Itremo Group Research Articles

Zircon U–Pb, oxygen and hafnium isotopic characteristics of the Neoarchean–Paleoproterozoic Betsiboka Suite, Madagascar: tracing source to sink pathways in Proterozoic and Phanerozoic provenance studies

Abstract The Neoarchean to Paleoproterozoic Betsiboka Suite of Madagascar mainly consists of granitoid orthogneiss and migmatite variably metamorphosed to amphibolite and granulite facies during the late Neoproterozoic. New U–Pb zircon data yield emplacement ages of 2512 ± 17, 2507 ± 17, 2493 ± 14 and 2485 ± 16 Ma. Zircon δ 18 O data indicate involvement of the continental crust during magma genesis. Mostly positive ε Hf ( t ) values plot slightly below the depleted-mantle curve. The isotopic data favour mixing between an older Archean crustal source and a juvenile depleted-mantle source. Possible local crustal candidates for the older component in the exposed Malagasy basement are Neoarchean mafic gneisses in the Tsaratanana Complex and the c. 3200 Ma Nosy Boraha orthogneiss in the Antongil Domain. The new isotopic data support interpretations from previous studies investigating the genesis of Tonian plutonic rocks advocating crustal melting of a Mesoarchean crustal source in the Antananarivo Domain. In addition, detrital zircon in the Proterozoic Ambatolampy Group and Itremo Group probably had a local source with the same isotopic composition as the Betsiboka Suite. Our study demonstrates the significance of multi-isotopic data from zircon for constraining the sources of detrital zircon grains in sedimentary provenance studies.

Tracing remnants of Ediacaran S-type granitoids from beach placers in SE Madagascar

Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran−Cambrian zircons (650−450 Ma) and monazites (574−484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650−450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900−1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P>25 μmol g−1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic−Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.

Tectonic evolution of an Early Cryogenian late- magmatic basin in central Madagascar

Central and southern Madagascar comprise a number of distinctive Archaean crustal blocks (the Antongil-Masora and Antananarivo domains) overlain by Proterozoic supracrustal sequences, preserved in the East African Orogen. Here, we present U–Pb and Lu–Hf isotopic data for two supracrustal units from detrital and metamorphic zircon grains. The lower sequence is comprised of quartzite and calc-silicate units with a major Palaeoproterozoic detrital zircon source and a minor Archaean contribution with a maximum depositional age of ca. 1780 Ma. This sequence reflects a stable shelf sedimentation within the Antananarivo Domain and is correlated with the Itremo Group. U–Pb and Hf data are equivocal in determining the direct sources for the Archaean and early Palaeoproterozoic detrital zircon grains. However, the abundant ca. 2.3–1.8 Ga detrital grains are correlative with the Congo-Tanzania-Bangweulu Block and as these are close to the inferred age of the Itremo Basin, these are interpreted to be single cycle detritus. This implies that the Congo-Tanzania-Bangweulu craton was close to central Madagascar at ca. 1.8–1.6 Ga and the lower sequence would correspond to an originally contiguous late Palaeoproterozoic to early Mesoproterozoic sedimentary basin across central Madagascar. The upper metasedimentary unit has contrasting detrital sources and is represented mostly by biotite-plagioclase paragneiss, with an inferred psammitic protolith interleaved with volcanic/subvolcanic andesitic/rhyolitic dikes. The predominant Tonian-aged population (ca. 860-710 Ma) are igneous zircon grains with εHf(t) values varying from −15.1 to −29.2 and TDM Hf model ages between ca. 3.4 and 2.6 Ga. These grains were derived from the ca. 850-750 Ma Imorona-Itsindro magmatic suite. Their Neoarchaean-Palaeoproterozoic cores are interpreted as xenocrysts, reinforcing that the Imorona-Itsindro magmatism has a prominent continental reworking component. The probable tectonic setting for this Early Cryogenian sedimentary basin would represent a transition from an intra-arc to an intracontinental setting related to an outboard subduction, partially jammed at ca. 710 Ma due to the subduction of a ridge-transform system. The analogue would be the western US, where the Basin and Range region corresponds to a wide rift associated with a major mantle thermal anomaly. The absence of geological units and structures between ca. 720 and 635 Ma in central Madagascar corroborate with this model for a transition to a transform continental setting. The pre-Gondwana amalgamation convergence in the Ediacaran-Cambrian, that deformed and metamorphosed all units in central Madagascar units, is accounted for by ca. 550 Ma metamorphic rims on zircon grains from the quartzites in the Itremo Group.

