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    Geological Survey of Finland, Special Paper 48 Igneous and tectonic setting of the allochthonous Tete Gabbro-Anorthosite Suite, Mozambique

    GTK Consortium Geological Surveys in Mozambique 2002–2007, edited by Yrjö Pekkala, Tapio Lehto & Hannu Mäkitie Geological Survey of Finland, Special Paper 48, 191–210, 2008

    IGNEOUS AND TECTONIC SETTING OF THE ALLOCHTHONOUS TETE GABBRO-ANORTHOSITE

    SUITE, MOZAMBIQUE

    by A. B. Phil Westerhof 1,2, André Tahon2, Tapio Koistinen3,

    Tapio Lehto3 & Christer Åkerman4 .

    Westerhof, A. B. Phil., Tahon, A., Koistinen, T., Lehto, T. & Åkerman, Ch. 2008. Igneous and tectonic setting of the allochthonous Tete Gabbro-Anorthosite Suite, Mo- zambique. Geological Survey of Finland Special Paper 48, 191–210, 10 figures.

    The Tete (Gabbro-Anorthosite) Suite in Tete Province, western Mozambique, forms an elongate, sub-horizontal sheet-like layered intrusive with a surface area of close to 6 000 km² and supposedly not exceeding 2–3 km in its thickest sections.

    The crystalline basement of Tete Province can be attributed to three major Pan-Af- rican lithospheric plates, called East, West and South Gondwana. The Tete Suite was emplaced into the Tete-Chipata Belt (TCB), a newly defined multi-terrane structural domain that forms part of West Gondwana since its collision and amalgamation during the Grenvillian orogenic cycle at ~1.06 Ga. The TCB continues into southern Zambia (as the South Irumide Belt) and is composed of an assemblage of stacked small ter- ranes, comprising a wide variety of supracrustal to mid-crustal rocks of oceanic floor/ back arc and/or continental-margin-arc setting with ages between ~1.3 and 1.08 Ga (and possibly older fragments), together with ~1.05 to 1.04 Ga partly stitching grani- toid plutons of subalkaline affinity and subordinate Pan-African intrusives.

    In more detail, the Tete Suite, together with smaller gabbro-anorthosite bodies, was emplaced at 1.05 Ga, approximately coeval with charnockites of the Castanho Granite, into undated granulites of the Chidzolomondo Group of the Luia terrane. This bimodal igneous association can be correlated with an important number of worldwide associa- tions referred to as Anorthosite-Mangerite-Charnockite-Granite (AMCG) Complexes, thought to have formed by melting of the (dehydrated) lower crust. This means that the Chidzolomondo granulites were already “dry” when the Castanho Granite charnockite and layered gabbro-anorthosite magmas formed, thus prior to 1.05 Ga.

    A large part of the Tete Suite lithologies (mainly gabbro, subordinate leucotroctolite and anorthosite, minor pyroxenite) are unmetamorphosed and show magmatic textures and mineralogy. The Tete Suite footwall, however, is a strongly deformed thin-skinned blasto- to ultramylonite. The crystalline basement below the Tete Suite has also been modified by contact-parallel tectonism and manifests the development of a variety of deformation fabrics, including blastomylonitic Augen gneiss, mylonite and phyllonite, in rocks of predominantly granitoid composition (Chacocoma and Mussata granites). Field observations indicate that Mussata granitoid mylonites mark the horizontal inter- section of a thick, subhorizontal to slightly south-dipping, ductile shear zone with S- to SE-directed top-over-bottom sense of shear. Based on geometric considerations, the thickness of the mylonite zone is estimated to be in the order of one to two kilometres. Strain is not uniform but the compound shear zone is composed of a sequence of rela- tively narrow, anastomosing, high-strain shear zones, wrapping around lower-strain “pods” or lenses.

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    Geological Survey of Finland, Special Paper 48 A. B. Phil Westerhof, André Tahon, Tapio Koistinen, Tapio Lehto & Christer Åkerman

    A hypothetical reconstruction of the original position of the Tete Suite, prior to SE- directed tectonic transport, suggest a displacement over a distance of ~ 40-45 km with the Chipera massif representing the western autochthonous extremity of the Tete Suite and the Rio Chiticula stocks possibly qualifying as a feeder conduits. Two phases of tectonic transport – syn- and late Pan-African – have been tentatively distinguished. This was followed by a long period of uplift and erosion and possibly gravity-driven transport during the formation of the mid-Zambezi rift. Finally, Cretaceous carbonatite magma has been emplaced into the thrust plane. Associated hydrous and ferruginous solutions have been subsequently injected along numerous second-order shear zones below the Tete Suite footwall, giving rise to the formation of calc-silicate rocks and a variety of hydrothermal mineralizing processes.

