafiniparany, A.

doi:10.1144/gsl.sp.1989.043.01.20

Cited by 26 publications

(16 citation statements)

References 11 publications

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“…These mostly have peak P-T estimates of 0.6-1.0 GPa and 850-1000°C with decompression-dominated clockwise P-T paths (Ackermand et al, 1989;Markl et al, 2000;Jöns and Schenk, 2011;Boger et al, 2012). Horton et al (2016) argued that peak T was >900°C in the south-central Anosyen Domain and decreased to the north and west, whereas earlier estimates of 0.3-0.5 GPa and 650-850°C for the Anosyen Domain (Nicollet, 1990;Martelat et al, 1999) are likely a retrograde overprint.…”

Section: Metamorphism Between the Suturesmentioning

confidence: 98%

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Fitzsimons

2016

Journal of Mineralogical and Petrological Sciences

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Granulites of the southern East African Orogen formed by continental collision during Gondwana assembly. The highest metamorphic gradients of 25-50°C km −1 were attained at 0.58-0.53 Ga in a microcontinental block that was sandwiched between two collisional sutures and is now exposed in Madagascar and southern India. The 50 Myr duration of extreme pressure-temperature (P-T) conditions and lack of coeval mantle magmatism suggest that metamorphism was driven by radiogenic heat accumulation beneath a long-lived orogenic plateau. Bounding sutures most likely record transfer of this microcontinent across the Neoproterozoic Mozambique Ocean, analogous to Gondwanan terranes that crossed Tethys before final India-Asia collision and, like Tibet, these sutures mark the edges of a plateau that formed following terminal ocean closure and collision. Both sutures record moderate metamorphic gradients of 15-25°C km −1 but with quite different ages. Metamorphism along the western suture at 0.65-0.61 Ga followed the end of magmatism in an adjacent 0.85-0.65 Ga ocean-arc terrane. It has an anti-clockwise P-T path that reflects preferential thickening of the hot arc during early stages of collision, and dates ocean closure at the western suture. Metamorphism along the eastern suture at 0.53-0.51 Ga has a clockwise P-T path and is widely assumed to date terminal collision in the East African Orogen. However, this event was coeval with rapid exhumation of granulites in the adjacent plateau and is more likely to reflect reactivation of a much older eastern suture during plateau collapse. Great care should be taken when using metamorphism to date ocean closure in ancient orogens. Rocks with hot metamorphic gradients give poor age constraints on initial collision because peak T is attained >50 Myr after ocean closure if radioactivity is a major part of the heat budget. Suture zone rocks with moderate metamorphic gradients can provide more reliable estimates for the time of ocean closure but are also prone to later reactivation in orogens with protracted histories.

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Section: Metamorphism Between the Suturesmentioning

confidence: 98%

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Fitzsimons

2016

Journal of Mineralogical and Petrological Sciences

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“…At the northern part of this belt south of Beraketa we have observed abundant discordant, partly deformed amphibolite dykes in hypersthene gneiss; these relations point to an early granulite facies metamorphism followed by dyke intrusion and then by amphibolite facies metamorphism. Ackermand et al (1989) reported a steep, nearly isothermal P-T trajectory from 10 to 4 kbar at 1000-800°C at Vohidava (Fig. 5); the reaction of enstatite + sillimanite to sapphirine + cordierite indicates early, very high temperature, granulite facies metamorphism above 9.5 kbar and 950°C.…”

Section: Southern Madagascarmentioning

confidence: 98%

“…The Betroka belt contains amphibolite dykes that post-date granulite facies gneisses, and also petrographic evidence of relict (undated), very early high temperature metamorphism with orthopyroxene above 950°C and 9.5 kbar (Ackermand et al, 1989). The gneisses of the Tranomaro belt contain gneissic, migmatitic granite sheets that crystallized at 565 Ma contemporaneous with granulite facies metamorphism associated with the synmetamorphic emplacement of the 570 Ma Anosyan charnockites and granites in the nearby Fort Dauphin-Anosyan belt (Paquette et al, 1994).…”

Section: Southern Madagascarmentioning

confidence: 99%

Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal

et al. 1994

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The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six n o r t h -s o u t h trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100-150 km wide dextral shear zone of probable PanAfrican age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic-ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.

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“…Windley et al 1994;Martelat 1998;Martelat et al 2000;Nédélec et al 2000) that formed at high-grade metamorphic conditions during the late stages of the East African Orogen (Nicollet 1990;Martelat et al 1997;de Wit et al 2001). Although the steep ductile zones predominantly strike N-S (present-day coordinates), previous workers also postulated a NW-SE striking ductile shear zone that traverses the entire crystalline basement of southern Madagascar (e.g., Katz and Premoli 1979;Ackermand et al 1989;Martelat 1998). This zone is generally referred to as the Ranotsara shear zone, but has previously also been termed Ranotsara-Ranomena lineament (Katz and Premoli 1979), Ranotsara shear belt (Ackermand et al 1989) or Bongolava-Ranotsara shear zone (Caen-Vachette 1979;Martelat 1998;Lardeaux et al 1997Lardeaux et al , 1999Müller 2000).…”

Section: Introductionmentioning

confidence: 99%

“…Although the steep ductile zones predominantly strike N-S (present-day coordinates), previous workers also postulated a NW-SE striking ductile shear zone that traverses the entire crystalline basement of southern Madagascar (e.g., Katz and Premoli 1979;Ackermand et al 1989;Martelat 1998). This zone is generally referred to as the Ranotsara shear zone, but has previously also been termed Ranotsara-Ranomena lineament (Katz and Premoli 1979), Ranotsara shear belt (Ackermand et al 1989) or Bongolava-Ranotsara shear zone (Caen-Vachette 1979;Martelat 1998;Lardeaux et al 1997Lardeaux et al , 1999Müller 2000). It is generally regarded as a *20-km-wide intra-crustal strike-slip shear zone (Martelat et al 2000) with a sinistral sense of shear (e.g.…”

Section: Introductionmentioning

confidence: 99%

A new perspective on the significance of the Ranotsara shear zone in Madagascar

Schreurs

Giese

Berger³

et al. 2009

Int J Earth Sci (Geol Rundsch)

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The Ranotsara shear zone in Madagascar has been considered in previous studies to be a [350-km-long, intracrustal strike-slip shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent NW-SE trending brittle faulting along most of its length. We therefore prefer the more neutral term ''Ranotsara Zone''. Lithologies, tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of E-W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550-520 Ma). The Ranotsara Zone shows significant NW-SE striking brittle faulting that reactivates part of the NW-SE striking ductile structures in the flexure zone, but also extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions.

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The Precambrian mobile belt of southern Madagascar

Cited by 26 publications

References 11 publications

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Pan–African granulites of Madagascar and southern India: Gondwana assembly and parallels with modern Tibet

Tectonic framework of the Precambrian of Madagascar and its Gondwana connections: a review and reappraisal

A new perspective on the significance of the Ranotsara shear zone in Madagascar

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