11 research outputs found
The architecture of the âBetsimisaraka Suture Zoneâ : a record of oceanic arcs and associated metasedimentary successions between the âIndianâ and âAfricanâ parts of Madagascar
Geochemical signatures of stream sediments within the main geological domains and terranes of North and Central Madagascar
Geochemical mapping of North and Central Madagascar was carried out using stream sediments at an average density of one sample per 11 km2. Over 50 elements were determined from some 13,300 stream sediments using a sample size fraction of <150 ÎŒm following hot aqua regia digestion. Partially extractable concentrations of six major elements and seven base metals reveal that the Andriamena `greenstone' Belt of the Tsaratanana Complex is geochemically distinct from the other geological divisions within the Precambrian basement of central and northern Madagascar. In particular, this study has shown the detailed spatial distribution of anomalous concentrations of base metals which confirm known areas of chromite mineralisation within the Andriamena Belt. Base metal anomalies also occur in relation to Cretaceous and Neogene volcanic rocks, for example at Nosy Be, and mafic-ultramafic intrusions such as along the Maroala deformation zone. The distribution of anomalous Au in stream sediments showed good correspondence with known gold districts of central and northern Madagascar. Highest concentrations were measured in stream sediments from within the Ampasary-Mananjary (southern Androna-Mandritsara) and Andavakoera (northern margin of North Bemarivo) gold districts. The results of the stream sediment geochemistry show that these new data provide valuable information for current and future mineral exploration and environmental studies in Madagascar, at both regional and local scale
Polyphase Neoproterozoic orogenesis within the East Africa-Antarctica Orogenic Belt in central and northern Madagascar
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 AfricaâAntarctica 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, 820â
760 Ma, 630â595 Ma and 560â520 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 âTerminal Pan-Africanâ event (560â490 Ma) finally stitched together
the Mid-Neoproterozoic cratons to form Gondwana
Post-collisional magmatism in the central East African Orogen: the Maevarano Suite of north Madagascar
Late tectonic, post-collisional granite suites are a feature of many parts of the Late Neoproterozoic to Cambrian East African Orogen (EAO), where they are generally attributed to late extensional collapse of the orogen, accompanied by high heat flow and asthenospheric uprise. The Maevarano Suite comprises voluminous plutons which were emplaced in some of the tectonostratigraphic terranes of northern Madagascar, in the central part of the EAO, following collision and assembly during a major orogeny at ca. 550 Ma. The suite comprises three main magmatic phases: a minor early phase of foliated gabbros, quartz diorites, and granodiorites; a main phase of large batholiths of porphyritic granitoids and charnockites; and a late phase of small-scale plutons and sheets of monzonite, syenite, leucogranite and microgranite. The main phase intrusions tend to be massive, but with variably foliated margins. New UâPb SHRIMP zircon data show that the whole suite was emplaced between ca. 537 and 522 Ma. Geochemically, all the rocks of the suite are enriched in the LILE, especially K, and the LREE, but are relatively depleted in Nb, Ta and the HREE. These characteristics are typical of post-collisional granitoids in the EAO and many other orogenic belts. It is proposed that the Maevarano Suite magmas were derived by melting of sub-continental lithospheric mantle that had been enriched in the LILE during earlier subduction events. The melting occurred during lithospheric delamination, which was associated with extensional collapse of the East African Orogen
Geological evolution of the Antongil Craton, NE Madagascar
The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental
Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-
Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paperwegive a synthesis of the geology
of the Antongil Craton and present previously published and new geochemical and UâPb zircon analyses
to provide an event history for its evolution.
The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-
Mesoarchaean cratons globally, including phases dated between 3320±14Ma to 3231±6Ma and
3187±2Ma to 3154±5 Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a
sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation
of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon
to around 3178±2Ma and subject to migmatisation at around 2597±49 Ma. A subsequent magmatic
episode generated voluminous, weakly foliated granitic rocks, that also included additions from both
reworked older crustal material and younger source components. An earlier granodiorite-dominated
assemblage, dated between 2570±18Ma and 2542±5 Ma, is largely exposed in xenoliths and more
continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated
between 2531±13Ma and 2513±0.4Ma is more widely developed. Together these record the stabilisation
of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained
by a sample of metagabbro dated at 2147±6 Ma, providing the first evidence for Palaeoproterozoic rocks
from the Antongil Craton.
The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism,
neocrystallisation and Pb-loss at 792±130Ma to 763±13Ma and 553±68 Ma. These events are
interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to
the assembly of the tectonic blocks that comprise the island
Provenance and tectonic significance of the Palaeoproterozoic metasedimentary successions of central and northern Madagascar
New detrital zircon U-Pb age data obtained from various quartzite units of three spatially
separated supracrustal packages in central and northern Madagascar, show that these units were
deposited between 1.8 and 0.8 Ga and have similar aged provenances. The distribution of detrital
zircon ages indicates an overwhelming contribution of sources with ages between 2.5 and 1.8 Ga.
Possible source rocks with an age of 2.5 Ga are present in abundance in the crustal segments
(Antananarivo, Antongil and Masora Domains) either side of a purported Neoproterozoic suture
("Betsimisaraka Suture Zone"). Recently, possible source rocks for the 1.8 Ga age peak have been
recognised in southern Madagascar. All three supracrustal successions, as well as the Archaean blocks
onto which they were emplaced, are intruded by mid-Neoproterozoic magmatic suites placing a
minimum age on their deposition. The similarities in detrital pattern, maximum and minimum age of
deposition in the three successions, lend some support to a model in which all of Madagascar's
Archaean blocks form a coherent crustal entity (the Greater Dharwar Craton), rather than an
amalgamate of disparate crustal blocks brought together only during Neoproterozoic convergence.
However, potential source terranes exist outside Madagascar and on either side of the Neoproterozoic
sutures, so that a model including a Neoproterozoic suture in Madagascar cannot be dispelled outright