Robert J. Thomas

Council for Geoscience

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Carte géologique de Madagascar 1:100 000. Feuille No S36 Anaboriana

Robert J. Thomas W. Bauer

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The chronology of central and north Madagascar

Bert De Waele Roger Key P.E.J. Pitfield Robert J. Thomas Robert D. Tucker

Sutures between continental plates/blocks provide key evidence for the existence of former oceans and for collisional tectonics throughout the geological record. The correct identification and delineation of these sutures is of critical importance for regional tectonic models and, as these suture zones themselves host economically important lithologies (ultramafic pods) and are important conduits for the mobilisation of precious metals, such identification can be of tremendous economic value. Unfortunately, the correct identification of suture zones in the Precambrian is hampered by various factors. The majority of sutures recorded in the literature are located in the upper crust of orogens, where the sutures have a greenschist grade of metamorphism and therefore have an identifiable anatomy that can be directly related to present-day suture zones. Older orogens have, however, invariably been highly eroded to expose predominant granulite and high amphibolite tonalite-trondhjemitegranodiorite (TTG) gneisses, as in Madagascar, and lack modern equivalents for comparison. This, together with the poor preservation of diagnostic rock assemblages (e.g. eclogite, whiteschist) adds to the difficulty in identifying ancient sutures. In current tectonic models, Madagascar comprises two Archaean toPalaeoproterozoic crustal provinces, the Anatananarivo and Antongil blocks, separated by a purported north-south Betsimisaraka suture. An east-west suture separates these older units from a younger arc terrane, the Bemarivo Block, to the north. Due to deep weathering profiles and a lack of accessible critical sections, there is very little tangible evidence for the presence of these sutures in the field. Moreover, the age data reported in the literature show a desperate shortage of data for central and northern Madagascar. Recent zircon U-Pb SHRIMP dating has confirmed that the Antongil Block comprises igneous components of 3.2 and 2.5 Ga, while in the Antananarivo Block only magmatic units of around 2.5 Ga occur. In the Antananarivo Block these Archaean units have been intruded by younger plutons at ~800 and ~550 Ma. Both the Antananarivo and Antongil blocks were strongly affected by a thermal/metamorphic event at around 550 Ma. Ages on granitoids and volcanic units in the Bemarivo Block to the north indicate a plutono-volcanic event at ~720 Ma, with no record of inheritance. The data so far thus shows some similarities between the Antongil and Antananarivo Block, which both record profuse crustal growth at 2.5 Ga. The lack of recorded 3.18 Ga dates to the west of the possible suture zone may merely reflect a scarcity of data on the Antananarivo Block, upon which further geochronological work is ongoing. The Bemarivo Block has been confirmed as a ~720 Ma arc terrane, in which a lack of Archaean inheritance suggests it to be exotic to the Antananarivo-Antongil blocks until its final docking at ~550 Ma. Work is in progress to characterise the detrital age populations of supracrustal units of central and northern Madagascar, in an effort to fingerprint the different terranes and thus reveal the cryptic sutures between the Antananarivo, Antongil and Bemarivo blocks.

Greenschist

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Vergleich ~1,1 Ga alter Terranes von Natal (Südafrika) und der Heimefrontfjella (West - Neuschwabenland / Antarktika)

Robert J. Thomas J. Jacobs Wilfried Bauer Karrie A. Weber Gerhard Spaeth

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A review of the geochronology of the Margate Terrane reveals a history of diachronous terrane docking and arc accretion across the Mesoproterozoic Natal belt, southeastern Africa

Journal of African Earth Sciences (2018)

P. Mendonidis Robert J. Thomas

Geochronology

10.1016/j.jafrearsci.2018.07.021

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Geochronology and structure of the eastern margin of the Tanzania Craton east of Dodoma

Robert J. Thomas Nick M.W. Roberts Joachim Jacobs Alphonce M. Bushi Matthew Horstwood