Proterozoic Basin Evolution and Tectonic Geography of Madagascar: Implications for an East Africa Connection During the Paleoproterozoic

AbstractMadagascar hosts several Paleoproterozoic sedimentary sequences that are key to unraveling the geodynamic evolution of past supercontinents on Earth. New detrital zircon U–Pb and Hf data, and a substantial new database of ∼15,000 analyses are used here to compare and contrast sedimentary sequences in Madagascar, Africa, and India. The Itremo Group in central Madagascar, the Sahantaha Group in northern Madagascar, the Maha Group in eastern Madagascar, and the Ambatolampy Group in central Madagascar have indistinguishable age and isotopic characteristics. These samples have maximum depositional ages >1700 Ma, with major zircon age peaks at c. 2500 Ma, c. 2000 Ma, and c. 1850 Ma. We name this the Greater Itremo Basin, which covered a vast area of Madagascar in the late Paleoproterozoic. These samples are also compared with those from the Tanzania and the Congo cratons of Africa, and the Dharwar Craton and Southern Granulite Terrane of India. We show that the Greater Itremo Basin and sedimentary sequences in the Tanzania Craton of Africa are correlatives. These also tentatively correlate with sedimentary protoliths in the Southern Granulite Terrane of India, which together formed a major intra‐Nuna/Columbia sedimentary basin that we name the Itremo‐Muva‐Pandyan Basin. A new Paleoproterozoic plate tectonic configuration is proposed where central Madagascar is contiguous with the Tanzania Craton to the west and the Southern Granulite Terrane to the east. This model strongly supports an ancient Proterozoic origin for central Madagascar and a position adjacent to the Tanzania Craton of East Africa.

U–Pb geochronology of detrital zircon in metasediments from Sri Lanka: Implications for the regional correlation of Gondwana fragments

Recent tectonic models of Sri Lankan basement suggest that the Highland Complex (HC), which is dominantly composed of metasedimentary rocks, forms part of an accretionary complex developed during Neoproterozoic subduction-collision tectonics between the Wanni and Vijayan Complexes. Here we present new U–Pb geochronological data on detrital zircon in metasediments from the HC and adjacent regions to investigate the source characteristics, deposition and regional correlations with similar rocks in adjacent Gondwana fragments. Cores of detrital zircon grains in twelve metasediment samples (quartzite, pelitic gneiss, leptynite, and metamorphosed banded iron formation) characterized by higher Th/U ratios indicating derivation from magmatic protoliths yield Neoarchean to Paleoproterozoic ages (ca. 2700–1700Ma) with minor Paleoarchean (ca. 3500Ma), Mesoproterozoic (ca. 1200Ma), and Early to Middle Neoproterozoic (ca. 800–600Ma) components. The rims of most grains as well as homogeneous grains show Late Neoproterozoic ages (582–533Ma) with lower Th/U ratios that can be interpreted as metamorphic ages. Our results constrain the depositional age of the HC sediments to be Late Neoproterozoic (ca. 600–550Ma) based on the ages of the youngest detrital zircon grains and metamorphism. Although exposed Archean magmatic units are absent in Sri Lanka, several crustal blocks including the Dharwar Craton, Coorg Block, and Salem Block in the adjacent Gondwana fragment of India carry Archean (ca. 3400–2500Ma) basement, which we consider to be the major source for the Sri Lankan detrital zircons. The Paleoproterozoic zircons might have been derived from the Congo–Tanzania–Bangweulu Block in East Africa that includes Archean to Paleoproterozoic blocks (e.g., ca. ∼2700Ma Tanzania Craton; ca. 2300–1800Ma Usagaran–Ubendian belt). The Kibaran belt (ca. 1400–1000Ma) in the Congo–Tanzania–Bangweulu Block might have served as the source of minor Mesoproterozoic zircons. Early to Mid Neoproterozoic grains might have come from the adjacent Wanni, Vijayan, and Kadugannawa Complexes in Sri Lanka (ca. 1100–750Ma). The similarity in detrital age spectra of the metasediments in the HC and those of the northern Madurai Block in southern India suggests that they could have similar provenances (Archean cratons in India and/or East Africa). Similar age distributions of detrital zircon cores reported from the Itremo Group (central Madagascar) indicate that the metasedimentary terranes in central Madagascar, southern India, and Sri Lanka could have been juxtaposed, and with sediment source including those in the East African cratons. Similar magmatic features in central Madagascar, southern India, Sri Lanka, and eastern Antarctica also suggest a series of volcanic arcs developed through a complex subduction-accretion process in these regions before the final amalgamation of Gondwana.