    Key words (GeoRef Thesaurus AGI): igneous rocks, gabbros, anorthosite, carbonati- tes, geochemistry, tectonics, augen gneiss, mylonites, shear, Precambrian, Cretaceous, Jurassic, Mozambique.

    1 Westcourt GeoConsult, Kon. Wilhelminalaan 11, 2264BL, Leidschendam, the Netherlands; corresponding author. 2 Formerly: International Institute for Geo-Information Science and Earth Observation (ITC), Hengelosestraat 99, Enschede, the Netherlands. 3 Geological Survey of Finland (GTK), P.O. Box 96, FI-02151 Espoo, Finland. 4 Sveriges Geologiska Undersökning (SGU), P.O. Box 670, S-751 28 Uppsala, Sweden

    E-mail: [email protected]

    The Tete (Gabbro-Anorthosite) Suite, formerly referred to as the Tete Gabbro-Anorthosite Com- plex, is a layered intrusive named after the town of Tete, situated some 10 km to the south. The Tete Suite forms an elongate, subhorizontal sheet-like body with a surface area of close to 6000 km². It stretches E-W over a distance of ~150 km. Its maxi- mum N-S dimension is ~60 km. The intrusive un- derlies a dissected plateau having altitudes between 200 and 400 m, with local elevations culminating at 700–1000 m. Its thickness is unknown. Thickness estimates have been made in the past on the basis of aeromagnetic data, but these are severely hampered by strong short wavelength anomalies. The 10–20 km estimate reported by Hunting (1984a, b) is irrec- oncilable with considerations from different sources (topography, structure, geophysics, field observa- tions), which indicate that the layered igneous body forms a lopolith with a thickness that probably does not exceed 2–3 km in its thickest sections.

    The contours of the Tete Suite (and surround- ing areas) have recently been mapped (Fig. 1) using modern mapping tools such as satellite ima-

    gery, airborne geophysical data and GPS-control- led field verification. Mapping was carried out by a GTK-led consortium as part of a regional resource mapping program (2002–2006) sponsored by the Nordic Development Fund and covering most of southern and central Mozambique (GTK Consor- tium 2006a, b, c, d). Being characterized by low Th- U-K radiometric signatures, airborne radiometric data proved particularly powerful in outlining ba- sic and ultrabasic rocks of the Tete Suite. The dark colours in the ternary image contrast strongly with the higher count rates in both the surrounding and underlying crystalline basement and Karoo cover (Fig. 2). High to very high, Th(-U)- and U-domi- nated radiometric point anomalies, both in the base- ment windows and within the intrusive, correspond to areas of hydrothermal alteration and associated mineralization at the base of or along fractures in the suite. The Tete Suite is further characterized by strong aeromagnetic anomalies (Fig. 2). Both geo- physical data sets show that the igneous intrusive has a block-faulted lopolith, in places covered by Karoo rocks.

    INTRODUCTION

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    Geological Survey of Finland, Special Paper 48 Igneous and tectonic setting of the allochthonous Tete Gabbro-Anorthosite Suite, Mozambique

    Fig. 1. Simplified geological map of the Tete Suite and surroundings (GTK Consortium 2006d). Two windows in the Tete Suite are named the Rio Chacocoma dome (west) and Rio Mázoè dome (east).

    Recent resource mapping (GTK Consortium 2006 a, b, c, d) favours a geodynamic model whereby the territory of Mozambique can be attributed to three major lithospheric plates – conveniently named East, West and South Gondwana – that collided and amalgamated during the Pan-African orogenic cycle (see also De Waele et al. 2003; Grantham et al. 2003, 2007; Johnson & Oliver 2004; Johnson

    REGIONAL GEOLOGY

    et al. 2005, 2006a, b, 2007). Each of these major terranes is present in the Tete Province of western Mozambique (Fig. 3). Based on structural and pet- rologic criteria, a northern crystalline domain can be separated from a southern crystalline basement. The northern crystalline domain can be divided into a small NW-SE trending fraction belonging to East Gondwana (i.e., the Angónia Group), with the

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    Geological Survey of Finland, Special Paper 48 A. B. Phil Westerhof, André Tahon, Tapio Koistinen, Tapio Lehto & Christer Åkerman

    bulk part, characterized by a WSW-ESE to SW-NE structural grain, attributed to West Gondwana (GTK Consortium 2006d). The southern crystalline base- ment, exposed south of the Sanangoè Shear Zone (SSZ) and mainly south of the mid-Zambezi Karoo rift and along the eastern margin of the Zimbabwe Craton, is attributed to South Gondwana (GTK 2006b, d, Westerhof et al. 2008, this issue).

    The West Gondwana crystalline basement in northern Tete Province belongs to a newly defined triangular structural element, called the ‘Tete-Chipa- ta Belt (TCB)’, bounded by two major Pan-African steeply-inclined shear zones and superposed Karoo rift structures - Mwembeshi Dislocat

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