The precise position, nature and U-Pb zircon geochronology of the eastern margin of the Tanzania Craton has been studied in the Mpwapwa area, some 60 km east of Dodoma, central Tanzania, in a number of field transects over a ca. 45 km strike length of the craton margin. The rocks to the east of the Tanzania Craton in this area either belong to the Palaeoproterozoic Usagaran belt, or the “Western Granulite” terrane of the Neoproterozoic East African Orogen, according to different authors. The eastern part of the craton is underlain by typical Neoarchaean migmatitic grey granodioritic orthogneisses dated by ICP-MS at 2674 ± 73 Ma. There is a gradual increase in strain eastwards in these rocks, culminating in a 1 to 2 km wide, locally imbricated, ductile thrust/shear zone with mylonites indicating an oblique top-to-the-NW, transpressional sense of movement. East of the craton-edge shear zone, a series of high-grade supracrustal rocks are termed the “Mpwapwa Group”, in view of uncertain age and regional lithostratigraphic correlations. There is an apparent east-west lithological zonation of Mpwapwa Group parallel to the craton margin shear zone. In the west, immediately adjacent to the craton, the group consists of typical “shelf facies” metasediments (marbles, calc-silicates, quartzites etc.). U-Pb dating of detrital zircons from two Mpwapwa Group quartzite samples from this marginal zone contain only Archaean detritus, constraining their maximum depositional age to > ca. 2.6 Ga and suggesting that the group is Neoarchaean in age. The shelf rocks pass eastwards into garnet and kyanite-bearing semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, where the mafic amphibolite gneisses may represent meta-basalts and the felsic rocks may have meta-rhyolite, -granite or –psammite protoliths. Massive garnet-clinopyroxene amphibolite layers in the Mpwapwa Group gneisses may have been intrusive mafic sills and possibly correlate with the Palaeoproterozoic Isimani Suite, which outcrops south of the study area and includes 2 Ga eclogites. Zircons from a quartzo-feldsapthic gneiss sample from the bimodal gneisses were dated and showed it to be a probable Neoarchaean rock which underwent metamorphism during the Palaeoproterozoic Usagaran event at ca. 1950 Ma. This event was broadly coeval with subduction, closure of an ocean basin and eclogite formation further south and led to the initial juxtaposition of the two Archaean blocks. The metamorphism probably dates the tectonic event when the Archaean Mpwapwa Group rocks were juxtaposed against the orthogneissic Tanzania Craton. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1871 ± 35 Ma. Zircons in the granite have metamorphic rims dated between 550 and 650 Ma that grew during the East African orogenic event.

Mylonite

Geochronology

Yilgarn Craton

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Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

Precambrian Research (2015)

Christopher J. Spencer Robert J. Thomas Nick M.W. Roberts Peter A. Cawood Ian Millar

Continental arc

Underplating

Back-arc basin

10.1016/j.precamres.2015.05.011

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Citations (41)

Geological mapping of the ∼1.1 Ga Cape Meredith complex (west Falkland)

Journal of African Earth Sciences (1996)

Robert J. Thomas Joachim Jacobs

Cape

10.1016/0899-5362(96)86006-4

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Geology of part of the eastern margin of the Tanzanian Craton in the Mpwapwa area and its relationships with an off-craton, high-grade supracrustal gneiss sequence (Mpwapwa Group) of possible Palaeoproterozoic age