Towards unravelling the Mozambique Ocean conundrum using a triumvirate of zircon isotopic proxies on the Ambatolampy Group, central Madagascar

Madagascar occupies an important location within the East African Orogen (EAO), which involves a collection of Neoproterozoic microcontinents and arc terranes lodged between older cratonic units during the final assembly of the supercontinent Gondwana. The detrital zircon record of Proterozoic metasedimentary rock packages within the Antananarivo Domain (the Ambatolampy, Manampotsy, Vondrozo, Itremo and Ikalamavony Groups) is used to identify pre-orogenic tectonic affiliations of the region—affiliations that control interpretations of the evolution of the Mozambique Ocean and the formation of this part of central Gondwana. Here we focus on the Ambatolampy Group, a previously poorly known group of high-grade siliciclastic metasedimentary rocks, and compare these data to similar data from the other sedimentary packages in central Madagascar (the Vondrozo, Sofia, Manampotsy, Itremo, Molo, Ikalamavony, Androyen, Ambodiriana, Iakora and Maha Groups). New U–Pb (SHRIMP) zircon data for the Ambatolampy Group yield detrital age maxima of ~3000Ma, ~2800–2700Ma, ~2500Ma, ~2200–2100Ma and ~1800Ma. The youngest near-concordant detrital zircon age is 1836±25Ma, which we suggest represents the maximum depositional age of the Ambatolampy Group, in contrast to younger ages reported elsewhere. The detrital spectra are very similar to those from the Itremo Group and we suggest that they correlate with each other. Metamorphic zircons and zircon rims constrain the minimum depositional age to be ~540Ma; the interpreted age of metamorphism. New δ18O data (SHRIMP-SI) and hafnium (MC-LA–ICP–MS) isotopic data complement the U–Pb data and provide new constraints on the age, geochemistry and provenance of the metasedimentary rocks. We suggest that central Madagascar contained a Mesoproterozoic (to possibly Tonian) siliciclastic sedimentary basin, within which the Ambatolampy Group, and the Itremo and Maha Groups (and possibly part of the Iakora Groups) were deposited. Later sedimentary systems are represented by the Ikalamavony, Ambodiriana, Manampotsy and Molo Groups.

Geology of bastnaesite and monazite deposits in the Ambatofinandrahana area, central part of Madagascar: An overview

Bastnaesite and monazite deposits of the Ambatofinandrahana region of Madagascar are closely associated to an alkaline to peralkaline complex of Panafrican age (∼540–570Ma; Ambalavao suite), composed essentially of syenite and granite that intrude Paleoproterozoic Itremo Group metasediments, orthogneiss and metagabbro of the ∼800Ma Imorona-Itsindro suite. The peralkaline syenite is characterized by aegirine-augite and Na-amphiboles. Primary REE mineralization consists of monazite, allanite and chevkinite, however, economically exploitable mineralizations contain secondary bastnaesite and monazite that formed by late- to post-magmatic hydrothermal processes. Evidence includes silicification of microsyenite; chalcedony–barite–fluorine–bastnaesite and/or monaziteveins; phlogopite–bastnaesite veins; alteration of chevkinite to bastnaesite; monazite-bearing marble. Bastnaesite, monazite and chevkinite have similar REE spectra, which are comparable to those obtained from other hydrothermal occurrences of these minerals and from carbonatites. Phlogopite associated to bastnaesite has a particular chemical signature (Si>6 pfu and high XF) that resembles that of phlogopite from the Bayan Obo REE deposit in China. These observations, and the similarities of these mineralizations with deposits such as Gakara (Western Rift Valley, Eastern Africa) and Mountain Pass (California), incite to better define the geodynamic and magmatic context of the alkaline magmatism in this region, and to prospect for the presence of carbonatite.

A new geological framework for south-central Madagascar, and its relevance to the “out-of-Africa” hypothesis

The Precambrian shield of south-central Madagascar, excluding the Vohibory region, consists of three geologic domains, from north to south: Antananarivo, Ikalamavony-Itremo, and Anosyen–Androyen. The northern Antananarivo domain represents the Neoarchean sector of the Greater Dharwar Craton amalgamated at 2.52–2.48Ga. The Greater Dharwar Craton is overlain by several groups of Meso- to Neoproterozoic supracrustal rocks (Ambatolampy, Manampotsy, Ampasary, Sahantaha, and Maha Groups) each with a common and diagnostic signature of Paleoproterozoic detrital zircons (2.2–1.8Ga). The central domain (Ikalamavony-Itremo) consists of two distinct parts. The Itremo Sub-domain, in the east, is a structurally intercalated sequence of Neoarchean gneiss and shallow marine metasedimentary rocks of Paleo-Mesoproterozoic age (Itremo Group), the latter with Paleoproterozoic detrital zircons ranging in age between 2.2 and 1.8Ga. The Ikalamavony Sub-domain, to the west, contains abundant volcano-clastic metasediments and lesser quartzite (Ikalamavony Group), formed between 1.03Ga and 0.98Ga, and intruded by igneous rocks (Dabolava Suite) of Stenian–Tonian age. Structurally intercalated with these are sheets of Neoarchean gneiss (∼2.5Ga) and Neoproterozoic metaclastic rocks (Molo Group). Like the Itremo Group, quartzite of the Ikalamavony Group has detrital zircons of Paleoproterozoic age (2.1–1.8Ga). The southern domain of Anosyen–Androyen consists of a newly recognized suite of Paleoproterozoic igneous rocks (2.0–1.8Ga), and stratified supracrustal rocks also having Paleoproterozoic detrital zircons (2.3–1.8Ga). The contact between the Anosyen–Androyen and Ikalamavony-Itremo domains, formerly known as the Ranotsara–Bongolava shear zone, is a tightly folded and highly flattened boundary that was ductilely deformed in Ediacaran time. It is roughly equivalent to the Palghat–Cauvery shear zone in south India, and it defines approximately the boundary between the Archean Greater Dharwar Craton (to the north) and the Paleoproterozoic terrane of Anosyen–Androyen (to the south).

Discovery of itacolumites in the central highlands of Madagascar

The presence of itacolumites, or flexible quartzites, and the precise geological descriptions of the same have hitherto been reported only in India, Brazil, and the Appalachian region of the USA. In this paper, we report the discovery of itacolumites from three localities in the central highlands of Madagascar. The localities are underlain by the Mesoproterozoic Itremo Group. The occurrence and petrography of these itacolumites and the results of SEM observations are summarized in this paper. The itacolumites from Madagascar contain very coarse grained one beside the fine-to-medium-grained varieties that are common to India, Brazil, and the Appalachians. Furthermore, the itacolumites of Madagascar occur over a wide area of exposure, supporting the opinion that their formation was a result of chemical weathering.