Robert J. Thomas Alphonce M. Bushi Y. Juma J. Kalimenzi A. Kankila

A number of traverses have been undertaken across a ca. 45 km section of the north-south oriented eastern margin of the Tanzanian Craton between Dodoma and Mpwapwa, Central Tanzania. The boundary is a SE-dipping zone of high strain between about 1 and 2 km wide. The rocks of the eastern craton are uniformly composed of coarse-grained grey granodioritic, migmatised orthogneisses which are heterogeneous at outcrop scale, but are regionally homogeneous. The orthogneisses have no regionally consistent fabric and foliations are variably oriented at outcrop scale. However, there is a gradual increase in strain eastwards towards the edge of the craton, manifest as an increasingly strong, regionally consistent, SE-dipping foliation. This strain increase eventually leads to mylonitic and porphyroclastic planar fabrics and strong, uniformly SE-plunging, linear fabrics. The kinematics of the high-strain mylonites show a consistent top-to-the-NW sense of movement. The frontal thrust zone grades laterally into steep sinistral and dextral oblique strike-slip shear zones to the north and south respectively. The contact is a single wide thrust zone in the north and south section of the studied area, with an imbricate belt in the central part. To the east, the cratonic rocks are in contact with a high-grade supracrustal succession, termed here the Mpwapwa Group in the light of uncertain regional correlations (= “Isimani Suite”?). It consists of a thick sequence of leucocratic quartzo-feldspathic gneisses and migmatites, semi-pelitic two mica-kyanite-garnet schists/gneisses, quartzites, marbles and calc-silicate rocks and abundant metabasic layers. There appears to be an east-west zonation of Mpwapwa Group lithological units, with most of the quartzites, calcareous rocks and pelitic schists/gneisses tending to occur close to the craton margin, with semi-pelitic gneiss/migmatite to the east, along with interlayered repetitions of bimodal acid, quartzo-feldspathic leucogneisses and mafic gneisses (amphibolite, mafic garnet amphibolite). Mineral assemblages, as evidenced by garnet-kyanite in pelitic rocks, garnet-clinopyroxene in some metabasites suggest metamorphism under moderate to high pressure amphibolite facies, as might be expected at the base of a thrust stack and resulting crustal thickening. Possibly, therefore, the Mpwapwa Group was deposited in a rifted passive-margin setting at the edge of the Tanzanian Craton, with shallow marine environments at the immediate continental margin and bimodal volcanic rocks more distally. During collision orogeny, thrusting took place at this rifted margin, inverted the Mpwapwa Group basin and transported the supracrustal rocks over the craton margin, an event which telescoped, but did not obliterate the original depositional zonation. The group may thus be viewed as a parauthochthonous succession. The rocks were intruded by plutons of largely unfoliated biotite granite, two-pyroxene charnockite and tonalite, the ages of which are unknown. With the above hypothesis and uncertainties in mind, a suite of samples are undergoing U-Pb zircon dating in order to constrain the timing of these events.

Mylonite

Thrust fault

Sillimanite

Lineation

Transpression

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Crustal architecture of the Mozambique belt in Northeastern Mozambique : perspective from U-Pb geochronology and Lu-Hf isotopes in zircon

B. Bingen Giulio Viola William L. Griffin J. Jacobs Rognvald Boyd

B. Bingen, G. Viola, W. L. Griffin, J. Jacobs, R. Boyd, R.J. Thomas, E. Daudi, I.H.C. Henderson, E. Beyer, O. Skar, A. Engvik, R.M. Key, A. Solli, J.S. Sandstad, M. Smethurst, E. Tveten, T. Bjerkgard, V.A. Melezhik, D. Jamal, R. Smith, L.M. Hollick, P. Feito 1. Geological Survey of Norway, 7491 Trondheim, Norway, bernard.bingen@ngu.no 2. Department of Earth and Planetary Sciences, Macquarie University, NSW 2109 Australia 3. Department of Earth Science, University of Bergen, 5007 Bergen, Norway 4. British Geological Survey, NG125GG Keyworth, UK 5. National Directorate for Geology, Maputo, Mozambique 6. Eduardo Mondlane University, Maputo, Mozambique

Geochronology

Geological survey

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Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

Precambrian Research (2011)

Wilfried Bauer Gregory J. Walsh Bert De Waele Robert J. Thomas Matthew Horstwood

Basement

Conglomerate

Felsic

10.1016/j.precamres.2011.07.018

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Citations (28)