Neoproterozoic nappes and superposed folding of the Itremo Group, west-central Madagascar

The Precambrian geology of west-central Madagascar is reviewed and re-interpreted in light of new field observations, Landsat Thematic Mapper image analysis, and U–Pb geochronology. The bedrock of the area consists of: (1) late Archean (to Paleoproterozoic) migmatite gneiss and schist; (2) Mesoproterozoic stratified rocks (Itremo, Amborompotsy, and Malakialina Groups) perhaps deposited unconformably on the older metamorphic rocks (1, above); (3) Proterozoic (∼ 1000 Ma–720 Ma) plutonic rocks emplaced into both units above (1 and 2), and; (4) latest Neoproterozoic to middle Cambrian (∼ 570–520 Ma) granitoids emplaced as regionally discordant and weakly foliated plutons throughout the regions. The effects of Neoproterozoic orogenic processes are widespread throughout the region and our observations and isotopic measurements provide important constraints on the tectonic history of the region: (i) Archean gneisses and Mesoproterozoic stratified rocks are the crystalline basement and platformal sedimentary cover, respectively, of a continental fragment of undetermined tectonic affinity (East or West Gondwanan, or neither). (ii) This continental fragment (both basement and cover) was extensively invaded by subduction-related plutons in the period from ∼ 1000 Ma to ∼ 720 Ma that were emplaced prior to the onset of regional metamorphism and deformation. (iii) Continental collision related to Gondwana's amalgamation began after ∼ 720 Ma and before ∼ 570 Ma. Collision related deformation and metamorphism continued throughout the rest of the Neoproterozoic with thermal effects that lasted until ∼ 520 Ma. The oldest structures produced during continental collision were km-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). (iv) Later upright folding of the nappes (and related folds and thrusts) produced km-scale interference fold patterns. The geometry and orientation of these younger upright folds is consistent with E–W horizontal shortening (present-day direction) within a sinistral transpressive regime. We relate this final phase of deformation 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.

Mineralogy, geochemistry and petrology of an amphibolite-facies aluminum-phosphate and borosilicate (APB)-bearing quartzite from the Mesoproterozoic Itremo Group (Central Madagascar)

The present paper reports the whole rock and mineral chemistry, textural relations and the mineral reactions under greenschist to amphibolite facies conditions of the Aluminum-Phosphate and -Borosilicate (APB)-bearing Mesoproterozoic quartzite of the Itremo Group (central Madagascar) and discusses the genesis of the protolith as well as the metamorphic P-T-conditions. The Itremo Group consists from bottom to top of a micaschist formation (metapelite and metasiltstone) (approximate to 500 m), a quartzite (approximate to 1000 m), and a stromatolitic dolomitic marble formation (> 1000 m). In the quartzite cross bedding and ripple marks document a deposition in a shallow continental shelf environment. Lazulite, augelite, trolleite, svanbergite, goyazite, crandallite, berlinite, tourmaline, apatite, celestine, anhydrite, muscovite and dumortierite with the minor constituents zircon, monazite and xenotime in addition to quartz are the rock-forming minerals. Boron and Sr have been supplied to the protolith of the quartzite by submarine hydrothermal fluids or by rivers fed by B and Sr-rich thermal springs. Using textures, stability data for augelite, trolleite, berlinite, lazulite, scorzalite, dumortierite, muscovite, kyanite and temperature data from the literature the P-T conditions of Panafrican metamorphism, which produced the prograde APB-parageneses can be approximated as 3.8 kbar and about 475 degrees C. Deposition and metamorphism of the protolith can be summarized as: i.) deposition of clay-bearing sand in a shallow continental shelf environment, ii.) chemical precipitation of phosphates and of phosphorus-rich iron hydroxides by oxidizing marine pore waters, iii.) reductive dissolution of the Fe-oxyhydroxides with crystallization of authigenic Al-phosphates in early diagenesis, iv) metamorphic overprint and crystallization during the Panafrican tectonothermal event, v) hydration reactions, element mobilisation and consequent retrograde resetting of prograde mineral equilibrium by postorogenic fluid circulation at the end of the Panafrican tectonothermal event.

Discussion on tectonic evolution of the Proterozoic Itremo Group metasediments in Central Madagascar Special Publication 206, 2003, 381–399

Ronadh Cox writes: Fernandez & Schreurs (2003) confirm previous findings about the sedimentology of the Itremo Group (Moine 1967; Cox et al . 1998). They also report a generalized Itremo Group stratigraphy which agrees (except for some thickness differences) with that measured in the Iheninkenina synclinorium south of Ibity (Daso 1986), and also matches that described by Collins et al . (2003) for the same region. This is a more complete stratigraphy than that reported by Cox et al . (1998) and Moine (1967), and should therefore be accepted as the standard for the eastern Itremo region. The authors also make some highly controversial contributions to the ongoing debate about the tectonics of central Madagascar. I disagree with some of their interpretations, and therefore offer the following alternative points of view. ### There is no evidence for tidal currents or beach sedimentation in the Itremo Group. Fernandez & Schreurs (2003) show sedimentary structures (their fig. 4a) that they interpret as herringbone cross-bedding, based on which they infer tidal currents; but herringbone cross-stratification is characterized by planar set boundaries marking the tide-reversal surfaces, whereas the set boundaries in this photograph are concave up, and parallel to the overlying foresets. The photograph in fact shows trough cross-stratification, produced by the migration of crescentic or linguloid dunes. Such structures are very common in Itremo Group quartzites. The palaeocurrent direction is not reversing, but unidirectional and approximately normal to the plane of the photograph. Conglomerates are interpreted as possible beach deposits by Fernandez & Schreurs (2003), but the presence alone of conglomerate is insufficient for this interpretation. Thin lenticular pebble and cobble deposits like those found in the Itremo Group can accumulate in many current-swept shelf environments (e.g. Johnson & Baldwin 1996), and the association with fining-upward quartzite (their fig. 3) would be more consistent with a tempestite than with beach sedimentation. The abundant large-scale dune …

Proterozoic Tectonostratigraphy and Paleogeography of Central Madagascar Derived from Detrital Zircon U‐Pb Age Populations

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

The Central Granites-Gneiss-Migmatite Belt (CGGMB) of Madagascar: the Eastern Neoproterozoic Suture of the East African Orogen

The northeastern part of Madagascar is characterized by Archaean to early Proterozoic rocks composed principally of Archaean granite and greenstone/amphibolite as well as reworked migmatite with subordinate Proterozoic paragneisses. The southern part is mostly occupied by Proterozoic rocks, composed mostly of Meso to Neo-Proterozoic and less metamorphic metasediments (Itremo Group) in the northwest, para- and ortho-gneisses in most other areas, with minor granitic gneisses with some Archaean components in the southeast. The north-northwest trending Central Granite-Gneiss-Migmatite Belt (CGGMB) is situated at the western margin of the Archaean-early Proterozoic terrain. The CGGMB is composed of granite, gneiss and migmatite with distinct lithologies and structures. They are: i) many types of granites including alkaline to mildly alkaline granites, and calc-alkaline granites; ii) batholitic granites, migmatitic granites and granite dyke swarm, iii) eclogite, and iv) the Ankazobe-Antananarivo-Fianarantsoa Virgation. The CGGMB was formed by the collision of the palaeo-Dharwar Craton to the east and the East African Orogen to the west at ca. 820-720 Ma and suffered indentation by a part of the western part of the East African Orogen at ca. 530 Ma that produced the Ankazobe-Antananarivo-Fianarantsoa Virgation at the centre of the CGGMB. Thus, the CGGMB is proposed to be the continuation of the eastern suture between the palaeo Dharwar Craton and the East African Orogen, and carries the main feature of the Pan-African collisional event in Madagascar.

Age constraints on the tectonic evolution of the Itremo region in Central Madagascar

The Itremo region in Central Madagascar comprises a deformed metasedimentary sequence (Itremo Group) that has undergone greenschist to lower amphibolite facies metamorphism. During a first phase of deformation (D1) Itremo Group sediments were deformed into a fold-and-thrust belt and transported toward the E to NE on top of migmatitic gneisses rocks of Anatananarivo block. A second phase of deformation (D2) affected both the fold-and-thrust belt and structurally underlying units, and formed large-scale N-S trending folds with steeply dipping axial planes. A Late Neoproterozoic Th–U–Pb XRF monazite age (565±17 Ma) dates the emplacement of a granite that truncates first-phase structures in the Itremo Group, and indicates that the fold-and-thrust belt formed prior to ≈565 Ma. Th–U–Pb electron microprobe dating was applied to elongated monazites that lie within the first-phase foliation of Itremo Group metapelites. The detrital cores of zoned monazites reveal two distinct age populations at ∼2000 and 1700 Ma, the latter age giving a maximum depositional age for the Itremo Group. Statistical analysis of ages determined from the rims of zoned monazites and from unzoned monazites indicates three Late Proterozoic–Early Paleozoic monazite growth events at about 565–540, 500 and 430 Ma. The oldest age population is contemporaneous within error, with the intrusion of the dated granite. The two younger age populations are found both in the Th–U–Pb and Ar–Ar data; together with the perturbation of the Rb–Sr system we interpret both ages as due to alteration related to fluid circulation events, possibly connected to the emplacement of pegmatite fields in Central Madagascar. Syn-D1 tectonic growth of contact metamorphism minerals such as andalusite has been observed locally in metapelites along the margin of Middle Neoproterozoic (≈800 Ma) granites, suggesting that D1 in the Itremo Group is contemporaneous with the intrusion of granites at ≈800 Ma. The N-S trending D2 folds are associated with ≈E-W shortening during the final assembly of Gondwana in Late Neoproterozoic–Early Cambrian times.

Quartz fabric evidence for early Pan‐African penetrative east‐directed shearing in the Itremo Supracrustal Group of central Madagascar

ABSTRACT The Pan‐African orogeny left a strong imprint on the basement rocks of Madagascar, which were metamorphosed up to granulite facies conditions. The supracrustal Itremo Group of central Madagascar, comprising quartzites, schists and carbonates of lower metamorphic grade, has to date been described as a folded sedimentary sequence. Despite their fine‐grained ‘sugary’ appearance, most quartzites are plastically deformed tectonites. Quartzite microstructures are mainly of the elongate mosaic type, indicating significant grain boundary migration, and are compatible with dynamic recrystallization under lower amphibolite facies conditions. Consistent asymmetric quartz c‐axis fabrics indicate a dominant top‐to‐the‐east shear sense. Hence, the Itremo quartzites bear evidence for a major eastward‐directed tectonic event of Pan‐African age, possibly resulting from an early Pan‐African thrust motion. Younger deformational events, responsible for localized mylonites with top‐to‐the‐WSW sense of motion and N–S‐trending folds and shear zones, were superimposed on this first fabric.

Tectonic evolution of the Proterozoic Itremo Group metasediments in central Madagascar

Abstract The major geologic units of the Itremo region in central Madagascar include: (1) upper amphibolite to granulite facies (higher grade) Precambrian rocks, mainly para- and orthogneisses, and migmatites; (2) the newly defined Itremo Nappes, a fold-and-thrust belt containing the Proterozoic Itremo Group sediments, metamorphosed at greenschist to lower amphibolite facies (lower grade) conditions: (3) Middle Neoproterozoic and Late Neoproterozoic-Cambrian intrusives. The stratigraphic succession of the Itremo Group in the eastern part of the Itremo region is, from bottom to top: quartzites, metapelites, metacarbonates and metapelites overlain by metacarbonates. During D1 the Itremo Group sediments were detached from their continental substratum, deformed into a fold-and-thrust nappe (Itremo Nappes), and transported on top of higher grade rocks that are intruded by Middle Neoproterozoic ( c. 797–780 Ma) granites and gabbros. A second phase of deformation shortening (D2) affected both the Itremo Sedimentary Nappes and structurally underlying higher-grade rocksunits, and formed large-scale N-S-trending F2 folds. S1 axial plane foliations in Itremo Group sediments are truncated by Late Neoproterozoic-Cambrian granites ( c. 570–540 Ma). The age of the formation of the Itremo Nappes is not well constrained: they formed in Neoproterozoic times between 780 and 570 Ma.

Juxtaposition of India and Madagascar: A Perspective

Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar ( ca 2.6–3.0 Ga) in the north and the older Sargur ( ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments.

Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana palaeogeography

Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform. SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69 Ga, and that the depositional age of the Itremo Group must be less than 1855 ± 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 ± 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland.